Photophysical parameters DCM [7, 8, 9, 10, 11, 12, 13, 14, 15, 16].
Abstract
4-(Dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) is, commonly known as red dye, an electron donor-acceptor molecule that exhibits very interesting photophysical properties such as high molar absorption coefficients, tunable electronic absorption and fluorescence emission energies, and high fluorescence quantum yields. Several DCM analogous have been synthesized and explored for various practical applications that include solid-state lasers, organic light-emitting diode (OLED), fluorescent sensors, logic gates, photovoltaics, nonlinear optics (NLO), and bioimaging of cells. In recent years, a significant amount of research work has been devoted for developing optical sensors based on DCM dye for detection of various guest analytes. The first part of this book chapter describes comprehensive photophysical properties of the DCM dye which include the results of steady-state and time-resolved absorption and fluorescence studies. The second part of the book chapter summarizes the recent developments of DCM-based optical sensors that exhibit colorimetric, ratiometric, and fluorosensing towards selective detection of metal cations, anions, and neutral species.
Keywords
- red dye
- electron donor-acceptor molecules
- photophysical properties
- optical sensors
- NIR fluorescence
- dicyanomethylene-4H-benzopyran
- dicyanomethylene-4H-pyran
- chemosensors
1. Introduction
The molecule, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4
2. Photophysical properties of DCM
2.1 Absorption
Electronic absorption spectrum of DCM in polar medium, dimethylsulphoxide (DMSO) was reported for the first time by Hammond in 1979 [7]. Later on, the DCM dye absorption spectra in different medium were studied in various contexts, and it is observed that the electronic absorption behavior is quite similar to many charge transfer (CT) dyes [7, 8, 9, 10, 11, 12, 13, 14, 15, 16]. The DCM dye has very broad absorption, typically in between 200 and 600 nm, and the nature of the absorption strongly depends upon polarity of the medium (Figure 1 and Table 1) [11, 12, 15, 16]. The DCM exhibits two absorption bands where the longer-wavelength band is found to be more intense than the shorter-wavelength band. In non-polar solvents, the shape of the longer-wavelength band is found to be more structured (vibronic structure) like any other CT dye molecules, and in polar solvents the structured nature disappears [17]. For example, the electronic absorption spectrum of DCM in cyclohexane shows two bands: structured longer-wavelength band with a maximum at 451 nm and shoulder at 340 nm [11]. However, when the same DCM dye is present in highly polar solvent like DMSO, a structureless longer-wavelength band is observed with maxima at 482 nm with a shoulder at 350 nm. Interestingly, the electronic absorption maximum of DCM undergoes a redshift upon increasing with the polarity of the medium which is commonly known as solvatochromic shift of the absorption. The solvatochromic behavior is more prominent in polar aprotic solvents than that of polar protic solvents with that of non-polar solvents. For example, the absorption maxima of DCM in cyclohexane and DMSO in the solvatochromic shift is found to be ~30 nm, which is relatively less (~20 nm) when compared to cyclohexane and ethanol absorption maxima. From the Lippert-Mataga theory [18, 19, 20], ground-state dipole moment (μa) of the DCM was estimated to be 5.6 D which suggests that the DCM is a dipolar molecule [11]. It is well-known in the literature that an electronic state of a dipolar molecule is more stabilized in polar solvents rather than in less polar or non-polar solvents. So, the observed solvatochromic behavior in different solvents is attributed to the extent of dipole–dipole interactions in the respective solvents. Dipole–dipole interactions are prominent in polar solvents and aromatic solvents, and corresponding energy state will be relatively more stabilized; thereby a redshift of the absorption maxima is quite obvious. Similarly, the structured longer-wavelength absorption band, observed in non-polar solvents, is primarily due to the vibronic coupling where the vibrational energy levels are well separated and thus their vibrational transitions become prominent. The vibronic coupling is even more prominent at the low-temperature (77 K) experiments and can be attributed to the absence of dipole–dipole interactions [15]. Molar absorption coefficients of DCM in ethanol are estimated to be 4.2 × 104 mol−1 cm−1 at its absorption maxima (470 nm) and 1.2 × 104 mol−1cm−1at the shoulder (337 nm).
Sl. no | Solvent | ϕf | τ (ns) | |||
---|---|---|---|---|---|---|
1 | n-Hexane | 451 | 530 | 79 | 0.05 | 0.015 |
2 | Cyclohexane | 454 | 533 | 79 | — | — |
3 | 1,4-Dioxane | 456 | 566 | 100 | — | — |
4 | Toluene | 461 | 567 | 106 | 0.08 | 0.022 |
5 | Chloroform | 471 | 565 | 94 | 0.35 | — |
6 | Tetrahydrofuran | — | — | — | 0.49 | 1.24 |
7 | Dichloromethane | 468 | 587 | 109 | — | — |
8 | Acetonitrile | 463 | 617 | 154 | 0.45 | 1.95 |
9 | DMF | 475 | 626 | 151 | — | — |
10 | DMSO | 481 | 644 | 163 | 0.80 | 2.25 |
11 | Methanol | 466 | 623 | 157 | 0.3 | 1.36 |
12 | EtOH | 470 | 614 | 144 | — | — |
13 | n-Propanol | 472 | 614 | 142 | 0.57 | 2.10 |
14 | PMMA | 453 | 550 | 97 | 0.76 | 2.00 |
2.2 Steady-state fluorescence
Fluorescence emission behavior of DCM laser dye in a variety of solvents has been measured [11, 16]. DCM dye molecule exhibits a single structured fluorescence band in non-polar aprotic solvents. For example, the fluorescence maxima of DCM in isooctane and cyclohexane are found to be at 533 nm and 530 nm, respectively (Stoke shift of ~80 nm), which shifts its maxima (to 566 nm) upon increasing polarity of the solvent (1,4-dioxane). On the other hand, in dipolar aprotic and protic solvents, the fluorescence maxima shift towards the red region of the visible light, undergo a little change in shape of the band, and are accompanied by a new fluorescence band with its maximum above 610 nm. Similarly, the DCM emits at 635 and 626 nm in DMSO and DMF, respectively, that gives rise to ~150 nm Stokes shift. Furthermore, from the systematic fluorescence study, it was observed that the short-wavelength fluorescence intensities depend upon solvent polarity and that the intensity of the longer-wavelength band enhanced monotonically with increasing polarity of the solvent. The structured fluorescence emission band in non-polar solvent is attributed to the Franck-Condon or locally excited (LE) state where the DCM molecular structure/configuration is almost same as the ground-state configuration. The dynamic Stokes shift of the fluorescence emission maxima in polar solvents indicates that the nature of the emitting state is changing to a highly polar state and the solvation of DCM molecules further stabilizing the emitting state. From Stokes shift values obtained in different solvents and by using Lippert-Mataga theory, the excited-state dipole moment (μe) was estimated to be 26.3 D [16], which further supports the high dipolar nature of DCM emitting state. A large change in dipole moment (~20 D) from ground state to the excited state resulted in a large Stokes shift (~150 nm) from non-polar solvents to the polar solvents. The estimated μe and large change in dipole moment upon photoexcitation also explain why Stokes shift is more than the solvatochromic shift. Since μe is very high, it is likely that the DCM molecule mostly exists in the planar confirmation in charge-transfer (CT) state which will be relatively more stabilized by polar solvents rather than non-polar solvents. Further, it was observed that both the spectral shifts are correlating with Lippert-Mataga solvent parameter, ∆
In order to understand the nature of the emitting state, titration experiments were carried out in which aliquots of pure ethanol solvent are added gradually to DCM and dioxane solution [9]. It was observed that the original fluorescence band in pure dioxane is redshifted upon gradual addition of ethanol to the DCM-dioxane solution and concurrently produces initially a longer-wavelength fluorescence band with a maximum at 610 nm which reduces its intensity beyond certain ethanol concentration (10−4 M) and emerged to a new fluorescence band with a maximum at 630 nm. However, further increase of ethanol concentration beyond this limit did not shift the position of the longer-wavelength fluorescence maximum but increases intensity of fluorescence band despite the fact that there is significant increase in the polarity of the binary solvent mixture. Fluorescence quantum yield (ϕf) of DCM highly depends upon the polarity of the solvent. For example, in n-hexane solvent, DCM quantum yield is calculated to be 0.05, and in polar DMSO solvent, quantum yield is estimated to be 0.81 [15]. Therefore, the quantum yield of DCM in non-polar solvents are less and in polar solvents high (Table 1). The observed high fluorescence quantum yields in polar solvents can be understood in terms of the CT character of the DCM dye. From the initial steady-state fluorescence studies, it was proposed that the DCM dye molecule emits a single fluorescence, and a three-state model was proposed in order to explain the fluorescence spectral behavior, and all the solvents and the emitting state would be either LE state. Therefore, solvatochromic behavior of DCM was attributed to the change in their dipole moment of the ground state and excited state where fluorescence spectral shift increases due to an increased dipole moment upon excitation and to the interaction of this dipole with the polar solvent cage.
As can be understood from the molecular structure (Figure 1), the DCM dye can present in either cis-confirmation or trans-configuration because of π-spacer. So, the photophysics of cis- and trans-isomerization of DCM were studied by Drake and co-workers [10]. The DCM solutions were analyzed by high-pressure liquid chromatography (HPLC) and nuclear magnetic resonance (NMR), and they found that in the freshly prepared solutions, the DCM exists in trans-configuration (in dark). However, DCM solution when exposed to ambient light, trans-DCM converts in to cis-DCM whose ratio depends on the solvent. From HPLC study, absolute absorption cross sections for both isomers were measured for the first time. The fluorescence quantum yield of trans-isomer is found to be more than that of the cis-isomer because of the less non-radiative rate of the trans-DCM. Temperature dependence of the fluorescence emission spectra of both isomers in methanol, dimethylsulphoxide (DMSO), and lipid bilayers was studied [14]. These results suggest that the fluorescence spectral behavior of the two isomers is almost overlapping while their fluorescence decay times are found to be distinct. Furthermore, cis-DCM fluorescence was measured for the first time in DMSO solvent along with the trans-DCM, and it is observed that the cis-isomer fluorescence quenches to give the trans-DCM.
Based on the steady-state absorption and fluorescence studies in a variety of solvents, a mechanism has been proposed to understand photophysical properties (Figure 2) [9]. The DCM dye may be thought of an ionic merocyanine-like electron donoracceptor (EDA) dye molecule in which an electron-donating N,Ndimethylaniline moiety is covalently connected with a conjugated π-electron spacer and an electron-accepting dicyanomethylene moiety. Electronic excitation of DCM molecules leads to the formation of locally excited (LE) state immediately after photoexcitation. So, the fluorescence emission of DCM in non-polar solvents predominantly occurs from LE state, formed via π-π* transitions, and has an electronic configuration similar to that of the ground state, which is evident from the vibronic fluorescence emission band. However, in polar solvents, excited DCM molecules emitted from ICT state, which are characterized by a planar molecular conformation, are formed immediately after photoexcitation under the influence of the electric polarization of the surrounding solvent molecules, and it is argued that the short-wavelength fluorescence primarily originated from ICT state. This also explains why a gradual shift in the position of the fluorescence band is observed from a non-polar aprotic solvent to a polar solvent. Further, interpretation of the additional long-wavelength fluorescence was not that easy as expected; however, the preliminary fluorescence lifetime data suggest that it is generated from excited DCM in a new ICT state which is formed during the lifetime of the lowest excited singlet state and equilibrates with the ICT state emitting at 610 nm. It was suggested that the dual fluorescence originates from the excited DCM in the ICT state with a twisted conformation formed by internal rotation of the donor moiety with simultaneous ICT from this group to a suitable acceptor orbital. The new state is commonly known as twisted intramolecular charge transfer state (TICT) which was first reported by Grabowski and co-workers [21] to explain dual fluorescence of structurally different compounds such as p-cyano and p-(9-anthryl) derivatives of N,N-dimethylaniline in polar solvents [17, 22]. Typically, the TICT state is characterized by a perpendicular conformation of donor and acceptor moieties which is responsible for dual fluorescence of p-N,N-dimethylaminobenzonitrile (DMABN). However, unlike DMABN molecule, it should be noted that the difference between the short- and long-wavelength maxima of the dual fluorescence of DCM is somewhat smaller than that calculated for DMABN. This may be because the larger separation between the D and A moieties in DCM leads to a smaller fraction of charge transfer than that of DMABN.
Contrary to the above three-state model, a combined experimental and theoretical study revealed quite different results from the measured absorption and steady-state emission spectra of DCM dye upon its comparison with Nile red in a series of aprotic solvents with similar refractive index and different polarity [16]. Unlike many other studies reported earlier, the observed spectral behavior is interpreted to two-state electronic model accounting for the coupling to internal molecular vibrations and to an effective solvation coordinate. This study pointed out that change in band shapes upon varying solvent cannot be accounted as an evidence for two different emitting states and explained all the observed solvatochromic behavior of absorption and fluorescence spectra. Based on the consistency between experimental and calculated spectral data, a two-state model was suggested for understanding DCM photophysical properties which is generally also valid for most of the of the electron donor-acceptor (EDA) molecules.
2.3 Fluorescence lifetimes
Fluorescence lifetimes of DCM were measured in six different solvents for the first time, and it is found that the fluorescence times (τ) depend upon the polarity of the solvent [8]. Later on, wavelength dependent fluorescence decay profiles of DCM in protic-polar solvent (ethanol) and other solvents were measured, and it is found that all the decays profiles are fitting with single exponential function despite the strong overlap between the two fluorescence bands [9]. Moreover, these studies clearly reveal that the fluorescence lifetime value of DCM in a given solvent is independent of the fluorescence wavelength at which the measurement was made. In order to obtain more information about the nature of the emitting states of DCM in polar solvents, the fluorescence spectra of DCM in DMSO were recorded at various times after excitation. From typical time-resolved emission spectral data, it was observed that both short- and long-wavelength fluorescence bands appear within the 0.75 ns after excitation. Further, their relative intensities change with time until a time-independent intensity ratio is reached, at about 2.25 ns. Wavelength-dependent time-resolved fluorescence measurements also suggest that DCM exhibits dual fluorescence in polar solvents which is assigned to the two well-separated different emitting states. Based on the steady-state and time-resolved fluorescence data, Hsing-Kang and co-workers suggested two different intramolecular charge transfer (ICT) emitting states for DCM which are in dynamic equilibrium with each other, where a short-wavelength emission was assigned to a planar conformation and a longer-wavelength emission to a twisted (TICT) conformation.
Fluorescence decay measurements of cis- and trans-isomers of DCM were carried out in six solvents using PRA photon counting system [10]. The fluorescence decays of DCM are fitting with mono-exponential despite the presence of two isomers. On the contrary, quite different results were observed when lifetime measurements are carried out using picosecond time-correlated single photon counting technique [23]. The fluorescent decay profiles in methanol, acetonitrile, and chloroform are fitting in bi-exponential. A short component (~25–48 ps) is having longer lifetime in methanol and acetonitrile solvents than that in chloroform. On the other hand, long component has a lifetime (τ) of 1.38 ns in methanol and chloroform solvents and τ ~1.94 ns in acetonitrile. Furthermore, fluorescence decay is fitting with single exponential function in DMSO with a lifetime ~2.25 ns. Thus, it is proved that solvent plays an important role in the non-radiative decay processes of the DCM in excited state which ultimately changes the fluorescence lifetimes. Bi-exponential nature of DCM clearly suggests the presence of two fluorescent species, and similarly, single exponential decay fitting in DMSO indicates single fluorescence species. The long-lived species are predominant in methanol, and acetonitrile solvent attributed to a trans-isomer which is produced while synthesizing DCM. From the relative weight component ratio (
2.4 Ultra-fast spectroscopic studies of DCM
As described in previous sections, since steady-state absorption and fluorescence studies were not conclusive about the nature of emitting state, one would always ask whether fluorescence emission is from direct charge-transfer state (CT) or relaxed CT state which is originated from locally excited state as shown in Figure 3. To answer this question, it is not necessary to have ultra-fast spectroscopy data; in fact simple steady-state fluorescence data would be sufficient to explain the nature of the emitting state. Suppose if it is encountered that the transition dipole moments for absorption (CT ← S0) and emission (CT → S0) are the same, one can conclude that the DCM photophysics are involved in two states (ground and CT states) [27]. Further, in such a case, the solvatochromism of absorption and emission should be consistent with ground- and excited-state dipole moments and their difference. On the other hand, if fluorescence anisotropy of DCM is substantially smaller than 0.4, then it is possible that the fluorescence emission could be from a different state than that of populated by photoexcitation, perhaps it is direct indicative of a three-state system (ground, LE, and CT states). Therefore, explicit evidence of such a three-state system can only be obtained by time-resolved spectroscopy through the direct observation of the LE → CT transition. However, because of the interference of both population transfer and relaxation (solvent, vibration) in spectral dynamics, the interpretation of the transient spectra can sometimes be sensitive and may to lead confusion. Easter et al. have investigated ultra-fast dynamics of DCM for the first time and observed temporal evolution of its stimulated emission in methanol and ethylene glycol at several wavelengths using sub-picosecond pump-probe spectroscopy [28]. The observed temporal changes of the fluorescence intensity measured during the first 100 ps after excitation were assigned to the dynamic Stokes shift of the fluorescence emission from the CT state following its direct optical excitation. Time-resolved transient absorption spectroscopic studies of DCM solutions in weakly polar and polar were carried out by Martin and co-workers, and corresponding data exhibits an isosbestic point in the net gain spectra within a few picoseconds after excitation which suggest rapid evolution of an emissive intermediate state from the initial excited S1 state [29]. Solvatochromic behavior of the gain spectral position and its time-resolved redshift in slowly relaxing solvents support the CT character of the emissive intermediate state. Further, the overall intramolecular CT process is observed to take place within 30 ps in all solvents, and solvent relaxation time appears as an important parameter in the observed kinetics. Moreover, it was also found that the time constants associated with these changes depend upon the solvent polarity and vary from 2 ps (in acetonitrile) to 8 ps (in methanol). All these dynamics of DCM were interpreted to a transition that occurs from optically populated LE state to the CT state. However, there was no evidence of the twisted nature of this CT state which was suggested earlier [26].
Population relaxation within the fluorescent state was selectively monitored by Glasbeek and co-workers using femtosecond fluorescent up-conversion technique with a time-resolution of ~150 fs which does not permit to probe any influence of the dynamics within the electronic ground state [30]. It has been shown that intramolecular charge separation is taking more than 300 fs after the pulsed excitation. Following the pulsed excitation of the molecule, the integrated intensity of the spontaneous fluorescence decreased to approximately 50% of its initial value within few picoseconds. Moreover, it was observed that a significant portion of the charge separation trajectory (~30%) is controlled by the solvation process on a picosecond time scale. Therefore, it is inferred that LE and CT states of photoexcited DCM strongly coupled adiabatically in the inverted region where a large extent of the charge separation process occurs on a picosecond time scale controlled by the excited state solvation process. However, subsequent high-resolution (<100 fs) fluorescence up-conversion studies of the DCM dye molecule in methanol and chloroform reveal that there is no change of the integrated spectral intensity during the first 25 ps after vertical excitation for the LE → CT transition [31]. Besides, for all times only one fluorescent excited state was noticeable, and the observed dynamic Stokes shift is attributed to solvent relaxation. Mean position of the time-resolved fluorescence spectrum of DCM in methanol shifts towards the red side with bi-exponential (175 fs and 3.2 ps) behavior, while in chloroform the spectral position remains practically unchanged for all times. The collected time-resolved data concluded that DCM has a single emitting state, which is directly populating upon photoexcitation.
A binodal dynamic Stokes shift was observed with time constants, one is about 100 fs, and another is of few picoseconds, respectively, when DCM is present in highly polar solvent media (methanol, ethylene glycol, ethyl acetate, and acetonitrile) [32]. The initial fast component is attributed to the free streaming motions of the solvent molecules and the second slow time component to the rotational diffusion motions of the solvent molecules. However, from the rapid sub-picosecond rise of the integrated emission intensity, it was suggested that the excited state electron transfer is preferentially taking place within about 100 fs from a higher-lying less emissive state to a lower-lying more emissive CT state. That is, the charge separation process in DCM is completed within about 100 fs. The LE and CT states are pictured as strongly coupled in the inverted region which is already reported earlier by Gustavsson et al. [31], and the gradual charge separation is treated as diffusional motion on the resulting barrierless potential. On the other hand, transient absorption spectra of DCM dye in methanol were measured using pump-supercontinuum probe technique with 40 fs time resolution and also revealed two components [33]. Initially (before 70 fs), a prominent spectral structure is observed which is primarily due to resonance Raman processes. At longer times (>70 fs), the spectrum undergoes a significant redshift, and shape of the band changes with a well-defined isosbestic point, and these observations are quite similar to earlier study done by Martin and co-workers [29]. The early transient component has been assigned to the locally excited state of DCM. Further, it was found that LE → CT transition is much faster than that suggested by Martin et al. and concluded that a substantial fraction of the intramolecular charge separation (≥70%) is completed within 300 fs of the pulsed excitation.
Later, time-resolved visible pump and infrared (IR) probe transient absorption measurements of the DCM and its isotopomer DCM-
Excited state non-radiative relaxation dynamics of DCM in hexane have been investigated using femtosecond fluorescence up-conversion technique at three excitation wavelengths [35]. The S1 lifetime was observed to be 9.8 ps which is found to be independent of the excitation wavelengths. The observed S1 lifetime of DCM is less by one order of magnitude as compared to julolidyl DCM dyes DCJT and DCJTB, indicating the significance of the twisting motion of the N,N-dimethylamino group affecting the S1 non-radiative dynamics. Further, TDDFT calculations suggest that an intersystem crossing is responsible for the observed S1 dynamics of DCM in non-polar solvent.
2.5 What is understood about DCM dye?
The ground state and dipole moments of DCM are estimated to be very high (5.6 D and 26.6 D) which suggests that the charge is highly polarized even in the ground state. The steady-state absorption and fluorescence spectra of DCM reveal that the molecule exhibit solvatochromic shift and large Stokes shifts depending on the polarity of the solvent [10, 16, 24]. Solvatochromic shift of the electronic absorption is due to high ground-state dipole moment. The dramatic Stokes shift is attributed to the change of the dipole moment upon photoexcitation and fluorescent emitting state to a charge-transfer (CT) state [23, 24]. The fluorescence lifetime of DCM is measured to be of the order of a few nanoseconds, and the solvent relaxation occurs in between sub-picoseconds and picoseconds [9, 10, 23, 28, 29, 30, 31, 32, 33]. Both fluorescence lifetime and relaxation depend on the solvent polarity.
Photoexcitation of DCM to its first absorption band put the excited molecule in the S1/LE state, and subsequently two conformational changes may happen. Firstly, –C=C bond rotation leading to trans and cis isomerization via a phantom singlet state which is a typical photochemical process occurring on trans-stilbene [36] and many olefin molecules [37]. Secondly, twisting of the N,N-dimethylamino group may give rise to a highly polar twisted intramolecular charge-transfer (TICT) state which can be stabilized in polar media like 4-dimethyl-aminobenzonitrile (DMABN) molecule [37, 38]. However, the transition from the LE state to the CT (or TICT) state is under debate, and from both experimental and theoretical calculations [39], the following widely accepted dynamical behavior has been proposed to understand the excited-state dynamics of DCM dye. The potential energy surface (PES) of the LE state (S1) for twisting motion of the central C=C bond (which bridges N,N-dimethylamino group with pyran group) is calculated to be very small (0.2 eV), and the barrier height is insensitive to the polarity of solvent. However, the shape of excited-state PESs of for the twisting motion of the CN single bond of the N,N-dimethylamino group of DCM is strongly influenced by the polarity of the solvent [39]. Moreover, in a polar media, the energy of the S1/LE state increases, whereas the energy of the S2/CT state decreases by twisting the CN single bond of the dimethylamino group and leads to a nonadiabatic curve crossing between the two states. Therefore, the formation of an emissive TICT state along the amino group twisting coordinate is more favored with increasing the polarity of the solvent. Trans and cis isomerization is dominated in polar solvents because of the increased the energy barrier in the TICT state along the torsional coordinate of the C=C double bond when the TICT state is formed at the perpendicular geometry where the energy of the S1/LE state is higher than that of the S2/CT state.
3. DCM derivatives as optical sensors
A
Any chemosensor consists of three components: a
Binding site-signaling approach
Displacement approach
Chemodosimeter approach
These approaches only differ in the arrangement of two units (receptor and signaling) with respect to each other. In the ‘binding site-signalling subunit’ approach, two parts are linked through a covalent bond. The interaction of the analyte/guest with the binding site induces changes in the electronic properties of the signaling subunit that results sensing of the target anion. The displacement approach is based on the formation of molecular assemblies of binding site-signaling subunit, which in coordination of a certain anion with the binding site results in the release of the signaling subunit into the solution with a concomitant change in their optical properties. In the chemodosimeter approach, a chemical reaction results in an optical signal when a specific anion approaches the receptor. Depending on the type of signals that are produced upon the recognition event, chemosensors are classified into two categories: optical sensors and electronic sensors. While the former sensors change optical signals, the latter change electrochemical properties. Based on the type of optical signal, the optical sensors further can be classified into two categories.
It has been demonstrated that the colorimetric sensors are simple and low-cost and offer both qualitative and quantitative information without any need of sophisticated spectroscopic instrumentation, and most often the colorimetric response can be visualized with the naked eye. On the other hand, the fluorescence measurement is a bit expensive but relatively more sensitive and versatile and offers micro- to nanomolar estimation of guest species. A wide variety of optical chemosensors have been reported for the cation, anion, and neutral molecules. Based on the nature of analyte being detected, irrespective of the photophysical phenomenon the receptors follows, the chemosensors may be broadly classified into three categories: cations sensors, anions sensors, neutral sensors.
The ICT mechanism has been exploited quite extensively in ion sensing and molecular switching applications [45, 46]. A fluorosensor is generally designed to have two units: a signaling unit typically a fluorophore and a receptor (recognition unit) which are covalently connected with a π-spacer for rendering the recognition event to the fluorophore that ultimately changes fluorescence signal. A group of fluorogenic sensors which has either weak fluorescence or no fluorescence (off state) by nature and that becomes fluorescent (on state) upon the receptor recognizes the analyte/guest molecule, and this type of fluorogenic sensors are called as off–on sensors. Similarly, on–off sensors can also be designed, where a sensor initially exhibits fluorescence (on state) and after the recognition event, the sensor becomes nonfluorescent/weakly fluorescent (off state). A schematic representation of off–on fluorogenic sensors is shown in Figure 4.
As discussed in the previous section, the DCM molecule and its derivatives are having unique advantages in terms of their photophysical properties such as red light emission, high quantum yield, and highly tunable fluorescence that is sensitive not only by solvent polarity but also structure modification. Unlike visible light fluorogenic sensors, red and NIR fluorogenic sensors (600–950 nm) have received considerable interest due to minimum fluorescence background, less light scattering, and less photodamage and are having certain advantages in bioimaging applications of live cells. Therefore, in recent years, there is a consistent growth of the colorimetric and fluorogenic sensors based on DCM and its analogues (Figure 5) for sensing cations, anions, and neutral species, which are summarized below.
3.1 DCM derivatives as metal sensors
Valeur and Bourson designed a DCM derivative,
A red fluorosensor (
In general, most of the fluorosensors exhibit on–off sensing behavior in solution phase because quenching of fluorescence emission is quite easy. However, developing off–on fluorosensor with processible technology is relatively a tedious and challenging task. Such fluorescence off–on sensors can be tailored to meet the specific needs via rational design approaches and have been paid much attention in recent years due to growing demand of various chemical and biological species detection by exploiting energy transduction principles such as radiant, electrical, mechanical, and thermal processes [49, 50]. Tian and co-workers have extended their previous research work [48] and developed a polymeric
3.2 DCM derivatives as anion sensors
As discussed in Section 3.1, the molecules
A near-infrared (NIR) fluorescent chemosensor,
3.3 DCM derivatives for detection of neutral species
3.3.1 Hydrogen sulphide (H2S)
Hydrogen sulphide (H2S) is involved as a signaling molecule in various physiological processes that include modulation of neuronal transmission, regulation of release of insulin, relaxation of the smooth muscle, and reduction of the metabolic rate [56, 57]. From the animal model study of critical illness, it was realized that the H2S donor protect from lethal hypoxia and reperfusion injury and exert anti-inflammatory effects [58]. Physiological H2S concentration is estimated to vary from nano- to millimolar levels [59], and once this limit is crossed, the cells release H2S that can cause certain diseases, such as Alzheimer, Down syndrome, diabetes, and other diseases of mental deficiency [60]. Hence, a reliable in vivo study is essential to measure accurately H2S concentration thereby preventing deceases. A NIR probe,
3.3.2 Dopamine
A catecholamine compound dopamine is known as a neurotransmitter that regulates a wide range of cognitive functions such as behavior, learning, motivation, and memory [63, 64, 65]. The dopamine content in the human brain is an important factor that can cause various diseases that include Parkinson’s disease, and in fact it is used as a marker in the diagnosis of several conditions related to neurotransmitters. Therefore, there is a strong quest for developing efficient and rapid methods that can selectively determine and continuously sense the dopamine levels on a real-time basis. The DCM fluorosensor (
3.3.3 Hydrogen peroxide (H2O2)
Zhang et al. have synthesized a new NIR and colorimetric fluorescent molecular probe,
3.3.4 Hydrazine (N2H4)
Hydrazine is used as a common precursor in synthetic chemistry of many polymers, pharmaceutical intermediates, hydrazine fuel cells in power generation sector, and materials science [68, 69]. It is often used in rocket propulsion systems as an important propellant for its flammable and detonable characteristics. Moreover, hydrazine serves as an important metal corrosion inhibitor because of its strong reducing properties; hydrazine scavenges oxygen in water boilers that are used for feed and heating systems. However, hydrazine and its aqueous solutions are highly toxic to all living organisms when inhaled or in contact. It has been shown that hydrazine is mutagenic and carcinogenic which causes serious damage to the human central nervous system, kidneys, liver, and lungs [70]. Therefore, it is of great interest and importance to develop a reliable method for hydrazine detection with selectivity and sensitivity. With a view to develop efficient DCM-based NIR fluorophore for selective detection of hydrazine, a phenyl ring baring
3.4 DCM derivatives for sensing biothiols and selecysteine
3.4.1 Biothiols
There is quest for developing molecular probes for rapid, selective, and sensitive detection of the highly toxic thiophenols which are of great importance in both environmental and biological science. James and co-workers have developed a novel near-infrared (NIR) and colorimetric fluorescent molecular probe,
Slightly similar molecular structure
Recently, a red-emitting fluorescent probe
3.4.2 Selenocysteine
Selenocysteine (Sec) is a cysteine (Cys) analogue which consists of selenol group in place of the thiol group in Cys and considered as a major form of biological selenium and known as the 21st proteinogenic amino acid that is specifically incorporated into selenoproteins (SePs). More than 50 human proteins are known to contain Sec [78]. Therefore, detection of Sec in physiological conditions is very important. In order to achieve NIR turn-on fluorescent detection of Sec selectively, the molecule
3.5 DCM derivatives as pH sensor
A pH-sensitive fluorescent chemosensor,
3.6 DCM derivatives as polarity sensor
Kwak et al. developed different types of copolymers by decorating with the DCM moiety into a certain polymer chain which are sensible to external environment and useful to probe dye molecules [82]. The photophysical properties in solution, solid film, and aggregation revealed that ICT characteristics of the copolymers are modifying. More interestingly, it was observed that the fluorescent properties of DCM-type dyes within the polymers are significantly dependent upon the polarity of the polymer matrix. Three copolymers (P(St-
Acknowledgments
RKK acknowledges the Science and Engineering Research Board (SERB), New Delhi, for the research funding (EEQ/2016/000736).
References
- 1.
Webster GF, McColgin WC. Arylidene dye lasers. US Patent: 3852683; 1974 - 2.
Marason EG. Laser dye DCM: CW, synchronously pumped, cavity pumped and single frequency performance. Optics Communications. 1981; 37 (1):56-58 - 3.
Mau AW-H. Broadband tunability of dye lasers. Optics Communications. 1974; 11 (4):356-359 - 4.
Chen C-T. Evolution of red organic light-emitting diodes: Materials and devices. Chemistry of Materials. 2004; 16 :4389-4400 - 5.
Guo Z, Zhu W, Tian H. Dicyanomethylene-4H-pyran chromophores for OLED emitters, logic gates and optical chemosensors. Chemical Communications. 2012; 28 :6073-6084 - 6.
Zarins E, Vembris A, Kokars V, Muzikante I. Chapter 8: Synthesis and physical properties of red luminescent glass forming pyranylidene and isophorene fragment containing derivatives. In: Singh J, editor. Organic Light Emitting Devices. Croatia: IntechOpen; 2012. pp. 197-232 - 7.
Hammond PR. Laser dye DCM, its spectral properties, synthesis and comparison with other dyes in the red. Optics Communications. 1979; 29 (3):331-333 - 8.
Lesiecki M, Asmar F, Drake JM, Camaioni DM. Photoproperties of DCM. Journal of Luminescence. 1984; 31&32 :546-548 - 9.
Hsing-Kang Z, Ren-Lan M, Er-Pin N, Chu G. Behaviour of the laser dye 4-dicyanomethylene-2-methyl-6-dimethylaminostyryl-4h-pyran in the excited single state. Journal of Photochemistry. 1985; 29 :397-404 - 10.
Drake JM, Lesiecki ML, Camaiont DM. Photophysics and Cis-trans isomerization of DCM. Chemical Physics Letters. 1985; 113 (6):530-534 - 11.
Meyer M, Mialocq JC. Ground state and singlet excited state of laser dye DCM: Dipole moments and solvent induced spectral shifts. Optics Communications. 1987; 64 (3):264-269 - 12.
Bourson J, Doizi D, Lambert D, Sacaze T, Vaveur B. A derivative of laser dye DCM highly soluble in alcohols. Optics Communications. 1989; 72 (6):367-370 - 13.
Meyer J-C. Photophysical properties of the DCM and DFSBO styryl dyes consequence for their laser properties. Laser Chemistry. 1990; 10 :277-296 - 14.
Birch DJS, Hungerford G, Imhof RE, Holme AS. The fluorescence properties of DCM. Chemical Physics Letters. 1991; 178 (2,3):177-184 - 15.
Bondarev SL, Knyukshto VN, Stepuro VI, Stupak AP, Turban AA. Fluorescence and electronic structure of the laser dye DCM in solutions and in polymethylmethacrylate. Journal of Applied Spectroscopy. 2004; 71 (2):194-201 - 16.
Boldrini B, Cavalli E, Painelli A, Terenziani F. Polar dyes in solution: A joint experimental and theoretical study of absorption and emission band shapes. Journal of Physical Chemistry A. 2002; 106 :6286-6294 - 17.
Grabowski ZR, Rotkiewicz K, Rettig W. Structural changes accompanying intramolecular electron transfer: Focus on twisted intramolecular charge-transfer states and structures. Chemical Reviews. 2003; 103 (10):3899-4031 - 18.
Mataga N, Kaifu Y, Kolzumi M. Solvent effects upon fluorescence spectra and the dipole moments of excited molecules. Bulletin of the Chemical Society of Japan. 1956; 29 (4):465-470 - 19.
Lippert VE. Dipolmoment und elektronenstruktur von angeregten molekülen. Zeitschrift für Naturforschung A. 1955; 10a :541-545 - 20.
Lippert VE. Spektroskopische bestimmung des dipolmomentes aromatischer verbindungen im ersten angeregten singulettzustand. Zeitschrift für Elektrochemie. 1957; 61 (8):962-975 - 21.
Rotkiewicz K, Grellmann KH, Grabowski ZR. Reinterpretation of the anomalous Fluorescense of p-N,N-Dimethylaminobenzonitrile. Chemical Physics Letters. 1973; 19 :315-318 - 22.
Kanaparthi RK, Sarkar M, Samanta A. Probing the aggregated state of 4-(9-anthryl)-N,N-dimethylaniline by UV-Vis absorption and fluorescence spectroscopy, microscopy, and crystallography. Journal of Physical Chemistry B. 2009; 113 :15189-15195 - 23.
Meyer M, Mialocq JC, Rougee M. Fluorescence lifetime measurements of the two isomers of the laser dye DCM. Chemical Physics Letters. 1988; 150 (6):484-490 - 24.
Meyer M, Mialocq J-C, Perly B. Photoinduced intramolecular charge transfer and trans-cis isomerization of the DCM styrene dye. Picosecond and nanosecond laser spectroscopy, high-performance liquid chromatography, and nuclear magnetic resonance studies. Journal of Physical Chemistry. 1990; 94 :98-104 - 25.
Rullière C. Laser action and photoisomerisation of 3,3′-diethyl oxadicarbocyanine iodide (DODCI): Influence of temperature and concentration. Chemical Physics Letters. 1976; 43 (2):303-308 - 26.
Rettig W, Majenz W. Competing adiabatic photoreaction channels in Stilbene derivatives. Chemical Physics Letters. 1989; 154 (4):335-341 - 27.
Kumpulainen T, Lang B, Rosspeintner A, Vauthey E. Ultrafast elementary photochemical processes of organic molecules in liquid solution. Chemical Reviews. 2017; 117 :10826-10939 - 28.
Easter DC, Baronavski AP. Ultrafast relaxation in the fluorescent state of the laser dye DCM. Chemical Physics Letters. 1993; 201 :153-158 - 29.
Martin MM, Plaza P, Meyer YH. Ultrafast intramolecular charge transfer in the Merocyanine dye DCM. Chemical Physics. 1995; 192 :367-377 - 30.
Zhang H, Jonkman AM, Meulen PVD, Glasbeek M. Femtosecond studies of charge separation in photo-excited DCM in liquid solution. Chemical Physics Letters. 1994; 224 (5-6):551-556 - 31.
Gustavsson T, Baldacchino G, Mialocq JC, Pommeret S. A femtosecond fluorescence up-conversion study of the dynamic stokes shift of the DCM dye molecule in polar and non-polar solvents. Chemical Physics Letters. 1995; 236 :587-594 - 32.
Meulen PVD, Zhang H, Jonkman AM, Glasbeek M. Subpicosecond solvation relaxation of 4-(dicyanomethylene)-2-methyl-6-(p-(dimethylamino)styryl)-4h-pyran in polar liquids. Journal of Physical Chemistry. 1996; 100 :5367-5373 - 33.
Kovalenko SA, Ernsting NP, Ruthmann JF. Femtosecond hole-burning spectroscopy of the dye DCM in solution: The transition from the locally excited to a charge-transfer state. Chemical Physics Letters. 1996; 258 :445-454 - 34.
Van Tassle AJP, Fleming GR. Investigation of the excited state structure of DCM via ultrafast electronic pump/vibrational probe. Journal of Physical Chemistry B. 2006; 110 :18989-18995 - 35.
Chang C-W, Kao Y-T, Diau EW-G. Fluorescence lifetime and nonradiative relaxation dynamics of DCM in nonpolar solvent. Chemical Physics Letters. 2003; 74 :110-118 - 36.
Baskin JS, Banares L, Pedersen S, Zewail AH. Femtosecond real-time probing of reactions. 20. Dynamics of twisting, alignment, and IVR in the trans-Stilbene isomerization reaction. Journal of Physical Chemistry. 1996; 100 :11920-11933 - 37.
Klessinger M, Michl J. Excited States and Photochemistry of Organic Molecules. New York: VCH Publishers; 1995 - 38.
Grabowski ZR, Dobkowski J. Twisted intramolecular charge transfer (TICT) excited states: Energy and molecular structure. Pure and Applied Chemistry. 1983; 55 :245 - 39.
Marguet S, Mialocq JC, Millie P, Berthier G, Momicchioli F. Intramolecular charge transfer and trans-cis isomerization of the DCM styrene dye in polar solvents. A CS INDO MRCI study. Chemical Physics. 1992; 160 (2):265-279 - 40.
Wu D, Sedgwick AC, Gunnlaugsson T, Akkaya EU, Yoon J, James TD. Fluorescent chemosensors: The past, present and future. Chemical Society Reviews. 2017; 46 (23):7105-7123 - 41.
Wang B, Anslyn EV. Chemosensors: Principles, Strategies, and Applications. Hoboken, New Jersey: John Wiley & Sons; 2011 - 42.
Rice AP et al. Signaling recognition events with fluorescent sensors and switches. Chemical Reviews. 1997; 97 (5):1515-1566 - 43.
Bissell RA, Silva APD, Gunaratne HQN, Lynch PLM, Maguire GEM, Sandanayake KRAS. Molecular fluorescent signalling with ‘fluor–spacer–receptor’ systems: approaches to sensing and switching devices via supramolecular photophysics. Chemical Society Reviews. 1992; 21 :187-195 - 44.
McDonagh C, Burke CS, MacCraith BD. Optical chemical sensors. Chemical Reviews. 2008; 2 :400-422 - 45.
Martínez-Máñez R, Sancenón F. Fluorogenic and chromogenic chemosensors and reagents for anions. Chemical Reviews. 2003; 103 (11):4419-4476 - 46.
Callan FJ, Silva APD, Magri CD. Luminescent sensors and switches in the early 21st century. Tetrahedron. 2005; 61 (36):8551-8588 - 47.
Bourson J, Valeur B. Ion-responsive fluorescent compounds. 2. Cation-steered intramolecular charge transfer in a crowned merocyanine. Journal of Physical Chemistry. 1989; 93 :3871-3876 - 48.
Huang X, Guo Z, Zhu W, Xiea Y, Tian H. A colorimetric and fluorescent turn-on sensor for pyrophosphate anion based on a dicyanomethylene-4H-chromene framework. Chemical Communications. 2008; 44 :5143-5145 - 49.
Liu Y, Schanze KS. Conjugated polyelectrolyte-based real-time fluorescence assay for alkaline phosphatase with pyrophosphate as substrate. Analytical Chemistry. 2008; 80 :8605-8612 - 50.
Zhao X, Liu Y, Schanze KS. A conjugated polyelectrolyte-based fluorescence sensor for pyrophosphate. Chemical Communications. 2007; 43 :2914-2916 - 51.
Guo Z, Zhu W, Tian H. Hydrophilic copolymer bearing dicyanomethylene-4H-pyran moiety As fluorescent film sensor for Cu2þ and pyrophosphate anion. Macromolecules. 2010; 43 :739-744 - 52.
Gu B, Huang L, Xu Z, Tan Z, Hu M, Yang Z, et al. A reaction-based, colorimetric and near-infrared fluorescent probe for Cu2+ and its applications. Sensors and Actuators B. 2018; 273 (10):118-125 - 53.
Wang P, Xia J, Gu Y. A novel NIR fluorescent probe for palladium detection based on Pd(0) mediated reaction. Tetrahedron Letters. 2015; 56 :6491-6494 - 54.
Zhu W, Huang X, Guo Z, Wu X, Yu H, Tian H. A novel NIR fluorescent turn-on sensor for the detection of pyrophosphate anion in complete water system. Chemical Communications. 2012; 48 :1784-1786 - 55.
Cao J, Zhao C, Zhu W. A near-infrared fluorescence chemodosimeter for fluoride via specific Si–O cleavage. Tetrahedron Letters. 2012; 53 (16):2107-2110 - 56.
Kimura H. Hydrogen sulfide: Its production, release and functions. Amino Acids. 2011; 41 :113-121 - 57.
Blackstone E, Morrison M, Roth MB. H2S induces a suspended animation–like state in mice. Science. 2005; 308 :518 - 58.
Wallace JL. Hydrogen sulfide-releasing anti-inflammatory drugs. Trends in Pharmacological Sciences. 2007; 28 :501-504 - 59.
Han Y, Qin J, Chang X, Yang Z, Du J. Hydrogen sulfide and carbon monoxide are in synergy with each other in the pathogenesis of recurrent febrile seizures. Cellular and Molecular Neurobiology. 2006; 26 :101-107 - 60.
Eto K, Asada T, Arima K, Makifuchi T, Kimura H. Brain hydrogen sulfide is severely decreased in Alzheimer’s disease. Biochemical and Biophysical Research Communications. 2002; 293 :1485-1488 - 61.
Sun W, Fan J, Hu C, Cao J, Zhang H, Xiong X, et al. A two-photon fluorescent probe with near-infrared emission for hydrogen sulfide imaging in biosystems. Chemical Communications. 2013; 49 :3890-3892 - 62.
Zheng Y, Zhao M, Qiao Q , Liu H, Lang H, Xu Z. A near-infrared fluorescent probe for hydrogen sulfide in living cells. Dyes and Pigments. 2013; 98 :367-371 - 63.
Gingrich JA, Caron MG. Recent advances in the molecular biology of dopamine receptors. Annual Review of Neuroscience. 1993; 16 :299-321 - 64.
Tritsch NX, Sabatini BL. Dopaminergic modulation of synaptic transmission in cortex and striatum. Neuron. 2012; 76 :33-50 - 65.
Vandecasteele M, Glowinski J, Deniau J-M, Venance L. Chemical transmission between dopaminergic neuron pairs. Proceedings of the National Academy of Sciences. 2008; 105 :4904-4909 - 66.
Suzuki Y. Design and synthesis of fluorescent reagents for selective detection of dopamine. Sensors and Actuators B. 2017; 239 :383-389 - 67.
Zhang X, Zhang L, Liu Y, Bao B, Zang Y, Li J, et al. A near-infrared fluorescent probe for rapid detection of hydrogen peroxide in living cells. Tetrahedron. 2015; 71 (29):4842-4845 - 68.
Serov A, Kwak C. Direct hydrazine fuel cells: A review. Applied Catalysis B: Environmental. 2010; 98 (1-2):1-9 - 69.
Ragnarsson U. Synthetic methodology for alkyl substituted hydrazines. Chemical Society Reviews. 2001; 30 :205-213 - 70.
Reilly CA, Aust SD. Peroxidase substrates stimulate the oxidation of hydralazine to metabolites which cause single-strand breaks in DNA. Chemical Research in Toxicology. 1997; 10 :328-334 - 71.
Ma J, Fan J, Li H, Yao Q , Xia J, Wang J, et al. Probing hydrazine with a near-infrared fluorescent chemodosimeter. Dyes and Pigments. 2017; 138 :39-46 - 72.
Li M, Wu X, Wang Y, Li Y, Zhu W, James TD. A Near-infrared colorimetric fluorescent chemodosimeter for the detection of glutathione in living cells. Chemical Communications. 2014; 50 :1751-1753 - 73.
Yu D, Huang F, Ding S, Feng G. Near-infrared fluorescent probe for detection of thiophenols in water samples and living cells. Analytical Chemistry. 2014; 86 (17):8835-8841 - 74.
Yu D, Zhai Q , Yang S, Feng G. A colorimetric and near-infrared fluorescent turn-on probe for in vitro and in vivo detection of thiophenols. Analytical Methods. 2015; 7 (18):7534-7539 - 75.
Dehuan Yu QZ, Ding S, Feng G. A colorimetric and near-infrared fluorescent probe for biothiols and its application in living cells. RSC Advances. 2014; 4 :46561-46567 - 76.
Qian M, Zhang L, Wang J, Peng X. A red-emitting fluorescent probe with large Stokes shift for real-time tracking of cysteine over glutathione and homocysteine in living cells. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy. 2019; 214 :469-475 - 77.
Weerapana E, Wang C, Simon GM, Richter F, Khare S, Dillon MBD. Quantitative reactivity profiling predicts functional cysteines in proteomes. Nature. 2010; 468 :790-795 - 78.
Romagné F, Santesmasses D, White L, Sarangi GK, Mariotti M, Hübler R, et al. SelenoDB 2.0: Annotation of selenoprotein genes in animals and their genetic diversity in humans. Nucleic Acids Research. 2014; 42 (D1):D437-D443 - 79.
Li M, Feng W, Zhai Q , Feng G. Selenocysteine detection and bioimaging in living cells by a colorimetric and near-infrared fluorescent turn-on probe with a large Stokes shift. Biosensors and Bioelectronics. 2017; 87 (15):894-900 - 80.
Yang J, Li M, Zhu W-H. Dicyanomethylene-4H-pyran based NIR fluorescent ratiometric chemosensor for pH measurement. Research on Chemical Intermediates. 2018; 44 :3959-3969 - 81.
Yang Y, Wang L, Xu M, Chen J, Qu Y. Triphenyl phosphate end-capped dicyanomethylene-4H-pyran as a near infrared fluorescent sensor for lysozyme in urine sample. Sensors and Actuators B: Chemical. 2019; 284 :553-561 - 82.
Kwak G, Kim H, Kang I-K, Kim S-H. Charge transfer dye in various polymers with different polarity: Synthesis, photophysical properties, and unusual aggregation-induced fluorescence changes. Macromolecules. 2009; 42 (5):1733-1738