The three aryl rings of triarylmethanes are free to rotate. However this free rotation can be restricted on either by bridging the aryl rings through covalent bonds or through heteroatoms resulting in the formation of Conformationally Restricted Triarylmethanes (CRT). The photophysics and photochemistry of these CRTs, like 9-arylxanthenes (oxygen bridging), 9-arylthioxanthenes (sulfur bridging), 9, 10-dihydro-9-arylacridines (nitrogen bridging), 9-arylfluorenes (bridging through carbon–carbon covalent bond) have recently been the subject of number studies. Various applications of CRT molecules have been developed out of which application as photoredox catalyst is undoubtedly the most important. In this chapter, we have highlighted recent development of various CRT molecules, their photophysics, photochemistry and an application in the field of photoredox catalysis.
- conformationally restricted triarylmethanes (CRTs)
- photoredox catalysis
Triarylmethanes are compounds in which a carbon atom is linked to three aryl rings (both aromatic as well as heteroaromatic) which may be same or different. The most simple triarylmethane namely, triphenylmethane was first synthesized by August Kekule in 1872 by heating diphenyl mercury and benzal chloride . Currently, more than thousand references can be found citing this molecule . The development in the synthesis as well as application of these molecules have attracted various scientists with diverse field of research on this molecular scaffold leading to further high-end applications of these molecules. For example, simple triarylmethane derivatives have shown significant bio-activity against intestinal helminthes, filariae, trichomonads and trypanosomes . Moreover, hydroxy substituted triarylmethanes are known for their antioxidant properties, antitumor activities as well as inhibitors of histidine protein kinases [4, 5]. Letrozole, Vorozole are effective Non-Steroidal Aromatase Inhibitors used commercially for the treatment of breast cancers [6, 7, 8, 9, 10]. (
Through various non-covalent interactions, extended networks of hydrogen bonds, steric interactions, cycles/polycycles present in a molecule, the rotation of groups attached to the central carbon are restricted thereby decreasing the conformational mobility enjoyed by the molecules. This change in the conformation of the molecule in general not only modulates different physical properties of the molecule but also influences different photophysical and photochemical properties of the molecule [16, 17]. For example in open chain molecules due to rapid rotation of various bonds the vibrational relaxation of these molecules from the excited state to the ground state through non-radiative pathways are very rapid and these compounds are seldom fluorescent while in cyclic molecules due to the restriction of free rotation in the molecules there is a decrease in the vibrational relaxation through non-radiative pathways and hence there is a increase in the fluorescent quantum yield. To study the effect of this decrement in the conformational freedom and the properties that arise due to this restriction scientists term the molecules with such reduced conformational mobility as “conformationally restricted analogues” .
Due to the various non bonded interactions among the ortho protons/substituents in triarylmethanes, the triarylmethanes exhibit ‘molecular propeller conformation’ in the ground state [17, 18, 19, 20, 21]. However, the three aryl rings attached to the central carbon atom rotate freely. This free rotation is restricted as shown in Figure 2, on bridging the two aryl rings with heteroatoms or bridging the aryl rings through bonds forming various types of molecules like 9-arylxanthenes (oxygen bridging) , 9-arylfluorenes (bridging through C-C bond) .
The presence of three aryl rings as well as the non-fluxional nature of the molecule results in easy abstraction of the corresponding methine hydrogen and thus faster generation of the corresponding free radicals; carbocations as well as carbanions . In fact, Arnett and coworkers have described 9-arylxanthenes (a typical example of CRTs) as a subset of triarylmethanes [25, 26]. Both triarylmethanes, and 9-arylxanthenes are therefore can be considered as amphihydric compounds. These CRT compounds are reported to exist in two forms; one benzenoid and other quinoid structures due to this amphihydric nature as shown in Figure 3 .
This benzenoid and quinoid structures along with the conformational restriction results in various interesting photophysical properties namely, the benzenoid form in the 9-arylxanthene derivatives being colorless while the quinoid form is intensely colored. In the benzenoid form the π electron delocalization in the chromophore is interrupted causing the absorption to be in the ultraviolet region and hence colorless while in the quinoid system this is uninterrupted causing the absorption to be in the visible region and hence is colored . In the presence of base in (
2. Photophysical studies of CRTs: effect of conformational restriction on the photophysical properties
2.1. Photophysics of unrestricted triarylmethanes
The UV–Vis spectrum of the conformationally flexible triarylmethanes are very interesting with predominately two types of absorption bands being observed namely, the x band and the y band as shown (Figure 4). The x band arises through the transition of electron from the nonbonding molecular orbital of the molecule to the lowest unoccupied molecular orbital while the y band arises through the transition of the electron from the second highest occupied molecular orbital of the dye to the lowest unoccupied molecular orbital of the dye. This UV–Vis spectrum is found to depend strongly depends on various factors like the structure of the molecule [31, 32, 33, 34, 35], concentration , pH of the solution  as well as temperature . Further due to the conformational flexibility and synchronous rotation among the attached aryl rings, the relaxation of the vibronically excited triarylmethanes to the ground state can occur through various radiation less processes. Due to this vibrational cascading the luminescence intensity and lifetime decrease in conformationally flexible triarylmethanes. However, this vibrational cascading is strongly affected by various factors like concentration, presence of other molecules (proteins/polymers, etc.), pH of the solution, as well as temperature thereby affecting the intensity of various luminescent processes .
2.2. Photophysics of conformationally restricted triarylmethanes
Decreasing the synchronous rotation of the aryl rings and hence the conformational flexibility in triarylmethanes has a profound influence in the observed photophysics of the triarylmethanes. The most significant change observed is the increase in the intensity of various luminescence processes. This is due to the fact that on conformational restriction, the relaxation of the excited state molecule to the ground state through various radiation less transitions decreases causing an increase in the intensity of the various luminescent processes . Furthermore, the rate of the intersystem crossing also increases  thereby increasing the population of the triplet state .
2.2.1. Effect of structure
The functional groups/ the substituent atoms attached to a molecule modulate the photophysical properties of the molecule to a large extent. For example electron withdrawing groups like nitro groups affects the intramolecular charge transfer processes in the molecule , a tertiary amine group increase the rate of relaxation to the ground state through various non-radiative transitions arising due to the rapid rotation around the substituents attached to nitrogen and hence causes a decrease in fluorescence quantum yield [43, 44], heavy atom substituents like iodo, bromo increase the rate of intersystem crossing  through efficient spin-orbit coupling decreasing the fluorescence lifetime and quantum yield and concomitantly increasing the triplet emission yield .
In the CRTs the substituted aryl rings have a strong influence in the absorption-emission spectra of these compounds. For example depending on the pH of the solution the hydroxy xanthene dyes like (
2.2.2. Effect of solvent, concentration and temperature
The different structural forms of the hydroxy xanthene dyes have different hydrogen bonding ability with the solvent molecules which is reflected by the change in the UV–Vis absorption spectra on changing the hydrogen bonding ability of the solvent. For example, as observed by Martin and coworkers, with the increase in the hydrogen bonding effect of the solvent both the absorption as well as the fluorescence spectra of hydroxy xanthene dyes shows a blue shift . This is due to the fact that hydrogen bonding interactions between the dye molecule and the solvent stabilizes the ground state of the hydroxy xanthene molecules more than the excited state causing an increase in the HOMO–LUMO energy gap and hence a blue shift in the UV–Vis absorption spectra is observed (Figure 6). Also the fluorescence quantum yield of these hydroxy xanthene dyes increase in such hydrogen bonding solvents. Moreover, in polar aprotic solvents in which no solute solvent interaction can take place through hydrogen bonding a red shift in the absorption-emission spectra is observed .
The hydrogen bonding interactions between the solute and the solvent molecules also stabilizes the singlet as well as the triplet energy states of the solute molecules with the solvent stabilization energy being most in the singlet ground state of the solute followed by the first excited singlet state. The triplet excited state, is least stabilized through hydrogen bonding. Thus the energy gap between the singlet excited state and the triplet excited state decrease in the hydrogen bonding solvent thereby increasing the rate of intersystem crossing. This interaction also organizes the dye molecules in a particular order changing the distortion required for internal conversion. This influences the rate constants as well as quantum yields for the internal conversion [50, 51].
Various non-covalent interactions between the dye molecule and the solvent as well as between the dye molecules result in the formation of dimer and/or higher order aggregates. These aggregates modulate the absorption-emission spectra CRTs strongly depend on the concentration of the CRT molecules. In fact, at higher concentration, the fluorescence spectra of many CRT molecules like
Temperature has a profound influence on the fluorescent as well as phosphorescent properties of the CRT molecules [53, 54, 55, 56]. This is not only due to the dependence of the population of molecules in a particular electronic state in a molecule depends on the temperature of the molecule (according to Boltzmann distribution factor) but also due to the change in the probability of de-excitation from various vibrational levels through non-radiative transitions changing the fluorescent quantum yield on change in the temperature. Moreover, at low temperature,
2.2.3. Photoinduced electron transfer (PET) in CRTs
Excitation of molecules from the ground state to the excited state increases the redox activity of the compound. This is due to the fact that on excitation of an electron from the ground state creates an “electron hole” in its highest occupied molecular orbital as well as equivalent amount of electron occupancy in the lowest unoccupied molecular orbital. However, the electrons in the LUMO energy level are loosely bound and hence can be easily detached (hence greater reducing ability) while the electron hole in the HOMO results in greater electron affinity (hence greater oxidizing ability).
The fluorescence quantum yield of the 9-arylxanthenium ion type of CRT derivatives have been found to be high (fluorescence Φ= 0.45) . However, this high fluorescence quantum yield is effectively quenched in various CRTs molecules having electron-rich aromatic substituents due to a PET from the electron-rich aryl rings to the fluorophore takes place, quenching the observed fluorescence . For example the fluorescence quantum yield of (
2.2.4. Photophysics of molecules formed through replacement of the bridging atom of CRT with Si atom and other groups 14 and group 16 elements
Molecules formed through the replacement of the bridging heteroatoms in the CRT molecules with Si atom or other groups 14 elements such as Ge, Sn, modulate the energy levels of the parent CRT molecules through various stereoelectronic effects causing a high bathochromic shift in the U.V-Vis absorption spectra. This is because the substituents attached to the group-14 atom has a strong electron pushing inductive effect as well as has a strong σ*-π* conjugation between the group 14 atom substituent σ* orbital and π* orbital of the fluorophore modulating the HOMO as well as LUMO energy levels. Moreover, this bathochromic shift in the absorption maxima among the group 14 substituted CRTs decreases down the group opposite to that observed when the oxygen atom is replaced by group 16 elements. This is due to the fact that in case of group 16 elements the resonance effect takes place between the lone pair of electrons in the chalcogens and the fluorophore with the positive charge being efficiently delocalized throughout the fluorophore resulting in a concomitant decrease in the energy gap between the frontier orbitals. For example, the absorption maxima, fluorescence maxima, fluorescence quantum yield for (
3. Photochemistry of CRTs
3.1. Photooxidation and photoreduction
CTRs with photocleavable appendages dissociate into corresponding cations on irradiation of ultraviolet light. The corresponding ions display a quinoid structure (vide supra, Figure 2) and are highly colored. The resulting carbocation takes part in various photooxidation or photoreduction processes. The photooxidation process involves ejection of an electron from the excited state to form the respective triarylmethane carbocation (presumably in the triplet state) which may further react with molecular oxygen to form organic peroxides. It may proceed to further oxidized products or may lose an electron to the solvent molecule to form the respective triarylmethane radical ion or solvated electrons which further reacts with molecular oxygen to form the peroxides (vide Scheme 2). The photoreduction process involves either hydrogen atom abstraction or electron abstraction process through a photoexcited triplet state.
3.2. Photoredox catalysis
For practitioners of synthetic chemistry in the pharmaceutical industry, materials industry as well as in the academia the ultimate goal is to develop practical, scalable processes with minimum impact on the environment. On this line, if different sources of energy available to humanity are compared then the light is the most abundant, endless, renewable, clean form of energy and using light to transform raw materials to value-added products will always be of high demand. Moreover, this mode of catalysis provides various new reactions through the facile generation of reactive intermediates which are otherwise difficult/ impossible to obtain. Though transition metal chromophores were used initially as photoredox catalysts, recently organic dyes are being used.
3.3. CRTs as photoredox catalysts
The rich photophysics of CRT derivatives (e.g., facile intersystem crossing due to heavy atom effect, photooxidation, photoreduction, PET, etc.) allow these molecules to be efficiently used as photocatalysts. Especially, the hydroxy xanthene dyes (
3.3.1. (CRT-2) as a Photoredox catalyst
Due to the heavy atom effect in the (
3.3.2. Use of (CRT-2) in reduction reactions
18.104.22.168. Nitrobenzene to aniline
Under green light irradiation and in the presence of sacrificial reducing agents like triethanolamine, strong electron acceptors like nitrobenzenes are reduced to aniline derivatives (Scheme 3) . Through flash photolysis experiments the mechanism of the reaction was established. (
A metal-free green protocol for the removal of sulfonyl group was reported by Wu and coworkers (Scheme 4) . Blue light irradiation to bis tetrabutylammonium (
3.3.3. Use of (CRT-2) in oxidation reactions
22.214.171.124. Benzylic oxidations
In a formal Kornblum oxidation, when benzyl bromides are heated at 80°C in the presence of (
126.96.36.199. Oxidative iminium ion formation
Single electron transfer from tertiary amines to the excited state of the (
188.8.131.52. Benzylic bromination
Using carbon tetrabromide as an efficient source of bromine, morpholine as a reducing agent and (
3.3.4. Use of (CRT-2) in redox neutral transformation
184.108.40.206. Arylation reaction using aryldiazonium salts
220.127.116.11. (CRT-11) as photoredox catalyst
The, (CRT-11) a class of triarylmethane derivatives, with electron-rich aryl rings in the 9th position, display a unique ability to form a donor-acceptor type of dyad where the electron-rich aromatic ring acts as electron donor while the acridinium ring acts as an electron acceptor. Moreover, the presence of sterically hindered aryl group in the 9th position shields the benzylic position from any nucleophilic attack and also shifts the absorption maxima to the visible range through electron delocalization with the aryl ring. When irradiated with light of suitable frequency (approx. 450 nm absorption maxima) an intramolecular electron transfer from the electron-rich aromatic nucleus to the singlet excited state of the acridinium ring takes place (PET) leading to the formation of the long lived electron transfer excited state. As evident from the reduction potential of the excited state of this molecule, this molecule can easily accept electrons to oxidize organic substrates to its radical cations which can further react with nucleophiles to form the radical adducts while the dye molecule itself loses electrons to electron acceptors like oxygen to form hydroperoxide radical. This radical may further react with the adduct to form its hydroperoxide and further to its oxidized species and hydrogen peroxide (Figure 7). Thus in the presence of air (
18.104.22.168. Use of 9-Mes acridinium ions as oxidizing agent
22.214.171.124.1. Oxidation of toluene to benzaldehyde
In the presence of (
126.96.36.199.2. C-H oxidation of cycloalkanes
Fukuzumi and coworkers reported a photocatalytic process to oxidize the inert C-H bonds of cycloalkanes . Using the photoredox catalyst (
188.8.131.52. Use of (CRT-11) for alkene hydrofunctionalization reaction
The regioselective alkene hydrofunctionalization is an essential yet challenging task in synthetic chemistry. Nicewicz developed an efficient protocol for alkene hydrofunctionalization using the (
The free rotation among the three aryl rings of the triarylmethane molecule is restricted on bridging the aryl rings with heteroatoms or through bonds. The resulting conformationally restricted triarylmethane molecules have very rich photophysics which is the topic of this chapter. This conformational restriction decreases the rate of relaxation of the excited molecules through vibrational cascading increasing the fluorescent intensity of the resulting compound. Heavy atom substituted CRTs shown a high rate of intersystem crossing thus increasing the triplet quantum yield and also gives rise to unique photochemistry which is discussed. This property is used currently to develop small organic molecules catalyzing the conversion of light energy to value-added products which is also discussed. We anticipate that this chapter will benefit readers interested to develop novel photocatalytic systems to synthesis various value-added products.
SM gratefully acknowledges Science and Engineering Research Board, India for the award of National Postdoctoral Fellowship (File number: PDF/2016/001146). Authors also acknowledges department of chemistry, Institute of Science, Banaras Hindu University for providing infrastructural facilities.
Conflict of interest
The authors declare no conflict of interest.