1. Introduction

Fluorescent organic dyes have emerged as suitable and highly recommended photoactive media in many light-driven devices and photonic applications overall [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Their success relies mainly on their chemical versatility, thanks to the recent advances in organic synthesis [12, 13]. Among them, functional fluorophores are in the spotlight. These pivotal molecules feature chromophoric cores, whose photophysical signatures can be finely modulated by tailored chemical modifications of the molecular structures through well-established synthetic protocols. Therefore, a given molecular scaffold can be applied for different and selective purposes just adjusting the molecular structure by a rational design of the fluorophores. The search and generation of sophisticated fluorophores has boosted dye chemistry as a powerful tool for tunable photonics. Indeed, there are several families of organic dyes (such as coumarins, acridines, pyronines, rhodamines, oxazines, and styryl and cyanines-based dyes), with absorption/fluorescence bands along the whole visible spectral region, which could be suitable scaffolds to this aim [2]. However, there are still some problems that should be fixed. For instance, coumarins are endowed with low photostability, while styryl- and cyanine-based dyes usually display modest fluorescence efficiencies owing to their push–pull chromophores. Such photonic drawbacks are solved in other dyes, but their access is not straightforward from a synthetic point of view; their purification is tedious or the options for post-functionalization are limited. Therefore, the challenge in dye chemistry is to find out a bright and stable molecular platform easily accessed and amenable to a wide sort of synthetic routes to decorate the chromophore with functional groups, which enable a deep but controlled modulation of their photonic performance.

The archetype of modern fluorophores is the renowned BOron DIPYrromethene dyes (trademark BODIPY) [14, 15, 16]. This cyclic cyanine features a dipyrrin (or dipyrromethene) chromophoric core chelated by a difluoroboron bridge (Figure 1) [17], which offsets most of the aforementioned shortcomings from both a photonic and synthetic point of view. The first reports of BODIPY date from the sixties [18], but it was not until the early nineties when they gain recognition as photoactive media of tunable lasers, yielding high long-lasting efficiencies [19, 20]. After such pioneer work, BODIPY witnessed a booming growth and many researchers paid attention to this fluorophore and incorporate it as the cornerstone of their research projects [21, 22, 23, 24, 25, 26, 27, 28]. As a result, nowadays BODIPY is qualified as a top-ranked dye successfully applied in biotechnology, and a myriad of optoelectronic and photovoltaic devices. Indeed, there is a huge chart of BODIPYs reported and patented, bearing exhaustive chemical transformations and functionalizations for specific photonic demands.

Figure 1.
Molecular structure of the BODIPY laser dyes purchased by exciton.

As aforementioned, BODIPYs became renowned owing to their laser performance. Since then, several attempts were conducted to modulate the laser emission wavelength [29] and ameliorate the efficiency and photostability [30, 31], including their confinement in solid hosts to develop solid-state dye lasers (SSDL) [32, 33]. Actually, some BODIPY laser dyes are available commercially. Exciton is one of the main stockiest and provides technical sheets with information about their laser performance (maximum wavelength, tunability range, conversion efficiency, and stability as half lifetime) at different concentrations (up to millimolar), environments (liquid and solid), and pumping lasers. Indeed, the available BODIPYs from Exciton are named as PyrroMethenes followed by their laser emission wavelength (for instance, PM546, PM567, PM597, PM605, and PM650 in Figure 1). However, in some cases, important data are missed (like photostability in PM605 and PM650 or efficiencies in some solvents for the rest of the dyes), and for each BODIPY, the data are collected under different experimental conditions or setups. Therefore, it is difficult to establish a well-grounded comparison between them to outline master guidelines about the molecular structure–laser relationship.

To offset such shortcoming and enable a direct comparison of the laser performance of these reference BODIPYs, we have measured their laser properties at the same experimental conditions, transversally pumped with a wavelength-tunable optical parametric oscillator (OPO) coupled to the third harmonic (355 nm) of the Nd:YAG laser. Furthermore, we aim to understand the complex interplay between molecular structure and photonic properties, a basic knowledge, which should enable the development of improved laser dyes. To accomplish this task, we have revisited their photophysical properties and conducted an exhaustive computationally aided spectroscopic and electrochemical study. This analysis is focused on identifying the main photophysical parameters, which triggers the laser signal, mainly the efficiency and photostability, as the key properties to determine the workability of the BODIPYs as long-lasting and effective photoactive media of tunable lasers.

2. Light absorption

The reference PM546 dye displays a sharp and intense (reaching molar absorption at the maximum of 10⁵ M⁻¹ cm⁻¹) absorption band at around 500 nm (Figure 2), showing the trademark low negative solvatochromism (around 7 nm) of BODIPYs [34]. The attachment of linear (ethyl in PM567) and branched (tert-butyl in PM597) alkyls at β-positions (Figure 1) induces a spectral bathochromic shift (around 20 and 30 nm, respectively, Figure 2), according to an inductive electron releasing effect (higher in tert-butyl owing to the hyperconjugative effect). In contrast, the branched substituent decreases the absorption probability (6–7·10⁴ M⁻¹ cm⁻¹). Such red-shift is more pronounced upon the incorporation of electron acceptor moieties at meso-position (Figure 2). Thus, the acetoxy moiety at the 8-methyl (PM605, Figure 1) places the absorption at 550 nm, whereas the stronger electron-withdrawing cyano at such chromophoric position (PM650, Figure 1) further shifts the absorption to 590 nm (Figure 2), together with a decrease of the molar absorption (5–6·10⁴ M⁻¹ cm⁻¹).

Figure 2.
Normalized absorption spectra of the BODIPY laser dyes in diluted solutions (2 μM) of cyclohexane.

3. Computational chemistry

To understand the substituent-induced spectroscopic changes, we ran theoretical calculations at the density functional theory (DFT) level, in particular using the CAM-B3LYP method and the 6-311 + g* basis set for the geometry optimization, and the time dependent approach (TD DFT) for the simulation of the vertical excitations. TD DFT fails in the description of the energy of the S₁ state but gives a good qualitative simulation of the substituent effect on the energy of the absorption gap. Besides, it is amenable to large molecules, where advance calculation methods are unaffordable [35, 36]. To estimate the TD DFT error and correct the calculated values [37, 38], we compare the TD DFT results of the pristine simplest BODIPY with those calculated with the accurate CASPT2 method [39]. Such comparison reveals that the S₁ energy of BODIPYs is overestimated at around 0.55 eV. Therefore, we applied this correction factor to the absorption energies calculated for the rest of herein studied BODIPY laser dyes using TD DFT. As a result, the predicted absorption energies perfectly match those experimentally recorded (Figure 3), supporting the accuracy of TD DFT to reproduce the substituent effects.

Figure 3.
Correlation between the corrected calculated absorption energies by TD DFT (TD CAM-B3LYP/6-311 + g*) and those spectroscopically recorded in cyclohexane.

Such electronic transition is a result of the promotion of an electron from the HOMO to the LUMO. Therefore, the substituent-induced preferential stabilization/destabilization of these molecular orbitals (OMs) rules the energy gap and the position of the absorption band. On the one hand, the presence of alkyls at positions 2 and 6 raises preferably the energy of the HOMO (Figure 4). Indeed, the contribution of these chromophoric positions to the HOMO is slightly higher than in LUMO. Accordingly, electron donor substituents (such as ethyl and tert-butyl) destabilize the HOMO with the respect to the LUMO, leading to a slight reduction of the energy gap (from PM546 to PM567 and PM597, Figure 4). On the other hand, electron-withdrawing substituents at position 8 induce a more pronounced decrease in the energy gap owing to a marked reduction of the LUMO energy. In this meso position, a node is predicted, being is contribution to the HOMO almost negligible but in contrast very important in the LUMO. Thus, it is a position very sensitive to the substituent effect. Accordingly, the higher the electron acceptor character of the functionalization (acetoxy and cyano) the higher the stabilization of the LUMO (from PM605 to PM650,Figure 4). It is noteworthy that even dim changes in the absorption band position are well reproduced by the theoretically predicted energy of the OMs, being a suitable tool to guide the synthesis of novel BODIPYs with tailored absorption.

Figure 4.
Representative HOMO and LUMO contour maps and evolution of their energies (in eV) for the studied BODIPYs.

4. Electrochemistry

Further support for the substituent effect in the spectral bands’ position is gathered by the redox potentials provided by cyclic voltammetry (Figure 5). The simplest BODIPY herein considered (PM546) shows two well-defined cathodic and anodic waves at high potentials [40]. The alkylation at positions 2 and 6 makes such waves more reversible and reduces mainly the oxidation potential (from 1.12 V in PM546 to 0.98 in PM597, Figure 5). That means, that the ionization potential decreases, in line with the predicted increase of the HOMO energy as the main reason for the recorded bathochromic shift. However, the substitution at position 8 affects mainly the reduction potentials, which become systematically lower as the electron acceptor character increases (from −1.32 V in PM567 to −1.09 with acetoxy in PM605 and −0.63 with cyano in PM650, Figure 5). Thus, these groups increase the electron affinity in agreement with the predicted decrease in the LUMO energy as the source of the recorded large bathochromic shift upon such 8-functionalization.

Figure 5.
Cyclic voltammograms in acetonitrile (0.1 m TBAPF₆) of the studied BODIPYs (dye concentration 2 mM).

5. Fluorescence

The evolution of the fluorescence band position with the substituent follows the same trend recorded for the absorption band, but with one exception, PM597 dye (Figure 6). In this case, the bathochromic shift is larger than expected and the profile is broader. Consequently, the Stokes shift is three times higher than in the rest of the dyes (from around 500 to 1500 cm⁻¹, Table 1). This feature suggests a marked geometrical rearrangement upon excitation. Another issue to remark is the solvent effect. Most of the dyes show the expected negative solvatochromism, except PM650 where the opposite positive solvatochromism is detected in polar solvents and just in fluorescence (Table 1).

Figure 6.
Normalized fluorescence spectra of the BODIPY laser dyes in diluted solutions (2 μM) of cyclohexane.

	λ_ab (nm)	ε_max·10⁻⁴ (M⁻¹ cm⁻¹)	λ_fl (nm)	Δν_St (cm⁻¹)	ϕ	τ (ns)	k_fl (10⁸ s⁻¹)	k_nr (10⁸ s⁻¹)	E_ox (V)	E_red (V)
PM546
c-hex	499.0	10.3	514.0	490	0.99	5.42	1.82	0.02
EtOAc	494.0	9.2	506.0	475	0.96	5.58	1.72	0.07
Acn	492.0	8.7	504.0	490	0.90	5.72	1.57	0.17	1.12	−1.27
PM567
c-hex	522.5	11.7	541.0	650	0.93	5.98	1.55	0.12
EtOAc	527.0	8.4	536.0	685	0.83	6.08	1.36	0.28
Acn	515.0	7.7	534.0	685	0.82	6.31	1.30	0.28	0.98	−1.32
PM597
c-hex	529.0	7.5	576.0	1545	0.47	4.00	1.17	1.33
EtOAc	523.0	6.7	564.0	1390	0.48	4.38	1.09	1.18
Acn	521.0	6.5	563.0	1430	0.51	4.13	1.23	1.19	0.98	−1.26
PM605
c-hex	547.5	8.2	562.0	470	0.69	6.33	1.09	0.49
EtOAc	543.0	6.8	560.0	560	0.74	6.71	1.10	0.32
Acn	541.5	6.6	558.0	550	0.68	6.90	0.98	0.43	1.06	−1.09
PM650
c-hex	589.0	6.6	598.0	255	0.53	4.73	1.12	1.00
EtOAc	588.0	6.0	605.0	475	0.50	2.48	0.80	3.22
Acn	587.5	5.1	608.0	575	0.14	1.67	0.84	8.38	1.28	−0.63

Table 1.

Photophysical properties of BODIPY laser dyes in diluted solutions (2 μM). Electrochemical data in acetonitrile are also provided.

Absorption (λ_ab) and fluorescence (λ_fl) wavelength, molar absorption at the maximum (ε_max), Stokes shift (Δν_St), fluorescence quantum yield (ϕ) and lifetime (τ), radiative (k_fl) and non-radiative (k_nr) rate constants, oxidation (E_ox) and reduction (E_red) potentials. c-hex: cyclohexane; EtOAc: ethyl acetate; and Acn: acetronitrile.

Alkylated dyes, PM546 and PM567, are extremely bright, featuring fluorescence efficiencies close to 100% and long monoexponential lifetimes (around 5–6 ns, Table 1). However, in PM597, bearing branched alkylation, such efficiency decreases to half. Owing to the quasi-aromatic boron-dipyrrin framework, the spin–orbit coupling is low and the intersystem crossing probability is almost negligible [17]. Thus, the main channel of non-radiative deactivation from the excited state is due to internal conversion, which is related to conformational freedom and planarity [34, 41]. Theoretical calculations predict a chromophoric core essentially planar (slight bending around the transversal axis). Such butterfly-like bending increases in PM597, being remarkably higher in the optimized excited state (deviation from planarity up to 15° in the dihedral angle comprising the central ring and the pyrrole). It seems that to accommodate the bulky tert-butyl in the peralkylated chromophore and relieve geometrical stress, the planarity is distorted due to steric reasons, and mainly upon excitation, supporting the recorded different bathochromic shift in absorption and fluorescence, and the ensuing high Stokes shift (Table 1).

With regard to the meso position, the acetoxy group shifts the emission bathochromically, while maintaining a reasonably high fluorescence efficiency (Table 1). Compared with the unconstrained alkylated BODIPYs PM546 and PM567, the radiative rate constant of PM605 is lower and the non-radiative one higher. Likely, the electron-withdrawing effect of the acetoxy retrieves electronic density from the chromophoric core decreasing the aromaticity, as reflected in lower molar absorptions (Table 1). Further increase in such electron acceptor character by the attachment of cyano implies more prominent changes in the fluorescence properties. Apart from the mentioned deep bathochromic shift displayed by PM650 (Figure 6), the fluorescence efficiency decreases mainly in polar media (from 50–15%), where the lifetimes are fast (1–2 ns) (Table 1). The chromophoric framework of PM650 is fully planar and the cyano is linear and small, so the fluorescence quenching cannot be assigned to internal conversion as in the constrained alkylated BODIPYs, but rather to the cyano-induced extra non-radiative deactivation pathway, which is favored in polar media. All these trends pinpoint a photoinduced intramolecular charge transfer (ICT) from the BODIPY to the cyano, giving its high electron acceptor ability [42]. In fact, the previously recorded voltammogram for PM650 reveals a drastic change in the cathodic wave, being this dye prone to be easily reduced (Figure 5). Moreover, the recorded positive solvatochromism exclusively in the fluorescence spectra of PM650 envisages an increase of the dipole moment upon excitation, according to charge separation processes. ICT states are characterized by high dipole moments owing to the induced charge separation and are further stabilized in polar media. Thus, in these solvents, ICT can become a low-lying state with regard to the locally excited (LE) one, efficiently quenching the fluorescence emission from the latter state (Figure 7). This ICT seems to be a dark state, or at least a weakly emissive state, because no new emission bands are recorded, or perhaps the ICT emission is masked under the LE emission. Therefore, the cyano-induced charge separation arises as the non-radiative channel funneling the excited electrons mainly in polar media, where the fluorescence quenching is more notable (Figure 7).

Figure 7.
Sketch of the cyano-induced charge transfer in PM650 depending on the solvent polarity. The optimized first excited state geometry with the atoms colored by the charge (positive in red and negative in green) is also included.

6. Laser

Once revisited the fundamental photophysical signatures, we registered the laser properties at high concentrations (millimolar) under transversal pumping by a wavelength-tunable optical parametric oscillator (OPO) coupled to the third harmonic (355 nm) of the Nd:YAG laser. Thus, all the dyes were pumped at their corresponding absorption maximum wavelength. At these conditions, all of them display strong and sharp laser emission signals as expected (Figure 8). The laser emission appears within the fluorescence band, but at the end of the long-wavelength absorption tail, where the overlap with the absorption band has vanished, its position nicely correlates with the fluorescence on. Thus, alkylation at positions 2 and 6 induces a bathochromic shift, especially for PM597, reinforced upon substitution at position 8 with electron acceptor groups like in PM650 (Figure 8).

Figure 8.
Laser emission spectra of the BODIPY laser dyes at a high concentration (0.75 mM) in ethyl acetate.

However, the correlation of the emission efficiency (laser vs. fluorescence) is more complex, and it cannot be done directly, since other factors like the Stokes shift should be considered altogether. We should bear in mind that the laser action is recorded at high optical density media, whereas the photophysical signatures at diluted solutions. At the high concentrations required for the gain overcomes the losses in the resonator cavity, other competitive processes can appear. Dye aggregation is discarded owing to the low tendency of these dyes to self-associate in organic media (the spectral profiles remain unaltered even at high concentrations) [34]. However, reabsorption/reemission phenomena are ubiquitous owing to the spectral overlap between the absorption and fluorescence bands (BODIPYs usually feature low Stokes shift). Their influence is notorious in the laser emission wavelength (bathochromic shift of 10–15 nm with the dye concentration), as well as in the laser efficiency since an increase of the dye concentration does not necessarily imply higher efficiency. Instead, a plateau is reached and there is an optimal concentration for each dye, in which the maximum efficiency is recorded (Figure 9).

Figure 9.
Laser efficiency of the BODIPY laser dyes at different concentrations in ethyl acetate.

Thus, PM546 and PM567 display similar laser efficiencies, albeit slightly higher in the latter (59 vs. 64%, Figure 9). Attending to their photophysics, PM567 is slightly less fluorescence, but it shows a faintly higher Stokes shift, which compensates for such lower efficiency. Further evidence of the key role of the Stokes shift is provided by PM597, which displays higher lasing efficiencies (74%, Figure 9), albeit its fluorescence efficiency decreases to half (Table 1). As mentioned previously, this dye outstands by large Stokes shift (three times that of PM546). Therefore, the impact of the reabsorption/reemission is less harmful and counteracts its lower fluorescence yielding very bright laser emission. Indeed, whereas the optimal concentration was 0.5 mM for PM546 and PM567, in PM597 it is 0.75 mM. Moreover, the laser efficiency drastically decreases for PM546 (the dye with the shortest Stokes shift, Table 1) at high concentrations (1 mM) (Figure 9). On the other hand, the substitution at meso position has a marked impact on the photophysics (Table 1). PM605 displays reasonably high-lasing efficiencies (up to 66%, Figure 9) similar to PM567, in concordance with their similar fluorescence efficiencies (even slightly lower in the former, Table 1). Finally, PM650 is the dye endowed with the lowest laser efficiency (54% at the optimal dye concentration, Figure 9) as anticipated by the photophysics owing to the ongoing ICT (Figure 7). However, the laser performance is much better than envisioned by the low fluorescence efficiency (just a 20%). A remarkable feature of this dye is the shortening of the lifetime (down to 2 ns, Table 1), which could enhance the stimulated emission once the population inversion is achieved, and, in this way, counteract partially its low spontaneous emission probability. Therefore, several parameters (fluorescence efficiency, lifetime, and Stokes shift) have to be simultaneously considered to account for the molecular structure effect on the laser output efficiency.

Another critical parameter for the practical implementation of the dyes as active media of lasers is the photostability, in other words, the tolerance to endure strong and continuous irradiation regimes [43]. High photostabilities are required to ensure a long-operative lifetime of the laser. In this regard, the laser emission of PM546 decreases to half after 50,000 pulses (Figure 10). Higher resistance is achieved upon substitution at positions 2 and 6, where PM597 retains the 80% of its emission after 70,000 pulses (Figure 10). Therefore, substitution at the chromophoric positions is recommended to enhance photostability. On the other hand, the substitution at the meso position has a marked impact on the photostability. Thus, in PM605, the emission completely vanishes after just 40,000 pulses, whereas PM650 withstands 70,000 pulses with an emission loss of just 20%. The photodegradation mechanism is not fully unraveled, but it seems to involve the oxidation of the dye by the in situ-generated reactive oxygen species (ROS, such as singlet oxygen) upon irradiation, taking place mainly at the meso position [44, 45, 46]. Thus, steric and electronic reasons can account for the photodegradation rate promoted by oxidative singlet oxygen or related ROS. In PM650, the meso position is accessible, since the geometry of cyano is linear, and it is more exposed to the singlet oxygen than the rest of dyes bearing 8-methyl. However, this dye is the BODIPY with the highest oxidation potential (1.28 V in Figure 5) and hence is less prone to be oxidized by ROS, supporting its drastic increase in photostability owing to electronic reasons (Figure 10). Conversely, in PM605, the oxidation potential decreases to 1.06 V (Figure 5), being easier oxidized. Perhaps, the acetoxy itself contributes also to the generation of ROS under light irradiation and enhances the photooxidation at the meso position, leading to cation radicals, and explaining its fast degradation rate (Figure 10). Therefore, considering both parameters (efficiency and photostability) alkylated PM567 and PM597 are the most recommended laser dyes as powerful and long-lasting photoactive media.

Figure 10.
Photostability of the BODIPY laser dyes under continuous irradiation at dye concentration (0.75 mM) in ethyl acetate.

7. Conclusions

We have performed a comprehensive and comparative analysis of the laser performance of commercially available BODIPYs, as well as the photophysical properties, which sustain them. This study highlights that a deep knowledge of the underlying photophysical phenomena, as well as their dependency on the molecular structure, is fundamental to understand the ulterior laser performance. Fully alkylated BODIPYs are suited to yield bright and stable green–yellow lasers, whereas the surprising PM650 shows good performance as red laser. The interpretation of the relationship between molecular structure and laser behavior is complex, and several photophysical parameters should be considered altogether. For instance, the laser efficiency does not depend solely on the fluorescence efficiency, and other properties, as the Stokes shift and the lifetime, play a key role. Therefore, the combination, which seems to optimize the laser action, endows high fluorescence efficiency and Stokes shift, and fast lifetimes. However, these three properties are antagonisms. Most of the brighter fluorophores are characterized by small Stokes shifts and long lifetimes, while fluorescence quenching usually involves large Stokes shifts and short lifetimes. Therefore, an equilibrium between them should be reached to optimize the laser action.

The herein-reported master lines could serve as a guide to design new fluorophores or modify the available ones to build up benchmark dyes for lasers. In this regard, computational chemistry is a recommended tool to save time and effort in their design, since reproducing nicely the spectral shifts toward the development of cost-effective visible-NIR tunable dye lasers.

Acknowledgments

We acknowledge Spanish MICINN (PID2020-114755GB-C33) and Gobierno Vasco (IT1639-22) for financial support.

Conflict of interest

The authors declare no conflict of interest.

Ibrahim El Baba

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