Xue
Zhang‡
a,
Andrey A.
Sukhanov‡
b,
Xi
Liu‡
c,
Maria
Taddei
d,
Jianzhang
Zhao
*a,
Anthony
Harriman
*e,
Violeta K.
Voronkova
*b,
Yan
Wan
*c,
Bernhard
Dick
*f and
Mariangela
Di Donato
*dg
aState Key Laboratory of Fine Chemicals, Frontiers Science Center for Smart Materials, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, P. R. China. E-mail: zhaojzh@dlut.edu.cn
bZavoisky Physical-Technical Institute, FRC Kazan Scientific Center of Russian Academy of Sciences, Kazan 420029, Russia. E-mail: vio@kfti.knc.ru
cCollege of Chemistry, Beijing Normal University, Beijing 100875, P. R. China. E-mail: wanyan@bnu.edu.cn
dLENS (European Laboratory for Non-Linear Spectroscopy), Via N. Carrara 1, 50019 Sesto Fiorentino (FI), Italy. E-mail: didonato@lens.unifi.it
eMolecular Photonics Laboratory, School of Natural and Environmental Sciences, Newcastle University, Newcastle Upon Tyne, NE1 7RU, UK. E-mail: anthony.harriman@newcastle.ac.uk
fLehrstuhl für Physikalische Chemie, Institut für Physikalische und Theoretische Chemie, Universität Regensburg, D-93053 Regensburg, Germany. E-mail: Bernhard.Dick@chemie.uni-regensburg.de
gICCOM, Istituto di Chimica dei Complessi OrganoMetallici, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy
First published on 15th April 2023
To explore the relationship between the twisted π-conjugation framework of aromatic chromophores and the efficacy of intersystem crossing (ISC), we have studied a N,N,O,O-boron-chelated Bodipy derivative possessing a severely distorted molecular structure. Surprisingly, this chromophore is highly fluorescent, showing inefficient ISC (singlet oxygen quantum yield, ΦΔ = 12%). These features differ from those of helical aromatic hydrocarbons, where the twisted framework promotes ISC. We attribute the inefficient ISC to a large singlet-triplet energy gap (ΔES1/T1 = 0.61 eV). This postulate is tested by critical examination of a distorted Bodipy having an anthryl unit at the meso-position, for which ΦΔ is increased to 40%. The improved ISC yield is rationalized by the presence of a T2 state, localized on the anthryl unit, with energy close to that of the S1 state. The electron spin polarization phase pattern of the triplet state is (e, e, e, a, a, a), with the Tz sublevel of the T1 state overpopulated. The small zero-field splitting D parameter (−1470 MHz) indicates that the electron spin density is delocalized over the twisted framework. It is concluded that twisting of π-conjugation framework does not necessarily induce ISC, but S1/Tn energy matching may be a generic feature for increasing ISC for a new-generation of heavy atom-free triplet photosensitizers.
(1) |
Within this context, we note that ISC induced by a helical π-conjugation structure might offer new perspectives.27,28,52–58 It is known that for helicenes, ISC is due to the non-vanishing SOCMEs at each molecular orbital component.27 However, helicenes are not ideal triplet PSs because of their poor visible light absorption, and their challenging derivatization.59,60 Recently, twisted π-conjugation framework-induced ISC was observed for certain chromophores, including perylenebisimide (PBI).28,52–54 We and other groups also found that twisted Bodipy derivatives show efficient ISC, as for instance, the naphtha[b]-fused Bodipy (triplet state quantum yield, ΦT = 52%)58 and the dihydronaphto[a]-fused Bodipy (singlet oxygen quantum yield, ΦΔ = 55%).53,55,61 Moreover, the electron spin selectivity of the ISC process, manifested by the electron spin polarization (ESP) phase pattern of the triplet state, is highly dependent on molecular structure.55,58,61,62 Intriguingly, twisted Bodipy structures do not always give efficient ISC, in marked contrast to the helicenes,57 and recent work by Vennapusa reports the same situation with PBI.63
In seeking to expand the range of structurally distorted chromophores absorbing in the red region, our attention has returned to the helical N,N,O,O boron-chelated dipyrromethenes (BO).64,65 The synthesis of such compounds is relatively straightforward and there remains the strong possibility that the twisted geometry will promote triplet formation.27,66 Although the helical structure was introduced as a mean to enhance fluorescence of the 3,5-arylated Bodipy derivatives,64 the fluorescence quantum yields are lower than those of the native Bodipy. Coordination of two 3,5-ortho-phenolic substituents to the central boron atom reduces the molecular symmetry and enforces helicity, thereby leading to circularly polarized luminescence.65 The same synthetic approach can be applied to develop near-IR absorbing compounds,67–71 and to construct supramolecular assemblies.72 To the best of our knowledge, however, the ISC characteristics of these compounds have not been investigated. Now, using a variety of time-resolved spectroscopic tools, together with quantum chemical calculations, we compare the photophysical properties of a N,N,O,O-boron-chelated Bodipy (Ph-BO) possessing a severely twisted π-conjugation framework with those of the corresponding compound bearing an anthryl moiety at the meso-position of the dipyrrin core (An-BO). This latter functionalization adds an additional triplet level (T2) to the excited state manifold which turns out to be crucial in terms of the propensity for ISC.
Scheme 1 Synthetic scheme for preparation of An-BO. The molecular formulae of the reference compounds are also presented, BO is a reference compound used in the computational work. (a) (i) NaH, THF, (ii) ZnCl2, (iii) Pd(OAc)2, JohnPhos, 2-chloroanisole, reflux, 24 h, yield: 49%.73,74 (b) 9-Anthraldehyde, trifluoroacetic acid, rt, 2 h, reflux, overnight; then DDQ, reflux, overnight; then triethylamine, 30 min, BF3·Et2O, reflux, 4 h, THF, overall yield: 21%.75 (c) BBr3, DCM, yield: 49%.65 |
Single crystals of Ph-BO were obtained by slow diffusion of n-hexane (HEX) into a dichloromethane (DCM) solution. Subsequent X-ray diffraction studies show that the angle between the planes defined by the two pyrrolic rings is 48.8°.
The two phenyl rings deviate from the mean plane of the dipyrrin by ca. 22.1° and 26.7°, confirming that the molecular structure is highly twisted (Fig. 1b). The O–B–O, N–B–N and O–B–N bond angles are 107.6°, 105.6°, 106.8°, respectively (Fig. 1c). These data show a distorted tetrahedral boron center, while angles are similar to those of previously reported analogues.64,81
Fig. 2 UV/vis absorption spectra recorded for An-BO and Ph-BO in acetonitrile. c = 1.0 × 10−5 M, 25 °C. |
The corresponding fluorescence spectra are presented in Fig. 3. Ph-BO has a strong emission band (fluorescence quantum yield, ΦF = 73%, Table 1), whose position and intensity are independent of solvent polarity (Fig. 3c). The strong fluorescence is noteworthy because ISC is usually efficient for helicenes. Recent results from our laboratory,55–58 and Hasobe's laboratory53 show that twisted Bodipys can also display efficient ISC. A similar fluorescence spectrum is observed for An-BO (Fig. 3b), although the fluorescence quantum yield is lower (ΦF = 48%, Table 1). Furthermore, no charge-transfer emission band could be observed for An-BO, the fluorescence emission intensity and wavelength do not change with solvent polarity, these properties are uncommon compared to previously reported anthryl-Bodipy dyads.48,62,84 The singlet state energy (E00) of the compounds, determined as the crossover point of normalized absorption and fluorescence spectra, is 1.96 eV and 1.99 eV for An-BO and Ph-BO, respectively.
Compounds | λ abs [nm] | ε | λ em [nm] | τ F [ns] | τ T [μs] | Φ Δ | Φ F |
---|---|---|---|---|---|---|---|
a In acetonitrile (1.0 × 10−5 M). b UV/vis absorption maximum. c Molar absorption coefficient, ε: 104 M−1 cm−1. d Fluorescence emission maximum. e Fluorescence lifetime. f Intrinsic triplet lifetime. g Singlet oxygen quantum yield (λex = 630 nm, methylene blue used as standard compound, ΦΔ = 57% in dichloromethane). h Absolute fluorescence quantum yield, determined with optical integrating sphere. i Parameters for BDP are taken from the literature.82 j Parameters of BDP-Me are from the literature.31 k Not observed. | |||||||
An-BO | 623 | 5.9 | 636 | 8.3 | 226 | 40 | 48 |
Ph-BO | 613 | 5.4 | 643 | 11.3 | 215 | 12 | 73 |
BDP | 499 | 6.1 | 521 | 0.2 | −k | −k | 5 |
BDP-Me | 503 | 8.2 | 515 | 3.9 | 0.02 | −k | 71 |
The fluorescence lifetimes of An-BO and Ph-BO were measured by time-correlated, single photon counting (TCSPC), using picosecond pulsed laser excitation (Fig. S13†). Both compounds gave mono-exponential decay curves, with lifetimes of 8.3 ns and 11.3 ns in ACN, respectively, for An-BO and Ph-BO. These lifetimes were found to be independent of solvent polarity. It is interesting to note that the measured lifetimes are considerably longer than those associated with conventional Bodipy emitters (τF ≈ 4.0 ns),31,85,86 and of a recently reported Bodipy with twisted structure (τF ≈ 3 ns),53,55,58 The radiative rate constants for An-BO and Ph-BO are lowered to ca. 6 × 107 s−1, which is roughly half of that reported for conventional Bodipy-based chromophores. The photophysical properties of the target compounds are summarized in Table 1.
As a preliminary evaluation of the ISC efficiency of these compounds, the quantum yields for formation of singlet molecular oxygen (ΦΔ) were measured (Table 2). For Ph-BO, ΦΔ ≤ 12% and dependent on solvent polarity; the highest yield was observed in ACN, with a much lower yield in HEX. The poor sensitization behaviour contrasts with that of the helicenes.27,60,66 and with that of the twisted Bodipy, which shows ΦΔ up to 55%.53,55,57,58 Interestingly, an improved photosensitizing capability was observed for An-BO, where ΦΔ = 40% in ACN and ΦΔ = 12% in HEX (Table 2). Based on the absorption and fluorescence data, we propose that the weak dependence of the ΦΔ values on the solvent polarity does not support the SOCT-ISC mechanism, as also demonstrated by femtosecond transient absorption spectroscopy (fs-TA, see later). However, the polarity of the solvent might affect the S1/Tn energy matching, thereby influencing the yield of ISC, as found previously for anthracene (An).87
Compounds | HEXb | TOLc | THFd | DCMe | ACNf | MeOHg |
---|---|---|---|---|---|---|
a In percentage, λex = 630 nm, methylene blue was used as standard, ΦΔ = 57% in dichloromethane. b n-Hexane, ET(30) = 30.9 kcal mol−1. c Toluene, ET(30) = 33.9 kcal mol−1. d Tetrahydrofuran, ET(30) = 37.4 kcal mol−1. e Dichloromethane, ET(30) = 41.1 kcal mol−1. f Acetonitrile, ET(30) = 46.0 kcal mol−1. g Methanol, ET(30) = 55.5 kcal mol−1. | ||||||
An-BO | 12 | 17 | 15 | 28 | 40 | 12 |
Ph-BO | 5 | 7 | 6 | 10 | 12 | 5 |
Redox potentials were also determined for An-BO. At +1.12 eV there is a reversible oxidation wave, attributed to one-electron oxidation of the anthryl moiety,88 compared to the known oxidation potential of anthracene (+0.9 V vs. Fc/Fc+).88,89 Making use of the Weller equation (eqn (S1)–(S3)†),90–92 the Gibbs free energy change (ΔGCS) accompanying intramolecular electron transfer for An-BO (with E00 = 1.96 eV) was calculated (for details please refer to the ESI†). The main finding is a positive ΔGCS in all solvents, a result in full agreement with the solvent-independent fluorescence properties found for An-BO (Fig. 3 and Table 3). Finally, spectroelectrochemistry was used to record absorption spectra for the BO radical anion (BO−˙), with peaks at 540 nm and 700 nm, and the anthracene radical cation (An+˙) with peaks at 540 nm and 580 nm (Fig. 4b and c). This latter information, used in conjunction with ultrafast transient absorption spectral measurements, helps to rule out the occurrence of light-induced charge separation.
E RED/V | E OX/V | ΔGCS (eV)/ECT (eV) | ||||
---|---|---|---|---|---|---|
HEX | TOL | DCM | ACN | |||
a Cyclic voltammetry in N2-saturated ACN containing 0.1 M Bu4N[PF6] as supporting electrolyte. Redox potentials of the compounds were determined with ferrocene (Fc) as internal standard (0 V). Counter electrode is Pt electrode and working electrode is glassy carbon electrode, with Ag/AgNO3 couple as the reference electrode. b E 00 = 1.96 eV. E00 is the energy level of singlet excited state localized on BO moiety (1BO*) approximated as the crossing point of normalized absorption and fluorescence spectra. c Not observed. d Not applicable. | ||||||
An | − c | +0.90 | —d | —d | —d | —d |
Ph-BO | −1.35/−2.17 | +0.67 | —d | —d | —d | —d |
An-BO | −1.39/−2.17 | +0.67/+1.12 | 1.22/3.17 | 1.06/3.01 | 0.61/2.56 | 0.49/2.44 |
Fig. 5c presents the fs-TA spectra of Ph-BO recorded in ACN following excitation at 610 nm. The transient spectra present an intense negative band centred at 630 nm, interpreted as the convolution between ground state bleaching (GSB) of the BO chromophore and stimulated emission (SE), with a weaker transient absorption band in the 380–600 nm range. The SE contributes mainly to the low-energy tail of the negative signal, extending beyond 740 nm. The transient bleaching gradually recovers over the timescale of the measurement, while the positive absorption band does not change significantly over time. Global analysis of the kinetic traces was applied to extract relevant time constants associated with evolution of the system (species associated difference spectra, SADS), using a target-model decay scheme comprising four species (Fig. 5d); the S1 state of Ph-BO produces the triplet state with a low probability and relaxes to the ground state by radiative and non-radiative transitions.
The first component (black dashed curve) is assigned to a singlet excited state localized on the N,N,O,O-boron-chelated Bodipy. It is expected that excitation at 610 nm will populate the S1 state with excess vibrational energy. The first ultrafast process, close to the detection limit of the instrument (155 fs) is assigned to a fast electronic relaxation bringing the system out of the initially populated Franck Condon state. Within 50 ps, the system evolves towards the third component, which retains similar spectral characteristics and is therefore ascribed to the S1 state. Over this time interval, there is a small increase in the bleaching signal, that might be related to a red shift of the emission band, together with a slight increase of ESA between 550 and 600 nm. These spectral perturbations can be assigned to relaxation of the excited state. Evolution to the final species occurs within ca. 9 ns, but the residual transient species is much longer lived than the maximum time window of the instrument. The profile of this residual signal is consistent with that of the triplet state observed in nanosecond transient absorption spectra (see below). This result indicates that the time constant for the decay of S1 state is 8.8 ns, which is roughly in agreement with the long fluorescence lifetime of Ph-BO (11.3 ns, Table 1). Ph-BO has been studied also in HEX (Fig. S14†). The recorded transient spectra and the excited state evolution are similar to those observed in ACN, shown by the comparison between the kinetic traces recorded at the maximum of the GSB signal in ACN and HEX (Fig. S16†).
The fs-TA spectra recorded for An-BO in different solvents are comparable to those recorded for Ph-BO, thereby suggesting that formation of an intramolecular charge-transfer state is unimportant for this dyad (Fig. 6 and S15†). The spectral changes, however, do not rule out fast electronic energy transfer from anthracene to BO following excitation at 360 nm. Indeed, the component A with a lifetime close to the instrument detection limit, presents no SE signal, indicating that the higher singlet excited state is located on the anthryl unit (Fig. 6b). Subsequent spectra show a negative signal, with a minimum at 630 nm, attributable to a combination of GSB and SE of the BO unit. In addition, there is a weak absorption band extending from 380 to 600 nm. Target analysis performed in both HEX and ACN shows that spectral evolution occurs on different timescales compared to Ph-BO. The two components B and C have similar spectral character and are also similar to the singlet excited state signal of Ph-BO (Fig. 5d), so that, they can be assigned to the S1 state and vibrationally relaxed S1 state, respectively. The relaxation times, possibly including a contribution from intramolecular electronic energy transfer, are 24 ps and 31 ps, respectively in HEX and ACN. The residual species D shows ESA peaks at ca. 460 nm and 660 nm, and GSB peaks at 550 nm–650 nm (olive line in Fig. 6b); this spectral pattern can be attributed to the triplet-excited state of An-BO (see below). From the kinetics study, the time constants for the decay of S1 of An-BO in HEX and ACN are 7.7 ns and 8.0 ns, respectively. When An-BO is excited at longer wavelength (610 nm, Fig. 6c and d), the first three components observed have similar shape. The GSB of the first component (black dotted line, component A, Fig. 6d) is slightly narrower than that of other two components, and has a somewhat stronger absorption at 590 nm; we attribute A to the unrelaxed singlet state. Solvation and vibrational relaxation occur within 250 fs and 50 ps, then the triplet state is produced (olive line). The lifetime of S1 state is 6.7 ns. Transient absorption spectra for An-BO have been recorded also in polar solvents (Fig. S17 and S18†). The evolution of the transient spectra and the associated kinetics show only a minimal dependence on the solvent polarity.
Assuming internal conversion to the ground state is insignificant, the time constant for ISC can be approximated by τISC = τs1/(1 − ΦF). An estimate for τs1 is available from the fs-TA studies, leading to the time constant for ISC for Ph-BO in acetonitrile as being ca. 33 ns, and that for An-BO as ca. 13 ns. This indicates that the rate of ISC for An-BO is roughly twice that for Ph-BO, which helps to account for the higher ΦΔ for the former. Importantly, the ISC time constant is significantly shorter than that for conventional Bodipy derivatives.
For An-BO, a strong bleaching signal is observed at 622 nm, together with weaker absorption centred at 457 nm (Fig. 7). The intrinsic triplet lifetime was determined as 226 μs. This was shortened to 0.32 μs in aerated solution (Fig. S20†), confirming that the observed transient species is a triplet excited state localized on the Bodipy core. The ESA band of the N,N,O,O-boron coordinated Bodipy is drastically different from that observed for a bis-styrylBodipy, although the latter shows a similar absorption maximum to the N,N,O,O-boron coordinated Bodipy.40,99,100 The intrinsic triplet lifetime of An-BO is much longer than that of Bodipy accessed with the heavy atom effect (153 μs),101 or of the diiodobisstyrylBodipy (1.8 μs).98 This is an important advantage of facilitating triplet population through the heavy atom-free protocal, e.g. by distorted framework approach, rather than the more conventional heavy-atom perturbation methodology.17,102
Fig. 7 (a) Nanosecond time-resolved transient absorption spectra recorded for An-BO upon pulsed laser excitation (λex = 610 nm), (b) decay trace of An-BO at 605 nm in c = 1.0 × 10−6 M in deaerated acetonitrile, the intrinsic triplet state lifetime (τintrinsic) was obtained by fitting the decay curves to the kinetic model taking into account triplet–triplet annihilation (TTA) (see the ESI† for details), the lifetime data in (b) were measured at 5 × 10−6 M, apparent lifetime is 209 μs. 25 °C. |
Neither of the target compounds is phosphorescent at room temperature or at 77 K. Estimates of the triplet state energies of An-BO and Ph-BO as being 1.35 eV and 1.38 eV, respectively, were obtained by TD-DFT computations (see ESI† for details). Also, an experimental determination of the triplet energy was made by intermolecular sensitization,103 using meso-tetraphenylporphyrin (TPP, ET = 1.44 eV) as triplet donor or perylenediimide (PBI, ET = 1.20 eV) as triplet acceptor (Fig. S21 and S22†).104 With this method, the triplet state energy of An-BO was determined to be ca. 1.24 eV. The T1 state energies of these compounds are lower than that of the pristine Bodipy (ca. 1.6 eV),105 but much higher than the bis-styrylBodipy (ca. 1.0 eV).40,100 As such, promotion of the triplet-excited state through a twisted molecular structure55,58 offers a promising route towards increasing absorption in the red region whilst maintaining a viable triplet state energy and long lifetime.106
The TR-EPR spectra of Ph-BO and An-BO recorded at 600 ns after the laser flash are presented in Fig. 8. The dependence of the TR-EPR and magnetophotoselection TR-EPR spectra of Ph-BO and An-BO on the delay time after photoexcitation are shown in the Fig. S25–S27†. The spectra are fully consistent with the involvement of a solitary triplet state. Furthermore, both the spectra and the consequent fitting parameters are comparable for the two compounds, clearly indicative of the localization of the triplet state on the BO fragment. The electron spin polarization (ESP) pattern for both compounds conforms to (e, e, e, a, a, a), this situation is reminiscent of that found for 2,6-diiodoBodipy,49,97,113 and 2,6-diiodobisstyrylBodipy.102 We previously observed the TR-EPR spectrum of the Bodipy triplet state, generated with the SOCT-ISC in compact anthryl-Bodipy orthogonal dyads, showing ESP phase pattern of (e, e, e, a, a, a), the SOCT-ISC mechanism can be excluded for An-BO based on the steady state fluorescence spectral study (Fig. 3b) and the fs-TA spectra (Fig. 6). Recently we found that the ESP of the triplet state TR-EPR spectrum of a twisted naphthalene-fused Bodipy is (a, e, a, e, a, e),58 this indicates that the ESP of the triplet state of twisted Bodipy derivatives is highly dependent on the molecular structures.56 This result shows the rich electron spin selectivity of the ISC of the twisted Bodipy derivatives.62,114
Fig. 8 Experimental TR-EPR spectra for Ph-BO, recorded at a delay time of 600 ns after laser excitation, the frozen samples (c = 3.0 × 10−4 M) were excited at 585 nm with a pulse energy of 1 mJ. For An-BO, the frozen samples (c = 2.0 × 10−4 M) were excited at 613 nm. The model TR-EPR spectrum of An calculated for D = 2150 MHz, E = 244 MHz, Pz = 0.00, Py = 0.56, Px = 0.44 (ref. 49 and 109) is also presented for comparison. Samples were dissolved in TOL/2-MeTHF (1/1, v/v), and the spectra were recorded at 80 K. Simulation parameters are presented in Table 4. |
Simulation of the TR-EPR spectra gives ZFS D and E parameters for Ph-BO as −1470 MHz and 477 MHz, and almost the same for An-BO (Table 4). Similar D and E parameters indicate that the T1 state of the molecules is confined on the BO chromophore. These parameters are different from those for 2,6-diiodoBodipy, for which the D and E values are −2940 MHz and 655 MHz (Table 4), and the rhombicity (non-axiality) of the spatial distribution of the triplet state wave function |E/D| is increased compared to the native Bodipy.49,113 These results show that the triplet state wave functions are delocalized over the twisted molecular framework. Interestingly, the ZFS D and E parameters of Ph-BO and An-BO are similar to the recently reported naphthalene-fused Bodipy derivative, which also has a twisted molecular structure.58 The population ratios of the sublevels of the T1 state of An-BO remain similar to those of Ph-BO (Table 4), Tz sublevel of the T1 state being overpopulated for both, which is supported by theoretical computations (see ESI† for more details).
Compound | D (MHz) | E (MHz) | P x :Py:Pz |
---|---|---|---|
a Obtained from simulation of the triplet-state TR-EPR spectra of the indicated molecules in a toluene/2-MeTHF glass (1/1, v/v) at 80 K. b The sign of the D parameter is negative for Bodipy. | |||
An-BO | −1470 | 467 | 0.00:0.00:1.00 |
Ph-BO | −1470 | 477 | 0.00:0.00:1.00 |
2,6-diiodoBodipy | −2940 | 655 | 0.00:0.15:1.00 |
The electron spin density of the T1 state was computed (Fig. 9c and d). For An-BO, the spin density is almost exclusively localized on the twisted Bodipy moiety, with negligible distribution on the anthryl unit. This seems consistent with the large dihedral angle between the anthryl moiety and the twisted Bodipy core (77°), which effectively decouples the two components. For Ph-BO, part of the spin density is distributed over the meso-phenyl ring, presumably because of the smaller dihedral angle. However, this disparate distribution of the triplet state electron spin density surface between the two triplets does not affect either the transient absorption or the TR-EPR spectra. For both compounds, the difference in electron density of the triplet excitation (corresponding to a HOMO → LUMO transition, Fig. S27†) is mainly localized on the twisted dipyrrin core.68
In order to further understand the difference in triplet state quantum yield of the compounds, we also studied the native oxo-Bodipy (BO), which has a hydrogen atom at the meso position. All electronic structure calculations were performed with the ORCA programs,115,116 using DFT with the CAM-B3LYP functional and the def2-SVP atomic orbital basis set. The electronic ground state S0 was optimized with restricted DFT, the T1 state with unrestricted DFT, and all other excited states with TD-DFT. For each excited state the Hessian matrix was calculated and checked for positive definiteness. These optimized geometries and their Hessians are subsequently used in an excited state dynamic (ESD) calculation that yields the rate constant for ISC. Both Franck–Condon and Herzberg–Teller contributions were calculated for all three sublevels of each triplet state. We used a purpose-written program applying the formulae of Baiardi et al.117 or of de Souza et al.,118 which yield identical results.
With the CAM-B3LYP functional, the T2 state is found below the S1 state, hence the ISC rates for S1 → T2 ISC were also calculated. The excitation energies calculated with the CAM-B3LYP functional for S1 are substantially higher than the experimental values (see below), TD-DFT usually highly overestimates the low-lying excited states of Bodipy derivatives, hence it is possible that the true S1 → T2 energy gap is small or even negative. An exception is the T2 state of An-BO which is a locally excited state on the anthryl unit and has no counterpart in the two other compounds. At each optimized geometry, the vertical excitation energies were calculated, resulting in the level diagrams shown in Scheme 2 (refer to Table S2† for specific values). For Ph-BO, both HOMO and LUMO are localized on the dipyrrin unit. The S1 state is predominantly of HOMO → LUMO excitation (i.e., a locally excited singlet state). Optimization of the S1 state lowers its energy by ca. 1000 cm−1 with respect to the vertical excitation and shifts the S0 state by about the same amount to higher energies (Table S2†), thus predicting a rather small Stokes shift. The geometrical change is small, and the dipole moment almost unchanged on excitation. For all three compounds, the T1 state is found at about half of the energy of the S1 state, but at almost the same geometry. A rather large exchange integral is indicated by the observation that T1 also has a large contribution from the HOMO → LUMO excitation.
For An-BO, the T2 state corresponds to excitation from the HOMO to the LUMO of the anthryl unit, i.e., it is a locally excited state (3An*). Optimization of the geometry of this T2 state leads to a strong stabilization accompanied by significant structural changes at the anthryl unit. At its minimum, the T2 state is almost degenerate with the Bodipy-localized T1 state. Since both the lower and the upper singly occupied orbitals in S1 and T2 are different, we expect small SOCMEs. Hence vibrationally induced SOC should be considered.28,119 Indeed, we find that Herzberg–Teller coupling accounts for 100% of the ISC rates to the M = +1/−1 substates of the triplets, and for a substantial fraction of the M = 0 substate.
The program allows calculation of the ISC rate constant as a continuous function of the energy gap. Input data were the geometries and Hessians from the ORCA optimizations, as well as the SOCMEs and their derivatives with respect to all normal coordinates. Fig. 10a shows the resulting ISC rate constants (kISC) for the S1 → T1 transition for each of the three compounds, the calculated energy gaps are indicated by a dot for each curve. We note that the rates of the S1 → T1 transition are comparable for the three compounds, indicating that the anthryl substituent is not involved in this particular process. For energy gaps of ca. 10000 cm−1, these rates are smaller than 105 s−1 and hence cannot explain the observed triplet yields. This is most probably a consequence of the fact that S1 and T1 correspond to the same HOMO → LUMO excitation and therefore have rather small SOCME. However, for An-BO, the kISC of S1 → T2 is greatly increased, which is consistent with the higher triplet yield. The S1 → T2 transition in An-BO leads directly to the triplet state localized on the anthryl moiety.
Fig. 10b shows the corresponding data for ISC from the S1 state to the second locally excited triplet on the BO unit, the relevant transitions are S1 → T2 for BO and Ph-BO, and S1 → T3 for An-BO. As in the case of the S1 → T2 transitions, these curves are comparable, indicating that these triplet states are associated with the BO unit and have similar orbital excitation character. The substituents appear not to be involved. Since orbitals other than those associated with S1 are involved, the SOCMEs are increased, thereby enhancing the rate constants by two orders of magnitude compared to S1 → T1. The rate constant for the S1 → T2 transition for An-BO shows significantly different behaviour to that of the other compounds (dotted line in Fig. 10). The final state T2 is a locally excited triplet of the anthryl unit which has no low-energy counterpart in the parent compound or the phenyl-derivative (i.e., ISC corresponds to intramolecular energy transfer). The transition is accompanied by a strong geometrical relaxation within the anthryl unit, giving a broad profile to the energy gap dependence.
The second local triplet state on the BO unit is probably at higher energy than the S1 state, i.e. the energy gap is negative. A gap in the range (1500–1000) cm−1 would still allow ISC to occur with a rate constant of ca. 108 s−1. There is little experimental information on the true energy gap between S1 and T2 in these compounds. It is hence possible that the T2 states of BO and Ph-BO in reality are higher than S1. On the other hand, the strong relaxation of T2 observed for An-BO suggests that the state is near T1 and below S1. Unsubstituted anthracene has a T1 state energy of 1.8 eV.104 This is below the experimental energy for the S1 state of Ph-BO (ca. 2.0 eV). For an energy gap in the range 4000–6000 cm−1 the calculations anticipate ISC rate constants near 5 × 108 s−1. These are in good agreement with a triplet yield in the range of 50%. It must then be concluded that the actual S1 → T2 energy gap in Ph-BO is negative, i.e., T2 is above S1. Thus, the increased rate of ISC for An-BO can be ascribed to transient population of the T2 state localized on the anthryl unit.
This is an interesting example whereby the initial and the final states of ISC are confined on different chromophores, yet the matching of the S1 and T2 state energies (at the geometry of S1) still induces efficient ISC. This result may present new understanding of singlet-to-triplet energy transfer, which is much less common than singlet–singlet or triplet–triplet energy transfer.120,121 Our theoretical studies are able to correctly predict that the Tz sublevel of the T1 states of the compounds are overpopulated, which is in agreement with the TR-EPR spectral observations (see ESI for more details, Fig. S30 and S31†). We found that the SOCME favours the transition to the Tx-substate, while the much larger vibrational contributions favour the transition to the Tz-state, which is in agreement with the TR-EPR spectral experimental results (Fig. 8 and Table 4). This principal is applicable to other twisted molecules, i.e. the ESP phase pattern of the T1 state TREPR spectrum of a compound, should be directly related to the ISC rate constants of S1 → Tx, S1 → Ty and S1 → Tz.
Because of the stated uncertainties about the S1 state energy obtained by the TD-DFT calculation, we prefer to rely on the values obtained experimentally. The absence of detectable levels of phosphorescence, however, precludes any experimental T1 state energy, and we must rely on the computed results. This leads to the simplified energy diagrams for An-BO and Ph-BO shown in Scheme 3. For Ph-BO, the T2 state energy is higher than that of S1, and there is a large energy gap between S1 and T1 states, making ISC inefficient. For An-BO, the T2 state (i.e., 1.81 eV), localized on the anthryl moiety, is lower in energy than the S1 state (i.e., 1.96 eV), thereby facilitating faster ISC through intermediary population of the anthryl triplet state. Notably, triplet population via this route involves two separate electronic energy transfer events rather than the conventional SOC mechanism.
To overcome the large S1 → T1 energy gap, the meso-phenyl ring has been replaced with an anthryl moiety which has a locally-excited triplet state of comparable energy to the S1 state associated with the twisted Bodipy chromophore. For this dyad, An-BO, charge recombination-induced ISC is insignificant, as demonstrated by steady-state optical spectra and fs-TA measurements. The triplet quantum yield is increased substantially and An-BO exhibits a much improved quantum yield for formation of singlet molecular oxygen (ΦΔ = 40%). The greater propensity towards triplet formation must arise from increased coupling between the S1 and T2 states. It is important to stress that these two excited states are localized on different chromophores and held in a near orthogonal geometry but still manage to promote ISC. Transient population of the anthryl-based triplet state is followed by triplet–triplet energy transfer to form the T1 state localized on the Bodipy fragment. The latter process is spin-allowed and likely to be fast but the former step is relatively slow because of the molecular orientation and limited spectral overlap. Nonetheless, the process competes with fluorescence and internal conversion from S1. The overall ISC rate for An-BO is faster (ca. 13 ns) than for Ph-BO. In this respect, it is important to note that these Bodipy derivatives based on twisted π-conjugation pathways show unusually long-lived S1 states and this is a considerable benefit in terms of applications. Our results show that the twisted π-conjugation system cannot be relied upon to induce efficient ISC. An improved strategy is based on energy matching of S1/Tn (n > 1) states. Moreover, since the anthryl triplet is far from optimal as an acceptor, there is considerable scope to design a new range of heavy atom-free triplet sensitizers along these lines.
Footnotes |
† Electronic supplementary information (ESI) available: General experimental methods, synthesis of compounds, molecular structure characterization, X-ray crystallographic data, computational details and additional spectra. CCDC 2210911. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc00854a |
‡ These authors contributed equally to this work. |
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