In contrast to the situation for the NDIs, the locally excited state of carbazole-substituted triarylboranes (TABs) already possesses a full CT character. This can be deduced from the clear solvatochromic shift of the lowest energy absorption band [58]. As shown in Fig. 3.3, the steady state fluorescence of donor-substituted TABs is subject to an even more pro- nounced shift that indicates the emergence of a highly polar excited state from a relatively nonpolar ground state. This strong charge redistribution upon photoexcitation makes TABs ideal candidates to study solute-solvent-correlated dynamics. By using TABs with multiple donor substituents, we investigated in particular the influence of molecular symmetry and the related charge mobility to the solvation dynamics. The results of this study are summa- rized in the following publication (appendix A4):
Symmetry-dependent solvation of donor-substituted triarylboranes U. Megerle, F. Selmaier, C. Lambert, E. Riedle, S. Lochbrunner
Physical Chemistry Chemical Physics 10, 6245-6251 (2008).
Fig. 3.3 Steady-state absorption and emission spectra of (a) CB and (b) TCB in the nonpolar sol- vent cyclohexane (dotted lines) and the polar solvent benzonitrile (solid lines). The CT-absorption (shaded area) corresponds to an electron transfer from a carbazole moiety (blue ellipse in the chemical structure) to the boron center.
In our investigation we compared the single carbazole-substituted borane (CB) shown in Fig. 3.3a and its highly symmetric counterpart, the triple carbazole-substituted borane (TCB) shown in Fig. 3.3b. For all polar solvents under study we found an accelerated solvation process for TCB compared to the less symmetric CB. We explained these findings by the possibility of an intramolecular charge delocalization over the different carbazole sites of TCB. This allows for a response of the excited state dipole moment of TCB to the local field of the solvation shell. In contrast to CB, where the relaxation is only possible by motions of the solvent molecules, the TCB system thus acquires an additional efficient pathway to reach the minimum energy configuration.
Fig. 3.4 Spectral decomposition to model solvatochromic band shifts: (a) TA spectra of TCB in benzonitrile solution after 360 nm excitation. The spectral evolution is governed by the redshift of the stimulated emission band. The more complicated TA spectra of CB (b) can be decomposed into a strongly shifting SE band and a slightly shifting broad ESA band (c). The reconstructed transient spectra (d) resulting from this minimal parameter global fit are in very good agreement with the experimental curves.
These results are based on the evaluation of the solvation dynamics of the two chromo- phores, i.e. the kinetics of the spectral shifts observed in the ultrafast TA experiments. The occurrence of strong band shifts in polar media can already be predicted from the cw spectra. By comparing the cw emission maxima in the nonpolar cyclohexane (dotted lines in Fig. 3.3) with, for example, the ones in benzonitrile (solid lines), one can expect a dynamic Stokes shift of up to 3000 cm-1 for the stimulated emission bands of CB and TCB in a time-
times from the extensive study by Maroncelli and coworkers on a coumarin dye [49]. Typi- cally, the characteristic times for the solvation lie in the ps range. Since the investigated TABs were designed as long-lived emitters and charge transport materials for organic light emitting diodes (OLEDs), the excited state has a lifetime on the order of nanoseconds [58] which implies that only the onset of a deactivation will be observed in the time range of our experiment.
Indeed, the TA spectra we obtained after 360 nm excitation of CB and TCB in polar sol- vents show a strong evolution on the ps timescale (see Fig. 3.4a and b). In cyclohexane how- ever, the TA spectra remain virtually unchanged for several tens of picoseconds. Only for larger delay times approaching the ns fluorescence lifetime does the decay to the ground state cause the signals to decrease. This slow, collective decay is also observed in polar envi- ronments. However, since its dynamics is (i) understood and (ii) completely separated from the ps evolution, it can be neglected for the further discussion.
Intuitively, the interpretation of the TCB spectra is easier. As seen in Fig. 3.4a, the ob- served spectral evolution is clearly governed by a red-shifting SE band. The ~7 mOD change in the amplitude of the local minimum between 0.6 and 50 ps can be understood from the superposition with the falling edge of an ESA band in this region. On the other hand, the CB spectra in Fig. 3.4b on first sight suggest a reaction from a species absorbing around 440 nm to one absorbing around 400 nm. This, however, would require an isosbestic point in be- tween, which is not observed. A closer look also reveals the occurrence of spectral shifts: for example, the local minimum around 450 nm shifts by ~10 nm between 3 and 50 ps. From the discussion above it is much more likely that for CB as well solvation is the only driving force for the spectral changes.
To validate this assumption we aimed for a quantitative modeling of the TA data. As shown in section 2.3 with artificial data, exponential fits to kinetic traces at fixed wave- lengths are not suitable to describe the dynamics of spectral shifts. For the TA data of CB and TCB in polar solvents, a reasonable parameterization requires up to three exponential components with strongly varying time constants depending on the probe wavelength. We therefore followed a different approach and decomposed the TA spectra in a global fit rou- tine.
In the case of a non-reactive system, the three signals contributing to the TA spectra are the stimulated emission (SE), the excited state absorption (ESA) and the ground state bleach (GSB). Here as in most cases, the GSB is not subject to spectral shifts: it represents the ab- sorption of ground state molecules that is not time-dependent. The global fit of the solvation dynamics is therefore restricted to λ > 400 nm for TCB and to λ > 385 nm for CB. For the spectral signatures of ESA and SE we assume an evolution that comprises an exponential shift of the spectral bands with time on the energy scale (see Fig. 3.4c):
(
)
(
SE)
t
ESA (t, ) ESA , ESA e ;
t SE (t, ) SE , e . − τ ⎡ ⎤ ν = ∞ ν − Δν⎢⎣ ⋅ ⎥⎦ − τ ⎡ ⎤ ν = ∞ ν − Δν⎢⎣ ⋅ ⎥⎦ (3.1)
As always for the modeling of TA spectra, one should try to use previous knowledge to limit the parameter range as much as possible. First of all, the SE signature must coincide with the known steady-state emission spectrum in the respective solvent at t → ∞, i.e. at quasi-equilibrium. The spectral shape of the ESA for large delay times can then be inferred from the final transient spectrum by subtracting the steady state emission with a proper scal- ing factor*. Secondly, the dynamics of the shift should be the same for ESA and SE. There- fore, a common time constant τ can be used reflecting the averaged solvation dynamics of the system. Moreover, the solvation model requires that the SE is initially blue-shifted, ac- counting for the unfavorable energetic situation directly after the excitation. As a conse- quence, Δν SE will always have a positive sign. Since the dipole moment of the higher elec-
tronic states contributing to the ESA band is unknown at first, Δν ESA can in principal have
both signs.
All of this was implemented in a least-square fit algorithm written in LabView. The tran- sient spectra resulting from this fit are in very good agreement with the experimental curves, including in particular the rather complex CB spectra (see Fig. 3.4d). As expected, the shift amplitudes for the SE band are on the order of several 1000 cm-1 and the corresponding shift times are in the same regime as the average solvation times known from literature [49], at least for the less symmetric CB. The consistently shorter times found for TCB led to the model of symmetry-dependent intramolecular contributions to the solvation as described above and in appendix A4.
It is worth noting that the TA spectra of CB in benzonitrile are a striking example that even complicated looking spectral evolutions might have fairly simple explanations. One only has to take into account what is known from independent sources, e.g. from cw spec- troscopy, literature, TA experiments in a nonpolar solvent or with a related molecule such as TCB, to rule out any unlikely processes. In the case of CB, the existence of several discrete intermediate states and the interconversion between them could be excluded from the discus- sion, even though the apparent rise and decay features in the TA spectra would suggest such a scheme on first glance. Instead, the solvation induced band shifts sufficed to reproduce the TA signatures.
* For a quantitative analysis, the spontaneous steady state emission spectra have to be corrected for the spectral
3.3 From absorption changes to state populations: