Time-resolved spectroscopy of DMABN and its cage derivatives 6-cyanobenzquinuclidine (CBQ) and benzquinuclidine (BQ)

Do fluorescence and transient absorption probe the same intramolecular charge transfer state of 4-(dimethylamino)benzonitrile?

“…Consistent with the TICT model, the ICT-state absorptions at about 420 and 320 nm very closely resemble the absorption spectrum of the benzonitrile radical anion in both peak positions and relative intensities. 5 Moreover, picosecond time-resolved resonance Raman spectra of the ICT state, obtained by using probe ͑Raman inducing͒ wavelength of 330 nm, exhibit several characteristic modes that are very similar to the modes observed for the benzonitrile radical anion. 6 An alternative ICT-state structure, known as PICT ͑P for planar͒, 7 is not supported by time-resolved laser spectroscopies ͑vide infra͒ or by a great majority of quantum chemical calculations.…”

Direct Observation of a Hydrogen‐Bonded Charge‐Transfer State of 4‐Dimethylaminobenzonitrile in Methanol by Time‐Resolved IR Spectroscopy

“…Ground state bleach recovery can be seen in Figure 1 b, and the enhancement relative to the behavior in MeCN indicates much stronger internal conversion in MeOH. By using the fluorescence quantum yield of DMABN in MeOH (0.017) [9] in fits to the bleach areas from 20 ps to 3 ns as before (not shown), the fluorescence, IC, and ISC rates are found to be 1.1 10 7 , % 31 10 7 , and % 30 10 7 s À1 , respectively. The LE equilibrium population fraction is again estimated to be very small and these rates are taken to apply to the charge-transfer state(s).…”

“…For example, the fluorescence quantum yield of DMABN in protic solvents is lower and the fluorescence spectrum is further red-shifted and broadened, relative to measurements in aprotic solvents of the same polarity, [8,9] and the fluorescence decay kinetics are difficult to interpret.…”

“…in MeOH appears to be largely due to the increase of about 2000 cm À1 in the width (FWHM), [9] rather than a large shift of electronic origin of the kind separating LE from ICT. It may be possible to resolve these two components experimentally by measuring the transient resonance Raman excitation profiles (REPs) of the ICT and HICT states, thus providing new opportunities for studying the differences between them.…”

“…The small level of decay back to the ground state is shown by the small recovery in the bleached signal and is due to fluorescence and internal conversion (IC) to the ground state, the lack of triplet decay providing a constant remainder on the 3 ns time scale. From fits to the bleached band areas from 20 ps to 3 ns (not shown) using the absorption decay time constant, and from the fluorescence quantum yield (0.030), [9] we estimate the rates for fluorescence (1.1 10 7 s À1 ), IC (% 6 10 7 s À1 ), and intersystem crossing (ISC) to the triplet (% 30 10 7 s À1 ). We have observed similar TRIR spectra in THF and triacetin (1,2,3-propanetriol triacetate), but with different time constants.…”