Ultrafast photoinduced relaxation dynamics of the indoline dye D149 in organic solvents

Abstract

The relaxation dynamics of the indoline dye D149, a well-known sensitizer for photoelectrochemical solar cells, have been extensively characterized in various organic solvents by combining results from ultrafast pump-supercontinuum probe (PSCP) spectroscopy, transient UV-pump VIS-probe spectroscopy, time-correlated single-photon counting (TCSPC) measurements as well as steady-state absorption and fluorescence. In the steady-state spectra, the position of the absorption maximum shows only a weak solvent dependence, whereas the fluorescence Stokes shift Dñ F correlates with solvent polarity. Photoexcitation at around 480 nm provides access to the S 1 state of D149 which exhibits solvation dynamics on characteristic timescales, as monitored by a red-shift of the stimulated emission and spectral development of the excited-state absorption in the transient PSCP spectra. In all cases, the spectral dynamics can be modeled by a global kinetic analysis using a time-dependent S 1 spectrum. The lifetime t 1 of the S 1 state roughly correlates with polarity [acetonitrile (280 ps) o acetone (540 ps) o THF (720 ps) o chloroform (800 ps)], yet in alcohols it is much shorter [methanol (99 ps) o ethanol (178 ps) o acetonitrile (280 ps)], suggesting an appreciable influence of hydrogen bonding on the dynamics. A minor component with a characteristic time constant in the range 19-30 ps, readily observed in the PSCP spectra of D149 in acetonitrile and THF, is likely due to removal of vibrational excess energy from the S 1 state by collisions with solvent molecules. Additional weak fluorescence in the range 390-500 nm is observed upon excitation in the S 0 -S 2 band, which contains short-lived S 2 -S 0 emission of D149. Transient absorption signals after excitation at 377.5 nm yield an additional time constant in the subpicosecond range, representing the lifetime of the S 2 state. S 2 excitation also produces photoproducts.

Ultrafast dynamics of the indoline dye D149 on electrodeposited ZnO and sintered ZrO2 and TiO2 thin films

“…The peak position of the S 0 -S 1 band in organic solvents shows only a weak variation (530 nm in acetonitrile and 543 nm in dichloromethane, both solvents containing tetrabutylammonium perchlorate at a concentration of 0.1 M for the spectroelectrochemical measurements reported in Section 3.4), in agreement with our previous study. 34 For all thin films, the spectra have a blue-shifted maximum (518 nm in the case of ZnO and ZrO 2 , 524 nm for TiO 2 ) and exhibit a significant broadening which is probably due to heterogeneous binding environments and a certain fraction of D149 aggregates. In all cases, the background absorption of the unsensitized thin films was subtracted, which features a slow rise towards the UV (due to scattering).…”

“…[34][35][36] An overview of the principle of PSCP spectroscopy can be found elsewhere. 37 The sample was excited using a NOPA with a center wavelength of 510 or 630 nm and probed by a supercontinuum (370-770 nm) generated in a 1 mm thick CaF 2 plate.…”

“…The shape resembles transient spectra of the S 1 state of D149 in organic solvents at early times measured in our previous PSCP study. 34 Yet, the appearance of characteristic spectral features in the range 600-750 nm already signals the very fast formation of an additional species, which has an absorption resembling the doublet ground state D 0 of the D149 + radical cation produced after electron injection. 28,45 Therefore, it is reasonable to assume that a substantial part of electron injection already occurs within the 70 fs time resolution of the current experiments.…”

Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile

“…23,[25][26][27] For the experiments in acetonitrile, a flow cell (1 mm path length) with a 200 mm thick quartz entrance window was used. Thin films were mounted on an x/y translation stage and moved in a plane perpendicular to the probe light path to expose a fresh spot for every laser shot.…”

“…the case of the indoline dye D149). 25 Moreover, for such a large molecule, isomerization should be very slow, because the excess energy is distributed over many vibrational modes, so it will take considerable time until enough energy ends up in the ''critical mode'' driving the isomerization. Finally, very recent STM experiments suggest that the yield of photoinduced isomerization of D35 is very low (o0.3%), at least on a TiO 2 surface.…”