Excited-state relaxation of the solar cell dye D49 in organic solvents and on mesoporous Al₂O₃ and TiO₂ thin films

Abstract

We present an ultrafast UV-Vis-NIR transient absorption study of the donor-acceptor solar-cell dye D49 in diisopropyl ether, THF and acetonitrile, as well as on mesoporous AlO and TiO thin films. Photoexcitation at 505 nm initially populates the first electronically excited state of the dye having significant intramolecular charge transfer character ("S/ICT"). On AlO and in the three organic solvents, the dynamics are fully explained in terms of S/ICT stabilisation (by reorientation of adjacent solvent or D49 molecules and collisional cooling), intramolecular vibrational redistribution and S/ICT → S electronic decay. A substantial decrease of the S/ICT lifetime is observed with increasing polarity of the surrounding medium suggesting an acceleration of internal conversion. In agreement with these results, the addition of the nonpolar co-adsorbent deoxycholic acid (DCA) to the AlO surface leads to a substantial increase of the S/ICT lifetime. DCA spacers reduce the local polarity around the dye molecules, thus interrupting D49 "self-solvation". These results are in contrast to a recent experimental study for the indoline dye D131 on AlO, where charge transfer from electronically excited D131 to adjacent dye molecules was proposed (Cappel et al., Sci. Rep., 2016, 6, 21276). We do not see evidence for charge transfer processes between D49 molecules and also not for electron injection from D49 into AlO trap states. Charge separation is only observed for D49 bound to TiO thin films, with efficient injection of electrons into the conduction band of the semiconductor via formation of a [D49˙e] complex and a transient Stark effect signalling the formation of mobile electrons upon dissociation of the complex.

Spectroscopic Investigations Provide a Rationale for the Hydrogen-Evolving Activity of Dye-Sensitized Photocathodes Based on a Cobalt Tetraazamacrocyclic Catalyst

“…Applying a three-component exponential fit to the data, a characteristic time constant of τ1 < 1 ps is obtained for this process, in line with the ultrafast hole injection observed in literature for other organic dyes grafted on NiO. 15,25,[71][72][73][74][75][76][77][78][79] As this class of push-pull organic dyes are known to undergo a fast relaxation cascade to the relaxed ICT state on the same subps time scale (Figure S17), 40,50,70,71,[80][81][82] it seems likely that hole injection and the relaxation cascade occur simultaneously and that τ1 therefore reflects a convolution of the two. A second process, with a characteristic time constant 2 ≈ 8 ps, can be assigned to the decay of residual excited state to the ground state since the spectral change it causes is a loss of the typical features of the T2R dye in the excited state.…”

Investigating Light-Induced Processes in Covalent Dye-Catalyst Assemblies for Hydrogen Production

“…The photoinduced processes at work in two novel dye-catalyst assemblies designed for hydrogen production in dye-sensitized photocathodes have been investigated by steady-state and ultrafast spectroscopic techniques. Comprehensive studies on the excited state dynamics of push-pull organic dyes are not common in the literature [30][31][32][33][61][62][63]. Compared to metallo-organic photosensitizers such as Ru(bpy) 3 2+ , various conformers indeed coexist in solution for these structures, increasing the number of potential relaxation pathways, thus rendering the photophysical analysis much more complex.…”

“…The observed red-shift of the SE, from the initial dip at 600 nm to the negative band at >690 nm, further supports this assignment. The exact nature of these relaxation processes is not clear in this class of push-pull dyes [30][31][32][33]. Quite generally, it is ascribed to a solvation process in response to the photoinduced intramolecular shift of electron density [33], coupled to a relaxation cascade through molecular motion such as flattening and rotation around bonds [30].…”

Ultrafast Broadband Transient Absorption and Circular Dichroism Reveal Relaxation of a Chiral Copolymer

“…Ultrafast Transient Absorption Spectroscopy . Femtosecond broadband transient absorption spectra were recorded on two setups covering the UV–vis (260–700 nm) and NIR (850–1600 nm) range using single-shot referencing. , They employed the PSCP method . PFPh was excited at 370 nm.…”