Femto to millisecond observations of indole-based squaraine molecules photodynamics in solution

Abstract

We present femto-to-millisecond studies of the photodynamics of seven types of indole-based squaraine molecules (SQs) in solvents of different H-bonding ability and viscosity. These SQs can be classified into two families: SQs with two carboxylic groups in the side indole groups (symmetrical SQs) and with only one carboxylic group (asymmetrical SQs). Steady-state absorption and fluorescence techniques show narrow absorption and emission bands, with a small Stokes shift (about 300 cm⁻¹). The femtosecond transient absorption spectra give a very short (∼100 fs) dynamics (assigned to IVR) and the associated spectra show two excited species assigned to two stereoisomers. A trans–cisphotoisomerization occurs in a very fast time through a conical intersection. Pico-to-nanosecond emission experiments also reveal the presence of two fluorescing trans stereoisomers whose lifetimes show similar sensitivities to the nature of solvent. For example, lifetimes of 1.72, 0.46 and 0.29 ns were determined for the trans photoisomer of the SQ 41 in triacetin, dichloromethane and acetonitrile, respectively, reflecting the short decay of the S₁ state in highly polar and low viscous solvents. Flash photolysis experiments gave the transient absorption signals of the cis photoisomer that is formed after the twisting process at S₁. The cis-to-transphotoisomerization at the ground state happens in the μs time scale (1–4 μs), and it depends on the H-bonding ability and viscosity of the solvent. Thus, combining fs–ns and ns–μs experiments suggests that in the conical intersection region, only a small fraction of the twisted trans isomers are converted to the cis ones in the excited states. These results bring detailed and global insight into the large time window photodynamics of this family of SQs in solution.