Identification of effective exciton–exciton annihilation in squaraine–squaraine copolymers

Abstract

Ultrafast time-resolved transient absorption spectroscopy is able to monitor the fate of the excited state population in molecular aggregates or polymers. Due to many competing decay processes, the identification of exciton-exciton annihilation (EEA) is difficult. Here, we use a microscopic model to describe exciton annihilation processes in squaraine-squaraine copolymers. Transient absorption time traces measured at different laser powers exhibit an unusual time-dependence. The analysis points towards dynamics taking place on three time-scales. Immediately after laser-excitation a localization of excitons takes place within the femtosecond time-regime. This is followed by exciton-exciton annihilation which is responsible for a fast decay of the exciton population. At later times, excitations being localized on units which are not directly connected remain so that diffusion dominates the dynamics and leads to a slower decay. We thus provide evidence for EEA tracked by time-resolved spectroscopy which has not been reported that clearly before.

From wavelike to sub-diffusive motion: exciton dynamics and interaction in squaraine copolymers of varying length

“…By this we obtain a diffusion length of about 44 dimeric units, which corresponds to roughly 130 nm. This is in line with the previously reported diffusion length in these polymers, 27 signifying their excellent transport properties, as this value is much higher than many comparable materials. 42 Reporting a value for the diffusion length facilitates comparison with other materials known from the literature.…”

“…All the parameters used for the calculations (Table S1 in the ESI †) are well in the range used in previous work on squaraine dimers and copolymers. 27,51,60 Results and discussion…”

“…When applied carefully, studying the excitation power dependence of transient absorption can reveal valuable information about EEI. [25][26][27][28][29][30] The EEI signal is, however, present merely as a power-dependent perturbation of the single-exciton time-dependent spectra. It is here that one of the key advantages of higher-order nonlinear spectroscopy comes into play.…”

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…1-3 ps −1 ) while exciton diffusion is extremely fast (10-100 ps −1 ). [234] The latter rate constants are about one order of magnitude large than in, e.g., MEH-PPV. The physical origin for this behaviour is the very high density of interacting initial and final states which is visible in the small Stokes shift of strongly allowed absorption and emission bands of squaraine dyes.…”

Mapping of exciton–exciton annihilation in a molecular dimer via fifth-order femtosecond two-dimensional spectroscopy

“…10,43,44 This applies to exciton-exciton annihilation processes as well. 10,[45][46][47][48][49] What is not described in detail is the internal conversion and relaxation which proceed via vibrational states. 50 Also, the appearance of the 2D spectra will change when including the vibrational motion.…”