Charge transfer relaxation in donor–acceptor type conjugated materials

Abstract

The development of conjugated materials bearing electron-rich and electron-poor units along their backbone introduces new possibilities to control functionality for organic electronic applications through charge transfer character in ground and excited states. A thorough understanding of intramolecular dipoles and their evolution during excited state relaxation is necessary in order to fully exploit this opportunity. PCDTBT is an alternating donor-acceptor copolymer with high photovoltaic efficiency in bulk heterojunction solar cells. We use time-resolved femtosecond transient absorption spectroscopy in solution and in the solid state to study PCDTBT and the dTBT and CDTBT model compounds, fragments of the polymer chain. Higher solubility and slower relaxation make CDTBT particularly suitable to understand the mechanism of charge transfer relaxation in this class of materials.A progressive increase of charge transfer character from the initially moderately polar excited state is mainly driven by solvent reorganization and some torsional rearrangements. Similar relaxation in solid state CDTBT might ultimately lead to the formation of separate charges.

Sub-picosecond delocalization in the excited state of conjugated homopolymers and donor–acceptor copolymers

“…7A. 150 There is excellent agreement with the experimental spectra in non-polar decane (DEC, most comparable to gas phase). The electron density difference between ground and excited states for the S 0 / S 1 transition, which dominates the rst absorption band, is depicted in Fig.…”

“…7A), which are quite delocalized with some weak charge transfer components. 137,142,150 The contribution of ICT to the higher absorption band of the PCDTBT polymer has recently been experimentally challenged.…”

“…The synthesis of the dTBT and CDTBT monomers allowed a more direct comparison between theory and experiment. 137,150 The vertical absorption and emission transitions, calculated by TD-DFT at the M062X/6-31+G* level of theory following gas phase geometry optimization in the ground and excited state, are plotted as lines in Fig. 7A.…”

“…150 In non-polar DEC there was practically no evolution of the ground state bleach (GSB), stimulated emission (SE) and excited state absorption (ESA) features within the 1 ns window of the experiment, pointing to negligible relaxation (Fig. 7D).…”

“…153 Banerji et al used the isolated repeat unit CDTBT to overcome those difficulties. 150 Not only is CDTBT soluble in most organic solvents, but relaxation is slowed down in a reduced density of states compared to the polymer. The solvatochromic behavior that Scholes et al observed for PCDTBT was conrmed in the monomer, with a correlation of the emission maximum with solvent polarity (Fig.…”