Singlet–Singlet Exciton Annihilation in an Exciton-Coupled Squaraine-Squaraine Copolymer: A Model toward Hetero-J-Aggregates

Abstract

Low-band-gap polymers with broad spectral absorption are highly sought after for application in organic photovoltaic cells and other optoelectronic devices. Thus, a conjugated copolymer based on two different indolenine squaraine dyes SQA and SQB was synthesized by Suzuki coupling, and its steady-state and time-resolved optical properties were investigated in detail. In CHCl3 the copolymer [SQA-SQB] n shows a strongly broadened and red-shifted absorption compared to that of its monomers, which was explained by exciton coupling of localized transition moments. The theoretical background of exciton coupling theory for copolymers was worked out in detail. In toluene, [SQA-SQB] n displays a spectral narrowing of the lowest excitation band which resembles the exchange narrowing effect found in cyanine J-aggregates. In this way [SQA-SQB] n behaves like a one-dimensional covalently bound hetero-J-aggregate. The photoinduced dynamics of the copolymer was investigated by transient absorption pump–probe spectroscopy with femtosecond resolution. Because of the unusually high exciton diffusion constant, singlet–singlet annihilation is the rate-limiting step for deactivation of the copolymer in solution at high laser fluencies. This is unlike the situation for many conjugated polymers in the solid state, where diffusion-limited annihilation is usually found. Thus, the [SQA-SQB] n copolymer is a unique model system which combines the excitonic features of J-aggregates with the chemical robustness of a polymer.

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

“…The absorption spectra of this polymer show the typical shape of a J-aggregate both in CHCl 3 and in toluene which can also nicely be modelled by exciton coupling theory albeit with some more elaborate mathematics than in case of homopolymers. [233] Because the transition moments of the highest-energy states do not cancel for heteropolymers there are strong absorption peaks at lowest and at highest energy of the exciton manifold ( Figure 13). However, in toluene, the lowest-energy absorption peak shows spectral exchange narrowing indicating a large coherence length over ca.…”

Exciton‐Vibrational Couplings in Homo‐ and Heterodimer Stacks of Perylene Bisimide Dyes within Cyclophanes: Studies on Absorption Properties and Theoretical Analysis

“…with DE o;r as the energy difference between the excited states of the different PBI chromophores S 1,o and S 1,r , respectively (Figure 3b). [28] Importantly,i nc ontrastt ot he homodimer,t he transition to the lowest excited state becomes partially allowed in such ah eterodimer,even for ap erfect collinearly stacked H-typed imer.T his effect may be attributedt o differentm agnitudeso ft he resp. transition dipole moments of the chromophores or simply to differences in the excited-state energieso ft he two coupled chromophores.…”

Energy Transfer Between Squaraine Polymer Sections: From Helix to Zigzag and All the Way Back

“…[72][73][74][75][76][77][78][79][80] Unlike conjugated polymers that are based on very small monomers such as styrene (e.g., MEH-PPV) or thiophene (e.g., P3HT) and whose polymer properties are totally different from those of the monomers, 81 squaraine homo-and copolymers are based on squaraine dyes which already show a strong absorption in the red region of the visible spectrum. [32][33][34][35][36]82 The optical properties of the squaraine polymer, although distinct from those of the monomeric dye, can be explained by exciton coupling of localized squaraine chromophore transition moments which in general leads to broadened and red-shifted spectra reflecting the excitonic manifold of states.…”