Symmetry breaking induced by indirect charge transfer governs photophysics of TIPS-pentacene

초록

Understanding how molecular structure and the surrounding environment shape the electronic states of organic chromophores is crucial for controlling their photophysical responses. In this study, we examine the solvent-polarity-dependent excited-state behavior of TIPS-pentacene (TIPS-Pc), a formally centrosymmetric acene derivative widely employed in organic optoelectronics. Although the absorption spectra of TIPS-Pc show little dependence on solvent polarity, the emission spectra exhibit clear positive solvatochromism, broadened line shapes, and changes in vibronic intensity ratios, indicating subtle yet distinct modifications in the nature of the S1 state. To account for these effects, we invoke the concept of symmetry breaking, typically associated with quadrupolar donor-acceptor-donor systems, and apply it to TIPS-Pc. Femtosecond transient absorption measurements reveal that in apolar solvents, the excited-state absorption in the near-infrared region is dominated by the Laporte-allowed S1 -> S2 transition. In polar solvents, however, solvent-induced fluctuations perturb the molecular centrosymmetry, enabling both the S1 -> S2 and the otherwise Laporte-forbidden S1 -> S3 transitions. Quantum-chemical analysis indicates that although the TIPS substituents do not directly contribute to the pi-pi* excitation, the experimental results apparently suggest that their inductive effects indirectly enhance charge-transfer character and could promote exciton desymmetrization.

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