Charge Delocalization and Vibronic Couplings in Quadrupolar Squaraine Dyes

• DOI: 10.1021/jacs.2c08682
• Publication Date: 2022-10-07T19:33:52Z
• Authors: Daniel Timmer; Fulu Zheng; Moritz Gittinger; Thomas Quenzel; Daniel C. Lünemann; Katrin Winte; Yu Zhang; Mohamed E. Madjet; Jennifer Zablocki; Arne Lützen; Jin-Hui Zhong; Antonietta De Sio; Thomas Frauenheim; Sergei Tretiak; Christoph Lienau
• Keywords: Vibronic coupling; Vibronic spectroscopy; Chromophore; Oscillator strength; Franck–Condon principle; Molecular electronic transition

Squaraines are prototypical quadrupolar charge-transfer chromophores that have recently attracted much attention as building blocks for solution-processed photovoltaics, fluorescent probes with large two-photon absorption cross sections, and aggregates circular dichroism. Their optical properties often rationalized in terms of phenomenological essential state models, considering the coupling two zwitterionic excited states to a neutral ground state. As result, transitions lowest S1 one-photon allowed, whereas next higher S2 can only be accessed by transitions. A further implication these models is substantial reduction vibronic ubiquitous high-frequency vinyl-stretching modes organic materials. Here, we combine time-resolved vibrational spectroscopy, two-dimensional electronic quantum-chemical simulations test rationalize predictions nonaggregated molecules. We find small Huang-Rhys factors below 0.01 high-frequency, 1500 cm-1 particular, well noticeable those lower frequency general S0 → transition. The allowed separated energetically from has weak signatures well. Thus, resulting pronounced concentration oscillator strength narrow region relevant transition makes squaraines their exceptionally interesting strong ultrastrong excitons localized light external resonators chiral largely controlled molecular architecture.