Intramolecular Charge‐Transfer Dynamics in Benzodifuran‐Based Triads

Keller, Stephan Sylvest; Hankache, Jihane; Yushchenko, O.; Daku, Latévi Max Lawson; Sun, Qinchao; Ding, Jie; Decurtins, Silvio; Vauthey, Eric; Häner, Robert; Hauser, Andreas; Liu, Shixia

doi:10.1002/hlca.202100099

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“…Organic chromophores bearing electron donating (D) and electron accepting (A) groups linked through a π-conjugated bridge have attracted great attention from both a scientific and a technological point of view. The Intramolecular Charge Transfer (ICT) in these molecules is the most crucial and fundamental process, playing a critical role and determining their optical and electronic properties. − The dynamics and strength of ICT upon excitation together with the potential coexistence with a Locally Excited (LE) state define the use of such molecules in optoelectronics, lasers, nonlinear optics (NLO), energy storage, and sensor applications. − …”

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confidence: 99%

Exploring Solvent and Substituent Effects on the Excited State Dynamics and Symmetry Breaking of Quadrupolar Triarylamine End-Capped Benzothiazole Chromophores by Femtosecond Spectroscopy

et al. 2022

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We investigate herein the excited state dynamics and symmetry breaking processes in three benzothiazole-derived two-photon absorbing chromophores by femtosecond fluorescence and transient absorption (fs-TA) spectroscopies in solvents of various polarity. The chromophores feature a quasi-quadrupolar D-π-A-π-D architecture comprised of an electron-withdrawing benzothiazole core and lateral triphenylamine donors (Qbtz-H), while the acceptor strength of the central unit is enforced by attached cyano groups (Qbtz-CN) and the electron-donating strength of the arylamine moieties by introduction of peripheral methoxy groups (Qbtz′-CN). Steady state spectroscopy reveals positive solvatochromism, which is mostly pronounced for Qbtz′-CN. Femtosecond spectroscopy of Qbtz-H reveals the coexistence of the Franck–Condon (FC) state and states populated after symmetry breaking (SB) in low-polarity solvents such as toluene and tetrahydrofuran, while the SB state becomes favorable in polar acetonitrile. For the other two molecules possessing a stronger electron-accepting unit and thus more polar excited state, SB takes place even in low-polarity solvents, as shown by fs-TA spectroscopy. Global fitting of the fs-TA spectra together with investigation of the evolution associated spectra (EAS) reveals the existence of an initial FC state in Qbtz-H, in all studied solvents, which relaxes toward Intermediate Charge Transfer (I-CT) and SB states. On the other hand, for Qbtz-CN and Qbtz′-CN in more polar solvents, the FC state undergoes ultrafast relaxation toward symmetry-broken charge transfer (SB-CT) states which in turn show very fast recombination to the ground state. Our measurements confirm that the extent of symmetry breaking is larger for D-π-A-π-D systems with the stronger acceptor core and increases further by increasing electron-donating strength of triarylamine moieties, giving rise to symmetry breaking in these nonionic quadrupolar molecules with ethynylene (triple bond) π-spacers also in less polar solvents.

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