title: Comprehensive investigation of the excited-state dynamics of push-pull triphenylamine dyes as models for photonic applications.
abstract: A series of emitting push-pull triarylamine derivatives, models of their widely used homologues in photonics and organic electronics, was investigated by steady-state and time-resolved spectroscopy. Their structural originality stems from the sole change of the electron-withdrawing substituent X (-H: 1, -CN: 2, -NO2: 3, -CHC(CN)2: 4), giving rise to efficient emission tuning from blue to red upon increasing the X electron-withdrawing character. All compounds are highly fluorescent in alkanes. The more polar compounds 2-4 undergo considerable Stokes shift and emission quenching in polar solvents. Femtosecond transient absorption data allowed us to identify the nature of the emissive state which varies as a function of the compound and surrounding polarity. A long-lived ππ* excited state with weak charge transfer character was found for 1. This excited state evolves into a long-lived ICT state with red-shifted emission for 2 in polar solvents. For 3 and 4, the ICT state is directly populated in all solvents. Long-lived and emissive in n-hexane, it relaxes in toluene to a new ICT' conformation with stronger charge transfer character and enhanced Stokes shift. In more polar THF, ethanol, and nitrile solvents, ICT relaxes to a dark excited state ICT'' with viscosity-dependent kinetics (<10 ps). The ICT'' state lifetime drops with increasing solvent polarity (150 ps for 3 in THF, 8.5 ps in butyronitrile, 1.9 ps in acetonitrile), denoting an efficient radiationless deactivation to the ground state (back charge transfer). This result reveals a very small S0-S1 energy gap at the relaxed ICT'' geometry, with a possible close-lying S0-S1 conical intersection, which suggests that the ICT → ICT'' process results from a structural change involving a large-amplitude molecular distortion. This fast structural change can account for the strong fluorescence quenching observed for 3 and 4 in polar solvents. Finally, the magnitude of intersystem crossing between the singlet and triplet excited states largely depends on the electron-deficient X unit and the solvent itself. These observations help one conclude on the prevailing role played by the electron-withdrawing groups and the surrounding polarity in the photophysical performances of triphenylamine derivatives, largely employed in numerous emissive solid-state devices.
authors.affiliation: CEISAM-UMR CNRS 6230, Université de Nantes, Nantes, France. elena.ishow@univ-nantes.fr
authors.forename: Eléna
authors.initials: E
authors.lastname: Ishow
authors.forename: Gilles
authors.initials: G
authors.lastname: Clavier
authors.forename: Fabien
authors.initials: F
authors.lastname: Miomandre
authors.forename: Mateusz
authors.initials: M
authors.lastname: Rebarz
authors.forename: Guy
authors.initials: G
authors.lastname: Buntinx
authors.forename: Olivier
authors.initials: O
authors.lastname: Poizat
mesh_terms: D000814:Aniline Compounds / Q000737:chemistry*; D004583:Electrons; D005456:Fluorescent Dyes / Q000737:chemistry; D007700:Kinetics; D008958:Models, Molecular; D017785:Photons; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics
chemical_list: D000814:Aniline Compounds; D005456:Fluorescent Dyes; D012997:Solvents