Abstract
We report the excited-state intramolecular charge transfer (ICT) characteristics of four tetrahydro[5] helicene-based imide (THHBI) derivatives with various electron-donating substitutes in different polarity of solvents using steady-state, time-resolved transient absorption (TA) spectroscopy. It is found that, the small bathochromic-shift of the absorption spectra but large red shift of the emission spectra for all dyes with increasing solvent polarity indicates the larger dipole moment of the excited state compared to ground state. The results of theoretical calculations exhibit the charge transfer from the terminal donors to helical backbone, which accounts for the degrees of red shift of the emission spectra from different extent of ICT nature. Time-resolved TA spectra recorded as a function of electron-donating substitutes and solvent polarity show the dye with stronger donors (THHBI-PhNPh2) in more polar solvent behaves faster excited-state ICT relaxation, leading to the formation of solvent-stabilized ICT state (ICT’ state) from the excited ICT state; The dyes (THHBI-Ph, THHBI-PhCF3 and THHBI-PhOMe) with relative weaker donors show weaker dependence on solvent polarity, and instead of that intersystem crossing (ISC) becomes possible from ICT state to triplet state.
Highlights
Organic molecular materials with electronic push-pull properties have attracted considerable attention because their photophysical properties based on intramolecular charge transfer (ICT) could be widely tuned through appropriate modification of the chemical structures of donor, acceptor substitutes and/or the π -conjugated linkers for practical applications[1,2,3,4,5,6]
The aim of this work is to explore whether ICT properties are improved by increasing the electron-donating abilities of donor based on helicene, for which, we performed a comparative study of the dyes containing the nonplanar tetrahydro[5] helicene-based imide (THHBI) as the central acceptor with different donors
Upon excitation, the size and direction of the dipole moment of the chromophore with ICT characteristic are changed from ground state to excited state
Summary
Organic molecular materials with electronic push-pull properties have attracted considerable attention because their photophysical properties based on intramolecular charge transfer (ICT) could be widely tuned through appropriate modification of the chemical structures of donor, acceptor substitutes and/or the π -conjugated linkers for practical applications[1,2,3,4,5,6]. The photophysical properties, such as fluorescence quantum yield, fluorescence lifetime, especially the band gap of these organic molecules are strongly dependent on the ICT characteristic from different push-pull abilities between donor and acceptor[7,8,9,10]. The studies presented here provide a fundamental understanding of the structure-property relationships and solvent-dependent photophysical performances and a guide for designing new compounds with favorable CT properties for applications as materials in electroluminescence devices, solar cells and thin film transistors[3,4,23]
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