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| - Spectroscopy and Photophysics of 1,4-Bis(phenylethynyl)benzene: Effects of Ring Torsion and Dark πσ* State
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| - A combination of supersonic-jet laser spectroscopy and quantum chemistry calculation was applied to 1,4-bis(phenylethynyl)benzene, BPEB, to study the role of the dark πσ* state on electronic relaxation and the effect of ring torsion on electronic spectra. The result provides evidence for fluorescence break-off in supersonic jet at high S1(ππ*) ← S0 excitation energies, which can be attributed to the ππ*−πσ* intersection. The threshold energy for the fluorescence break-off is much larger in BPEB (∼4000 cm−1) than in diphenylacetylene (∼500 cm−1). The high-energy barrier in BPEB accounts for the very large fluorescence quantum yield of the compound (in solution) relative to diphenylacetylene. The comparison between the experimentally derived torsional barrier and frequency with those from the computation shows overall good agreement and demonstrates that the low-energy torsional motion involves the twisting of the end ring in BPEB. The torsional barrier is almost an order of magnitude greater in the ππ* excited state than in the ground state. The finding that the twisting of the end ring in BPEB is relatively free in the ground state, but strongly hindered in the excited state, provides rationale for the well-known temperature dependence of the spectral shape of absorption and the lack of mirror symmetry relationship between the absorption and fluorescence at elevated temperatures.
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| - Spectroscopy and Photophysics of 1,4-Bis(phenylethynyl)benzene
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