Do Hydrogen Bonds Influence Excitonic Splittings?
Keywords:Benzonitrile, Meta-cyanophenol, Excitonic splitting, Hydrogen bonds, Vibronic coupling
AbstractThe excitonic splitting and vibronic quenching of the inversion-symmetric homodimers of benzonitrile, (BN)2, and meta-cyanophenol, (mCP)2, are investigated by two-color resonant two-photon ionization spectroscopy. These systems have very different hydrogen bond strengths: the OH···N?C bonds in (mCP)2 are ?10 times stronger than the CH···N?C hydrogen bonds in (BN)2. In (BN)2 the S0(1Ag) ? S1(1Ag) transition is electric-dipole forbidden, while the S0(1Ag) ? S2(1Bu) transition is allowed. The opposite holds for (mCP)2 due to the different transition dipole moment vector alignment. The S0 ? S1S2 spectra of the dimers are compared and their excitonic splittings and vibronic quenchings are investigated by measuring the 13C-substituted heterodimer isotopomers, for which the centrosymmetry is broken and both transitions are allowed. The excitonic splittings are determined as ?exc = 2.1 cm–1 for (BN)2 and ?exc = 7.3 cm–1 for (mCP)2. The latter exhibits a much stronger vibronic quenching, as the purely electronic splitting resulting from ab initio calculations is determined to be ?calc = 179 cm–1, while in (BN)2 the calculated splitting is ?calc = 10 cm–1. The monomer site-shifts upon dimerization and comparing certain vibrations that deform the hydrogen bonds confirm that the OH···N?C hydrogen bond is much stronger than the CH···N?C bond. We show that the H-bonds have large effects on the spectral shifts, but little or no influence on the excitonic splitting.
How to Cite
F. A. Balmer, P. Ottiger, S. Leutwyler, Chimia 2016, 70, 284, DOI: 10.2533/chimia.2016.284.
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