Do Hydrogen Bonds Influence Excitonic Splittings?

Balmer, Franziska; Ottiger, Philipp; Leutwyler, Samuel (2016). Do Hydrogen Bonds Influence Excitonic Splittings? CHIMIA, 70(4), pp. 284-287. Schweizerische Chemische Gesellschaft 10.2533/chimia.2016.284

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The 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 S 0(1 A g) → S 1(1 A g) transition is electric-dipole forbidden, while the S 0(1 A g) → S2(1 B u) transition is allowed. The opposite holds for (mCP)2 due to the different transition dipole moment vector alignment. The S 0 → S 1 S 2 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 Δexc = 2.1 cm–1 for (BN)2 and Δexc = 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 Δcalc = 179 cm–1, while in (BN)2 the calculated splitting is Δcalc = 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.

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