Cross-conjugation vs. linear conjugation in donor-bridge-acceptor nitrophenol chromophores

The influence of cross-conjugation vs. linear conjugation on the electronic communication between donor and acceptor groups in phenol(ate)–bridge–nitrobenzene chromophores was investigated by solution and gas-phase absorption spectroscopy, fluorescence spectroscopy, and quantum chemical calculations. The compounds studied include, among others, geminally and trans-substituted diethynylethenes prepared by stepwise Sonogashira cross-coupling reactions, and the cross-conjugated analogue of stilbene. A butadiyne-bridged donor–acceptor chromophore was prepared by an unsymmetrical Pd-catalysed coupling between a chloroalkyne and a terminal alkyne. While the linearly conjugated chromophores showed a strong and redshifted charge-transfer (CT) absorption maximum upon deprotonation in solution, a new redshifted absorption was either absent or present as a weak shoulder for the cross-conjugated derivatives. Calculations on the nonplanar 1,1-diaryl-substituted ethylene derivative revealed that the S0–S1 absorption has a very low oscillator strength due to insignificant coupling between the donor and acceptor ends. The energy of this CT absorption for a nonsolvated molecule is smaller than that of the linearly conjugated stilbene analogue by 0.4 eV. This result cannot directly be inferred from the solution studies. A gas-phase absorption spectrum of this chromophore was measured by action spectroscopy, which only revealed a higher-energy absorption band. Based on calculations, this band was assigned to a higher-lying π–π* transition. For the planar diethynylethene chromophores, cross-conjugation leads to a decrease of 0.3 eV in the excitation energy, according to the calculations. Finally, in contrast to the linearly conjugated chromophores, the cross-conjugated ones did not show any fluorescence, which may be due to deexcitation via the low-lying CT “dark” state.The Danish Council for Independent Research, Natural Sciences (#10-082088), the Lundbeck Foundation, and the University of Copenhagen are gratefully acknowledged for financial support. M. W. and A. R. acknowledge financial support from the European Research Council Advanced Grant DYNamo (ERC-2010-AdG –267374) Spanish Grants (FIS2011-65702-C02-01 and PIB2010US-00652), Grupo Consolidado UPV/EHU del Gobierno Vasco (IT578-13), Ikerbasque and European Commission project CRONOS (280879-2).Peer Reviewe