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 S-0-S-1 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 -* 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.