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. A selection of phenol(ate)–bridge–nitrobenzene chromophores was prepared and investigated for their optical properties. Cross‐conjugated bridges were found to strongly reduce the electronic coupling between the donor and acceptor ends, complicating experimental measurements of the charge‐transfer absorptions. Calculations clearly showed that the inefficient conjugation reduces the HOMO–LUMO gap.