Tuning the Rainbow: Systematic Modulation of Donor–Acceptor Systems through Donor Substituents and Solvent

A series of donor–acceptor compounds is reported in which the energy of the triarylamine donor is systematically tuned through para substitution with electron-donating methoxy and electron-withdrawing cyano groups. The acceptor units investigated are benzothiadiazole (btd), dipyridophenazine (dppz), and its [ReCl­(CO)3(dppz)] complex. The effect of modulating donor energy on the electronic and photophysical properties is investigated using 1H NMR spectroscopy, DFT calculations, electrochemistry, electronic absorption and emission spectroscopies, ground state and resonance Raman spectroscopy, and transient absorption spectroscopy. Qualitative correlations between the donor energy and the properties of interest are obtained using Hammett σ+ constants. Methoxy and cyano groups are shown to destabilize and stabilize, respectively, the frontier molecular orbitals, with the HOMO affected more significantly than the LUMO, narrowing the HOMO–LUMO band gap as the substituent becomes more electron-donatingobservable as a bathochromic shift in low-energy charge-transfer absorption bands. Charge-transfer emission bands are also dependent on the electron-donating/withdrawing nature of the substituent, and in combination with the highly solvatochromic nature of charge-transfer states, emission can be tuned to span the entire visible region.