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.