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Transitioning from Intraligand π,π* to Charge-Transfer Excited States Using Thiophene-Based Donor-Acceptor Systems.

A series of electron donor-acceptor compounds are reported in which both the donor and acceptor strengths are systematically altered using mono-, bi-, and terthiophene as donors and benzo[c][1,2,5]thiadiazole (btd), dipyrido[3,2-a:2',3'-c]phenazine (dppz), and the corresponding rhenium(I) complex, [ReCl(CO)3(dppz)], as acceptors. The electronic properties of the compounds are characterized using electrochemistry, electronic absorbance and emission spectroscopies, and transient absorption spectroscopy. The effect of donor and acceptor strengths on frontier molecular orbital localization and on the charge-transfer (CT) character of optical transitions is modeled using density functional theory (DFT) calculations. The electronic absorption spectra of the compounds investigated are dominated by intraligand charge-transfer (ILCT) transitions, where the CT character is shown to increase across the series from mono- to bi- to terthiophene but not significantly across the acceptor series. Emission is shown to originate from the absorbing state. Long-lived nonemissive states have been observed using transient absorption spectroscopy and assigned using triplet-state DFT calculations, which indicate that the lowest energy excited state has more thiophene-localized π,π* character with an increasing number of appended thiophenes.