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

Abstract

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.

Impact of Internal Charge Transfer on the Photophysical Properties of Pyridine‐Fused Phosphorus‐Nitrogen Heterocycles

“…As would be expected, the intensity of the ICT band relative to the π‐π* band is proportional to the electron donating capabilities of the aryl substituent (Figure 3). [27] A summary of the absorption data between both the higher and lower energy bands and the emissions are given in Table 1.…”

Dissecting Conjugation and Electronic Effects in the Linear and Non-Linear Optical Properties of Rhenium(I) Carbonyl Complexes

“…32 Gordon and co-workers have recently reported on a series of tricarbonyl Re(I) complexes with thiophenebased donor-acceptor systems with varying degrees of ILCT and localised π, π* characters. 33 They have also reported on a series of Pt(II) bis(acetylide) and Re(I) tricarbonyl complexes with triphenylamine-substituted phenanthroline ligands, in which the MLCT and ILCT characters were switched as a function of the substituents on the triphenylamine group. 34 .…”

Dissecting conjugation and electronic effects on the linear and non-linear optical properties of rhenium(i) carbonyl complexes

“…Gordon and co-workers have recently reported on a series of tricarbonyl Re( i ) complexes with thiophene-based donor–acceptor systems with varying degrees of ILCT and localised π,π* characters. 38 They have also reported on a series of Pt( ii ) bis(acetylide) and Re( i ) tricarbonyl complexes with triphenylamine-substituted phenanthroline ligands, in which the MLCT and ILCT characters were switched as a function of substituents on the triphenylamine group. 39 Further reports from the Gordon group on [Ru(N∩N) 3 ] 2+ complexes with N∩N ligands derived from substituted 1,10-phenanthroline showed that appending aromatic groups to the ligand changes the character from MLCT to a ligand-centred triplet ( 3 LC).…”

“…[34][35][36][37] Gordon and co-workers have recently reported on a series of tricarbonyl Re(I) complexes with thiophenebased donor-acceptor systems with varying degrees of ILCT and localised π, π* characters. 38 They have also reported on a series of Pt(II) bis(acetylide) and Re(I) tricarbonyl complexes with triphenylamine-substituted phenanthroline ligands, in which the MLCT and ILCT characters were switched as a function of the substituents on the triphenylamine group. 39 .…”