Charge stabilization via electron exchange: excited charge separation in symmetric, central triphenylamine derived, dimethylaminophenyl-tetracyanobutadiene donor-acceptor conjugates

Abstract

Photoinduced charge separation in donor-acceptor conjugates plays a pivotal role in technology breakthroughs, especially in the areas of efficient conversion of solar energy into electrical energy and fuels. Extending the lifetime of the charge separated species is a necessity for their practical utilization, and this is often achieved by following the mechanism of natural photosynthesis where the process of electron/hole migration occurs distantly separating the radical ion pairs. Here, we hypothesize and demonstrate a new mechanism to stabilize the charge separated states via the process of electron exchange among the different acceptor entities in multimodular donor-acceptor conjugates. For this, star-shaped, central triphenylamine derived, dimethylamine-tetracyanobutadiene conjugates have been newly designed and characterized. Electron exchange was witnessed upon electroreduction in conjugates having multiple numbers of electron acceptors. Using ultrafast spectroscopy, the occurrence of excited state charge separation, and the effect of electron exchange in prolonging the lifetime of charge separated states in the conjugates having multiple acceptors have been successfully demonstrated. This work constitutes the first example of stabilizing charge-separated states via the process of electron exchange.

Chart 1. Structure and abbreviation of star-shaped, central triphenylamine derived, dimethylaminophenyl–tetracyanobutadiene conjugates, 1–4 and the control compounds, C1–C2 newly designed, synthesized to demonstrate charge stabilization via electron exchange in the present study.

Scheme 1. Synthetic scheme of compounds (NND)₃–TPA 1, and (NND–TCBD_1–3)₃–TPA, 2–4.

Fig. 1. (a) Absorption and (b) fluorescence spectra of indicated compounds in DCB. Compound 1 was excited at 386 nm. No measurable emission was observed for compounds 2–4 upon exciting the samples at either the locally excited or charge transfer absorption peak positions.

Fig. 2. DPVs (left panel) and CVs (right panel) of indicated compounds in DCB containing 0.1 M (TBA)ClO₄. For DPV: scan rate = 5 mV s⁻¹, pulse width = 0.25 s, and pulse height = 0.025 V. For CV: scan rate = 100 mV s⁻¹. The ‘*’ in the left panel represents the oxidation peak of ferrocene used as the internal standard. Note: the first reduction corresponding to TCBD in 3 and 4 is a split wave (see the text for details).

Fig. 3. Frontier HOMO, LUMO and LUMO+1 of the investigated compounds from the B3LYP/6-31G** optimized structures (see the ESI for the coordinates of computed structures†).

Fig. 4. Spectral changes observed during the (a) first oxidation and (b) first reduction of 3 in DCB containing 0.2 M (TBA)ClO₄. (c) Spectrum deduced for the charge separation state using spectroelectrochemical data (see the text for details, and Fig. S20 in the ESI for complete results). (d) Energy level diagram showing possible charge transfer and charge separation events upon photoexcitation of the compounds 2–4. NND without linked TCBD in 2–3 is not shown in the abbreviated formula for simplicity.

Fig. 5. fs-TA spectra at the indicated delay times, (a–d, panel i), species associated spectra (a–d, panel ii), and population kinetics (a–d, panel iii) of compounds 1–4 (a through d) in benzonitrile. The samples were excited at 350 nm corresponding to the locally excited state.

Fig. 6. fs-TA spectra at the indicated delay times of compounds 2–4 in benzonitrile. The samples were excited at 500 nm corresponding to the charge transfer band. Right hand panel shows the population kinetics. The dip at 500 nm is due to the excitation laser.