The excited-state dynamics of the radical anions of cyanoanthracenes

Abstract

The radical anion of 9,10-dicyanoanthracene (DCA) has been suggested to be a promising chromophore for photoredox chemistry, due to its nanosecond excited-state lifetime determined from indirect measurements. Here, we investigate the excited-state dynamics of the radical anion of three cyanoanthracenes, including DCA˙-, produced by photoinduced electron transfer in liquid using both pump-probe and pump-pump probe transient electronic absorption spectroscopy. All three excited radical ions are characterised by a 3-5 ps lifetime, due to efficient non-radiative deactivation to the ground state. The decay pathway most probably involves D₁/D₀ conical intersection(s), whose presence is favoured by the enhanced flexibility of the radical anions relative to their neutral counterparts. The origin of the discrepancy with the nanosecond lifetime of DCA˙-* reported previously is discussed. These very short lifetimes limit, but do not preclude, photochemical applications of the cyanoanthracene anions.

Fig. 1. Structures of the cyanoanthracenes.

Fig. 2. Stationary absorption spectra of DCA, TrCA and TCA in acetonitrile.

Fig. 3. Stationary absorption spectra of DCA˙−, TrCA˙⁻ and TCA˙⁻ in the presence of TEA + TBADHP in acetonitrile after 405 nm irradiation and subtraction of the absorption bands of the neutral precursor. The signal in the shaded area is attributed to an artefact.

Fig. 4. Transient absorption spectra measured 10 ps after 400 nm excitation of the cyanoanthracenes in acetonitrile.

Fig. 5. (A) Transient absorption spectra measured various time delays after 710 nm excitation of DCA˙⁻ generated upon continuous irradiation of a DCA/TEA/TBADHP solution. (B) Evolution-associated difference absorption spectra and time constants obtained from a global analysis of the transient absorption data assuming three successive exponential steps, A → B → C → D, D being the ground state, and negative stationary absorption spectrum (bottom).

Fig. 6. (A) PPP spectra measured with DCA and 1 M TMB in ACN at different delays, Δt₂₃, after the second pump pulse at 710 nm. The delay between the actinic pulse at 400 nm and the 710 nm pulse was 1 ns. (B) Evolution-associated difference absorption spectra and time constants obtained from a global analysis of the PPP data assuming two successive exponential steps A′ → B′ → C′, C′ being the D₀ state.

Fig. 7. (A) Transient absorption spectra measured various time delays after 710 nm excitation of TrCA˙⁻ generated upon continuous irradiation of a TrCA/TEA/TBADHP solution, and negative stationary absorption spectrum. (B) Evolution-associated difference absorption spectra and time constants obtained from a global analysis of the transient absorption data assuming four successive exponential steps, A →…→E, E being the ground state. (C) Time profiles of the transient absorption at the wavenumbers shown by the dashed lines in (A).

Fig. 8. (A) Transient absorption spectra measured various time delays after 710 nm excitation of TCA˙⁻ generated upon continuous irradiation of a TCA/TEA/TBADHP solution, and negative stationary absorption spectrum. (B) Evolution-associated difference absorption spectra and time constants obtained from a global analysis of the transient absorption data assuming four successive exponential steps, A →…→E, E being the ground state. (C) Time profiles of the transient absorption at the wavenumbers shown by the dashed lines in (A).

Fig. 9. Energy-level diagrams illustrating the most probable decay pathways of the photopopulated state of the radical anions, with the associated time constants. The grey lines designate electronic states that are dark with respect to the ground state.