Excitation-dependent electron exchange energy and electron transfer dynamics in a series of covalently tethered N,N-bis(4′-tert-butylbiphenyl-4-yl)aniline – [C60] fullerene dyads via varying π-conjugated spacers

Abstract

Femtosecond time-resolved fluorescence and transient absorption studies are reported for three newly synthesized covalently linked N,N-bis(4′-tert-butylbiphenyl-4-yl)aniline (BBA) and pyrrolidinofullerenes (C₆₀)-based donor–π conjugated bridge–acceptor dyads (D–B–A) as functions of the bridge length (7.1, 9.5 and 11.2 Å for Dyad-1, Dyad-2 and Dyad-3), dielectric constants of the medium and pump wavelengths. In polar solvent, ultrafast fluorescence quenching (k_EET ≥ 2 × 10¹² s⁻¹) of the BBA moiety upon excitation of the BBA moiety (320 nm) is observed in the dyads and is assigned to a mechanism involving electron exchange energy transfer (EET) from ¹BBA* to C₆₀ followed by electron transfer from BBA to ¹C₆₀*. Cohesive rise and decay dynamics of conjugated BBA˙⁺–C₆₀˙⁻ anion pairs confirm the involvement of a distance independent adiabatic charge-separation (CS) process (k_CS ≥ 2.2 × 10¹¹ s⁻¹) with near unity quantum efficiency (φ_CS ≥ 99.7%) and a distance-dependent non-adiabatic charge-recombination (CR) process [k_CR ∼ (10¹⁰–10⁸) s⁻¹]. In contrast, excitation of the C₆₀ moiety (λ_ex = 430 to 700 nm) illustrates photoinduced electron transfer from BBA to ¹C₆₀*, involving non-adiabatic (diabatic) and distance-dependent CS (k_CS in the range of 0.59–1.78 × 10¹¹ s⁻¹) with 98.86–99.6% (Dyad-3–Dyad-1) quantum efficiency and a CR process with k_CR values [k_CR ∼ (10¹⁰–10⁸) s⁻¹] up to three orders greater than k_CS of the respective dyads. Both the processes, CS and CR, upon C₆₀ excitation and the CR process upon BBA excitation show distance dependent rate constants with exponential factor β ≤ 0.5 Å⁻¹, and electron transfer is concluded to occur through a covalently linked conjugated π bridge. Global and target analysis of fsTA data reveal the occurrence of two closely lying CS states, thermally hot (CS_hot) and thermally relaxed (CS_eq) states, and two CR processes with two orders of different rate constants. Careful analysis of the kinetic and thermodynamic data allowed us to estimate the total reorganization energy and electronic coupling matrix (V), which decrease exponentially with distance. These novel features of the distance independent adiabatic CS process and the distance-dependent diabatic CR process upon donor excitation are due to extending the π-conjugation between BBA and C₆₀. The demonstrated results may provide a benchmark in the design of light-harvesting molecular devices where ultrafast CS processes and long-lived CS states are essential requirements.