Near-Quantitative Triplet State Population via Ultrafast Intersystem Crossing in Perbromoperylenediimide

Perylenediimide (PDI) derivatives are essential organic semiconductor materials in a variety of photofunctional devices. By virtue of the large energy gap between the singlet and triplet excited states (ΔE_ST = 1.1 eV), augmentation of the triplet state population in monomeric PDI is a challenging task. We report the metal atom-free approach in engendering a near-quantitative triplet yield in perbromoperylenediimide/octabromoperylenediimide (OBPDI), absorbing in the visible region of the electromagnetic spectrum. Perbromination of PDI causes significant out-of-plane distortion (θ = 39°) in the aromatic core of OBPDI as compared to the planar PDI (θ = 0°). A substantial decrease (ΔE⁰_red = 0.377 V) in the reduction potential of OBPDI, E_1/2(OBPDI/OBPDI^·-) = -0.170 V, when compared to the reduction potential, E_1/2 (PDI/PDI^·-) = -0.547 V, of bare PDI makes OBPDI a promising electron acceptor. As a consequence of incorporating eight bromine atoms, the fluorescence quantum yield of a bare PDI chromophore (ϕ_f = 97 ± 1%; τ_f = 4.54 ns) decreases to a very low value in OBPDI (ϕ_f = 3 ± 1%; τ_f = 13.78 ps). Femtosecond transient absorption measurements of OBPDI reveal intersystem crossing (ISC) occurring at an ultrafast time scale (τ_ISC = 14.20 ps), leading to a near-quantitative triplet population (ϕ_T = 97 ± 1%). Theoretical investigations performed to decode the excited state dynamics in OBPDI propose that (i) cumulative addition of eight bromine atoms enhances the magnitude of spin-orbit coupling (SOC) and (ii) twist on the perylene core moderately reduces the energy gap between the singlet-triplet states. Understanding the structural alterations that control the electronic parameters in accessing the triplet excited states of organic chromophores, like PDI, can lead to the design and fabrication of efficient optoelectronic devices and energy storage materials.