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 (ΔEST = 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 (ΔE0red = 0.377 V) in the reduction potential of OBPDI, E1/2(OBPDI/OBPDI·-) = -0.170 V, when compared to the reduction potential, E1/2 (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 (ϕf = 97 ± 1%; τf = 4.54 ns) decreases to a very low value in OBPDI (ϕf = 3 ± 1%; τf = 13.78 ps). Femtosecond transient absorption measurements of OBPDI reveal intersystem crossing (ISC) occurring at an ultrafast time scale (τISC = 14.20 ps), leading to a near-quantitative triplet population (ϕT = 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.

Metastable Chiral Azobenzenes Stabilized in a Double Racemate.

The self-assembly of chiral organic chromophores is gaining huge significance due to the abundance of supramolecular chirality found in natural systems. We report an interdigitated molecular assembly involving axially chiral octabrominated perylenediimide (OBPDI) which transfers chiral information to achiral aromatic moieties. The crystalline two-component assemblies of OBPDI and electron-rich aromatic units were facilitated through π-hole⋅⋅⋅π donor-acceptor interactions, and the charge-transfer characteristics in the ground and excited states of the OBPDI cocrystals were established through spectroscopic and theoretical techniques. The OBPDI cocrystals entail a remarkable homochiral segregation of P and M enantiomers of both molecular entities in the same crystal system, leading to twisted double-racemic arrangements. Synergistically engendered cavities with the stored chiral information of the twisted OBPDI stabilize higher-energy P/M enantiomers of trans-azobenzene through non-covalent interactions.