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

Abstract

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 (ΔE 0 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.

Organic room-temperature phosphorescence from halogen-bonded organic frameworks: hidden electronic effects in rigidified chromophores

“…62 Broadly speaking, the more bromo atoms in a molecule, the faster the S 1 → T n and T 1 → S 0 processes, in line with the stronger heavy-atom enhanced SOC. 63 Unexpectedly, however, the S 1 → T n ISC for Br 4 NTE is noticeably slower than its less brominated analogues.…”

Intersystem Crossing and Triplet-State Property of Anthryl- and Carbazole-[1,12]fused Perylenebisimide Derivatives with a Twisted π-Conjugation Framework

“…Thus, for PBI-1 , the ISC rate or the triplet rise time is 8.2 ns, nearly identical to the 1 ICT* decay time (τ F = 8.1 ns). The brominated twisted PBI derivative and doubly phenanthrene-fused PBI derivatives were previously reported to have ultrafast ISC (0.38 or 0.25 ps). , In contrast, singly phenanthrene-fused PBI (τ ISC = 1 ns) and triply linked DIPBIs derivatives (τ ISC = 970 ps) showed ISC at a relatively slower timescale. , Thus, the ISC rate significantly varied (from few picoseconds to nanoseconds) in the different twisted derivatives having different extents of the π-conjugated framework.…”

Symmetry-Breaking Charge Separation in Phenylene-Bridged Perylenediimide Dimers

“…The relatively slow time constants for the appearance of 3 *PDI are sufficient to discount singlet exciton fission as a triplet formation mechanism. Previous studies of rylene dye derivatives have shown intersystem crossing may results from the radical pair (RP-ISC) or spin–orbit charge-transfer (SOCT-ISC) mechanisms. − However, in p- PDI 2 and m- PDI 2 , there is no direct evidence for ion pair state formation, so that RP-ISC and SOCT-ISC are not operative here. The PDI chromophores in p- PDI 2 and m- PDI 2 have far more conformational freedom about the single bonds joining them to their phenylene spacers than does o- PDI 2 .…”