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Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States.

ABSTRACT: The design and regulation of multiple room-temperature phosphorescence (RTP) processes are formidably challenging due to the restrictions imposed by Kasha's rule. Here, we report a general design principle for materials that show multiple RTP processes, which is informed by our study of four compounds where there is modulation of the linker hybridization between donor (D) and acceptor (A) groups. Theoretical modeling and photophysical experiments demonstrate that multiple RTP processes can be achieved in sp³ C-linked D-A compounds due to the arrest of intramolecular electronic communication between two triplet states (T₁^H and T₁^L) localized on the donor and acceptor or between two triplet states, one localized on the donor and one delocalized across aggregated acceptors. However, for the sp² C-linked D-A counterparts, RTP from one locally excited T₁ state is observed because of enhanced excitonic coupling between the two triplet states of molecular subunits. Single-crystal and reduced density gradient analyses reveal the influence of molecular packing on the coincident phosphorescence processes and the origin of the observed aggregate phosphorescence. These findings provide insights into higher-lying triplet excited-state dynamics and into a fundamental design principle for designing compounds that show multiple RTP.

SUBMITTER: Wang T

PROVIDER: S-EPMC9880999 | biostudies-literature | 2023 Jan

REPOSITORIES: biostudies-literature

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Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States.

Wang Tao T Gupta Abhishek Kumar AK Wu Sen S Slawin Alexandra M Z AMZ Zysman-Colman Eli E

Journal of the American Chemical Society 20230113 3

The design and regulation of multiple room-temperature phosphorescence (RTP) processes are formidably challenging due to the restrictions imposed by Kasha's rule. Here, we report a general design principle for materials that show multiple RTP processes, which is informed by our study of four compounds where there is modulation of the linker hybridization between donor (D) and acceptor (A) groups. Theoretical modeling and photophysical experiments demonstrate that multiple RTP processes can be ac ...[more]

PMID: 36638828

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Project description:Organocatalyzed ATRP (O-ATRP) is a growing field exploiting organic chromophores as photoredox catalysts (PCs) that engage in dissociative electron-transfer (DET) activation of alkyl-halide initiators following absorption of light. Characterizing DET rate coefficients (kact) and photochemical yields across various reaction conditions and PC photophysical properties will inform catalyst design and efficient use during polymerization. The studies described herein consider a class of phenoxazine PCs, where synthetic handles of core substitution and N-aryl substitution enable tunability of the electronic and spin characters of the catalyst excited state as well as DET reaction driving force (ΔGET0). Using Stern-Volmer quenching experiments through variation of the diethyl 2-bromo-2-methylmalonate (DBMM) initiator concentration, collisional quenching is observed. Eight independent measurements of kact are reported as a function of ΔGET0 for four PCs: four triplet reactants and four singlets with kact values ranging from 1.1 × 108 M-1 s-1, where DET itself controls the rate, to 4.8 × 109 M-1 s-1, where diffusion is rate-limiting. This overall data set, as well as a second one inclusive of five literature values from related systems, is readily modeled with only a single parameter of reorganization energy under the frameworks of the adiabatic Marcus electron-transfer theory and Marcus-Savéant theory of DET. The results provide a predictive map where kact can be estimated if ΔGET0 is known and highlight that DET in these systems appears insensitive to PC reactant electronic and spin properties outside of their impact on the driving force. Next, on the basis of measured kact values in selected PC systems and knowledge of their photophysics, we also consider activation yields specific to the reactant spin states as the DBMM initiator concentration is varied. In N-naphthyl-containing PCs characterized by near-unity intersystem crossing, the T1 is certainly an important driver for efficient DET. However, at DBMM concentrations common to polymer synthesis, the S1 is also active and drives 33% of DET reaction events. Even in systems with low yields of ISC, such as in N-phenyl-containing PCs, reaction yields can be driven to useful values by exploiting the S1 under high DBMM concentration conditions. Finally, we have quantified photochemical reaction quantum yields, which take into account potential product loss processes after electron-transfer quenching events. Both S1 and T1 reactant states produce the PC•+ radical cation with a common yield of 71%, thus offering no evidence for spin selectivity in deleterious back electron transfer. The subunity PC•+ yields suggest that some combination of solvent (DMAc) oxidation and energy-wasting back electron transfer is likely at play and these pathways should be factored in subsequent mechanistic considerations.

Dataset Information

Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States.

Publications

Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States.

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