Observation of Two Triplet-Pair Intermediates in Singlet Exciton Fission

Abstract

Singlet fission is an excitation multiplication process in molecular systems that can circumvent energy losses and significantly boost solar cell efficiencies; however, the nature of a critical intermediate that enables singlet fission and details of its evolution into multiple product excitations remain obscure. We resolve the initial sequence of events comprising the fission of a singlet exciton in solids of pentacene derivatives using femtosecond transient absorption spectroscopy. We propose a three-step model of singlet fission that includes two triplet-pair intermediates and show how transient spectroscopy can distinguish initially interacting triplet pairs from those that are spatially separated and noninteracting. We find that the interconversion of these two triplet-pair intermediates is limited by the rate of triplet transfer. These results clearly highlight the classical kinetic model of singlet fission and expose subtle details that promise to aid in resolving problems associated with triplet extraction.

Unified model for singlet fission within a non-conjugated covalent pentacene dimer

“…If the rate at which the two triplets within 1 (T 1 T 1 ) diffuse apart in the material is faster than singlet-quintet spin-mixing, a process that usually takes place in nanoseconds, then the direct formation of (T 1 +T 1 ) from 1 (T 1 T 1 ) may occur. However, if the triplet excitons separate on a timescale comparable to spin-mixing, the exponential decrease in J SQ as the distance between the two spin-correlated triplet states increases will result in singlet-quintet mixing4041. In contrast, if two pentacenes are covalently linked, J SQ is controlled by the electronic structure of the linkage between them.…”

Triplet transport in thin films: fundamentals and applications

“…Current investigations into the intermediates in singlet fission, e.g. bound triplet pair excimers 99 or strongly interacting triplet pairs, 136 etc., enabled by various monomer or dimer configurations 97,[137][138][139][140][141][142] will shed light on the dielectric environment needed to stabilize triplet excitons. Such interdisciplinary approaches will reveal the fundamental structure-property relationships affecting triplet diffusion in thin films, and ultimately enable the engineering of optoelectronic devices making use of multi-excitonic processes.…”

Distinct properties of the triplet pair state from singlet fission

“…The most widely used technique to probe singlet fission has been transient absorption (TA) spectroscopy, but most studies to date have only identified spectral features assigned to S 1 and T 1 states, and there has been little explanation as to why these TA peaks nearly always overlap ( 12 – 16 , 35 , 36 ). Exceptions to this prevalent practice can be found in the recent work of Sanders et al ( 11 ) who found, in pentacene dimers, an excited-state absorption (ESA) peak at ~690 nm whose magnitude is strongly correlated with the strength of intertriplet electronic coupling and in the work of Pensack et al ( 37 ) who observed near-infrared (IR) (1200 to 1400 nm) ESA in pentacene aggregates assigned to 1 (TT) but not to the triplet pair labeled 1 (T…T), which has lost electronic coherence but retained spin coherence. These two examples reveal the presence of spectroscopic signatures for the 1 (TT) state in TA, but the origins of these transitions and their relationships to the energetics of the 1 (TT) state remain unknown.…”