Singlet Fission and Excimer Formation in Disordered Solids of Alkyl-Substituted 1,3-Diphenylisobenzofurans

Abstract

We describe the preparation and excited state dynamics of three alkyl derivatives of 1,3-diphenylisobenzofuran (1) in both solutions and thin films. The substitutions are intended to disrupt the slip-stacked packing observed in crystals of 1 while maintaining the favorable energies of singlet and triplet for singlet fission (SF). All substitutions result in films that are largely amorphous as judged by the absence of strong X-ray diffraction peaks. The films of 1 carrying a methyl in the para position of one phenyl ring undergo SF relatively efficiently (≥75% triplet yield, Φ) but more slowly than thin films of 1. When the methyl is replaced with a t-butyl, kinetic competition in the excited state favors excimer formation rather than SF (Φ = 55%). When t-Bu groups are placed in both meta positions of the phenyl substituent, SF is slowed further and Φ = 35%.

Striking the right balance of intermolecular coupling for high-efficiency singlet fission

“… 34 A large matrix element coupling the parent singlet exciton and nascent triplet pair is often considered desirable for highly efficient singlet fission, and it is expected that maximizing this matrix element correspondingly maximizes triplet pair yields. We note that although several factors could potentially complicate a direct interpretation of the results in terms of the intrinsic timescale of triplet pair formation, including the concentration of singlet fission sites in the amorphous nanoparticles and singlet diffusion to them, 20 , 32 , 61 qualitative arguments can be made such that our interpretation regarding coupling strengths relevant to the initial step of singlet fission still holds (Section S7 † ).…”

“… 66 , 68 , 89 While differences between experiment and theory remain, the studies above could be unified by considering a more fundamental view where losses can be explained by considering how molecular packing influences the extent to which the constituent triplets act independently, either through ground-state intermolecular packing geometries 9 , 66 or through those introduced via excited-state processes such as excimer relaxation. 61 , 68 , 90 …”

“…, nanoparticles and films) lack the conformational flexibility in other disordered media, such as solution-phase covalently-tethered molecular pairs, yet retain the propensity to adopt multiple packing configurations. Although various strategies have been implemented to vary molecular packing in amorphous systems, such as by embedding molecules in an inert host matrix to increase the intermolecular spacing between chromophores, 91 we (and others 61 ) posited that side chains—which are known to play a profound role in the packing 92 – 95 —could be used as an incisive handle to influence local molecular packing, couplings relevant to singlet fission, and ultimately singlet fission dynamics.…”

Structure and photophysics of indigoids for singlet fission: Cibalackrot

“…The lower yield of triplets further supports the notion of the excimer as a trap state rather than a facilitator of triplet formation, which is consistent with our prior work on engineered films of 1,3-diphenylisobenzofuran. 57 The small but nonzero triplet yield in 2a is not surprising given the likelihood of small crystalline domains not detectable by XRD that produce triplets with yields similar to the 2á or 2â forms.…”

“…The presence of excimer formation pathways has previously been implicated as deleterious in endothermic SF situations. 57,58 Excimer formation is less likely at low temperature, which is evident from the structured fluorescence and the slowing of the CT decay, from which at least a portion of the excimers arise ( Figure S7). The CT yield can be determined with the aid of the known radical cation extinction coefficients ( Figure S8) to be roughly 20% for 2a, 30% for 2á and 40% for 2â at room temperature.…”