Long-Range Exciton Diffusion via Singlet Revival Mechanism

Triplet and singlet exciton diffusion in disordered rubrene films: implications for photon upconversion

Triplet and singlet exciton diffusion plays a decisive role in triplet-triplet annihilation (TTA) and singlet fission (SF) processes of rubrene (Rub) films at low excitation power, and therefore has an important implication for TTA-mediated photon upconversion (UC). Although triplet diffusion in crystalline Rub was studied before, there is no quantitative data on diffusion in disordered Rub films most widely employed for NIR-to-Vis UC. The lack of these data hinders the progress of TTA-UC applications relying on a Rub annihilator (emitter). Herein, a time-resolved PL bulk-quenching technique was employed to estimate the exciton diffusion coefficient (D) and diffusion length (L_D) in the neat Rub films as well as Rub-doped PS films at 80 wt% doping concentration, previously reported to be optimal in terms of UC efficiency. The impact of commonly utilized singlet energy collector (sink) DBP on exciton diffusion was also assessed, highlighting its importance exclusively on the dynamics of singlets in Rub films. Our study revealed that triplet diffusion lengths (LTD) of 25-30 nm estimated for the disordered Rub films are sufficient for encountering triplets from the neighboring sensitizer molecules at a low sensitizer PdPc concentration (0.1 wt%), thereby enabling the desired TTA domination regime to be reached. Essentially, the performance of Rub-based UC systems was found to be limited by the modest maximal LTD (up to ∼55 nm) in disordered films resulting from a short maximum triplet lifetime τ_T (∼100 μs) inherent to this emitter. Thus, to enhance the NIR-to-Vis TTA-UC performance, new emitters with a longer triplet lifetime in the solid state are required.

Sweet Spot of Intermolecular Coupling in Crystalline Rubrene: Intermolecular Separation to Minimize Singlet Fission and Retain Triplet-Triplet Annihilation

Singlet fission is detrimental to NIR-to-vis photon upconversion in the solid rubrene (Rub) films, as it diminishes photoluminescence efficiency. Previous studies have shown that thermally activated triplet energy transport drives singlet fission with nearly 100% efficiency in closely packed Rub crystals. Here, we examine triplet separation and recombination as a function of intermolecular distance in the crystalline films of Rub and the t-butyl substituted rubrene (tBRub) derivative. The increased intermolecular distance and altered molecular packing in tBRub films cause suppressed singlet dissociation into free triplets due to slower triplet energy transport. It was found that the formation of correlated triplet pairs ¹(TT) and partial triplet separation ¹(T···T) occurs in both Rub and tBRub films despite differences in intermolecular coupling. Under weak intermolecular coupling as in tBRub, geminate triplet annihilation of ¹(T···T) outcompetes dissociation into free triplets, resulting in emission from the ¹(TT) state. Essentially, increasing intermolecular distance up to a certain point (a sweet spot) is a good strategy for suppressing singlet fission and retaining triplet-triplet annihilation properties.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

Triplet Exciton Transfers and Triplet-Triplet Annihilation in Anthracene Derivatives via Direct versus Superexchange Pathways Governed by Molecular Packing

Triplet exciton transfer (TET) and triplet-triplet annihilations (TTAs) in anthracene derivatives, namely, one of the polymorphs of 9,10-bis(triisopropylsilylethynyl)anthracene (TIPS-ANTp) and 1,2,3,4-tetrafluoro-5,8-bis(trimethylsilylethynyl)anthracene (F4-TMS-ANT), are analyzed theoretically. The electronic couplings for TET and TTA are evaluated by means of the diabatization scheme in conjunction with the time-dependent density functional theory and the multireference second-order Møller-Plesset method. The TET rate is estimated on the basis of Fermi's golden rule considering the Franck-Condon factor of intramolecular modes. TTA is analyzed by means of quantum dynamics calculations with the multiconfiguration time-dependent Hartree method. TET in the cofacially stacked F4-TMS-ANT is faster than that of the slip-stacked TIPS-ANTp. In the anthracene derivatives, a singlet exciton is lower in energy than a pair of triplets. F4-TMS-ANT can exhibit an ultrafast TTA via the superexchange pathway mediated by higher lying charge transfer (CT) states, owing to strong electronic couplings. In contrast, TIPS-ANTp exhibits an inefficient TTA via the direct pathway with a small two-electron coupling. The cofacial stacking decreases the energy gap to the intermediate CT states, thereby facilitating TET and TTA via the superexchange pathway.