Evolution of Diffusion Coefficient of Photoexcited Species in Excimer Forming Organic Thin Films

Abstract

The diffusion of photogenerated excitons in organic semiconductors represents often a rate-determining process in the exciton population evolution in ultrafast transient absorption experiments, due to an increased mutual exciton interaction probability at typical excitation densities. A simple description of a diffusion process with a time-independent diffusion coefficient often fails to explain the observed kinetics in the highly nonequilibrium system. In this work, we used the concept of time-dependent diffusion and determined the evolution of the diffusion coefficient of photogenerated excited species in thin films of bis(terpyridine-4′-yl)terthiophene from the experimental ultrafast transient absorption spectroscopic data at a subnanosecond time scale. We show that the diffusion coefficient decreases in time more rapidly with increasing intensity of the photoexcitation. We explain the dependences by the planarization of photoexcited molecules and excimer formation.

Ultrafast Dynamics and Photoresponse of a Fungi‐Derived Pigment Xylindein from Solution to Thin Films

“…In particular, the ESA intensity from 710–740 nm shows a gradual increase with increasing concentrations, albeit a sharp decrease is apparent for the highest concentration (Figure 2 c) and thin films (Figure 2 d), primarily due to ground‐state bleaching (GSB) of the aggregated species. Most strikingly, we observed a sign reversal of the xylindein TA signal from about 850–900 nm: the negative SE band in solution (Figure 2 a–c) is replaced by a positive ESA band in thin films (Figure 2 d), generally attributable to an excimer→charge‐transfer band transition (see below) [9, 13] …”

“…These results support our calculation approach (Supporting Information). In contrast, the xylindein thin films display one dominant emission peak at a redder wavelength (909 nm) that originates from an excimer state [7a, 9] . The quantum yield (QY) in both cases is lower than that in Draconin Red, another pigment produced by wood‐decay spalting fungi (Figure S4a,b) [10] …”

“…Self‐trapping is commonly recognized as a quenching process (fs to μs) for excitons that could be detrimental to optoelectronic device performance [23, 25] . On the ps timescale, the excimers undergo structural rearrangement to optimize the coupling and decrease the intermolecular distance, [9] achieving a more relaxed structure. The sidechains of xylindein thus play a pivotal role throughout these processes by limiting the degree of coupling and distance between molecules [15] .…”

Elucidating Inner Workings of Naturally Sourced Organic Optoelectronic Materials with Ultrafast Spectroscopy

“…The sub‐ps self‐trapping hinders optoelectronic performance by confining electronic delocalization, which could be mitigated by more ordered xylindein thin films [20b,41a] . Moreover, small‐scale molecular rearrangements within the excimer can rapidly reach a more relaxed structure by decreasing the intermolecular distance to improve the electronic and vibrational coupling, [23b] while electronic delocalization splits the excited‐state energy relative to monomer (Figure 3f) [33] . Given that such distance/coupling is regulated by the tail extensions of xylindein, a targeted design principle could involve extended or branched tail sidechains to hinder excimer formation while still maintaining a certain degree of π‐π stacking (see Figure 3g insets), thus improving the photoconductivity of xylindein thin films [15b,31] …”

Kinetics of the Photoexcited States in Thin Films of Metallo-Supramolecular Polymers With Ditopic Thiophene-Bridged Terpyridine Ligands

“…Although the evolution of singlet and triplet exciton populations was experimentally well-characterized in the study by Rais et al, (2017b ), the origin of the bi-molecular singlet annihilation and subsequent triplet formation and decay remained unexplained. Meanwhile, we have elaborated a novel mathematical technique ( Menšík et al, 2018a ; Rais et al, 2018 ; Menšík et al, 2019 ), which makes the determination of the time-resolved diffusion coefficient from the transient absorption kinetics of photoexcited species possible. In this article, we applied the developed procedure on the previously acquired experimental data ( Rais et al, 2017b ) and proved that it is capable of elucidating in detail the process of the bi-molecular singlet annihilation and the subsequent formation and de-excitation of the triplet states.…”