The excitation intensity dependence of singlet fission dynamics of a rubrene microcrystal studied by femtosecond transient microspectroscopy

Abstract

When a rubrene crystal was excited with an intense fs pulse (397 nm), transient local heating (∼ps) of the crystal through nonradiative relaxation from SN to S1 accelerates singlet fission of the S1.

In optimized rubrene-based nanoparticle blends for photon upconversion, singlet energy collection outcompetes triplet-pair separation, not singlet fission

“…28 Previous studies of rubrene-DBP TTA-UC systems include in some form the assertion that FRET from rubrene to DBP outcompetes singlet fission. 18,[26][27][28] This simple statement appears to overlook the complexity of singlet fission and triplet fusion that occurs in solid rubrene, dynamics that have been extensively studied in vapour-grown orthorhombic single crystals [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] and to a lesser degree in more complex thin films. 25,30,[46][47][48][49][50][51] Furthermore, transient absorption spectroscopy of rubrene single crystals has shown that S 1 -1 (TT) occurs extremely rapidly, with reported time constants varying from 25 fs 35,36 to 2 ps.…”

“…25,30,[46][47][48][49][50][51] Furthermore, transient absorption spectroscopy of rubrene single crystals has shown that S 1 -1 (TT) occurs extremely rapidly, with reported time constants varying from 25 fs 35,36 to 2 ps. 33,34 FRET from rubrene to DBP is therefore very unlikely to outcompete this first singlet fission step for the majority of excitons. In ESI, ‡ Section S11, we estimate the rubrene-DBP FRET rate to be 1.5 ps for nearest neighbours based on previous calculations of the Fo ¨rster radius.…”

“…35,36,55 As stated above, reported time constants for this first step range from 25 fs 35,36 to 2 ps. 33,34 Next, 1 (TT) separates into (TÁ Á ÁT) over tens of picoseconds, [33][34][35][36] a process thought to occur by thermally-activated triplet hopping. [56][57][58][59] The overall singlet fission process is therefore commonly written as 60,61 S 1 -…”

Photoluminescence dynamics in singlet fission chromophore liquid melts

“…Femtosecond transient absorption experiments on crystalline RUB suggested that the majority of SF occurs within 1 ps 27 , but later experiments indicated that this rate may depend on excitation density and thermal load. 23 Our excitation densities are below those typically used in transient absorption experiments, and our SF time of 60 ps approaches the 35 ps time seen at the lowest fluences in such experiments. 23 But the distorted PL spectrum makes it impossible to unequivocally identify the emitting species as the high energy species, so it is also possible that we are measuring the dynamics of a subset of sites within the crystal rather than the majority species seen in the transient absorption experiments.…”

“…Our samples were powdered in order to pack the melting point capillary, and this process leads to an unstructured emission peaked at around 600 nm, shown in Figure 1, the same wavelength observed for most single crystals. 21,23 This redshifted emission probably reflects strong selfabsorption in the sample 19 , but may also include contributions from defect states created by powdering. After the crystal melts, this emission shifts slightly to higher energy and broadens, but does not undergo dramatic changes in shape or intensity.…”