Temperature dependence of the exciton dynamics inDCM2:Alq3

Abstract

We study the temperature dependence of the triplet and singlet exciton dynamics in the archetype small molecule fluorescent guest-host system, tris(8-hydroxyquinolinato) aluminum (Alq 3 ) doped with 4-(dicyanomethylene)-2-methyl-6-julolidyl-9-enyl-4H-pyran (DCM2). We develop a comprehensive model of the exciton dynamics, and use it to fit the transient photoluminescence under different pulsed optical pumping in the temperature range of 80 K< T< 295 K. The triplet decay has a significantly different temperature dependence than that of singlets. From 295K to 80K, the triplet-triplet annihilation (TTA) rate decreases by two orders of magnitude, whereas the singlet-triplet annihilation rate decreases by <50% primarily a result of the different energy transfer mechanisms of singlets and triplets. The temperature dependence of TTA rate reveals two regimes separated by a transition at 180K from Marcus to MillerAbrahams transfer. This work deepens our understanding of exciton dynamics and energy transfer in small molecule organic materials.2

Triplet exciton diffusion in metalorganic phosphorescent host-guest systems from first principles

“…E.g., Hoffmann et al [36] found that the temperature dependence of triplet diffusion in conjugated polymers is best described by Marcus theory, with a transition to MA theory at lower temperatures. A similar conclusion was drawn by Liu et al for diffusion in the disordered fluorescent small-molecule host-guest system consisting of tris(8-hydroxyquinolinato) aluminum (Alq 3 ) doped with 4-(dicyanomethylene)-2methyl-6-julolidyl-9-enyl-4H-pyran (DCM2) [21]. However, for metalorganic phosphorescent emitter molecules, it is presently not clear which of these approaches is most accurate, which type of transfer process (Förster or Dexter) is predominant, and to what extent information about the detailed diffusion mechanism can be gained by studying the guest concentration and temperature dependence of the diffusion-related efficiency loss due to, e.g., concentration quenching or TTA.…”

“…Triplet exciton diffusion has been extensively studied for purely organic neat-film materials that are commonly used as a host, as an electron or hole transport layer, or a fluorescent emissive layer [20][21][22][23][24]. Due to the long triplet lifetime in such materials and the small intermolecular distances, the diffusion lengths can be of the order of 100 nm.…”

“…Within simplified approaches, this factor is approximated using a semiclassical approach to the triplet-vibron coupling (Marcus theory [30]), using a quantum-mechanical single-mode approach (Marcus-Levich-Jortner (MLJ) theory [31,32]), or using the Miller-Abrahams (MA) theory [33]. For some classes of materials, the validity of these various approaches has been studied theoretically and experimentally [21,24,[34][35][36]. E.g., Hoffmann et al [36] found that the temperature dependence of triplet diffusion in conjugated polymers is best described by Marcus theory, with a transition to MA theory at lower temperatures.…”

Exciton diffusion in organic semiconductors

“…5). 68,74,75,79,81,[88][89][90]92,93,[96][97][98][99][100][101][102][103][104][105][106][107][108][109][110][111][112] Since triplet excitons undergo diffusion via short range Dexter energy transfer, the number of available hopping sites quickly becomes limited during the downhill migration. In strongly disordered systems sometimes one observes a blue shift of the maximum of phosphorescence spectrum upon cooling below 100 K. 103,104 Using theoretical modeling Beljonne, Ko ¨hler, and co-authors showed that in materials with high degree of disorder, certain thermal activation is needed for triplets to find lower energy sites, resulting in the blue shift of PL spectrum upon cooling.…”