The dependence of singlet exciton relaxation on excitation density and temperature in polycrystalline tetracene thin films: Kinetic evidence for a dark intermediate state and implications for singlet fission

Abstract

The excited state dynamics of polycrystalline tetracene films are studied using femtosecond transient absorption in combination with picosecond fluorescence, continuing work reported in an earlier paper [J. J. Burdett, A. M. Muller, D. Gosztola, and C. J. Bardeen, J. Chem. Phys. 133, 144506 (2010)]. A study of the intensity dependence of the singlet state decay is conducted to understand the origins of the discrepancy between the broadband transient absorption and fluorescence experiments seen previously. High-sensitivity single channel transient absorption experiments allow us to compare the transient absorption dynamics to the fluorescence dynamics measured at identical laser fluences. At high excitation densities, an exciton-exciton annihilation rate constant of ~1 × 10(-8) cm(3) s(-1) leads to rapid singlet decays, but at excitation densities of 2 × 10(17) cm(-3) or less the kinetics of the transient absorption match those of the fluorescence. At these lower excitation densities, both measurements confirm that the initially excited singlet state relaxes with a decay time of 80 ± 3 ps, not 9.2 ps as claimed in the earlier paper. In order to investigate the origin of the singlet decay, the wavelength-resolved fluorescence dynamics were measured at 298 K, 77 K, and 4 K. A high-energy J-type emitting species undergo a rapid (~100 ps) decay at all temperatures, while at 77 K and 4 K additional species with H-type and J-type emission lineshapes have much longer lifetimes. A global analysis of the wavelength-dependent decays shows that the initial ~100 ps decay occurs to a dark state and not via energy transfer to lower energy bright states. Varying the excitation wavelength from 400 nm to 510 nm had no effect on the fast decay, suggesting that there is no energy threshold for the initial singlet relaxation. The presence of different emitting species at different temperatures means that earlier interpretations of the fluorescence behavior in terms of one singlet state that is short-lived due to singlet fission at high temperatures but long-lived at lower temperatures are probably too simplistic. The presence of a rapid singlet decay at all temperatures indicates that the initially created J-type singlet exciton decays to an intermediate that only produces free triplets (and delayed fluorescence) at high temperatures.

Identification of a triplet pair intermediate in singlet exciton fission in solution

“…We can compare the results here with studies from films of tetracene, where the first step of highly efficient endothermic singlet fission is temperature independent (22,23) and has been proposed to involve barrier-free formation of an intermediate state (32,33) or tunnelling into a bound triplet pair (22). We find that our model is in qualitative agreement with these other studies; our endothermic system reveals rapid formation of a bound intermediate with triplet character that goes on to produce two free triplet excitons over a longer timescale.…”

“…Whereas singlet fission in pentacene is exothermic by ∼100 meV (1), in tetracene it is found to be endothermic by ∼180 meV (1,(19)(20)(21). Accordingly, triplet formation is significantly faster in pentacene films (∼80 fs) (5) than in tetracene (∼90 ps) (22), although curiously in the latter material the process remains highly efficient and fully independent of temperature (22,23). In this study, we look at solutions of TIPStetracene, a tetracene derivative also capable of singlet fission (24).…”

Harvesting singlet fission for solar energy conversion via triplet energy transfer

“…5c shows TR-2PPE spectrum for Tc only, without the CuPc overlayer. For Tc, there is an equilibrium between singlet and triplet pair states owing to the quantum superposition on short timescales (7 ps) and to triplet-triplet annihilation on longer times; the triplet population decays on this timescale mainly because of radiative recombination (fluorescence emission) [15][16][17] . Figure 5d summarizes the triplet populations as a function of pump-probe delays for the 2-nm/20-nm CuPc/Tc sample (red line), 2-nm/2-nm CuPc/Tc (blue line) and 20-nm Tc only (grey dots), all exited at hn 1 ¼ 2.43 eV.…”

“…The quantum superposition in Tc possesses a lifetime of 7 ps (refs 11,17) after which the triplet pair state decouples from the singlet. The triplet excitation energy in Tc is T 1 ¼ 1.25 eV (refs [15][16][17] and that in CuPc is T 0 1 ¼ 1.15 eV (ref. 18); the near-resonant but slightly downhill energetics should facilitate efficient triplet energy transfer from Tc to CuPc.…”

Microscopic theory of singlet exciton fission. I. General formulation

“…The authoritative review by Smith and Michl 9 effectively summarizes the state of the field up to 2010. Since then, singlet fission has been further investigated by TA in thin films of diphenylisobenzofuran, 16 by DF and TA in crystalline tetracene, 17,18 by time-resolved two-photon photoemission 19 and TA 20 in crystalline pentacene, by TA in solution and crystalline rubrene, 21 and even by DF and TA in amorphous films of diphenyl tetracene. 22 Although still far away from commercial use in solar cells, singlet fission has been investigated in a) Electronic mail: tcb2112@columbia.edu b) Electronic mail: mhyberts@bnl.gov c) Electronic mail: drr2103@columbia.edu pentacene-perylene blend films 23 and even successfully incorporated into heterojunction solar cells utilizing phthalocyanine, tetracene, and C 60 11 .…”