Experimental and theoretical study of temperature dependent exciton delocalization and relaxation in anthracene thin films

Abstract

The spectroscopy of solid anthracene is examined both experimentally and theoretically. To avoid experimental complications such as self-absorption and polariton effects, ultrathin polycrystalline films deposited on transparent substrates are studied. To separate the contributions from different emitting species, the emission is resolved in both time and wavelength. The spectroscopic data are interpreted in terms of a three-state kinetic model, where two excited states, a high energy state 1 and a low energy state 2, both contribute to the luminescence and are kinetically coupled. Using this model, we analyze the spectral lineshape, relative quantum yield, and relaxation rates as a function of temperature. For state 1, we find that the ratio of the 0-0 vibronic peak to the 0-1 peak is enhanced by roughly a factor of 3.5 at low temperature, while the quantum yield and decay rates also increase by a similar factor. These observations are explained using a theoretical model previously developed for herringbone polyacene crystals. The early-time emission lineshape is consistent with that expected for a linear aggregate corresponding to an edge-dislocation defect. The results of experiment and theory are quantitatively compared at different temperatures in order to estimate that the singlet exciton in our polycrystalline films is delocalized over about ten molecules. Within these domains, the exciton's coherence length steadily increases as the temperature drops, until it reaches the limits of the domain, whereupon it saturates and remains constant as the temperature is lowered further. While the theoretical modeling correctly reproduces the temperature dependence of the fluorescence spectral lineshape, the decay of the singlet exciton appears to be determined by a trapping process that becomes more rapid as the temperature is lowered. This more rapid decay is consistent with accelerated trapping due to increased delocalization of the exciton at lower temperatures. These observations suggest that exciton coherence can play an important role in both radiative and nonradiative decay channels in these materials. Our results show that the spectroscopy of polyacene solids can be analyzed in a self-consistent fashion to obtain information about electronic delocalization and domain sizes.

Charge Transfer–Mediated Singlet Fission

“…Time-and wavelengthdependent emission spectroscopy has been used to establish that singlet excitons can be delocalized across approximately 10 molecules in anthracene (40) and tetracene (41) thin films, and this number can rise to approximately 100 under appropriate conditions (8). The extent of delocalization has implications for the nature of electronic states in molecular solids: No longer is it appropriate to consider low-energy excitations in such systems purely Frenkel-like in character with both the electron and hole localized on a single molecule.…”

Microscopic theory of polariton lasing via vibronically assisted scattering

“…where g = √ S ∼ 1 is the strength of the exciton-phonon coupling, 30 E 11 is the energy quantum of a vibration of the excited state. Even if the Franck-Condon model which we are using prescribes E 11 = E 01 , this is not necessarily true in general.…”

Molecular Weight Dependence of Exciton Diffusion in Poly(3‐hexylthiophene)

“…[30,31] The on-site energies (diagonal elements) Hii were sampled from a Gaussian distribution with standard deviation σ = 0.1eV to account for intrastack static energetic (conformational) disorder. Assuming a simple Gaussian-like shape for the exciton absorption and emission spectra [32] In Figure 7(a), we show the stack-to-stack energy transfer Förster-like rates for two different pairs of identical stacks (1360 and 6800 gmol -1 , or 8 and 40 repeated monomers, correspondingly) as a function of the stack size (i.e., the number of polymer chains in the stack). Independently on the stack size, the Förster hopping rate kF is approximately 3.5 times larger for stacks with the higher molecular weight.…”