Exciton Delocalization and Superradiance in Tetracene Thin Films and Nanoaggregates

The structure and dynamics of luminescent excitons in tetracene thin films and nanoaggregates are investigated using time-resolved spectroscopy and theoretical calculations. The orientation of tetracene’s transition dipole moment along its short molecular axis leads to properties qualitatively different from those observed in aggregates of phenylene-vinylene and thiophene oligomers, despite similar crystal structures. The spectral shape, temperature dependence, and radiative lifetime are consistent with a short-lived superradiant exciton delocalized over $\sim 10$ tetracene molecules.

Figure 1

(a) Absorption spectra of dilute tetracene in THF (solid line), tetracene nanoparticles in aqueous solution (dashed line), and a vacuum-deposited film of tetracene (dotted line). Also shown is the tetracene molecular structure and orientation of its transition dipole. (b) Fluorescence spectra for samples in (a). The labels 0-0 and 0-1 denote the vibronic peaks.

Figure 2

(a) Fluorescence decay, integrated over all wavelengths, for dilute tetracene in THF (dotted line) and vacuum-deposited tetracene (dashed line) at 290 K, and vacuum-deposited tetracene at 4 K (solid line). The lower panels show the evolution of the fluorescence spectrum at 4 K integrated over the following time windows: (b) 0–50 ps; (c) 1.0–1.4 ns; (d) 16–17 ns.

Figure 3

(a) Experimental emission spectra of vacuum-deposited tetracene, integrated over the 0–50 ps time window, at 290, 150, and 4 K. Inset: Unit cell of tetracene crystal. (b) Theoretical emission spectra from $3\times 3$ pinwheel calculation and Boltzmann distributions at 4, 150, and 290 K. Inset: Top view of $3\times 3$ pinwheel aggregate used in the calculations. Arrows indicate the transition dipole moments for the $b$-polarized lowest energy exciton.

Figure 4

Temperature dependence of the ratio of radiative rates, $k_{\mathrm{aggregate}}/k_{\mathrm{monomer}}$ from theoretical calculations for $2\times 2$ (dashed line), $3\times 3$ (solid line), and $4\times 4$ (dot-dashed line) pinwheels, along with experimental values obtained by multiplying the ratio measured at 290 K by the relative increase in integrated fluorescence within the 0–50 ps time window. The error bars are due to uncertainty in the 290 K radiative rate measurement.