Fig. 2: Temperature-dependent optical measurements.

a, b PL spectra measured at various temperatures for WL (a) and 1L (b). For 1L, a sharp peak at ~680 nm (designated as FR excitonic emission) rises sharply as the temperature decreases. c Time-resolved PL emission (normalised) from WL and 1L pentacene samples at 77 and 298 K. The orange/green balls represent the measured decay curve from WL, which arises from CT exciton emission. An effective long lifetime of 2.64 ns (2.89 ns) was extracted from the orange (green) decay curve at 77 K (298 K) by fitting with deconvolution of the instrument response function (IRF) (green dots). The red/blue balls represent the decay curve from 1L, which arises from FR exciton emission. The red (blue) decay curve was fitted (solid line) with deconvolution of the IRF, giving an effective short lifetime of 4.1 ps (12.7 ps) for 1L at 77 K (298 K). The inset shows the deconvoluted curves obtained from 1L. d, e Time- and spectrally resolved transient differential reflectance of 1L (c) and WL (d) recorded using ~200 fs degenerate pump and probe with spectra centred at 680 nm. f Slices of the TR for 1L (blue) and WL (red) at 677 nm and 673 nm, respectively. A lifetime of 4.2 ps was extracted from the 1L decay, while the WL response was effectively constant across the range. The feature at 3.5 ps is due to interference between the pump pulse and a weak reflection of the probe pulse. g Measured polar plot of PL intensity as a function of emission polarisation angle θ from 1L (blue) and WL (red) pentacene samples at 77 K, revealing the anisotropic (isotropic) excitonic nature of emission from 1L (WL) pentacene. In the experiment, the excitation polarisation angle was fixed, and the polarisation angle of the emission θ was determined by an angle-variable polarizer located in front of the detector. The solid lines are curves fitted using a cos²θ function