Pentacene fluorescence lifetime

Pirotta M, Guttler F, Gygax FI, Renn A, Sepiol J and Wild U P 1993 Single molecule spectroscopy fluorescence lifetime measurements of pentacene in p-terphenyl Chem. Phys. Lett. 208 379-84... 

Fluorescence lifetimes have been measmed directly by time-correlated single-photon counting for pentacene in p-terphenyl [100]. This experiment requires careful selection of the laser pulse eharaeteristies sueh that the pulse duration is short enough to resolve the 23 ns decay time, yet has a bandwidth narrow enough to allow speetral selection of individual molecules. Four different molecules had the same lifetime to within experimental uncertainty, indicating that the principal contributions to the Sj state decay (radiation and internal conversion to Sq) are not strongly sensitive to the local environment in this relatively homogeneous crystalline matrix. 

In Fig. 18 we exhibit the results of both a two-pulse and a three-pulse echo measurement at low temperature on pentacene in p-terphenyl. The important point to note is that the echo decay times are identical but shorter than the fluorescence lifetime for this more concentrated crystal. The implication is that at higher concentration, optical dephasing is also caused by energy-transfer processes in this system and that the process is irreversible (7 ,-type). 

Aartsma and Wiersma were the first to report on the temperature dependence of the photon echo of pentacene in p-terphenyl. These initial experiments were performed using a nanosecond pulsed dye laser and measuring the echo intensity as a function of temperature for a fixed time separation of the exciting pulses. Figure 19 shows the latest result using this method for the 0,-site of pentacene in p-terphenyl. In a separate experiment it was ascertained that the fluorescence lifetime of 23.5 ns remained constant up to 110 This change in echo intensity as a function of temperature is thus a manifestation of a pure (7 ) dephasing contribution to the echo hfetime. Experimentally it was foimd that an... 

Furthermore, Payer and co-workers have recently reported" detailed results of the concentration dependence of the photon echo decay in the mixed crystal of pentacene in naphthalene. At low guest concentration they find that at 1.4K r, whereby ry, is the fluorescence lifetime. 

Table 3. Fluorescence lifetimes of four investigated pentacene molecules in a p-terphenyl host crystal [10], The data were analyzed according to the deconvolution algorithm described in the text and in Ref. 12. The second column represents the time constant of a single exponential decay function with a constant background. The third and fourth columns were obtained by fitting two exponential decay functions without background. The time constant of the faster decay is slightly smaller than in the first case. The longer contribution features a time constant of about 2 ps.

Figure Cl.5.2. Fluorescence excitation spectra (cps = counts per second) of pentacene in /i-teriDhenyl at 1.5 K. (A) Broad scan of the inhomogeneously broadened electronic origin. The spikes are repeatable features each due to a different single molecule. The laser detuning is relative to the line centre at 592.321 nm. (B) Expansion of a 2 GHz region of this scan showing several single molecules. (C) Low-power scan of a single molecule at 592.407 nm showing the lifetime-limited width of 7.8 MHz and a Lorentzian fit. Reprinted with pennission from Moemer [198]. Copyright 1994 American Association for the Advancement of Science.

In fact, single molecule spectroscopy (SMS) experiments have recently become a reality. The first experiments were performed on pentacene (the chromophore) in a p-terphenyl crystal [8-10]. I will focus here on the experiments of Ambrose, Basche, and Moemer [9, 10], which involved repeated fluorescence excitation spectrum scans of the same chromophore. For each chromophore molecule they found an identical (except for its center frequency) Lorentzian line shape whose line width is determined by fast phonon-induced fluctuations (and by the excited state lifetime), as discussed above. However, for each of a number of different chromophore molecules Moemer and coworkers found that the chromophore s center frequency changed from scan to scan, reflecting spectral dynamics on the time scale of many seconds The transition frequencies of each of the chromophores seemed to sample a nearly infinite number of possible values. Plotting the transition frequency as a function of time produces what has been called a spectral diffusion trajectory (although the frequency fluctuations are not necessarily diffusive ). These fascinating and totally... 

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