Pentacene fluorescence spectra

The absorption spectrum of a carbazole-acceptor cyclophane shows transannular 7t-7t electronic interaction and little charge transfer interaction, while the fluorescence spectrum exhibits intramolecular exciplex emission around 525 nm <2003CL910>. 6,13-Bis(9-ethyl-9//-carbazol-3-yl)pentacene (a pentacene derivative bearing two carbazole moieties) displays an absorption spectrum that consists of the bands due to the carbazole (352 nm) and the pentacene moieties (483, 523, 561, 605 nm), indicating that the electronic transition level of pentacene is not affected by the carbazole chromophore <2006MI185>. 

Here pis the quantum efficiency of the sensitizer (ti = Tp/xp = l/3forpentacene)in the O4 site of p-terphenyl at 4 K, n is the index of refraction (n = 1.7 for the p-terphenyl crystal), and Na is the Avogadro s number. The integral in (H9) is calculated from the normalized fluorescence spectrum/(v) and the decadic molar extinction coefficient e(v) of pentacene at O4 site. The critical interaction distance is the sensitizer-activator separation for which the transfer rate is equal to the intrinsic decay time. Although derived for low temperatures. Equation H9 is also vaUd for arbitrary temperatures. In fact, the temperature dependence of the resonant energy transfer rate is contained in the spectral overlap integral. 

Chapter 7 introduces the reader to solutions of many selected problems in molecular physics. In particular, the following important problems are studied in detail the fluorescence spectrum ofp-terphenyl crystal, the vibrational fine structure of the spin-allowed absorption band of rans-[Co(CN)2(f )2]Cl3H20, and transport phenomena of electronic excitation in pentacene-doped molecular crystals. It is followed by an analysis of phosphorescence and radiationless transition in aromatic molecules with nonbonding electrons as well as predissociation of the 82 state of H2O+ by nonadiabatic interaction via conical intersection. 

The first SMS experiments in 1989 utilized either of two powerful doublemodulation FM absorption techniques, laser frequency-modulation with Stark secondary modulation (FM-Stark) or frequency-modulation with ultrasonic strain secondary modulation (FM-US) [3,26]. The secondary modulation was required in order to remove the effects of residual amplitude modulation produced by the imperfect phase modulator. In contrast to fluorescence methods, Rayleigh and Raman scattering were unimportant. Figure 2.3B (specifically trace d) shows examples of the optical absorption spectrum from a single molecule of pentacene in p-terphenyl using the FM-Stark method. 

Fig. 14. Low-temperature (at 1.5 K) (r-polarized fluorescence excitation spectrum of pentacene in p-terphenyl. Note that the absorption of the O3, O4 sites in this picture is artificially reduced by the detection setup. In a straight absorption experiment all four sites absorb equally.

Fig. 15. Low-temperature (1.5 K) fr-polarized fluorescence excitation spectrum of pentacene in naphthalene. 

Figure 9. Fluorescence spectra of two single pentacene molecules in />-terphenyl recorded with a confocal setup (see text). Spectrum A was taken in 33 minutes with a sample contacted to a silica plate. The excitation frequency of this molecule coincided with the unperturbed 0 site maximum. The spectrum presents all features of an Oi molecule when compared to a bulk spectrum with only some small deviations [39]. Spectrum B was recorded over 40 minutes with a sample glued to a pinhole. The excitation frequency of tlie molecule was in the middle of the 0 -O2 interval. According to its spectrum which shows some small differences to spectrum A it clearly belonged to site O2 (from Ref. 39).

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... 

Figure 10. Fluorescence excitation spectrum of the Oi spectral site of pentacene inp-terphenyl. The satellites 1-5 result from pentacene molecules which contain a single nucleus (natural abundance).

Figure 12. Fluorescence-excitation spectrum of pentacene-d 4 in / terphenyl-d 4. The strong lines, labelled Oi and O2, coimected by the brackets, correspond to pentacene-du molecules in the 0 and O2 spectra sites of p-terphenyl. The O2 line appears weaker in the spectrum because the polarization of the incident laser was adjusted to give a maximum signal for the Oi line. The two lines connected by the dashed bracket correspond to the Oi and O2 ensemble lines of pentacene molecules which contain a single proton bound to the carbon in the 7 position. Similarly the two lines connected by the dashed-dotted bracket result from pentacene molecules containing two protons each of them bound to a carbon in the 7 position of pentacene.

Fig. 18.15 Spectroscopy of single pentacene molecules in p-terphenyl crystal (W. P. Ambrose, Th. Basche and W. E. Moemer, 1. Chem. Phys. 95, 7150 (1991). (a) Fluorescence excitation spectrum of a single molecule at 1.5 K (0 MHz detuning = 592.407 nm, at the wing of the inhomogeneous hneshape) (b) Fluorescence excitation spectrum of the full inhomogeneous line at 1.5 K. (c) The dependence of the single molecule homogeneous hnewidth on temperature (the solid line is a fit to the data), (d) Two views of spectral diffusion The upper panel shows a time sequence of excitation spectra (each taken over a period of 1 s). The lower panel shows the jumps in the peak frequency as a function of time.

At liquid helium temperature, the absorption spectrum of pentacene at each site reveals an intense zero-phonon line associated with the electronic transition (Figure 7.14) and an accompanying phonon sideband (not shown in Figure 7.14). The latter appears as a mirror image in fluorescence and excitation and stems from pseudo-local phonons due to guest-host interactions [155]. The B2U pentacene transition in p-terphenyl is strongly b-axis polarized [250,251]. 

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