Terrylene in polyethylene

The most detailed investigation of single-molecule vibrational spectra has been carried out on the terrylene in polyethylene system by Tchenio et al. [38, 43, 44]. For this study the setup shown in Fig. 8 was used. Single molecule fluorescence spectra were obtained with excitation wavelengths both to the red and slightly to the blue of Aniax (569 nm) of the inhomogeneous distribution. 

Since the chromophore terrylene is an almost unknown molecule spectroscopically, the interpretation of the fluorescence spectra had to rely entirely on comparison with the results of quantum chemical calculations (QCFF/PI -I- CISD [45]) that were performed by Myers [38] and colleagues. While the calculated frequencies were found to be unifomJy too high, there was a fairly good correspondence between calculated and observed frequencies and intensities, particularly in the low 

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Figure Cl.5.8. Spectral jumping of a single molecule of terrylene in polyethylene at 1.5 K. The upper trace displays fluorescence excitation spectra of tire same single molecule taken over two different 20 s time intervals, showing tire same molecule absorbing at two distinctly different frequencies. The lower panel plots tire peak frequency in tire fluorescence excitation spectmm as a function of time over a 40 min trajectory. The molecule undergoes discrete jumps among four (briefly five) different resonant frequencies during tliis time period. Arrows represent scans during which tire molecule had jumped entirely outside tire 10 GHz scan window. Adapted from...

Tchenio P, Myers A B and Moerner W E 1993 Vibrational analysis of the dispersed fluorescence from single molecules of terrylene in polyethylene Chem. Phys. Lett. 213 325-32... 

Figure 10. Vibrationally resolved fluorescence spectra of terrylene in polyethylene (r= 1.4 K). Tlie bulk spectrum was obtained with excitation near the peak of the inhomogeneously broadened origin band. A-E represent spectra of different molecules which are described in the text. The average time to collect the single molecule spectra was several hundred seconds (from Ref. 38).

Fluorescence spectra of single terrylene molecules were also recorded recently in the crystalline matrix of p-terphenyl [29] using again a similar setup as shown in Fig. 8. Most of the observed vibrational frequencies coincide with those of terrylene in polyethylene with the exception of small frequency shifts. In Fig. 11 the fluorescence spectrum of a single terrylene molecule in site Xz [29], which is the most photostable site, is displayed. The most important feature is the appearance of two lines of similar intensity at 244 and 256 cm, respectively. This doublet has not to be confused with type 1 and type 2 spectra in terrylene/polyethylene as discussed in the preceding section because those stem from different molecules. In contrast, in the p-terphenyl crystal the doublet around 250 cm is an inherent property of a single absorber, a fact which also can be deduced from bulk spectra through the appearance of the combination band at 500 cm. ... 

Figure 7. Sample of single-molecule resonances of differing widths for the system terrylene in polyethylene at 1.8 K (from Ref. 76).

Figure 8. Distribution of the widths at half maximum of around 200 molecules of terrylene in polyethylene. The lower cut-off corresponds to the lifetime limited width, indicating molecules by chance isolated from tunneling systems, the peak to molecules in an average environment and the tail at large widths to molecules probably influenced by more than the average number of tunneling systems.

Figure 14. Temperature dependence of the decay rates of the fluorescence correlation (tunneling rate of the tunneling system coupled to the probe molecule) of three different molecules of terrylene in polyethylene. The smooth lines are fits showing proportionality to T (a), to (b)...

Fleury et al. [9] measured for a single terrylene molecule coupled to single TLS in polyethylene matrix. They showed that their experimental results are well described by... 

Linear Stark effect of smgle terrylene molecules in polyethylene... 

The correlation method was applied to a thorough study of spectral diffusion of single terrylene molecules in polyethylene [44, 76], and later on to other systems [81]. Once a single molecule line was identified in the excitation spectrum, the laser was brought into resonance with the line, and the correlation function of the emitted fluorescence was recorded. For most single molecules studied (around 80%), no clear correlation appeared between 1 is and 100 s, i.e. the contrast of the correlation was weaker than experimental noise. This is in general agreement with bulk studies of... 

Figure 11. The auto-correlation function of the fluorescence intensity reveals the characteristic times of the random telegraph signal in Fig. 10. Here, three examples of correlation functions of different terrylene molecules in polyethylene at 1.8 K illustrate (note the logarithmic time axis) (a) an exponential decay (with fit, smooth line) (b) a bi-exponential decay and (c) a decay with many timescales. Such different behaviours may reflect molecules coupled strongly to one, two or many tunneling systems.

Similar spectral dynamics of individual chromophores from repeated fluorescence excitation scans have subsequently been seen in amorphous hosts, for the systems Tr in polyethylene (PE) [14] and tetra-r-butyl-terrylene (TBT) in polyisobutylene (PIB) [15, 16]. In at least one instance [16] the chromophore samples far fewer frequencies than in the case of pentacene in p-terphenyl. The spectral difl usion trajectories are assumed to result from the flipping of those TLSs whose dynamics is slower than the scan time. 

Figure 11. Statistical distribution of the values of Sue for 60 terrylene molecules in a polyethylene matrix [15). The distribution is centered around 0, but not symmetric. Thus, a small average value A/tg = 0.44 D remains. Some of the terrylene molecules exhibit a very strong change of the dipole moment Sue ...

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