Pentacene in p-terphenyl

Wild U P, Guttler F, Pirotta M and Renn A 1992 Single molecule spectroscopy stark effect of pentacene in p-terphenyl Chem. Phys. Lett. 193 451-5... 

Guttler F, Croci M, Renn A and Wild U P 1996 Single molecule polarization spectroscopy pentacene in p-terphenyl Chem. Phys. 211 421-30... 

Tchenio P, Myers A B and Moerner W E 1993 Dispersed fluorescence spectra of single molecules of pentacene in p-terphenyl J. Chem. Phys. 97 2491-3... 

Fig. 2.2. (A) Illustration of the source of statistical fine structure (SFS) using simulated absorption spectra with different total numbers of absorbers N, where a Gaussian random variable provides center frequencies for the inhomogeneous distribution. Traces (a) through (d) correspond to N values of 10, 100, 1,000, and 10,000, respectively, and the traces have been divided by the factors shown. For clarity, yjj = Fi/10. Inset several guest impurity molecules are sketched as rectangles with different local environments produced by strains, local electric fields, and other imperfections in the host matrix. (B) SFS detected by FM spectroscopy for pentacene in p-terphenyl at 1.4K, with a spectral hole at zero relative frequency for one of the two scans. Note the repeatable fine structure...

Fig. 2.3. (A) Illustration of the crystal structure of p-terphenyl, with a single substitutional impurity of pentacene. (B) Single molecules of pentacene in p-terphenyl detected by FM-Stark optical absorption spectroscopy, (a-c) Simulated traces for absorption, FM and FM-Stark, respectively. The W -shaped structure in the center of trace (d) is the absorption from a single pentacene molecule, acquired multiple times to show repeatability, (e) Average of traces in (d) with expected hneshape. Trace (f) is the signal from a region of the spectrum with no molecules, while trace (g) is a region with many molecules showing SFS. For details, see [3,26]...

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. 2.4. (A) Sketch of the cryostat insert for single-molecule spectroscopy by fluorescence excitation. The focus of lens L is placed in the sample S by the magnet/coil pair M, C. (B) Scan over the inhomogeneous line (a) with a 2 GHz region expanded (b) to show isolated single-molecule absorption profiles. (C) Three-dimensional pseudo-image of single molecules of pentacene in p-terphenyl. The measured fluorescence signal (z-axis) is shown over a range of 300 MHz in excitation frequency (horizontal axis, center = 592.544 nm) and 40 pm in spatial position (axis into the page). (D) Rotation of the data in (c) to show that in the spatial domain, the single molecule maps out the shape of the laser focal spot. Bar, 5 pm. For details, see [33]...

Fig. 2.5. Examples of single-molecule spectral diffusion for pentacene in p-terphenyl at 1.5 K. (A) A series of fluorescence excitation spectra each 2.5 s in duration spaced by 0.25 s showing discontinuous shifts in resonance frequency, with zero detuning = 592.546 nm. (B) Trend or trajectory of the resonance freqnency over a long time scale for the molecule in (a). For details, see [34]...

Fig. 2.7. (A) Measured distribution of time delays between successive detected fluorescence photons for a single molecule of pentacene in p-terphenyl showing antibunching at r = 0. For details, see [53]. (B) Magnetic resonance of a single molecular spin. Reductions in fluorescence as a function of microwave frequency for four different single molecules of pentacene in p-terphenyl. For details, see [59]...

Fig. 2.8. Left (A) Optical configuration for exciting a single molecule with a nearfield fight source, an Al-coated pulled optical fiber. Application of a potential V to the A1 coating produces a highly anisotropic DC local electric field. (B) Spectra of pentacene in p-terphenyl molecules at various applied potentials, (a-b) saturation method to identify molecules close to the tip, (c-1) transverse dithering with Stark shift. For details, see [63]...

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. 

Kohler J, Brouwer A C J, Groenen E J J and Schmidt J 1994 Fluorescence detection of single molecule magnetic resonance for pentacene in p-terphenyl. The hyperfine interaction of a single triplet spin with a single C nuclear spin... 

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. 18. Results of a two-pulse and a three-pulse photon-echo measurement at low temperature in a mixed crystal of pentacene in p-terphenyl. Note that this mixed crystal is not dilute in the sense that =27] at low temperature (1.5 K).

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... 

Ftg. 21. Absorption (upper) and temperature modulation (lower) spectrum of pentacene in p-terphenyl. The temperature jump was ca. 2.7°. 

Excited State Vibrational Lifetimes of Pentacene in p-Terphenyl and Naphthalene at 1.5 K... 

H. Talon, L. Fleury, J. Bernard, and M. Orrit, J. Opt. Soc. Am. B 9, 825 (1992). In this work, the motional narrowing phenomenon has been observed in the single molecule spectrum of pentacene in p-terphenyl crystal as the temperature of the system is increased. 

The first single-molecule spectra were recorded in the pentacene in p-terphenyl system in 1989 using a sophisticated zero-scattering-background absorption technique. 

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