Pentacene in naphthalene

Chang T-C and DIott D D 1989 Vibrational cooling in large molecular systems pentacene in naphthalene J. Chem. Phys. 90 3590-602... 

Fig. 3. Jablonski energy-level diagram typical for chromophores involved in ONP processes (specifically for pentacene in naphthalene, after Ref. [39]), showing the respective roles of optical excitation, ISC, OEP, electron-nuclear polarization transfer (here driven by mw radiation), and subsequent decay to the diamagnetic ground state. The relative populations of the triplet magnetic sublevels are given at the far right (assuming the crystal is oriented such that the long molecular axis of pentacene is parallel to the external magnetic field ).

Hesselink W H and Wiersma D A 1980 Optical dephasing and vibronic relaxation in molecular mixed crystals a picosecond photon echo and optical study of pentacene in naphthalene and p-terphenyl J. Chem. Phys. 73 648-63... 

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

Picosecond Punyt-Probe Experiments in Pentacene in Naphthalene... 

Fig, 22. Results of psec pump-probe experiments on the origin (a), 261 cm vibronic band (b), and 748 cm vibronic band (c) in the absorption spectrum of pentacene in naphthalene. The zero-time point was determined in a separate pump-probe optical mixing experiment. The autocorrelation of the excitation pulses was 4.3 ps (a), 19 ps (b), and 8.5 ps (c). 

Fig. 24. The low-temperature transient absorption (a) and photon-echo relaxation (b) of the 5767 A vibronic transition of pentacene in naphthalene at 1.3 K. The dashed line in (a) is the correlation between the pump and probe pulse. In (b) the wavelengh of the probemixing pulse was 5570 A. (After Ref. 60.)...

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. 

Much of our knowledge of the frequency dependence of VER rates in polyatomic molecules stems from low-temperature studies of molecular crystals [2] such as pentacene (PTC 221 4) guest molecules in a crystalline naphthalene (N C,., H ) host. In naphthalene, the phonon cut-off frequency is -180 cm [97]. At low temperature,... 

Figure C3.5.10. Frequency-dependent vibronic relaxation data for pentacene (PTC) in naphthalene (N) crystals at 1.5 K. (a) Vibrational echoes are used to measure VER lifetimes (from [99]). The lifetimes are shorter in regime I, longer in regime II, and become shorter again in regime III. (b) Two-colour pump-probe experiments are used to measure vibrational cooling (return to the ground state) from [1021.

A number of attempts to transfer the spin polarization directly from the triplet electrons to the 13C spins that are present in the sample have been unsuccessful. Nevertheless, it is possible to polarize the 13C spins indirectly, by polarizing the 1H spins first, and then transporting the enhanced 1H polarization to the I3C spins by the conventional crosspolarization technique. Figure 18B shows the enhanced 13C spectra obtained in this way using the same crystal of pentacene-doped naphthalene. 

However, the recent experiments demonstrating (MI)ONP in polycrystalline samples were actually achieved by selective implementation of the ISE. In this work, an enhancement factor of 3160 was achieved in a poly crystalline (ground and pressed) sample of pentacene-doped naphthalene at 100 K in a field of 0.32 T (Fig. 8). However, instead of sweeping over the entire triplet ESR line (as is normally done with the ISE), the field sweep was designed to cover only a partial area of the ESR spectrum. With this approach (as implemented), only those crystallites with the long axis of the pentacene chromophores... 

Fig. 8. MIONP enhancement of nuclear spin polarization in a polycrystalline sample of pentacene doped naphthalene/ (a) Build-up curve for the enhanced spin polarization, with an MIONP repetition rate of 50 Hz (determined by the laser) the time constant for the build-up was 3730 s. TCP is an acronym for integrated cross-polarization - the authors term for their variant of the ISE. (b) 13.6 MHz single-shot H NMR spectra, taken with (top) and without bottom) MIONP enhancement (respectively, acquired at 100 K and room temperature). The top spectrum was obtained with a 10,000 s MIONP build-up time. The enormous sensitivity enhancement obtained with ONP is clear from comparing the two spectra. (Figures courtesy of K. Takeda and T. Terao, reprinted from Ref. [71], copyright 2001, with permission from Elsevier.)...

The accumulated 3-pulse stimulated photon-echo method " was used in order to monitor vibrational relaxation times of the first excited electronic state of pentacene. Two amplified dye lasers were used to perform ps photon-echo measurements on pentacene and naphthalene samples, which established that pseudo-local photon scattering was responsible for optical dephasing in vibronic transitions. A mode-locked cavity-dumped synchronously pumped dye laser system was used to demonstrate long coherence times for the delocalized optical excitation of dimer states, by ps photon-echo spectroscopy. ... 

Figure C3.5.10. Frequency-dependent vibronic relaxation data for pentacene (PTC) in naphthalene (N) crystals at...

Origin Wavelengths and Lifetimes at 1.5 K of Pentacene in -Terphenyl and Naphthalene... 

Fig. 20. Results of picosecond photon-echo decay measurements on the origin of pentacene in />-terphenyl and naphthalene as a function of temperature. Note that the echo decay term isir,.

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

Silinsh et al. (1989) applied their thermalization procedure to naphthalene and anthracene at low temperatures, 35 K or less. A stationary state was envisaged in the presence of an external field. Calculations have been performed for the saturation drift velocity, friction coefficients, and effective mass as functions of the external field. The conclusions are almost the same as for pentacene. 

To characterize the relative gas-chromatographic retentions of condensed aromatics and heteroaromatics, inclu g thienothiophenes, benzo[b]thiophene, dibenzothiophene, naphthobenzothiophenes, and anthrabenzothiophenes, a system of indices. In, was proposed, In this system a series of similar linearly condensed hydrocarbons (such as benzene, naphthalene, anthracene, tetracene, pentacene,...) was used as a reference scale. The logarithm of the corrected retention volume (adjusted to 0°), log Ft, depends linearly upon the number of condensed benzene rings (z) in the molecule, both in the polar and nonpolar phases. In is expressed by Eq. (58) ... 

Derivatives of the linear polyacenes, naphthalene,46 anthracene,41,90,93 naphthacene,91,92 and pentacene,92 form stable photodimers M2 when irradiated in concentrated Oa-free solution or (exceptionally) in the crystalline state.94,95 Transannular a-bonding of the molecular dimer components results in a folding of the aromatic planes about the bonded atoms and a reduced -electron delocalization reflected in a shift of the absorption spectrum to much higher frequencies.46,92,96... 

Give the number of lines in the ESR spectrum of the anion radical of each of the following molecules. (Assume all lines are resolved.) (a) Naphthalene (b) anthracene (c) pentacene (d) azulene (e) o-xylene (f) w-xylene (g) p-xylene (h) nitrobenzene (i) />-fluoronitrobenzene. 

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