Fluorescence of nonlinear optical organic single crystal of 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) excited by a nonabsorbed light pulses from Q-switched Nd:YAG laser λ = 1064 nm as well as absorbed λ = 532 nm light is reported. Two mechanisms of two-photon excited fluorescence are considered: (i) direct two-photon excited fluorescence and (ii) single-photon excitation due to reabsorption of light generated in process of second harmonic generation (SHG) by the crystal due to its nonlinear optical properties. Strong anisotropy of fluorescence that has been observed is linked with uniaxial molecular alignment. Fluorescence decay profile shows two- exponential decay with lifetimes of emitting species of 3.7 and 5.6 ns at 293 K. The excitation and fluorescence spectra of the DCNP single crystal have been measured at 294 K and in function of temperature down to 77.4 K. The strong bathochromic shift of fluorescence spectrum in crystal with respect to fluorescence of DCNP molecule in solution is observed and interpreted with possible formation of molecular aggregates.
Articles you may be interested inLow temperature single molecule spectroscopy using vibronic excitation and dispersed fluorescence detection The Shpol'skii system perylene in n-hexane: A computational study of inclusion sitesWe examine experimentally and computationally the nature of substitution of perylene in polycrystalline solid alkane matrices ͑Shpol'skii systems͒. The technique of low temperature excitation-emission matrix spectroscopy is used to determine all substitution sites in alkane matrices from hexane to decane. A theoretical method from the group of Jortner ͓Shalev et al., J. Chem. Phys. 95, 3147 ͑1991͔͒, which was extended and applied by us to this problem in the past ͓Wallenborn et al., J. Chem. Phys. 112, 1995 ͑2000͔͒, allows one to separate the perylene sites in all alkanes into normal and defective sites. Normal sites are obtained by direct substitution of two alkane molecules by a perylene molecule, while defective sites are derived from normal sites by eliminating one of the four nearest neighbors of perylene in the lattice planes parallel to the chromophore. We discuss the strengths and limitations of the present theoretical treatment, which can serve as a valuable supplement and guide to line-narrowing and single-molecule spectroscopic investigations of impurity centers in low-temperature solids.
We present a combined quantum mechanics/molecular mechanics study of the Shpol'skii system perylene/n-hexane. The system was modeled utilizing a customized pcff-derived force field optimized with a balanced set of optimization criteria based on geometry, vibrational modes, and the energies and forces in an ensemble of molecular geometries. Spectral shifts were calculated perturbatively using the method of Shalev et al. ͓J. Chem. Phys. 95, 3147 ͑1991͔͒. The calculated shifts are within the experimental uncertainty of the observed 0-0 lines and allow an unambiguous assignment of the three most prominent sites. The proposed assignment differs from that of a previous study.
Fluorescence emission and excitation spectra of tenylene in several n-alkane Shpol'skii matrixes have been studied at liquid helium temperatures. The presence of two dominant Shpol'skii sites has been detected in all matrixes; we tentatively attribute one of the sites to tenylene substitutionally entering the host lattice, and the other one to guest molecules situated in the region of defects. The experiments, supported by quantumchemical calculations, point to the existence of two stereoisomers of the solute. We found n-decane to form the best matrix for tenylene; the system seems to be suitable for single molecule spectroscopy studies.
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