Andrea L. Motyka

Picosecond time-resolved S2→S0 fluorescence of xanthione in different fluid solvents

Time-correlated single-photon counting experiments at ≈ 25 ps resolution have been used to measure the S2 fluorescence decay time of xanthione in different fluid solvents. Systematic differences in S2 lifetime in the range 20–30 ps are revealed for a homologous series of n-alkane solvents, and an increase in lifetime for extensive chain branching is also noted. Solutions of xanthione in aromatic solvents and in alcohols are found to exhibit fluorescence decay limes < 20 ps. Little evidence is seen which would support a nonradiative relaxation mechanism involving Franck-Condon selective energy transfer to high-energy vibrational modes of the solvent. Instead, the current results provide support for a mechanism in which a reversible solvent-solute chemical reaction, probably involving bond insertion, is responsible for the solvent dependence of the S2 fluorescence decay time.

Two-photon threshold ionization spectroscopy of perylene and van der waals complexes

Abstract Ionization thresholds have been determined for perylene and some of its van der Waals complexes. The Ar 1 complexes have an ionization energy 46 cm −1 lower than bare perylene. This number compares favorably with recent data for tetracene, but is 4–6 times smaller than values obtained for argon complexes of single-ring aromatics. The (CH 4 ) 1 complex of perylene shows a rather similar shift to the argon case, 52 cm −1 , the n -pentane complex has a red-shift of 10 cm −1 and the n -octane complex exhibits a blue -shift of 23 cm −1 (all values ±4 cm −1 ). In contrast, benzene/perylene has a large ionization red-shift of 700 cm −1 . These data, together with results for larger clusters of Ar and CH 4 , indicate a strong dependence of the ionization energy shift on the spatial distribution of the adduct species with respect to the perylene substrate. With the possible exception of benzene complexation, the δ I values are much smaller than might have been expected on the basis of previous work.

Mode- and complexation-specific singlet-triplet coupling in jet-cooled perylene studied by time-resolved fluorescence spectroscopy

Abstract Fluorescence spectral data for jet-cooled perylene indicate little or no vibrational coupling for most levels up to 1057 cm −1 . Fluorescence time profiles have been measured for several levels, and evidence presented for several cases of coupling to the triplet manifold. The principal Franck-Condon-active mode 353 1 exhibits a single-exponential decay, although the emission spectrum indicates little or no singlet vibrational level mixing. The (CH 4 ) 1 complex of perylene, also excited into 353 1 , exhibits a single-exponential profile with a fast decay, again indicating strong singlet-triplet coupling, while the 2 : 1 and 3 : 1 complexes at the same energy are strongly fluorescent. These results imply that singlet-triplet coupling is level-selective, and can be induced by complexation.

Time-resolved desorption of argon and methane from the surface of a perylene molecule

Abstract Time-resolved fluorescence measurements have been made of the predissociation of van der Waals complexes of perylene with Ar and CH 4 . At 705 cm −1 , predissociation of Ar 1,2 complexes occurs in competition with efficient singlet-triplet coupling. At 900 cm −1 , both Ar and CH 4 complexes show little contribution from singlet-triplet coupling. The predissociation of CH 4 complexes at this energy is very rapid, largely as a result of extensive vibrational coupling.

Weak vibrational coupling in a large van der Waals complex: Fluorescence spectroscopy of perylene/naphthalene

Fluorescence excitation and dispersed fluorescence spectra are reported for 1:1 and 2:1 complexes of naphthalene with perylene under supersonic jet conditions. Confirming preliminary results, the fluorescence spectra of the 1:1 complex following excitation of an ag (in‐plane) mode at 353 cm−1 and its first overtone show unusually weak vibrational coupling. Although excitation of combination levels of 3531 with out‐of‐plane modes at 74, 79, and 93 cm−1 gives rise to emission which is predominantly ‘‘relaxed,’’ the residual ‘‘unrelaxed’’ component indicates a significant degree of mode‐selective vibrational coupling. It is notable that the vibrational coupling for 3532 excitation (i.e., at ≈700 cm−1) is substantially less extensive than for excitation into the 3531 combination bands nearly 300 cm−1 lower in energy. A similar comparison has been made between a second ag mode, at 550 cm−1, and a perturbed b3g (out‐of‐plane) mode, at 540 cm−1. In this case, the data indicate a difference in coupling, which is ...

Non-radiative relaxation in jet-cooled xanthione and some molecular complexes studied by fluorescence and phosphorescence excitation

Abstract The S 0 →S 2 excitation spectra for S 2 →S 0 fluorescence and T 1 →S 0 phosphorescence are reported for jet-cooled xanthione and some molecular complexes. Phosphorescence and fluorescence excitation were monitored simultaneously, and from their difference spectra it has been determined that the phosphorescence/fluorescence ratio is affected both by predissociational cooling of the triplet state and by non-radiative deactivation of the S 2 state. Evidence is presented for mode-specificity in the S 2 non-radiative relaxation of the bare molecule.

The effect of molecular symmetry in alkane complexes of 2,5-diphenylfuran and related species

Abstract Fluorescence excitation spectra are reported for van der Waals complexes of 2,5-diphenylfuran (PPF), 2,5-diphenyloxazole (PPO) and 2,5-diphenyloxa-1,3,4-diazole (PPD), with a number of normal alkanes ranging from pentane to dodecane, measured under supersonic jet conditions. It is shown that referencing the spectral red shifts for alkane complexes against the corresponding pyrene S0 → S2 data provides a useful means of detecting structural variations. The PPF complexes with alkanes from heptane to dodecane show an odd—even effect, where n-alkane guests of different symmetries give rise alternately to single and doubled resonances. Thus, even-alkane complexes have two indistinguishable isomers of C1 symmetry, which contribute to a single spectrum. On the other hand, odd-alkane complexes also have a Cs isomer, which has a slightly different electronic origin resonance, so that a double spectrum results, with spacing on the order of 10 cm−1. These conclusions are consistent with the general predictions of atom—atom pair-potential calculations. We also show that further splitting may be induced in the spectra by complexation of the asymmetrical molecule 2,5-diphenyloxazole (PPO). Complexes with the shorter alkanes pentane and hexane show increased complexity in the spectrum, which is attributed to greater flexibility in the complexation geometry.

Jet spectroscopy of organic scintillation compounds: low-frequency modes of 2,5-diphenylfuran and related species

Abstract The fluorescence excitation spectra of a number of furan-based scintillation compounds, including 2,5-diphenylfuran (PPF), have been measured under jet-cooled conditions. While many of the species are effectively planar, steric crowding is found to force diphenylisobenzofuran into a non-planar ground state, so that two rotameric forms are observed. The molecule PBD is also found to be non-planar, consistently with literature data for biphenyl itself. Finally, evidence of torsional mode coupling is observed for α-naphthyl-substituted compounds.

Electronic coupling and conformational barrier crossing of 9,9’‐bifluorenyl studied in a supersonic jet

Fluorescence excitation, dispersed fluorescence, and picosecond time‐resolved fluorescence spectroscopies have been used to study the dimeric molecule 9,9’‐bifluorenyl, isolated under supersonic jet conditions. The excitation spectrum reveals a splitting in several principal resonances of the S0→S1 excitation spectrum, which can be attributed to electronic coupling between the two fluorenyl moieties. The splitting, which for different vibronic transitions correlates with Franck–Condon factors, is consistent with an exciton model that includes higher electronic states. The splitting is reasonably well reproduced by a multipole interaction potential. Calculations have verified that the electron exchange coupling is small. Furthermore, the relative intensities of the resonances allow an estimate of the equilibrium geometry, suggesting that the molecular long axes have an angular displacement of 60°–70°, consistent with the geometry found by x‐ray diffraction. A most interesting feature of this species is tha...

Picosecond time resolution of the S2 fluorescence of jet-cooled xanthione

Abstract The fluorescence lifetime of the S 2 electronic level of xanthione has been measured under supersonic jet conditions to be 345±10 ps. Fluorescence lifetimes ranging from 240 to 370 ps were also measured, following excitation into different S 2 vibrational levels. This confirms that, in addition to a well known solvent and internal CH mode dependence, the non-radiative coupling of S 2 to lower electronic states is also a function of the vibrational mode excited.