S0 Fluorescence Excitation Research Articles

Electronic spectra of jet-cooled tropolone–Mn (n=1,2) clusters. Microscopic solvent effects on proton tunneling in the S1 state

The S1←S0 fluorescence excitation spectra of jet-cooled tropolone (TRN)–Mn (M=Ar, Kr, Xe, N2, CH4/CD4, C2H6, C3H8, CCl4; n=1,2) clusters have been measured in the wavelength region near the electronic origin to investigate the effects of van der Waals interactions on proton tunneling in the electronic excited S1 state. The solvation of TRN with the rare gas atom(s) has small effect on the 000 tunneling splitting, while the solvation with the molecule(s) considerably decreases the tunneling splitting. The decrease in the tunneling splittings of the TRN clusters has been explained by strong coupling of intermolecular vibration with intramolecular vibration of TRN, increasing the effective potential barrier height and/or tunneling distance. The anisotropy in the intermolecular interactions, and the configuration and number of solvent molecules are suggested to be important factors for the changes in the tunneling splitting.

Ar and CH4 van der Waals complexes of 1- and 2-fluoronaphthalene: A perturbed spherical top attached to a surface

We compare and contrast the low and high resolution S1←S0 fluorescence excitation spectra of four van der Waals complexes, Ar–1FN, CH4–1FN, Ar–2FN, and CH4–2FN (where 1FN and 2FN are 1- and 2-fluoronaphthalene, respectively) in the gas phase. Whereas the Ar and CH4 complexes exhibit comparable low resolution spectra, their high resolution spectra are significantly different. The CH4–1/2FN complexes exhibit origin bands that are each split into three distinct subbands with different intensities and separations of less than 1 cm−1. No such splittings are observed in Ar–1/2FN. The relative intensities of the three subbands in both CH4 complexes are 1:2:2. These are identical, within experimental error, to the total statistical weights of the J=0, 1, and 2 rotational levels of CH4. Both Ar and CH4 are weakly attached to 1/2FN at a distance of ∼3.5 Å above the aromatic plane. This distance decreases slightly (∼0.1 Å) on S1←S0 excitation. Thus, the splittings observed in CH4–1/2FN are attributed to ‘‘surface-induced’’ perturbations of the normally isotropic rotational motion of methane whose magnitudes depend on the electronic structure of the surface to which it is attached. A model is proposed that accounts for these observations. Comparison of the numerical predictions of this model with the experimental results shows that the rotational motion of the attached CH4 is nearly the same as that of the free molecule.

Discovery of two rotational isomers of 4-methoxystyrene by laser-induced fluorescence in a supersonic jet

The S1–S0 fluorescence excitation spectrum of 4-methoxystyrene seeded into a supersonic free jet shows two 000 bands, corresponding to the cis and trans rotational isomers. Dispersed fluorescence, with excitation in each of these bands, show that some of the resulting ground-state vibration wavenumbers are appreciably different. This confirms the 000 band assignments to different isomers. One prominent vibration has a wavenumber of 373 or 400 cm–1, depending on the isomer involved. These two bands are also observed in the Raman spectrum of the liquid and the fact that the 373 cm–1 band disappears in the solid-state Raman spectrum at 269 K identifies the 000 band of the more stable isomer. Ab initio calculations at the 6-31G* level indicate that the trans isomer is the more stable but by only 77 cm–1. This compares with a possible value of 174 cm–1 derived from experimental data.

Electronic spectra of o-, m- and p-tolunitrile—substituent effect on internal rotation of the methyl group

The S1 ← S0 fluorescence excitation spectra and the S1→S0 dispersed fluorescence spectra of o-, m-and p-tolunitrile were measured in supersonic jets. Low-frequency bands due to internal rotation of the methyl group were observed in m- and p-tolunitrile. Observed band positions and relative intensities of the internal rotational bands were reproduced by a calculation using a free rotor basis set. From the analysis, the potential curve of the internal rotation was determined in both S1 and S0. It was found that the barrier height increases in going from S0 to S1 in m-tolunitrile, while it decreases in p-tolunitrile. In contrast, no low-frequency band was found in o-tolunitrile. It is concluded that the potential curve in o-tolunitrile does not change in going from S0 to S1. The change of the barrier height by electronic excitation in tolunitriles differs greatly from that observed in other toluene derivatives. It is suggested that the electronic properties of a substituent are important for the methyl rotation in the excited state.

High resolution electronic spectroscopy of p-toluidine. A precessing rotor model for G12 molecules

Based on a study of the high resolution S1←S0 fluorescence excitation spectrum of p-toluidine (p-methylaniline) and related G12 molecules, we propose that the threefold axis of the methyl group is tilted slightly with respect to the symmetry axis of the molecular frame, and exhibits a kind of precessional motion in the course of its hindered internal rotation. We derive a new Hamiltonian to describe this motion and show that it is consistent with previous modifications of the traditional torsion–rotation Hamiltonian first proposed by Wilson, Lin, and Lide [J. Chem. Phys. 23, 136 (1955)]. Applying the new Hamiltonian to the S1←S0 spectrum of p-toluidine, we have determined the sixfold barrier heights V6(S0) = ( − ) 5.6 and V6(S1) = ( − ) 43.9 cm−1, values that are similar to those of toluene and other 4-substituted toluenes.

The fluorescence excitation spectrum of 1-naphthoic acid at rotational resolution: S and S1 potential energy surfaces along the R–COOH torsional coordinate

Several vibronic bands that appear within 300 cm−1 of the electronic origin in the S1←S0 fluorescence excitation spectrum of 1-naphthoic acid (1NA) and carboxyl-deuterated 1NA have been examined at full rotational resolution. The data show that all bands belong to the s-cis isomer of 1NA. They also show that all bands are torsional in nature; i.e., that they involve displacements along either the S0 or the S1 carboxyl torsional coordinate, φ, or both. Unambiguous assignments of the bands follow from the observed inertial defects, from which the torsional potential energy surfaces of both electronic states are derived. In S0 s-cis-1NA, the naphthalene and carboxyl groups are not coplanar. The s-cis minima are at φ=±17°; the barrier to planarity is 13.2 cm−1. In contrast, S1 s-cis-1NA is a completely planar molecule, with φ=0°. The barrier for s-cis to s-trans isomerization is ∼1450 cm−1 in the S0 state and ∼2200 cm−1 in the S1 state. The validity of the two derived potential energy surfaces of s-cis-1NA has been tested by comparing the observed inertial defects with those computed using the torsional eigenfunctions. Excellent agreement is obtained, confirming the one-dimensional approach employed.

ChemInform Abstract: High Resolution S1 ← S0 Fluorescence Excitation Spectra of Hydroquinone. Distinguishing the cis and trans Rotamers by Their Nuclear Spin Statistical Weights.

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High resolution S1←S fluorescence excitation spectra of hydroquinone. Distinguishing the cis and trans rotamers by their nuclear spin statistical weights

Hydroquinone (HYQ) has two rotational isomers, differing in the orientation of the two O–H bonds with respect to the benzene ring. In the S1←S0 fluorescence excitation spectrum of HYQ, two electronic origins, one for each rotamer, are present with a separation of 34.7 cm−1. Rotationally resolved spectra of both origins have been obtained and analyzed. This analysis reveals that the rovibronic lines in the two origins exhibit different alternating intensity patterns due to nuclear spin statistical weights, thereby providing a means for distinguishing the two rotamers. On this basis, we assign the origin at 33 500 cm−1 to the trans, C2h rotamer and the origin at 33 535 cm−1 to the cis, C2v rotamer of HYQ. Coordinate determinations of the hydroxy–hydrogen atoms of both rotamers, in both electronic states confirm this assignment.

Exploiting quantum interference effects for the determination of the absolute orientation of an electronic transition moment vector in an isolated molecule

Quantum interference effects have been observed in the O00 band of the fully resolved S1←S0 fluorescence excitation spectrum of the hydrogen bonded complex of trans-2-naphthol with ammonia (t-2HNA). The effects owe their origin to the simultaneous existence of (1) ab hybrid band character in the spectrum; (2) perturbations in the spectrum that are produced by a coupling of overall rotation with the hindered internal rotation of the attached NH3 group; and (3) low barriers along the torsional coordinate in both electronic states. Analysis of the resulting interference patterns makes possible an unambiguous determination of both the magnitude and the sign of the orientation of the transition moment vector with respect to the a inertial axis in t-2HNA, θTM=−66±2°. Comparison of this result with that for the S1←S0 transition of the bare molecule shows that the orientation of its transition moment in the molecular frame is unaffected by complex formation. Additionally, symmetry-based arguments are given that may be used to predict the existence of quantum interference effects of this type in other systems.

Electronic spectra of jet-cooled 5-bromotropolone and 5-chlorotropolone. Influence of symmetrical substitution on proton tunneling in the S1 state

The S1–S0 fluorescence excitation and dispersed fluorescence spectra have been measured for jet-cooled 5-bromotropolone (5BTR-h), 5-chlorotropolone (5CTR-h), and their OD derivatives in order to investigate substituent effects on proton tunneling. Both the 0++ and 0−− transitions were identified in the fluorescence spectra of these molecules. The tunneling doublet splittings in the electronic origin band of 5BTR-h (16 cm−1) decreased by 4 cm−1, whereas that of 5CTR-h (23 cm−1) increased by 3 cm−1 as compared with tropolone (TRN-h). Relatively small deviation in the tunneling separations for these molecules from that for tropolone is ascribed to conjugation in the S1 state. An electron-withdrawing character of chlorine and bromine atoms is almost canceled by a conjugative electron-releasing effect.

Fluorescence excitation spectra of the S1 states of isolated trienes

First observation of fluorescence for simple, linear trienes is reported. S1←S0 fluorescence excitation spectra of hexatriene and octatriene indicate large differences between the S0 and S1 potential energy surfaces. Activation energy of <200 cm−1 for the S1 state nonradiative decay is tentatively ascribed to isomerization.

Spectroscopic and dynamical studies of the S1 and S2 states of decatetraene in supersonic molecular beams

Fluorescence and fluorescence excitation spectra of all-trans-2,4,6,8-decatetraene have been obtained in free jets and in inert-gas clusters. In isolated decatetraene, excitation into 1 1Bu (S2) results in emission from both S2 (1 1Bu→1 1Ag) and S1 (2 1Ag→1 1Ag) on time scales that are faster than the 10 ns experimental resolution. In clusters, rapid electronic and vibrational relaxation leads to long-lived (360 ns) emission from thermally relaxed levels of S1. Direct excitation of low-lying, S1 vibronic levels in cold, isolated molecules also results in long-lived S1→S0 fluorescence, as expected for this symmetry-forbidden transition. The detection of S1 emission in free decatetraene has permitted the first detailed study of the vibronic structure and kinetics of the 2 1Ag state of an isolated, all-trans linear polyene. The S1←S0 fluorescence excitation spectrum is rich in low-frequency vibronic progressions. Analysis of this spectrum suggests that the transition not only is made allowed by vibronic coupling involving low-frequency bu skeletal modes (Herzberg–Teller coupling), as for polyenes in condensed phases, but also gains intensity from interactions between the electronic motion and the hindered rotations (torsions) of the terminal methyl groups. Preliminary analysis suggests that the barriers to internal rotation of the methyl groups must be substantially reduced in the 2 1Ag (S1) state. For isolated decatetraene, the 2 1Ag fluorescence lifetimes show a monotonic decrease with increasing vibrational energy, presumably due to increased mixing with the 1 1Bu state.

Electronic Spectra of 1‐Phenylethylamine and its Derivatives in Supersonic Jets

The S1 ← S0 fluorescence excitation spectra and dispersed fluorescence spectra of jet‐cooled (+)‐, (‐)‐ and (±)‐1‐phenylethylamine and their derivatives (amides) have been observed. The 0‐0 band of the amine locates at 37,641 cm‐1. The amides which were synthesized from (+)‐amine or (‐)‐amine with (+)‐tartaric acid are diastereomers. It was found that the two diastereomers give the identical spectra with the 0‐0 band at 34,757 cm-1. No difference in the spectrum indicates that the excitation is localized in the phenyl group which is far from the asymmetric carbon causing diastereoism. It was also found that 1‐phenylethylamine has a fast nonradiative relaxation process in the Sstate, but such a process is removed by the formation of the amide.

ChemInform Abstract: Ring Torsional Dynamics and Spectroscopy of Benzophenone: A New Twist.

The low energy portion of the high resolution S1←S0 fluorescence excitation spectrum of benzophenone recently reported by Holtzclaw and Pratt [J. Chem. Phys. 84, 4713 (1986)] is modeled here using a simple two‐degree‐of‐freedom vibrational Hamiltonian. The Hamiltonian features a 1:1 nonlinear resonance between the two low frequency ring torsional modes of the molecule in its S1 state. Line positions and intensities of the two major spectral progressions are well reproduced using parameters similar to those derived from earlier matrix diagonalizations. The comparison of the theory and experiment results in a determination of the displacement of the S1 surface relative to the ground electronic state along the symmetric torsional coordinate and permits a calculation of the excitation spectra of various isotopically substituted molecules not yet measured in the laboratory. A clear picture of the relationship between the dynamics on the S1 surface and the spectroscopy of benzophenone is revealed by comparing a...

Ring torsional dynamics and spectroscopy of benzophenone: A new twist

The low energy portion of the high resolution S1←S0 fluorescence excitation spectrum of benzophenone recently reported by Holtzclaw and Pratt [J. Chem. Phys. 84, 4713 (1986)] is modeled here using a simple two-degree-of-freedom vibrational Hamiltonian. The Hamiltonian features a 1:1 nonlinear resonance between the two low frequency ring torsional modes of the molecule in its S1 state. Line positions and intensities of the two major spectral progressions are well reproduced using parameters similar to those derived from earlier matrix diagonalizations. The comparison of the theory and experiment results in a determination of the displacement of the S1 surface relative to the ground electronic state along the symmetric torsional coordinate and permits a calculation of the excitation spectra of various isotopically substituted molecules not yet measured in the laboratory. A clear picture of the relationship between the dynamics on the S1 surface and the spectroscopy of benzophenone is revealed by comparing a time domain analysis of the experimental data with wave packet dynamics on the model S1 surface. This comparison provides new insight into energy flow in the isolated molecule and permits a qualitative simulation of the effects of collisional quenching on the fluorescence spectrum. We also discuss, using a classical trajectory analysis, the resonance dynamics of the torsional modes and note the existence of heretofore undetected local modes in the high resolution spectrum.

C i s-glyoxal in effusive and supersonic beams. Spectroscopy, selective pumping, and c i s–t r a n s interconversion

S 1–S0 fluorescence excitation and dispersed fluorescence spectra from molecular beams containing both trans- and cis-glyoxal have been used to extend the characterization of the 1A1 (S0) and 1B1 (S1) states of cis-glyoxal. Explorations using both effusive and supersonic beams with rotational temperatures ranging from 350 to 30 K have revealed no conditions where cis can be pumped (S1←S0) without simultaneous excitation of trans. Selective cis excitation at low beam temperatures is hampered by highly efficient cis→trans conformational interconversion in the molecular beam expansions. Under conditions of optimal cis:trans contrast (cool expansions with Ar carrier gas), four new S1–S0 cis absorption bands (510,520,610, and 720 ) are identified, yielding cis frequencies ν′5 =303 cm−1, ν6 =713 cm−1, and 2ν′7 =688 cm−1. Single vibronic level fluorescence spectra have been obtained from the levels 00, 51, and 61 of cis-glyoxal, from which values of two cis S0 fundamentals are newly established: ν4 =826 cm−1 and ν′′6 =1049 cm−1. Previous assignments of ν4 and ν′′8 are shown to be incorrect and ν8 now joins the list of unknown frequencies. The 1B1–1A1 system of cis-glyoxal contains forbidden transitions, vibronically induced by Δv=±1 changes in the a2 mode ν6. A remeasurement of the cis–trans energy separation in the ground electronic state gives ΔH=1350±200 cm−1, matching to within experimental uncertainty a previous experimental determination. As an aside, the trans-glyoxal fundamental ν′′3 =1352 cm−1 has been obtained from observations of the trans 301 and 301510 transitions. With this addition, all trans S0 fundamentals have now been directly measured.