S0 Fluorescence Excitation Research Articles

Rotationally resolved electronic S1 spectra of tryptoline and oxindole: Reversal of the 1La and 1Lb state character

Rotationally resolved S1←S0 fluorescence excitation spectra of the lowest electronic transitions (origin bands) of tryptoline (TRY) and oxindole (OXI) have been recorded and analyzed. A higher vibronic band of OXI has also been observed and assigned. The derived inertial parameters from the fits provide detailed information about their conformational or tautomeric structures in both the ground and excited electronic states. These molecules exhibit significantly different transition dipole moment orientations relative to the parent reference frame of indole that signify S1 state characters having principal parentage in a 1La state for OXI and a 1Lb state for TRY.

The structure of 2-methylpropanal molecule in the S1 lowest excited singlet electronic state: theoretical and experimental studies

We have obtained and analyzed the S1 ← S0 fluorescence excitation spectrum of jet-cooled 2-methylpropanal ((CH3)2CHCHO). In addition, the ab initio calculations of the 2-methylpropanal structure in the ground (S0) and lowest excited singlet (S1) electronic states have been carried out by the CASPT2/def2-TZVPP method. The calculations have shown that the S1 ← S0 electronic excitation of the 2-methylpropanal conformers (cis and gauche) causes large conformational changes: the isopropyl group rotates around the central C–C bond and the carbonyl fragment CCHO becomes non-planar. The potential energy surface of 2-methylpropanal in the S1 state has the six minima, corresponding to three pairs of mirror symmetry conformers: 1ab, 2ab, and 3ab. In the S1 ← S0 jet-cooled spectra of (CH3)2CHCHO and (CH3)2CHCDO three systems of vibronic transitions from the gauche (S0) conformer to vibrational levels of the 1, 2, and 3 conformers (S1) with the origins ( $$ {0}_0^0 $$ ) at 29,887; 30,198; and 29,963 cm−1 for (CH3)2CHCHO and 29,926; 30,221; and 29,990 cm−1 for (CH3)2CHCDO have been identified. The high activity of torsional and inversional vibrations of the 1, 2, and 3 conformers has been found for both molecules. Intensities of bands in torsional sequences pass through a maximum; it confirms the theoretical prediction about the minima shifts between the gauche and 1, 2, and 3 conformers along the torsional coordinate. Using the experimental inversional energy levels, the 1↔3 and 2a↔2b potential functions of inversion have been determined for both molecules. The corresponding calculated and experimental data are in satisfactory agreement.

The S1 ← S0 fluorescence excitation spectrum and structure of propanal in the S1 excited electronic state.

We have obtained and analyzed the S1 ← S0 fluorescence excitation spectra of jet-cooled propanal-h1 (CH3CH2CHO) and -d1 (CH3CH2CDO). Using the results of theoretical studies of the structure of propanal molecule in the S1 lowest excited singlet electronic state, we have assigned the bands of both spectra to the vibronic transitions of the cis conformer (in the S0 ground electronic state) to the 1 and 3 conformers (in the S1 state) differed by the angle of the C2H5 ethyl group rotation around the central C-C bond. The origins of the 1 ← cis and 3 ← cis electronic transitions have been observed at 29 997 and 30 075 cm(-1) for propanal-h1 and at 30 040 and 30 115 cm(-1) for propanal-d1, respectively. The high activity of torsional (C2H5 ethyl groups) and inversional (CCHO/CCDO carbonyl fragments) vibrations and the intensity distribution of the bands in torsional sequences (passing through maximum) are in agreement with the theoretical prediction that the S1 ← S0 electronic excitation of the cis conformer causes (after geometrical relaxation) the pyramidalization of carbonyl fragments and the rotation of ethyl groups around the central C-C bond. A number of energy levels have been found for torsional and inversional vibrations, and also fundamentals of ν10 (CCO bend) and ν13 (CCC bend) for the both 1 and 3 conformers of propanal-h1 and -d1 have been found. Then the "experimental" potential functions of inversion for the pair of the 1 and 3 conformers have been determined. The heights of potential barriers to inversion and the angle values corresponding to the minima of potential functions of inversion are 900 cm(-1) and 35° for propanal-h1 and 820 cm(-1) and 34° for propanal-d1, respectively.

Lifetime Broadening in the Rotationally Resolved Electronic Spectra of Dibenzothiophene, 2,5-Diphenylfuran, and 2,5-Diphenyl-1,3,4-oxadiazole in the Gas Phase. Intersystem Crossing Dynamics in the Statistical Limit.

The fluorescence lifetime of the zero point vibrational level of the first excited electronic state of dibenzothiophene (DBT) has been determined to be 1.0 ns by analysis of its rotationally resolved S1 <-- S0 fluorescence excitation spectrum. The S1 lifetime of DBT is substantially shorter than those observed for fluorene (FLU), carbazole (CAR), and dibenzofuran (DBF), analogs of DBT in which the heavy sulfur atom is replaced by lighter ones. The electronic origin bands through the series CAR, FLU, DBF, and DBT exhibit a monotonic increase in Lorentzian broadening in their Voigt line shape profiles. Two other heterocyclic molecules manifest similar photophysical properties; 2,5-diphenylfuran and 2,5-diphenyl-1,3,4-oxadiazole. Lorentzian line shape broadenings of approximately 76 MHz were observed in the high-resolution spectra of their origin bands. Possible reasons for the short fluorescence lifetimes of these heterocycles are discussed.

Tryptamine in the gas phase. A high resolution laser study of the structural and dynamic properties of its ground and electronically excited states

High resolution S1←S0 fluorescence excitation spectra of tryptamine have been observed in the collision-free environment of a supersonic molecular beam. Each origin band has been assigned to a unique conformer of the isolated molecule based on its observed rotational constants. For the first time, subbands have been detected in the rotationally resolved spectra of bands Cblue and D. A possible hindered motion is proposed to account for the appearance of these subbands. This motion connects the minima associated with the Antipy and Antiph conformers, and thus explores new regions of the energy landscape of this important biomolecule.

Rotationally resolved electronic spectroscopy of tryptophol in the gas phase

High resolution S1-S0 fluorescence excitation spectra of tryptophol have been observed in the collision-free environment of a supersonic beam. Each origin band has been assigned to a unique conformer based on its observed rotational constants. Unlike its close relative tryptamine, which exhibits seven distinguishable conformers under similar conditions, tryptophol exhibits only four (GPy-in, GPh-in, and two anti structures). Possible reasons for this difference in behavior are discussed.

High resolution electronic spectroscopy of three n-alkylbenzenes: ethyl-, propyl-, and butylbenzene

Rotationally resolved S1−S0 fluorescence excitation spectra of ethylbenzene, two conformers of n-propylbenzene, and two conformers of n-butylbenzene have been observed and assigned. The data obtained provide information about the equilibrium properties of each molecule, including their geometries in the S1−S0 states, their electronic distributions, and their dynamical behavior following the absorption of light. Trans structures are found to have S1 states that are Lb1 in character with relatively long fluorescence lifetimes. Gauche structures are found to have S1 states that are mixed (1Lb/1La) in character with relatively short fluorescence lifetimes. Possible reasons for these differences in properties are discussed.

Probing nuclear quadrupole interactions in the rotationally resolved S1←S electronic spectrum of 2-chloronaphthalene

Rotationally resolved S1←S0 fluorescence excitation spectra of 2-chloronaphthalene (2ClN) are measured using a newly constructed ultraviolet (UV) laser/molecular beam spectrometer. More than 1000 well-resolved rotational lines are recorded at a resolution of ∼3 MHz for the two overlapping band origins of the Cl35 and Cl37 isotopomers (Δνorigin∼3 GHz) over a 3.5 cm−1 spectral region at a rotational temperature of 10 K. Both spectra are hybrid bands, having 18% a-/82% b-type character in accordance with La1←1A1 excitations observed for other 2-substituted naphthalenes. Additionally, Cl nuclear quadrupole splittings are observed as broadened asymmetric line shapes with linewidths from 5 to 20 MHz. Least-squares fits of these line shapes coupled with high-precision Fourier-transform microwave measurements of the ground state have provided accurate S1 quadrupole coupling constants. Specified relative to the principal quadrupole tensor orientation of S0, the S0 and S1 values of eQqzz are −71.198(5) MHz and −67.8(30) MHz, and the asymmetry parameters, η=(eQqyy−eQqxx)/eQqzz, are 0.076 38(5) and 0.13(5), respectively. The experimental line shapes are fit to Voigt functions, yielding 4.9(3) MHz homogeneous (lifetime) and 3.0(2) MHz inhomogeneous (Doppler) contributions. All of the observed structure are well reproduced by more than 20 000 transitions predicted for the nuclear-quadrupole-split rotational spectra of the two hybrid-band origins. Furthermore, the homogeneous linewidths show no dependence on either the upper state energy or angular momentum quantum numbers. The absence of additional level structure at the resolution of 1 MHz or more, from heavy-atom-enhanced intersystem crossing mechanisms, is discussed within the framework of radiationless transition theory. The quadrupole tensor components are used in simple theoretical models to investigate the orbital character of the S1 state and its radiationless decay mechanisms.

Stark Effects in Gas-Phase Electronic Spectra. Dipole Moment of Aniline in Its Excited S1 State

Measurements of the Stark effect on the rotationally resolved S(1)<--S(0) fluorescence excitation spectrum of aniline are reported, providing quantitative information about the degree of charge transfer in the electronic transition. We find that mu(a)(S(1)) = 2.801 +/- 0.007 D, a value that is approximately 150% larger than the ground state, mu(a)(S(0)) = 1.129 +/- 0.005 D. The enhanced value of the dipole moment in the S(1) state is attributed to more efficient electron donation by the quasi-planar amino group to the aromatic ring.

Toluene internal-rotation: Measurement and simulation of the high-resolution S1–S fluorescence excitation spectrum at 0.5 K

Rotational structure in the origin band of the S1–S0 fluorescence excitation spectrum of toluene has been measured at 0.012 cm−1 resolution with a rotational temperature of 0.5 K using a pulsed beam apparatus. Such spectra have been obtained for the parent isotopomer and for the isotopomers with mono- and di-deuterated methyl groups. These, and previously known forbidden bands in which the internal-rotation quantum number Ki changes, are simulated here using ab initio internal-rotation-angle-dependent geometries, potential functions, and electronic transition moment function. An adjustment of some of the ab initio parameters allows a close fitting of the spectra to be made, and this can only be achieved if bond lengths and angles are allowed to vary with internal rotation. The resulting geometries for ground and excited-state toluene are the most accurate to date.

Polarized fluorescence of β-carotene and related polyenes

The spectral and polarization characteristics of trans-isomers ofβ-carotene, lycopene, retinal, and 1,6-diphenylhexatriene (DPH) in solutions have been investigated by the methods of polarized fluorescence and semiempirical calculations of AM1 and RM3. The high and constant values of the degree of polarization P ∼0.46, closely coinciding with the limiting value P0=0.5, according to the S2→S0-fluorescence and S2←S0-fluorescence excitation spectra ofβ-carotene and lycopene in n-hexane at room temperature and also high and coinciding according to the S1→S0-fluorescence and S2←S0-fluorescence excitation spectra for retinal in EPA at 77 K and DPH in a polyvinylbutyral film, point to the coincidence of the directions of absorption and emission oscillators. These oscillators are aligned along the major axis of the polyene molecule, and the longwave S2←S0-absorption band is formed only by one electronic1B u + ←1A g − -transition. The oscillators of shortwave Sa←S0-absorption bands are aligned at an angle to the oscillators of the S2←S0-transitions, which is related to the distortions in the structure of molecules (β-carotene, lycopene) or, as in the case of DPH, to the influence of transitions localized on the orbitals of phenyl rings and polarized perpendicularly to the major axis of the molecule.

Torsional symmetry dependence of S1 dynamics in molecules that undergo methyl internal rotation

The single-rovibronic-level fluorescence of “intermediate-case” molecules that undergo methyl internal rotation is strongly influenced by the torsional symmetry of the lowest excited singlet state (S1). The most dramatic example of such symmetry dependence comes from our recent finding that the intensities of the e–e transitions in the high-resolution S1←S0 fluorescence excitation spectra of jet-cooled acetaldehyde become very weak relative to the a–a transitions at higher beam temperatures. In this study, we rationalize this remarkable torsional symmetry dependence of electronic relaxation in acetaldehyde on the basis of internal-overall rotation coupling that leads to symmetry-selective increase in the density of states for singlet-triplet coupling. Related observations by others on aliphatic carbonyls and diazabenzenes are also discussed within the context of the coupling between the internal and overall rotation. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 167–176, 1999

Indole−H2O in the Gas Phase. Structures, Barriers to Internal Motion, and S1 ← S0 Transition Moment Orientation. Solvent Reorganization in the Electronically Excited State

Rotationally resolved S1 ← S0 fluorescence excitation experiments have been performed on a solute−solvent complex of indole and water in a molecular beam. The results show that the complex whose S1 ← S0 origin is shifted by 132 cm-1 below the bare molecule origin is a 1:1 complex, with the water molecule linked to the indole frame via a quasi-linear N−H···OH2 σ hydrogen bond. The results also show that both the position and the orientation of the attached water molecule change when the photon is absorbed. The properties of the intermolecular potential-energy surfaces that govern these motions in both electronic states are derived from an analysis of the high-resolution spectrum.

S2–S0Spectroscopy of Azulene and Its 2-Chloro, 2-Methyl, and 1,3-Dimethyl Derivatives

The laser-inducedS2–S0fluorescence excitation and dispersed emission spectra of azulene and three azulene derivatives ofC2vskeletal symmetry, 2-chloro-, 2-methyl-, and 1,3-dimethylazulene, have been measured under ultracold, isolated conditions in a supersonic expansion. The FT-IR spectra of the derivatives have been measured, and the ground state vibrational frequencies have been calculated, with scaling, from the force fields by anab initiomethod at the RHF/CEP-31G level for all four compounds to aid in the assignments of these spectra. To achieve global consistency, the vibrational assignments of the ground and second excited singlet states of azulene have required refinement; the reassignments have implications for the vibronic and vibrational coupling models used in existing interpretations of itsS2–S0spectra and theS2photophysics. The results suggest that small changes of the energy gap between theS2andS1states which result from substitutions in the ring produce large changes in both theS2–S0fluorescence excitation spectra and the photophysics of azulene. Substitution at the 2-position accelerates the rate of radiationless relaxation, whereas this rate remains nearly unaffected by substitution at the 1,3-positions when compared with azulene itself. The threshold for intramolecular vibrational redistribution is significantly reduced in the methyl-substituted azulenes, consistent with an increase in the density of coupled states.

Hydrogen bonding in acid–base complexes: The trans-hydroquinone-NH3 complex in its S and S1 electronic states

We deduce information about the geometry of the hydrogen bond between hydroquinone and ammonia from an analysis of the fully resolved S1←S0 fluorescence excitation spectrum of the 1:1 complex in the gas phase. The complex is planar in both electronic states, with NH3 forming a nearly linear hydrogen bond to one of the hydroxy hydrogen atoms of hydroquinone. The O–H…N heavy atom separation is R=2.85 Å and the barrier to internal rotation of the NH3 group about its C3 axis is V3=35.5 cm−1 in the S0 state. Excitation of the complex to its S1 state decreases the heavy atom separation to R=2.77 Å and increases the torsional barrier to V3=58.8 cm−1. These changes are a direct consequence of the increased acidity of hydroquinone in its S1 state.

The Relative Influences of Ring Modes and Methyl Internal Rotation on the Cold Beam IVR of p‐Fluorotoluene

AbstractRoom temperature (300 K) experiments have earlier established that the replacement of a fluorine atom in p‐difluorobenzene (pDFB) with a methyl group to make p‐fluorotoluene (pFT) generates a qualitative difference in intramolecular vibrational redistribution (IVR) characteristics as seen in the S1 states. Here we report S1–S0 fluorescence excitation and dispersed single vibronic level (SVL) fluorescence spectra that have been obtained for IVR comparisons in the cold (5 K) environment of supersonic expansions. As in the 300 K experiments, the cold beam S1 vibrational energy threshold for IVR is substantially lower in pFT. The vibrational congestion in dispersed fluorescence that reveals extensive S1 level interactions first appears after pumping an S1 pFT level near 800 cm−1. In contrast, congestion in pDFB spectra is still absent from levels with twice this energy. Attention is directed to the relative S1 ring level structures as a potential contributor to the distinctive IVR behaviors. Dispersed fluorescence from the S1 zero point levels and fluorescence excitation spectra are combined with published information about S1 fundamentals to show that the S1 vibrational level structures of the two molecules are as closely related as those of an isotopomer pair. It is argued that the small differences in S1 fundamentals cannot be a principal contribution to the qualitative IVR differences.

S2–S0Spectra of Jet-Cooled Xanthione and Benzopyranthione: Microscopic Solvation by Rare Gases

The laser-inducedS2–S0fluorescence excitation and dispersed emission spectra of two aromatic thiones, xanthione (XT) and benzopyranthione (BPT), have been measured under jet-cooled conditions. These emission spectra, together with the corresponding FT-IR and FT-Raman spectra andab initioCEP-31G calculations, have been used to achieve a complete vibrational analysis of BPT in its ground state, complementing that of XT, previously published. The 1:1 and 1:2 van der Waals complexes of XT and BPT with He, Ne, Ar, Kr, and Xe have been synthesized and characterized using their microscopic solvation shifts and the frequencies of their intermolecular (van der Waals) vibrations. In all cases the adatom preferentially occupies a position over the pyranthione ring in the 1:1 complex, and the 1:2 complex has a symmetrical sandwich (1|1) structure. No (2|0) isomer is observed. An analysis of the microscopic solvent shifts of the origin bands of the complexes indicates that dipole-induced dipole interactions contribute significantly to the binding energies. The binding energies and geometries of the 1:1 and 1:2 van der Waals complexes of XT, BPT, and pyranthione (PT) have been calculated using pairwise atom-atom Lennard–Jones 6–12 potentials and are consistent with the structures derived from experiment. The out-of-plane intermolecular stretching vibrations can be clearly observed in the excitation spectra of most complexes, and their assignments have been confirmed using model vibrational potentials.Ab initioSCF calculations including second order Møller–Plesset correlation are also reported for the PT·rare gas systems, and show nice agreement with the Lennard–Jones calculations and experiment.

Acid–base chemistry in the gas phase: The trans-1-naphthol⋅NH3 complex in its S and S1 electronic states

We deduce information about the dynamics of a proton transfer reaction between an acid and a base. Our probe is the fully resolved S1←S0 fluorescence excitation spectrum of the 1:1 complex of 1-naphthol and ammonia in the gas phase. Analysis of this spectrum shows that the complex is planar in both electronic states, with the NH3 forming a nearly linear hydrogen bond to the hydroxy hydrogen atom of 1-naphthol. The O–H...N heavy atom separation is R=2.86 Å and the barrier to rotation of the NH3 group about its C3 axis is V3=39.9 cm−1 in the S0 state. Excitation of the complex to its S1 state increases the acidity of 1-naphthol, decreases the heavy atom separation to R=2.72 Å, and increases the torsional barrier to V3=46.5 cm−1. Modeling these changes using the Lippincott–Schroeder potential for the hydrogen bond shows that the photoinitiated heavy atom motion produces a significant decrease in the barrier to proton transfer in the S1 state.

Erratum: High resolution S1←S fluorescence excitation spectra of hydroquinone: Distinguishing the cis and trans rotamers by their nuclear spin statistical weights [J. Chem. Phys. 99, 5078 (1993)

Erratum: High resolution <i>S</i>1←<i>S</i> fluorescence excitation spectra of hydroquinone: Distinguishing the <i>cis</i> and <i>trans</i> rotamers by their nuclear spin statistical weights [J. Chem. Phys. <b>99</b>, 5078 (1993)

S2−S0 Spectroscopy of the van der Waals Complexes of Azulene with Rare Gases

The S2−S0 fluorescence emission and excitation spectra of jet-cooled azulene and its complexes with the rare gases, Ne, Ar, Kr, and Xe, have been measured. Features due to AZ·RGn (n = 1−4 when RG = Ar, Kr, Xe and n = 1 when RG = Ne) are observed in the S2−S0 fluorescence excitation spectra when azulene is coexpanded with the rare gases. The microscopic solvent shifts, δν, of the origin bands for each complex scale linearly with the polarizability of the adatom(s), indicating that binding is dominated by dispersive interactions. This conclusion has been confirmed in calculations of the separate contributions of dispersion and induction to δν. Mildly anharmonic progressions in very low frequency excited-state intermolecular (van der Waals) modes are attached to the origin bands of each AZ·Krn and AZ·Xen complex. The dominant progression is assigned to single quantum changes in that excited-state bending mode which involves motion of the adatom(s) in the plane perpendicular to the azulene ring containing t...