Electronic to vibrational energy transfer and relaxation in matrices. II. Hg in mixed N2/Kr matrices

The electronic fluorescence spectrum of Hg-doped N₂/Kr mixed matrices obtained by laser excitation of the Hg³P₁ level is composed of the Hg “atomic” (³P₁—¹S₀ and ³P₀—¹S₀) fluorescence and the exciplex fluorescence due to the (Hg—N₂)^* complex stable in the excited state. The temperature dependence of their intensities and lifetimes was studied in the temperature range 12–24 K. It is argued that the essential part of the ³P₀ and exciplex emission is due to two types of Hg sites with one N₂ nearest neighbor, differing probably in the orientation of the N₂ molecular axis. Strong irreversible effects due to the diffusion of N₂ molecules induced by laser irradiation are observed.     

Electronic spectra of 2-acetylanthracene

The electronic spectrum of 2-acetylanthracene in a supersonic free jet reveals the existence of two rotational isomers with ≈ 196 cm⁻¹difference between their S₁ origins. In low-temperature rigid glasses, 2-acetylanthracene forms stable dimers. The probable structure of this dimer is also briefly discussed.     

Two electronic transitions in the near-UV region of six isomeric difluorobenzonitriles—I

A new technique—photoacoustic spectroscope (PAS)—is applied to the study of the electronic transitions in the six isomers 3,5-; 2,4-; 2,5-; 3,4-; 2,3-; and 2,6-difluorobenzonitriles. The PAS spectra are compared with solution spectra recorded. The two π-π* transitions analogous to the benzene strong 200 nm, and weak and forbidden 260 nm transitions could be identified in these molecules. An interesting observation is that the origins of the electronic transitions in these molecules are in fairly good agreement with the additive rule which is routed through different starting points and also a prediction of the origin of meta fluorobenzonitrile at 37,536.1 cm⁻¹ from the data of 3,4- and 2,5-difluorobenzonitriles.The fluorescence spectrum for all six isomers is reported here for the first time.     

Methyl torsional potentials of rotational isomers of m-methylanisole studied by spectral hole-burning spectroscopy

Laser-induced fluorescence (LIF) excitation spectra of m-methylanisole in a supersonic jet were measured. Two series of progressions were observed in the spectrum, originating at 36048 and 36115 cm⁻¹, which were successfully assigned to the transitions to the methyl internal rotational vibronic levels of the two isomers, i.e. cis and trans isomers, with the aid of hole-burning spectrum measurements and quantum-chemical calculations. The progression for the trans isomer was observed up to the 6a₁ band, while only the 3a₁ band in addition to the 0a₁ and 1e bands was observed for the cis isomer. This finding can be explained by the conformational change upon the electronic excitation; the 60° rotation of the methyl torsional angle takes place for the trans isomer but not for the cis isomer.     

Jet spectroscopy of pyrene complexes: Unusual complexation shifts in the S0 → S1 spectrum

The fluorescence excitation spectra of pyrene complexes with n-alkanes are reported for the S₀ → S₁ and S₀ → S₂ transitions. The S₂ spectral shifts are predictable from theory, and by comparison with other molecules, such as perylene. On the other hand, the S₁ Resonances of pyrene complexes show unusually small displacements from those of the parent species. The spectra of the butane, pentane and hexane complexes actually exhibit net blue shifts. This behaviour provides good evidence for a repulsive interaction in the S₁ state, which is not observed in S₂. Moreover, because the butadiene and benzene complexes give predictable red shifts ⩾ 100 cm⁻¹, and these are found to have plane-parallel geometries, the blue shift correlates with a host carbon—guest hydrogen interaction in the repulsive regime. We also report that the ethylene complex of pyrene exhibits a net blue shift on the S₀ → S₁ transition, and a red shift on S₀ → S₂ only 75% of the predicted value, based on measurements with perylene complexes. This behaviour strongly indicates that ethylene-pyrene has a T-shaped configuration, as predicted by potential energy calculations.     

Rotational relaxation in S1 glyoxal: Cross sections and a search for propensity rules

Details of rotational energy transfer from a few selected K′J′ levels in the zero point vibrational level of ¹Au(S₁) glyoxal vapor have been studied. The cross section for destruction of an initial K′J′ level by rotational relaxation in collision with ground electronic state glyoxal is about 240 A² or 4.5 times gas kinetic. Much of the rotational transfer within the S₁ state occurs with large ΔK′ and ΔJ′. No strong propensities for △K′ = 0, ± 1, ± 2, or ± 3 with small ΔJ′ changes occur in collisions with ground electronic state glyoxal. The study was made by examination of the rotational structure in the 5₁⁰ emission band at various pressures after excitation in the 0,0 band of the S₁—S₀ system with the 454.5 nm argon ion line.     

High-resolution electronic spectra and luminescence properties of some benzonaphthyridines at 77 K. absorption and fluorescence spectra of 1,5-, 1,6- a

Absorption and fluorescence spectra of 1,5-, 1,6- and 4,6-benzo[h]naphthyridines (BN) were examined in Shpolskii matrices and in n-butanol at 77 K. Vibrational analysis of quasilinear spectra of 1,6- and 4,6-BN in n-hexane matrices was performed. Calcula- tions of the electronic structure of the isomers examined were done using a modified INDO CI method. The results of the experiments and calculations prove the π, π^* state to be the lowest excited singlet state of 1,6- and 4,6-BN molecules; in 1,5-BN molecule the S₁ (n, π^*) state is strongly perturbed by the nearby s₂(π π^*) state.     

Inter- and intramolecular hydrogen bond in methyl 2-hydroxy-9H-1-carbazole carboxylate: effect of solvents and acid concentration

The photo-physics of methyl 2-hydroxy-9H-1-carbazazole carboxylate (MPCC) in different solvents and cyclohexane-trifluoroethanol (TFE) mixtures has been studied by means of absorption, fluorescence, fluorescence excitation spectra, time dependence spectrofluoremetery and AM1 semi-empirical quantum mechanical calculations. Only one small Stoke’s shifted fluorescence band is observed under all the environments, indicating that the geometry of the molecule is not changed much on excitation to the first singlet state (S₁) and excited state intramolecular proton transfer (ESIPT) is not viable both in the ground (S₀) and S₁ states at the room temperature. AM1 calculation shows that the ESIPT is still endothermic in S₁ state. Single exponential decay is observed in the fluorescence from MPCC in all the solvents except acetonitrile and methanol. This suggests that in these two solvents, at least two different conformers are present in the S₀ state, whose absorption spectra are not different from each other. Spectral characteristics of MPCC in cyclohexane as a function of TFE have shown a slight blue shift in the λ_max^ab, decrease in the ε_max, red shift in the λ_max^fl and decrease in the φ_fl. This suggests that intermolecular hydrogen bonding is playing a major role in the deactivation of the fluorescence intensity than the intramoleuclar hydrogen bonding (IHB). Spectral properties of MPCC were also studied as a function of acid–base concentrations. pK_a values for different prototropic equilibriums were determined in S₀ and S₁ states and discussed.     

Fluorescence spectroscopy of C60 in toluene solutions at 5 K

We have recorded the dispersed fluorescence and the fluorescence excitation spectra of C₆₀ in toluene matrices at 5 K. Upon excitation with the green Ar⁺ laser line (λ=514 nm) we obtained for the first time in this matrix well resolved visible fluorescence spectra which we have compared with those observed in other low temperature matrices. Our spectra were interpreted and assigned using theoretical assessments of vibronic activities of transitions between the three lowest excited electronic states ¹T_1g, ¹T_2g, ¹G_g and the totally symmetric ground state, and on the basis of a single 0⁰ level which has pseudo-Jahn–Teller (JT) components of the three near-degenerate excited states. The fluorescence spectra exhibit prominent JT induced h_g(1) progressions, Herzberg–Teller-induced h_u and other ungerade mode vibrations, including a very active t_1u(4) mode. Excitation wavelength independent bands are assigned to the fluorescence of C₆₀ molecules in toluene microcrystals embedded in the toluene glass whereas excitation wavelength dependent features are interpreted as originating from C₆₀ molecules isolated in the toluene glass itself. These interpretations are supported by the results of spectrally selective detected fluorescence excitation spectra.     

RESOLUTION OF THE FLUORESCENCE EXCITATION SPECTRUM OF INDOLE INTO THE 1La AND 1Lb EXCITATION BANDS*

Abstract— The fluorescence excitation spectrum and the excitation polarization spectrum of indole in propylene glycol were measured at — 58°C, after selecting by optical filters the emission originating from the ¹L_a electronic level. From the analysis of these spectra, the excitation spectrum was resolved into the ¹L_a and ¹L_a excitation bands. A similar resolution of the excitation spectrum of tryptophan is given. This method can also be applied to the resolution of the emission spectrum in cases of dual emission.     

Spectroscopy and decay dynamics of jet-cooled carbazole and N-ethylcarbazole and their homocyclic analogues

Fluorescence excitation spectra of supersonic jet-cooled carbazole (C) and N-ethylcarbazole (EC) are reported together with those of their homocyclic analogues fluorene (F) and 9-ethylfluorene (EF). Fluorescence spectra of C and EC have been measured following excitation at energies up to ≈ 1300 cm⁻¹ above the S₁ origin and reveal that the onset of intramolecular vibrational redistribution occurs at around 900 cm⁻¹ for both molecules, with redistribution being more extensive in the ethylated molecule. In C and EC, a number of modes are active in vibronically coupling the S₁ and S₂ states and Duschinsky mixing of these modes is apparent in the spectra. The fluorescence lifetimes of both C and EC show a slowly decreasing trend with increasing excitation energy in the range 0–1500 cm⁻¹ excess vibrational energy; vibrational redistribution does not appear to enhance the rate of non-radiative decay in either molecule. Comparison of lifetime values under supersonic jet conditions with solution phase results indicates that solvation produces a considerable increase in the rate of intersystem crossing in these molecules.     

Fluorescence and intersystem crossing as a function of rotational energy on the S1 state of benzene

The effect of rotational excitation on the electronic relaxation of the S₁ state of “isolated” benzene has been studied by examination of resonance fluorescence generated when narrow-band exciting light is swept through the rotational envelopes of four S₁ ← S₀ absorption bands. To within the ten percent uncertainty in measurement, the fluorescence yields are independent of the position of the exciting light within an absorption band. Thus neither radiative decay nor intersystem crossing to the triplet state displays large sensitivity to molecular rotation, at least when excited state relaxation proceeds from rotational distributions of moderate diversity.     

Jet-cooled styrene: spectra and isomerization

The excitation and dispersed fluorescence spectra for the ùA′ ← X?¹A′ transition of styrene, cooled in a supersonic jet are reported. Assignment of the vibrational modes in the ground and excited electronic states are made and the excess energy dependence of the fluorescence spectra are analyzed in terms of their relevance to the dynamics of photoisomerization. We compare our results with those of stilbene and discuss the influence of IVR and mode mixing in the S₁ surface.     

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

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 C_s 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.     

Vibrational spectra and structure of cis- and trans-1,2- dimethoxyethylenes

The vapor, liquid and CCl₄ solution infrared spectra of cis- and trans-1,2-dimethoxyethylene were recorded in the region 250–4000 cm^?1. The laser-Raman spectra were obtained in the liquid state only. The vibrational spectra show that at least two rotational isomers exist for each molecule. Further, the spectra indicate that for both the cis- and trans molecules, one of the rotational isomers has at least one planar conformer. Some vibrational assignments are made for the observed infrared and Raman bands of the cis- and trans- 1,2-dimethoxyethylenes.     

Microwave spectrum,structure, dipole moment and internal rotation of ethyl fluorosilane

Microwave spectra of the trans and gauche isomers of ethyl fluorosilane and their eleven isotopically substituted species have been measured. The r_s structures of the two isomers were determined from the observed moments of inertia. The molecular structures found for the two isomers in the present study are compared with those of analogous molecules. Dipole moments of the two isomers were determined by Stark-effect measurements and are also compared with those of analogous molecules. The energy difference between the trans and gauche isomers was obtained from the relative intensity measurements of the spectra and the barrier to internal rotation of the methyl group for the gauche isomer was obtained from the A—E splittings of the spectra in the first excited methyl torsional state. The V₃ value was 2775 ± 25 cal mol^?1.     

Tunable Dual Fluorescence of 3‐(2,2′‐Bipyridyl)‐Substituted Iminocoumarin

3‐(2,2′‐Bipyridyl)‐substituted iminocoumarin molecules (compounds 1 and 2 ) exhibit dual fluorescence. Each molecule has one electron donor and two electron acceptors that are in conjugation, which leads to fluorescence from two independent charge transfer (CT) states. To account for the dual fluorescence, we subscribe to a kinetic model in which both CT states form after rapid decays from the directly accessed S₁ and S₂ excited states. Due to the slow internal conversion from S₂ to S₁, or more likely the slow interconversion between the two subsequently formed CT states, dual emission is allowed to occur. This hypothesis is supported by the following evidence: 1) the emission at short and long ends of the spectrum originates from two different excitation spectra, which eliminates the possibility that dual emission occurs after an adiabatic reaction at the S₁ level. 2) The fluorescence quantum yield of compound 2 grows with increasing excitation wavelength, which indicates that the high‐energy excitation elevates the molecule to a weakly emissive state that does not internally convert to the low‐energy, highly emissive state. The intensity of the two emission bands of 1 is tunable through the specific interactions between either of the two electron acceptors with another species, such as Zn²⁺ in the current demonstration. Therefore, the development of ratiometric fluorescent indicators based on the dual‐emitting iminocoumarin system is conceivable. Further fundamental studies on this series of compounds using time‐resolved spectroscopic techniques, and explorations of their applications will be carried out in the near future.     

Influence of an identified dimer vibration on the emission spectrum of [2,2]paracyclophane

The emission spectrum of polycrystalline [2,2]paracylophane shows a resolved vibronic structure with a 241 cm^?1 progression at He temperatures. The dependence of the energy of this mode upon selective deuteration in combination with results from FIR and Raman spectra could be used to identify the mode as a torsional dimer vibration. The emission spectra could be simulated assuming a linear coupling of the torsional mode to the electronic transitions with coupling strengths of S = 10 (fluorescence) and S = 13 (phosphorescence). This corresponds to an equilibrium displacement of the benzene rings under electronic excitation by a torsional angle of 10.6° (S₁) and 12.1° (T₁), in addition to the small torsion in the ground state S₀ by about 3°.     

Laser-induced Fluorescence Spectroscopy of NiS: Identification of a Low-lying Electronic State

Laser-induced fluorescence excitation spectra of jet-cooled NiS molecules were recorded in the energy range of 12200-13550 cm^-1. Four vibronic bands with rotational structure have been observed and assigned to the [12.4]³∑₀^- -X³∑₀^- transition progression. The relevant rotational constants, significant isotopic shifts, and (equilibrium) molecular parameters have been determined. In addition, the lifetimes of the observed bands have also been measured.     

Theoretical study of structure,vibrational and electronic spectra of isomers of methyl-3-methoxy-2-propenoate

A systematic quantum mechanical study of the possible conformations, their relative stabilities, vibrational and electronic spectra and thermodynamic parameters of methyl-3-methoxy-2-propenoate has been reported for the electronic ground (S₀) and first excited (S₁) states using time-dependent and time-independent Density Functional Theory (DFT) and RHF methods in extended basis sets. Detailed studies have been restricted to the E-isomer, which is found to be substantially more stable than the Z-isomer. Four possible conformers c′Cc, c′Tc, t′Cc, t′Tc, of which the first two are most stable, have been identified in the S₀ and S₁ states. Electronic excitation to S₁ state is accompanied with a reversal in the relative stability of the c′Cc and c′Tc conformers and a substantial reduction in the rotational barrier between them, as compared with the S₀ state. Optimized geometries of these conformers in the S₀ and S₁ states are being reported. Based on suitably scaled RHF/6-31G^** and DFT/6-311G^** calculations, assignments have been provided to the fundamental vibrational bands of both these conformers in terms of frequency, form and intensity of vibrations and potential energy distribution across the symmetry coordinates in the S₀ state. A complete interpretation of the electronic spectra of the conformers has been provided.