Manifestation of intramolecular vibrational energy redistribution on electronic relaxation in large molecules

Some universal characteristics are discussed of the decay lifetimes and fluorescence quantum yields from the S₁ manifold of large molecules, which originate from the coupling between intrastate vibrational energy redistribution and interstate electronic relaxation. The time-resolved total fluorescence decay from the S₁ state of jet-cooled 9-cyanoanthracene exhibits non-exponential decay in the energy range E_v= 1200–1740 cm⁻¹ above the S₁ origin, which does not originate from dephasing but rather manifests the effects of intrastate intermediate level structure for vibrational energy redistribution on intersystem crossing.     

Vibrational spectra of tetramethyleyclobutane-1-one-3-thione and the fully deuterated derivative

Raman and i.r. spectra of tetramethylcyclobutane-1-one-3-thione (TMCBOT) and the fully deuterated derivative TMCBOT-_d¹² have been recorded. A fairly complete set of vibrational frequencies and assignments are given for the two molecules. The CO stretching mode was observed as a very strong Fermi doublet in the infrared spectrum of TMCBOT at 1811/1782 cm⁻¹. For TMCBOT-d₁₂ a similar doublet was observed at 1808/1775 cm⁻¹. The CS stretching mode was assigned to bands at 1303 cm⁻¹ for TMCBOT and 1307 cm⁻¹ for the deuterated molecule.     

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.     

Vibrational redistribution effects in the time resolved emission of isolated pyrene molecules

Arguments are given that the appearance of a short component in the fluorescence decay of pyrene after excitation in a vibronic origin of the S₀ → S₁ transition, has to be ascribed to a redistribution of vibrational energy of the inducing mode over the other vibrational modes in the molecule.     

Energy transfer rates for vibrationally excited gas-phase azulene in the electronic ground state

Gas-phase azulene molecules were prepared with 17200 cm^?1 vibrational energy in the S₀ state by laser excitation of the S₁ state and subsequent internal conversion. Rates of vibrational energy removal (for several collision partners) were determined from the decay of the CH-stretch fluorescence at 3.3 μm. A stepladder model indicates each azulene-azulene collision removes 3500 cm^?1 of vibrational energy.     

Determination of intermolecular energy transfer by time resolved CARS measurements

Energy transfer in methyl isocyanide following i.r. multiphoton excitation has been examined with a CARS probe. Results show that CARS is an effective technique both for measuring the fraction of molecules receiving energy from the laser and for monitoring intermolecular energy transfer following laser irradiation. In addition, the Raman transition at 2169 cm⁻¹ shows a vibrational progression due to hot bands of the ν₈ bend. Since the ν₈, bend is the only low energy mode of the molecule, it is an effective vibrational “thermometer”. Utilizing this thermometer, results show that the vibrational state distribution does not fit a Boltzmann distribution for 7 μs following i.r. multiphoton excitation.     

Spectroscopy and dynamics of jet-cooled 1,1′ binaphthyl

Fluorescence excitation and dispersed fluorescence spectra of jet-cooled 1,1′-binaphthyl are reported and analysed. The spectra indicate that in the ground and excited state the naphthalene rings are perpendicular to one another. The spectra can be further interpreted in terms of an exciton model with an exciton splitting of 21 cm^?1 in the origin. From the structureless emission spectrum and lifetime it is concluded that, in the isolated molecule, efficient vibrational relaxation occurs through conversion of vibrational into librationaI energy.     

Rotational effects on intramolecular radiationless transitions in a large molecule

We report the observation of a pronounced rotational-state dependence for the fluorescence quantum yield and fluorescence decay from the S₁(0,0+1160 cm⁻¹) state of jet-cooled 9-cyanoanthracene. We attribute these results to the effects of Coriolis rotational-vibrational interaction on intramolecular vibrational energy redistribution, which influence electronic relaxation.     

Spectroscopy and dynamics of 9,10-dichloroanthracene-Arn van der Waals complexes

In this paper we report on the measurements of the absorption spectra of large van der Waals complexes in planar supersonic jets. The absorption spectra and the fluorescence excitation spectra of the complexes of 9,10-dichloroanthracene (DCA) with Ar atoms are reported for the S₀ → S₁(0) vibrationless transition of DCA·Ar_n (n = 1?6), and from the S₀ → S₁ (1390 cm^?1) transition of DCA·Ar_n (n = 1?4). Information on the structure of these complexes was inferred from the additivity of the spectral shifts per added rare-gas atom (ASSRA) for DCA·AR_n (n = 1.2) and from deviations from the ASSRA for CDA-Ar_n (n = 3?6). The vibrational predissociation (VP) dynamics of DCA-Ar_n (n = 1?3) complexes was interrogated by fluorescence quantum yield, Y, measurements. The value of the S₁ (1390 cm^?1) state of DCA·Ar_n (n = 1?3) exhibits a dramatic enhancement relative to that of DCA. Utilizing the dependence of Y on the excess vibrational energy of bare DCA, we were able to estimate the internal energy of the fragments resulting from VP of DCA·Ar_n of DCA·Ar_n (n = 1?3). An upper limit of ? 100 ps was estimated for the VP (and/or vibrational energy redistribution) lifetime from the S₁ (1390 cm^?1) state of DCA·Ar₃.     

Interstate coupling and dynamics of excited singlet states of isolated diphenylbutadiene

In this paper, we report on absolute fluorescence quantum yields from photoselected vibrational states of jet-cooled 1,4-diphenylbutadiene for excess vibrational energies, E_v = 0−7500 cm⁻¹, above the apparent electronic origin of the S₁(2A_g) state. The pure radiative lifetimes, τ_r, of the strongly scrambled S₂(1B_u)—S₁(2A_g) molecular eigenstates (E_v = 1050−1800 cm⁻¹) show a marked dilution effect, (τ_r/τ_r(S₂) ≈ 40), being practically identical with the τ_r values from the S₁(2A_g) manifold (E_v = 0–900 cm⁻¹), which is affected by near-resonant vibronic coupling to S₂(1B_u) and exhibiting the dynamic manifestations of the intermediate level structure. Isomerization rates in the isolated molecule, which do not exhibit vibrational mode selectivity, were recorded over the energy range 0–6600 cm⁻¹ above the threshold.     

Metallomethanes: XII. Vibrational spectra and force fields of methylmercurimethanes

The vibrational spectra data, the assignments, and the results of normal coordinate calculations for CH_{4 − n}(HgCH₃)_n molecules (2 ⩽ n ⩽ 4) are reported. The central CHg valence force constants are 1.870, 1.653, and 1.582 N cm⁻¹ while the terminal ones are 2.121, 2,101, and 2.160 N cm⁻¹ for n = 2, 3 and 4, respectively. The latter values are 12, 21, and 27% higher than the central CHg force constants, but all of them are substantially lower than those in dimethylmercury (2.379 N cm⁻¹). These findings can be accounted for in terms of increasing shift of electron density towards the periphery of these molecules and increasing non-bonded metal-metal interaction. The nature of the normal modes is discussed.     

Rate constants and vibrational distributions for water-forming reactions of OH and OD radicals with thioacetic acid, 1,2-ethanedithiol and tert-butanethiol

Reactions of OH and OD radicals with CH₃C(O)SH, HSCH₂CH₂SH, and (CH₃)₃CSH were studied at 298 K in a fast-flow reactor by infrared emission spectroscopy of the water product molecules. The rate constants (1.3 ± 0.2) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ for the OD + CH₃C(O)SH reaction and (3.8 ± 0.7) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ for the OD + HSCH₂CH₂SH reaction were determined by comparing the HOD emission intensity to that from the OD reaction with H₂S, and this is the first measurement of these rate constants. In the same manner, using the OD + (C₂H₅)₂S reference reaction, the rate constant for the OD + (CH₃)₃CSH reaction was estimated to be (3.6 ± 0.7) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. Vibrational distributions of the H₂O and HOD molecules from the title reactions are typical for H-atom abstraction reactions by OH radicals with release of about 50% of the available energy as vibrational energy to the water molecule in a 2:1 ratio of stretch and bend modes.     

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.     

On the “anomalous” flourescence of 3,4-benzypyrene in the vapour phase

An analysis of the fluorescence of 3,4-benzpyrene in the vapour phase shows two contributions to the “anomalous” fluorescence: (i) the emission from the second excited state to the ground state and (ii) a vibronically induced S₁ → S₀ emission originating from the +520 cm^?1 vibrational level. A comparison between the intensities of the emissions indicates that in the vapour phase the vibrational redistribution from the 520 cm^? vibrational level of S₁ to modes of lower frequencies is relatively slow.     

Jet-cooled exciplexes: Cyanoanthracene with 2,5-dimethyl-2,4-hexadiene

Fluorescence excitation spectroscopy of a cyanoanthracene (CNA) seeded in helium and expanded in a supersonic jet in the presence of 2,5-dimethyl-2,4-hexadiene (DMHD), reveals two types of transitions. One consists of narrow bands in both excitation and emission, characteristic of van der Waals complexes. The other involves broad bands (≈200 cm⁻¹ in excitation, and about 2500 cm⁻¹ in emission), assigned to an exciplex. It is found that the decay time of the exciplex emission decreases steeply with excess energy, indicating an efficient radiationless process.     

Raman probing of overtone and combination bands to study the vibrational energy distribution produced by multiple-photon excitation

Vibrational energy distribution in CF₃Br, produced by multiple-photon excitation, is studied with the use of Raman probing of fundamental bands and also overtone and combinations bands. On a collision-free time scale, statistical energy distribution among vibrational modes is found at energies over 7200 cm⁻¹. Possible physical causes of this effect are discussed.     

Rotational isomers of several aromatic molecules studied by supersonic jet spectroscopy

The electronic spectra of m—substituted phenols and β-naphthol in supersonic free jets have been investigated. All the molecules studies exhibit two strong features in the fluorescence excitation spectra, which are due to two rotational isomers in which the hydrogen atom of the OH group is in cis and trans configurations with respect to the m—substituent. The difference between the S₁ — S₀ excitation energies of the two isomers is in the range from 100 to 300 cm^?1. This large difference indicates the potentiality of electronic spectroscopy in the discrimination of the rotational isomers. The dispersed fluorescence spectra of the molecules as well as the electronic spectra of the hydrogen—bonded complexes provide definite evidences for the existence of the two isomers     

Ultrafast fluorescence risetime of a polyatomic molecule in solution and under collision-free conditions

The fluorescence rise in solution and in the vapor phase was investigated for the same molecule, dimethyl-POPOP. After optical excitation the structured fluorescence spectrum of the solution evolves in a time shorter than 2 ps. Likewise the fluorescence in the vapor phase rises within less than 2 ps. This finding suggests rapid intramolecular vibrational relaxation in S₁ For excitation with a large excess energy of 11000 cm^?1 the relaxation to fluorescing states in S₁ is still fast under collision-free conditions.     

Isomer effects on vibrational energy relaxation in perylene-argon complexes

Fluorescence spectroscopic measurements have been carried out on jet-cooled complexes of perylene with a number of guest species ranging from argon to small hydrocarbons. In the case of argon, a sequence of red-shifted absorption bands is assigned to a group of aggregates involving up to five guest atoms. Calculations using empirical atom-atom pair potentials have allowed unambiguous assignment of eight different perylene-argon complexes. Spectrally dispersed fluorescence measurements have studied the effect of internal energy on the IVR and predissociation processes. In particular, the two perylene diargon isomers are shown to exhibit different rates for IVR.It has been shown that arson forms a series of organised, readily identifiable complexes with perylene in a supersonic jet. On excitation of the different isomeric forms of the 2:1 complex, different rates for vibrational energy redistribution have been found. The greatest difference was observed at low vibrational energies of the parent, which confirmed that low-frequency van der Waals modes were responsible. Although more detailed calculations will appear elsewhere, the most obvious difference between the two isomeric forms is the exchange of two low-frequency x-y plane vibrations (≈5 cm^?1) for a hindered rotation and a substantially higher-frequency (15–30 cm^?1) argon-argon stretching mode. Photodissociation is also readily observed, confirming the computed values for the argon-perylene binding energies. Finally, strong resonance fluorescence is observed, accompanied by (relaxed) emission from the singly dissociated species when either the 1:1 or either 2:1 isomer is excited with a (parent) vibrational energy of 705 cm^?1. Thus, in spite of the presence of the dissociative pathway, all observable emission from the undissociated species appears to originate from a state in the “small molecule” limit, which survives for about 4 ns.