Phosphorescence Excitation Spectra Research Articles

A theoretical study of the methyl torsional modes in the [formula omitted]3A u (T 1) ← [formula omitted]1A g (S 0) n → π* spectrum of biacetyl

The S 0→T 1 electronic band spectrum in biacetyl arising from the activity of the methyl torsion modes was simulated by RHF/UHF ab initio calculations. A fit of the calculated band spectrum to the cold jet phosphorescence excitation spectrum provided an assignment of the major bands and predicted that the system origin should lie at 19500 cm −1.

Singlet And Triplet n,π∗ Transitions of Jet‐Cooled p‐Benzoquinone

The fluorescence/phosphorescence excitation spectra, the dispersed fluorescence/phosphorescence spectra and the sensitized phosphorescence excitation spectrum have been measured for jet‐cooled p‐benzoquinone. The assignments of vibronic bands in the 1B1g(n-,π∗) ← 1Ag and 1Au(n+,π∗) ← 1Ag transitions have been definitely established. The phosphorescence spectra from vibronic levels of 3Au exhibit a feature of single vibronic level emission. The 3Au ← 1Ag transition was also vibrationally analyzed.

Phosphorescence spectra of R-phycoerythrin

The phosphorescence of R-phycoerythrin from the red alga Callithamnion corymbosum was detected. The phosphorescence spectrum of R-phycoerythrin has two main components with maxima at 720 nm and 820 nm and lifetimes of 2.5 ms and 6.0 ms respectively. The phosphorescence excitation spectrum exhibited three bands at 498, 541 and 566 nm, characteristic of the absorption spectrum of R-phycoerythrin, but their relative intensifies differed from those recorded in fluorescence excitation spectrum. The fluorescence quantum yield of the pigment was 0.8 at 77 K; the phosphorescence quantum yield was 1.6 × 10 −6 and it increased to 1.5 × 10 −4 for the R-phycoerythrin denatured with 8 M urea at pH 3.0. This might be explained partly by alteration of the chromophore conformation accompanying the perturbation of protein structure.

Laser-induced phosphorescence of jet-cooled pyrimidine

Laser-induced phosphorescence excitation spectra of pyrimidine molecules have been observed under molecular beam conditions. The origin of the lowest triplet state T 1( 3B 1) was measured to be at 28534.0 cm −1. More than 30 vibrational bands of the T 1←S o transition were observed in the region E = 0–1300 cm −1 from the origin. High-resolution (0.03 cm −1) laser scans of the most intense bands show parallel type contours. Lifetimes of selected T 1 state vibrational bands were measured to be ⩾ 400 μs for the origin, decreasing to 55 μs at E = 1261.0 cm −1.

Electronic Spectra and Luminescence Properties of Helicenes in Crystalline Matrices at 4.2K

Abstract Fluorescence, phosphorescence and excitation spectra, of [4]-, [5]-, [6]-,and [7]-helicenes in policrystalline matrices were obtained at 4.2 K. Highly-resolved electronic spectra were observed for [4]-, [5]-,and [6]-helicene whereas for [7]-helicene only broad bands were monitored in emission and excitation spectra. Irradiation of [5]-helicene at 4. 2 K leads to its photocy- clization to dihydrobenzoperylene. The observed concentration effects in [4]-, and [6]-helicene as well as influence of the crystallinity of the medium on the nuorescence-to-phosphorescence intensity ratios are explained assuming the presence of the fluorescent but not phosphorescent preaggregates. The vibronic structures of the site-selected [4]-, and [6]-helicene phosphorescence excitation spectra point to site-dependent distortions of the helicene molecules. Calculations of electronic transitions helicene molecules were performed with the modified Warshel-Karplus method.

Electronic relaxation induced by interaction between diatomics in low temperature matrices

Fluorescence (phosphorescence) excitation spectra as well as luminescence spectra and decay times were investigated for NO in N2 and mixed N2/Kr matrices. All Rydberg states rapidly relax to the ground state in mixed matrices with the N2 molar fraction exceeding xN2=0.35. This dependence on the N2 concentration is attributed to the complete fluorescence quenching of all NO molecules with at least one N2 nearest neighbor. The vibrational levels of the B 2Π valence states strongly coupled to the Rydberg state are also depopulated. The general picture of relaxation channels is discussed.

Dual phosphorescences of o-benzoylbenzoic acid and methyl-o-benzoylbenzoate in rigid glass matrixes at 77 K

In solvents with an oxygen atom such as EPA (ether-isopentane-ethanol 5:5:2 in volume ratio), ethanol, ethanol-water, and 2-methyltetrahydrofuran at 77 K, o-benzoylbenzoic acid (BBA) and methyl o-benzoylbenzoate (MBB) exhibit dual phosphorescences, and their decay curves can well be analyzed by biexponential functions with lifetimes almost equal to those of the triplet-triplet absorptions. Since phosphorescence excitation spectra in solvents stated above indicate the existence of two types of ground-state BBA (and MBB), we have proposed that the short-lived (T a ) and long-lived (T b ) triplet states are produced independently by excitation of the corresponding ground-state conformers

Phosphorescence emission and polarization of quinoline 3-carbonitrile

The phosphorescence emission and excitation spectra and their polarization have been studied for quinoline 3-carbonitrile in methylcyclohexane and ethanol glasses at 77 K. From the results obtained in this molecule (τ=1.15 sec in ethanol, τ=1.04 sec in methylcyclohexane and out-of-plane polarization) it is concluded that the emission is orginated from a triplet state of 3ππ * character. The two major spin-orbit coupling mechanisms through which the ππ * state of quinoline 3-carbonitrile acquires dipole-allowed character via nπ * states are: 1. In ethanol glass: ▪ 2. In methylcyclohexane glass: ▪

Lowest triplet state of 4-cyanopyridine studied by time-resolved EPR, phosphorescence, and ab initio calculation

The lowest triplet (T 1 ) state of 4-cyanopyridine (4CNP) is investigated by time-resolved EPR, phosphorescence, and ab initio calculations. The EPR study shows that the zero-field sublevel scheme is similar to that of pyridine in both polar and nonpolar media. A single-crystal EPR work using a p-dichlorobenzene host shows that the nitrogen hyperfine splittings also show a trend (A z >A x >A y ) similar to that found in pyridine. The vibrational structures of the low-temperature phosphorescence emission and excitation spectra show progressions of b 1 vibrations

An analysis of the methyl rotation and aldehyde wagging dynamics in the S (X̃ 1A′) and T1 (ã 3A″) states of thioacetaldehyde from pyrolysis jet spectra

Jet-cooled, laser induced phosphorescence (LIP) excitation spectra of thioacetaldehyde (CH3CHS, CH3CDS, CD3CHS, and CD3CDS) have been observed in the 15 800–17 300 cm−1 region in a continuous pyrolysis jet. The responsible electronic transition, T1 ← S0, ã 3A″← X̃ 1A′, results from an n→π* electron promotion and gives rise to a pattern of vibronic bands that can be attributed to activity of the methyl torsion and the aldehyde hydrogen out-of-plane wagging modes. Potential and kinetic energy surfaces were mapped out for the aldehyde wagging (α) and the torsional (Θ) internal coordinates by using 6–31G* Hartree–Fock calculations in which the structural parameters were fully relaxed. The potential and kinetic energy data points were fitted to double Fourier expansions in α and Θ and were incorporated into a two-dimensional Hamiltonian operator. The spectrum was simulated from the transition energies and the Franck–Condon factors and was compared to the observed jet cooled LIP spectra. It was concluded that while the RHF procedure gives a good description to the ground state dynamics, the triplet state surface generated by the UHF method is too bumpy and undulating.

Photophysical behavior of 3-methylquinoline and 3-bromoquinoline

The phosphorescence emission and excitation spectra and their polarizations have been studied for 3-methylquinoline and 3-bromoquinoline in methylcyclohexane and ethanol glasses at 77 K. From the results obtained in 3-methylquinoline (τ p = 1.30 s in ethanol, τ p = 1.05 s in methylcyclohexane and the out-of-plane polarization) it is concluded that the emission originates from a triplet state of 3ππ ∗ character. In 3-bromoquinoline, the phosphorescence lifetimes (ca. 8 ms in a non-polar solvent and 15 ms in a polar solvent) and the negative degree of polarization indicate an emitting state of ππ ∗ character that is strongly coupled to a higher-lying triplet state of the nπ ∗ type. The most likely phosphorescence mechanisms for both molecules are proposed.

Molecular structure effects on carbonyl photophysics: phosphorescence yields for n-alkyl benzoate esters

Dispersed phosphorescence and phosphorescence excitation spectra were obtained for gas-phase methyl benzoate. Relative phosphorescence quantum yields for methyl, ethyl, and n-butyl benzoate were determined for five excitation energies above the 0,0 band and of the second 1(π, π*) transition. The emitting state was assigned as the 3(n, π*) state. Relative phosphorescence yields were found to be inconsistent with trends expected for variation in either intersystem crossing rates or photodissociation rates as a function of alkyl chain length, strongly suggesting a competing molecular elimination process.

High-resolution electronic spectra of tetrahelicene and hexahelicene in low-temperature polycrystalline matrices

Highly resolved fluorescence, phosphorescence and phosphorescence excitation spectra of tetrahelicene (THL) and hexahelicene (HHL) in polycrystalline matrices were obtained at liquid helium temperature. The observed concentration effects as well as the influence of the crystallinity of the medium on the fluorescence-to-phosphorescence intensity ratios are explained assuming the presence of fluorescent but not phosphorescent preaggregates and aggregates of the helicenes in crystalline matrices. The vibronic structures of the site-selected THL phosphorescence excitation spectra point to site-dependent distortions of the THL molecules. Calculations of electronic transitions of the HHL molecule were performed with the modified Warshel-Karplus method. The activity of out-of-plane skeletal modes in S 1↔S 0 and T 1→S 0 HHL spectra correlates with the calculated HHL helix pitch changes in S 1 and T 1 states, respectively.

Electronic spectra and proton transfer in the phenol/(NH 3) n clusters in argon matrices

Absorption, fluorescence, phosphorescence and fluorescence- and phosphorescence-excitation spectra are recorded for the phenol/ammonia(Φ-OH/NH 3) complexes, to Φ-OH..(NH 3) 2 and large Φ-OH..(NH 3) n (n⩾3) clusters are identified. It is shown that the ionic form: Φ-O −..H in very large Φ-OH..(NH 3) n clusters formed in pure ammonia and mixed NH 3/Ar matrices with a high ammonia content. The results are compared to those obtained for free phenol-ammonia clusters formed in a supersonic expansion.

Phosphorescence of protochlorophyll(ide) and chlorophyll(ide) in etiolated and greening bean leaves

The assignment is presented for the principal phosphorescence bands of protochlorophyll(ide), chlorophyllide and chlorophyll in etiolated and greening bean leaves measured at -196°C using a mechanical phosphoroscope. Protochlorophyll(ide) phosophorescence spectra in etiolated leaves consist of three bands with maxima at 870, 920 and 970 nm. Excitation spectra show that the 870 nm band belongs to the short wavelength protochlorophyll(ide), P627. The latter two bands correspond to the protochlorophyll(ide) forms, P637 and P650. The overall quantum yield for P650 phosphorescence in etiolated leaves is near to that in solutions of monomeric protochlorophyll, indicating a rather high efficiency of the protochlorophyll(ide) triplet state formation in frozen plant material. Short-term (2-20 min) illumination of etiolated leaves at the temperature range from -30 to 20°C leads to the appearance of new phosphorescence bands at about 990-1000 and 940 nm. Judging from excitation and emission spectra, the former band belongs to aggregated chlorophyllide, the latter one, to monomeric chlorophyll or chlorophyllide. This indicates that both monomeric and aggregated pigments are formed at this stage of leaf greening. After preillumination for 1 h at room temperature, chlorophyll phosphorescence predominates. The spectral maximum of this phosphorescence is at 955-960 nm, the lifetime is about 2 ms, and the maximum of the excitation spectrum lies at 668 nm. Further greening leads to a sharp drop of the chlorophyll phosphorescence intensity and to a shift of the phosphorescence maximum to 980 nm, while the phosphorescence lifetime and a maximum of the phosphorescence excitation spectrum remains unaltered. The data suggest that chlorophyll phosphorescence belongs to the short wavelength, newly synthesized chlorophyll, not bound to chloroplast carotenoids. Thus, the phosphorescence measurement can be efficiently used to study newly formed chlorophyll and its precursors in etiolated and greening leaves and to address various problems arising in the analysis of chlorophyll biosynthesis.

A phosphorescence study of jet-cooled xanthone

We have observed a phosphorescence excitation spectrum of xanthone in a supersonic jet. In contrast with the results in some solid media, it has been shown that the T 1 state in the isolated xanthone molecule is 3A 2(nπ*) in character. The rotational envelope has been analyzed and it is concluded that the intensity of the T 1 origin band mainly comes from the direct spin—orbit coupling with the S 2 1A 1 (ππ*) state.

An analysis of the methyl rotation dynamics in the S (X̃ 1A1) and T1 (ã 3A2) states of thioacetone, (CH3)2CS and (CD3)2CS from pyrolysis jet spectra

Jet-cooled, laser-induced phosphorescence excitation spectra (LIP) of thioacetone (CH3)2CS/(CD3)2CS have been recorded over the region 16 800–18 500 cm−1 using the pyrolysis jet spectroscopic technique. The responsible electronic transition, T1←S0, ã 3A″←X̃ 1A1, results from an n→π* electron promotion and gives rise to a pattern of vibronic bands that were attributed to activity of the methyl torsion and the sulphur out-of-plane wagging modes. The intensities of the torsional and wagging progressions in the excitation spectra were interpreted in terms of a C2v–Cs molecular distortion of the triplet molecule from its singlet ground state equilibrium structure. A complete unrestricted Hartree–Fock (UHF) ab initio molecular orbital (MO) structural optimization of the T1 state predicted that the sulphur was displaced by 27.36° from the molecular plane and the methyl groups were rotated by 10.93° in clockwise–counterclockwise directions. Restricted Hartree–Fock (RHF) calculations were used to generate the V(θ1,θ2) potential surface governing methyl rotation for the S0 state. This was incorporated into a two-dimensional Hamiltonian, symmetrized for the G36 point group and solved variationally for the torsional frequencies. The calculated frequencies of 159.97/118.94 for the ν17(b1) mode of S0 (CH3)2CS/(CD3)2CS were found to agree with the experimental values, 153.2/114.7 cm−1.

Discrimination of Rotational Isomers of Jet-Cooled Acetophenone Derivatives as Studied by Sensitized Phosphorescence Excitation Spectroscopy

Abstract The S1(n,π*)←S0 and T(n,π*)←S0 sensitized phosphorescence excitation spectra of jet-cooled o- and m-substituted (fluoro- and methyl-) acetophenones have been measured. The two rotational isomers (O-cis and O-trans isomers) of each molecule arising from the orientation of the acetyl group with respect to o- or m-substituent have been discriminated. The energy difference in the 0–0 transition between the two isomers is less than 100 cm−1 for m-substituted acetophenone but more than 300 cm−1 for o-substituted acetophenone. The characteristic energy difference can be explained from the fact that the electronic excitation is localized on the C=O group.

Absorption and luminescence spectroscopic analysis of tautomeric forms of protonatedN,N-dimethyl-N?-(1-nitro-9-acridinyl)-1,3-propanediamine (nitracrine) and its nitro isomers in poly(vinyl alcohol) films

The electronic absorption, fluorescence, and phosphorescence excitation spectra, as well as the fluorescence and phosphorescence spectra, at either room or liquid nitrogen temperatures, were measured forN,N-dimethyl-N'-(1-nitro-9-acridinyl)-1,3-propanediamine and its three nitro isomers in acidified poly(vinyl alcohol) (PVA) film. The spectral characteristics obtained reveal the existence of the compounds studied in at least two structural forms. The results are interpreted in terms of the tautomeric phenomena which originate due to the migration of the hydrogen atom, which is bound to the nitrogen atom attached to the carbon atom (9), to the acridine ring nitrogen.

Methyl torsional barriers in the S 0, T 1, and S 1 electronic states of 2-methylpyrazine

The laser-induced fluorescence and phosphorescence excitation spectra of 2-methylpyrazine are reported. Vibrational and methyl torsional assignments have been made. Methyl torsional parameters for the S 0, T 1 and S 1 electronic states have been determined from these data and are discussed in terms of the local electronic distributions in each state.