Highly resolved electronic absorption and luminescence spectra of some phenanthrolines in Shpolskii matrices: Part II. Fluorescence and S1←S0 absorption spectra

Highly resolved fluorescence and S₁←S₀ absorption spectra of some phenanthrolines in n-alkane matrices were obtained at 77 K. The vibrational analysis of the spectra was carried out. Mirror symmetry distortions of the fluorescence and absorption spectra were attributed to vibronic coupling between ¹(π, π¹) states. It was shown that vibronic mixing of S₁(π, π¹) and S₂(π, π¹) states occurs in phenanthroline molecules with S₁-S₂ energy gap not exceeding 3000 cm^-1 (i.e., 4,7-, 1,7- and 1,10-Phen), whereas in other phenanthrolines the coupling of S₁ and S₃(π, π¹) plays a dominant role. Fluorescence quantum yields of phenanthroline liquid solutions were measured. Changes of luminescence efficiencies due to the change of the polarity of the media point to a weak coupling of the lowest ¹(π, π¹) and ¹(n, π¹) states.     

The vibrational structure of the electronic spectra of quinoxaline in shpolskii-type hydrocarbon matrices

Quasilinear spectrum of the S₁(n, π¹, ¹B₁) ← S₀ transition of quinoxaline in crystalline methylcyclohexane matrix and quasilinear phosphorescence spectra of quinoxaline in high- and low-temperature modifications of isooctane matrix were obtained at 77 K. The results of a full vibrational analysis of the spectra are presented and discussed. The diffuseness of the absorption spectrum in the range 3200-2900 Å is ascribed essentially to a small energy gap between the S₂ and S₃ states. The activity of out-of-plane vibration, 865 cm^-1, in the phosphorescence and the influence of matrices on the structure of the spectra suggest the possibility of out-of-plane distortion of the quinoxaline molecule in the T₁ state.     

Structural dynamics of 4‐cyanobenzaldehyde in S2(ππ*) state

The excited state structural dynamics of 4‐cyanobenzaldehyde (p‐CNB) were studied by using the resonance Raman spectroscopy and the quantum mechanical calculations. The experimental A‐ and B‐band absorptions were, respectively, assigned to the major n_O → π₃* and π₂ → π₃* transitions according to the B3LYP‐TD/6‐31G(d) and CIS/6‐31G(d) computations, and the resonance Raman spectra. It was determined that the actual S₂(π₂π₃) state was in energy lower than S₃(π₁π₃), which was just opposite to the B3LYP‐TD/6‐31G(d) calculated order of the S₂(π₁π₃) and S₃(π₂π₃). The vibrational assignments were carried out for the A‐ and B‐band resonance Raman spectra. The B‐band resonance Raman intensities of p‐CNB were dominated by the C2–C3/C5–C6 symmetric stretch mode ν₈, the overtones nν₈ and their combination bands with the ring C–H bend mode ν₁₇, the C9–N10 stretch mode ν₆, the C7–O8 stretch mode ν₇ and the remaining modes. The conical intersection between S₁(n_Oπ₃) and S₂(π₂π₃) states of p‐CNB was determined at complete active space self‐consistent field (CASSCF)(8,7)/6‐311G(d,p) level of theory. The B‐band short‐time structural dynamics and the corresponding decay dynamics of p‐CNB were obtained by analysis of the resonance Raman intensity pattern and CASSCF computations. The resonance Raman spectra indicated that CI[S₁(n_Oπ₃)/S₂(π₁π₂π₃π₄)] located nearby the Franck–Condon region. The excited state decay dynamics evolving from the S_{2, FC}(π₂π₃) to the S₁(n_Oπ₃) state was proposed. Copyright © 2013 John Wiley & Sons, Ltd.     

Studies of the temperature dependence of the phosphorescence spectra and decay rates of aromatic carbonyl molecules in mixed organic crystals

The temperature dependence of the phosphorescence spectra, decay rates and the S ₀ → T ₂(³ nπ*) absorption spectra were studied for aromatic carbonyl molecules in mixed organic crystals. The energy separation, ΔE_T , between the ³ ππ* lowest excited triplet (T ₁) states and the higher ³ π ^* states (T ₂) was estimated for several systems from the temperature dependence of the phosphorescence spectra and decay rates. It was found that the decay rates of the aromatic carbonyl molecules in the mixed crystal systems studied are determined by (1) thermal population to the ³ nπ* states and (2) increased radiationless transition rates at higher temperatures.     

Decay Dynamics of 3‐methyl‐3‐pentene‐2‐one from the light absorbing S2(ππ*) state ‐ Resonance Raman Spectroscopy and CASSCF Study

The photophysics of 3‐methyl‐3‐pentene‐2‐one (3M3P2O) after excitation to the S₂(ππ*) electronic state were studied using the resonance Raman spectroscopy and complete active space self‐consistent field (CASSCF) method calculations. The A‐band resonance Raman spectra were obtained in cyclohexane, acetonitrile, and methanol with excitation wavelengths in resonance with the first intense absorption band to probe the structural dynamics of 3M3P2O. The B3LYP‐TD/6‐31++G(d, p) computation was carried out to determine the relative A‐band resonance Raman intensities of the fundamental modes, and the result was used to reproduce the corresponding fundamental band intensities of the 223.1 nm resonance Raman spectrum and thus to examine whether the vibronic‐coupling existed in Franck‐Condon region or not. CASSCF calculations were carried out to determine the minimal singlet excitation energies of S_{1, FC}, S_1,min (nπ*), S_{2, FC}, S_2,min (ππ*), the transition energies of the conical intersection points S_n/S_π, S_n/S₀, and the optimized excited state geometries as well as the geometry structures of the conical intersection points. The A‐band short‐time structural dynamics and the corresponding decay dynamics of 3M3P2O were obtained by the analysis of the resonance Raman intensity pattern and CASSCF computations. It was revealed that the initial structural dynamics of 3M3P2O was towards the simultaneous C₃=C₄ and C₂=O₇ bond elongation, with the C₃=C₄ bond length lengthening greater at the very beginning, whereas the C₂=O₇ bond length changing greater at the later evolution time before reaching the CI(S₂/S₁) conical intersection point. The decay dynamics from S₂(ππ*) to S₁(nπ*) via S₂(ππ*)/S₁(nπ*) in singlet realm and from S₁(nπ*) to T₁(nπ*) via ISC[S₁(nπ*)/T₂(ππ*)/T₁(nπ*)] in triplet realm are proposed. Copyright © 2014 John Wiley & Sons, Ltd.     

Subject index for volume 88

The emission and excitation spectra of the aromatic thioketone xanthione have been measured in Shpolskii matrices at 15 K. Under these conditions a sharp and rich vibrational structure is observed in the lowest triplet and the first and second excited singlet states. The phosphorescence excitation spectrum places the origin of the T₁ ← S₀ transition at 15 143 cm^?1, while that of the $\text{S}{}_1\text{(n, π}{}^1\text{) ← S}{}_0$ absorption is tentatively assigned to the band at 16 093 cm^?1. The phosphorescence spectrum, which shows only a weak CS stretch vibrational band, is dominated by ring vibrations. In accordance with the previous analysis of ODMR measurements, it is suggested that T₁ and T₂ states are energetically very close, thereby resulting in a lowest triplet state of heavily mixed n, $\text{π}{}^1\text{-π}$, $\text{π}{}^1$ character. No mirror-image relationship is found between the relatively strong S₂ → S₀ fluorescence and the excitation spectrum of the $\text{S}{}_2\text{(π, π}{}^1\text{) ← S}{}_0$ transition. The latter is dominated by a long, pronounced 336-cm^?1 progression.     

Stereoelectronic properties of photosynthetic and related systems: Ab initio configuration interaction calculations on the ground and lower excited singlet and triplet states of magnesium chlorin and chlorin

Ab initio configuration interaction wavefunctions and energies are reported for the ground state and many low-lying singlet and triplet states of magnesium chlorin and chlorin, and are employed in an analysis of the electronic absorption spectra of these systems.In chlorin, the calculated visible spectrum consists of two ${}^1\text{(π, π}{}^1\text{)}$ states, the lower energy, y-polarized state exhibiting moderate absorption intensity in contrast to the very weak absorption of the higher energy x-polarized state. The configurational composition of both states is well described by the four-orbital model. Five ${}^1\text{(π, π}{}^1\text{)}$ states are responsible for the Soret band envelope. A moderately intense y-state lies under the low energy edge of the band envelope, while two x-polarized states of moderate and strong intensity, respectively, are responsible for the band maximum. The final two ${}^1\text{(π, π}{}^1\text{)}$ states lie at the high energy edge of the Soret band and introduce a measure of asymmetry into the band envelope. Two ${}^1\text{(n, π}{}^1\text{)}$ states of very weak oscillator strength are also found in this region of the spectrum. All the Soret states are of complex configurational composition, and several of the higher lying states contain contributions from doubly excited configurations.The calculated visible spectrum of magnesium chlorin also consists of two ${}^1\text{(π, π}{}^1\text{)}$ states, with the weakly absorbing x-polarized state lying approximately 200 cm^?1 lower in energy than the moderately intense y-polarized state. The configurational composition of both states is well described by the four-orbital model. Four ${}^1\text{(π, π}{}^1\text{)}$ states constitute the bulk of the intensity in the Soret band envelope. In distinction to chlorin, the moderately intense ${}^1\text{(π, π}{}^1\text{)}$ state at the low energy edge of the band envelope is x-polarized. Two intense ${}^1\text{(π, π}{}^1\text{)}$ states of y- and x-polarization, respectively, constitute the band maximum region, and a single x-polarized state of moderately strong intensity can be assigned to the high energy shoulder of the band envelope. Two other weakly absorbing ${}^1\text{(π, π}{}^1\text{)}$ states are also found in this region, along with another weakly absorbing state of mixed in-plane and out-of-plane polarization. No clearly defined ${}^1\text{(n, π}{}^1\text{)}$ states are observed. As was the case for chlorin, all the Soret states are of complex configurational composition, and some of the higher energy states contain significant contributions from doubly excited configurations.Chlorin and magnesium chlorin both possess three ${}^3\text{(π, π}{}^1\text{)}$ states which lie below S₁ and a single ${}^3\text{(π, π}{}^1\text{)}$ which lies slightly above S₂. All four of the low-lying ${}^3\text{(π, π}{}^1\text{)}$ states in each molecule are well described by the four-orbital model, with T₁ being essentially a single configuration in each case. The remainder of the ${}^3\text{(π, π}{}^1\text{)}$ states are clustered in the same energetic region as the comparable ${}^1\text{(π, π}{}^1\text{)}$ Soret states, with comparably complex configurational compositions.Dipole moments and charge distributions for low-lying singlet and triplet states are also reported, and are used to rationalize chemical reactivity characteristics.     

Temperature dependence of the emission spectra of p-benzoquinone in a p-dichlorobenzene matrix

Emission and excitation spectra of p-benzoquinone have been measured in a partially translucent p-dichlorobenzene host matrix at different temperatures from 77 up to 323 K. Successive occurrence of the T₁(n,π^?) and T₂(n,π^?) phosphorescence and S₁(n,π^?) fluorescence has been observed for p-benzoquinone upon increasing temperature. The S₁–T₂ and T₂–T₁ energy separations determined from the temperature dependence of the emission intensities agreed well with those obtained from the locations of the T₁ and T₂ phosphorescence and S₁ fluorescence origins. Occurrence of the emission from S₁ and T₂ is brought by the thermal population from the T₁ state. The symmetries of the T₁ and T₂ states were determined to be B_1g and A_u, respectively, based on the spectral data.     

Study of the π+π+π-π- system centrally 0 produced by incident π+ andp beams at 85 GeV/c

The reactions π⁺ p→π⁺(π⁺π⁺π^-π^-)p andpp→p(π⁺π⁺π^-π^-)p, where the (π⁺ π⁺ π^- π^-) system is centrally produced have been studied at 85 GeV/c. The π⁺π⁺π^-π^- mass spectrum shows evidence for thef ₁ (1285) meson with a mass of 1278±2 MeV and width 41±12 MeV which decays mainly through ρ⁰(770)π⁺π^-. Thef ₁(1285) is also observed in the ηπ⁺π^- channel. There is no significant evidence for ηππ or 4π decay modes of thef ₁(1420). The ρ⁰ρ⁰ production is found to be small. A Dalitz plot analysis of the 3π system shows evidence fora ₂ (1320) production and for a large contribution of theJ ^PC=1⁺⁺ ρπ wave.     

A study of the centrally produced π+π−π0 system formed in the reactionpp→p f (π+π−π0)p s at 300 GeV/c

The reactionpp→p _f (π⁺π^?π⁰)p _s , where the π⁺π^?π⁰ system is centrally produced, has been studied at 300 GeV/c. The π⁺π^?π⁰ mass spectrum shows evidence for the η, ω anda ₂ (1320) as well as an enhancement in thea ₁ (1260) region. A Dalitz plot analysis of the π⁺π^?π⁰ system has been performed. Thea ₁ (1260) parameters coming from the fit of the 1⁺ S wave arem=1208±15 MeV and Γ=430±50 MeV. No evidence is found for theh ₁(1170) orh ₁(1380).     

IDENTIFICATION AND SEPARATION OF THE TWO OVERLAPPING SINGLET 1(π, π*) ABSORPTION BANDS OF 1, 4-TETRACENEQUINONE

Absorption and fluorescence spectra of 1, 4-tetracenequinone (1, 4-TQ) have been measured in solution and the two overlapping 1(π, π^*) absorption bands with comparable intensities were separated. Based on the spectroscopic data, the energy levels of the low-lying 1(π, π^*) excited states of 6, 15-hexacenequinone were discussed in relation to those of 1, 4-TQ.     

Time-resolved EPR and ODMR studies on the lowest excited triplet states of α-silyl and α-germyl ketones

Sublevel properties of the lowest excited triplet (T¹) nπ* states of α-silyl and α-germyl ketones were examined by means of ODMR, time-resolved EPR and optical spectroscopy. The EPR parameters, D, E and g, population ratios, and triplet lifetimes were obtained. The D value and the triplet lifetime varied among the molecules. In contrast the E value and population ratio remains nearly the same. These properties together with their solvent dependence and emission properties are interpreted in terms of spin-orbit couplings between T₁ (nπ*) and higher S₁ (nπ*), T₂ (ππ*) and S₂ (ππ*) states. An origin of the remarkable red-shifts of ^1,3 (π*) are discussed based on a model of delocalized n, π and π* electrons over the Si and Ge atoms. This model is also consistent with all the triplet properties obtained.     

Features of dual fluorescence of 1, 2, 3, 4- tetrahydroisoquinoline

In polar and nonpolar solvents, tetrahydroisoquinoline emits S₂ (ππ¹) → S₀ and S₁ (ππ¹) → S₀ dual fluorescence and T₁ (ππ¹) → S₀ phosphorescence on S₂ (ππ¹) ← S₀ excitation. When excited by S₁ (ππ¹) ← S₀ the molecule yields S₁ (ππ¹) → S₀ fluorescence and T₁ (ππ¹) → S₀ phosphorescence in nonpolar solvent but only fluorescence in polar solvents. Probable participation of intermediate S₁ (nπ¹), T₁(nπ¹) states in intersystem crossing and internal conversion processes and its significance in interpreting the results are discussed.     

Electronic relaxation of troponoids: Tropolone fluorescence

Absorption spectra, excitation and fluorescence spectra of tropolone, in water at pH 3 and pH 12, and in cyclohexane, are reported. An assumption for the superposition of two (π, π¹) absorption transitions is made, and the oscillator strengths are determined for these transitions. Radiative and non-radiative processes following the excitation are investigated. Results are discussed in terms of the existence of an (n, π¹) fluorescent state lying below the first (π, π¹) state.     

Ab initio SCF CI study of the electronic spectrum of s-tetrazine

Configuration interaction (CI) studies of ground, n→ π* and π→ π* electronically excited states are reported for s-tetrazine. The first n→ π* singlet excited state (¹ B _3u ), which is responsible for the purple-red colour of the molecule, is calculated at 2·80 eV, compared to the experimental transition energy of 2·22–2·70 eV. The singlet-triplet split of the first n→ π* states (¹ B _3u and ³ B _3u states) is calculated to be 0·76 eV.

The interaction of nitrogen lone pair orbitals (n-orbitals) is studied in terms of the ordering of the n π* excited states and found that the SCF orbital ordering is qualitatively in accord with the ordering of the n π* excited states in the CI level.

The first π→ π* excited state (¹ B _2u ) is calculated at 5·99 eV, slightly above the observed range of absorption. Numerous other high-lying singlet states as well as the triplet states have been calculated and they are used to verify several proposals relating to the excited state dynamics in the photo-physical studies of s-tetrazine.     

Emission and absorption spectra of 2,3,4-, 2,3,5- and 2,4,5-trifluorobenzaldehyde vapors

Emission and absorption spectra have been measured for 2,3,4-, 2,3,5- and 2,4,5-trifluorobenzaldehyde vapors at different temperatures along with the Raman spectra of the pure samples. The observed emission is shown to consist of the T₁(n, π∗) phosphorescence accompanied by the weak S₁(n. π∗) delayed fluorescence for all the molecules. The vibrational analyses of the fluorescence and phosphorescence spectra based on the results of the DFT calculation at the B3LYP/6-311+G(p,d) level indicate that the observed emission and absorption originates from only the stable conformer (anti). The measured Raman spectra also agreed well with those calculated for the stable (anti) conformer.     

The electronic structure of the uranyl ion

Polarized single crystal absorption spectra at 4·2 K are reported for CsUO₂(NO₃)₃ and NaUO₂(CH₃COO)₃. Natural and magnetic circular dichroism and Zeeman effect measurements have been made. Measurements have also been made on crystals containing U¹⁸O₂ ²⁺. In the portion of the spectrum between 21 000 and 30 000 cm^-1 seven electronic states have been identified in both CsUO₂(NO₃)₃ and NaUO₂(CH₃COO)₃. In the majority of cases the symmetry of the excited states has been assigned.     

Carbon K-shell excitation spectra of linear and branched alkanes

The electron energy loss spectra of ethane, propane, n-butane, n-pentane, n-hexane, isobutane, isopentane and neopentane in the region of carbon K-shell excitation have been recorded under dipole-dominated conditions (2.8 ke V impact energy, small angle). The spectra are dominated by transitions to unoccupied valence π¹(CH₂, CH₃) and σ¹(C-C) levels. Additional weak features are assigned to Rydberg transitions. The position of the main continuum feature in each spectrum is consistent with the predictions of an empirical relationship with bond length. Systematic variations of spectral intensities are observed which support our assignments. The dominant feature in the K-shell spectrum of ethane, which was previously assigned to C 1s → 3p Rydberg transitions, is reassigned to excitation to a 3p/π¹(CH₃ ), mixed Rydberg/valence orbital (of antibonding σ-¹(C-H) character), in comparison to the other alkane spectra. An improved calibration value of 290.74(5) eV for the energy of the C 1s → π¹ transition in CO₂ is also obtained.     

Lowest excited states of 2-aminopurine

Absorption and fluorescene spectra and quantum yields of emission over a wide range of pH and solvent polarity were measured for 2-aminopurine and 2-dimethylaminopurine. An increase in dipole moment and a rise of pK values for N₁(N₃) ring atom protonation in the excited state were demonstrated. Thus, the lowest excited singlet state shows partially the (1, a_π¹) configuration. The polarization, lifetimes and the phosphorescence-to-fluorescence ratios indicate that spin-orbit coupling of ¹(n, π¹) and ¹(1, a_π¹) with the lowest triplet state is the main source of phosphorescence intensity. The matrix element of spin-orbit coupling of ¹(n, π¹) to the T₁ state is about 33 times larger than that of ¹(1, a_π¹). The temperature- dependent process of 2-aminopurine fluorescence quenching in either is presumably due to the thermal population of the second excited triplets state of (n, π¹) configuration, which can approximately be located at 370 cm^-1 above the lowest (1, a_π¹) singlet.