Qualitative considerations of the T1 → S0 intersystem crossing in benzene

We consider the equivalent of the S₀(¹A_1g → T₁(³B_1u absorption spectrum of benzene obtained by Burland et al. On the basis of this spectrum we suggest that the theoretical rate of the T₁(³B_1u) → S₀(¹A_1g) intersystem crossing in benzene is faster by several orders of magnitude than that obtained in recent theoretical work. Furthermore, it is suggested that the rate of this process is not retarded drastically upon deuteration, as claimed in the literature. A new interpretation of the S_o(¹A_1g) → T₁(³B_1u absorption spectrum is also given.     

Theoretical study of the absorption and emission spectrum and non-adiabatic excited state dynamics of gas-phase xanthone

The ground, singlet, and triplet excited state structures (S₁, S₂, T₁, and T₂) of xanthone have been calculated and characterized in the adiabatic representation by using time-dependent density functional theory (TDDFT). However, the fast intramolecular transition mechanisms of xanthone are still under debate, and so we perform non-adiabatic excited state dynamics of the photochemistry of xanthone gas phase and find that it follows El-Sayed's rule. Electronic transition mechanism of xanthone is sequential from the S₂ state: the singlet internal conversion (IC) time from S₂ (¹ππ*) to S₁ (¹nπ*) is 3.85 ps, the intersystem crossing (ISC) from S₁ (¹nπ*) to T₂ (³ππ*) takes 4.76 ps, and the triplet internal conversion from T₂ (³ππ*) to T₁ (³nπ*) takes 472 fs. The displaced oscillator, Franck–Condon approximation, and one-photon excitation equations were used to simulate the absorption spectra of S₀ → S₂ transition, with v₅₅ being most crucial for S₀ structure; the fluorescence spectra of S₁ → S₀ transition with v₄₇ for S₁; and the phosphorescence spectra of T₁ → S₀ transition with v₄ for T₁. Our method can reproduce the experimental absorption, fluorescence, and phosphorescence spectra of gas-phase xanthone.     

Large-scale configuration interaction calculations on the π-electron states of benzene

Ab initio extensive configuration interaction calculations were carried out on the π-electron states of benzene. Among the three π → π^*(e_1g → e_2u) singlet states, ¹B_2u(S₁). ¹B_1u(S₂), and ¹E_1u(S₃), the π^* orbital was found to be velence-like in S₁ and S₂, but diffuse in S₃. All three corresponding triplet states, ³B_1u(T₁) and ³B_2u(T₃), were found to be valence-like. The valence-like ¹E_2g(S₄) and ³E_2g(T₄) states were found to have significant double-excitation character, and were estimated to lie somewhat above S₃ and T₃, respectively. No low-lying S₅ and T₅ states were found. Several low-lying Rydberg states were identified.     

Selective excitation effects in microwave optical double resonance and phosphorescence spectroscopy of molecular crystals

The photoexcitation routes used to produce molecular crystal, triplet states are shown to have important optical and microwave spectral consequences. 2-benzoylpyridine crystals at 4.2 K have T₁ → S₀ phosphorescence spectra showing line width dependence on whether initial production of the T₁ state is through direct T₁ → S₀ absorption, or through S₁ ← S₀ absorption followed by S₁ → T₁ intersystem crossing. Striking differences are seen in the optically detected zero-field resonance spectra.     

Distinction of the delayed fluorescences S1 → S0 And S2 → S0 of pyrene by selective quenching Of S1 → S0

In the spectrum of the delayed fluorescence (DF) of pyrene, caused by triplet-triplet annihilation T₁ + T₁ → S_n + S_o (n = 1,2), a strong DF S₁ → S_o and a very weak DF S₂ → s₀ are observed. The DF S₁→ S_o is quenched selectively by compounds like N-diethylanine or triethylamine which do not quench T₁ of pyrene.     

Delayed fluorescence S2 → S0 of azulene in isopentane,due to hetero-triplet—triplet annihilation of 3azulene* and 3fluoranthene*

The lowest triplet state of azulene, T₁(Az), can be populated efficiently by triplet energy transfer from the lowest triplet state of fluoranthene, T₁(F1). In isopentane at temperatures 120 K ? T ? 193 K a delayed fluorescence S₂(Az) → S₀(Az) is found, caused by hetero-triplet—triplet annihilation T₁(Az) + T₁(Fl) → S₂(Az) + S₀(F1).     

Stochastic model for triplet yields

A stochastic model of triplet yields is considered where the singlet S₁ is initially excited and subsequently feeds the triplet T₁. Both S₁ and T₁ have Montroll—Shuler step ladder vibrational relaxation mechanisms and radiative and non-radiative decay rates that vary linearly with increasing vibrational energy. Assuming the S₁ → T₁ rates also have this linear variation, the kinetic model is exactly solved in terms of integrals of simple functions of hyperbolic functions. The predictions of the model are illustrated by application to naphthalene. The model parameters are chosen; wherever possible, from experimental data. The predictions are in gross qualitative agreement with available experiments on triplet yields, and they indicate more detailed future experiments to separate the S₁ → T₁ and S₁ → S₀ (ground singlet) decays (and their energy dependence) in aromatic hydrocarbons.     

The triplet state decay (T1(nπ*) → S0) of benzaldehydes in the dilute gas phase

The triplet T₁(nπ^*) decay of benzaldehyde (B) and its isotopomers

and

were investigated in the dilute vapour phase (≤0.5 Torr) at room temperature. Following

excitation the quantum yields of the phosphorescence and photodecomposition, and the rate constants of the phosphorescence and the radiationless T₁ → S_o process were determined. Based on these results and in conjunction with theoretical calculations of T₁ → S_o rates and previous data obtained on propynal, the decay mechanism of benzaldehyde was analyzed. It is shown that the important accepting modes of the non-radiative T₁ → S_a decay are the

wagging and the CO stretching modes. In spite of the close vicinity of the T₂(ππ^*) and the T₁(nπ^*) states, the non-adiabatic coupling (communication between ring and carbonyl vibrations is not sufficient to influence the relaxed T₁(nπ^*) decay significantly.     

Collision induced intersystem crossing the photophysics of glyoxal vapor excited at 4358 Å

The yields, lifetimes and spectra of singlet ¹A_u (S₁) and triplet ³A_u (T₁) emissions from glyoxal vapor (0.003 to 10 torr) have been measured after initially pumping levels about 1000 cm^?1 above the S₁ zero-point level with the 4358 A Hg line and with flash excitation centered at 4345 A. Only S₁ emission is observed at the lowest pressures. The singlet fluorescence contains appreciable structure from the zero-point level even when the hard sphere collision interval exceeds the radiative lifetime calculated from the absorption coefficient. Implications of long lifetimes (due to S₁ - T₁ vibronic interactions) are not confirmed by pulsed excitation studies. Both S₁ and T₁ emissions are observed at pressures above about 0.1 tert and both are self-quenched. However, added gases such as cyclohexane, argon, and helium selectively quench only S₁ emission. This quenching is collision-induced S₁→T₁ intersystem crossing with cross sections of order 0.1 hard sphere for transitions from the S₁ zero-point level. The triplet yield in 0.2 torr of pure glyoxal is probably near unity, and the subsequent crossing T₁ → S₀, if it occurs, lies in the statistical limit. Indications of fast nonradiative decay from high triplet vibrational levels are seen in the phosphorescence yields. Self-quenching of the triplet state appears to be associated with the photochemical activity of glyoxal.     

List of subject

We report the high resolution emission (S₁ → S₀, T₁ → S₀) and laser single site singlet excitation (S₁ ← S₀) spectra for the various insertion sites of coronene in n-heptane cooled to 1.5 K. The observation of site splitting of doubly degenerate vibrations and weak electric dipole forbidden 0, 0 bands in the S₁ → S₀ and T₁ → S₀ spectra indicates that the ground state, the first excited singlet and lowest triplet states are all distorted. In these spectra, the intensity distribution of the various sites in the 0, 0 bands suggests that the distortion is different from site to site but similar in S₀, S₁ and T₁. Identical ordering of the sites in S₁ S₀ and S₁ S₀ spectra as well as the observation of weak shifts in the vibrational frequencies in the two states implies the absence of strong pseudo Jahn-Teller forces in the first excited singlet state. We propose, further, that this is also true for the triplet state. This conclusion is supported by the similarity in zero-field splitting parameters of coronene and deuterated coronene. Taken together, these results indicate strongly that the distortion of coronene in n-heptane is primarily crystal field induced and is not greatly changed upon excitation of the molecule to its lowest excited states.     

Reversible intersystem crossing,fluorescence lifetime lengthening and collisionally induced phosphorescence in biacetyl

The emissions of biacetyl excited at 4200 Å were studied at pressures down to 10^?3 torr. Apart from the well-known nanosecond fluorescence, a new emission of the same spectral composition was found with a non-exponential decay in the microsecond range. Furthermore the phosphorescence, as defined by its spectral composition, was found to be collisionally induced.The results imply that after excitation, the molecule rapidly transfers (rate constant k_S→T) to the triplet state, giving rise to the nanosecond decay time; and can then transfer back to the singlet state (rate constant k_T→S), giving rise to the microsecond emission. At the same time internal conversion can occur (k_S→S0). From an analysis of the data we find for k_S→S0 = 2.4 × 10⁷ sec^?1, k_S→T = 7.6 × 10⁷ sec^?1, k_T→S = 1.9 × 10⁵ sec^?1. The kinetic treatment can be transformed to a quantum mechanical one, yielding values for the triplet level density (?_T), the coupling element V_ST and the number of triplet states (N) coupled to the singlet excited. At 4200 Å we find ?_T = 6.3 × 10⁵cm, V_ST = 1.0 × 10^?5 cm^?1, N = 400.Phosphorescence occurs only when the molecule is deactivated by collisions to a vibronic triplet state below the vibrationless excited singlet state. The efficiency of biacetyl collisions is 0.54.     

The study of the intramolecular heavy atom effect in 9,10-dichlorophenanthrene and 1,2,3,4-tetrachloronaphthalene using PMDR techniques

Using PMDR techniques, the top two zero-field triplet levels in 9,10-dichlorophenanthrene and 1,2,3,4-tetrachloronaphthalene are shown to be most favoured by S₁ → T₁ ISC and T₁ → S₀ phosphorescence. Spin—orbit vibronic coupling via CCl out-of-plane modes and static distortions of the heavy atom are responsible for such behaviour respectively.     

Laser excited S2 → S1 and S1 → S0 emission spectra and the S2 → Sn absorption spectrum of azulene in solution

We present the S₁ → S₀ fluorescence spectrum, between 740 and 940 nm, of azulene solutions (10^?3 M in methanol) excited with a Q-switched ruby laser. The nitrogen-laser excited S₂ → S₁ fluorescence spectrum, between 700 and 930 nm, is also reported. The transient S₁ → S_n spectrum between 500 and 650 nm was studied, using synchronous nitrogen laser and dye laser excitation. The S₅ (¹B₁(3)) state of azulene was found to be located at 45500 cm^?1 and the cross section σ₂₅ of the transient absorption S₂ → S₅ is estimated to be 3 × 10^?18 cm²/molecule.     

Nonlinear optical properties of polyvinylcarbazole composites with graphene

The study has focused on polyvinylcarbazole (PVK) composites with graphene. It has been shown that there is a noticeable nonadditive shoulder on the long-wavelength edge of the optical absorption of PVK in these samples, which can be attributed to the formation of a charge-transfer complex between PVK as a donor and graphene as an acceptor. The formation of the complex causes a significant nonlinear optical effect in the PVK/graphene composite. The revealed increase in both the nonlinearity coefficient with increasing laser intensity and the cross section with increasing incident energy density is due to the formation of the graphene^?· radical anion, an additional species contributing to nonlinear absorption, with an increase in the radiation energy density. Nonlinear optical properties of PVK composites with graphene isolated from a solution in tetrachloroethane after 1.5-h centrifugation (sample 1) have been considered. It has been suggested that a significant decrease in optical transmission of laser radiation by the composite T _OA = 0.4 at an energy density at focus of 502 J/cm² is due to the formation of the PVK/graphene charge-transfer complex responsible for the nonadditive shoulder on the long-wavelength optical absorption edge of PVK. During photoexcitation of graphene in the PVK/graphene composites at a laser wavelength of 1064 nm, mobile holes are generated in PVK, indicating the formation of graphene^?· radical anions as a result of charge transfer from PVK to photoexcited graphene. The observed increase in both β with an increase in the laser radiation intensity and the cross section (σ_exc — σ₀) with an increase in the incident energy density may be due to either the contribution of nonlinear transitions (S ₀ → S ₂, S ₀ → S ₁ → S ₂, T ₁ → T ₂) or the formation of the additional species, the graphene-· radical anions, participating in nonlinear absorption by increasing the energy density at the focus (F _foc, J/cm²).     

Photon-echo experiments on singlet-triplet transitions in organic mixed crystals

The homogeneous optical linewidth of a spin-forbidden S_o → T₁ transition in a chemically mixed organic crystal was directly measured in a two-pulse photon-echo experiment. The experimental values of T₂^Opt for the system dibromobenzophenone in dibromodiphenyl ether ranged from 1.4(3) to 3.8(2) μs, depending on the nuclear spins of the host. A temperature-dependent contribution to (T₂^Opt)^?1 revealed an activation energy ΔE of 13.8(5) cm^?1, which is tentatively attributed to a librational mode frequency.     

Theoretical study of the photochemical reaction mechanism of bicyclo[4.1.0]hept‐2‐ene upon direct photolysis

Ab initio multiconfigurational CASSCF/MP2 method with the 6‐31G* basis set has been employed in studying the photochemistry of bicyclo[4.1.0]hept‐2‐ene upon direct photolysis. Our calculations involve the ground state (S₀) and excited states (S₁, T₁, and T₂). The ground‐state reaction pathways corresponding to the formation of the six products derived from bicyclo[4.1.0]hept‐2‐ene via two important diradical intermediates (D1 and D2) were mapped. It was found that there are various crossing points (conical intersections and singlet–triplet crossings) in the regions near D1 and D2. These crossing points imply that direct photolysis can lead to two possible radiationless relaxation routes: (1) S₁ → S₀, (2) S₁ → T₂ → T₁ → S₀. Computation indicates that the second route is not a competitive path with the first route during direct photolysis. The first route is initiated by barrierless cyclopropane bond cleavage to form two singlet excited diradical intermediates, followed by efficient decay to the ground‐state surface via three S₁/S₀ conical intersections in the regions near the diradical intermediates. All six ground‐state products can be formed via the three conical intersections almost without barrier after the decays. The barriers separating the diradical minima on S₁ from the S₁/S₀ conical intersections were found to be very small with respect to the vertical excitation energy, which can explain why the product distribution is independent of excitation wavelength. Triplet surfaces are not involved in the first route, which agrees with the fact that the product contribution was unchanged by the addition of naphthalene. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Picosecond spectroscopic measurement of very fast intersystem crossing for 9,10-dioxa-anti-bimanes

Picosecond spectroscopy, following the buildup of T₁ → T_n absorption (maximum at 420 nm), shows that the T₁ state of 1,5-diazabicyclo[3,3,0]octa-3,7-diene-2,6-diones(9,10-dioxa-anti-bimanes) is formed within about 10 ps. The nature of the T₁ state was confirmed by decay rates of T₁ → T_n absorption in acetonitrile (n = 0.375 cP, k_nr = 4.5 × 10⁵ s^?1), 1,2-ethanediol (n = 26 cP, k_nr = 1.5 × 10⁴ s^?1 and glycerol (n = 1400 cP, k_nr = 1.3 × 10³ s^?1). The very fast intersystem crossing is ascribed to the proximity of a ³nπ* state to the ππ* (S₁ state produced by light absorption (El-Sayed rule).     

Consecutive two-photon absorption of azulene in solution utilizing dye lasers

We report a spectroscopic study of consecutive two-photon absorption of azulene excited in the range 32800–42000 cm^?1, which provides information concerning the cross sections for the S₁ → S₃ and the S₁ → S₄ transitions.     

Radiationless decay of the second excited singlet states of aromatic thiones: Experimental verification of the energy gap law

S₂ → S₀ fluorescence quantum yields and S₂ lifetimes of eight aromatic thiones in inert perfluoroalkane solutions at room temperature have been measured using picosecond laser techniques. Photostable, structurally rigid thiones undergo S₂ → S₁ internal conversion at rates consistent with the energy gap law of radiationless transitions. An average electronic coupling matrix element of 1.9 × 10² cm^?1 is found.     

Elektronisches Resonanz-Raman-Spektrum von Hexabromo-osmat(IV)

Electronic Resonance Raman Spectrum of Hexabromo Osmate(IV) Besides the vibrational bands there are other strong bands in the low-temperature Raman spectrum of [OsBr₆]^2?, which are independent from the excitation line and are interpreted as arising from transitions between the spin-orbit split components of the ³T_1g–Os⁴⁺ ground state. The band at 2800 cm^?1 is anomal polarized and attributable to Γ₁(³T_1g) → Γ₄(³T_1g), while the band at 4880 cm^?1 is depolarized and therefore assigned to Γ₁(³T_1g) → Γ₅(³T_1g). In the electronic Raman spectrum, too, a rigorous resonance-Raman effect is displayed and as far as six overtones of the stretching vibration A_1g and as many combination tones especially with T_2g are observed. Because of the dynamic Jahn-Teller effect Γ₁(³T_1g) → Γ₃(³T_1g) cannot be detected as an electronic Raman transition. Γ₁(³T_1g) → Γ₁(¹T_1g) at 15915 cm^?1 is obtained by luminescence absorption. The results are in good agreement with the absorption spectrum.