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.     

The photophysics of trans-stilbene

The fluorescence lifetime of trans-stilbene in dilute methylcyclohexane/iso-hexane solution has been measured and the mean S₁ radiative (k_F), radiationless (k_I) and cis-isomerization (k_C) rate parameters have been determined from ?90 to 60°C. S_i consists of a fluorescent trans (¹B_u^*) state (k_F⁰ = 6.0 × 10⁸ s^?1) which undergoes reversible thermal-activated rotational internal conversion (ΔH = 1.75 kcal mole^?1, ΔS = 10.6 cal deg^?1 mole^?1) to a non-fluorescent perp (¹A_g^*) state. p(¹A_g^*) lies 610 cm^?1 above t (¹B_u^*) with an intermediate S₁ potential maximum. p(¹A_g^*) undergoes internal conversion(k_I.^∞ = 5.8 × 10⁸ s^?1) to p (¹A_g) leading to cis-isomerization. This is the main isomerization channel over the whole temperature range.     

Emission spectrum of p-benzoquinone in a fluid solution

The emission spectrum of p-benzoquinone in carbon tetrachloride has been investigated at different temperatures. The results indicate that the emission consists of two types of phosphorescence, ³A _u (T₂) → ¹A _g (S₀) and ³B_1g (T₁) → ¹A _g, and thermally activated delayed fluorescence, ¹B_1g(S₁) → ¹A _g. The spectroscopically estimated singlet—triplet energy separations agree well with those determined from the temperature dependence of the emission spectrum.     

Some comments on vibronic intensities of naphthalene fluorescence spectra from single vibronic levels

It is shown that some features of intensity distribution among certain vibronic transitions in naphthalene molecule can be understood, when one takes into account adiabatic and nonadiabatic interaction between S₁(¹B_3u), S₂(^tB_2u), and S₃(^IB_3u) electronic states. the vibronic activity of the $\text{6}\text{?}$(b_1g) mode in naphthalene-d₈ can be explained in terms of an anharmonic coupling with the $\text{7}\text{?}$(b_1g) mode. The theoretical analysis suggests reinterpretation of some vibronic transitions.     

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.     

Exciton transitions in quasi-one-dimensional crystals. 9,10-dichloroanthracene

Polarized reflection spectra of the first singlet transition of the α-crystalline form of 9,10-dichloroanthracene are reported. Crystal faces (001), (011) and (010) were examined in spectral range 450 to 350 nm at two temperatures, 5 K and 300 K. Two systems of transitions were observed. The first system is assigned to neutral excitons. Spectral similarities with unsubstituted anthracene and arguments based on the one-dimensional stacking of molecules are used to construct a model of the exciten band structures. The M-polarized ππ* molecular transition gives rise to a four branch band with two allowed transitions. The 0-0_b (A_g → A_u) transition lies 50–100 cm^?1 above the bottom of the exciton band and the 0-0_c′ (A_g → B_u) transition lies at the top of the band. In the reflection spectrum the Davydov splitting c′b for transverse excitons is 210 cm^?1. The exciton band of the 00 molecular transition is not isolated but overlaps the two-particle manifold of the 0–1 vibronic transition. As a result of the 0–1_c′ transition is unexpectedly strong in the spectra of the (010) face. The second system is polarized along the stack-axis a and starts 2500 cm^?1 above the first system. It is tentatively assigned as |a(A_g → B_u) charge transfer exciton transition in agreement with earlier observations.     

The fluorescence of all-trans diphenyl polyenes

The temperature dependence of the fluorescence and fluorescence excitation spectra of all-trans diphenyl hexathene (DPH) and octatetraene (DPO) in six solvents confirms the S₁(¹A_g*) and S₂(¹B_u*) state assignment, and determines their energy difference ΔE. The S₁ fluorescence rate parameter k_F depends on ΔE, the solvent refractive index n, the S₂ (n = 1) fluorescence rate parameter k_F20 (2.23 × 10⁸ s^?1 for DPH, 2.33 × 10⁸ s^?1 for DPO), and the S₂-S₁ coupling matrix element V (745 cm^?1 for DPH, 500 cm^?1 for DPO). The S₁ fluorescence is induced by ¹B_u*-¹A_g* potential interaction (PI), via a b_u vibrational mode (≈ 900 cm^?1), and not by vibronic coupling. The main S₁ radiationless transition, rate parameter k_R, is thermally-activated internal rotation through an angle θ about the central ethylenic bond(s). The PI distorts the S₁ (θ) potential surface and thus influences k_R.     

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.     

Aspects of acyclic azine chemistry—I. The effect of structure and solvents on the electronic absorption spectra of benzaldazine,o-, m- and p-hydroxybenzaldazines

The electronic absorption spectra of benzaldazine, o-, m-, and p-hydroxybenzaldazines have been separately examined in various solvents (20 solvents and solvent mixtures). The observed three groups of bands designated as α (200–225 nm), β (289–307 nm) and γ (352–375 nm) are attributed to π*←π transitions although experimental evidence based on spectra obtained in concentrated sulphuric acid reveals that π*←σ(n) transitions constitute part of the γ-band. On the basis of group theoretic analysis, in the singlet manifold, all π*←σ(n) transitions (i.e. ¹A_u ← 1¹A_g) are out-of-plane (z) polarized while all π*←π transitions (i.e. ¹B_u ← 1¹A_g are in-plane (x, y) polarized. In the rationalization of data, the theory of resonance has been employed where deemed useful. The shifts of bands with solvents have been traced.     

The state energy and the displacements of the potential minima of the 2Ag− state in all-trans-β-carotene as determined by fluorescence spectroscopy

The fluorescence spectrum of all-trans-β-carotene was recorded at 170 K. The 1B_u⁺ → 1A_g⁻ fluorescence exhibited clear vibrational structures constituting a mirror image with those of the 1B_u⁺ ← 1A_g⁻ absorption, and the deconvolution of the entire spectrum identified the 2A_g⁻(0) → 1A_g⁻(0) transition at 14 500 cm⁻¹. The displacements of the 1B_u⁺ and 2A_g⁻ potential minima along ν₁ and ν₂ (the CC stretching and C–C stretching normal coordinates, respectively) were determined to be 1.2 and 0.9, and 1.6 and 1.5, respectively. Thus, much larger potential displacements in the 2A_g⁻ state than in the 1Bu⁺ state have been shown.     

Raman spectroscopy of the [TeX6]2− ions (X  Cl or Br) at resonance with their lowest 3T1u and 1T1u states: Evidence for tetragonal distortion in these excited states

The observation that the v₂(e_g) band is the most enchanced Raman band at resonance with the bands assigned to both the ³T_1u ← ¹A_1g and ¹T_1u ← ¹A_1g transitions of the [TeX₆]^2? ions indicates that the ions are tetragonally distorted in these excited states. The depolarisation ratio of 2v₂ band of [TeBr₆]^2? at resonance with the ¹T_1u ← ¹A_1g transition is found to be 0.18, in close agreement with that expected (3/14) for the first overtone of a doubly degenerate vibration coupled to a triply degenerate excite state.     

1 Bu–2 Ag conical intersection in trans-butadiene: ultrafast dynamics and optical spectra

The potential-energy functions of the 1 ¹B_u and 2 ¹A_g excited valence states of trans-butadiene have been characterised by the CASPT2 method. Based on these ab initio data, a vibronic-coupling model describing the conical intersection of the 1 ¹B_u and 2 ¹A_g states has been constructed. UV resonance-Raman and absorption spectra have been calculated, employing the time-dependent approach. The time-dependent wave-packet calculations reproduce the expected ultrafast (≈30 fs) radiationless decay of the optically bright 1 ¹B_u state into the dark 2 ¹A_g state.     

Absorption,fluorescence and phosphorescence spectra of the singlet and triplet states of s-tetrazine in the crystal and in mixed crystals at low temperatures

The electronic absorption, fluorescence and phosphorescence spectra of s-tetrazine at low temperatures (4.2-1.5 K) are reported and analyzed in the neat crystal and in several mixed crystals. The ³B_3u-¹A_g (nπ^*) origin is at 18414 ± 5 cm^?1 for neat tetrazine. In the mixed crystal several sites identified. The lowest energy origin is at 17453 cm^?1 for tetrazine in pyrazine; 17 701 cm^?1 in pyrimidine; and 17 676 cm^?1 in pyridazine. The ^eB_3u-¹A_g (nπ^*) origin is at 14 096 ± 2 cm^?1 for the neat crystal. The phosphorescence lifetime of neat tetrazine is measured to be 96.8 ± 2.1 μs at 4.2 and 1.8 K. All the spectra are predominately composed of members of progressions in a single totally symmetric mode (ν_6a) built upon site origins and vibrational fundamentals. The ν_6a interval is: 743 (¹A_g), 715 (³B_3u), and 709 cm^?1 (¹B_3u) in the neat tetrazine crystal; 732 (¹A_g) and 705 cm^?1 (¹B_3u in pyrazine host, 737 (¹A_g) and 701 cm^?1 (¹B_3u) in pyrimidine host, and 732 (¹A_g) and 703 cm^?1 (¹B_3u) in pyridazine host mixed crystals. All emission spectra may be analyzed by Oi → (ν″_6a)^o_n (i), i indicating the observed s     

Analysis of the vibronic structure of the 1A1g → 1B3g, 1B2u transition in biphenyl -h10 and -d10

The absorption spectra of biphenyl-h₁₀ and -d₁₀ crystals in the region of the ¹A_1g → 1B_3g, ¹B_2u transitions are interpreted in terms of vibronic coupling of the two closely spaced electronic states by six b_1u vibrations.     

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.     

Theoretical study on S<Subscript>1</Subscript>(<Superscript>1</Superscript>B<Subscript>3u</Subscript>) state electronic structure and absorption spectrum of pyrazine

Making use of a set of quantum chemistry methods, the harmonic potential surfaces of the ground state (S₀(¹ A _g)) and the first (S₁(¹ B _3u)) excited state of pyrazine are investigated, and the electronic structures of the two states are characterized. In the present study, the conventional quantum mechanical method, taking account of the Born-Oppenheimer adiabatic approximation, is adopted to simulate the absorption spectrum of S₁(¹ B _3u) state of pyrazine. The assignment of main vibronic transitions is made for S₁(¹ B _3u) state. It is found that the spectral profile is mainly described by the Franck-Condon progression of totally symmetric mode ν_6a. For the five totally symmetric modes, the present calculations show that the frequency differences between the ground and the S₁(¹ B _3u) state are small. Therefore the displaced harmonic oscillator approximation along with Franck-Condon transition is used to simulate S₁(¹ B _3u) absorption spectra. The distortion effect due to the so-called quadratic coupling is demonstrated to be unimportant for the absorption spectrum, except the coupling mode ν_10a. The calculated S₁(¹ B _3u) absorption spectrum is in reasonable agreement with the experimental spectra. Supported by Taiwan National Science Council (Grant Nos. NSC 96-2113-M-009-021 and NSC 96-2811-M-009-023)     

Pressure effects of a genuine organic crystalline ferromagnet dupeyredioxyl

We have revealed that the isothermal magnetization M of the genuine organic crystalline dupeyredioxyl (N,N′-dioxy-1,3,5,7-tetramethyl-2,6-diazaadamantane; T_c(0)=1.48 K) observed below 10 K converges on the S=1 Brillouin function B₁((H+λM)/k_BT) with λ=2.4±0.2 or 2zJ/k_B=3.6±0.3 K, where J and z are, respectively, the averaged exchange interactions and coordination numbers for the S=1 spin system. This fact suggests that S=1 is constructed within a molecule via a strong ferromagnetic coupling between two S=1/2 spins on each of the two NO moieties. The modified notation of the Rushbrooke and Wood theory, T_c=2AzJS(S+1)/k_B (A=0.23±0.02 for the three-dimensional Heisenberg systems), is found to quantitatively hold not only for this S=1 spin system but also for other S=1/2 ferromagnets β-phase p-NPNN (2zJ/k_B=3.6 K) and 2,5-DFPNN (2zJ/k_B=2.8 K). Pressure effects of this compound have been studied under the hydrostatic pressure (P) up to 15 kbar. T_c(P) is revealed to show a negative pressure effect with the initial gradient a=d(T_c(P))/dP=−0.047 kbar⁻¹, nearly the same value for other organic ferromagnets as β-phase p-NPNN (−0.048 kbar⁻¹) and p-Cl-TEMPO radical (−0.03 kbar⁻¹), in contrast to the positive pressure effect for genuine antiferromagnets such as TANOL (a=+0.15 kbar⁻¹). Microscopically, different from the above two ferromagnets, the pressure-induced destruction of the orthogonality of molecular orbitals associated with the two NO moieties plays an effective role in reducing the intramolecular ferromagnetic interaction J₀. The possible weaker intermolecular interactions other than J₀ and J are also expected to be more susceptible to the stress of pressure to result in the reduction of their values perhaps even changing their sign, just as in the case of β-phase NPNN or p-Cl-TEMPO.     

The lowest triplet state of tetramethyl-1,3-cyclobutanedithione. II. Calculation of spin-orbit coupling

A calculation of the spin-orbit coupling in the lowest excited triplet state of tetramethyl-1,3-cyclobutanedithione (TMCBDT) has been performed. The results show the following. (1) In the TMCBDT crystal the ground singlet-to-lowest triplet transition moment is predicted to be exclusively ? c polarized, as observed. (2) The assignment of the lowest triplet state should be ³A_u as found earlier for the oxygen analog. TMCBD. (3) The two largest contributions (～ 60%) to the isolated-molecule T₁ → S₀ transition moment come from the two triplet-triplet transitions.

and

, both of which are polarized along the CS bonds. (4) The total contribution to the transition moment parallel to the CS bonds is 76% from the T_l ← T₁ transitions and 24% from the S_γ ← S₀ transitions. And, finally, (5) the calculated oscillator strength of 2 × 10^?4 for the largest T₁ ← S₀ component (along y) falls within the range of typical spin-allowed, singlet-singlet n-⁼ transitions.     

Laser-induced fluorescence in N2 and N+2 by multiple-photon excitation at 266 nm

The fluorescence transitions corresponding to the second positive system of N₂ (C³Π_u → B³Π_g) for Δv = 0, 1 and the first negative system of N⁺₂(B²Σ⁺_u → X²Σ⁺_g) for Δv = 0, 1, 2 have been observed following laser-induced mul excitation of N₂.