The T3 (π, π) State of acridine

The 00 band maximum of the transition T₃(π, π^*) ← T₁ (π, π^*) of acridine occurs at ≈ 10200 ± 20 cm^?1 in inert (n-hexane, benzene, CCl₄), at 10220 ± 20 cm^?1 in polar (acetonitrile) and at 10170 ± 50 cm^?1 in hydrogen-bonding (methanol, 2-propanol and alkaline water) solvents. Based on the solvent-independent energy of T₁ (π, π^*), the T₃(π, π^*) state of acridine is estimated at 26050 ± 50 cm^?1 in all the solvents.     

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     

Excited state geometry of and radiationless processes in the lowest B3u(nπ*) singlet state of s-tetrazine

High resolution absorption and laser induced emission spectra of the lowest B_3u(nπ^*) singlet state of s-tetrazine-h₂ and -d₂ in a benzene crystal at 1.8 K are presented and discussed. The absorption spectrum with origin at 17231 cm^?1 (h₂) is dominated by a progression in ν_6a and a Herzberg-Teller origin which has been assigned as ν₁. The absence of ν₁ in the emission spectrum is explained as being due to a destructive vibronic interference effect. The Franck-Condon envelope of the unique ν_6a progression in emission is used for a determination of the excited state structure and the limitations of this procedure are examined. Direct lifetime measurements using a dye laser and single photon counting techniques show the fluorescence lifetime of s-tetrazine-h₂ and -d₂ to be shorter than 1.5 ns. From a deconvolution of the emission pulse of dimethyl s-tetrazine its fluorescence lifetime in the gas phase is found to be 6.0 ± 0.3 ns. Through a comparison of the fluorescence quantum yield of s-tetrazine-h₂ and dimethyl s-tetrazine we calculate for s-tetrazine-h₂ a fluorescence lifetime of 1.5 ± 0.2 ns and a fluoresence quantum yield of 1.8 × 10^?3. The ratio of the emissive lifetimes of s-tetrazine-d₂ and -h₂ was measured from relative fluorescence yields and found to be 1.18 ± 0.05. Photodissociation quantum yield studies on s-tetrazine-h₂, -d₂ and dimethyl for excitation into the origin of the ¹B_3u(nπ^*) state show this yield to be in the range of 1.3 ± 0.3, and this could explain the low fluorescence yields of the s-tetrazines. The fluorescence quantum yields in the gas phase are found to vary among the vibronic levels of the ¹B_3u state. This finding is in agreement with earlier measurements by Vemulapalli and Cassen, but the report by these authors that such quantum yield variations also occurred in the rovibronic structure is not confirmed.     

Triplet xanthone in n-pentane: multiple emission from a single site

The three phosphorescence components of xanthone in n-pentane originate from three states of one solvated species: From the z sublevel of the second triplet state, of ³nπ^* origin, and from two widely split sublevels of the lowest triplet state, of ³ππ_* origin. Its z sublevel is thermally depleted across the spin-orbit mixing induced zero-field sublevel splitting of 15.1 cm^?1.     

Self-quenching and electronic energy transfer of triplet molecules. The benzaldehyde-biacetyl system in the vapor phase

The triplet-triplet energy transfer from benzaldehyde to biacetyl and the competing self-quenching between triplets and ground state molecules of benzaldehyde were investigated in the dilute vapor phase by monitoring the phosphorescence (T₁(nπ^*)S_o) decay of benzaldehyde. Following excitation into the S₁(nπ^*)S₀ absorption band, a triplet self-quenching rate constant of k_SQ=(2.4±0.1) × 10⁴ s^?1 Torr^?1, corresponding to a gas-kinetic cross section of σ_SQ=0.22 A², was measured. The collision-free lifetime of the benzaldehyde triplet was found to be 2.3 ± 0.4 ms. Substitution of the aldehydic proton by deuterium reduces k_SQ by a factor of two: complete deuteration of the molecule has no further effect. Under the same excitation conditions, the energy transfer rate to biacetyl is k_ET=(2.8 ± 0.1) × 10⁶ s^?1 Torr^?1, with σ_ET = 24 A². This process is not influenced by deuteration.     

Temperature effects on the phosphorescence spectra and lifetimes of 2,4-, 2,5-, and 3,4-dimethylbenzaldehydes dispersed in durene single crystals

The phosphorescence spectra and lifetimes of 2,4-, 2,5-, and 3,4-dimethylbenzaldehydes dispersed in durene single crystals have been measured as a function of temperature between 10 and 200 K. For all the guests involved, the vibrational structures of the spectra are found to be temperature dependent. This is interpreted in terms of two emissions that proceed from a triplet state having predominantly a ππ* character at low temperatures and from a thermally populated triplet state having essentially a nπ* character at higher temperatures. The energy gaps ΔE_T between ³ππ* and ³nπ* states evaluated spectroscopically are found to be 100, 70, and 340 cm^?1, respectively for 2,4-, 2,5- and 3,4-dimethylbenzaldehydes.Activation energies ΔE* determined from the Arrhenius plots of the phosphorescence decay rate constants are in good agreement with the ΔE_T for the first two guests. In contrast, the ΔE* are higher than the ΔE_T for 3,4-dimethylbenzaldehydes as well as for 2,4,5-trimethylbenzaldehyde (where ΔE_T ≈ 400 cm^?1) because of the rapid increase of radiationless transitions in the temperature range where thermal population of the upper ³nπ* state is efficient. In the low and high temperature ranges, the phosphorescence decays for all these guests are exponential. In the intermediate range, these decays are non-exponential. The origin of these non-exponential decays is discussed.     

Diffusion coefficient measurement of O*2(1Δg) in ground state O2

The diffusion coefficient of O_*2(¹Δ_g) in O₂(³Σ^?_g) has been measured as a function of pressure, D^* = 0.201 ± 0.005 cm² s^?1 at 1 atmosphere and 298 K.     

Assignment and analysis of the absorption and emission spectra of the lowest nπ* triplet state in 9,10-anthraquinone

Polarized Zeeman absorption experiments on 9,10-anthraquinone crystals show the lowest triplet state in this molecule to be a g nπ^* state. The gap between this state and the higher u nπ^* state is found to be 410 cm^?1. The phosphorescence spectrum of an isotopically mixed crystal of AQ-h₈ in AQ-d₈ is analyzed in detail and confirms the orbitally forbidden nature of the emitting state. The results are compared with those previously obtained for p-benzoquinone.     

Absorption and emission spectra of the lowest triplet state in hexachloroacetone

T₁ ← S₀ absorption and T₁ → S₀ phosphorescence spectra of neat cystalline hexachloroacetone have been analyzed at 4.2°K. From the lifetime and energy the upper state is assigned as ³nπ^*. The spectra are sharp compared to other aliphatic ketones, with the 0-0 band at 26 248 ± 2 cm ^?1. The phosphorescence shows two strong progressions; one involving the CO stretching mode at 1784 cm^?1 (x), the other a long progression of at least 8 bands involving a mode at 143 cm^t-1 (a). The 143 cm^?1 progression forming mode can best be asigned to the CO out-of-plane wagging vibration. The absorption shows the same two strong progressions, reduced in frequency to 1270 cm^t-1 and 123 cm^?1, respectively, but with the progression in mode a broadened with increasing n. The broadening is interpreted as arising from inversion doublets; the close harmonicity up to n = 5 allowing the potential barrier to inversion to be estimated as > 700 cm^?1. A feature of the spectra is the absence of low frequency torsional modes suggesting lack of pseudo Jahn-Teller distortion of the triplet state potential surface. For comparison, the phosphorescence of crystalline hexafluoroacetone was also studied at 4.2°K. The spectrum exhibits broad bandedness with a 00 band tentatively assigned at 26 870 ± 20 cm^?1.     

Structure and dynamics of the lowest triplet state in p-benzoquinone: I. An isotope effect study on the optical absorption,emission and ODMR spectra

The results of detailed spectroscopic experiments on the lowest nπ* triplet state of p-benzoquinone-h₄, -dh₃, 2,6-d₂h₂, -d₄ and -CH₃ in mixed and isotopic mixed crystals are presented and analyzed. The origin of the lowest B_1g (nπ*) singlet-triplet transition in p-benzoquinone-h₄ (PBQ-h₄) is shown to be induced by asymmetric isotopic substitution and the oscillator strength of this origin is seen to be accounted for by a corresponding decrease in intensity of a level 16.9 cm^?1 higher in energy in the pure PBQ-h₄ crystal. The combined oscillator strength of these close lying levels is measured and found to be almost independent of deuteration.These results are discussed in reference to the previously proposed double minimum potential model for the lowest nπ* triplet state in PBQ-h₄ and the applicability for this model is critically examined.Optical absorption experiments on heavily doped isotopic mixed crystals of PBQ-h₄ in PBQ-d₄ show hydrogen (deuterium) bounding effects between translational inequivalent molecules to be primarily responsible for the observed cluster states. These hydrogen bounding effects also induce the electronic origin of the B_1g (nπ*) triplet state in case of a translational inequivalent dimer.A detailed vibrational analysis of the phosphorescence spectrum of PBQ-h₄ in a PBQ-d₄ host crystal at 1.8 K is presented and it is shown that the unobserved origin of the B_1g (nπ*) triplet state of PBQ-h₄ is located at 18609 ± 1 cm^?1 and that the inversion splitting in this lowest excited state amounts to 21 ± 1 cm^?1 in this mixed crystal system. An isotope effect is study on the vibronic structure in the emission spectrum further indicates that the excited state structure of PBQ is isotope dependent.The observed large isotope effect on the ZFS parameters of the lowest triplet state of PBQ-h₄ is demonstrated to be an intramolecular phenomenon and explained as an isotope dependent spin-orbit contribution to the ZI-S parameters, induced by localization of the nπ* excitation on oxygen.Finally the dynamics of energy migration in the dilute PBQ-h₄ in PBQ-d₄ isotopic mixed crystal is probed by concentration and temperature dependent phosphorescence intensity measurements and it is suggested that trap-exciton band communication effects are of importance in this system.     

The collisional quenching of O2*(1Δg) by NO and CO2

Rate coefficients for the collisional quenching of O₂^*(¹Δ_g) by NO and CO₂ at 2–8 torr and 300 K have been determined. k_NO = (2.48 ± 0.23) × 10^?17 cm³ molecule^?1 s^?1 and

= (2.56 ± 0.12) × 10^?18 cm³ molecule^?1 s^?1.     

The visible spectrum of jet-cooled CF3NO

The electronic origin of the à A″ ← X̃ A′ transition of trifluoronitrosomethane (CF₃NO) has been reassigned to the very weak feature near 717.9 nm in the fluorescence excitation spectrum of the jet-cooled molecule. The prominent torsional progression has been reinpreted and the height of the threefold torsional barier in the Ã( n,π^* ) state has been revised to 601.5 ± 10 cm⁻¹.     

Photodissociation of dimethylnitrosamine

Photodissociation of (CH₃)₂N-NO following S₁(nπ^*) ← S₀ excitation yields (CH₃)₂N^? and NO with a quantum yield of 1.03 ± 0.10. These fragments recombine leaving no stable photopioducts. A fraction of NO produced by photolysis is vibrationally excited. The rate of the NO(v = 1) relaxation in collision with (CH₃)₂N-NO, measured by IR fluorescence, is (1.47 ± 0.03) × 10⁴ s^?1 Torr^?1.     

The triplet state of photosynthetic pigments. I. Pheophytins

ZFS constants (in cm^?1) and decay rate constants for the lowest triplet state of pheophytins have been determined by ESR: pheophytin a: D = 341 ± 3,E = 33 ± 3, K_T = 1050 s^?1; pheophytin b: D = 358 ± 8, E = 46 ± 5, K_T = 630 s^?1; bacteriopheophytin: D = 256 ± 4, E = 54 ± 5/37 ± 5, K_T ≈ 4000 s^?1. In addition values for the decay rate constants and relative populating rates of the individual spin levels have been obtained; these numbers turn out to be appreciably different from those for the corresponding chlorophylls. For the series pheophytin a, b and bacteriopheophytin we find parallel behaviour with the corresponding chlorophylls. The effects of side group substitution and pyrrole ring reduction on the ZFS constant D can be understood by including configuration interaction between the excited states using the 4-orbital model. The change of the mean triplet decay constant K_T upon side group substitution and pyrrole ring reduction follows an energy gap law. Substitution of the central Mg-ion by two protons, however, causes K_T to increase; this is attributed to the introduction of an extra promoting mode - of the NH-group - and/or to the presence of low lying nπ^* states in pheophytins.     

Correlation of the zero-field splittings with the n,π* and π,π* triplet levels of benzaldehydes

The n,π^* and π,π^* triplet state energies of p-chlorobenzaldehyde and p-mathoxybenzaldehyde were determined in several hosts with the aid of phosphorescence and phosphorescence excitation spectra. A linear relationship expected from the theory considering spin-orbit interaction between the closely located n,π^* and π,π^* triplet states was found to be satisfied. The spin—orbit interaction parameter, <vb>G₂>vb>² was found to be 83 cm^?2 for benzaldehydes.     

Quenching of triplet states of diazines by H-atom donors. Formation of azyl radicals

The laser flash photolysis of pyrazine in water and in organic solvents has been examined. The ³(n, π^*) state in water has absorption bands at 230, ≈ 260, ≈ 295, ≈ 640, 700 and 810 nm, and decays with k = 2.2 × 10⁵ sec^?1. It is quenched by oxygen with k_q = 3.2 × 10⁹ M^?1 sec^?1 and by various H-atom donors, e.g., k_q = 1.3 × 10⁸ M^?1 sec^?1 for isopropyl alcohol. On reaction with H-atom donors, the chemistry of ³(n, π^*) pyrazine produces the neutral pyrazyl-radical and the dihydro radical cation, whose characteristic absorption spectra have been identified. These results are discussed by comparison with ³(π, π^*) diazines and with ³(n, π^*) aromatic carbonyl compounds.     

Zeeman experiments on the 3Eu α 1A1g transition of platinum porphein in an n-alkane single crystal at 4.2

We report absorption spectra from the ground state to the photoexcited triplet state of platinum porphin (PtP) in single crystals of n-octane (C₈) and n-decane (C₁₀) at 4.2 K, with and without a magnetic field. For PtP in C₁₀ the same transition was studied in emission. From the experiments, values are derived of the spin-orbit coupling parameter Z, the crystal field splitting δ and the orbital angular momentum A for PtP in the two hosts: Z = 76 ± 2 cm^?1 (C₈, C₁₀), δ = 71 ± 1 cm^?1 (C₈), 55 ± 1 cm^?1 (C₁₀) and A = 1.6 ± 0.1 (C₈, C₁₀). For the ratio of the in-plane and the z-polarized electric dipole transition moments we obtain ¦Mx,y¦/¦Mz¦=76± 0.3 (C₈).     

A Raman spectroscopic study of the low-frequency vibrations in anisole and anisole-d3

Low-frequency Raman spectra of solid anisole and of solid anisole-d₃ have been recorded at 130 K. The phenyl torsion observed at 148 cm^?1 is shifted to 133 cm^?1 upon deuteration of the methyl group. The twofold torsional barriers calculated from these frequencies are 4033 ± 110 cm^?1 and 4094 ± 123 cm^?1 indicating that coupling to other low-frequency modes in both cases is of the same order of magnitude. The methyl torsional mode was observed at 285 cm^?1 in the spectrum of solid anisole and at 183 cm^?1 in the spectrum of anisole-d₃. The threefold barriers calculated using these frequencies are 1847 ± 20 cm^?1 and 1465 ± 18 cm^?1 respectively. These barrier values indicate that the methyl torsion is coupled to another low-frequency mode. A doublet centered at 230 cm^?1 in anisole is shifted to 245 cm^?1 in anisole-d₃; it is proposed that this is due to a ring mode coupled to the methyl torsion. The splitting is interpreted as an example of Davydov splitting.     

Temperature dependence of radiationless processes. Isoquinoline in solution

The temperature dependence of the fluorescence quantum yield φ_f, the fluorescence lifetime τ_f, and the oscillator strength f(S₀→S₁) of isoquinoline in solution has been measured between room temperature and 77 K. Following an Arrhenius type expression, φ_f in ethanol increases from 0.012±0.002 at 295 K to 0.61±0.03 at 77 K paralleled by an increase of τ_f from 0.25±0.10 ns to 9.0±0.2 ns. Over the same temperature range f(S₀→S₁) and the radiative fluorescence lifetime remain constant. By analyzing the temperature dependent data, it is shown that a spin-allowed internal conversion process with an activation energy of ～1500 cm^?1 is responsible for the observed temperature effect. A mechanism is proposed based upon a thermally activated depopulation of the S₁(ππ^*) state of isoquinoline via a slightly higher state, presumably the S₂(ππ^*) singlet state. An extremenly fast process involving the dissociation of the hydrogen bond deactivates this latter state, by possing S₁.