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.     

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.     

Spin-forbidden electronic transitions in non-planar aromatic amines

Evidence is presented which indicates that the direct spin—orbit coupling between low-lying ππ^* states is largely responsible for the efficient S₁ → T₁ intersystem crossing and T₁ → S₀ radiative transition in non-planar aromatic amines.     

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.     

Jet spectroscopy of pyrene complexes: Unusual complexation shifts in the S0 → S1 spectrum

The fluorescence excitation spectra of pyrene complexes with n-alkanes are reported for the S₀ → S₁ and S₀ → S₂ transitions. The S₂ spectral shifts are predictable from theory, and by comparison with other molecules, such as perylene. On the other hand, the S₁ Resonances of pyrene complexes show unusually small displacements from those of the parent species. The spectra of the butane, pentane and hexane complexes actually exhibit net blue shifts. This behaviour provides good evidence for a repulsive interaction in the S₁ state, which is not observed in S₂. Moreover, because the butadiene and benzene complexes give predictable red shifts ⩾ 100 cm⁻¹, and these are found to have plane-parallel geometries, the blue shift correlates with a host carbon—guest hydrogen interaction in the repulsive regime. We also report that the ethylene complex of pyrene exhibits a net blue shift on the S₀ → S₁ transition, and a red shift on S₀ → S₂ only 75% of the predicted value, based on measurements with perylene complexes. This behaviour strongly indicates that ethylene-pyrene has a T-shaped configuration, as predicted by potential energy calculations.     

Electronic energy transfer from the S2 state of rhodamine 6G

In this paper we present the results of an experimental study of intermolecular electronic energy transfer (EET) from the short-lived Second excited singlet state of rhodamine 6G (R6G) to the ground state of 2,5-bis [5′-tert-butyl-2-benzoxazolyl] thiophene (BBOT). The S₂ state of the donor was excited by sequential, time-delayed, two-photon excitation (STDTPE) utilizing the second harmonic and the first harmonic of a mode-locked Nd³⁺: glass laser, while the EET process was interrogated by monitoring the enhancement of the S₁ → S₀ fluorescence of BBOT. The enhancement of the fluorescence intensity of BBOT was found to be linear in the energies of the two exciting pulses, and linear in the concentration of the energy acceptor (over the BBOT concentration range of (0.3–7) × 10^?5 M), which is in accord with the predictions of the Forster—Dexter mechanism for resonant EET from an ultrashort-lived donor state at low acceptor concentrations. Quantitative measurements of the S₂ → S₀ fluorescence yield in R6G solution directly excited by STDTPE and of the S₁ → S₀ fluorescence of BBOT from R6G + BBOT solutions resulting from EET led to the values of Y_D(S₂ → S₀) = (2.1 ± 0.5) × 10^?6 for the emission quantum yield of the S₂ state of R6G and τr_D(S₂) ≈ 3 × 10^?14 s for the lifetime of the metastable S₂ state of this molecule.     

Intracoil excited singlet state annihilation in polyvinyldiphenylanthracene

The intensity of fluorescence of poly(diphenylanthracene) (PDPA) has been found to be highly non-linear in excitation laser energy, while either diphenylanthracene or polystyrene with a low loading of covalently bound diphenylanthracene (PS-co-DPA) is linear under the same conditions. It is proposed that a Forster-type annihilation process occurs: S₁ + S₁ → S₀ + S_n → 2S₀. The R₀ for this process is estimated to be ≈ 35 Å. On the other hand the singlet exciton diffusion constant (Λ_S) is estimated to be very low, by the method of comparative quenching.     

On the photoisomerisation of stilbene in n-alcohols

Photoisomerisation of t-stilbene in n-alcohols is discussed on the basis of recent experiments by Sundstro¨m and Gillbro. The reaction is proposed to consist of three steps: rotational relaxation on S₁^1rans →_k1S₁^p (p = perpendicular configuration) in a barrierless potential U₁(θ); a fast electronic transition S₁^p → S₀^p due to strong non-adiabatic interactions, and a rotational relaxation S₀^p →_k2S₀^cis(trans) in a barrierless potential U₀(θ); k₁ ≈ k₂. In the diffusion limit the photoexcited state lifetime, τ = [(k₁ + k₂)/2]⁻¹, is a linear function of the solvent viscosity in qualitative agreement with experiment.     

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.     

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.     

Picosecond fluorescence decays from vibrational levels in the S1(n,π*) state of pyridine vapor

Fluorescence lifetimes of pyridine vapor were measured by exciting at various vibrational bands in the lowest-energy region of the S₁(n,π^*) ← S₀ transition. The lifetime varies between 35 and 60 ps, depending on the vibronic level excited. The non-radiative decay from S₁ is characterized by particularly fast S₁ → S₀ internal conversion.     

Moderately high resolution fluorecence spectrum of the S1 → S0 transition of azulene

We report an Ar/Kr ion laser induced spectrally resolved S₁ → S₀ emission from an azulene in a host naphthalene crystal observed at the helium λ-point and at 77 K. Less well resolved S₁ → S₀ emission from crystalline azulene dispersed in a KBr pellet at 300 K is also reported. For the 6471 Å excitation the emission from the azulene in naphthalene system is analyzed in terms of three components: a resonance enhanced Raman emission originating from a nonstationary laser photon energy state 800 cm^?1 above the S₁ origin, a partially relaxed fluorecence originating from the 665 cm^?1 vibrational level of S₁ and a totally relaxed fluorecence from the S₁ origin (14651 cm^?1). The interpretation of the spectral lines is based on totally symetric vibrational modes (406, 679, 825,902, 1203, 1269, 1401, and 1586 cm^?1) the most prominent of which is the progression forming 825 cm^?1 mode. On the basis of both energies and intensities, correlations are made between ground and excited state vibrations and are compared with earlier results. Based on our results, a discussion is given on a plausible relaxation scheme for our system including the influence of Franck—Condon factors on the observability of unrelaxed emission.     

Emission spectrum of 9,10-anthraquinone vapor

Emission and excitation spectra of 9,10-anthraquinone in the vapor phase have been measured and compared with those in the condensed phases. It is shown that the emission of 9,10-anthraquinone vapor consists of the S₁ (n,π*) → S₀ fluorescence and T₁ (n,π*) → S₀ phosphorescence, with the origins at 23 700 and 22 030 cm⁻¹, respectively. The present study presents new measurements that substantially clarify the emission behavior of this molecule in the vapor phase.     

Selective Population of the n, π* and π, π* Triplet States of 1-Indanones

The two components of the dual phosphorescence of 1-indanone ( 1 ) and six related ketones ( 2–7 ) possess different excitation spectra exhibiting the vibrational progression characteristic of the S₀ → S₁ (n, π*) transition (shorter-lived emission) and two bands of the S₀ → S_{2 and 3} (π,π*) 0–0 transitions, respectively. The most favorable intersystem crossing routes are S₁ (n, π*) → T (n, π*) and S_2,3 (π*) → T (π, π*). Internal conversion to S₁ competes more effectively with S (π, π*) → T (π, π*) intersystem crossing only from higher vibrational levels of the S₂ and S₃ states.     

Dissociation of Multisubunit Protein–Ligand Complexes in the Gas Phase. Evidence for Ligand Migration

The results of collision-induced dissociation (CID) experiments performed on gaseous protonated and deprotonated ions of complexes of cholera toxin B subunit homopentamer (CTB₅) with the pentasaccharide (β-D-Galp-(1→3)-β-D-GalpNAc-(1→4)[α-D-Neu5Ac-(2→3)]-β-D-Galp-(1→4)-β-D-Glcp (GM1)) and corresponding glycosphingolipid (β-D-Galp-(1→3)-β-D-GalpNAc-(1→4)[α-D-Neu5Ac-(2→3)]-β-D-Galp-(1→4)-β-D-Glcp-Cer (GM1-Cer)) ligands, and the homotetramer streptavidin (S₄) with biotin (B) and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(biotinyl) (Btl), are reported. The protonated (CTB₅ + 5GM1)ⁿ⁺ ions dissociated predominantly by the loss of a single subunit, with the concomitant migration of ligand to another subunit. The simultaneous loss of ligand and subunit was observed as a minor pathway. In contrast, the deprotonated (CTB₅ + 5GM1)^n- ions dissociated preferentially by the loss of deprotonated ligand; the loss of ligand-bound and ligand-free subunit were minor pathways. The presence of ceramide (Cer) promoted ligand migration and the loss of subunit. The main dissociation pathway for the protonated and deprotonated (S₄ + 4B)^n+/– ions, as well as for deprotonated (S₄ + 4Btl)^n– ions, was loss of the ligand. However, subunit loss from the (S₄ + 4B)ⁿ⁺ ions was observed as a minor pathway. The (S₄ + 4Btl)ⁿ⁺ ions dissociated predominantly by the loss of free and ligand-bound subunit. The charge state of the complex and the collision energy were found to have little effect on the relative contribution of the different dissociation channels. Thermally-driven ligand migration between subunits was captured in the results of molecular dynamics simulations performed on protonated (CTB₅ + 5GM1)¹⁵⁺ ions (with a range of charge configurations) at 800 K. Notably, the migration pathway was found to be highly dependent on the charge configuration of the ion. The main conclusion of this study is that the dissociation pathways of multisubunit protein–ligand complexes in the gas phase depend, not only on the native topology of the complex, but also on structural changes that occur upon collisional activation.

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.     

Fine structure excitation transfer between the potassium 42P states induced by collisions with caesium atoms

Applying diode-laser resonant fluorescence method, the cross sections for the excitation energy transfer of the collisional process K*(4² P _1/2)+Cs(6² S _1/2)?K*(4² P _3/2)+Cs(6² S _1/2) have been measured. The values we have obtained are σ(1/2→3/2)=77 Ų and σ(3/2→1/2)=48 Ų. These results complete the sequence of data for the fine-structure mixing of the first-resonance states of alkali atoms colliding with the ground-state caesium atoms.     

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.     

On the “anomalous” flourescence of 3,4-benzypyrene in the vapour phase

An analysis of the fluorescence of 3,4-benzpyrene in the vapour phase shows two contributions to the “anomalous” fluorescence: (i) the emission from the second excited state to the ground state and (ii) a vibronically induced S₁ → S₀ emission originating from the +520 cm^?1 vibrational level. A comparison between the intensities of the emissions indicates that in the vapour phase the vibrational redistribution from the 520 cm^? vibrational level of S₁ to modes of lower frequencies is relatively slow.     

Vibronic spectral behavior of molecules. XIII. Theoretical contribution to the vibronic coupling and the dushinsky effect on the S1 ← S0 absorption and the S1 → S0, S2 → S1, and S2 → S0 fluorescences of azulene

Based on the completely optimized S₀, S₁, and S₂ molecular geometries of azulene, the vibronic structure of the S₁ ← S₀ absorption as well as of the S₁ → S₀, S₂ → S₁, and S₂ → S₀ fluorescences is investigated theoretically within the adiabatic approximation. By means of theory-experiment comparisons, the influence of non-Condon terms and of the Dushinsky effect on the vibronic structure of azulene spectral behavior is discussed. Typical for the S₁ ← S₀ absorption and the S₁ → S₀ fluorescence are vibronic transition moment contributions of Condon type, whereas the interpretation of azulene S₂ → S₁ and S₂ → S₀ fluorescences is successful only within the scope of the Herzberg–Teller approach by taking into account vibronic coupling terms and, additionally, the Dushinsky effect in the latter case. An analysis of the relevant vibrational modes is given.