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.     

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.     

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).     

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.     

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.     

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     

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.     

Temperature dependence of the nonradiative decay of indoles in solution

The nonradiative decay rate k_nr. of some indole derivatives has been measured as a function of temperature in three solvents : n-heptane, methanol and water. A temperature-independent component (attributed to S₁ → T₁ intersystem crossing) and a temperature-dependent component (attributed to S₁ → S₀ internal conversion) are present in all case. In aqueous solutions, our results indicate the existence of a second temperature-dependent process which can be identified with photoionization.     

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.     

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 studies of spin state‐specific [2 + 2] and [5 + 2] photocycloaddition reactions of n‐(1‐penten‐5‐yl)maleimide

N‐alkenyl maleimides are found to exhibit spin state‐specific chemoselectivities for [2 + 2] and [5 + 2] photocycloadditions; but, reaction mechanism is still unclear. In this work, we have used high‐level electronic structure methods (DFT, CASSCF, and CASPT2) to explore [2 + 2] and [5 + 2] photocycloaddition reaction paths of an N‐alkenyl maleimide in the S₁ and T₁ states as well as relevant photophysical processes. It is found that in the S₁ state [5 + 2] photocycloaddition reaction is barrierless and thus overwhelmingly dominant; [2 + 2] photocycloaddition reaction is unimportant because of its large barrier. On the contrary, in the T₁ state [2 + 2] photocycloaddition reaction is much more favorable than [5 + 2] photocyclo‐addition reaction. Mechanistically, both S₁ [5 + 2] and T₁ [2 + 2] photocycloaddition reactions occur in a stepwise, nonadiabatic means. In the S₁ [5 + 2] reaction, the secondary C atom of the ethenyl moiety first attacks the N atom of the maleimide moiety forming an S₁ intermediate, which then decays to the S₀ state as a result of an S₁ → S₀ internal conversion. In the T₁ [2 + 2] reaction, the terminal C atom of the ethenyl moiety first attacks the C atom of the maleimide moiety, followed by a T₁ → S₀ intersystem crossing process to the S₀ state. In the S₀ state, the second C C bond is formed. Our present computational results not only rationalize available experiments but also provide new mechanistic insights. © 2017 Wiley Periodicals, Inc.     

Ultrafast Dynamics of the Transoid-cis Isomer Formed in Photochromic Reaction from 3H-Naphthopyran

Recent efforts in designing new 3H-naphthopyran derivatives have been focused on efficient coloration process with a short fading time of the colored transoid-cis TC isomer. It is desirable to avoid photoisomerization of TC leading to transoid-trans TT isomers in the photoreaction. Long lifetime of TT can hamper fast applications such as dynamic holographic materials and molecular actuators, the residual color is one of the serious issues for photochromic lenses. Herein we characterize the photophysical and photochemical channels of TC excited state deactivation competing with the unwanted TC → TT isomerization process. Transient absorption spectroscopy reveals a very short lifetime of the singlet excited TC (≈0.8 ps) and its deactivation channels as S₁→S₀ internal conversion (major), intersystem crossing S₁→T₁, pyran ring formation, photoenolization and TC → TT isomerization. Computations support the S₁→S₀ and T₁→S₀ channels as responsible for photostabilization of the TC form.     

Multiphoton tea CO2 laser excitation of the triplet state of propynal

Multiple infrared photon excitation of propynal triplet molecules gives rise to a strongly perturbed phosphorescence. Following absorption of a few IR photons per molecule the phosphorescence spectrum extends to higher energy, the intensity increases, the decay — deviating from the original exponential decay — accelerates and the emission quantum yield drops dramatically. These findings are explained in terms of temperature sensitive radiative (T₁ → S₀) and radiationless (T₁ → S₀) processes with the vibrational temperature as the determining factor. During the perturbed triplet decay, the IR excitation initially confined to the vibrational degrees of freedom becomes distributed among all degrees of freedom which results in a decrease in the vibrational temperature and thus a complex phosphorescence decay.     

Orientation dependence in collision induced electronic relaxation studied through van der Waals complexes with isomeric structures

Weakly bound molecular complexes with more than one well-defined structures provide us with an unique opportunity to investigate dynamic processes induced by intermolecular interactions with specific orientations. The relative orientation of the two interacting molecules or atoms is defined by the complex structure. The effect of the orientation in the spin changing collisions glyoxal(S₁)+Ar → glyoxal(T₁)+Ar and acetylene (S₁)+Ar → acetylene(T)+Ar have been studied by measuring the intersystem crossing (ISC) rates of the glyoxal(S₁)·Ar and acetylene(S₁)·Ar complexes with different isomeric structures. Results show that there is a strong orientation dependence in the ISC of glyoxal(S₁) induced by interaction with the Ar atom: the Ar atom positioned in the molecular plane is much more effective than in the out-of-plane position in inducing the S₁ → T₁ transition of glyoxal. On the other hand, studies of acetylene(S₁)·Ar complexes indicate that the Ar-induced ISC rates are nearly identical for the in-plane and out-of-plane positions. Orientation dependence in the collision induced vibrational relaxation process C₂H₂(S₁,v _i )+Ar → C₂H₂(S₁,v _f <v _i )+Ar is also studied by measuring the vibrational predissociation rates of the acetylene(S₁)·Ar complex isomers. The results indicate that collisions of C₂H₂(S₁,v ₃=3, 4) with Ar at two orthogonal orientations are equally effective in causing vibrational relaxation of C₂H₂.     

Nuclear Coordinate dependence of electronic transition moments in orbitally forbidden transitions: A new interpretation of deuterium isotope effects

The nuclear coordinate dependence of electronic transtion moments has been investigated for the purpose of finding new interpretations of deuterium isotope effects on spectral intensities and radiative decay rates in orbitally forbidden electronic transitions. By using “AO following nuclei” wavefunctions as the building block for the electronic wavefunction in the adiabatic BO vibronic wavefunction, the spin-free hamiltonian is diagonalized to generate eigenfunctions and eigen-energies. It is found that the electronic transtion moments based on these eigenfunctions show dependences upon the vibrational modes which are not directly involved in vibronic coupling. This leads to interpretations of the deuterium isotope effects in T₁ → S₀ radiative transitions of aromatic hydrocarbons and S₀ → S₁ absorption in pyrazine which are not based on the conventional Herzberg—Teller or non-BO coupling.     

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.     

A highly variable opto-acoustic phase angle within the So → S1 band of glyoxal

The S₀ → S₁ transition in the opto-acoustic spectrum of glyoxal vapor exhibits a variable phase angle which follows the pattern of absorption intensity. This unusual phenomenon results from a quenching of T₁ by S_o to form a long-lived intermediate, M, which then produces heat via an M + M reaction.     

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.     

Excited-states decay dynamics of phenylosazone d-glucose in the liquid phase

The singlet and triplet lifetime and the quantum yield of intersystem crossing S₁ → T₁ for phenylosazone D-glucose (PH) were determined. A diagram of molecular electronic energy levels and the transition between them was obtained.