Xanthone. II. Vibronic coupling analysis from high resolution phosphorescence spectra

High resolved phosphorescence spectra of xanthone have been recorded in four host matrices in order to study vibronic coupling between the lowest tripl origin in all hosts, and the energetically close-lying second triplet state T₂, which is of nπ* orbital origin. In three hosts, there is thermall to phosphorescence from T₁. Vibrational analyses of the two emissions are reported. The vibrational structure of both emissions depends little on t vibronic mixing between the lowest two triplet states is weak, in spite of the small energy separation in some hosts. The importance of the different i ₁ and its sublevels is discussed, and it is concluded that across energy separations smaller than about 200 cm^?1 spin—orbit mixing is more orbital mixing between the ³ππ* and ³nπ* configurations of xanthone.     

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.     

Vibronic interaction and triplet state photophysics of phenylisatin and oxindole

Photophysical study of phenylisatin and oxindole triplet states have been made at room temperature and in different glasses at 77K. Qualitatively, in all respects the compounds have identical spectroscopic characteristics. Phosphorescence emission, excitation along with their polarization and lifetime suggest that a perturbation of the zero-point level of emitting state (³ππ*) by a close-lying triplet state (³nπ*) leads to a number of new spectral features. The experimental observations have been interpreted satisfactorily in terms of a switch (³ππ* state to ³nπ*) in the character of the lowest triplet states (T₁ and T₂) and also a similar switch in the character of the excited singlet states S₁ and S₂ for a change of glass matrix from MCH to ethanol. Invoking of first order and second order spin-orbit coupling explains the phosphorescence emission unambiguously.     

MULTI-SITE PHOSPHORESCENCE OF α, β-ENONES:LEVEL INVERSION IN 6β, 19-EPOXYCHOLEST-4-EN-3-ONE BY SOLVATION AND AGGREGATION

Millisecond time-resolved emission spectroscopy was used to probe the phosphorescence kinetics of the α-β-enone 6β, 19-epoxycholest-4-en-3-one (1) as a function of concentration in several paraffinic and hydroxylic glasses at 77 K. Only in methylcyclohexane/methylcyclopentane glass at low concentration (10^?4M) does the phosphorescence decay exponentially. It is interpreted as emission from the ³n* state. Upon increasing the concentration a second emission grows which is characterized by a longer lifetime, a decreased fine structure and a hypsochromically shifted S₀→¹nπ* excitation spectrum. This phosphorescence is ascribed to ³ππ* emission of aggregates of 1. In hydroxylic glasses the phosphorescence decay is multiexponential, even at 10^?4M concentration; from emission band shapes and lifetimes it follows that both ³nπ* and ³ππ* type emissions are present, the latter increasing with the alcohol concentration in the solvent. The two types of phosphorescence have different excitation spectra: that of the structureless and long-lived ³ππ* emission is shifted to the blue in the S₀→¹nπ* region and to the red in the S⁰→¹ππ* region. This emission is ascribed to complexes of 1 with the alcoholic solvent. The results of time-resolved measurements of the circular polarization of the luminescence are consistent with the assignments given above and indicate that in the H-bonded and possibly also in the free species ³ππ* and ³nπ* states are intermixed to a considerable extent.     

Analysis of vibronic intensities in the phosphorescence spectrum of dimethylbenzaldehydes in durene

An attempt is reported to explain the main intensity patterns in the phosphorescence spectra of 2,4-, 2,5- and 3,4-dimethyl-benzaldehyde-1h₁ and -1d₁, observed previously. The analysis is based on CNDO and MINDO calculations of (transition) dipole moments, spin-orbit couplings, vibronic couplings, state energies, normal coordinates and vibrational frequencies. Where possible these quantities are empirically checked and corrected. Additional information, especially about the separation of the closely spaced T₁(³ππ^*) and T₂(³nπ^*) states, is obtained from phosphorescence excitation spectra reported here for all six isomers. The phosphorescence spectra consist of two components, an “allowed” component of ³ππ^* and a “forbidden” component of ³nπ^* symmetry. It is concluded that the allowed component is partly induced by the crystal field. The forbidden component is vibronically induced by out-of-plane vibrations among which the aldehydic CH(CD)-wag mode is the most active. The observed intensity patterns for this component are ascribed to interference between two mechanisms, one involving vibronic coupling between S₀ and S₁(¹nπ^*) and spin-orbit coupling between S₁ and T₁, the other involving vibronic coupling between T₁ and T₂ and spin-orbit coupling between S₀ and T₂. Within the groups of either 1h₁ or 1d₁ isomers, the main changes in the spectrum are shown to be due to the change in T₁–T₂ energy separation. The changes observed upon deuterium substitution in the aldehyde group involve, in addition to changes in the T₁–T₂ gap, changes in vibronic coupling due to normal-coordinate mixing. All these spectral changes are reproduced by calculations based on a mixture of theoretical and empirical input parameters, derived from, or at least consistent with, other observations, including excitation spectra, dipole moments and zero-field splittings. It is concluded that the mechanisms underlying these calculations offer a satisfactory explanation of the observed intensity patterns in the phosphorescence spectra of dimethylbenzaldehydes.     

A simple theoretical model for dual phosphorescence

A simple theoretical model is presented to explain the observed anomalous dual phosphorescences of certain aromatic carbonyl compounds in some rigid media. The phenomenon of dual phosphorescence for large molecules violates the well-known Kasha rule stating that the emission can occur only from the lowest excited electronic state of a given multiplicity. For a small energy gap between the second triplet state (T₂) and the first triplet state (T₁), the sparse density of T₁ vibronic levels, isoenergetic with the T₂ vibrationless level, leads to a rather slow T₂ → T₁ radiationless process which is unable to quench the T₂ emission completely. Two cases of T₁ = ³nπ^*, T₂ = ³ππ^* and T₁ = ³ππ^*, T₂ = ³nπ^* are discussed at both the low-temperature and the high-temperature limits.     

Simulated T ← S spectrum of 2,4,5-trimethylbenzaldehyde in durene

The T_1,2 ← S₀ phosphorescence excitation spectrum of 2,4,5-trimethylbenzaldehyde in durene has been simulated using forty-five zero-order Born-Oppenheimer product states of which thirty-two belong to T₁ (ππ*), the others to T₂ (nπ*). The spectrum is very complicated in the region 400–600 cm^?1 above the T₁ (ππ*) ←3 S₀ origin band at 24150 cm^?1. In this tangled region conventional vibrational analysis is not useful. Several comments on the physical properties of the excited triplet states of 2,4,5-trimethylbenzaldehyde are given.     

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.     

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.     

Photophysical Properties of Some Methyl-Substituted Angelicins: Fluorometric and Flash Photolytic Studies

This paper describes the results of a study of the photophysical properties of various methyl-angelicins (MA) in solvents of different polarity and proticity. The behavior of their excited singlet and triplet states was investigated by fluorometry and nanosecond laser flash photolysis. On the basis of semiempirical (ZINDO/S-CI) calculations and the solvent effect on the absorption and fluorescence properties, the lowest excited singlet state (S₁) is assigned to a partially allowed π, π* state. The close lying S₂ state is n,π* in nature. The efficiency of the decay pathways of S₁ (fluorescence, intersystem crossing and internal conversion) strongly depends on the energy gap between the S₁ and S₂ states consistent with the manifestation of “proximity effect.” Thus, MA in cyclohexane decay only through S₁→ S₀ internal conversion, while in acetonitrile and ethanol, where the n, π* state is located at higher energy, their fluorescence and intersystem crossing increase significantly. The lowest excited triplet states (T₁) were characterized in terms of their absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yields. The interaction of T₁ MA with molecular oxygen leads to an efficient formation of singlet oxygen, as evidenced by the appearance of characteristic IR phosphorescence centered at 1269 nm.     

Photoexcited Singlet and Triplet States of a UV Absorber Ethylhexyl Methoxycrylene

The excited states of UV absorber, ethylhexyl methoxycrylene (EHMCR) have been studied through measurements of UV absorption, fluorescence, phosphorescence and electron paramagnetic resonance (EPR) spectra in ethanol. The energy levels of the lowest excited singlet (S₁) and triplet (T₁) states of EHMCR were determined. The energy levels of the S₁ and T₁ states of EHMCR are much lower than those of photolabile 4‐tert‐butyl‐4′‐methoxydibenzoylmethane. The energy levels of the S₁ and T₁ states of EHMCR are lower than those of octyl methoxycinnamate. The weak phosphorescence and EPR B_min signals were observed and the lifetime was estimated to be 93 ms. These facts suggest that the significant proportion of the S₁ molecules undergoes intersystem crossing to the T₁ state, and the deactivation process from the T₁ state is predominantly radiationless. The photostability of EHMCR arises from the ³ππ* character in the T₁ state. The zero‐field splitting (ZFS) parameter in the T₁ state is D** = 0.113 cm^?1.     

Excited-state intramolecular proton transfer driven by conical intersection in hydroxychromones

Nonradiative decay pathways associated with vibronically coupled S₁(ππ*)–S₂(nπ*) potential energy surfaces of 3- and 5-hydroxychromones are investigated by employing the linear vibronic coupling approach. The presence of a conical intersection close to the Franck–Condon point is identified based on the critical examination of computed energetics and structural parameters of stationary points. We show that very minimal displacements of relevant atoms of intramolecular proton transfer geometry are adequate to drive the molecule toward the conical intersection nuclear configuration. The evolving wavepacket on S₁(ππ*) bifurcates at the conical intersection: a part of the wavepacket moves to S₂(nπ*) within a few femtoseconds while the other decays to S₁ minimum. Our findings indicate the possibility of forming the proton transfer tautomer product via S₂(nπ*), competing with the traditional pathway occurring on S₁(ππ*).     

The electronic spectra of 10, 10-dimethylanthracen-9-one crystals: Spin—orbit/vibronic coupling in 3nπ3 states

10, 10-dimethylanthracen-9-one single crystal emission and absorption spectra have been recorded at low temperatures, as well as Raman spectra on the melt. The absorption spectra of both the lowest triplet and lowest excited singlet states clearly show the absorption origins of the three different molecular sites in the triclinic unit cell of the crystal. The emission spectra indicate that substantial spin—orbit/vibronic state mixing occurs, giving rise to transitions between the z sub-level of the lowest ³nπ³ state (T₁) and both totally and non-totally symmetric vibrations of the ground state. The preferred intensity stealing route is from T₁ (z) to S₂(¹ππ³) by spin—orbit coupling with vibronic mixing of S₃ and S₄(¹ππ³, ¹B₂).     

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.     

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.     

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.     

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.     

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.     

STEREOELECTRONIC PROPERTIES OF PHOTOSYNTHETIC AND RELATED SYSTEMS — V. AB INITIO CONFIGURATION INTERACTION CALCULATIONS ON THE GROUND AND LOWER EXCITED SINGLET AND TRIPLET STATES OF ETHYL CHLOROPHYLLIDE a AND ETHYL PHEOPHORBIDE a

Abstract— Ab initio configuration interaction wavefunctions and energies are reported for the ground state and many low-lying excited singlet and triplet states of ethyl pheophorbide a (Et-Pheo a) and ethyl chlorophyllide a (Et-Chl a), and are employed in an analysis of the electronic absorption spectra of these systems. In both molecules the visible spectrum is found to consist of transitions to the two lowest-lying ¹(π, π*) states, S₁ and S₂. The configurational compositions of S₁ and S₂ in both molecules are similar, and are described qualitatively in terms of a four-orbital model. The S₁← S₀ transition in each case is predicted to be intense, and is largely in-plane y-polarized, while the S₂← S₀ transition is predicted to be extremely weak and in-plane polarized. The orientation of the S₂← S₀ transition dipole is not conclusively established in the present calculations. The Soret band in both molecules is composed of transitions to no less than ten states (S_3-S₁₂ in Et-Chl a and S₃-S₇S₉-S₁₂. and S₁₄ in Et-Pheo a), which exhibit primarily (π, π*) character. The configurational compositions of these states are generally a complex mixture of excitations from both occupied macrocyclic π molecular orbitals and occupied orbitals with electron density in the cyclopen-tanone ring and the carbomethoxy chain. No clear correspondences are evident between respective Soret states of the two systems. Transitions to these states are generally intense and display a variety of in-plane polarizations. Two additional Soret states of Et-Pheo a, S₈ and S₁₃, exhibit primarily (n. π*) character. S₈ is characterized by excitations from u and non-bonding regions of the carbomethoxy chain, while S₁₃ is described by n →π* excitations involving the nitrogen atom of ring II. No corresponding (n, π*) states were found for Et-Chl a. In both molecules the lowest two triplet states, T₁ and T₂, are found to lie lower in energy than S₁. while T, and S₁ are approximately degenerate. The configurational compositions of T₁-T₄ of both molecules are nearly identical, and may be described by a four-orbital model. However, the compositions of T₁-T₄ differ sharply from those of S₁ and S₂. A number of higher-lying ³(π, π*) states of both molecules (T₅-T₁₃ in Et-Chi a and T₈-T₉, T₁₁-T₁₃ in Et-Pheo a) are found to have energies similar to the singlet Soret states, relative to S₀. They are characterized by a complex mixture of configurations which do not include significant contributions involving the four-orbital model. In addition, two ³(n, π*) states of Et-Pheo a, T₁₀ and T₁₄, are found, which are somewhat analogous to S₈ and S₁₃. Additional data presented include the charge distributions and molecular dipole moments of the S₀. S₁, and T₁ states of both molecules, as well as energies and oscillator strengths of computed S_n←S₁ and T_n←₁ transitions.     

Time-resolved EPR and laser photolysis investigations of photoinduced ω-bond dissociation in an aromatic carbonyl compound having triplet π,π* character

Photochemical properties of photoinduced ω-bond dissociation in p-phenylbenzoylbenzyl phenyl sulfide (PPS) having the lowest triplet state (T₁) of π,π* character in solution were investigated by time-resolved EPR and laser flash photolysis techniques. PPS was found to undergo photoinduced ω-bond cleavage in the excited lowest singlet state (S₁(n,π*)) with a quantum yield (Φ_rad) of 0.15 for the radical formation, which was independent of excitation wavelengths. Based on the facts of the observation of the absorption spectrum of triplet PPS upon triplet sensitization of xanthone, and absence of CIDEP signal, ω-cleavage was shown to be absent in the T₁(π,π*) state of PPS. Considering the electronic character of the excited and dissociative states of PPS, a schematic energy diagram for the ω-bond dissociation of PPS was shown.