Electron spin resonance in the photo-excited triplet state of free base porphin in a single crystal of n-octane

E.S.R. experiments have been performed on the lowest triplet state of free base porphin (H₂P) in a n-octane single crystal at 1·3 K. The results demonstrate that the large majority of guest molecules occur in two orientations. While the molecules in these two orientations are coplanar, they have their N-H H-N axes at right angles. The fine structure results show that the two molecular orientations have zero-field splittings that differ by a few per cent in magnitude. Further, the 65 cm^-1 doublet separation which appears in the fluorescence spectrum of H₂P is related to the occurrence of these two orientations.

Resolved hyperfine structure is obtained for the two in-plane canonical orientations of the magnetic field and also when the field bisects the angle between these two directions. From an analysis of the fine structure and hyperfine structure results it is established that the zero-field splitting pattern is described by the parameters (average over the two orientations) the x axis is taken along the N-H H-N direction and z is the out-of-plane axis. From a computer simulation of the hyperfine structure it further follows that this structure is dominated by a high spin density at the methine carbons; with the coupling constants of the C-H fragment proposed by Hirota et al. the methine density is found to be ρ_m = 0·163.

In order to interpret the experimental results the zero-field splitting parameters and spin density distribution have been calculated for the lower triplet states of H₂P on the basis of a set of PPP-SCF-MO-CI calculations. From these calculations it follows that the lowest triplet state must correspond to the excitation e_g ←a _2u in Gouterman's four-orbital model. In terms of the D _2h symmetry of the H₂P molecule the assignment is ³ B _2u (b _3g ←b _1u ). For this assignment the calculations yield .     

Photophysical and photochemical properties of the water-soluble porphyrin complexes of metals of the platinum group

Consideration is given to phosphorescence and photochemical properties of the coordination compounds Pt(II), Pd(II), and Rh(III) with three water-soluble porphyrins, i.e., meso-tetrakis (4-N,N,N-trimethylaminophenyl) porphin (H₂TTMAP⁴⁺), meso-tetrakis (4-N-methylpyridyl) porphin (H₂TMPyP⁴⁺) and meso-tetrakis (4-sulfonatophenyl) porphin (H₂TSPP⁴⁻). It is shown that the process of photoreduction by an irreversibly oxidized electron donor (EDTA) proceeds only for the complexes with TMPyP, which is attributed to the strongest oxidative properties of these metal poprhyrins in the triplet excited state. The end products of the photoreduction reaction (metal chlorins and metallophlorin-anions) are established and its possible mechanism is suggested. To whom correspondence should be addressed. Reported at the VIIIth International Conference on Spectroscopy of Porphyrins and Their Analogs, Minsk, September 22–26, 1998. A. I. Gertsen Russian State Pedagogical University, 48, Moika Embankment, St. Petersburg, 191186, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 528–531, July–August, 1999.     

Luminescence of phenylisocyanide and perdeuterophenylisocyanide at 77° K in polycrystalline matrices

Vibrational analyses are presented for the phosphorescence and fluorescence spectra of C₆H₅NC and C₆D₅NC in polycrystalline methylcyclohexane matrices at 77° K. Polarization measurements indicate that the lowest triplet state is ³A₁. The longest progression found for phosphorescence is that of a totally symmetric C-C stretching mode, and is interpreted as indicative of a planar, non-regular-hexagon benzene ring geometry of the lowest triplet state. The fluorescence spectra show a weak 0-0 band and two strong false origins, both based on one quantum of non-totally symmetric b₂ vibrations. The main progressions here, in the ring breathing mode and C-X stretching mode, suggest a planar slightly expanded regular hexagon ring with a changed C-X bond length for the geometry of S₁.     

The lowest triplet state of Zn porphin

The dynamics of populating and depopulating the phosphorescent triplet state of Zn porphin in an n-octane matrix is investigated by microwave induced delayed phosphorescence experiments at 1·2K. Values for the absolute rates of decay of the three spin levels are given and also the relative steady-state populations, radiative decay rates and probabilities for entry through intersystem crossing. Further, it is established that the phosphorescence is inplane polarized. The results are compatible with the decay modes expected for a lowest vibronic state that deviates from D ₄ symmetry by combined action of the Jahn-Teller effect and crystal field anisotropy.     

Determination of the rate constant of oxygen quenching of the excited states of molecules from the singlet oxygen luminescence

We have pioneered a method of determining the rate constant of quenching of the excited electronic states of molecules by molecular oxygen from measurements of the kinetics of photosensitized luminescence of singlet molecular oxygen (^lδ_g). The method can be used in the case where the lifetime of the excited electronic state in an air-saturated solution is comparable with or larger than the luminescence time of the singlet molecular oxygen in the given solvent. It is shown that this situation is implemented on quenching, by molecular oxygen, of the excited triplet states associated with the biopolymers of tetrapyrrole molecules in aqueous (H₂O and D₂O) solutions. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 3, pp. 401–404, May–June, 2000.     

Semiempirical determination of the vibronic constants of diatomic molecules. Optimal sets of constantsa 3Σg +,e 3Σu +,d 3Πu ±, and f3Σu + of states of H2

The possibility of describing the vibronic electron levels of diatomic molecules by means of a limited set of molecular constants is investigated. For the example of the low triplet states of H₂, it is shown that, in determining the set of vibronic constants of the given electron state and their covariance matrix, account must be taken of the specific features of the physical problem for which these constants are to be used. The optimal sets of constants obtained are significantly different from those in the literature. The reasons for the discrepancy are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 66–70, February, 1986.It remain to thank N. P. Penkin, T. K. Rebane, and P. A. Braun for their interest in the work and useful discussions.     

Isotopic effects on the control of molecular handedness of H 2 POSH by ultrashort laser pulses

The selective preparation of an enantiomer starting from a pure state of H₂POSH representing a 50%:50% mixture of two enantiomers with opposite chiralities is extended to its deuterated counterpart, H₂POSD. A one-dimensional model involving the torsional angle of S-H/D around a pre-oriented P-S axis is used. The design of an appropriate sequence of ultrashort infrared laser pulses to achieve molecular handedness for H₂POSH/D is based on the characteristic level splitting and tunneling times of both molecules. A simple scheme of two linearly polarized laser pulses involving a three level system serves to convert the different isotopomers to opposite enantiomers, for any given mixture of H₂POSH and H₂POSD molecules. Received 31 August 2000 and Received in final form 23 November 2000     

The reactions of Cl+ (3P) and Cl+ (1D) with hydrogen sulphide. A G2 molecular orbital study

G2 ab initio molecular orbital calculations have been performed to study the potential energy surfaces (PESs) associated with the reactions of Cl⁺ in its ³P ground state and in its ¹D first excited state with hydrogen sulphide. [H₂, Cl, S]⁺ singlet and triplet state cations present very different bonding characteristics. The latter are systematically ion-dipole or hydrogen-bonded weakly bound species, while the former are covalent molecular ions. As a consequence, although the Cl⁺(³P) is 34.5 kcal mol^?1 more stable than Cl⁺(¹D), the global minimum of the singlet PES lies 37.3 kcal mol^?1 below the global minimum of the triplet PES. Both singlet and triplet potential energy surfaces show significant differences with respect to those associated with Cl⁺ + H₂O reactions as well as with SH₂ reactions with F⁺. In both cases, the major product should be SH⁺ ₂; SH⁺ and HCl⁺ being the minor products, in agreement with the experimental evidence. The estimated heat of formation for the most stable H₂SCl⁺ singlet state species is 198 ± 1 kcal mol^?1.     

Fine‐Structure Fluorescence Spectrum of a Dication Form of Porphin in an Inorganic Gel Matrix

It is shown that the molecules of the free base of porphin (H₂P) incorporated into an inorganic tetraethoxysilane (TEOS) gel matrix produce a dication form H₄P²⁺. A finestructure spectrum of the fluorescence of H₄P²⁺ is obtained at 4.2 K on selective photoexcitation. A conclusion on the manifestation of the outofplane vibrations of the porphyrin macrocycle in the lowfrequency region of the vibronic spectrum of porphin dication is drawn and the assignments of the corresponding frequencies to the vibrations of a certain form are suggested.     

IR spectrum and localization of excitation of reaction center of photosystem II in triplet state

Using the density functional theory, vibrations of different model forms of the chlorophyll a molecule in the ground and triplet states have been calculated. The assignment of the experimental difference IR spectrum corresponding to the formation of the triplet state of the photosynthetic reaction center of photosystem II has been proposed on this basis. It has been shown that molecules of accessory chlorophyll B _A and B _B located between the special pair and H _A and H _B pheophytin molecules can be involved in the intermolecular hydrogen bond with the water molecule. The energy of this interaction in the triplet state of molecules for B _A is larger on 6 kcal/mol. This allows us to relate this pigment to the location of the triplet excitation at the reaction center of photosystem II.     

ESR Detection of Triplet States in Rare Earth Chelates

Abstract

Energy transfer from ligand molecule to ion in rare earth chelates has been assumed to involve a molecular triplet state. ^1,2 These triplet states manifest themselves optically as long-lived T_{π, π? →S}π,π emission if the lowest resonance level of the ion is of higher energy than the molecular triplet. ³ If the resonance level lies below the triplet, longlived emission is absent or severely reduced in intensity. ^4,5     

Calculation of Quantum‐Chemical Characteristics of the Ground and Excited Electronic States of Porphin and Its Tetrapyrrole Isomers

Using the CNDO/S method, we have performed quantumchemical calculations of the ground and excited electronic states of porphin molecules (symmetry D _2h ) and a number of porphin isomers: porphycene (C_2h ), hemiporphycene, corphycene (C_2v ), isoporphycene, and three nonsynthesized structural isomers of the porphin skeleton by its bridge groups —(CH) _meso —. The results of the calculations are compared with the corresponding data for the freebase porphin molecule (H₂P). Near the boundary between the occupied and vacant orbitals the isomers form rows of singleelectron levels with similar energy characteristics. For the MOs of these isomers, the H₂P MOs closest in distribution on the comparable atomic centers are given. The weak (Q) and strong (B) transitions in the visible and near UV regions point to the porphyrin type of spectra of all the isomers. The calculation data on the excited electronic states of N, g, and ntype H₂P near and above the B states are given; the presence of such states in other isomers is shown. The calculation does not point to a difference in the position of the first singlet transitions in the first five isomers. The calculation data on the lower singlettriplet transitions are also reported.     

Analysis of electronic vibration spectra of tetrabenzoporphin molecules on the basis of calculations of their normal vibrations

Quasi-line fluorescence and fluorescence-excitation spectra of tetrabenzoporphin (H₂TBP) and its deuterated derivative (D₂TBP) in n-octane at 77 K were measured and analyzed. A calculation of normal vibrations of the H₂TBP and D₂TBP molecules has been performed. The normal vibrations of these molecules were calculated with the use of the valence force fields of porphin and phenanthrene (the direct spectral problem was solved). Good agreement between the calculated and measured frequencies of the investigated normal vibrations has been obtained without correction of the transfered force constants. A detailed interpretation of the fluorescence spectra obtained is presented. It is shown that on deuteration of H₂TBP, the behavior of the quasi-lines in the fluorescence-excitation spectra in the region of resonance with the S₂←S₀ transition becomes more complex. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 375–382, May–June, 1999.     

Triplet exciton in a non-reactive bis-triphenyllead (or tin) diacetylene crystal. A spectroscopic study

The lowest triplet exciton state of crystals formed by (C₆H₅)₃?X?C ≡ C?C ≡ C?X?(C₆H₅)₃ where X is Pb or Sn and an equimolecular amount of CH₂Cl₂ solvent molecules, is studied through the analysis of absorption and of phosphorescence emission and excitation spectra at low temperatures. In the lead compound, the origins of absorption and emission are identical, showing the dominance of free exciton emission and the absence of crystal lattice relaxation in the excited state; the splitting is accounted for by symmetry considerations in the different crystalline phases; the triplet exciton band is several cm^-1 wide, although the nearest diacetylene-diacetylene distance is about 10 Å. There is evidence for triplet exciton motion, through the detection of trap (presumably X-trap) emission in the tin compound, and through the analysis of the excitation spectrum lineshape, showing the effect of exciton-exciton interaction in the lead compound.     

Theoretical Calculation of Products Distribution in the Reaction of Atomic Carbon with Pyridine

The elementary reaction of atomic carbon C(³P) with pyridine C₅H₅N is studied using quantum-mechanical calculations of the electronic structure and statistical calculations on the potential energy surface of the C6H5N triplet state. At least two most probable reaction ways associated with hydrogen atom abstraction reaction are revealed. The important role of the molecular fragmentation channel, including acetylene C₂H₂ molecule abstraction, is also shown.     

Fine-structure spectra of metal complexes of porphin in tetrahydrofuran at low temperatures: Manifestation of nonplanarity effects

The fine-structure fluorescence spectra of Mg and Zn porphins in solid tetrahydrofuran matrices at the liquid helium temperature are recorded for the first time. The fluorescence spectra of Mg porphin molecules deposited from the gas phase on a sapphire substrate simultaneously with tetrahydrofuran molecules are measured. Based on the data obtained from the fluorescence and fluorescence excitation spectra, it is ascertained that there are two noninteracting spectrally different long-and short-wavelength forms of metal complexes of porphins in the ground state. In the case of Mg porphin, the spectral gap for these two forms at 4.2 K amounts to 330 cm^?1. The short-wavelength form is attributed to the nonplanar saddle conformation of a porphin molecule, while the long-wavelength form is associated with the nonplanar domed conformation, in which the central metal ion is displaced out of the plane of the porphyrin ligand. The fine-structure fluorescence spectra of both forms of Mg and Zn porphins are measured in tetrahydrofuran at 4.2 K upon selective laser excitation. The frequencies of the normal vibrations in the ground electronic state are determined and the reasons for the differences in the vibrational frequencies of the forms are discussed.     

B-type delayed fluorescence of rubreneperoxide in solution.

B-type of delayed fluorescence was observed for the first time for rubreneperoxide. Rubreneperoxide molecules were excited in a two step process. In the first step an excited singlet S₁ is created, which undergoes intersystem crossing to T₁; then T-T absorption creates an excited triplet rubreneperoxide molecule, which returns to the first excited singlet level by intersystem crossing. The recreated first excited singlet of rubreneperoxide decays back to the ground state by emitting B-type of delayed fluorescence.     

Synthesis,photoluminescence and intramolecular energy transfer model of a dysprosium complex

The energy of the highest occupied molecular orbital and the lowest unoccupied molecular orbital as well as their energy gaps, and the singlet and triplet state energy levels of 4-benzoylbenzoic acid (HL=4-BBA) and triphenylphosphine oxide (TPPO) were calculated with the Gaussian03 program package. The singlet state and triplet state energy levels were also estimated from the UV–vis absorption spectra and phosphorescence spectra. The results suggest that the calculated values approximately coincided with the experimental values. A Dy(III) complex was synthesized with 4-BBA as primary ligand and TPPO as neutral ligand. The structure of the complex was characterized by elemental analysis, ¹H NMR spectrometry, and FTIR spectrometry. TG–DTG analysis indicates that the complex kept stable up to 305 °C. The photoluminescence properties were studied by fluorescence spectrometry. The results show that Dy(III) ion sensitized by 4-BBA and TPPO emitted characteristic peaks at 572 nm (⁴F_9/2–⁶H_13/2) and 480 nm (⁴F_9/2–⁶H_15/2), and its Commission Internationale de L'Eclairge coordinates were calculated as x=0.33 and y=0.38, being located in the white range. Intermolecular energy transfer process was discussed and energy transfer model was also proposed.     

Spectral-Luminescent and Electroluminescent Properties of Charge-transfer Systems Based On Electron-donating Diphenylamine Derivatives and Acceptors of Dibenzothiophene Sulfone and Phenanthridine

Spectral characteristics and luminescence under the photo- and electro-excitation of substituted dibenzthiophene sulfone and phenanthridine were studied in this paper. Diphenylamines are substituents introduced in the 2nd and 7th positions (linear configuration) or the 3rd and 6th positions (angular configuration) of dibenzthiophene sulfone or phenanthridine. All molecules show delayed fluorescence, both in solutions and films produced by thermal vacuum deposition. The value of the energy gap between the S₁ and T₁ states has been estimated and is shown to depend not only on the spatial arrangement of the fragments among themselves (linear or angular), but also on the nature of the substituent in diphenylamine. The highest electroluminescence brightness was found for the molecules, in which triplet levels are involved, both through the process of triplet–triplet annihilation and through thermally activated delayed fluorescence.

     

Addition complex and insertion isomers on the potential energy surface of the reaction of indium dimer with water studied with relativistic ECP

Stationary points on the lowest singlet and triplet In₂ + H₂O potential energy surfaces (PESs) have been explored using the coupled cluster method, including single and double excitations with perturbative triples (CCSD(T)), and the density functional theory (DFT), employing the effective core potential (ECP) for indium (In), which accounts for scalar relativistic effects, with the triple-zeta quality basis set. The CCSD(T) calculated binding energy and anharmonic ν₂-bending mode frequency for the triplet ground-state addition complex, In₂^… OH₂(³B₁), are consistent with the complex detected in the matrix isolation infrared (IR) spectroscopic study under the thermal conditions. The two minimum energy crossing points between the triplet and the singlet PESs that have been located between the structures of In₂^…OH₂ and the transition state for the O–H bond breakage are not likely to be thermally accessible under the low-temperature matrix conditions. With the CCSD(T)-calculated In₂ + H₂O reaction profile and anharmonic vibrational frequencies for several In₂(H)(OH) insertion product isomers, we support the IR matrix isolation detection (by two experimental groups) of the lowest energy singlet double-bridged In(μ-H)(μ-OH)In isomer. For the proposed two-step mechanism of H₂ elimination from the In₂(H)(OH) species, the estimated energy barriers are also compatible with experiment.