Excited Singlet Electronic States Research Articles

On the electronic states of the extended porphyrin family

The ground and lower excited electronic singlet and triplet states of eight macrocycles resulting from porphyrin by replacing pyrroles with pyridines or indoles and the meso methines by nitrogens (the extended porphyrin family ) have been treated by Skala's version of the CNDO/S method in view of providing systematic reference computational data. The results are especially useful for comparisons capable of throwing light on the structure-property relations that make some of those macrocycles especially active for energy and electron transfer. A brief discussion is given. The uniqueness of porphyrin stands out clearly, but some of its features may be strictly related to the biochemical environment of heme groups, so that other macrocycles of the family might be better suited for technological applications simulating the in vivo behavior of porphyrin derivatives. Hints concerning the effect of coordination to metal atoms are given, pending the completion of a parallel study on some metal complexes of the family. A few novel experimental results from our laboratories are mentioned in the discussion.

On the electronic states of the extended porphyrin family

The ground and lower excited electronic singlet and triplet states of eight macrocycles resulting from porphyrin by replacing pyrroles with pyridines or indoles and the meso methines by nitrogens (the extended porphyrin family) have been treated by Skala's version of the CNDO/S method in view of providing systematic reference computational data. The results are especially useful for comparisons capable of throwing light on the structure-property relations that make some of those macrocycles especially active for energy and electron transfer. A brief discussion is given. The uniqueness of porphyrin stands out clearly, but some of its features may be strictly related to the biochemical environment of heme groups, so that other macrocycles of the family might be better suited for technological applications simulating the in vivo behavior of porphyrin derivatives. Hints concerning the effect of coordination to metal atoms are given, pending the completion of a parallel study on some metal complexes of the family. A few novel experimental results from our laboratories are mentioned in the discussion.

Photolysis of methanolic solutions of benzyl acetate

Photolysis of solutions of benzyl acetate in CD3OD has been studied. As has been established based on the effects of CIDNP and the yields of products, decomposition of the starting compound occurs from both excited singlet and triplet electronic states. The contributions of homolysis and solvolysis to the overall photochemical process have been measured, and the lifetimes of the acetoxy radicals formed have been evaluated. Toluene-d has been shown to be formed mostly by disproportionation of benzyl radicals with hydroxymethyl radicals derived from the solvent molecules.

Theoretical investigation of monohalogenbenzenes vibronic spectra

The vibrational structure of the first singlet excited electronic states B1 of C6H5X (X = F, CL, Br) molecules is investigated. The calculated results are compared to experimental data [1] about vapour and crystal spectra analyzed in the polarized light at 20 and 4K. The vibronic structure analysis of spectra offered a possibility to assign the vibronic transitions in connection with excitation of definite normal modes.

Theoretical study of the interaction of AlH(X 1Σ+,A 1Π) with Ar: Potential energy surfaces and bend–stretch levels of the ArAlH(X,A) van der Waals complex

Multireference, configuration interaction potential energy surfaces (PES’s) are reported for the interaction of Ar with the AlH radical in its ground (X 1Σ+) and first singlet excited (A 1Π) electronic states. To assist in the assignment of the van der Waals bend–stretch vibrational quantum numbers in the accompanying experimental spectroscopic study of the ArAlH complex [E. Hwang and P. J. Dagdigian, J. Chem. Phys. 102, 2426 (1995)], slightly modified versions of these PES’s have been employed to calculate the vibrational energy levels of the ArAlH(X,A) van der Waals complex, both within the adiabatic bender model and in full variational calculations. Additionally, the band strengths for electronic transitions out of the ground and first vibrationally excited level of ArAlH(X) have been calculated for comparison with the relative band intensities measured by Hwang and Dagdigian.

A study of the N-inversion barrier and the circular dichroism spectra of 1-thia-10-aza[2.2]metacyclophane

The inversion barriers and the excited singlet electronic states of 1-this-10-aza[2.2]metacyclophane (X = NH 1) and two isovalent compounds (X = NCH32, X = PH 3) have been calculated with semiempirical AM1 and multireference configuration interaction (MRD-CI) methods. Owing to the inversion at the heteroatom N or P, all compounds investigated exhibit two different minima which are separated energetically by less than 1 kcal mol–1. The inversion barriers are calculated to be 5.5 kcal mol–1(X = NH), 4.5 kcal mol–1(X = NCH3) and 15.6 kcal mol–1(X = PH), respectively.Good agreement between the experimental and calculated circular dichroism (CD) spectra is obtained for the X = NH compound (1). The various bands can be characterized as separate and coupled ππ* transitions localized in the aromatic rings and by valence-type excitations involving the lone-pair orbital of the sulfur atom (3p) and/or antibonding σ*-MOs of C–S bonds. The dominant CD transitions do not change significantly if the nitrogen is replaced by a phosphorus centre.From a comparison of the CD spectra calculated for both inversion minima for X = NH, it is predicted that the energetically lowest lying conformation in solution or in the gas phase is not identical with the conformer found in X-ray studies. This supports earlier conclusions which show that the conformer found in the crystal is favoured by intermolecular hydrogen bondings.

Potential energy surfaces for the interaction of BH(X 1Σ+,A 1Π) with Ar and a theoretical investigation of the stretch-bend levels of the ArBH(A) van der Waals molecule

New multireference, configuration-interaction potential energy surfaces are reported for the interaction of Ar with the BH radical in its ground (X 1Σ+) and first excited (A 1Π) electronic states. These potential energy surfaces are then used with an adiabatic bender model for the calculation of the vibrational energy levels of the ArBH van der Waals complex in its ground and first excited singlet electronic states. Comparison of vibrational energies calculated using this adiabatic bender model with computed exact vibrational energies indicates that the former provides a very useful description of the bound levels of the ArBH complex. A qualitative discussion of the expected features in the A 1Π−X 1Σ+ electronic spectrum of ArBH is also presented, to facilitate comparison with the experimental ArBH spectrum reported in the following paper [E. Hwang and P. J. Dagdigian, following paper, J. Chem. Phys. 101, 2903 (1994)]. The most strongly bound ArBH(A) levels, with Ar–BH separations less than in the ground state ArBH(X) complex, correspond to motion described primarily by the more attractive VA″ potential energy surface and to a helicopterlike internal motion of the BH moiety. For the more weakly bound states supported by higher bender curves, the vibrational motion cannot be described as occurring on either the VA″ or VA′ potential energy surfaces separately.

Picosecond time–resolved Raman spectroscopy of S1p‐terphenyl and p‐terphenyl d14 in solution: Time‐dependent changes of Raman band shapes

AbstractThe picosecond time‐resolved Raman spectra of p‐erphenyl and its deuterated analogue (dl4) in the first excited singlet (S1) electronic state were observed in heptane and acetonirile solutions. The most intense band in the S1, p‐erphenyl at 750 cm−1 is assigned to an in‐plane ring vibration based on the shift on the deuteration. The peak position and the bandwidth of the 750 cm−1 band change as the time delay between the pump and probe pulses changes. The initial changes in the position and width appear between 0 and 5 ps, which can be attributed to the strong electric fields of the pump and probe light pulses. After the initial changes are over, the position and the width of the 750 cm−1 band change gradually to their stationary values until about 30 ps. These changes are probably due to the cooling of the solute–solvent system. Similar changes are recorded for two other Raman bands at 993 and 1018 cm−1.

Fragment Ion Formation in Resonance Enhanced Multiphoton Ionization (REMPI) of n-Propyl Phenyl Ether in a Supersonic Jet

Resonance enhanced multiphoton ionization (REMPI) mass spectra of different conformational isomers of n-propyl phenyl ether at 30 K give the same fragmentation patterns. Laser REMPI excitation spectra exhibit three principal conformers in a supersonic free jet expansion. The most abundant species in the jet happens also to contribute the longest wavelength 0,O band. SCF calculations suggest that the lowest energy structure corresponds to a gauche geometry in which all the carbons except that of the methyl group are essentially coplanar with the oxygen. This is confirmed by experimental observation of a predicted blue shift for a prominent vibrational overtone when the propyl group is partially deuterated (0,/3-d2 or a,a,T,T,T-d5). Time-of-flight mass spectra of deuterated analogues of each of three conformers exhibits propene expulsion to yield PhOH'+ and PhOD'+ (the principal fragment ions) in which all seven alkyl hydrogens have become randomized within the chain. REMPI of individual conformers (via intermediacy of the lowest vibrational levels of their excited singlet electronic states) therefore gives the same outcome as does field ionization or electron impact source mass spectra. The predominant decomposition mechanism of the radical cation involves an ion-neutral complex, nPrOPh*+ -. (iPr+ PhO'), in which the hydrogens of the iPr+ undergo rapid internal transpositions prior to the ultimate decomposition step. Ab initio computations on a model system concur with the experimental inference that this mechanism operates regardless of the conformation of the precursor neutral.

Proton transfer in excited electronic states: environmental effects on the tautomerization of 2-pyridone

Abstract The modulation of proton-transfer reactions by environmental effects has been investigated for the case of lactam—lactim isomerization in several electronic states to 2-pyridone. Specific interactions with a single water molecule strongly enhance the reaction rate and shift the equilibrium toward the lactam form. The effect of bulk solvent is smaller, although the lactam form is further stabilized. In the lowest singlet and triplet excited electronic states the lactim form is noticeably destabilized. In the latter case the planarity of the pyrimidinic ring is destroyed. Zero-point and entropy effects on the thermodynamics and kinetics of the reaction are generally small and do not modify the above general trends. Absorption and flourescence spectra are well reproduced by the semiempirical method of intermediate neglect of differential overlap for spectroscopy. As is always the case for π-π * transitions solvent-induced spectral shifts are negligible.

Carotenoids as accessory light-harvesting pigments

Abstract

Photoluminescence study of β relaxation of polyethylene blends

AbstractSecondary β relaxation of polyethylene blends (50/50 HDPE/LDPE) were studied by photoluminescence of anthracene molecules dissolved in the polymer bulk. The temperature of the β relaxation has been determined as Tβ = −40°C by the dependence of the ratio of vibronic components of the fluorescence band on the temperature. The molecular mechanism of this relaxation has been discussed considering the possibility of the energy migration involving anthracene molecules in the singlet electronic excited and ground states. © 1993 John Wiley & Sons, Inc.

The S 0→S 3 transition in 1, 2-benzanthracene. Experimental evidence for the statistical limit of vibronic coupling of S 3 with S 2

The spectroscopic properties of an excited singlet electronic state S n ( n⩾1) of an aromatic compound depend on the type of vibronic coupling of S n with S n−1 . The main spectroscopic criterion for the statistical limit of vibronic coupling is a definite 0,0 transition S 0,0→S n0 whose homogeneous lineshape is Lorentzian. This criterion is satisfied by the S 0→S 3 absorption band of 1,2-benzanthracene. In the excitation spectrum of the S 1→S 0 fluorescence from isolated 1,2-benzanthracene molecules in a supersonic jet, the origin of the S 0→S 3 absorption band consists of a single Lorentzian line with a width (fwhm) of 39 cm −1. Practically the same Lorentzian width is obtained with 1,2-benzanthracene in polymethylmethacrylate by transient spectral hole-burning. The corresponding lifetime of S 3,0 is τ 3,0=136 fs. The same order of magnitude of τ 3,0 is obtained from the quantum yield of the S 3→S O fluorescence and the calculated radiative lifetime of S 3.

Theoretical characterisation of the lowest n→π * electronic state of CF 2O. Is it planar or non-planar?

Structures, vibrational frequencies and energies for the lowest singlet electronic excited state of carbonyl fluoride CF 2O have been computed by ab initio methods (PMP4 (SDTQ) and QCISD (T) with a 6-311 + G (2d) basis and full-valence CASSCF with a 6-31 G * basis), showing this 1(n,π *) state to be non-planar and to lie 111 kcal mol −1 above the ground state, in good agreement with experiment. Nearby triplet states have also been studied.

Excited states, spectroscopic and generation properties, and transition scheme of the complex molecule 1,4-phenylene-2,2'-bisoxazole

We investigate the spectroscopic and generation properties of a new active complex molecule, 1,4-phenylene-2,2'-bisoxazole (OPO), which is capable of fluorescing and generating light in solvents of various kinds within the wavelength region λ≈340–420 nm with a high fluorescence quantum yield γ≈0.45–0.93 and a low threshold pump density Elp. Using the density matrix method, we show that the solvent affects the distribution of electron density among the individual atoms and fragments of the complex OPO molecule in the ground state, leading to systematic changes in geometry. As a result, there are changes in the distribution of bond lengths in the rings of the OPO molecule. Protonation of the nitrogen atoms in the OPO molecule changes the structure of the excited electronic singlet (Si*) and triplet (Ti) states and in the transitions S0 → Sn*, Sl* → S0, Sl* → Sn*, Tl → Tn , Tl → S0, which determine the spectroscopic and generation characteristics of the complex compounds. The spectral, fluorescence, and generation characteristics of this complex heteroatomic molecule in solvents of various types and also in the vapor phase were measured and calculated. The results show that the decrease or increase of Elp in the switch from solution to vapor, or as the properties of the solvent change, stems from dynamic separation or overlap of the lasing and induced absorption bands of the Sl* → Sn* and Tl → Tn transitions. Lasing does not occur in concentrated acids because of the nearly complete overlap of the limiting-gain and induced absorption (Tl → Tn) bands. We show that the method of a priori structural modeling for quantum mechanical control of the properties of molecular excited states and transitions can be used in practical applications.

Stark effect in dye-doped polymers studied by photochemically accumulated photon echo

We study theoretically and experimentally the influence of a homogeneous static electric field on a photochemically accumulated stimulated photon echo in dye-doped polymers. The dependence of the time profile of the echo signal on the strength of the electric field is evaluated in the case of a linear Stark effect. Experiments using polymers doped with organic dye molecules are carried out at low temperature, and our new technique is applied to determine the difference of the permanent dipole moment in the ground and the first excited singlet electronic state of the impurity molecules.

Solvent effects on the torsional dynamics of a twisted intramolecular charge transfer (TICT) molecule: bianthryl in acetonitrile

Molecular dynamics simulations have been used to study the torsional potential surface and dynamics of 9,9′-bianthryl (BA) in the polar aprotic solvent acetonitrile. In the gas phase, the ground state (S 0) torsional potential has a minimum at the perpendicular conformation with a twist angle of 90°, while for the first excited singlet electronic state (S 1), the 90° twist angle corresponds to a local maximum with symmetric minima located at ≈ 70° and ≈ 110°, respectively. The solvent coupling to the torsional coordinate in both the ground and excited states results in increased twisting with respect to the gas phase potential. Two models for the charge distribution in the electronically excited state were studied corresponding to a locally excited (LE) and a charge transfer (CT) state, respectively. The partial atomic charges representative of the S 0, LE, and CT states of BA were obtained from fits to molecular electrostatic potentials created by electronic structure calculations employing the semi-empirical INDO/1S Hamiltonian. The transfer of an electron between the two anthracene rings does not have a large effect on the torsional potential of mean force in acetonitrile. This surprising result is partly attributed to delocalization of the electronic charge occurring over a large molecular region without substantial atomic charge buildup or depletion occurring on any specific atomic site fully exposed to the solvent. The torsional dynamics of bianthryl following photoexcitation was modeled by running nonequilibrium trajectories on the excited state surfaces with initial configurations chosen from the ground state distribution. The relaxation time τ t for bianthryl on the torsional surface in the excited state is estimated to be between 1.5 and 2 ps in acetonitrile. We have also calculated the time correlation function C( t) for the fluctuations in the bianthryl-acetonitrile electrostatic potential. The relaxation time is estimated to be τ s≈ 200 fs. The possible consequences of the relation τ s<τ t are discussed.

A trimer vibronic coupling model for triptycene: The Jahn–Teller and Barnett effects

The first singlet electronic excited state of triptycene, as measured by resonant two-photon ionization in a supersonically cooled beam, has been found to be a textbook example of the E′⊗e′ Jahn–Teller effect. Here it is shown that this E′⊗e′ vibronic coupling can be profitably viewed as a subset of a (A′1⊕E′)⊗(a′2⊕e′) vibronic coupling scheme which results from a simple trimer model. The enlarged coupling scheme has a simple physical interpretation where the wagging coordinates of the benzene subunits are strongly coupled to their excimer formation. The previously obtained parameters, in which there is a large reduction between the ground and excited electronic state frequencies of the lowest frequency e′ mode as well as an unusually large second-order vibronic coupling constant, are shown to arise naturally from a trimer viewpoint. Features of the spectra have been found which are attributed to the involvement of an a′2 vibration which couples through nonzero momentum rather than coordinate matrix elements. A coupling of this type has been termed the ‘‘molecular Barnett effect’’ and has been predicted to appear in Jahn–Teller systems when certain conditions are fulfilled. This effect has not been previously observed in molecular spectroscopy. It indicates that crude adiabatic basis functions are inadequate to describe the electronic states of the present system.

Excited electronic states and generation in series of complex polyatomic molecules

The photophysical properties of some new and also some known complex organic molecules which emit in the wavelength interval 340–410 nm have been studied in a wide range of organic solvents. Specifically, these molecules are based on phenyl-, furyl-, and thienyl-oxazoles and oxadiazoles. Density-matrix methods are used to study the properties of a broad spectrum of excited singlet and triplet electronic states. There are relationships among the structure, fluorescence spectra, and emission properties of the complex molecules forming a quasihomologous series. The method of a priori structural modeling of active complex molecules is useful for achieving quantum-mechanical control over the properties of the excited states and the transitions.

Excited state relaxation of crown ether styryl dyes: photoisomerization

Photophysical processes are studied in a new class of dyes containing a chromophoric part and a crown ether part (crown ether styryl dyes). Photoisomerization is proposed to be the main channel of nonradiative decay of the singlet excited electronic state. The temperature dependence of fluorescence decay was measured and the lifetime of the trans-isomer in excited electronic state was found to decrease with temperature in ethanol solutions. Isomerization around the CC bond is discussed, and the activation barrier is estimated as 2.3 and 1.5 kcal/mol for the two crown ether styryl dyes studied.