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Phenalenyl radical is an odd-alternant hydrocarbon radical of high symmetry, D3h and is extremely attractive as the constituent of molecular magnets. But it has not been characterized in detail. Recently, 2,5,8-tri-tert-butylphenalenyl radical has successfully been synthesized. In this work the gas phase He(I) photoelectron spectrum of this radical has been measured and analyzed with the aid of UHF MO and RHF MO SECI calculations. The first band has been assigned to the ionization from the SO–π–MO of the neutral radical. The second band group has been ascribed to the ionized states relevant to three triplet ionic states and one singlet ionic state of the monocation, the third band group being ascribed to the two singlet ionic states of the monocation.

The transient difference absorption spectrum of a substituted α-linked oligothiophene 13-mer was measured by sub 5-fs pump–probe spectroscopy. This molecule is the longest oligothiophene studied under time-resolved investigation. The spectrum observed in the sub-ps range exhibited a feature of the dynamic Stokes shift. A simple numerical calculation based on the Franck–Condon (FC) overlaps induced by molecular vibration reproduced the modulation on the difference absorption spectrum. These results verify the time-dependent FC overlap in this molecule.

Melatonin is a hormone structurally regarded as being composed of a 5-methoxyindole group and an N -ethylacetamide group; its various physiological activities have attracted a great deal of attention recently. The gas phase He(I) photoelectron spectra of melatonin (M) and its related compounds including N -acetylserotonin have been studied with the aid of molecular orbital calculations. The first photoelectron spectral band group of compound M is ascribed to ionizations from the two π orbitals localized on the methoxyindole group. The second band group is quite complicated and is regarded as being composed of several bands. The lower energy part of the second band group is ascribed to the three orbitals relevant to the third highest occupied π orbital of 5-methoxyindole and the highest occupied π and the n CO orbitals of N -ethylacetamide. The interactions among the three orbitals have been found to operate on the basis of the molecular orbital calculations; these interactions depend strongly on the conformations. The high energy end of the second band group is relevant to the π orbital mainly localized on the 5-methoxyindole group and is ascribed to the fourth highest occupied π orbital of 5-methoxyindole.

The Disilane Chromophore: Photoelectron an dElectronic Spectra of Hexaalkyldisilanes and 1, (n+,)-Disila[n.n.n]propellanes

Electroreflection spectra have been measured to investigate the electric field effects on excitonic modulation for several subbands in a ZnSe–Zn 0.8 Cd 0.2 Se superlattice structure. In photoluminescence spectra, three subbands originating from the n =1 heavy-hole ( E 1hh ), n =2 heavy-hole ( E 2hh ) and light-hole ( E 2lh ) excitonic peaks were observed at 17 K. In a weak electric field of only 3.5 kV/cm the induced excitonic transitions corresponding to the above three peaks were all clearly modulated both at 77 K and at room temperature. The polarization dependence of the excitonic transition in ZnSe–Zn 0.8 Cd 0.2 Se superlattices was studied by low-field electroreflection spectroscopy. A new peak observed in the lower energy side of the n =2 heavy-hole exciton E 2hh at room temperature is attributed to the forbidden transition in zero field from the n =2 heavy-hole state to the n =1 electron state ( E 2h,1e ). This reflection change is found to be polarization dependent.

Time-resolved difference absorption spectra of polycrystalline thin films of C 60 were measured by femtosecond pump–probe spectroscopy. The decay dynamics of the excited state can be explained in terms of a model of localized excitations. Self-trapped excitons (STEs) are formed after photoexcitation and then converted to polarons. The polarons mainly recombine when they encounter each other after intersite hopping. The decay times of the STEs and polarons are estimated to be 570±120 fs and 54±7 ps, respectively.

Femtosecond transient absorption of a poly(3-dodecylthiophene) film is measured in a wide spectral range from visible to near-infrared. Our results show that the transient absorption consists of four decay components. The two faster components are assigned to self-trapping and the subsequent relaxation process of photogenerated excitons. The relaxation time constant of the self-trapped exciton is evaluated as 680±50 fs. The excited species corresponding to the third slow component is identified as a pair of intrachain, oppositely-charged polarons on the basis of its power-law decay and its spectral peak around 1.4 eV. A long-lived transient absorption due to interchain polarons is also observed.

The lifetimes of self-trapped excitons (STE) in poly-[8-(4-bromophenylthio)-1-(3-chlorophenylcarbamoyloxy)-5,7-octadiyne] (PDA-TPC) with excitonic transition energy E ex =1.75 eV and poly-[1,6-dicarbazolyl-2,4-hexadiyne] (PDA-DCHD) with E ex =1.90 eV were measured to be 0.6±0.1 and 1.3±0.5 ps, respectively, at room temperature. The value of E ex in PDA-TPC is the smallest among all polydiacetylenes (PDA) studied so far because of the electron donor nature of the side substituent group. From the data, together with those of seven other PDAs, the shortest lifetime in PDA-TPC is explained in terms of its low potential barrier between the minima of the STE and the ground state. A comparison of the lifetime and E ex among many PDAs shows that a previous model of STE needs slight modification.

The gas-phase He(I) photoelectron spectra of docosahexaenoic acid (I) and its ethyl ester, docosahexaenoic acid ethyl ester (II), have been studied with the aid of MNDO calculations. These molecules have six CC double bonds and a carboxyl or an ethoxycarbonyl end-group. The six CC double bonds in these molecules are not directly adjacent to one another, but are placed with great regularity; that is, there is one methylene group between each pair of double bonds adjacent to one another. The first photoelectron spectral band groups of compound I show almost the same vertical ionization energy values as those of compound II and are ascribed to the ionizations from the six CC double bond π orbitals. The six CC double bonds of both compounds seem to be isolated from one another, as judged by their molecular structure, but the occupied π orbitals have been found to be delocalized over several CC double bond groups on the basis of the photoelectron spectra and the molecular orbital calculations. In compound II an independent second band is apparently well separated from the following band group. This second band group is ascribed to the carbonyl oxygen atom n band and the ethoxycarbonyl group π band overlapping each other.

Crystal symmetry and lattice constants of BaPb1−xBixO3 were analyzed by X-ray diffraction using synchrotron radiation as a light source. Measurements with high resolution could be carried out and crystal system of BaPb1−xBixO3 could be determined clearly. Monoclinic and orthorhombic symmetry were observed in the x range 0.5 ≦ x ≦ 1.0 and 0.0 ≦ x ≦ 0.5, respectively, showing agreement with the electrical conduction behavior and optical reflection property of BaPb1−xBixO3.

9,10-Disilatriptycene is of high symmetry and structurally very close to triptycene. In the latter case it is known that the through-space orbital interactions among the π electron systems of the three benzene rings predominate, the effect of the through-bond interactions being negligibly small. In this study, the vapor-phase He(I) photoelectron spectra of triptycene and 9,10-disilatriptycene were measured and studied comparatively in order to investigate the relative importance of the Jahn–Teller effects in the cationic states of these compounds, the through-space and the through-bond interactions. Simple LCBO model calculations could reproduce almost quantitatively the features of their observed spectral patterns. The spectral pattern is quite complex in 9,10-disilatriptycene in contrast to that of triptycene due to the back-donation from the benzene rings to the Si atoms in addition to the Jahn–Teller effects in the cationic states and the through-space interactions. © 1997 Elsevier Science B.V.

The He I photoelectron spectra of xanthone, thioxanthone, and acridone, which are system iso-π-electronic with one another, have been measured and are comparatively discussed on the basis of the HMO, MINDO/3, MNDO, and PM3 methods.

The gas phase He(I) photoelectron spectra of uracil (I) and its derivatives, 5-fluorouracil (II), ftorafur (tegafur, 5-fluoro-1-(tetrahydro-2-furyl)uracil) (III), 2′,3′-dideoxyuridine (IV) and 2′,3′-dideoxy-5-fluorouridine (V), have been studied by means of MNDO calculations. The first four photoelectron spectral bands of I and II are π1, n1, π2 and n2 from the top, respectively. In contrast to these cases it is extremely difficult to assign the photoelectron spectral bands n1, π2 and n2 of III because of their complete overlapping with the oxygen atom n orbital band of the tetrahydrofuryl group. In order to solve this problem the difference spectrum method has been used, and the exact peak positions have been successfully detected. The photoelectron spectral band shifts were reasonably analyzed in terms of the conjugative, the short-range inductive and the long-range molecular polarization effects. Although for I, II and III the keto forms are stable, it has been revealed that in the case of IV or V both a keto and an enol form (at least) coexist predominantly in the gas phase. The π1 ionization energy of the predominant enol form of IV is completely indifferent to the 5-fluorination, as is the case of the keto form of I. This unusual finding can be explained as follows: the molecular polarization and the electron-withdrawing inductive effects of the 5-fluorine atom are almost cancelled by the π conjugative effect.

The He I photoelectron spectra (PES) of 2- and 4-ethynylpyridine were measured, and the origin of each ionization band was clarified. For instance, 4-ethynylpyridine shows five PES bands at 9.58, 9.9, 10.0, 10.75 and 11.84 eV in the lower energy region, the 9.9 eV and 10.75 eV bands in particular being assigned to the ionizations from the a 1 n orbital of the pyridyl group and the in-plane b 2 π orbital of the ethynyl group, respectively. Furthermore, with the aid of first order perturbation theory, the 10.59 eV band of 2-ethynylpyridine has been ascribed to the ionization from the a 2 -like π orbital, and the 10.8 eV band to that from the in-plane b 2 -like π one. Through the process of the band assignments for the ethynylpyridines, it has been revealed that there is a weak interaction between the n orbital on the orbio-substituted nitrogen atom and the in-plane π orbital localized on the ethynyl group of 2-ethynylpyridine. The photoelectron spectra of 1,4-bis(2-pyridyl)-1,3-butadiyne and 1,4-bis(4-pyridyl)-1,3-butadiyne, whose π-electron systems are regarded electronically as double ones of the corresponding ethynylpyridines, were also measured, and we have succeeded in making band assignments of these compounds on the ground of the correlation energy diagram with the ethynylpyridines.

Permethylated 3,4,7,8-tetragermacycloocta-1,5-diyne ( 1 ) and 3,4-disila-7,8-digermacycloocta-1,5-diyne ( 2 ) were prepared. The photoelectron spectra of 1 and 2 were compared with that of their tetrasila analogue 3 . The first ionization energy decreases in the order 3 > 2 > 1 . Charge-transfer spectra of 1 -tetracyanoethylene (TCNE), 2 -TCNE and 3 -TCNE complexes were observed. In CH 2 Cl 2 containing TCNE 1 underwent oligomerization to give its higher homologues, and in CH 3 CN containing a catalytic amount of TCNE both 1 and 2 were polymerized to give the corresponding polymers almost quantitatively, whereas the sila analogue 3 was stable under these conditions. All compounds 1, 2 and 3 cycloadded to 2,3-dichloro-5,6-dicyano-1,4-benzoquinone to give the corresponding 1 : 1 adducts in a [2 + 6] manner.

Ultrafast optical response in two quasi-one-dimensional halogen-bridged mixed-valence complexs [Pt(en)2][PtBr2(en)2](ClO4)4 (en=ethylenediamine) (abbreviated as PtBr) and [Pt(en)2][PtBr2(en)2](ClO4)4 (as PdCl) has been investigated by femtosecond absorption spectroscopy at room temperature by pump-probe spectroscopy. The photo-induced absorption around 1.3 eV and the bleaching from 1.5 eV to 2.7 eV were observed in PtBr. Both consist of a fast-decay component due to STEs and a slow-decay component due probably to polaron pairs. The former decays exponentially with the time constant of 1.4 ± 0.2 ps. The latter decays as erf(t β) with β = -0.22 ± 0.02, indicating the geminate recombination of an electron polaron and a hole polaron after moving freely along the chain. The deviation of β from the ideal random-walk model (β = -0.5) is explained by introducing the effect of potential barrier between the polarons hindering the recombination. A pump-probe absorption spectrum of PdCl is obtained from the ...

Quantum beats in optical nutation are calculated in a system, such as a polydiacetylene, in which photon energies of stimulated emission and photoinduced absorption are nearly equal. It is pointed out that the transversal relaxation time constant and the dipole moment of the transition between self-trapped excitons and m 1Ag excitons, observed as photoinduced absorption, can be determined by a measurement of quantum beats in the optical nutation.

The ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pd(en)2] [PdCl2(en)2] (ClO4)4 (en = ethylenediamine) has been investigated by the use of a femtosecond absorption spectrum, calculated from a pump—probe reflection spectrum at room temperature by the Kramers—Kronig relations. The time dependence of the transient photoinduced absorption around 1.7 eV and the bleaching from 1.9 to 2.5 eV were calculated for three decay components. They are free excitons with a lifetime of about 800 fs, self-trapped excitons with a lifetime of about 3 ps, and polaron pairs which relax within a 100 ps time period.

Ultrafast optical response in a quasi-one-dimensional halogen-bridged mixed-valence complex [Pt(en) 2 ][PtBr 2 (en) 2 ](ClO 4 ) 4 (en=ethylenediamine) has been investigated by femtosecond absorption spectroscopy at room temperature using pump-probe spectroscopy. The photo-induced absorption around 1.3 eV and the bleaching from 1.5 eV to 2.7 eV were observed. Both consist of a fast-decay component due to STEs and a slow-decay component due to polaron pairs. The former decays exponentially with the time constant of 1.4±0.1 ps. The latter decays as erf( t β ) with β =−0.22±0.02, indicating the geminate recombination of an electron polaron and a hole polaron after moving freely along the chain. The deviation of β from the ideal random-walk model ( β = −0.5) is explained by introducing the effect of potential barrier between the polarons hindering the recombination.

Time dependence of a photoinduced absorbance change in a thin film of polydiacetylene PDA-3BCMU (poly-4,6-decadiyne-1, 10-diol-bis( n -butoxycarbonyl-methylurethane)) was separated from the effect of increasing sample temperature by a newly designed method. The resultant nonthermal contribution relaxes via a geminate recombination process, probably due to polarons.

Linearly-condensed polythiophenes (4 and 5) have been prepared and their characteristic aggregation behaviour as hydrogen-poor π compounds in the solid state has been investigated in comparison with those of the lower homologues (2 and 3); the photoelectron spectra of the series 2–5 were measured in the solid state as well as in the gas phase. For 4 the difference in the ionization energies for the gaseous (adiabatic; 7.22 eV) and solid (threshold; 4.86 eV) states is significant, giving rise to a large polarization energy (2.36 eV), which is larger than the calculated value (1.83 eV) based on the packing density and molecular polarizability. This has been interpreted in terms of the intermolecular orbital interactions in the solid state of 4. Such an interaction has not been observed in the lower homologues (2 and 3).

Time-resolved photoinduced absorption spectra of vacuum-evaporated thin films of α-conjugated thiophene oligomer (α- n T) exhibit a transient bleaching with strong oscillations of almost constant energy spacing (Δ A =0.2 eV) in the intra-bandgap region that can be related to the fine structures currently observed in the unperturbed absorption spectra. Exponential decay of the time-resolved absorbance change with time constants of 1.3±0.1 μs, 550±20 ns and 310±10 ns, due to the lowest triplet state in the α-4T, α-5T and α-6T respectively, was identified.

A CdS/p-Si:H heterojunction is formed by the evaporation of CdS on a hydrogen passivated Si wafer. It is found that the substrate passivation procedure has a strong influence on the photocurrent properties of the CdS/p-Si interface. In fact, in addition to the photoresponse in the red and infrared spectral ranges, the CdS/p-Si:H heterodiode also reveals photocurrent in the blue and green. This is in contrast to CdS/p-Si and CdS/p-InP devices, which do not exhibit photocurrent in the absorption region of CdS. The positive influence of the passivation on the optoelectronic properties of the CdS/p-Si:H heterodiode is explained by the prevention of interface reactions during the formation of the device. The dependence of the photocurrent of the CdS/p-Si:H heterodiode on an applied bias gives evidence of a homojunction-like behavior.

By molecular beam epitaxy, a thin (≈200 nm) ZnSe film was grown on a p -type Si substrate covered with an InSe buffer layer. The InSe buffer is used to bypass the huge lattice mismatch of 4.4% between ZnSe and Si in order to ensure the growth of a stress-free optically smooth ZnSe film. Reflection, photoluminescence and photocurrent properties at 300 K demonstrated that the thin ZnSe film grown represents a high quality etalon with the bandgap of cubic bulk ZnSe. Furthermore, it is shown that the ZnSe/InSe/Si heterojunction exhibits rectification and a nearly constant intrinsic photoconductivity between 500 and 900 nm. This appreciable behavior makes the ZnSe/InSe/Si heterojunction very attractive for applications to sophisticated light detectors.

Vertical photocurrent in a ZnSe Zn 0.78 Cd 0.22 Se structure consisting of three sequent superlattices with 2.0, 3.8 and 8.2 nm well width was investigated. At 77 K, only the superlattice with the 2 nm wells remained thermally activated. By means of excitation spectroscopy, we have demonstrated that those thermally activated carriers performed a carrier injection into the superlattices with the wider wells causing a significant enhancement of their excitonic emission. We call the phenomenon of carrier injection from a narrow superlattice into a wider one, directional carrier injection.

The near degeneracy of the two lowest ionization energies of Dewar benzene can be understood in terms of closely competitive ‘through-bond’ and ‘through-space’ interaction. Empirical, semiempirical, and open-shell ab initio procedures converge to require mutually consistent symmetry assignments.

The photoelectron spectra (PES) of 1- and 2-substituted naphthalenes (naphthols, aminonaphthalenes, N,N-dimethylaminonaphthalenes, acetylnaphthalenes, and cyanonaphthalenes) were measured in the region from 6 to 20 eV. The observed PES were assigned with the aid of the CNDO/2 calculations and the substituent effect. The lower energy bands of the compounds with the electron-donating group were found to be due to the ionization from the orbitals caused by the interaction of the highest three occupied π orbitals of the naphtha ene ring with the nonbonding orbital of the substituent group. The second bands of the 1-substituted naphthalenes are commonly assigned to π orbitals almost completely localized on the naphthalene ring. The dimethylammo group of 1-dimethylaminonaphthalene was found to be twisted to a great extent from the ring plane.

Vapor phase HeI photoelectron spectra of o- and m-terphenyl were measured, and conformational analysis of biphenyl, o-, m-, and p-terphenyl was made by comparing the observed photoelectron spectral patterns with the calculated ones by the CNDO/S method, the inter-ring twist angles in these compounds being estimated to be around 40°, 60°, 40°, and 40°, respectively. The result that the inter-ring twist angles in o-terphenyl are far smaller than 90° is completely in contrast to the long believed perpendicular model of o-terphenyl suggested by the gas electron diffraction study.

As a model for detecting the intramolecular n-π type charge-transfer interaction, a homologous series of the compounds 3 was chosen and subjected to ultraviolet, fluorescence, and photoelectron spectroscopic study. The interaction was confirmed in the lower homologs of n=1, 2, and 3.

The intramolecular orbital interactions in N,N′-diphenylcarbodiimide were revealed by photoelectron spectroscopy and the phenyl rings in this compound were found to be twisted by ca. 25° from the perpendicular model.

Supporting evidence for the ring-opened structure of the radical cation of dicyclopentadiene (DCP) proposed by Roth et al. is presented: geometry optimization procedures predict that the ring-opened structure is more stable than the ring-closed by about 0.4 eV for both the exo- and endo-DCP isomers and that the optimized geometries are symmetric with the C1 and C2 symmetries for the respective isomers. The allylic nature in the cyclopenta ring is clearly disclosed. The observed electronic absorption spectrum of the radical cations produced in a γ-irradiated rigid solution is inconsistent with the observed photoelectron spectrum of the parent molecules, but the discrepancy is removed by assuming that the observed electronic absorption spectrum is associated with the ring-opened structure.

A new modified CNDO—MO method is presented and applied to transbutadiene, cyclopentadiene, fulvene, 3,4-dimethylenecyclobutene, spiro[4.4]nona-1,3-diene and spiro [4.4]nonatetraene. Though the method contains only one arbitrary adjustable molecular parameter, it gives reasonable explanations for the dipole moments, photoelectron spectra, and electronic spectra of the molecules treated here. The ionization potentials were estimated not only via Koopmans' theorem but also by the restricted Hartree—Fock SCF—CI treatment of radical cations.

The photoelectron spectrum (PES) of malonaldehyde was measured and assigned in comparison with those of acetylacetone, trifluoroacetylacetone, and hexafluoroacetylacetone. The first and the second bands of malonaldehyde were attributed to the ionizations from the b2π and the b1n orbitals, respectively, on the basis of the methyl and trifluoromethyl substituent effects and the PES result of acetylacetone.

Photoelectron spectra of isoxazole, 5-methylisoxazole, and 3,5-dimethylisoxazole were studied by the technique of photoelectron angular distribution measurements combined with a first order perturbation theoretic approach. Photoelectron spectra of oxazole, 4-methyloxazole, furan, and 2-methylfuran, which are closely related to isoxazole, were also discussed at the same time for the sake of comparison. The first three bands of the isoxazoles were assigned to the π, π, and n bands from the top, respectively, while those of the oxazoles were identified as the π, n, and π bands from the top.

The angular distributions for the He I photoelectron spectra of some aliphatic alcohols, amines, and halides were measured in order to reexamine the assignments of the so-called p-type photoelectron spectral bands in the He I region of these molecules. The second band of methanol and the third band of ethanol were assigned to the in-plane n orbitals localized mainly on the oxygen atom. Furthermore, it was found that the angular distributions of photoelectrons were useful for the conformational analysis of molecules and that the sum of β values was conserved for these compounds.

Photoelectron spectra (PES) of nitrophenols and nitroanisoles were measured and interpreted with the aid of previously reported PES of nitrobenzene, phenol and anisole. The effect of the intramolecular hydrogen bond is discussed in the case of o-nitrophenol.

Photoelectron spectra of tetrathiafulvalene (TTF) and 6,6′-bi(1,4-dithiafulvenyl) (BDTF) were measured and compared with each other. The first photoelectron spectral band of BDTF is only slightly shifted relative to that of TTF, though the BDTF molecule is lengthened compared with the TTF molecule by two sp2 carbon atoms. From the LCGO model orbital interaction analysis, this apparent anomaly is ascribed to the near cancellation of the effects of two main factors, the energy and the terminal atomic orbital coefficient differences between the central olefinic group π orbitals. Orbital interaction analysis as demonstrated in this work may be useful for molecular design, especially for orbital energy control.

A new method for molecular conformational analysis using photoelectron spectral data was developed and applied to alkyl-substituted styrenes as an example. The dihedral angle between the phenyl ring and the olefinic group planes, θ, in each molecule is evaluated by comparing the observed difference between the first and the third vertical ionization energies, ΔEiv1.3, with the empirically estimated ΔEiv1.3 vs. θ curve, the alkyl substituent effect being taken into account by the first order perturbation theory. The new method gives reasonable results consistent qualitatively with those given by other former methods.