Bryan E. Kohler

Polyene spectroscopy: The lowest energy excited singlet state of diphenyloctatetraene and other linear polyenes

Optical absorption and emission spectra at liquid helium temperatures of solutions of all‐trans‐1,8‐diphenyloctatetraene in single crystals of bibenzyl and polycrystalline n‐paraffin matrixes are presented which show an excited singlet state at a lower energy than the 1Bu state responsible for the strong absorption of all linear polyenes. The transition from the ground state to this level has an oscillator strength of about 0.05. The location of this low energy weak transition removes a number of discrepancies between the expected and observed fluorescence properties of diphenyloctatetraene. The observed vibronic pattern is consistent with the Raman spectrum which is also reported. Substantial indirect evidence is presented which indicates that many, and perhaps all, linear polyenes have a similar ordering of excited states. A review of the available information on diphenylpolyenes is presented. A correlation of the oscillator strength of the fluorescence transition as deduced from the emission lifetime w...

A low-lying weak transition in the polyene α,ω-diphenyloctatetraene

Optical absorption and emission spectra at liquid helium temperatures of solutions of α,ω-diphenyloctatetraene (DPO) in single crystal bibenzyl and polycrystalline n-paraffins have been measured. These measurements show that (1) there is an excited singlet state at lower energy than the 1B1u state responsible for the strong absorption at 4100 A: (2) the presence of this state rationalizes a number of distinctive characteristics of polyene spectra which had previously been explained on the basis of “Franck-Condon forbiddeness”. Evidence that these results describe a general property of polyene electronic structure which is not specific to DPO is presented.

The origin of the chemiluminescence of phagocytosing granulocytes.

Granulocytes engaged in the phagocytosis of opsonized zymosan emit light by a process that is inhibited by superoxide dismutase and catalase. In the present report is is shown that light emission is the result of reactions between certain unspecified constituents of the ingested particles and some or all of the oxidizing agents (H2O2, O2),and possibly the hydroxyl radical and singlet oxygen) produced by the activated cells. This conclusion is based on a study of light emission by both activated cells ans artificial O2 generating system containing xanthine oxidase and purine. With these two systems light production required the presence of both zymosan and oxidizing agent, suggesting that the oxidation of particle components is necessary for luminescence to occur. The characteristics of the emission spectrum as well as the finding that granulocytes activated by a nonparticulate agent (F-) fail to liminesce show that light emission by the relaxation of singlet oxygen to the ground state does not contribute in a major way to the chemiluminescence of phagocytosing granulocytes; whether singlet oxygen contributes to chemiluminescence in other ways cannot be decided from the data available. Inasmuch as the oxidation of constituents of ingested particles is an important bacterial killing mechanism in the granulocyte, chemiluminescence may be viewed as a manifestation of the microbicidal activity of the cell.

High‐resolution one and two photon excitation spectra of trans, trans‐1,3,5,7‐octatetraene

One and two photon excitation spectra for the lowest energy singlet transition (2 1Ag←1 1Ag) of trans, trans 1,3,5,7‐octatetraene in n‐octane at 4.2 K(0–0 at 28 56l cm−1) have been measured and analyzed. The one photon spectrum is found to be vibronically induced while the two photon spectrum is electronically allowed. The contribution of the higher lying 1 1Bu←11Ag transition (0–0 at approximately 32 100 cm−1) to the observed two photon excitation spectrum is found to be negligible allowing vibrational overtones of the 2 1Ag←1 1Ag system to be observed. On the bases of frequency, intensity, and one or two photon allowedness, 128 measured lines are assigned as combinations of 4 antisymmetric and 11 symmetric normal modes.

Franck–Condon analysis of the 1 1Ag→1 1Bu absorption in linear polyenes with two through six double bonds

The vibronic intensity distribution at room temperature and 77 °K of the 1 1Ag→1 1Bu transition for the series of linear polyenes with two through six conjugated double bonds has been measured and analyzed. These inhomogeneously broadened solution spectra can be described reasonably using only two harmonic normal modes of vibration: single and double bond stretches at ∼1200 and ∼1600 cm−1, respectively. Excited state normal coordinate displacements were determined by a least‐squares fitting procedure. For both the double and single bond modes the estimated change in bond length upon excitation decreases as the length of the polyene chain increases. The bandwidth of these spectra decreases with increasing polyene length. This is well described by assuming that in solution a range of conformations differing by very small torsions about the terminal single bonds has been frozen in. The Franck–Condon analysis indicates that under these conditions all of the polyenes are approximately planar in the 1Bu state, ...

LOW RESOLUTION OPTICAL SPECTROSCOPY OF RETINYL POLYENES: LOW LYING ELECTRONIC LEVELS AND SPECTRAL BROADNESS*

Abstract— Optical absorption and emission spectra of retinal, axerophtene, anhydrovitamin A, and 2,10‐dimethylundecapentaene in rigid glass matrixes at 77°K are presented. An interpretation of the observed spectra which provides a rationalization on the basis of electronic structure for the diffuse nature of the retinal spectrum is discussed. For the undecapentaene a weakly absorbing 1Ag state was found 3250 cm‐1 lower in energy than the level responsible for the strong visible absorption.

Vibronic coupling in polyenes: High resolution optical spectroscopy of 2,10‐dimethylundecapentaene

High resolution fluorescence and absorption spectra of 2,10‐dimethylundecapentaene in 4.2 °K n‐nonane matrices are presented. Analysis of these spectra indicates that the transitions between the ground and first excited state are induced by low frequency, nontotally symmetric vibrations. This is in agreement with Herzberg‐Teller expectations for forbidden transitions and supports the 1Ag assignment for the lowest lying excited singlet state. Vibrational analysis of the absorption indicates increases in the carbon–carbon double bond stretching frequencies in the 1Ag state. This is unexpected given the decreases in carbon–carbon double bond orders predicted by molecular orbital calculations. Rationalization of this apparent discrepancy may be found in substantial mixings of excited state vibrations. The photochemical implications of these findings also are discussed.

The crystal structure of trans,trans-1,3,5,7-octatetraene as a model for fully-ordered trans-polyacetylene

Abstract The crystal structure of trans,trans -1,3,5,7-octatetraene has been determined using x-ray diffraction techniques. This crystal structure, the first for an unsubstituted polyene, is used to predict the molecular and crystallographic structures of trans -polyacetylene. The structure of the octatetraene crystal is monoclinic, P2 1 /c, and has two centrosymmetric molecules in a unit cell with dimensions a = 10.228(8) A , b = 4.120(1) A , c = 8.313(3) A , and β = 96.53(4)°. The carbon skeleton is planar to within the experimental error and the carbon-carbon bond lengths starting from the chain end are 1.336(4), 1.451(3), 1.327(4), and 1.451(5) A. The average CCC angle is 125.0(3)°. From the intermolecular packing mode of octatetraene, the predicted structure of trans -polyacetylene is monoclinic, P2 1 /n, with a = 7.38 A , b = 4.12 A , c = 2.47 A ( chain axis ), β = 97.8° , and two chains per unit cell (2C 2 H 2 units). These axial lengths deviate at most by 1% from the average of three experimental determinations using x-ray diffraction and suggest that alternate proposed cells are less likely. Likewise, the predicted setting angle of the polymer chain (52.7° relative to the (010) of the present cell) is in good agreement with the results of crystal energy calculations (51.0°) and x-ray diffraction analyses (55°). Using the octatetraene data, the predicted single-bond length, double-bond length and CCC angle in trans -polyacetylene are 1.451(5) A, 1.327(4) A, and 125.3(4)°. These results are in reasonable agreement with bond lengths derived from nutation n.m.r. studies on trans -polyacetylene ( 1.44 ± 0.02 a and 1.36 ± 0.02 A ). The dimerization parameter ( μ 0 ) derived from the octatetraene results is 0.035(1) A, which can be compared with values previously estimated from the analysis of the limited available electron and x-ray diffraction intensities for poorly ordered trans -polyacetylene ( 0.012 ± 0.001 A and 0.03 A , respectively.

Narrow‐line fluorescence spectra of perylene as a function of excitation wavelength

Experiments on the laser excited fluorescence spectrum of perylene in n‐octane and ethanol hosts at low temperatures are reported. Monochromatic excitation leads to a dramatic sharpening of the emission lines. The dependence of the spectrum on both excitation wavelength and temperature are presented and discussed qualitatively in terms of a model which applies the basic concepts of the theory of mixed crystal spectra to systems with inhomogeneous broadening.

Electronic structure and spectra of finite linear polyenes

Abstract A brief review of the current status of our knowledge of the excited electronic states of finite linear polyene chains is presented. The main conclusion of recent studies is that the neutral molecular species with an even number of carbons greater than or equal to six have a state of Ag symmetry as their lowest excited singlet state. Theoretical studies lead to the conclusion that the proper description of this excited state requires inclusion of electronic configurations involving two or more excitations. The low energy of this state is due to extensive electron correlation in these molecules. This excited Ag state gives rise to transitions that are electric dipole forbidden from the ground state which has the same symmetry. The ordinary absorption spectrum is dominated by the very strong transition to the Bu singlet state, a state well described as a one-electron excitation corresponding to the band gap for the infinite chain. This lack of intensity for the lowest transition and the good agreement obtained between predicted and observed spectra led to the long held, but erroneous, conclusion that the states of these linear chains were well described by simple one-electron theories.