The phosphorescence spectra and lifetimes of acetophenone, aceto-d3-phenone and acetophenone-d5 were recorded in a variety of close packed and relaxed matrices. The variable vibronic lifetime changes, temperature dependence, and deuteration effects on the acetophenone phosphorescence from solvent to solvent are manifestations of perturbation between close lying 3ππ* and 3nπ* states. The acetophenone phosphorescence problem is marked by environmental perturbations superimposed on pseudo-Jahn-Teller interactions. The result is an environmental sensitive potential surface for the lowest triplet state marked by failure of the Condon approximation. Five cases are distinguished. (i) Only the dynamical pseudo-Jahn-Teller effect is operative as in acetophenone vapor. The emission here is interpreted to be from a distorted, i.e., largely torsionally twisted acetophenone about the COCH3 group. (ii) and (iii) There are environmental (static) interactions larger than dynamical interactions leading to strong characteristic out-of-plane vibrations and medium lifetime, e.g., COCH3 group modes in methylbenzoate matrix, where 3nπ* is lowest, and ring out-of-plane modes in acetophenone-d5 matrix, where 3ππ* is lowest. (iv) There is a dynamical Jahn-Teller interaction stronger than the environmental interaction. In some instances the resulting triple minimum potential curve results in dual phosphorescence emissions even at low temperatures (1.68°K) from the lowest 3nπ* state—one with a very short lifetime from a highly distorted region of the potential surface (the zero point region) and the other with a medium lifetime from an excited vibronic region, e.g., in pentane and other relaxed hydrocarbon matrices. The upper vibronic region is kept from rapidly cascading to the lower region of the triplet state by a potential barrier leading to temperature dependent phosphorescence. Breakdown of the Condon approximation for the lowest triplet state is observed, probably due to rapid change of the singlet—triplet transition moment with the distortion coordinate in the vicinity of near degeneracy of the zeroth order states. (v) Again the dynamical Jahn-Teller interaction is stronger than the environmental interaction except that the zeroth order 3ππ* state is now lowest, e.g., at 77°K in a benzene matrix the emission with its vibrational structure and lifetime characteristic of 3nπ is from the zeroth order region—whose electronic wavefunction is described largely by 3nπ. At 4.2°K, the vibrational structure and lifetime represent phosphorescence from an excited vibronic region of the lowest triplet state whose electronic wavefunction is largely 3ππ*. The rapid change in transition moment with vibrational coordinate also renders the Condon approximation inappropriate in this case.
Polarized IR spectra of partially oriented benzimidazole crystals grown from the melt pressed between salt plates were recorded. The depolarization ratios of the Raman bands in the melt and the polarized Raman spectra of a single crystal in all possible orientations were measured. The experimental assignment of the fundamental vibrational bands is compared with a set of calculated values.
The absolute intensity of absorption parallel to the a and b axis of a fluorene crystal has been measured from 3000 to 2300 Å and allowed the following interpretation to be placed on the solution spectrum: The 3000-Å system of medium intensity and the stronger 2600-Å system are long-axis polarized, and a rather weak band at about 2730 Å is short-axis polarized. The onset of a stronger short-axis polarized transition above 2400 Å was observed. This analysis shows that fluorene has quite different electronic properties from phenanthrene and carbazole. The low-temperature absorption and fluorescence was entirely consistent with these electronic assignments. Vibrational analyses of the absorption and fluorescence spectra of fluorene in a polycrystalline n-heptane matrix at about 15°K are presented; intervals that formed overtones were tentatively assigned as totally symmetric fundamentals. Crystalline fluorene did not measurably phosphoresce when pure, but intense, blue phosphorescence of fluorene could be induced at about 6°K by the deliberate addition of dibenzthiophene as an impurity. A vibrational analysis of this phosphorescence revealed intervals identical with those in the fluorescence and phosphorescence of fluorene in n-heptane.
The infrared, Raman, and fluorescence spectra of carbazole are reported with as complete polarization information as possible. An experimental assignment of fundamentals is given and is reliable only for A1 species, being incomplete or uncertain for the other symmetry species. Some lattice modes of the carbazole crystal were found both in the infrared and Raman spectra, and a partial assignment made. A crude normal-coordinate calculation was made using a force field transferred from phenanthrene for the in-plane problem and from benzene and anthracene for the out-of-plane problem. The observed and calculated frequencies are compared, and good agreement is found for the A1 fundamentals.
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