Biphenyl transition energies

Figure 13. Orbital energy-level scheme for the biphenyl anion radical (A) and biphenyl cation radical (C) based on the SCF calculations (59) by the Pople and Longuet-Higgins method. Pairing of MO s is indicated. Thick lines with arrows represent the five lowest transition energies. All entries are given in eV.

Table 14.5 Transition energy A (in eV), osdUator strength (in parentheses), collectivity number K, excitation distribution La, CT numbers (14.37) for intra-fragment (/i = h->-]) and interfragment (/i- ii) interactions, and CT,ot (in %) in the lowest p-transitions of biphenyl...

Figure 11.9 shows the Pariser-Parr-Pople model predictions of the Al-dependence of the transition energies of some key states. Also shown are experimental results for biphenyl, oligomers, and polymer thin films. To analyze these results it is useful to classify the excited states into a number of types. 

Numerous systematic studies of the ultra-violet absorption characteristics of pyridine derivatives have been made, as e.g. of the halogenopyridines Table 5.6) and the arylpyridines. In the latter series , comparison with biphenyl shows that the hetero-atom lowers the transition energy to the first excited state, and change in the hetero-atom is effective in this respect in the order... 

According to Bartell (1961a), the relative motion of the interacting non-bonded atoms is described by means of a harmonic oscillator when the two atoms are bonded to the same atom, and by means of two superimposed harmonic oscillators when the atoms are linked to each other via more than one intervening atom. It is the second case which is of interest in connection with the biphenyl inversion transition state. The non-bonded interaction will of course introduce anharmonicity, but since a first-order perturbation calculation of the energy only implies an... 

JThese are conformations different from the lowest energy conformation in the absence of the restraints imposed by the crystal lattice, in solution or in the gas phase. The two rings of biphenyl, for example, are coplanar in the crystal, though not in solution. The solid state structure thus formally represents the transition state for rotation about the central C-C bond. 

In biphenyls bridged at the 2 and 1 positions, the inversion center of the ideal planar biphenyl is removed and transitions can be allowed in both IPA and 2PA cases. As a consequence, the IPA and 2PA peaks are observed at approximately the same energy for fluorene (the lowest 2PA peak is located at 586 nm) [52]. Similar observations hold true for carbazole, dibenzofuran, and dibenzothiophene. However, the fine details of the spectra are hard to interpret. In a number of cases, 2PA peaks actually appear at the edge of the timing... 

Fig. 3. Schematic potential energy diagram for the ground- and first-triplet states of biphenyl as a function of the angle of twist between the rings. The vertical lines represent allowed spectroscopic T <-> S0 transitions, while the slanted line represents the nonvertical transition possible during energy transfer.

Oxygen quenching of phenanthrene phosphorescence involves an exchange mechanism which also operates for triplet state energy transfer involving phenanthrene in biphenyl 2i. Radiationless processes of excited states of 5,6-diazaphenanthrene in hexane and hexafluoroisopropanol have been studied by both triplet and singlet lumi ne s cen ce 2 2 extremely fast radiationless transition in the... 

Table 1. Comparison of the atomic energies of biphenyl in the planar transition state geometry (TS, = 0°) vs. the corresponding atomic energies in the equilibrium twisted geometry (EQU,...

A comparison of the TS" and TSIV pathways is illustrated for the 1,4-migration in the biphenyl system (Table 9, entry 2). The two pathways are close in energy and thus are competing (Fig. 2). Both pathways are single transition state, single step in nature and no intermediate is involved. Although the structures of the two transition states look very similar, the imaginary frequencies of the transition states differ... 

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