Bu state

The induced absorption band at 3 eV does not have any corresponding spectral feature in a(co), indicating that it is most probably due to an even parity state. Such a state would not show up in a(co) since the optical transition IAK - mAg is dipole forbidden. We relate the induced absorption bands to transfer of oscillator strength from the allowed 1AS-+1 (absorption band 1) to the forbidden 1 Ak - mAg transition, caused by the symmetry-breaking external electric field. A similar, smaller band is seen in EA at 3.5 eV, which is attributed to the kAg state. The kAg state has a weaker polarizability than the mAg, related to a weaker coupling to the lower 1 Bu state. 

Figure 7.7 Renner-Teller PES for ethylenedione radical anion. Geometrical data for the Au equilibrium structure are provided for various levels of theory using an augmented polarized triple-f basis set (TZP+). Barriers to linearity (AE. kcal rnol ) are from CCSD(T) calculations using, from top to bottom, DZP, DZP+, TZP, and TZP+ basis sets. If the initial guess is for the Bu state instead of the Au state, what will happen ...

Another aspect worth mentioning is that it is usually the case that the 2Ag state has a covalent (neutral) character while the 1BU state has a more ionic character. As a result, in polyenes end-capped by strong donor and acceptor groups, the 1 Bu state becomes very much stabilized and passes below the 2Ag state24 substitution of the... 

The origin of these effects has been debated. One possibility is the Peierls instability [57], which is discussed elsewhere in this book In a one-dimensional system with a half-filled band and electron-photon coupling, the total energy is decreased by relaxing the atomic positions so that the unit cell is doubled and a gap opens in the conduction band at the Brillouin zone boundary. However, this is again within an independent electron approximation, and electron correlations should not be neglected. They certainly are important in polyenes, and the fact that the lowest-lying excited state in polyenes is a totally symmetric (Ag) state instead of an antisymmetric (Bu) state, as expected from independent electron models, is a consequence... 

The aromaticity of benzene is linked, in spin-coupled theory, to the particular mode of coupling of the electron spins, and so it seems reasonable to suppose that the orbital descriptions of Dih cyclobutadiene and of benzene could be fairly similar, but for these to be associated with very different modes of spin coupling. To a first approximation, this indeed turns out to be the case. With benzene-like orbitals ordered a,b,c,d around the ring, the symmetry requirements of an overall Bu state are such that the electron spins associated with each diagonal (ale and bid) must be strictly triplet coupled. These two triplet subsystems combine to a net singlet. A characteristic feature of antiaromatic situations in spin-coupled theory is the presence of such triplet-coupled pairs of electrons. 

The ground state of most of the luminescent molecules and polymers which are used as the emitters in OLEDs and PLEDs is the symmetric singlet 11 Ag state.22 Figure 1.4 shows thebasic processes which may occur following photoexcitation of the molecule or conjugated segment of the polymer. Since the material is assumed to be luminescent, the antisymmetric 11 Bu state must lie below the symmetric 2-photon 21 Ag state. Otherwise, photoexcitation will still populate the 11 Bu state, but that state will quickly decay to the 21 Ag, and the latter will decay nonradiatively to the ground state, with lifetimes as short as 2 ps.23... 

Our laboratory (21) has analyzed the variation in the P chemical shifts and H3 -p coupling constants in terms of fractional populations of the two thermodynamically stable Bi and Bn states. We have determined that the P chemical shift and Jh3 -p coupling constant of a phosphate in a purely Bi conformational state is estimated to be ca. -4.6 ppm and 1.3 Hz respectively. Similarly the P chemical shift and Jh3 -p coupling constants of a phosphate in a purely Bu conformational state should be ca. -3.0 ppm and 10 Hz respectively. The dispersion in the P chemical shifts of oligonucleotides is attributable to different ratios of populations of the Bi and Bu states for each phosphate in the sequence. The phosphate makes rapid jumps between these two states (30). Thus the measured coupling constant (and P chemical shift) provides a measure of the populations of these two conformational states. 

We have collected 12 parameter sets with yo>0 where the error A4 is optimal between 0.7 and 1.3 meV. An additional criterion for the choice of these sets was also that the excitation energies especially for Bu and 3BU are not too far from experiment. For the 2 Ag state also the experimental situation is not clear, and in addition this state is a true more-determinantal one. Thus one cannot expect that HF excitation energies could be correct. The Bu state is most probably not planar (see the above discussion) and it is not clear whether the excitation occurs... 

They find a large photoexcitation signal at around 2.17 eV for a6T thin films measured at 4.2 K which is attributed to the excitation into the Bu state. 

Initially, the calculated results for finite phenylene vinylene, which is A in Fig. 2, are shown. The first virtual transition of the sequence is from ground to Bu state. This transition is mainly composed of that from the highest occupied molecular orbital to the lowest unoccupied orbital, where a Jt-electron which is distributed on C=C double bonds and their alternating bonds, moves into another orbital which is distributed mainly on the other alternating bonds. This type of transition occurs not only for this finite phenylene vinylene but also for all of the molecules we have calculated in Fig 2. The next virtual transition in the sequence is from By to Ag excited state. This mainly comprises three orbital transitions, because the Ag excited states of this energy area are expressed mainly as a linear combination of three configurations according to the result of Cl. The three orbital transitions are HOMO-1 to HOMO, HOMO to LUMO and LUMO to LUMO+1, as shown in Fig. 3, where HOMO-1 represents the otbital level just below HOMO, and LUMO+1 represents the level above LUMO. 

Electroabsorption (Martin et al. 1999), and two-photon absorption and photoinduced absorption (Frolov et al. 2002) indicate a dipole forbidden state at ca. 0.7 eV higher in energy than the l Bu state in PPV derivatives. This state is labelled the m Ag state, and is indicated in Fig. 11.1(b). Electroabsorption in PFO indicates that the m Ag state is 0.8 eV higher in energy than the l Bu state (Cadby et al. 2000). 

Third harmonic generation indicates a Bu state at 3.2 eV in PPV (Mathy et al. 1996). Modelling of the electroabsorption data by Martin et al. (1999) indicates that this state is ca. 0.1 eV higher in energy than the m Ag state. This Bu state is labelled the state. 

Phosphorescence indicates a triplet state at ca. 0.7 eV lower in energy than the l Bu state for a wide variety of systems. (See Kohler and Beljonne (2004) for a review of the data.) This triplet state is the 1 B state. 

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