Naphthalene HOMO—►LUMO transition

The ionization potential and electron affinity of naphthalene were determined experimentally as IP = 8.2 eV and EA 0.0 eV. According to Koopmans theorem it is possible to equate minus the orbital energies of the occupied or unoccupied MOs with molecular ionization potentials and electron affinities, respectively (IP, = - s, and EA = - ). Thus, in the simple one-electron model, the excitation energy of the HOMO->LUMO transition in naphthalene may be written according to Equation (1.22) as... 

An example is shown in Figure 2.15 where an even perturbation produces the orbitals of azulene and an odd perturbation (/ j) those of naphthalene from the perimeter orbitals of cyclodecapentaene. (Cf. Example 2.5.]) The energies of 0, and 0,. are not affected to the first order by the odd perturbation producing naphthalene, whereas 0s is stabilized and 0,- destabilized. Therefore, 0, becomes the HOMO and 0. the LUMO, and the HOMO-LUMO splitting AEhomo-lumo he same as for cyclodecapentaene. The HOMO- LUMO transition is referred to as L according to Platt. In azulene, on the other hand, 0, is destabilized and becomes the HOMO, whereas 0s- is stabilized and becomes the LUMO. The HOMO-LUMO splitting is markedly smaller than for cyclodecapentaene, and the HOMO-LUMO transition is of the Lb type. 

For alternant hydrocarbons cj, = cj,. for all p, and there is no first-order energy change for the HOMO LUMO transition, as is apparent from Figure 2.26. This result is remarkably well confirmed by the absorption spectra of naphthalene, quinoline, and isoquinoline shown in Figure 2.27. 

The HOMO of 6,13-TCPQ is located on the naphthalene moieties and does not correlate with the HOMO of TCNQ. It actually corresponds to the HOMO of the naphthalene molecule which has an energy (—8.49 eV) higher than that of the folded TCNQ moiety (—8.79 eV). This means that the HOMO -> LUMO electronic transition implies an electronic transfer from the naphthalene units, acting as a donor, to the acceptor TCNQ moiety. For largely 7t-extended TCNQs, the HOMO belongs to the lateral... 

The HOMO —> LUMO electronic transition observed in the electronic spectra corresponds to a photoinduced electron transfer from the donor naphthalene moiety to the acceptor TCNQ ring. Since donor and acceptor moieties linked by a Ti-system can provide a strong hyperpolarizability in such molecules, these compounds could be used, in principle, to devise nonlinear optical materials. 

Figure 4.89 (a) Orbital energy diagram of a closed-shell molecule (C) and its radical anion, illustrating the much smaller first transition in the latter (R). (b) Absorption spectra of naphthalene and of its radical cation H = HOMO, L = LUMO, S = SO MO (singly occupied molecular orbital)... 

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