HOMO—LUMO transition

The absorption in the visible region is assigned to the HOMO-LUMO transition of adsorbed iodine molecules. This band is shifted from the value of 540 nm, typical of iodine in gas-phase, to 490 nm, because of the interaction with the pore walls. 

HOMO-LUMO) transition. The higher energy absorption observed in Me PS Nj, Ph,re,N, and S NjOj" (but objured in the other derivatives) is assigned to a... 

The sharpness of the UV absorption and emission bands corresponding to HOMO<-> LUMO transition is also a measure of delocalization. The transition energy depends on the location of a polymer skeleton where a photon is absorbed or emitted, if electron orbitals are localized on parts of the skeleton. A sharp UV band of poly(n-decyl-(S)-2-methylbutylsi-lane) indicates the delocalization of HOMO and LUMO. Table 2 summarizes the location... 

Figure 3.20 Charge transfer in the HOMO-LUMO transitions of (a) nitrobenzene and (b) aniline...

Cyclopenta[2, l -k 3,4-l/]d ithiophen-4-one 581 was considered as a first approximation to it. Ketone 581 displays its lowest n n transition at max = 472 nm (s = 1250). Upon electropolymerization, this long wavelength absorption shifts to 740 nm in the neutral polymer, a red shift of > 200 nm compared to PT. A strong absorption at 425 nm is also present in poly-581. If we assume that the lower energy absorption in neutral poly-581 is derived primarily from the aromatic HOMO-LUMO transition, and the higher energy transition arises between some deeper level and the LUMO. The Egap is < 1.2 eV. 

It is interesting to note that the HOMO-LUMO transition in the emission spectrum at 2.20 eV is almost dark while an important excitonic peak is evident at about 2.75 eV (see Figure 18), red-shifted with respect to the first absorption peak. 

FIGURE 6. The HOMO-LUMO transition for the model La[Ag2Au(CN)2]3 3H2Q. 

Since the C6o molecule is a stronger acceptor in the excited than in the ground state, photoexcitation of C6o (Fig. 2, IF) can also result in electron transfer from the donor to the excited C6o molecule (Fig. 2, II2). Due to that the direct HOMO-LUMO transitions are symmetry forbidden in the C6o, photoexcitation is realized mainly at energies higher than 2 eV. 

In summary, the electronic structure of all triads confirms the donor-bridge-acceptor character of the exTTF-oPPE -C60 systems and suggests that the HOMO - LUMO transition would represent a nearly complete charge-transfer excitation with a very low extinction coefficient. Since these findings are based on... 

Orbital energies (ev) HOMO LUMO Transition energiesb (ev) Singlet Triplet Oscillator strengths ... 

From the data reported in Fig. 2.2 and from spectroscopic ellipsometry measurements, the anisotropic complex optical constants of oriented PPV have been determined [32,69]. Several different data analyses, described previously, were carried out on the 7Z and T spectra in order to extract n, both below and above the HOMO-LUMO transition (transparent free-standing film and bulk material, respectively). In order to evaluate n below 1.6 eV, where the sum of 1Z and T is equal to 1 within experimental error, a numerical inversion of the 7Z and T spectra was performed by assuming k = 0... 

Figure 24 displays the high energy (E > 25,000 cm-1) region of the room temperature electronic absorption spectrum for Zn(bpy)(tdt), where bpy = 2,2 -bipyridine. The LLCT transition occurs at 22,470 cm-1 (445 nm) with very weak absorption intensity (e = 72 M 1cm 1). The origin of the weak LLCT is a function of the symmetry of this psuedo-tetrahedral complex. A MO diagram for Zn(bpy)(tdt), derived from extended Hiickel calculations, is presented in Fig. 25. Irrespective of whether the metallo(diimine)(dithiolene) complex is square-planar or psuedo-tetrahedral, the point symmetry is C2V, and all intermediate geometries possess C2 symmetry. When the dithiolene and diimine planes are orthogonal (psuedo-tetrahedral geometry) the HOMO — LUMO transition represents a b2 —> b one-electron promotion and is electric dipole forbidden. However, the HOMO —> LUMO transition in a square-planar...

Molecular complexes frequently exhibit two intermolecular charge-transfer transitions.256 The energy difference between the two such transitions in 1-alkylpyridinium iodides has been correlated with the HOMO-LUMO transition in the corresponding pyridinyl radicals, supporting the contention that the charge-transfer transitions have their origin in dual electron-acceptor levels.257... 

---