Electronically delocalized chromophores

The appearance of the square root in Eq. [147] is an indication of one important feature of delocalized CT systems the existence of a lower limiting frequency of the incident light that can be absorbed by a donor-acceptor complex. This effect results in asymmetries of CT absorption and emission lines as discussed below. 

A real root of Eq. [146] exists only if the following condition holds  

The dash-dotted lines indicate the lower boundary for the energy of the incident light vm n (Eq. [149]). 

Equation [48] gives the Franck-Condon factor that defines the probability of finding a system configuration with a given magnitude of the energy gap between the upper and lower CT free energy surfaces. It can be directly used to define the solvent band shape function of a CT optical transition in Eq. [134] 

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Figure 4.35 shows the CD and UV spectra for 48 in isooctane. The observation of the Cotton effect indicates that the chromophore in the polymer, which is a main-chain segment over which the silicon a and o orbitals are electronically delocalized, exists in a chiral structure a helix. 

The reducing character of the ligand is linearly related to the relative position of the first spin-allowed transition and the first electron transfer transition. Jorgensen 233) calculated the optical electronegativity of ethyl-dsep as 2.6 which is close to that of ethyl-dtp, 2.7. Jorgensen 233) has also discussed electron delocalization in M(X2P)3 (X = S, Se) chromophores in terms of a molecular orbital model. 

Magnetic measurements have mainly been used to clarify the structures of the Ni and Co complexes. The isolated Ni complexes are diamagnetic, as would be expected for pseudo-square planar coordinated Ni11. The magnetic moments of the Co complex are essentially closer to spin-only values than to values of typical compounds with a tetrahedral CoS4 chromophore, because of the strong electron delocalization M -> M04 S2. This behavior is even more clearly expressed in [Fe(MoS4)2]3-. 

Coordination of metal ions often has a dramatic effect on the n delocalization in porphyrins and porphyrinoids. It has particularly conspicuous influence on the electronic spectra of metalloporphyrins, which show a dependence on the identity of the metal ion, axial ligation, oxidation level, and spin state. In regular porphyrins, metal coordination reduces the number of observed Q bands from four to two, reflecting the higher symmetry of the chromophore relative to the free base. However, detailed quantitative information on the Jt-electron delocalization is more easily accessible from other physical methods. 

It is interesting to observe the influence of the electronic systems of the substituents on the Si-Si bond. The Si-Si bond is a chromophor. With increasing chain length the UV maxima shift to longer wavelengths. In polymeric forms, the absorption maxima reach the visible region and the compounds become yellow (see Chapter 10 Polymeric Compounds). An electron delocalization seems to exist between the Si-Si bonds. 

The challenges outlined above still await a solution. In this section, we show how some of the theoretical limitations employed in traditional formulations of the band shape analysis can be lifted. We discuss two extensions of the present-day band shape analysis. First, the two-state model of CT transitions is applied to build the Franck-Condon optical envelopes. Second, the restriction of only two electronic states is lifted within the band shape analysis of polarizable chromophores that takes higher lying excited states into account through the solute dipolar polarizability. Finally, we show how a hybrid model incorporating the electronic delocalization and chromophore s polarizability effects can be successfully applied to the calculation of steady-state optical band shapes of the optical dye coumarin 153 (C153). We first start with a general theory and outline the connection between optical intensities and the ET matrix element and transition dipole. 

Our collaborators, Kim, Lee and coworkers have also synthesized a series of novel two-photon chromophores by utilizing dithienothiophene (DTT) as the n-center [15]. The experimental result has shown that the DTT moiety leads to an enhancement of molecular two-photon absorptivity. We hypothesized that the dramatic improvement of 03 values is due to this rigid, planar and polarizable fused-terthiophene structure, which provides a significant reduction of the band gap and extension of Tt-electron delocalization. The chemical structures and the measured 02 values of these chromophores are depicted in Fig. 5. 

The bathochromic shifted, intense UV maxima of the partially saturated heterocyclic /J-enamino esters (lmax 270-297.5 nm log e = 4.07-4.28) confirm a considerable -electron delocalization (p-Tt-overlap) in the enaminocar-bonyl chromophore,57 which can be depicted by dipolar mesomeric canonical formulae 66a-66e possessing a high transition moment58 (Scheme 14). A strong participation of the ring heteroatom can be discerned. 

Upon folding, the pyrochlorophyll a and the chlorophyll a dimer undergo a red shift in their absorption spectrum to 696 nm (Fig. 12). The cation formed by chemical oxidation appears to have its unshared electron delocalized over the two molecules. The oxidation potential is reduced by about 100 mV from that of monomeric pyrochlorophyll a. Judging from the zero field splitting parameters, the triplet state of the dimer also appears to be delocalized over the two chromophores. Such a triplet delocalization is found in reaction centers from photosynthetic bacteria. Thus both the synthetic pyrochlorophyll a and chlorophyll a dimers in... 

The later discovery of the iron(II) complexes of aliphatic a-diimi-nes (IV),e. g. glyoxalbismethylimine (GMI) and biacetylbismethylimine (BMI) (/7) and of 2-pyridinMimines (V) 12—16) with spectral characteristics very similar to those of the bipy and phen complexes, corroborated Sone s chromophore concept. Electron delocalization within... 

Dyes and colorants are complex organic molecules with conjugated or delocalized iT-electron systems (chromophores). Based on the different relevant chemical functionalities that act as chromophores in specific dyes, the latter can be grouped into azo dyes, carbonyl dyes, methine dyes, and phthalocyanines. The dye indigo shown in Table 5.3.19 belongs to the group of carbonyl dyes. 

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