Electronic transition MLCT

The near IR spectra of the tetrakis(cumylphenoxy)phthalocyanines have not been reported before. The absorption in the Cu complex and one of the absorptions in the Co complex lie close to bands which have been tentatively assigned to trip-multiplet transitions in other phthalocyanines.(14) However, the other absorption bands shown in Table 1 have not been previously reported for phthalocyanines with no peripheral substitution. The small absorption cross sections of these bands in the cumylphenoxy phthalocyanines suggest that they are forbidden transitions. Possible assignments for these bands include a symmetry forbidden electronic transition (like the MLCT transitions in NiPc discussed above) becoming vibronically allowed, d-d transitions on the metal ion, or trip-multiplet transitions. Spectroscopic studies are in progress to provide a more definitive assignment of these absorptions. 

Figure 11.4 Energy level diagram for an octahedral transition metal complex showing the various kinds of electronic transition. MC = metal-centred, LC = ligand-centred, MLCT = metal-to-ligand charge transfer, LMCT = ligand-to metal-charge transfer.

Recent work also has focused on theoretical investigations of the electronic structure and electronic transitions of Os(II) imine complexes. Baranovskii and Lyubimova have employed ab initio methods to obtain vibrational frequencies and distortions following excitation to determine rate constants for nonradiative transitions of complexes having MLCT excited states [29]. 

Fig. 3 a d orbitals in octahedral field b orbital description of MC, MLCT, and LC transitions S is a substituent group capable of exerting electron withdrawing or releasing effects (resulting in stabilization or destabilization, respectively, of the energy level of the filled d and n orbitals) c electronic transitions involving MC, MLCT, and LC excited states the MC levels are not emissive... 

Redox Photochemistry of MLCT The existence of intense absorption bands in the spectra of MZL complexes where the redox potentials of the M1 /M1 couples, for example, Cu(I), Fe(II), and L/L , are not too negative was associated by spectroscopists with MLCT electronic transitions.125 128 The MLCT electronic transition causes a radial displacement of charge from the metal to the ligand. There... 

Fig. 1. Schematic orbital energy diagram representing various types of electronic transitions in octahedral complexes. A line connects an atomic orbital to that molecular orbital in which it has the greatest participation. 1 metal centered (MC) transitions 2 ligand centered (LC) transitions 3a ligand-to-metal charge transfer (LMCT) transitions 3b metal-to-ligand charge transfer (MLCT) transitions...

A relatively frequent mode is mixing of MLCT and LLCT characters in low-lying electronic transitions and excited states recently it was reported in the case of/ c-[Re(NCS)(CO)3(bpy)] [48,49,82],... 

CT and II.(NN) states mix very little (if at all), because of a large energy gap (However, this situation is profoundly different in the triplet manifold, see Sect. 4). There are not many Re complexes in which the lowest allowed electronic transition is IL(NN). Atypical case is presented by the family of complexes with N,N = pyridylimidazol l,5-a pyridinc derivatives [35, 36], where the IL(N,N) state occurs below a MLCT/XLCT state. [Re(L)(CO)3(R-dppz)] 1 complexes are another example [28, 29],... 

Electronic transitions and excited states of metal complexes are traditionally described in terms of text-book categories such as MLCT, LLCT (XLCT), IL, MC (=LF or dd), LMCT, etc. It was mentioned several times above that MLCT, LLCT, and IL characters in the case of [Re(L)(CO)3(N,N)] represent only limiting cases. In reality, electronic transitions and excited states have mixed character owing to two factors (1) delocalization of the optical orbitals (i.e., frontier orbitals involved in electronic transitions), and (2) combining several one-electron excitations in an electronic transition. 

Metal to ligand charge transfer (MLCT) Transition An electronic transition of a metal complex that corresponds to excitation populating an electronic state in which considerable electron transfer from the metal to a Hgand has occurred. Compare ligand to metal charge tranter transition. 

A typical electronic spectrum of a M(4-TCPyP) complex is shown in Fig. 16 (39,123,170,176,182,183). In general, the electronic transitions in the porphyrin center exhibit many similarities with those observed in the spectra of the M(4-TRPyP) species, with the Soret band typically in the range of 414- 75 nm, and the Qi and Qo o bands in the range of 557-584 and 611-645 nm, respectively. In the formal Ru(III)Ru(III)Ru(III) oxidation state, the characteristic intracluster band is observed in the 685-707-nm range, while the RusO py MLCT band can be found in the 314—351 -nm range. The spectral data of a series of M-4TCPyP derivatives are listed in Table II. 

Figure 2 Simplified MO schemes showing the effect of jt-interaction between the rhenium atom and the halide axial ligand on the character of the lowest electronic transition of [Re(CO)3(N-N)X]. The two highest-occupied molecular orbitals of the chloro (a) and iodo (b) complexes were predominantly of metal dTr(Re) and iodo pji character, respectively, giving rise to essentially (d 7r(Re) — jr (N-N)) MLCT and (p(I) — 7r (N-N)) LLCT lowest transitions, respectively. (Reprinted fi om Ref. 15, 1998, with permission from Elsevier)...

For the sake of simplicity, electronic transitions in metal complexes are usually classified on the basis of the predominant localization, on the metal or on the ligand(s), of the molecular orbitals involved in the transition (4). This assumption leads to the well-known classification of the electronic excited states of metal complexes into three types, namely, metal-centered (MC), ligand-centered (LC), and charge-transfer (CT). The CT excited states can be further classified as ligand-to-metal charge-transfer (LMCT) and metal-to-ligand charge-transfer (MLCT). 

The electronic transition to the MLCT and nn excited states are optically allowed transitions, and they have relatively large transition moments. They do not involve the population of an orbital that is antibonding with regard to the M—L bonds, in contrast to the forbidden transitions to the LF excited states. This is one of the reasons for photostability of Red) complexes. However, as discussed in the following sections, photoinduced chemical reactions have been reported in some cases, where transitions to reactive higher-energy states arise from photoexcitation with shorter wavelength irradiation or thermal activation from lowest excited state. 

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