Charge-transfer excited state

Having these severe approximations in mind, SCC-DFTB performs surprisingly well for many systems of interest, as discussed above. However, it has a lower overall accuracy than DFT or post HF methods. Therefore, applying it to new classes of systems should be only done after careful examination of its performance. This can be done e.g. by conducting reference calculations on smaller model systems with DFT or ab initio methods. A second source of errors is related to some intrinsic problems with the GGA functionals also used in popular DFT methods (SCC-DFTB uses the PBE functional), which are inherited in SCC-DFTB. This concerns the well known GGA problems in describing van der Waals interactions [32], extended conjugate n systems [45,46] or charge transfer excited states [47, 48],... 

Direct probe of ligand field and charge transfer excited states Greater sensitivity than ABS in observing weak transitions and greater resolution due to differences in circular polarization complimentary selection rules aiding in assignment of electronic transitions... 

Table 1. Charge Transfer Excited State Lifetimes. ...

Ti-beta at 77 K exhibits a photoluminescence spectrum at about 465 nm (95). The excitation was at 260 nm. Addition of H20 and C02 quenches the photoluminescence, H20 being more effective than C02 (Fig. 4). The lifetime of the charge transfer excited state was also shortened by such additions, indicating that H20 and C02 interact with the Ti4+ ions in both the ground and excited states. 

Dias FB, King S, Monkman AP, Perepichka n, Kryuchkov MA, Perepichka IF, Bryce MR (2008) Dipolar stabilization of emissive singlet charge transfer excited states in polyfluorene copolymers. J Phys Chem B 112 6557-66... 

Polypyridine rhodium(III) complexes (RM ) may be reduced by one-electron reductants The reductants which have been successfully employed include Ru(bpy)32+, the luminescent charge-transfer excited state of Ru(bpy)32+ (J, 9)... 

J Ru(bpy), the metal-to-ligand charge-transfer excited state... 

W. J. Vining, J. V. Caspar, and T. J. Meyer, Influence of environmental effects on excited state lifetimes. The effect of ion pairing on metal-to-ligand charge transfer excited states, J. Phys. Chem. 89, 1095-1099(1985). 

Such a stabilization is precisely provided by a superexchange mechanism implying the charge-transfer excited state (P B H). When this mechanism prevails, Tlj (Q) is obtained from Eq. (20) or the equivalent expression derived in Sect. 2.2.3 ... 

No reaction at all is observed upon irradiation oil 4, R =4-NH2 in methanol or 74, R=4- (or 5-) -OH in aqueous alkaline solution ). From solvent shifts in the absorption spectra and lack of reactivity a lowest (7r,n )- or charge transfer excited state is implied. 

Such reactions are commonly found as a result of the decomposition of charge transfer excited states. For example, while excitation of the MC bands of Co(NH3)5X (X = Cl, Br, I) leads to photosolvation and the formation of Co(NH3)5(0112) and Co(NH3)4(OH2)X, shorter wavelengths yield the LMCT state which decomposes into Cc II) ions and halogen atoms. The quantum yield for the reaction is found to depend on the excitation energy, indicating a role for the initially formed radical pair (Eq. 5). This may reform the starting complex (Eq. 6) or decompose to the redox products stabilised by the solvent or some other species (Eq. 7). The Co(II) complexes eventually decomposes (Eq. 8). 

The lowest excited states of paramagnetic metal complexes are described by configuration interactions of the porphyrin (7T,tt ) excited singlet and triplet states and the "porphyrin-to-metal" or metal-to-porphyrin charge-transfer excited states (35,36). Thus T (phosphorescence) emission of paramagnetic metal complexes decays... 

Charge transfer excited states internal charge transfer (ICT) states, metal-to-ligand charge transfer (MLCT) and twisted internal charge transfer (TICT) states... 

Charge Transfer Excited States (ICTand TICT)... 

Figure 2. Schematic representation of some relevant ground and excited-state properties of Ru(bpy)j. MLCT and MLCT are the spin-allowed and spin-forbidden metal-to-ligand charge transfer excited states, responsible for the high intensity absorption band with = 450 nm and the luminescence band with = 615 nm, respectively. The other quantities shown are intersystem crossing efficiency energy (E°°) and lifetime (x) of the MLCT state luminescence quantum yield ( ) quantum yield for ligand detachment (O,). The reduction potentials of couples involving the ground and the MLCT excited states are also indicated.

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