Charge-transfer states

Kovalenko S A, Ernsting N P and Ruthmann J 1996 Femtosecond hole-burning spectroscopy of the dye DCM in solution the transition from the locally excited to a charge-transfer state Chem. Phys. Lett. 258 445-54... 

The transfer of PCSs from solutions into the solid state may be accompanied by the origination of hydrogen and salt bonds, by associations in crystalline regions, or by charge transfer states and some other phenomena. These effects are followed by some conformational transformations in the macromolecules. The solution of the problem of the influence of these phenomena on the conjugation efficiency and on the complex of properties of the polymer is of fundamental importance. 

It should be noted that the properties of a CTC depend to a considerable degree on the conditions of their preparation. Temperature increase, in particular, favors the accumulation of complete charge transfer states in a CTC. In the case of a CTC obtained in solution, the increase of dielectric constant of the solvent has the same effect. The method of preparation of a CTC also affects the kinetic curves of the accumulation and depletion of complete transfer states arising at protoirradiation. 

The products of the thermal transformation of PAN were used for the examination of the effect of stereoregularity of the initial polymer on the CTC properties. At equal transformation degrees, the concentration of complete charge transfer states in a bromine-complexed PCS, obtained on the basis of PAN-c (t e., a polymer with elevated content of isotactic sequences), exceeds by two orders of magnitude this parameter in the polymer obtained on the basis of PAN-r. 

The second example in Fig. 4-4 shows how a (spin-allowed or spin-forbidden) band lying close to a charge transfer band may acquire unusually high intensity. We shall discuss charge-transfer bands more in Chapter 6. For the moment, we note that they involve transitions between metal d orbitals and ligands, are often fully allowed and hence intense. On occasion, the symmetry of a charge transfer state... 

The ML species may interact with a species in its second coordination sphere. Therefore one distinguishes inner-sphere charge-transfer and outer-sphere charge-transfer states. 

Fig. 23. Schematic representation of quenehing by eleetron transfer. Curves a, b and c are levels of the pair Level d is a charge-transfer state...

M.L) it may be a MMCT state (see text). The arrow indicates the nonradiative transition from the charge-transfer state d to the ground state, so that c b and c - a emission is quenched... 

Several base pairs of adenine-thymine, including the WC pair, have also been studied [238], It is found that a charge transfer state exists about 1.5 eV higher than the local nn states. Proton transfer between the bases stabilizes a charge transfer state which then crosses with the ground state facilitating radiationless relaxation. This mechanism is not energetically favorable for non WC pairs. 

Bases stacked rather than hydrogen bonded have also been studied with quantum chemical methods [182, 244-247]. The nature of excited states in these systems has been debated and theoretical calculations are called to decide on the degree of excited state localization or delocalization, as well as the presence and energy of charge transfer states. The experimentally observed hypochromism of DNA compared to its individual bases has been known for decades [248], Accurate quantum chemical calculations are limited in these systems because of their increased size. Many of the reported studies have used TDDFT to calculate excited states of bases stacked with other bases [182, 244, 246, 247], However, one has to be cautious when us-... 

Rettig, W. Photoinduced Charge Separation via Twisted Intramolecular Charge Transfer States. 169, 253-300 (1994). 

Kopczynski, M., F. Ehlers, T. Lenzer, and K. Oum. 2007. Evidence for an intramolecular charge transfer state in 12 -apo-beta-caroten- l 2 -al and 8 -apo-beta-caroten-8 -al Influence of solvent polarity and temperature. J. Phys. Chem. A 111 5370-5381. 

Grabowski ZR, Rotkiewicz K, Rettig W (2003) Structural changes accompanying intramolecular electron transfer Focus on twisted intramolecular charge-transfer states and structures. Chem Rev 103 3899 4031... 

When the electron coupling between locally excited-state (LE) and charge transfer state (CT) is weak, the electron transfer rate kcl can be expressed as (7)... 

Seo J, Kim S, Park SY (2004) Strong solvatochromic fluorescence from the intramolecular charge-transfer state created by excited-state intramolecular proton transfer. J Am Chem Soc 126 11154-11155... 

Anthracene has also been used as an acceptor (Fig. 10). In solution, 26 emits a single fluorescence band that is somewhat structured in nonpolar solvents and becomes broad and structureless with increasing polarity [58]. The strongly hindered molecule 27 also exhibits a similar behavior, but its absorption spectrum is better structured [59]. The rate of formation of a charge transfer state is higher for 27 than for 26. Based on this observation, it appears that the twist around the anthryl-phenyl C-C bond plays a significant role in the fluorescence profile of the probes [60]. Acridines, such as 28, behave similarly to anthracene except that acridine is a better electron acceptor [61]. 

Herbich J, Kapturkiewicz A (1991) Radiative and radiationless depopulation of the excited intramolecular charge transfer states aryl derivatives of aromatic amines. Chem Phys 158 143-153... 

Hara K, Bulgarevich DS, Kajimoto O (1996) Pressure tuning of solvent viscosity for the formation of twisted intramolecular charge-transfer state in 4, 4 -diaminophenyl sulfone in alcohol solution. J Chem Phys 104(23) 9431-9436... 

Table I lists the excited state lifetimes for 0s(phen)3 + at three temperatures. The striking result is the lack of any significant change in the LMCT statefs lifetime on going from 295 to 10 K. Table I also presents data on the effects of deuteration on the charge transfer state lifetimes of Os(bpy) 3 /3+ complexes. The lifetimes are lengthened, but only by factors of 2 and 2.5 respectively, for Os(bpy)33+ and 0s(bpy)3 +.

While the lifetimes of the Osl 2 and Osl " charge transfer states differ by a factor of 105-106, the energy gaps between their ground and charge transfer states are similar,... 

Similarly the small increase in charge transfer state lifetime upon deuteration implies that high frequency C-H stretching modes are not critical to the radiationless decay process in OsL 2 /3 complexes. The above considerations imply that mid-frequency... 

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