Local excited state

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)... 

From the excited-state conversion rates (ka and kd), it is possible to calculate the ratio of the quantum yields between emission from the twisted and LE states. The ratio of the steady-state quantum yields from the locally excited state (f>LE and the TICT state r/>TI, T for DMABN and other aminobenzonitrile derivatives is described in (1) [12],... 

In acetonitrile-dichloromethane 1 1 v/v solution, their absorption spectra are dominated by naphthalene absorption bands and they exhibit three types of emission bands, assigned to naphthyl localized excited states (/Wx = 337 nm), naphthyl excimers (Amax ca. 390 nm), and naphthyl-amine exciplexes (/lmax = 480 nm) (solid lines in Fig. 3). The tetraamine cyclam core undergoes only two protonation reactions, which not only prevent exciplex formation for electronic reasons but also cause strong nuclear rearrangements in the cyclam structure which affect excimer formation between the peripheral naphthyl units of the dendrimers. 

Strongly coupled systems are characterized by large differences between their absorption spectra and those of their components. For a two-component system, two new absorption bands are observed due to transitions of the in-phase and out-of-phase combinations of the locally excited states. These two transitions are separated in energy by 2 ll. ... 

The relationship between the phases of the locally excited states and Fd Fa is... 

V,/V-Dimethylamino)benzonitrile (DMABN) and its derivatives, as a class of organic donor-acceptor compounds, exhibit dual fluorescence, one related to the local excited state ( B band) and the other ascribed to the twisted intramolecular charge transfer (TICT) state ( A band).17 As expected, compound 818 exhibits dual fluorescence, showing two fluorescence bands centered at 350 and 432 nm, which can be ascribed to the corresponding band (from the local excited state) and A band (from the TICT state), respectively. After oxidation of TTF unit in 8, the fluorescence intensity of A band decreases while that of band increases slightly. As expected, further reduction of TTF" + into neutral TTF unit leads to the restoration of the fluorescence spectrum of 8. Therefore, the dual fluorescence spectrum of 8 can be reversibly modulated by redox reactions of TTF unit in 8. 

The compound bis-(4,4 -dimethylaminophenyl)-sulfone (DMAPS) and related compounds show multiple fluorescences in polar solvents due to excited state charge transfer (Rettig and Chandross [144]). Su and Simon [84,85] have examined the intramolecular electron transfer reaction in DMAPS, in alcohol solution over the temperature range from — 50°C to + 30°C. They observe that the decay of the local excited state is nonexponential and significantly faster than the longitudinal relaxation time of the solvent. In addition, they observed that the emission spectrum of the TICT state... 

The excited state properties of bis-a-9-anthrylmethyl ethers 23 closely resemble those of l,3-di-9-anthrylpropane derivatives. The photoexcited parent compound 23a deactivates by fluorescence (0 = 0.03) from the locally excited state only, and it isomerizes by intramolecular 4ji+4ji cycloaddition with a quantum yield of 0.32 [66]. By contrast, excimer emission (see Table 4) does characterize the excited state properties of the 10,10 -diphenyl derivative 23b, which does not undergo intramolecular cycloaddition for steric reasons [66,67]. 

TABLE 19 The Effect of Solvents and Substituents on the Quantum Yields of Emission from the Locally Excited State of co-9-Anthrylpropiophenones 97 109]... 

In polar solvents, the quantum yields for the emission from the locally excited state of anthronyl-anthracenes 98 and 99 decrease drastically (see Tables 20 and 21), and a structureless, red-shifted exciplex emission is observed (see Figure 23). For the parent compound 98a in dichloromethane, for example, the quantum yield of emission from the exciplex state is 0.012, but that of emission from the locally excited state has decreased to 0.00058 (cf. Tables 20 and 22). Thus, intramolecular exciplex formation between the photoexcited anthracene moiety and the aromatic ketone in its electronic ground state represents the major mode of deactivation in polar solvents. 

Photoexcitation of lepidopterene 118 (L Y = H in cyclohexane solution results in cycloreversion and gives the electronically excited product E whose deactivation to ground state is characterized by the structureless emission around 600 nm (see Figure 32). The quantum yield of the E emission is 0.58 (0.80), while that of the emission from the locally excited state L is only 0.005 (0.016). (The lower quantum yield data have been reported by... 

Photoexcitation of lepidopterene in solution also gives rise to a structured emission of low intensity around 400 nm. This emission is attributable to the deactivation of the locally excited state of the E rotamer A, formed mainly by inadvertent direct excitation of the ground state cycloreversion product 114 [131]. The absorption and emission spectra of 114 are typical of the anthracene chromophore (see Figure 33). Selective excitation of 114, experimentally possible because of the suitable ground state [L]/[A] equilibrium ratio, gives rise to locally excited A, which in cyclohexane solution at room temperature has a fluorescence quantum yield of 0.84 [131]. The adiabatic conversion of A into E is difficult to detect because it proceeds at 298 K... 

A photolytic study with trifluoromethyl-substituted phenyldisilanes in the presence of alcohols revealed the nature of the excited states responsible for the photoreactions the 1,3-silyl migration occurs from an aromatic mt (locally excited) state, while the direct alcoholysis of the aryldisilane takes place from the an orthogonal intramolecular charge transfer state138. 

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