Locally excited state deactivation

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]. 

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... 

The first experiments in this direction were carried out on the triads 244+ and 254+ (Fig. 8), by exciting preferentially the metal centre around 340-355 nm. Excitation at this wavelength region produces to a predominant extent the excited state localized on the iridium complex unit, the ligand centered triplets PH2 - 3Ir - PAu or PZn - 3Ir - PAu [48]. Energy transfer to the porphyrin triplets dominates the deactivation of PH2 - 3Ir - PAu in 244+, with rate constants of 2.9 x 1010 s 1 for the transfer to the gold porphyrin localized excited state and ca. 1011 s 1 to the free base porphyrin localized excited state, respectively (Scheme 9). 

The photoluminescence of dipyridophenazine complexes of ruthenium ) in the presence and absence of DNA has been well-characterized (38-40, 46-52). Excitation of the dppz complexes with visible light (440 nm) leads to localized charge transfer from the metal center (39, 40). In aqueous solution, the emission resulting from the metal-to-ligand charge-transfer excited state is deactivated via nonradiative energy transfer... 

The CT excited states usually are not emissive because the low energy gap and the strong distortion with respect to the ground state favor the occurrence of radiationless deactivation. Furthermore, the presence of the low-energy CT excited states causes rapid radiationless decay of the upper lying, potentially luminescent excited states localized on the molecular components. Therefore, rotaxanes and catenanes based on CT interactions usually do not exhibit any luminescence. For example, the intense emission band with maximum at 320 nm (t = 2.5 ns) exhibited by macrocycle 7 [1 la] is no longer present in catenane [23]. 

Intercomponent CT interactions introduce low energy excited states which are responsible for the presence of broad and weak absorption bands in the visible region (and therefore, for the color) exhibited by pseudorotaxanes, rotaxanes, and catenanes based on this kind of interactions. Furthermore, the low energy (non-emissive) CT excited states offer favorable deactivation paths to the potentially luminescent excited states localized on the molecular components, so that this kind of pseudorotaxanes, rotaxanes, and catenanes usually does not exhibit luminescence [11, 18]. 

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