Fluorescence naphthalenophanes

The naphthalenophanes that have been synthesized to date are listed in Table 6, in order of their discovery. The [m.n] isomers for which m,n > 3 have not yet been synthesized. References for the UV absorbance, fluorescence, and other properties of existing naphthalenophanes are given in Table 6. The UV absorption spectra of all the naphthalenophanes are red-shifted and broadened relative to their respective open-chain analogs, similar to the [2.2] and [3.3] paracyclophanes. Moreover, broad and structureless emissions have been observed for the naphthalenophanes in all references cited in Table 6 except one.107) The structural aspects of naphthalenophane photobehavior will be discussed in detail in the following paragraphs. 

The naphthalenophanes that are fully eclipsed, i.e. the sj>n-[2.2](l,4), achiral [2.2](1,5), achiral [3.3](2,6),. n -[3.3](l,4), and syn-[2>,2] A) isomers, share certain traits in absorption and fluorescence. The UV absorbance spectra of these compounds between 260 and 310 nm retain all of the structure shown in the spectra of the open-chain analogs. Also, new absorption shoulders not seen in the open-chain spectra appear strongly at 245 and weakly at 340 nm. The fluorescence peak of these fully eclipsed naphthalenophanes occurs near 22,000 cm-1, as seen in Table 7. This represents a red shift of 2600 cm-1 relative to the solution excimer of the dimethylnaphtha-lenes.71)... 

New absorption shoulders appear strongly at 250 and 345 nm. Given this evidence of ground-state interaction, the fluorescence band of the noneclipsed naphthalenophanes should be red-shifted below the peak emission of the dimethylnaphthalene solution excimer. In fact, the emission of chiral [2.2](2,6) naphthalenophane is blue-shifted 900 cm 1, and the emissions of the onh -[2.2](l,4), onfi-[3.3](l,4), and chiral [2.2](1,5)... 

The (1,4) substituted naphthalenophanes undergo [4 + 4] photocycloaddition when irradiated at X > 280 nm, in addition to fluorescence. This photoreaction is competitive with fluorescence, and requires a conformational change that can be suppressed at low temperature 93). The few reports of the lifetime or quantum yield of naphthalenophane fluorescence indicate the effects of photocycloaddition. For the anti-[2.2](1,4) isomer, kpu/ku = 0.021 in cyclohexane 93) the lifetime of syn-[3.3](l,4) naphthalenophane fluorescence was given as 15.3 ns107). Both values are low relative to the naphthalene solution excimer (kpu/kjj 0.2 xD 80 ns 71)), and this may be due in part to the photoreaction of the (1,4) naphthalenophanes. 

As noted earlier, the limiting lifetime of pyrene excimer fluorescence from concentrated solutions in PS and PMMA glasses was found to be the same as that of pyrene in cyclohexane solution. There have been no similar studies of naphthyl compounds in rigid glasses. Values of k and Q for the [2,6]-naphthalenophanes have not yet been determined for any solvent system. The bis(2-naphthyl) compounds have not been quantitatively characterized in rigid matrices, probably because excimer fluorescence is weak and difficult to detect under such conditions. Given such limited data, it can only be assumed that the values of QD and kD of 2-naphthyl excimers remain the same in rigid solution as in fluid solution. 

In the emission spectra (31, 34, 35), the orientational effects are differentiated much more clearly (Figure 5). At 1.3 K, monomeric 2,6-dimethylnaphthalene emits a sharply structured fluorescence from the excited singlet state Si however, only structureless red-shifted bands, typical of excimers and dimers, appear in the case of naphthalenophanes 1, 2, 7, and 8. The fluorescence red shift of excimers relative to the monomer, as mentioned... 

Figure 5. Fluorescence (F) and phosphorescence (P) of 2,6-dimethylnaphtha-lene and naphthalenophanes 1, 2, 7, and 8 in octane or methylcyclohexane (MCH c < 10 3 mol/L) at 1.3 K. In the fluorescence of 7, contamination by 8 apparently causes the shoulder at 23,700 cm-I, which is not observed in the emission of crystals of 7 (31).

As shown by the fluorescence red shifts of the syn- and anti-isomers of [2.2]- and [3.2](l,4)naphthalenophane (5, 6 and 11, 12, respectively), the it—it electronic interaction in the excited singlet state is also diminished by a translation so that the overlap of the naphthalenes is reduced from two to only one six-membered ring. The small angle, by which the naphthalene planes in 11 and 12 are inclined to each other because of the different lengths of the two bridges, apparently has only a minor effect on the excited singlet state. 

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