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The photochemical behavior of 2-nitrobiphenyl (NBP) derivatives was extensively studied by the electron paramagnetic resonance (EPR) technique. The molecules studied mainly are methyl-, fluoro-, carboxy-, ethoxycarbonyl-, hydroxymethyl- and methoxymethyl-NBPs and ethers possessing two NBP groups. Most of the triplet species produced were stable in ethanol glasses at 77 K after UV irradiation. The zero-field splittings (ZFSs) of these species are fairly close to those of the stable triplet species produced from NBP. Two sets of EPR signals were observed for the NBP derivatives with a relatively large substituent at the 2′-position of the biphenyl skeleton. The ZFS |D| parameters of the stable triplet species giving strong EPR signals are a little larger than those giving weak signals. On the other hand, the ZFS |E| parameters of the stable triplet species giving strong EPR signals are slightly smaller than those giving weak signals. For 3′-substituted NBPs, the EPR signals of methoxycarbonyl- and carboxy-NBPs were observed only during UV irradiation. These signals are attributed to the lowest excited triplet states of the precursors. For 4′-substituted NBPs studied except the methyl- and fluoro-NBPs, no stable triplet signals could be observed at 77 K in ethanol glasses after UV irradiation. From the precursors possessing two NBP groups, no signals of the quintet state could be observed, although the stable triplet signals were detected. The present study has no contradictions to the previous estimation that the stable triplet species is formed through the abstraction of a hydrogen atom from the 2′-position of the biphenyl skeleton by one of the oxygen atoms of the nitro group. Such a hydrogen-transfer process is inferred to occur for the nonplanar derivatives where the above hydrogen atom can approach one of the oxygen atoms rather easily. |
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