Photodenitrogenation of the diazenes 4 affords exclusively the housanes 5 through intramolecular cyclization of the spectrally detected and characterized singlet diradicals 3. The lifetime of singlet diradical 3, determined by transient absorption measurements, depends on the Y and Z substituents at the para position of the phenyl ring and has the following order: Y, Z = OMe, OMe > OMe, CN > CN, CN > OMe, H > Cl, Cl approximately CN, H approximately Me, Me > H, H. This unprecedented substituent effect reveals stabilization of the singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals 3 through radical, zwitterionic, pi-bonding, and hyperconjugative structures.

Effects of base pairing on the one-electron oxidation rate of guanine derivatives, guanine, 8-bromoguanine, and 8-oxo-7,8-dihydroguanine have been studied. The one-electron oxidation rate of guanine derivatives was determined by triplet-quenching experiments, using N,N'-dibutylnaphthaldiimide (NDI) in the triplet excited state (3NDI*) and fullerene (C(60)) in the triplet excited state ((3)C(60*)) as oxidants. In all three guanine derivatives studied here, acceleration of the one-electron oxidation was observed upon hydrogen bonding with cytosine, which demonstrates lowering of the oxidation potential of guanine derivatives by base pairing with cytosine. When a methyl or bromo group was introduced to the C5 position of cytosine, acceleration or suppression of the one-electron oxidation relative to the guanine:cytosine base pair was observed, respectively. The results demonstrate that the one-electron oxidation rate of guanine in DNA can be regulated by introducing a substituent on base pairing cytosine.

Rapid cleavage of the naphthylmethyl-oxygen bond of 1- and 2-[(4-benzoylphenoxy)methyl]naphthalenes in higher triplet excited states occurred within a laser flash of 5 ns to give 1- and 2-naphthylmethyl radicals with formation quantum yields of 0.042 +/- 0.004 and 0.020 +/- 0.002, respectively, during two-colour two-laser flash photolysis.

The absorption spectrum of benzophenone ketyl radicals in the D 1 excited state (BPH•(D 1 )), generated by hydrogen abstraction of triplet benzophenone from cyclohexane as a solvent and the sequential excitation of the benzophenone ketyl radical in the ground state (BPH•(D 0 )), was directly observed using nanosecondpicosecond two-color two-laser flash photolysis. The whole spectral shape of BPH•(D 1 ) with peaks at 350 and 480 nm was detected for the first time. The absorption spectra and lifetimes were obtained for a series of ketyl radicals (BPDH•) in the D 1 excited state (BPDH•(D 1 )) of benzophenone derivatives (BPDs) such as 4-methyl-, 4,4′-dimethyl-, 4-methoxy-, 4,4′-dimethoxy-, 4-trifluoromethyl-, 4-fluoro-, 4,4′-difluoro-, 4-chloro-, and 4-bromobenzophenones. The absorption spectra of BPDH•(D 1 ) were found to be significantly affected by the number and electronic character of the substituents on the phenyl ring. BPDH• which has a large conformational change between the ground and emitting states showed a shorter D 1 -state lifetime. The decay processes of the D 1 state including the radiative and nonradiative relaxation processes and the chemical reaction are discussed quantitatively.

Transient phenomena of benzophenone (BP) in the higher triplet excited state (Tn) have been investigated by the two-colour two-laser excitation method. Triplet energy transfer from BP(Tn) to quenchers (Q) occurred within the duration of a laser pulse (5 ns) to give Q(T1) with higher triplet energy than that of BP(T1). The quantum yield of the triplet energy-transfer quenching of BP(Tn) by CCl4 was found to be 0.0023 +/- 0.0002 from the bleaching of the transient absorption of BP(T1) and the absorbed photon number. It appears that internal conversion from BP(Tn) to BP(T1) is the predominant process. The lifetimes (tauTn) of BP(Tn) and several substituted benzophenones (BPs) in the higher triplet excited state [BPs(Tn)] were estimated from the dependence of the Q concentration on the efficiency of the triplet energy-transfer quenching of BP(Tn) by Q, and found to be 110-450 ps, depending on the nature of the substituents on the BPs. The effect of the substituents on tauTn may be explained by the energy gap between the Tn and T1 states, because the main deactivation pathway for BPs(Tn) is the internal conversion process. In contrast, the substituent effect on the lifetimes of BPs(T1) cannot be explained by the energy gap law. The transient behaviour of Q(T1) depends on the properties of the quencher. Sequential triplet energy transfer from Q(T1) to BP occurred for p-dichlorobenzene and tert-butylbenzene as quenchers, while Q(T1) reacted partly with Q to form triplet excimers (3Q2*) for benzene, chlorobenzene, and o-dichlorobenzene as quenchers. When CCl4 was used as the quencher, the homolytic cleavage of a C-Cl bond of CCl4(T1) occurred to give Cl* and Cl3C* radicals.