Despite the number and variety of their biological applications, the mechanisms of action of the photoactive naphthalenic imides have not yet been fully elucidated. In order to provide mechanistic insight, the photochemistry of several N-substituted 1,8-naphthalimides (NI) and 1,4,5,8-naphthaldiimides (NDI) has been studied using absorption and fluorescence spectroscopy and by laser flash photolysis (λexc = 355 nm). The lowest singlet state (S1) is mainly ππ* in nature for NI whereas nπ* character predominates for the NDI. This difference exerts a profound effect on subsequent reaction mechanisms:  upon irradiation, only the NDI molecules can undergo intramolecular γ hydrogen abstraction. In the case of NP-III, a bishydroperoxy NDI derivative, this photoprocess (Φ = 0.03) leads to concomitant formation of an oxygen-centered radical (ε = 21 600 M-1 cm-1 at 465 nm in acetonitrile) and release of the hydroxyl radical (•OH). All the compounds studied produce the triplet state (in acetonitrile, εT ≈ 10 500−11 500 M-1 cm-1 at 470 nm for NI and 485 nm for NDI). The quantum yield of intersystem crossing was determined to be close to unity except where intramolecular γ hydrogen abstraction was possible (Φisc < 0.5). The triplet states were found to efficiently sensitize the formation of singlet oxygen (with S Δ > 0.8 for NI and > 0.5 for NDI). In the absence of quenchers, the triplet states react with the ground-state of starting material via electron-transfer with a high rate constant [k = (4−6) × 109 and 5 × 108 M-1 s-1 for NDI and NI, respectively] to give the radical anion and radical cation of the corresponding naphthalenic derivative. The high reactivity of the triplet states toward electron donors such as DABCO and their low ability for hydrogen abstraction are typical of a ππ* configuration. These mechanistic photochemistry results are discussed with regard to the photobiological effects observed for these compounds and show that the actual reaction leading to biological damage will depend on the microenvironment of the naphthalenic molecule.

The photophysical properties of benzoporphyrin derivative monoacid ring A (BPD-MA), a second-generation photosensitizer currently in phase II clinical trials, were investigated in homogeneous solution. Absorption, fluorescence, triplet-state, singlet oxygen (O2 (1 delta g)) sensitization studies and photobleaching experiments are reported. The ground state of this chlorin-type molecule shows a strong absorbance in the red (lambda approximately 688 nm, epsilon approximately 33,000 M-1 cm-1 in organic solvents). For the singlet excited state the following data were determined in methanol: energy level, Es = 42.1 kcal mol-1, lifetime, tau f = 5.2 ns and fluorescence quantum yield, phi f = 0.05 in air-saturated solution. The triplet state of BPD-MA has a lifetime, tau T > or = 25 microseconds, an energy level, ET = 26.9 kcal mol-1 and the molar absorption coefficient is epsilon T = 26,650 M-1 cm-1 at 720 nm. A dramatic effect of oxygen on the fluorescence (phi f) and intersystem crossing (phi T) quantum yields has been observed. The BPD-MA presents rather high triplet (phi T = 0.68 under N2-saturated conditions) and singlet oxygen (phi delta = 0.78) quantum yields. On the other hand, the presence of oxygen does not significantly modify the photobleaching of this photostable compound, the photodegradation quantum yield (phi Pb) of which was found to be on the order of 5 x 10(-5) in organic solvents.