Trace concentrations of polycyclic aromatic hydrocarbons (PAHs) have been successfully detected using surface-enhanced Raman scattering (SERS) spectroscopy. For such purpose, new SERS substrates have been developed, consisting of Ag nanoparticles, either in suspension or immobilized on glass, obtained by a new method and covered by adsorbed self-assembled calix[4]arene molecules. Among the assayed calix[4]arenes, the 25,27-dicarboethoxy-26,28-dihydroxy-p-tert-butylcalix[4]arene host molecule displays analytical selectivity to the PAH guest systems bearing four benzene rings, mainly pyrene. The host-guest interaction mechanism seems to take place through a π-π stacking interaction leading to a charge transfer between the complex and the metallic surface, which may also induce a notable influence on the surface charge of the metallic nanoparticle.

Photoreduction by amines of oxoisoaporphine dyes occurs via a stepwise mechanism of electron-proton-electron transfer that leads to the metastable N-hydrogen oxoisoaporphine anion. During photoreduction that occurs from the triplet manifold of the oxoisoaporphine, a radical ion A(-)(*), a neutral-hydrogenated radical A-NH(*), and the metastable ion A-NH(-) of the oxoisoaporphine are formed. We present time-resolved spectroscopic data and quantum mechanical semiempirical PM3 and ZINDO/S results for the transient species formed during the flash photolysis of oxoisoaporphines in the presence of amines. These calculations reproduce adequately the experimental spectra of the triplet-triplet absorption near 450 nm, and that of neutral hydrogenated radical of the studied oxoisoaporphines centered at 390 nm. A transient absorption observed near 490 nm, for all of the studied systems, was explained by considering the formation of radical ion pair between the radical anion of the oxoisoaporphine, A(-)(*), and the radical cation of the amine, whose ZINDO/S calculated spectra generate the strongest transition near the experimentally observed absorption maximum at 490 nm, supporting the formation of a radical ion pair complex as the first step of the photoreduction.

The photophysical and photochemical behavior of 1-methyl-3-phenylquinoxalin-2-one (MeNQ) and 3-phenylquinoxalin-2-one (HNQ) in the presence of amines is reported. While HNQ fluorescence shows an auxochromic effect and a bathochromic shift with added amines, explained by association of HNQ with amine in the ground state and emission from both excited species HNQ and [HNQ-amine], both MeNQ and HNQ are photoreduced efficiently on irradiation in the presence of amines, leading to the semireduced quinoxalin-2-ones, MeNQH(-) and HNQH(-), respectively, via an electron-proton-electron transfer, with unit quantum yields at high amine concentrations. The semireduced quinoxalin-2-ones XNQH(-) (X = H, Me) revert almost quantitatively to the parent XNQ in a dark thermal reaction with an activation free energy for MeNQH(-) of 17.4 and 25.9 kcal/mol in acetonitrile and benzene, respectively. Kinetic and spectroscopic (UV and NMR) evidence supports the proposed reaction mechanism for the reversible photoreduction.

Photoreduction of 3-phenylquinoxalin-2-ones, XNQ, by triethylamine, TEA, gives a metastable semireduced quinoxalin-2-one via an electron-proton-electron transfer, with unit quantum yields at high amine concentrations. During the photoreduction, an ion-radical pair, XNQ -• /TEA +• , a neutral-radical pair, XNQH • / TEA-H • , and an ion-pair, XNQH -/TEA-H + , are formed. We present time-resolved spectroscopic data and quantum mechanical semiempirical AM1, PM3, and ZINDO/S results for the transient species formed during the flash photolysis of quinoxalin-2-ones in the presence of amines. These calculations show that the neutralradical pair and the ion-pair are similar in energy, and that the calculated spectra of all the transient species should have similar absorption bands near 400 nm in agreement with experimental results. The ZINDO/S calculated spectra of the XNQH -/iminium ion pair fit the experimental spectra and explain the lack of visible or near-IR absorption of the metastable compound. Energy changes between the species involved are of interest with regard to the possible use of quinoxalin-2-one/amine systems for light to chemical energy conversion or as temporal data storage devices.