The rate constants for electron transfer from guanosine 5‘-monophosphate (GMP), adenosine 5‘-monophosphate (AMP), cytidine 5‘-monophosphate (CMP), and thymidine 5‘-monophosphate (TMP) to the triplet excited states of N-(3-propanol)-1,8-naphthalimide (NI), N,N ‘-(3-propanol)-1,4,5,8-naphthaldiimide (NDI), and N,N ‘-(3-propanol)-3,3‘,4,4‘-benzophenonediimide (BPDI) have been determined in 1:1 H2O/CH3CN solution. Upon 355-nm (8 ns) laser flash excitation of each of the imide or diimides in solution, the triplet states decayed by first-order kinetics under conditions of low excitation energy. Photoinduced electron transfer to the lowest electronically excited triplet state of N-(3-propanol)-1,8-naphthalimide from GMP occurred with a rate constant of 2.0 × 107 M-1 s-1. Electron-transfer quenching by the other nucleotides was almost 2 orders of magnitude slower. In the case of BPDI, photooxidation rate constants ranged from 2.3 × 108 M-1 s-1 for quenching by CMP to 1.1 × 109 M-1 s-1 by GMP. In all cases, the imide radical anion was observed by laser flash photolysis, and the yields were quantified. From these investigations, nucleotide oxidation by the triplet state of a series of redox-active photosensitizers has been demonstrated. The results represent a systematic study of nucleotide oxidation by the triplet states of a series of structurally related organic photosensitizers in which the reduction potential can be tuned by ca. 800 mV. The greater than 100-fold variation in bimolecular rate constants for oxidation of base monophosphates by these photosensitizers offers the prospect of kinetic “selectivity” of oxidative damage in random-sequence DNA.

Despite the growing number of redox-active chromophores utilized to photoinduce oligonucleotide cleavage, detailed correlations between the degree of ground-state complexation and product yields have not been developed. To elucidate the specific role of singlet and triplet excited states in nucleotide photooxidation, the photochemical reactivities of N-(2-(N-pyridinium)ethyl)-1,8-naphthalene imide (NI) and N,N‘-bis-[2-(N-pyridinium)ethyl]-1,4,5,8-naphthalene diimide (NDI) with calf-thymus DNA have been explored as a function of ground-state complexation with the DNA polymer. Upon addition of calf-thymus DNA to a phosphate buffered solution of the naphthalene imide derivatives, distinct changes in the UV absorption spectrum of the chromophores, along with single isosbestic points, are observed. Analysis of these changes using the noncooperative model of McGhee and von Hippel yield association constants of (2.46 ± 0.42) × 104 M-1 and (7.78 ± 0.11) × 105 M-1 for NI and NDI, respectively. Pulsed 355 nm excitation of either NI or NDI in the presence of calf-thymus DNA produced the reduced NI•- and NDI•- species that absorbed maximally at 400 and 480 nm, respectively, from the triplet excited states. For both compounds, the yield of radical anion from self-quenching processes was substantial (φI •- = 0.11 ± 0.01 and 0.25 ± 0.01 for NI and NDI, respectively). However, pulsed excitation of NI in the presence of DNA resulted in the production of radical species that were not attributed to self-quenching processes. For both compounds, the fraction of associated imide was systematically varied between 0 and 1. The intersystem crossing yield was found to decrease linearly with the fraction bound to DNA from 0.71 to 0.08 for NI and 0.35 to 0.004 for NDI.

We have examined the UV resonance Raman and the VUV absorption spectra of aqueous glycylglycine and other dipeptides. We observe strong resonance Raman enhancement of the amide I, II, and III bands and the amide CαH bending mode in a manner similar to that we observed previously with excitation within the π→π* transition of N-methylacetamide (Chen, X. G.; Asher, S. A.; Schweitzer-Stenner, R.; Mirkin, N. G.; Krimm, S. J. Am. Chem. Soc. 1995, 117, 2884). However, in addition, we observe strong resonance Raman enhancement of the ca. 1400 cm-1 symmetric COO- stretching vibration, whose 206.5 nm Raman cross section is increased 20-fold compared to that of the carboxylate in sodium acetate, for example. Addition of a methylene spacer between the amide and carboxylate groups causes the resonance Raman enhancement of this symmetric COO- stretch to disappear. The UV resonance Raman excitation profiles, the Raman depolarization ratio dispersion, and the VUV absorption spectra of glycylglycine and other dipeptides demonstrate the existence of a new 197 nm charge transfer band which involves electron transfer from a nonbonding carboxylate orbital to the amide-like π* orbital. This transition occurs at the penultimate carboxylate end of all peptides and proteins.

Molecular sensors have received an enormous amount of attention in the current literature. Chemical sensors for peptides and proteins, 1 sugars, 2 and neutral or charged species 3 have been recently reviewed. However, the development of molecular sensors to detect changes in "environmental" properties, such as temperature and pressure, has received very little attention in the chemical literature. In the literature, we found just one example of a two-component fluorescent temperature sensor. 4 In this report, the naphthalene fluorescence intensity is modified by the temperature-dependent nickel(II) high-spin/low-spin interconversion in a covalently attached macrocycle. The studies were performed in solution phase, and the response is at a single wavelength region of the UV (λ max ) 340 nm).The incorporation of a molecular thermometer into a solid coating has utility in mapping temperature changes on twodimensional surfaces. In the aerodynamic community, these materials are referred to as temperature-sensitive paints, or TSPs. 5

A series of polymethylene-linked 1,8-naphthalimide/viologen diads has been synthesized. The number of intervening methylenes was varied from 2 to 6. For comparison, a series of N-alkylpyridiniumyl-1,8-naphthalimide “parent” compounds was prepared and photophysically characterized. Relative to the parent compounds, the electronically excited singlet state of the 1,8-naphthalimide was found to be quenched by the covalently attached viologen. From Stern−Volmer analyses of the steady-state fluorescence spectra, along with the singlet-state lifetime of the pyridinium-substituted 1,8-naphthalimide, the rate constants for intramolecular quenching were calculated to range from 1.5 × 1010 s-1 (2 intervening methylenes) to 8.3 × 107 s-1 (6 intervening methylenes) in aqueous buffered solution. For comparison, the intermolecular reactivity of the excited singlet state of N-alkylpyridiniumyl-1,8-naphthalimides with methylviologen was assessed. In 0.5 M phosphate buffer (pH 7.0), the bimolecular rate constant was found to be 3.2 × 109 M-1 s-1. Nanosecond laser flash photolysis studies were carried out to identify the quenching products. From these studies, reduced methylviologen was identified as a singlet-state quenching product. From these results, we attribute both the intra- and intermolecular quenching process to electron transfer from the singlet excited state of 1,8-naphthalimide to methylviologen. Within the covalently linked series, the rate constant for intramolecular electron transfer was found to vary exponentially with the number of intervening methylenes. Linear least-squares analysis of the results yielded an apparent β value of 1.04 Å-1 for electron transfer through the polymethylene bridge.