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Electron transfer (ET) processes in DNA are of current interest because of their involvement in oxidative strand cleavage reactions and their relevance to the development of molecular electronics. Two mechanisms have been identified for ET in DNA, a single-step tunneling process and a multistep charge-hopping process. The dynamics of tunneling reactions depend on both the distance between the electron donor and acceptor and the nature of the molecular bridge separating the donor and acceptor. In the case of protein and alkane bridges, the distance dependence is not strongly dependent on the properties of the donor and acceptor. In contrast, we show here that the distance decay of DNA ET rates varies markedly with the energetics of the donor and acceptor relative to the bridge. Specifically, we find that an increase in the energy of the bridge states by 0.25 eV (1 eV ‫؍‬ 1.602 ؋ 10 ؊19 J) relative to the donor and acceptor energies for photochemical oxidation of nucleotides, without changing the reaction free energy, results in an increase in the characteristic exponential distance decay constant for the ET rates from 0.71 to 1.1 Å ؊1 . These results show that, in the small tunneling energy gap regime of DNA ET, the distance dependence is not universal; it varies strongly with the tunneling energy gap. These DNA ET reactions fill a ''missing link'' or transition regime between the large barrier (rapidly decaying) tunneling regime and the (slowly decaying) hopping regime in the general theory of bridge-mediated ET processes. E lectron transfer (ET) processes in which an electron donor and acceptor are separated by a molecular spacer or bridge (D-B-A systems) are encountered widely in biological systems (proteins and DNA; refs. 1-4) and molecular wires (5-7). The dynamics of such processes are known to depend, inter alia, on the length and nature of the bridge (8, 9). The dynamics of single-step photoinduced ET in D-B-A systems, a process referred to as tunneling, is generally found to display an exponential dependence on the D-A distance as described by Eq. 1,where k o is a temperature-dependent prefactor, r da is the D-A separation, and ␤ characterizes the steepness of the experimental distance dependence are observed for bridges consisting of conjugated polyenes (5, 12), which probably arise from delocalization of the donor and acceptor states onto the bridge. DNA systems apparently are unique in that a wide range of values of ␤ (0.1-1.5 Å Ϫ1 ) have been observed for similar duplex DNA bridge structures (13,14). The very smallest ␤ values for DNA ET may arise from an alternative ET mechanism, multistep hole hopping (15-18). The distance dependence of the reorganization energy may enhance some of the observed ␤ values (19). A question in DNA ET is how much the donor and acceptor energetics can influence the ␤ value by tuning the electronic coupling strength. By making donor-acceptor modifications that do not change the ETactivation free energies, we find that ␤ values can be changed by more than 50%. We interpret...

The photochemical behavior of 3-dimethylaminostilbene (3DS), 3-amino-3‘-cyanostilbene (3A3‘CS), and 3-dimethylamino-4‘-cyanostilbene (3DCS) has been investigated and compared to that of the analogous 4-amino derivatives. The absorption spectra of the 4-aminostilbenes display a single strong band whereas the spectra of the 3-aminostilbenes display multiple bands or lesser intensity as a consequence of configuration interaction which results from loss of symmetry. The 3-aminostilbenes have substantially higher fluorescence quantum yields and longer fluorescence lifetimes when compared to the 4-aminostilbenes. Based on analysis of the solvatochromic shifts of the absorption and fluorescence spectra, the 3- and 4-aminostilbenes have similar excited state dipole moments which are substantially larger than their ground state dipole moments. The long lifetimes of the 3-aminostilbene charge transfer singlet states are a consequence of low fluorescence rate constants and a large barrier for singlet state torsion. Thus the “meta-amino effect” reported previously for 3-aminostilbene is also observed in the case of donor−acceptor substituted stilbenes. In nonpolar solvents the 3-aminostilbenes decay predominantly by fluorescence and intersystem crossing to the triplet state which decays to yield a mixture of trans and cis isomers. In polar solvents a second nonradiative decay channel is operative and is tentatively assigned to internal conversion on the basis of its energy gap dependence. The primary 3-amino-3‘-cyanostilbene displays specific solvation in alcohol solvents at room temperature and aggregation in a nonpolar solvent at low temperature.

We report the chemistry and photophysics of atomic gold and silver particles in inorganic glasses. By synchrotron irradiation of gold-doped soda-lime silicate glasses we could create and identify unambiguously the gold dimer as a stable and bright luminescing particle embedded in the glassy matrix. The gold dimer spectra coincide perfectly with rare gas matrix spectra of Au(2). The glass matrix is, however, stable for years, and is hence perfectly suited for various applications. If the irradiated gold-doped sample is annealed at 550 degrees C a bright green luminescence can be recognized. Intense 337 nm excitation induces a decrease of the green luminescence and the reappearance of the 753 nm Au(2) emission, indicating a strong interrelationship between both luminescence centers. Time-dependent density functional theory (TD-DFT) calculations indicate that the green luminescence can be assigned to noble metal dimers bound to silanolate centers. These complexes are recognized as the first stages in the further cluster growth process, which has been investigated with small-angle x-ray scattering (SAXS). In silver-doped glasses, Ag(0) atoms can be identified with electron paramagnetic resonance (EPR) spectroscopy after synchrotron activation. Annealing at 300 degrees C decreases the concentration of Ag(1), but induces an intense white light emission with 337 nm excitation. The white luminescence can be decomposed into bands that are attributed to small silver clusters such as Ag(2), Ag(3) and Ag(4), and an additional band matching the green emission of gold-doped glasses.

The effects of 4-cyano and 3-cyano substituents on the spectroscopic properties and photoacidity of 3- and 4-hydroxystilbene have been investigated. In nonpolar solvents, the 3-hydroxycyanostilbenes have much longer singlet lifetimes and larger fluorescence quantum yields than do the 4-hydroxycyanostilbenes. The longer lifetimes of 3-hydroxystilbene and its cyano derivatives are attributed to a "meta effect" on the stilbene torsional barrier, similar to that previously observed for the aminostilbenes. The cyano substituent causes a marked increase in both ground state and excited-state acidity of the hydroxystilbenes in aqueous solution. The dynamics of excited-state proton transfer in methanol-water solution have been investigated by means of femtosecond time-resolved transient absorption spectroscopy. Assignment of the transient absorption spectra is facilitated by comparison to the spectra of the corresponding potassium salts of the conjugate bases and the methyl ethers, which do not undergo excited-state proton transfer. The 4-cyanohydroxystilbenes undergo excited-state proton transfer with rate constants of 5 x 10(11) s(-1). These rate constants are comparable to the fastest that have been reported to date for a hydroxyaromatic photoacid and approach the theoretical limit for water-mediated proton transfer. The isotope effect for proton transfer in deuterated methanol-water is 1.3 +/- 0.2, similar to the isotope effect for the dielectric response of water. The barrier for excited state double bond torsion of the conjugate bases is small for 4-cyano-4-hydroxystilbene but large for 4-cyano-3-hydroxystilbene. Thus the "meta effect" is observed for the singlet states of both the neutral and conjugate base.

The photophysical properties of several acceptor substituted 1-arylpyrene derivatives were investigated. The fluorescence spectra strongly depend on the nature of the aryl moiety and the position and number of methoxycarbonyl acceptor groups. Dual fluorescence, originating from a locally excited and a charge transfer state, was observed for the diester derivatives. The solvent dependence of the dual fluorescence and the slightly curved solvatochromic plots indicate a change of the character of the excited states from solvents of low to high polarity. The rate constants for fluorescence and nonradiative decay were calculated to reveal the nature of the excited-state relaxation, that is, the increase of the mesomeric interactions by geometrical flattening or stabilization of the CT state by further twisting toward perpendicularity. Flattening is the major relaxation pathway of the diester phenylpyrene derivative in nonpolar solvents whereas in highly polar solvents the low value for the fluorescence transition dipole moment (M f ) indicates stabilization of the charge transfer state by further twisting. The fluorescence of the diester biphenylpyrene derivative originates from a locally excited state (LE) in nonpolar solvents. The low value for M f in polar solvents and the change of the relative intensity of the dual fluorescence signals with the temperature indicate that the red-shifted fluorescence can be assigned to a twisted intramolecular charge transfer state.

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The spectroscopy and photochemistry of several 3-substituted trans-diaminostilbenes have been investigated and the results compared with those from previous studies of 4,4′-diaminostilbenes. The excited-state properties of 3,5-diaminostilbene and 3,3′-diaminostilbene are similar to those for the parent 3-aminostilbene. These amines display long, temperature-independent singlet lifetimes and large fluorescence quantum yields. The long singlet lifetimes are a consequence of large barriers for torsion about the central double bond on the singlet state surface and low fluorescence rate constants. Thus, these diaminostilbenes display the "metaamino effect" previously identified for 3-aminostilbene and for some amine-donor-acceptor substituted stilbenes. The 4,4′-diaminostilbenes also have larger torsional barriers in nonpolar solvents but shorter singlet lifetimes as a consequence of larger fluorescence and intersystem crossing rate constants. The photophysical properties of 3,4′-dimethylaminostilbene are similar to those of 4,4′-aminostilbenes; however, it has a lower torsional barrier, similar to that of the parent 4-aminostilbene.