Photonics is the study of the interaction of light with molecules or matter. The absorption of light by single molecules creates electronically excited molecules, whereas absorption by assemblies of chromophores creates delocalized excited complexes - excitons or exciplexes. Following excitation, the excited state can luminesce or undergo a variety of chemical processes including electron transfer, energy transfer, bond making, and bond breaking. The objective of our research is to determine how molecular structure and environment affect photonic behavior. The molecular structures of interest range in size and complexity from aryl olefins to designer-DNA. Current projects include studies of the distance and angular dependence of electron transfer, energy transfer, and exciton coupling in DNA-based systems and in urea-linked self-organized π-stacked aromatics.

Photoinduced electron transfer in DNA can occur via one of two mechanisms, single-step superexchange and multi-step hole hopping. In our laboratory, synthetic DNA hairpins, in which an organic chromophore serves both as a linker and electron acceptor, have been employed to investigate electron transfer dynamics in DNA. Studies in which a guanine or deazaguanine nucleobase serves as the electron donor, separated from the acceptor by a variable number of A:T base pairs, indicated that the π-stacked bases of DNA provide a better medium for electron transfer than the sigma bonded pathways of proteins and saturated hydrocarbons but do not function as a molecular wire. However, these studies do not distinguish between superexchange and hopping mechanisms. Recent studies of capped hairpins in which acceptor and donor chromophores are separated by a variable number of A:T base pairs have established that a crossover in mechanisms occurs when two or three base pairs separate the donor and acceptor.
      Capped hairpin and dumbbell structures which possesses two chromophores separated by a variable number of base pairs have also been used to investigate the distance and angle dependence of fluorescence resonance energy transfer and exciton coupled circular dichroism. The DNA base pair domain serves as a helical ruler which controls both the distance and the angle between the electronic transition moments of the two chromophores. The use of perylenediimide chromophores as hairpin linkers and capping groups permits the self-assembly of novel DNA hairpin dimer and oligomeric structures without the aid of hydrogen bonding. A combination of spectroscopic and computational methods are used to characterize these structures.
      We are seeking to develop organic models for the π-stacked bases in duplex DNA that might find applications in the construction of molecular electronic devices. Tertiary amide and tertiary urea groups can serve as conformational control elements that allow the assembly of aromatic donor, acceptor, and bridging chromophores into well-defined geometries. Oligomeric poly(arylureas) with donor and acceptor chromophores at their termini have been prepared and self-organize into π-stacked geometries. Electronic interactions in simple di-, tri-, tetra-, and pentachromophoric molecules are being studied.