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The report that the central bond photoisomerization of the 1,3,5-hexatrienes (Hts) is highly inefficient has encouraged theoreticians to seek conical intersections (CIs) at geometries that can explain rapid nonradiative return to the initially excited isomer. Because they are photochemically silent, torsional relaxations about the terminal double bonds of the Hts have not been evaluated as significant radiationless decay pathways. Study of the photoisomerization of trans,trans,trans- and trans,cis,trans-1,6-dideuterio-1,3,5-hexatrienes ( ttt- and tct-Htd2) addresses this issue. Degassed cyclohexane- d12 (C6D12) and CD3CN solutions were irradiated at 254 nm in quartz NMR tubes, and the progress of the reactions was followed by 1H NMR. Photoisomerization rates based on the integration of terminal hydrogen NMR peaks are in reasonable agreement with rates obtained by fitting pure isomer NMR spectra to the phase shift and baseline corrected experimental NMR spectra. The results show that terminal bond isomerization is highly efficient, especially when one considers that central bond isomerization is much more efficient than previously reported and is mainly observed together with terminal bond isomerization. A mechanism involving terminal one-bond-twist (OBT) in competition with a bicycle pedal (BP) process accounts for all terminal and most central bond photoisomerization. OBT central bond isomerization is a minor reaction that is observed primarily in the tct to ttt direction. Most surprising is the prominent role of the BP process in central bond photoisomerization. Proposed initially to account for photoisomerization in free volume constraining media, it is observed here in the absence of medium constraints.

In glassy isopentane at 77 K, 1-phenyl-4-(4-pyridyl)-cis-1,cis-3-butadiene (cc-PPyB) and 1-phenyl-4-(4-pyridyl)-cis-1,trans-3-butadiene (ct-PPyB) can undergo simultaneous two-bond photoisomerization. Under the same conditions, 1-phenyl-4-(4-pyridyl)-trans-1,cis-3-butadiene (tc-PPyB) gives tt-PPyB, the ultimate photoproduct in all cases.

A number of N,N′-disubstituted perylenetetracarboxylic diimides have been reported to bind effectively to DNA that adopts G-quadruplex motifs. In some cases, this binding may actively drive the transition from single-strand DNA to the quadruplex form. The perylenediimides in the reported cases all have amine-containing side chains, which are thought to interact with the grooves of the quadruplex and help dictate the selectivity of these compounds for quadruplex versus duplex DNA. We synthesized a polyethyleneglycol-swallowtailed (PEG-tailed) perylenediimide that is water-soluble even though it is uncharged. Binding to duplex and quadruplex DNA of this perylenediimide was studied by fluorescence quenching titrations under a variety of salt conditions, and the compound’s effect on quadruplex formation was studied by non-denaturing gel electrophoresis. Our results indicate that while the molecule binds to single-stranded DNA quite effectively and with selectivity, it does not drive the transition of the DNA to the tetrameric quadruplex structure, supporting the idea that charge neutralization is a key component of perylene compounds that stabilize tetrameric quadruplexes.

Donor-σ-Acceptor (D-σ-A) molecules, arrayed in a monolayer between electrodes, can serve as molecular rectifiers. Using perylene-3,4,9,10-tetracarboxylic bisimide (PBI) as the acceptor allows the attachment of the donor group to one imide nitrogen and a solubilizing swallowtail, normally a long (e.g., C(19)) alkane connected at midchain, on the other. Such an alkyl tail facilitates the formation of Langmuir-Blodgett (LB) monolayers. We have employed several modified swallowtails to make new D-σ-A molecules: poly(ethylene glycol) (PEG) swallowtails with 6 ether oxygens or with 4 ether oxygens to promote hydrophilicity in orienting LB monolayers, and alkyl swallowtails ending with sulfur anchors (thioacetate, thiol, or methyl disulfide) to stabilize attachment of the D-σ-A molecules to gold electrodes. The preparation and characterization of D-σ-A molecules containing combinations of these swallowtails with pyrene, ferrocene, and tetramethylphenylenediamine donor groups is described.