Ionic Stabilization of the Polythiophene-Oxygen Charge-Transfer Complex.

A substantial reduction in the rate of irreversible polymer photo-oxidation was observed through the ionic stabilization of the polymer-O2 charge-transfer complex (CTC) in amorphous polythiophene thin films. Through the incorporation of anionic functionality containing mobile cations, it was found that CTC stability increases with increasing cation charge density. This results in an increased rate of electron transfer to molecular oxygen relative to photosensitization and reaction of 1O2, leading to a reduction in the overall rate of polymer degradation. UV-vis and FTIR spectroscopy were utilized to determine the identity of intermediate and irreversible photodegradation products as well as the effect of ion identity on the dominant photo-oxidation mechanism. As polymer-O2 CTCs are common in the photo-oxidation of many conjugated polymers, these results have significant implications for the ongoing effort to produce commercially viable organic electronic and photonic devices.

Oligothiophene functionalized dimethyldihydropyrenes I: syntheses and photochromicity.

The syntheses of 2,7-di-tert-butyldimethyldihydrobenzo[e]pyrenes with thienyl (6), terthienyl (7), and pentathienyl (14) side chains at the 4,5- positions, ter- and pentathienyl side chains at the 4-position with ter- (39) and pentathienylcarbonyl (40) side chains at the 10- and 11-positions, 2-naphthoyl-7-tert-butyldimethyldihydropyrenes with ter- (53), penta- (54), and septithienyl (55) side chains at the 4,9-positions are described. These compounds are all photochromic and open to the corresponding cyclophanedienes with long wavelength (>490 nm) light, and as such, the conjugative path could change considerably, making them suitable to investigate as potentially switchable conducting molecules. In this paper, the syntheses and the photochemical and thermal isomerizations are studied; in the accompanying paper, the electrochemical and conductive properties are studied. Here, a comparison of the relative opening rates to that of the benzo[e]pyrene 4 (with no thienyl substituents) is made, and all of the above photochromes show considerably enhanced photo-opening of the DHPs to the CPDs. As examples, 14, 40, and 54 were cycled between the open and closed forms, and no decomposition was observed; however, when 54 was irradiated for 40 h with 254 nm light, some radicals did form, which enhanced the thermal closing rate, and so extensive irradiation with short wave UV is better avoided. The thermal closing reactions were also studied, and all of the above compounds close faster than benzo-CPD 4', though for the highly photochromic ter- and pentathienyl benzo-CPDs 39' and 40', the rate was not too enhanced from that of 4' and so are probably the best compromise between fast photochromicity and slow thermal reversion.

Oligothiophene functionalized dimethyldihydropyrenes II: electrochemical and conductive properties.

Three different types of oligothiophene functionalized dihydropyrene photoswitches, (A) 2-naphthoyldimethyldihydropyrenes functionalized at the 4,9-positions with oligothiophenes, (B) benzodimethyldihydropyrenes functionalized at the 4-positions with oligothiophenes and the 10(11)-position with oligothienylcarbonyl groups, and (C) benzodimethyldihydropyrenes functionalized at the 4,5-positions with oligothiophenes, were studied with the goal of using the change in pi-conjugation between the open and the closed forms of the dihydropyrene (DHP)-metacyclophane (CPD) switch to control electrical conductivity. UV-vis absorption studies were performed to measure the extent to which the attached thienyl oligomers were conjugated with the switch and the ability of the switch when opened or closed to affect conjugation. Of the three types of switches studied, those of type A showed the greatest effect. Solution cyclic voltammetry (CV) for the closed isomers indicated that the first few oxidation peaks were quasi-reversible, but that later ones were irreversible, leading to polymerization. Solution CV experiments on the open CPD form led to electrochemically induced closing to the DHP form. Conductivity studies were performed on undoped thin films of the A-type compound 4 and showed that opening the switch caused a decrease in electrical conductivity, and closing the switch caused an increase in electrical conductivity through the film. Doped films were studied by dual-electrode voltammetry and spectroelectrochemistry and showed that while doping led to an increase in the conductivity of the film it also led to the closing of the open form, preventing the conductivity of the open doped form from being measured.

Stable ion NMR and GIAO-DFT study of novel cations from 8,16-dicyano[2.2]metacyclophanedienes and from strategically substituted/benzannelated dihydropyrenes: charge-induced tropicity modulation and pi-switching.

The dicyanometacyclophanediene 1 is diprotonated at the cyano groups (1H2 2+) in various superacid media. Upon quenching, intact 1 and the ring-closed CPD 2 were obtained in a 3:2 or 3:1 ratio, depending on the superacid system. Compound 2 undergoes ring opening in the superacid to give the ipso-monoprotonated 2H+, which on quenching furnishes 1-cyanopyrene as a major product together with 2 and 1. The dication 3 2+, with strongly deshielded internal methyls, was generated from the epoxyannulene 3. Ketones 4-6 and ester 7 are O/C diprotonated to give paratropic carboxonium-annulenium dications (4H2 2+, 5H2 2+, 6H2 2+, and 7H2 2+, respectively). Ester 8 gives a trication by two-electron oxidation and O-protonation. Conjugated carboxylic acid 9 gives a mixture of two dications by CO and ring protonation. The dibromo derivatives 10 and 11 form carboxonium ions, whereas the monobromo derivative 12 is O/C diprotonated to give an oxonium-annulenium dication. Charge delocalization modes and tropicity in the resulting species are evaluated by NMR and GIAO-DFT. Facile formation of 2 from 1 in quenching experiments indicates that thermal closing can be achieved with the diprotonated dinitrile, without imposing skeletal rearrangement.

Suppressing the thermal metacyclophanediene to dihydropyrene isomerization: synthesis and rearrangement of 8,16-dicyano[2.2]metacyclophane-1,9-diene and evidence supporting the proposed biradicaloid mechanism.

Synthesis of 8,16-dicyano-anti-[2.2]metacyclophane-1,9-diene, 1b, was achieved in five steps from 1,3-bis(bromomethyl)benzonitrile. Unlike most metacyclophanedienes which easily thermally isomerize (tau 1/2 = minutes to days at 20 degrees C) to dihydropyrenes 2, dinitrile 1b shows no tendency to convert thermally to 2b at room temperature (tau 1/2 > 30 years), consistent with predictions based on calculation of activation barriers. Irradiation of cyclophanediene 1b with UV light readily forms the dinitriledihydropyrene 2b, which unexpectedly shows a much more facile (50 degrees C) 1,5-sigmatropic rearrangement of the internal nitrile groups to give dihydropyrenes 9b and then 10b than is the case for internal methyl substituents, 2a, which forms 9a at temperatures above 190 degrees C. Synthesis of the 2-formyl derivative 1c and the 2-naphthoyl derivative 1d are also described. These substituents were predicted to lower the activation barrier for the thermal closing reaction to the corresponding dihydropyrenes, and experimental evidence supports these calculations.

The effect of addition of fluorescent moieties to dihydropyrenes: enhancing photochromicity and fluorescence monitoring.

A series of dihydropyrenes with appending fluorescent moieties were synthesized with the objective of increasing the photochromic efficiency for this class of compounds and to establish how suitable fluorescence would be to follow their photochromic behavior. The ring opening quantum yields of dihydropyrenes with aroyl substituents at the 4-position showed increased ring opening quantum yields without a decrease of the half-life for the thermal reversion of the less stable open isomer, the metacyclophanediene to the dihydropyrene. The fluorescence of the appending naphthoyl or pyrenoyl moieties was not suitable to follow the photochromic cycling of the dihydropyrenes. However, the emission detected above 600 nm of the closed isomer of the dihydropyrene moiety was shown to be a good monitoring method for the photochromic cycling.

Reversible photo-regulation of a hammerhead ribozyme using a diffusible effector.

The potential utility of catalytic RNAs and DNAs (ribozymes and deoxyribozymes, respectively) as reagents in molecular biology as well as therapeutic agents for a variety of human diseases, has long been recognized. Although naturally occurring RNA-cleaving ribozymes are typically not subject to feedback control, rational methodologies for the creation of allosteric ribozymes, by functional combination of ribozyme and ligand-responsive aptamer RNA elements, have existed for some years. Here, we report the in vitro selection of RNA aptamers specific for binding one but not the other of two light-induced isomers of a dihydropyrene photo-switch compound, and the utilization of such an aptamer for the construction of the UG-dihydropyrene ribozyme, an allosteric hammerhead ribozyme whose catalysis is controllable by irradiation with visible versus ultraviolet light. In the presence of micromolar concentrations of the photo-switch compound, the ribozyme behaves as a two-state switch, exhibiting a >900-fold difference in catalytic rates between the two irradiation regimes. We anticipate that the UG-dihydropyrene, and other ribozymes like it, may find significant application in the developmental biology of model organisms such as Drosophila melanogaster and Caenorhabditis elegans, as well as in the biomedical sciences.

A DFT study of the thermal, orbital symmetry forbidden, cyclophanediene to dihydropyrene electrocyclic reaction. Predictions to improve the dimethyldihydropyrene photoswitches.

The orbital symmetry forbidden thermal electrocyclic equilibria between a series of cyclophanedienes and dimethyldihydropyrenes (CPD<==>DDPs) were studied using density functional theory (DFT). These reactions are important not only because of their fundamental interest but also in how they restrict the potential utility of the DDP photoswitches by limiting the thermal lifetime of the CPDs. The transition states (TSs) for these reactions could not be modeled using restricted DFT (RB3LYP) but were located using unrestricted DFT (UB3LYP). Each TS possesses significant biradical character as indicated by their spin contaminated wave functions, S2 not = 0. Specific substitution by nitrile or trifluoromethyl group(s) is predicted to strongly affect the magnitude of the activation barriers for these reactions. In particular, replacing the internal methyl groups of the CPDs/DDPs with nitrile groups is predicted to have the maximum effect and to raise the activation barriers and lifetimes of the CPDs considerably.