Photoswitching and Thermoresponsive Properties of Conjugated Multi‐chromophore Nanostructured Materials

Conjugated multi‐chromophore organic nanostructured materials have recently emerged as a new class of functional materials for developing efficient light‐harvesting, photosensitization, photocatalysis, and sensor devices because of their unique photophysical and photochemical properties. Here, we demonstrate the formation of various nanostructures (fibers and flakes) related to the molecular arrangement (H‐aggregation) of quaterthiophene (QTH) molecules and their influence on the photophysical properties. XRD studies confirm that the fiber structure consists of >95% crystalline material, whereas the flake structure is almost completely amorphous and the microstrain in flake‐shaped QTH is significantly higher than that of QTH in solution. The influence of the aggregation of the QTH molecules on their photoswitching and thermoresponsive photoluminescence properties is revealed. Time‐resolved anisotropic studies further unveil the relaxation dynamics and restricted chromophore properties of the self‐assembled nano/microstructured morphologies. Further investigations should pave the way for the future development of organic electronics, photovoltaics, and light‐harvesting systems based on π‐conjugated multi‐chromophore organic nanostructured materials.     

Comparative Studies on Optical,Redox, and Photovoltaic Properties of a Series of D–A–D and Analogous D–A Chromophores

A series of new symmetrical donor‐acceptor‐donor (D?A?D) dyes based on s‐indacene‐1,3,5,7(2H,6H)‐tetraone as an acceptor unit containing varying electron donating moieties and analogous donor‐acceptor (D?A) chromophores with indane‐1,3‐dione as an acceptor are synthesized. By employing these two sets of dyes, the influence of a scaffold change from unsymmetric push‐pull (D?A) to symmetrical (D?A?D) systems on optical, electrochemical, and photovoltaic properties are explored. Detailed comparative studies reveal favorable optical characteristics and considerably decreased bandgaps for the D?A?D dyes compared to those of the reference D?A chromophores. Accordingly, the evaluation of the present dyes as donor materials in bulk heterojunction (BHJ) solar cells in combination with fullerene derivatives PC₆₁BM or PC₇₁BM as acceptors afforded significantly improved performance for devices based on D?A?D blends (up to a factor of 4 compared to the respective D‐A reference) with power conversion efficiencies of up to 2.8%. In less polar solvents such as toluene, some of the novel D?A?D chromophores exhibit unexpectedly high fluorescence quantum yields Φ_em of up to unity, in striking contrast to their weakly fluorescent D‐A counterparts.     

Chromophore exchange in the blue light-sensitive photoreceptor YtvA from Bacillus subtilis

YtvA from Bacillus subtilis was found as the first prokaryotic phototropin-like blue-light-responsive photoreceptor. It is composed of two domains, the photoactive LOV (light, oxygen, voltage) domain, which binds a flavin mononucleotide (FMN) as a chromophore and a STAS (sulfate transporter/anti-sigma-factor antagonist) domain, which generates a physiological signal. Here we present a routine chromophore-exchange protocol that allows chemically synthesized, structurally modified chromophores instead of the naturally present flavin mononucleotide (FMN) chromophore to be introduced. FMN was exchanged for riboflavin (RF), flavin adenine dinucleotide (FAD), 7,8-didemethyl flavin mononucleotide (DMFMN), and 8-isopropyl flavin mononucleotide (iprFMN). LOV domains reconstituted with new flavins undergo the same photocycle as native YtvA LOV, consisting of triplet formation and covalent binding of the chromophore followed by a thermal recovery of the parent state, albeit with different kinetics and photophysical properties. Interestingly, the iprFMN chromophore, inducing steric hindrances to the protein, exhibits a very fast light-to-dark-conversion and shows a high fluorescence quantum yield (0.4). Incorporation of FAD causes an increase of its fluorescence quantum yield from 0.04 (H(2)O) to 0.2.     

The unique optical behaviour of bio‐related materials with organic chromophores

Molecularly designed materials based on macromolecules and organic dyes offer unique opportunities in connection with the possibility of preparing optically responsive ‘smart’ materials. Indeed macromolecules are able to transmit and amplify small signals reaching sites at interacting distance through the involvement of the whole chain. The corresponding materials can then acquire stimuli‐responsive properties in relation to specific features connected to primary structure and conformation. As a first approach to benefit from the above features for preparing eco‐compatible smart materials, bio‐related polypeptides, polysaccharides and polyesters can be used as the macromolecular partner in combination with a selected dye following different interaction methodologies. Two distinct routes were used to prepare optically responsive products from the above bio‐related polymers, respectively based either on the covalent bonding to the original macromolecules of photochromic molecular species, such as azobenzene and spiropyran, or on the morphology‐modulated dispersion of highly conjugated dyes in the polymer bulk. Examples related to the two different routes have been investigated in our laboratory and are presented and discussed also with reference to selected recent cases from the literature. Copyright © 2012 Society of Chemical Industry     

New side-chain polyurethanes with highly conjugated push–pull chromophores for second order NLO applications

Six new side-chain polyurethanes with large molecular hyperpolarizabilities were synthesized by reaction of ethylenic and/or azo bridged push–pull chromophoric monomers with tolylene 2,4-diisocyanate. The NLO-phores show different conjugation pathway but analogous donor/withdrawing substituents. The chemical and thermal properties of the chromophores and of the polymers were examined. Evaluation of the second order NLO properties was performed by EFISH determination of μβ on the monomers, and by SHG measurements on some amorphous thin films obtained from polymers through spin-coating technique.     

Preliminary characterization of light harvesting in E. coli DNA photolyase

E. coli DNA photolyase is a monomeric light-harvesting enzyme that utilizes a methenyltetrahydrofolate (MTHF) antenna cofactor to harvest light energy for the repair of thymine dimers in DNA. For this purpose, the enzyme evolved to bind the cofactor and red-shift its absorption maximum by 25 nm. Using the crystal structure as a guide, we mutated each protein residue that contacts the cofactor in an effort to identify the interactions responsible for this selective stabilization of the cofactor's excited state. Hydrogen bonding, packing, and electrostatic interactions were examined. Remarkably, a single residue, Glu109, appears to play an important, if not exclusive, role in inducing the observed red-shift. Thus, this protein, the simplest light-harvesting system known, appears to have evolved a remarkably simple mechanism to tune the photophysical properties of the antenna cofactor appropriately for biological function.     

Solution-state (15)N NMR spectroscopic study of alpha-C-phycocyanin: implications for the structure of the chromophore-binding pocket of the cyanobacterial phytochrome Cph1

The detailed structure of the chromophore-binding pocket in phytochrome proteins and the structural changes associated with its photocycle are still matters of debate. Insight into the structure and dynamics of the binding pocket has been gained through the comparison of a (15)N NMR spectrum of alpha-C-phycocyanin, which is often used as a model system for the study of phytochromes, with the previously described (15)N NMR spectrum of the cyanobacterial phytochrome Cph1. The former spectrum supports the hypothesis that all four nitrogen atoms of the alpha-C-phycocyanin chromophore are protonated, in analogy with the proposed protonation state for the P(r) and P(fr) forms of Cph1. The spectra show that the chromophores in both proteins exhibit a distinct dynamic behavior, as also indicated by a NOESY spectrum of Cph1. Finally, stereochemical arguments and a Cph1 homology model support the hypothesis that the chromophore in Cph1 is most likely in the ZZZssa conformation in the P(r) form of the protein.     

Mutagenic stabilization of the photocycle intermediate of green fluorescent protein (GFP)

The optical spectra of the Aequorea victoria green fluorescent protein (GFP) are governed by an equilibrium between three different chromophore states. Mutants that predominantly show either the protonated (A) or the deprotonated (B) form of the chromophore have previously been described. In contrast, the I form, which is formed by rapid excited-state deprotonation of the A form of the chromophore, has only been described as an obligatory photochemical intermediate. We report the design of a new GFP mutant with a stabilized I form. For this purpose, we introduced two isosteric point mutations, Thr203Val and Glu222Gln, that selectively raise the potential energy of both the A and the B form. Knowledge of the absorption spectrum of the I form at room temperature allows the detailed analysis of concentration dependent changes in bulk wild-type(wt)-GFP spectra, as well as the determination of the dimerization constant of GFP. This information expands the use of GFP to that of a spectral probe for protein concentration. We determined energy differences between the chromophore ground states in the monomer and the dimer and reconstructed part of the potential energy surface.     

Supramolecular Assemblies of Cucurbiturils with Photoactive, π-conjugated Chromophores

Supramolecular assemblies of cucurbituril (CBn) homologues with π-conjugated chromophores will be overviewed. Special emphasis will be given to the effect of CBn on the optical properties of conjugated oligomers and polymers. How supramolecular complexes of π-conjugated chromophores including porphyrin derivatives, conjugated oligomers and polymers with CBn could be utilized in the theranostic and photonic applications will also be discussed.     

Novel white-light-emitting polyfluorenes with benzothiadiazole and Ir complex on the backbone

Novel white-emitting polyfluorenes were synthesized by mixing fluorescence and phosphorescence emission. Benzothiadiazole(BT) and iridium(III)bis(2-(1-naphthalene)pyridine-C^2′,N)-2,2,6,6-tetramethyl-3,5-heptanedione[(1-npy)₂Ir(tmd)] units were incorporated into polyfluorene backbone as green and red chromophores by Suzuki polycondensation. The device from PFG03-IrR07 shows a maximum luminous efficiency (LE) of 5.3 cd/A, a maximum luminance of 9900 cd/m² at a current density of 453 mA/cm² and a CIE coordinate of (0.32, 0.34) with the configuration: ITO/PEDOT:PSS/PVK/emissive layer/CsF/Al. Besides, the EL efficiencies decline slightly with increasing the current density. All emissions located very close to the equi-energy white point (0.33, 0.33) when applied voltage change from 9 to 14 V. Furthermore, the white emission of devices based on these materials shows very good color quality, with high color rendering index range between 84 and 89. Our results indicate that, by incorporation of singlet and triplet species into polymer backbone, the obtained white-emitting materials and devices are promising candidates for display and solid-state-lighting purpose.