Vladimir Chukharev

Preparation and Photophysical and Photoelectrochemical Properties of a Covalently Fixed Porphyrin–Chemically Converted Graphene Composite

Chemically converted graphene (CCG) covalently linked with porphyrins has been prepared by a Suzuki coupling reaction between iodophenyl-functionalized CCG and porphyrin boronic ester. The covalently linked CCG-porphyrin composite was designed to possess a short, rigid phenylene spacer between the porphyrin and the CCG. The composite material formed stable dispersions in DMF and the structure was characterized by spectroscopic, thermal, and microscopic measurements. In steady-state photoluminescence spectra, the emission from the porphyrin linked to the CCG was quenched strongly relative to that of the porphyrin reference. Fluorescence lifetime and femtosecond transient absorption measurements of the porphyrin-linked CCG revealed a short-lived porphyrin singlet excited state (38 ps) without yielding the porphyrin radical cation, thereby substantiating the occurrence of energy transfer from the porphyrin excited state to the CCG and subsequent rapid decay of the CCG excited state to the ground state. Consistently, the photocurrent action spectrum of a photoelectrochemical device with a SnO(2) electrode coated with the porphyrin-linked CCG exhibited no photocurrent response from the porphyrin absorption. The results obtained here provide deep insight into the interaction between graphenes and π-conjugated systems in the excited and ground states.

Photophysics and photoelectrochemical properties of nanohybrids consisting of fullerene-encapsulated single-walled carbon nanotubes and poly(3-hexylthiophene)

Novel nanohybrids of single-walled carbon nanotubes (SWNTs) encapsulating C60 or C70 with poly(3-hexylthiophene) (P3HT) have been prepared and their photophysics and photoelectrochemical properties are studied in detail for the first time. Strong π–π interaction between the SWNT sidewalls and P3HT afforded successful dissolution of the so-called fullerene peapods into an organic solvent, as in the case of empty SWNTs (p-SWNTs). Fluorescence emission of P3HT in the SWNT–P3HT hybrids was completely quenched by the SWNTs regardless of the fullerenes insertion. Transient absorption and fluorescence up-conversion techniques revealed the excited state dynamics of the nanohybrids, where exciplex formation from the short-lived P3HT singlet excited state (∼0.2 ps) with the fullerene peapods and subsequent relaxation to the ground state within ∼1 ps occurred dominantly. Significant difference in the photodynamics upon encapsulation of C60 or C70 was not detected, implying little participation of the fullerenes in the excited state event and thus the inability of the encapsulated fullerenes to generate the charge-separated state between the fullerene peapods and P3HT. Photoelectrochemical devices based on the peapod–P3HT nanohybrids showed almost the same incident photon-to-current efficiencies as those for the p-SWNT–P3HT-based device, which is in good agreement with the results of the time-resolved spectroscopies. Thus, the results obtained here will give a deep insight into the photophysics and photoelectrochemical properties of fullerene peapod–conjugated polymer as well as SWNT–conjugated polymer hybrids and therefore provide valuable information on the design of peapod-based optoelectronic devices.

Effects of Carbon–Metal–Carbon Linkages on the Optical, Photophysical, and Electrochemical Properties of Phosphametallacycle-Linked Coplanar Porphyrin Dimers

5-(Diphenylphosphanyl)-10,15,20-triarylporphyrins (meso-phosphanylporphyrins) underwent complexations with palladium(II) and platinum(II) salts to afford phosphapalladacycle- and phosphaplatinacycle-fused coplanar porphyrin dimers, respectively, via regioselective peripheral β-C-H activation of the meso-phosphanylporphyrin ligands. The optical and electrochemical properties of these metal-linked porphyrin dimers as well as their porphyrin monomer/dimer references were investigated by means of steady-state UV-vis absorption/fluorescence spectroscopy, cyclic and differential pulse voltammetry, time-resolved spectroscopy (fluorescence and transient absorption lifetimes and spectra), and magnetic circular dichroism spectroscopy. All the observed data clearly show that the palladium(II) and platinum(II) linkers play crucial roles in the electronic communication between two porphyrin chromophores at the one-electron oxidized state and in the singlet-triplet intersystem-crossing process at the excited state. It has also been revealed that the C-Pt-C linkage makes more significant impacts on these fundamental properties than the C-Pd-C linkage. Furthermore, density functional theory calculations on the metal-linked porphyrin dimers have suggested that the antibonding dπ-pπ orbital interaction between the peripherally attached metal and adjacent pyrrolic β-carbon atoms destabilizes the highest occupied molecular orbitals of the porphyrin π-systems and accounts for the observed unique absorption properties. On the basis of these experimental and theoretical results, it can be concluded that the linear carbon-metal-carbon linkages weakly but definitely perturb the optical, photophysical, and electrochemical properties of the phosphametallacycle-linked coplanar porphyrin dimers.

Structural photoactivation of a full-length bacterial phytochrome

Time-resolved x-ray solution scattering reveals the conformational signaling mechanism of a bacterial phytochrome. Phytochromes are light sensor proteins found in plants, bacteria, and fungi. They function by converting a photon absorption event into a conformational signal that propagates from the chromophore through the entire protein. However, the structure of the photoactivated state and the conformational changes that lead to it are not known. We report time-resolved x-ray scattering of the full-length phytochrome from Deinococcus radiodurans on micro- and millisecond time scales. We identify a twist of the histidine kinase output domains with respect to the chromophore-binding domains as the dominant change between the photoactivated and resting states. The time-resolved data further show that the structural changes up to the microsecond time scales are small and localized in the chromophore-binding domains. The global structural change occurs within a few milliseconds, coinciding with the formation of the spectroscopic meta-Rc state. Our findings establish key elements of the signaling mechanism of full-length bacterial phytochromes.

Spectroscopy of a terthiophene–vinylbenzoate

A new terthiophene-vinylbenzoate compound has been synthesized for applications in molecular optoelectronic devices. The photophysical properties of the compound have been studied in a series of solvents The compound is characterized by a high emission yield (43% in cyclohexane) and a large solvent-dependent Stokes shift (90-120 nm). The shift is attributed to a considerable change in the dipole moment in the excited state as compared to that in the ground state. The emission spectra have been analyzed in the frame of semi-classic charge-transfer theory. This gave estimates for the emitting state free energy, the solvent and internal reorganization energies, and the vibrational frequency. Fast dynamics of the emitting state have been studied by using femtosecond pump-probe and up-conversion methods. In polar solvents, the intramolecular vibrational energy redistribution in the excited state takes place in a sub-picosecond time domain and may result in a molecular configuration different from the all-trans conformer in the ground state. The conformational difference between the excited and ground states makes it possible to use the compound for light amplification. The amplification coefficient can be greater than 2 cm(-1), as demonstrated by preliminary experiments.

Photophysical properties of Sn(IV)tetraphenylporphyrin-pyrene dyad with a β-vinyl linker

A Sn(IV)tetraphenylporphyrin (T) has been functionalized with a β-vinyl pyrene (P) and the photophysical properties of the formed dyad (T-P) with its corresponding precursors were studied in three solvents with different polarities using steady-state and time-resolved measurements in ps and fs timescales. When the pyrene moiety is excited at λex = 340 nm, the fluorescence spectroscopy experiments indicate in all the studied solvents, an efficient quenching of the pyrene emission. When excited at either λex = 340 nm or λex = 405 nm, where porphyrin absorbs, a new emissive excited state complex (T-P)* is observed at wavelenghts close to the parent porphyrin emission. The emission is more pronounced in nonpolar hexane showing a mono-exponential decay, but bi-exponential decays are observed in more polar dicloromethane and acetonitrile. When the porphyrin moiety is excited at λex = 425 nm, the fs transient absorption analysis shows two different intermediate species (~ 7–11 ps and 80–100 ps) with broad absorption in the near-IR region. This implies either the existence of two different excited conformers (T-P)*, which decay to the ground state via a charge separated state (CSS), or the formation of the (T-P)* state via the second excited state of the porphyrin moiety, yielding first an excited emissive v(T-P)* state, with a lifetime of 80–100 ps.

Photochemical Behavior and Photolysis of Protonated Forms of Levofloxacin

The effect of intermolecular proton transfer on the spectral properties of levofloxacin in the ground and excited electronic states was studied. The preferred direction of possible protolytic reactions induced by UV irradiation in this compound was studied. It was found that the proton transfer processes have a considerable effect on the capability of the compound to emit light and occur on the nanosecond timescale. The photochemical reactions of the tree forms of levofloxacin (pH: 4.0, 7.0, 10.0) were studied by laser flash photolysis and product studies. Irradiation at pH 4 yielded a pulse and transient (λmax = 395, 515, 575 nm) assigned to the protonated triplet. Irradiation at pH 7 yielded a transient species (λmax = 525, 610 nm) assigned to the neutral form. Protonation of the anionic singlet excited state was also observed (λmax = 440, 570, 680 nm).

Vectorial photoinduced electron transfer in multicomponent film systems of poly(3-hexylthiophene), porphyrin–fullerene dyad, and perylenetetracarboxidiimide

Multistage electron transfer in a film system consisting of a hole-transporting layer (HTL), donor-acceptor pair (D-A), and an electron-transporting layer (ETL) was studied by photovoltage and flash-photolysis techniques. Poly(3-hexylthiophene) (PHT) was used as the HTL, while a symmetric porphyrin-fullerene dyad (P-F) and perylenetetracarboxidiimide (PTCDI) layers were functioning as the D-A pair and ETL, respectively. The photoexcitation of this three-component film system causes charge separations in the monomolecular P-F film, followed by electron transfer from the PHT polymer film and the fullerene anions to the porphyrin cations and the PTCDI layer, respectively. The final transient state is a charged PHT(+)|P-F|PTCDI(-) system, with significantly increased amplitude and lifetime of the photoelectrical signals compared to previously studied P-F|PTCDI and PHT|P-F systems, due to the its increased charge-separation distance. The study promotes the knowledge on the charge transfer mechanism in multilayered film systems.

Directed photocurrent in Langmuir-Schaefer organic molecular films

In the three-layer Langmuir-Schaefer films consisting of a monolayer of donor-acceptor phtalocyanine-fullerene dyad molecules (Bu3ProM2F) and two layers of poly(3-hexylthiophene) (PHT), placed into an electrochemical cell, the electric current resulted from optical excitation of sampleswas measured with no external voltage source. Samples were excited by the xenon lamp light passed through amonochromator or by a source simulating the solar spectrum. It was shown that the current density in samples reached 40–50 µA/cm2 in the latter case. Current-voltage characteristics of samples were measured. Photovoltaic responses of three-layer structures were studied by the time-resolved Maxwell displacement charge method. The preferred orientation of dyad molecules in the film was demonstrated.

Photochemical properties of porphyrin films covering surfaces of tapered optical fibers

We report the fabrication and characterization of tapered fibers covered with porphyrin monolayer films prepared by Langmuir-Blodgett (LB) deposition method. The studied molecule was 10 mol-% 5,10,15,20- tetrakis(pentafluorophenyl)porphyrin (PFP) entrapped in an octadecylamine (ODA) matrix. PFP molecules, deposited on plane glass surfaces, have relatively long fluorescence lifetime (~ 4 ns) together with high fluorescence efficiency. Therefore, such photoactive materials, example of which are PFP molecules, hold much promise for the development of chemical sensors and efficient light harvesting devices.