Electron transfer in oriented donor-acceptor dyads, intralayer charge migration, and formation of interlayer charge separated states in multi-layered Langmuir-Schäfer films.

Photoinduced intra- and interlayer electron transfer (ET) of doubly bridged donor-acceptor molecule, porphyrin-fullerene dyad (PF), was studied in single- and multi-layered Langmuir-Schäfer (LS) films and in LS films, where PF and an efficient electron donating polymer polyhexyltiophene (PHT) formed a bilayer PHT/PF and multi-layered PHT/PF structures. The ET through layers were investigated by a method, which measures the photovoltaic (PV) response proportional to the number of charge-separated (CS) states and to the CS distance between the electrons and holes formed in pulsed photo-excitation. Primary conclusions were, that ET starts as formations of CS dyads (P+F-) in single-layers, continues as long-range intra-layer charge migrations following interlayer CS between two adjacent monolayers. Quantitative conclusions were, that the interlayer ET efficiency is 100% in the bi-layered PF structure (2PF), where two CS dyads in adjacent layers forms CS complexes (P+F/PF-) and that the probability to form longer or higher order of CS complexes follows an expression of a convergent geometric series, with a converting factor of 2/3. In the PHT/PF bilayer structure the ET efficiency was one order of magnitude higher, than that for the 2PF structure due to the ET from the CS dyads to ground state electron donor PHT, with an acceptor density, much higher than that of (P+F-).

Structural photoactivation of a full-length 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.

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).

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.

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 ß-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 ß-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.

Effects of fullerene encapsulation on structure and photophysical properties of porphyrin-linked single-walled carbon nanotubes.

Fullerene-encapsulating single-walled carbon nanotubes (C(60)@SWNT) linked with porphyrins by a short bridge have been prepared for the first time. Steady state and time-resolved spectroscopies demonstrated the initial formation of an exciplex state, followed by a charge-separated state.

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.

Photoinduced electron transfer in thin films of porphyrin-fullerene dyad and perylenetetracarboxidiimide.

Photoinduced intra- and intermolecular electron transfer (ET) in thin films of porphyrin-fullerene dyad (P-F) and perylenetetracarboxidiimide (PTCDI) was studied by means of photoelectrical and spectroscopic methods. Films consisting of smooth 100 mol% layers of P-F and PTCDI were prepared by the Langmuir-Schäfer (LS) technique and thermal evaporation, respectively. The time-resolved Maxwell displacement charge (TRMDC) and laser flash-photolysis methods were utilized to demonstrate photoinduced ET from P-F to PTCDI regardless of which chromophore is photoexcited. Finally, the information about the electron movement in the respective thin films was used to build a layered organic solar cell, whose internal quantum yield (Φ(I)) of collected charges was 13%.

Langmuir-Schaeffer films from a pi-pi stacking perylenediimide dye: organization and charge transfer properties.

The organization of pi-pi stacking perylenediimide (PDI) derivative, PDI12, was studied in solution and in thin films. Films were prepared with the Langmuir-Schaeffer (LS) method and characterized by means of AFM, optical profilometry, steady-state absorption, emission, fluorescence lifetime, and transient photovoltage measurements. The columnar aggregates observed previously in PDI12 solutions and in spin-coated films persist also in LS films. Because of the specific conditions during the preparation of the LS film, i.e., hydrophobic interactions and lateral compression, the columnar aggregates seem to organize with their long axis perpendicular to the layer plane whereas in spin-coated films the columns were oriented parallel to the layer plane. According to AFM and profilometer results, the thickness of LS monolayer of PDI12 is 10 nm, indicating that it consists mainly of aggregates, each containing approximately 30 monomers. Intermolecular photoinduced energy and electron transfer processes in C(60)|PDI12 double layer junction were studied. The fluorescence lifetime of PDI12 film is exceptionally long, but the quenching is very efficient in the presence of C(60). In charge transfer studies, long-lived photovoltage signal was observed for the double layer. Results of this work indicate that PDI12 acts as an electron acceptor and fullerene C(60) as an electron donor.

Kinetics of photoinduced electron transfer in polythiophene-porphyrin-fullerene molecular films.

Photoinduced electron transfer (ET) processes were studied by the time-resolved Maxwell displacement charge (TRMDC) method in bilayer structures consisting of an electron donor-acceptor and conductive polymer monolayers, porphyrin-fullerene dyad and polyhexylthiophene, respectively, both layers prepared by the Langmuir-Blodgett (LB) method. The charge separation involves two fast steps: an intramolecular ET in the dyad molecule followed by an interlayer ET from the polymer to the formed porphyrin radical cation. These fast vertical intra- and interlayer processes could not be time-resolved by the TRMDC method. The lifetime of the charge separated state in the system was extended to hundreds of milliseconds by lateral electron and hole transfers in fullerene and polymer sublayers. The kinetics of the system was described by a model involving two long-living energetically different complete charge separated states. The data analysis indicates that the charge separation has a recombination time of 0.5 s. This is a promising result for possible applications.

Photoinduced electron transfer in self-assembled monolayers of porphyrin-fullerene dyads on ITO.

Two porphyrin-fullerene dyads were synthesized to form self-assembled monolayers (SAMs) on indium-tin oxide (ITO) electrode, with either ITO-porphyrin-fullerene or ITO-fullerene-porphyrin orientations. The dyads contain two linkers for connecting the porphyrin and fullerene moieties and enforcing them essentially to similar geometries of the donor-acceptor pair, and two linkers to ensure the attachment of the dyads to the ITO surface with two desired opposite orientations. The transient photovoltage responses (Maxwell displacement charge) were measured for the dyad films covered by insulating LB films, thus ensuring that the dyads interact only with the ITO electrode. The direction of the electron transfer was from the photoexcited dyad to ITO independent of the dyad orientation. The response amplitude for the ITO-fullerene-porphyrin structure, where the primary intramolecular electron-transfer direction coincides with the direction of the final electron transfer from the dyad to ITO, was 25 times stronger than that for the opposite ITO-porphyrin-fullerene orientation of the dyad. Static photocurrent measurements in a liquid electrochemical cell, however, show only a minor orientation effect, indicating that the photocurrent generation is controlled by the processes at the SAM-liquid interface.

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.