Intramolecular Electronic Energy Transfer Research Articles

Intramolecular interactions and optical properties of indole-containing alkaloids extracted from Arundo donax L.

Three optically active natural alkaloids were computationally investigated. The electronic ground state was found to be characterised by numerous intramolecular interactions, among which a hydrogen bond. These interactions are responsible for the different rate constants of the kinetics of conversion from one atropisomer to the other. The complex molecular electrostatic potential reveals a strong polar nature of these compounds, which strengthens when the electronic excited states are populated. The absorption spectra were interpreted and the intramolecular charge transfer nature of the Sn ← S0 transitions was revealed. The ECD spectra for the different atropisomers of these compounds indicate that the assignment proposed in the literature for 1 should be revised. The geometry of the excited state shows dramatic changes with respect to the ground state; some intramolecular interactions get lost. The fluorescence spectrum, proposed for the first time, shows de-excitation energies in the blue-green region (488 nm to 518 nm). Internal Conversion and intramolecular Electronic Energy Transfer may be fluorescence quenching channels.

Spectro-luminescent characterization of molecular complex based on tryptanthrin for design of new effective drugs

Optical absorption (at room temperature), fluorescence (at room temperature and T = 78 K) and phosphorescence (at T = 78 K) spectra of solution of the molecular complex based on tryptanthrin with platinum chloride and DMSO [Pt(Try)(DMSO)Cl2] (Try-Pt) was investigated. The positions of the first excited singlet (S1) and triplet (T1) energy levels were obtained. It was shown that in the excited state intramolecular singlet excitations energy transfer inside tryptanthrin π-electron system between two centers of optical absorption (that absorb equally) exists. It was found out the optical response of toxic influence of platinum chloride on tryptanthrin π-electron system inside this molecular complex as it is shown by us for a number of polynucleotides. Contrary to singlet energy transfer no intramolecular triplet electronic excitations energy transfer inside tryptanthrin π-electron system between both optical centers is observed. Taking into account the facts that such complexes can easy bind to telomeric G4 DNA the studied phenomena can be used for the design of new effective antitumor and anti-inflammatory drugs.

Vibrational Kinetics of NO and N2 in the Earth's Middle Atmosphere During GLE69 on January 20, 2005

AbstractThe mechanisms of the production of vibrationally excited NO and N2 molecules at the altitudes of the middle atmosphere of the Earth during high‐energetic proton precipitation on 20 January 2005 are considered. The study of vibrational populations N2(X1Σg+,v′ > 0) during high‐energetic proton precipitation has shown different principal mechanisms in the N2(X1Σg+,v′ > 0) excitation. First, the excitation by secondary electrons is principal for vibrational levels v′ = 1−10. Second, it is obtained that intramolecular electron energy transfer process in N2(A3Σu+)+N2 collisions dominates in vibrational excitation of high vibrational levels v′ = 20−30. It is shown that the chemical reaction of metastable atomic nitrogen with molecular oxygen is the main production mechanism of vibrationally excited NO(X2Π,v > 0) and of the radiation of 5.3 and 2.7 μm infrared emissions at these altitudes. The calculated intensities of the 5.3 μm emission are compared with experimental data from SABER instrument on TIMED spacecraft received at the time of the proton precipitation. The role of VV′‐processes in the radiation of 5.3 μm infrared emission is discussed.

The study of electronic kinetics of molecular nitrogen in the Titan’s middle atmosphere during the precipitation of cosmic rays

The kinetics of the \({{{\text{A}}}^{3}}\Sigma _{u}^{ + },\) B3Πg, W3Δu, \({{{\text{B}}}^{{{\text{'}}3}}}\Sigma _{u}^{ - },\) and C3Πu triplet states of molecular nitrogen at the heights of the middle atmosphere of Titan during action of cosmic rays into the atmosphere has been studied. The calculations consider the intramolecular and intermolecular electron energy transfer during inelastic collisions of electronically excited molecular nitrogen with N2, CH4, and CO molecules. The interaction of electronically excited N2 molecules with molecules of acetylene C2H2 and ethylene C2H4 in the middle atmosphere of Titan at altitudes of 50–250 km has been studied. For the first time, the dominance of reactions with metastable molecular nitrogen N2(\({{{\text{A}}}^{3}}\Sigma _{u}^{ + }\)) in the formation of C2H and C2H3 radicals at these heights has been shown.

Tuning Emission Lifetimes of Ir(C^N)2(acac) Complexes with Oligo(phenyleneethynylene) Groups

Emissive compounds with long emission lifetimes (μs to ms) in the visible region are of interest for a range of applications, from oxygen sensing to cellular imaging. The emission behavior of Ir(ppy)2(acac) complexes (where ppy is the 2-phenylpyridyl chelate and acac is the acetylacetonate chelate) with an oligo(para-phenyleneethynylene) (OPE3) motif containing three para-rings and two ethynyl bridges attached to acac or ppy is examined here due to the accessibility of the long-lived OPE3 triplet states. Nine Ir(ppy)2(acac) complexes with OPE3 units are synthesized where the OPE3 motif is at the acac moiety (aOPE3), incorporated in the ppy chelate (pOPE3) or attached to ppy via a durylene link (dOPE3). The aOPE3 and dOPE3 complexes contain OPE3 units that are decoupled from the Ir(ppy)2(acac) core by adopting perpendicular ring-ring orientations, whereas the pOPE3 complexes have OPE3 integrated into the ppy ligand to maximize electronic coupling with the Ir(ppy)2(acac) core. While the conjugated pOPE3 complexes show emission lifetimes of 0.69-32.8 μs similar to the lifetimes of 1.00-23.1 μs for the non-OPE3 Ir(ppy)2(acac) complexes synthesized here, the decoupled aOPE3 and dOPE3 complexes reveal long emission lifetimes of 50-625 μs. The long lifetimes found in aOPE3 and dOPE3 complexes are due to intramolecular reversible electronic energy transfer (REET) where the long-lived triplet-state metal to ligand charge transfer (3MLCT) states exchange via REET with the even longer-lived triplet-state localized OPE3 states. The proposed REET process is supported by changes observed in excitation wavelength-dependent and time-dependent emission spectra from aOPE3 and dOPE3 complexes, whereas emission spectra from pOPE3 complexes remain independent of the excitation wavelength and time due to the well-established 3MLCT states of many Ir(ppy)2(acac) complexes. The long lifetimes, visible emission maxima (524-526 nm), and photoluminescent quantum yields of 0.44-0.60 for the dOPE3 complexes indicate the possibility of utilizing such compounds in oxygen-sensing and cellular imaging applications.

Novel axially substituted lanthanum phthalocyanines: Synthesis, photophysical and nonlinear optical properties

Materials with excellent nonlinear optical (NLO) properties are usually applied in optoelectronics and optical limiting devices. Photoinduced intramolecular electron transfer (PET) and energy transfer (ET) play an important role in enhanced NLO properties. Through axially connected anthraquinone to phthalocyanine, two types of novel lanthanum phthalocyanine (LaPc-Aqn and HLaPc-Aqn) were designed to effectively improve PET/ET process, and thus leading to excellent NLO properties. Hyperbranched phthalocyanines possess enhanced NLO properties owing to their special three-dimensional structures that have more donor–acceptor (D–A) systems in their cell structure than lanthanum phthalocyanines. The uniform polyphenylsulfone composite film(HLaPc-Aqn-PPSU) was prepared by a simple solution casting method according to the “like dissolves like” theory. The values of large third-order nonlinear susceptibility (Im[χ(3)]) and low limit threshold (Ilim) were 1.51 × 10−9 esu and 0.015 J/cm2, respectively, demonstrating that the film had great optical limiting performance. Such research can contribute to the development of novel phthalocyanine materials for nonlinear optics and optical limiting devices.

Ultrafast Non-Förster Intramolecular Donor-Acceptor Excitation Energy Transfer.

Ultrafast intramolecular electronic energy transfer in a conjugated donor-acceptor system is simulated using nonadiabatic excited-state molecular dynamics. After initial site-selective photoexcitation of the donor, transition density localization is monitored throughout the S2→S1 internal conversion process, revealing an efficient unidirectional donor→acceptor energy-transfer process. Detailed analysis of the excited-state trajectories uncovers several salient features of the energy-transfer dynamics. While a weak temperature dependence is observed during the entire electronic energy relaxation, an ultrafast initially temperature-independent process allows the molecular system to approach the S2-S1 potential energy crossing seam within the first ten femtoseconds. Efficient energy transfer occurs in the absence of spectral overlap between the donor and acceptor units and is assisted by a transient delocalization phenomenon of the excited-state wave function acquiring Frenkel-exciton character at the moment of quantum transition.

Homo- and Heterobimetallic Ruthenium(II) and Osmium(II) Complexes Based on a Pyrene-Biimidazolate Spacer as Efficient DNA-Binding Probes in the Near-Infrared Domain.

We report in this work a new family of homo- and heterobimetallic complexes of the type [(bpy)2M(Py-Biimz)M'(II)(bpy)2](2+) (M = M' = Ru(II) or Os(II); M = Ru(II) and M' = Os(II)) derived from a pyrenyl-biimidazole-based bridge, 2-imidazolylpyreno[4,5-d]imidazole (Py-BiimzH2). The homobimetallic Ru(II) and Os(II) complexes were found to crystallize in monoclinic form with space group P21/n. All the complexes exhibit strong absorptions throughout the entire UV-vis region and also exhibit luminescence at room temperature. For osmium-containing complexes (2 and 3) both the absorption and emission band stretched up to the NIR region and thus afford more biofriendly conditions for probable applications in infrared imaging and phototherapeutic studies. Detailed luminescence studies indicate that the emission originates from the respective (3)MLCT excited state mainly centered in the [M(bpy)2](2+) moiety of the complexes and is only slightly affected by the pyrene moiety. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations in the positive potential window and several reduction processes in the negative potential window. An efficient intramolecular electronic energy transfer is found to occur from the Ru center to the Os-based component in the heterometallic dyad. The binding studies of the complexes with DNA were thoroughly studied through different spectroscopic techniques such as UV-vis absorption, steady-state and time-resolved emission, circular dichroism, and relative DNA binding study using ethidium bromide. The intercalative mode of binding was suggested to be operative in all cases. Finally, computational studies employing DFT and TD-DFT were also carried out to interpret the experimentally observed absorption and emission bands of the complexes.

Stepwise photoconversion of an artificial light-harvesting array built from extended BODIPY units.

A molecular dyad, comprising two disparate extended boron dipyrromethene (BODIPY) units, has been identified as a potential component of artificial light-harvesting arrays. Highly efficient, intramolecular electronic energy transfer takes place under illumination but there is some competition from light-induced electron transfer along the molecular axis. The primary energy acceptor has a somewhat shortened excited-state lifetime and reduced emission quantum yield due to charge transfer from a terminal amine residue, the latter being required for the molecular system to operate in organic solar cells. Under continuous illumination with simulated solar light, the dyad undergoes very slow decomposition. In a protic solvent, both BODIPY units degrade at the same rate via an autocatalytic process. The products, one of which is a protonated analogue of the donor, degrade further by independent routes. In aprotic solvents or thin plastic films, the acceptor BODIPY dye absorbing at lowest energy undergoes photochemical degradation as above but the donor is much more stable under these conditions. At each stage of degradation, the molecule retains the ability to sensitize an amorphous silicon solar cell and the overall turnover number with respect to absorbed photons exceeds 10 million. The optical properties of the target compound nicely complement those of the solar cell and sensitization helps to avoid Staebler-Wronski photo-degradation.

Photochemical Bleaching of an Elaborate Artificial Light-Harvesting Antenna.

The target artificial light-harvesting antenna, comprising 21 discrete chromophores arranged in a logical order, undergoes photochemical bleaching when dispersed in a thin plastic film. The lowest-energy component, which has an absorption maximum at 660 nm, bleaches through first-order kinetics at a relatively fast rate. The other components bleach more slowly, in part, because their excited-state lifetimes are rendered relatively short by virtue of fast intramolecular electronic energy transfer to the terminal acceptor. Two of the dyes, these being close to the terminal acceptor but interconnected through a reversible energy-transfer step, bleach by way of an autocatalytic step. Loss of the terminal acceptor, thereby switching off the energy-transfer route, escalates the rate of bleaching of these ancillary dyes. The opposite terminal, formed by a series of eight pyrene-based chromophores, does not bleach to any significant degree. Confirmation of the various bleaching steps is obtained by examination of an antenna lacking the terminal acceptor, where the autocatalytic route does not exist and bleaching is very slow.

Cation-induced switching on–off luminescence in an imidazole 4,5-dicarboxylate-bridged RuIIOsII bipyridine complex: A combined experimental and DFT/TD-DFT investigation

A heterobimetallic complex of the composition [(bpy)2Ru(Imdc)Os(bpy)2](ClO4), where Imdc3−=imidazole-4,5-dicarboxylate(3−) ion and bpy=2,2′-bipyridine, was synthesized and characterized using standard analytical and spectroscopic techniques, including X-ray crystallography. Steady state and time resolved luminescence data at room temperature showed that an efficient intramolecular electronic energy transfer takes place from the Ru center to the Os based component in the dyad. The cation binding properties of the complex were thoroughly investigated in solution using UV–Vis absorption, steady state and time-resolved emission spectroscopic techniques. It is interesting to note that the luminescence intensity, quantum yield as well as the lifetime of the complex were enhanced substantially in the presence of selective cations, showing cation-induced molecular switching behavior. The effect of different cations on the rate of photoinduced intramolecular energy transfer in the dyad was also explored. Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies were utilized to investigate the structural and electronic properties of the dyad. The optimized geometrical parameters for the dyad, computed both in the gas phase and in solution, are reported and compared with the single crystal X-ray data. Computational (TD-DFT) studies also provide insight into the nature of the ground and excited states, with detailed assignments of the orbitals involved in the absorption and emission transitions. In particular, the blue-shifts of the absorption and emission bands in the presence of selective cations are reproduced by our calculations.

Photoinduced intramolecular energy transfer and anion sensing studies of isomeric RuIIOsIIcomplexes derived from an asymmetric phenanthroline–terpyridine bridge

Two heterobimetallic Ru(II)-Os(II) complexes of compositions [(bpy)2M(II)(phen-Hbzim-tpy)M'(II)(tpy-PhCH3)](4+), where M(II) = Ru and M'(II) = Os (4) and M(II) = Os and M'(II) = Ru (5), phen-Hbzim-tpy = 2-(4-(2,6-di(pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5][1,10]phenanthroline, bpy = 2,2'-bipyridine, and tpy-PhCH3 = 4'-(4-methylphenyl)-2,2':6',2''-terpyridine have been synthesized and characterized by elemental analyses, ESI mass spectrometry, and (1)H NMR and UV-vis absorption spectroscopy. The absorption spectra, redox behavior, and luminescence properties of the complexes have been thoroughly investigated and compared with that of monometallic model compounds [(bpy)2M(II)(phen-Hbzim-tpy)](2+) [M(II) = Ru (1) and M(II) = Os (2)] and [(phen-Hbzim-tpy)Ru(II)(tpy-PhCH3)](2+) (3). The complexes display very intense, ligand-centered absorption bands in the UV and moderately intense MLCT bands in the visible regions. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations, whereas the monometallic complexes display one-electron oxidation in the positive potential window. Steady state and time-resolved luminescence data at room temperature show that an efficient intramolecular electronic energy transfer takes place from the Ru-center to the Os-based component in both the heterometallic dyads in all the solvents. The complexes under investigation contain an imidazole NH proton which became appreciably acidic due to metal coordination and can be utilized for recognition of selective anions in solution either via hydrogen bonding interaction or by proton transfer. Accordingly, the anion binding properties of the two heterobimetallic complexes as well as parent bridging ligand, phen-Hbzim-tpy, have been studied in solutions using absorption, steady state and time-resolved luminescence spectral measurements. The metalloreceptors act as sensors for F(-), CN(-) and AcO(-) ions. It is evident from sensing studies that in the presence of excess of selective anions, deprotonation of the imidazole N-H proton occurs in all cases.

Photoinduced Electron and Energy Transfer and pH-Induced Modulation of the Photophysical Properties in Homo- and Heterobimetallic Complexes of Ruthenium(II) and Rhodium(III) Based on a Heteroditopic Phenanthroline–Terpyridine Bridge

Homo- and heterobimetallic complexes of compositions [(bpy)2Ru(II)(phen-Hbzim-tpy)Ru(II)(tpy/tpy-PhCH3/H2pbbzim)](4+) and [(bpy)2Ru(II)(phen-Hbzim-tpy)Rh(III)(tpy-PhCH3/H2pbbzim)](5+), where phen-Hbzim-tpy = 2-[4-(2,6-dipyridin-2-ylpyridin-4-yl)phenyl]-1H-imidazole[4,5-f][1,10]phenanthroline, bpy = 2,2'-bipyridine, tpy = 2,2':6',2"-terpyridine, tpy-PhCH3 = 4'-(4-methylphenyl)-2,2':6',2"-terpyridine, and H2pbbzim = 2,6-bis(benzimidazol-2-yl)pyridine, have been synthesized and characterized by elemental analyses, electrospray ionization mass spectrometry, and (1)H NMR spectroscopy. The absorption spectra, redox behavior, and luminescence properties of these bimetallic complexes have been thoroughly investigated and compared with those of monometallic [(bpy)2Ru(II)(phen-Hbzim-tpy)](2+) and [(tpy-PhCH3)Rh(III)(tpy-Hbzim-phen)](3+) model compounds. The electrochemistry of the complexes shows a reversible Ru(II/III) oxidation in the anodic region and an irreversible Rh(III/I) reduction and several ligand-based reductions in the cathodic region. Steady-state and time-resolved luminescence data at room temperature show that an efficient intramolecular electronic energy transfer from the metal-to-ligand charge-transfer (MLCT) excited state of the [(bpy)2Ru(II)(phen-Hbzim-tpy)] chromophore to the MLCT state of the tpy-containing chromophore [(phen-Hbzim-tpy)Ru(II)(tpy/tpy-PhCH3/H2pbbzim)] occurs in all three unsymmetrical homobimetallic complexes. On the other hand, for both heterometallic dyads, an efficient intramolecular photoinduced electron transfer from the excited ruthenium moiety to the rhodium-based unit takes place. The rate constants for the energy- and electron-transfer processes have been determined by time-resolved emission spectroscopy. The influence of the pH on the absorption, steady-state, and time-resolved emission properties of complexes has been thoroughly investigated. The absorption titration data were used to determine the ground-state pK values, whereas the luminescence data were utilized for determination of the excited-state acid dissociation constants. In effect, deprotonation of the azole NH moieties of the complexes leads to a substantial lowering of the MLCT absorption and emission band energies.

Nanomechanical properties of molecular-scale bridges as visualised by intramolecular electronic energy transfer

A series of molecular dyads has been synthesized and fully characterised. These linear, donor–spacer–acceptor compounds comprise terminal dyes selected to exhibit intramolecular electronic energy transfer (EET) along the molecular axis. The spacer is built by accretion of ethynylene–carborane units that give centre-to-centre separation distances of 38, 57, 76, 96, and 115 A respectively along the series. The probability of one-way EET between terminals depends on the length of the spacer but also on temperature and applied pressure. Throughout the series, the derived EET parameters are well explained in terms of through-space interactions but the probability of EET is higher than predicted for the fully extended conformation except in a glassy matrix at low temperature. The implication is that these spacers contract under ambient conditions, with the extent of longitudinal contraction increasing under pressure but decreasing as the temperature is lowered. Longer bridges are more susceptible to such distortion, which is considered to resemble a concertina effect caused by out-of-plane bending of individual subunits. The dynamics of EET can be used to estimate the strain energy associated with molecular contraction, the amount of work done in effecting the structural change and the Young's modulus for the bridge.

Synthesis and Photoluminescence of the Derivatives of Symmetrical Bi-oxadiazoles Containing Azobenzene

A series of derivatives of symmetrircal bi-oxadiazoles containing azobenzene was synthesized.Their molecular structures were determined by IR and 1H NMR spectroscopies.The effect of azobenzene on the photoluminescence of oxadiazole derivatives was carefully studied by fluorescence spectra.The emission peaks of F3,F6 and F10 are bimodal,at 348,356 nm and 345,357 nm and 345,357 nm,respectively,attributed to the oxadiazole groups of bi-oxadiazoles.The introduction of azobenzene has less impact on the emission maximum peak of oxadiazole derivatives Fn in solution,but the luminous intensity of oxadiazole derivatives Fn decreases.This is mainly because of the intramolecular electronic energy transfer between the azobenzene groups and the oxadiazole groups in the process of photoexcitation.

Influence of electronically excited N2 and O2 on vibrational kinetics of these molecules in the lower thermosphere and mesosphere during auroral electron precipitation

The rate coefficients of vibrational excitation of ground-state molecules N2(X1Σg+) and O2(X3Σg−) in processes of the quenching of electronically excited molecules N2(A3Σu+, B3Πg, W3Δu, В′3Σu−, a′1Σu−, a1Πg, w1Δu) and O2(a1Δg, b1Σg+, c1Σu−, A′3Δu, A3Σu+) are calculated according to quantum-chemical approximations. Applying the calculated rate coefficients we have studied the role of inelastic intermolecular and intramolecular electron energy transfer processes in the production of vibrationally excited molecules at altitudes of the lower thermosphere and mesosphere during auroral electron precipitation. The study of vibrational populations (v′=2–30) of the X1Σg+ state of N2 at the altitudes of the lower thermosphere and mesosphere during auroral electron precipitation has shown three principal excitation mechanisms: radiational transitions from the A3Σu+ and a1Πg states for vibrational levels v′>10, intramolecular electron energy transfer process in N2(A3Σu+,v=0–5)+N2 collisions for vibrational levels v′=25–30 and the excitation by auroral electrons. Calculations have shown that transitions from two singlet a1Δg, b1Σg+, and three Herzberg c1Σu−, A′3Δu, A3Σu+ electronically excited states of molecular oxygen through radiational and collisional quenching are important in vibrational excitation of ground-state molecule O2(X3Σg−,v′=2–37).

Resolving the contribution due to Förster-type intramolecular electronic energy transfer in closely coupled molecular dyads

This work examines the electronic energy-transfer (EET) processes inherent to a molecular dyad in which aryl polycycles attached to a boron dipyrromethene (Bodipy) dye act as ancillary light harvesters for near-UV photons. The solvent, being methyltetrahydrofuran, is compressed under applied pressure to such an extent that, over the accessible pressure range, there is a 25% decrease in molar volume. This effect serves to increase the effective concentration of the solute and increases fluorescence from Bodipy when this chromophore is excited directly. Illumination into the aryl polycycles, namely pyrene and perylene derivatives, leads to rapid intramolecular EET to Bodipy but fluorescence from these units is partially restored under high pressure. The argument is made that applied pressure restricts torsional motions around the linkages and imposes a near orthogonal geometry for transition dipole moment vectors on the reactants. In turn, this pressure-induced conformational restriction switches off Forster-type EET within the system, leaving the electron-exchange contribution. For the target dyad, the Forster component is ca. 5% for pyrene and ca. 25% for perylene. Such contributions are not inconsistent with calculations made on the basis of Forster theory but modelling is rendered difficult by the absence of accurate information about the nature of the conformational motion. Two possibilities have been considered. In the first case, the appendages remain stiff but pressure reduces the extent of displacement from the lowest-energy position. The results can be accounted for in a quantitative sense on the basis of small deviations from the lowest-energy conformation; the actual amount of displacement needed to explain the pressure effect depends on the method used to compute the Forster rates and ranges from ca. 4° for the ideal dipole approximation to only 0.5° for the extended dipole method. Secondly, pressure is assumed to bend each appendage into a banana-like shape. Again, the full effect of applied pressure can be accounted for by way of minor curvature of the linkage.

Photoinduced Processes in Some Mechanically Interlocked Supramolecules as Studied by Time-Resolved Electron Paramagnetic Resonance

Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used for studying photoinduced intramolecular electron transfer (ET) and energy transfer (EnT) processes in three mechanically interlocked molecules in which zinc(II)porphyrin (ZnP) and C60 fullerene moieties are arrayed around a central Cu(I) bisphenanthroline core used to assemble these donor–acceptor (D–A) systems. The specific molecules studied include a “long” (ZnP)2–Cu(I)(phen)2–C60 rotaxane as well as ZnP–Cu(I)(phen)2 and ZnP–Cu(I)(phen)2–C60 catenanes, embedded in different phases of nematic liquid crystal and frozen isotropic solvents. It was demonstrated that the routes and rates of the transfer processes in these supramolecules strongly depend on the characteristics of their microenvironment and the molecular entity which was selectively photoexcited. This is reflected by formation of distinct long-lived charge-separated species such as the radical ion pair (ZnP)2•+–Cu(I)(phen)2•– and the metal-to-ligand charge-separated state Cu(II)–(phen)2•– under the various experimental conditions. The results are discussed in terms of the correlation between the chemical structure, conformational mobility, and relaxation pathways of the photoexcited states in these mechanically interlocked systems. Results are compared with previously reported TREPR data on related interlocked D–A porphyrin/fullerene systems.

Electronic kinetics of molecular oxygen in the nightglow

Altitude profiles of O2 nightglow emission intensities give information about composition of the atmosphere. Remote measurements of intensities of the emissions from satellites or rockets allow estimation of concentrations of atomic oxygen at the altitudes of the nightglow. Peculiarities of O2 electronic-state kinetics in the nightglow of the atmosphere are studied here. A model of the kinetics based on calculated quenching-rate coefficients for Herzberg states of O2 is suggested. The model considers intramolecular and intermolecular electron-energy transfer processes in molecular collisions as production and loss mechanisms of Herzberg states. The quenching-rate coefficients are applied in the calculation of vibrational populations of O2(A3Σu +), O2(A′3Δu) and O2(c1Σu −) in the nightglow. The function of the distribution of electronically excited molecular oxygen after the process of three-body recombination is estimated using experimental data and a least-squares fitting program. Calculated vibrational populations of A3Σu + and A′3Δu Herzberg states at an altitude of 95 km are compared with results of experimental measurements.

Photoexcited State Properties of H2-Porphyrin/C60-Based Rotaxanes as Studied by Time-Resolved Electron Paramagnetic Resonance Spectroscopy

Light-driven intramolecular electron transfer (ET) and energy transfer (EnT) processes in two rotaxanes, the first containing two free base porphyrins and C(60) fullerene moieties incorporated around a Cu(I)bisphenanthroline core ((H(2)P)(2)-Cu(I)(phen)(2)-C(60)) and a second lacking the fullerene moiety ((H(2)P)(2)-Cu(I)(phen)(2)), were studied by X-band (9.5 GHz) time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The experiments were performed in frozen toluene and ethanol and different phases of the nematic liquid crystal (E-7). It is demonstrated that the ET and EnT processes in the (H(2)P)(2)-Cu(I)(phen)(2)-C(60) rotaxane in different media result in the formation of the same charge-separated state, namely (H(2)P)(2)(•+)-Cu(I)(phen)(2)(•-)-C(60), while photoexcitation of the (H(2)P)(2)-Cu(I)(phen)(2) rotaxane does not induce noticeable transfer processes in these matrices. The results are discussed in terms of the high conformational mobility of the rotaxanes, which enables changes in the molecular topography and resultant modification of the rates and routes of photoinduced processes occurring in these systems. The parameters of the transfer processes are compared with those obtained in our previous study of (ZnP)(2)-Cu(I)(phen)(2)-C(60) and (ZnP)(2)-Cu(I)(phen)(2) rotaxanes under the same experimental conditions.