Photochemical Events Research Articles

ChemInform Abstract: Excitation of Nucleobases from a Computational Perspective I: Reaction Paths

AbstractReview: 8163 refs.].

Temperature Independence of Ultrafast Photoisomerization in Thermophilic Rhodopsin: Assessment versus Other Microbial Proton Pumps.

Primary photochemical events in the unusually thermostable proton pumping rhodopsin of Thermus thermophilus bacterium (TR) are reported for the first time. Internal conversion in this protein is shown to be significantly faster than in bacteriorhodopsin (BR), making it the most rapidly isomerizing microbial proton pump known. Internal conversion (IC) dynamics of TR and BR were recorded from room temperature to the verge of thermal denaturation at 70 °C and found to be totally independent of temperature in this range. This included the well documented multiexponential nature of IC in BR, suggesting that assignment of this to ground state structural inhomogeneity needs revision. TR photodynamics were also compared with that of the phylogenetically more similar proton pump Gloeobacter rhodopsin (GR). Despite this similarity GR has poor thermal stability, and the excited state decays significantly more slowly and exhibits very prominent stretched exponential behavior. Coherent torsional wave-packets induced by impulsive photoexcitation of TR and GR show marked resemblance to each other in frequency and amplitude and differ strikingly from similar signatures in pump-probe data of BR and other microbial retinal proteins. Possible correlations between IC rates and thermal stability and the promise of using torsional coherence signatures for understanding chromophore protein binding in microbial retinal proteins are discussed.

ChemInform Abstract: Photochemical Synthesis of Azaheterocycles

AbstractReview: [66 refs.

Monitoring Nonadiabatic Electron-Nuclear Dynamics in Molecules by Attosecond Streaking of Photoelectrons.

Streaking of photoelectrons has long been used for the temporal characterization of attosecond extreme ultraviolet pulses. When the time-resolved photoelectrons originate from a coherent superposition of electronic states, they carry additional phase information, which can be retrieved by the streaking technique. In this contribution we extend the streaking formalism to include coupled electron and nuclear dynamics in molecules as well as initial coherences. We demonstrate how streaked photoelectrons offer a novel tool for monitoring nonadiabatic dynamics as it occurs in the vicinity of conical intersections and avoided crossings. Streaking can provide high time resolution direct signatures of electronic coherences, which affect many primary photochemical and biological events.

Photochemical Synthesis of Azaheterocycles

Synthetic organic photochemistry is a field that continues to offer unique routes to organic structures not typically accessed from thermal reactions. In this review, the photochemical synthesis of five-, six-, seven-, and eight-membered-ring azaheterocycles, as well as some aza-macrocycles, is discussed. The discussion will focus on methods where heterocycle formation occurs during or as a result of a photochemical event. Reaction mechanisms will be discussed also, as these provide a rationale for understanding the synthetic results. 1 Introduction 2 Five-Membered Rings 2.1 Cyclization Reactions 2.2 Cycloadditions 2.3 From Aromatic and Vinyl Azides 2.4 earrangements 2.5 Other Individual Methods 3 Six-Membered Rings 3.1 Cyclization Reactions 3.2 Isocyanide Insertions 3.3 Cycloadditions 4 Seven-Membered Rings and Larger 5 Conclusion

(Invited) Long-Lived Charge Separated States Exclusively from the Triplet Excited States in Palladium Porphyrin-Fullerene Conjugates

Molecular engineering of donor-acceptor conjugates to control their photoinduced processes to yield the much desired long-lived charge separated states has been one of the major goals in artificial photosynthesis. The charge separated states of sufficient lifetime and stored energy are key in ultimately driving the photocatalytic processes of either light-to-electricity conversion or light-to-fuel production. In simple donor-acceptor dyads, charge separation from the singlet excited donor or acceptor due to rapid charge recombination process results in a short-lived charge separated state of singlet spin character. Utilization of high energy triplet sensitizers instead of the traditionally employed singlet sensitizers could be an elegant approach to generate long-lived charge separated states. The radical ion-pairs of triplet spin character charge recombine to the ground state slowly due to spin forbidden nature of the process. However, witnessing such a process purely from the triplet excited state without the interference of the singlet excited state and other competing photochemical events such as excitation energy transfer has been challenging to establish. Here, we report our strategy to observe electron transfer exclusively from the triplet excited state of the donor in PdP-C60 dyads (see figure below). Using transient absorption spectral studies operating at different timescales and time-resolved electron paramagnetic resonance (TR-EPR) studies, photoinduced electron transfer from 1PdP* to C60 leading to the formation of PdP•+-C60 •- charge separated state of singlet spin character has been witnessed in 2 (instead of triplet-triplet energy transfer observed in 1 1) due to the close proximity of the donor and acceptor entities. In contrast, by lowering the energy of the [(TPA)3PdP]•+-C60 •- state owing to the presence of electron rich TPA entities (facile oxidation) compared to that of 3C60*, electron transfer from 3(TPA)3PdP* to C60 leading to [(TPA)3PdP]•+-C60 •- charge separated state of triplet spin character with lifetime in the microsecond range has been observed in 3 in polar solvents. Results of these findings and its significance will be highlighted. C. Obondi, G. N. Lim, F. D’Souza, Triplet-Triplet Excitation Transfer in Palladium Porphyrin – Fullerene and Platinum Porphyrin – Fullerene Dyads’ J. Phys. Chem. C 2015, 119, 176-185. Figure 1

Engaging Copper(III) Corrole as an Electron Acceptor: Photoinduced Charge Separation in Zinc Porphyrin-Copper Corrole Donor-Acceptor Conjugates.

An electron-deficient copper(III) corrole was utilized for the construction of donor-acceptor conjugates with zinc(II) porphyrin (ZnP) as a singlet excited state electron donor, and the occurrence of photoinduced charge separation was demonstrated by using transient pump-probe spectroscopic techniques. In these conjugates, the number of copper corrole units was varied from 1 to 2 or 4 units while maintaining a single ZnP entity to observe the effect of corrole multiplicity in facilitating the charge-separation process. The conjugates and control compounds were electrochemically and spectroelectrochemically characterized. Computational studies revealed ground state geometries of the compounds and the electron-deficient nature of the copper(III) corrole. An energy level diagram was established to predict the photochemical events by using optical, emission, electrochemical, and computational data. The occurrence of charge separation from singlet excited zinc porphyrin and charge recombination to yield directly the ground state species were evident from the diagram. Femtosecond transient absorption spectroscopy studies provided spectral evidence of charge separation in the form of the zinc porphyrin radical cation and copper(II) corrole species as products. Rates of charge separation in the conjugates were found to be of the order of 10(10) s(-1) and increased with increasing multiplicity of copper(III) corrole entities. The present study demonstrates the importance of copper(III) corrole as an electron acceptor in building model photosynthetic systems.

Influence of selenium in drought-stressed wheat plants under greenhouse and field conditions

Effects of selenium (Na2SeO4) was studied in two wheat genotypes under well-watered and drought conditions in greenhouse (15 µg Se L-1) and field (20-60 60 g ha-1) experiments. Application of Se improved dry matter and grain yield under both well-watered and drought conditions. Se increased leaf concentration of pigments and photosynthesis rate under both well-watered and drought conditions. Our results indicated that Se alleviates drought stress via increased photosynthesis rate, protection of leaf photochemical events, accumulation of organic osmolytes and improvement of water use efficiency. Under well-watered condition, Se-mediated growth improvement was associated with higher photosynthesis rate and water use efficiency, greater root length and diameter, and higher leaf water content.

Estimation of the driving force for dioxygen formation in photosynthesis

Photosynthetic water oxidation to molecular oxygen is carried out by photosystem II (PSII) over a reaction cycle involving four photochemical steps that drive the oxygen-evolving complex through five redox states Si (i=0,…, 4). For understanding the catalytic strategy of biological water oxidation it is important to elucidate the energetic landscape of PSII and in particular that of the final S4→S0 transition. In this short-lived chemical step the four oxidizing equivalents accumulated in the preceding photochemical events are used up to form molecular oxygen, two protons are released and at least one substrate water molecule binds to the Mn4CaO5 cluster. In this study we probed the probability to form S4 from S0 and O2 by incubating YD-less PSII in the S0 state for 2–3days in the presence of 18O2 and H216O. The absence of any measurable 16,18O2 formation by water-exchange in the S4 state suggests that the S4 state is hardly ever populated. On the basis of a detailed analysis we determined that the equilibrium constant K of the S4→S0 transition is larger than 1.0×107 so that this step is highly exergonic. We argue that this finding is consistent with current knowledge of the energetics of the S0 to S4 reactions, and that the high exergonicity is required for the kinetic efficiency of PSII.

Rethinking the Concepts of Fluence (UV Dose) and Fluence Rate: The Importance of Photon-based Units - A Systemic Review.

After a critical review of the fundamental equations describing photobiological and photochemical processes occurring in a medium exposed to a quasi-collimated monochromatic UV light beam, the analysis in this review is extended to analogous processes driven by polychromatic UV light, such as that emitted by medium pressure mercury-vapor arc lamps. The analysis is based on the Second Law of Photochemistry, namely that all photochemical events must be independent, and the rate of such events must be proportional to the rate of photon absorption. A consistent application of the Second Law of Photochemistry leads to a concept change; hence it is proposed herein to use photon fluence and photon fluence rate, rather than fluence (UV dose) and fluence rate, respectively, in the analysis and interpretation of photobiological and photochemical processes. As a consequence, many equations that have been used in the past must be revised, and some experimental information (e.g. action spectra) needs to be re-analyzed.

Ultrafast Photoinduced Charge Separation in Wide-Band-Capturing Self-Assembled Supramolecular Bis(donor styryl)BODIPY-Fullerene Conjugates.

A new series of self-assembled supramolecular donor-acceptor conjugates capable of wide-band capture, and exhibiting photoinduced charge separation have been designed, synthesized and characterized using various techniques as artificial photosynthetic mimics. The donor host systems comprise of a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) containing a crown ether entity at the meso-position and two styryl entities on the pyrrole rings. The styryl end groups also carried additional donor (triphenylamine or phenothiazine) entities. The acceptor host system was a fulleropyrrolidine comprised of an ethylammonium cation. Owing to the presence of extended conjugation and multiple chromophore entities, the BODIPY host revealed absorbance and emission well into the near-IR region covering the 300-850 nm spectral range. The donor-acceptor conjugates formed by crown ether-alkyl ammonium cation binding of the host-guest system was characterized by optical absorbance and emission, computational, and electrochemical techniques. Experimentally determined binding constants were in the range of 1-2×10(5) M(-1) . An energy-level diagram to visualize different photochemical events was established using redox, computational, absorbance, and emission data. Spectral evidence for the occurrence of photoinduced charge separation in these conjugates was established from femtosecond transient absorption studies. The measured rates indicated ultrafast charge separation and relatively slow charge recombination revealing their usefulness in light-energy harvesting and optoelectronic device applications. The bis(donor styryl)BODIPY-derived conjugates populated their triplet excited states during charge recombination.

Effect of Blue Light on the Electronic and Structural Properties of Bilirubin Isomers: Insights into the Photoisomerization and Photooxidation Processes.

The central process of neonatal phototherapy by employing blue light has been attributed to the configurational conversion of (4Z,15Z)-bilirubin to (4Z,15E). Indeed, photoisomerization is the early photochemical event during this procedure. However, in this paper, we show that the bilirubin solutions under continuous blue light exposure undergo a photooxidation process. To ascertain the role of this photodegradation in the phototherapy, we evaluated UV–visible absorption spectra obtained from bilirubin solutions in CHCl3, milli-Q water, and physiological saline, as well as FTIR spectroscopy for bilirubin in CHCl3. These analyses also showed that the first 2 h of phototherapy are the most relevant period. In addition, quantum molecular modeling using B3LYP/6-31G(d,p) and ZINDO/S-CIS was performed to evaluate the electronic and structural properties of four bilirubin isomers, showing that the (4Z,15E)-bilirubin isomer is the most polar configuration. Therefore, it can be more soluble in aqueous environments than the other configurations. This clarifies why this is the faster isomer excreted during the phototherapy.

Directly Connected AzaBODIPY-BODIPY Dyad: Synthesis, Crystal Structure, and Ground- and Excited-State Interactions.

Directly connected, strongly interacting sensitizer donor-acceptor dyads mimic light-induced photochemical events of photosynthesis. Here, we devised a dyad composed of BF2-chelated dipyrromethene (BODIPY) directly linked to BF2-chelated tetraarylazadipyrromethene (azaBODIPY) through the β-pyrrole position of azaBODIPY. Structural integrity of the dyad was arrived from two-dimensional NMR spectral studies, while single-crystal X-ray structure of the dyad provided the relative orientation of the two macrocycles to be ∼62°. Because of direct linking of the two entities, ultrafast energy transfer from the (1)BODIPY* to azaBODIPY was witnessed. A good agreement between the theoretically estimated Förster energy transfer rate and experimentally determined rate was observed, and this rate was found to be higher than that reported for BODIPY-azaBODIPY analogues connected with spacer units. In agreement with the free-energy calculations, the product of energy transfer, (1)azaBODIPY* revealed additional photochemical events such as electron transfer leading to the creation of BODIPY(•+)-azaBODIPY(•-) radical ion pair, more so in polar benzonitrile than in nonpolar toluene, as evidenced by femtosecond transient spectroscopic studies. Additionally, the spectral, electrochemical, and photochemical studies of the precursor compound azaBODIPY-dipyrromethane also revealed occurrence of excited-state events. In this case, electron transfer from the (1)azaBODIPY* to dipyrromethane (DPM) yielded DPM(•+)-azaBODIPY(•-) charge-separated state. The study described here stresses the role of close association of the donor and acceptor entities to promote ultrafast photochemical events, applicable of building fast-response optoelectronic and energy-harvesting devices.

Photosensitizing nanoparticles and the modulation of ROS generation

The association of PhotoSensitizer (PS) molecules with nanoparticles (NPs) forming photosensitizing NPs, has emerged as a therapeutic strategy to improve PS tumor targeting, to protect PS from deactivation reactions and to enhance both PS solubility and circulation time. Since association with NPs usually alters PS photophysical and photochemical properties, photosensitizing NPs are an important tool to modulate ROS generation. Depending on the design of the photosensitizing NP, i.e., type of PS, the NP material and the method applied for the construction of the photosensitizing NP, the deactivation routes of the excited state can be controlled, allowing the generation of either singlet oxygen or other reactive oxygen species (ROS). Controlling the type of generated ROS is desirable not only in biomedical applications, as in Photodynamic Therapy where the type of ROS affects therapeutic efficiency, but also in other technological relevant fields like energy conversion, where the electron and energy transfer processes are necessary to increase the efficiency of photoconversion cells. The current review highlights some of the recent developments in the design of Photosensitizing NPs aimed at modulating the primary photochemical events after light absorption.

Multi-Step Energy and Electron Transfer in a 'v-Configured' Supramolecular Bodipy-Azabodipy-Fullerene Triad

Supramolecular multi-modular donor-acceptor systems are useful for solar-fuel and solar-electricity generation, and also for building optoelectronic devices. In the present contribution, a novel photosynthetic antenna-reaction center model compound composed of covalently linked BF2-chelated dipyrromethene (BODIPY), BF2-chelated azadipyrromethene (azaBODIPY) and fullerene (C60), in a ‘V-Configuration’, has been newly synthesized and characterized using a multi-step synthetic procedure.1 Optical absorbance and steady-state fluorescence, computational, and electrochemical studies were systematically performed in nonpolar, toluene and polar, benzonitrile solvents to establish the molecular integrity of the triad and establish an energy level diagram revealing different photochemical events. The geometry obtained by B3LYP/6-31G* calculations revealed the anticipated V-configuration of the BODIPY-azaBODIPY-C60 triad. The location of the frontier orbitals in the triad tracked the site of electron transfer determined from electrochemical studies. The different photochemical events originated from 1BODIPY* were realized from the energy level diagram. Accordingly, 1BODIPY* resulted in competitive ultrafast energy transfer to produce BODIPY-1azaBODIPY*-C60 and electron transfer to produce BODIPY·+-azaBODIPY-C60 ·- as major photochemical events. The charge separated state persisted for few nanoseconds prior populating 3C60 * which in turn revealed an unusual triplet-triplet energy transfer to produce 3azaBODIPY* prior returning to the ground state. These findings delineate the importance of multi-modular systems in energy harvesting, and more importantly, their utility in building multi-function performing optoelectronic devices. V. Bandi, F. P. D’Souza, H. Gobeze, F. D’Souza, Chem. Eur. J. 2015, in press.

Low-level light/laser therapy versus photobiomodulation therapy.

Low-level light/laser therapy versus photobiomodulation therapy.

Controlling Electron Transfer between the Two Cofactor Chains of Photosystem I by the Redox State of One of Their Components

Two functional electron transfer (ET) chains, related by a pseudo-C2 symmetry, are present in the reaction center of photosystem I (PSI). Due to slight differences in the environment around the cofactors of the two branches, there are differences in both the kinetics of ET and the proportion of ET that occurs on the two branches. The strongest evidence that this is indeed the case relied on the observation that the oxidation rates of the reduced phylloquinone (PhQ) cofactor differ by an order of magnitude. Site-directed mutagenesis of residues involved in the respective PhQ-binding sites resulted in a specific alteration of the rates of semiquinone oxidation. Here, we show that the PsaA-F689N mutation results in an ∼100-fold decrease in the observed rate of PhQA− oxidation. This is the largest change of PhQA− oxidation kinetics observed so far for a single-point mutation, resulting in a lifetime that exceeds that of the terminal electron donor, P700+. This situation allows a second photochemical charge separation event to be initiated before PhQA− has decayed, thereby mimicking in PSI a situation that occurs in type II reaction centers. The results indicate that the presence of PhQA− does not impact the overall quantum yield and leads to an almost complete redistribution of the fractional utilization of the two functional ET chains, in favor of the one that does not bear the charged species. The evolutionary implications of these results are also briefly discussed.

Estimating the contribution of photochemical particle formation to ultrafine particle number averages in an urban atmosphere

Ultrafine particles (UFPs, diameter<100nm) have gained major attention in the environmental health discussion due to a number of suspected health effects. Observations of UFPs in urban air reveal the presence of several, time-dependent particle sources. In order to attribute measured UFP number concentrations to different source type contributions, we analyzed observations collected at a triplet of observation sites (roadside, urban background, rural) in the city of Leipzig, Germany. Photochemical new particle formation (NPF) events can be the overwhelming source of UFP particles on particular days, and were identified on the basis of characteristic patterns in the particle number size distribution data. A subsequent segmentation of the diurnal cycles of UFP concentration yielded a quantitative contribution of NPF events to daily, monthly, and annual mean values. At roadside, we obtained source contributions to the annual mean UFP number concentration (diameter range 5–100nm) for photochemical NPF events (7%), local traffic (52%), diffuse urban sources (20%), and regional background (21%). The relative contribution of NPF events rises when moving away from roadside to the urban background and rural sites (14 and 30%, respectively). Their contribution also increases when considering only fresh UFPs (5–20nm) (21% at the urban background site), and conversely decreases when considering UFPs at bigger sizes (20–100nm) (8%). A seasonal analysis showed that NPF events have their greatest importance on UFP number concentration in the months May–August, accounting for roughly half of the fresh UFPs (5–20nm) at the urban background location. The simplistic source apportionment presented here might serve to better characterize exposure to ambient UFPs in future epidemiological studies.

Multistep energy and electron transfer in a "V-configured" supramolecular BODIPY-azaBODIPY-fullerene triad: mimicry of photosynthetic antenna reaction-center events.

A new photosynthetic antenna-reaction-center model compound composed of covalently linked BF2 -chelated dipyrromethene (BODIPY), BF2 -chelated azadipyrromethene (azaBODIPY), and fullerene (C60 ), in a "V-configuration", has been newly synthesized and characterized by using a multistep synthetic procedure. Optical absorbance and steady-state fluorescence, computational, and electrochemical studies were systematically performed in nonpolar, toluene, and polar, benzonitrile, solvents to establish the molecular integrity of the triad and to construct an energy-level diagram revealing different photochemical events. The geometry obtained by B3LYP/6-31G* calculations revealed the anticipated V-configuration of the BODIPY-azaBODIPY-C60 triad. The location of the frontier orbitals in the triad tracked the site of electron transfer determined from electrochemical studies. The different photochemical events originated from (1) BODIPY* were realized from the energy-level diagram. Accordingly, (1) BODIPY* resulted in competitive ultrafast energy transfer to produce BODIPY-(1) azaBODIPY*-C60 and electron transfer to produce BODIPY(.) (+) -azaBODIPY-C60 (.) (-) as major photochemical events. The charge-separated state persisted for few nanoseconds prior populating (3) C60 *, which in turn revealed an unusual triplet-triplet energy transfer to produce (3) azaBODIPY* prior returning to the ground state. These findings delineate the importance of multimodular systems in energy harvesting, and more importantly, their utility in building multifunction performing optoelectronic devices.

Night-time sinks, daytime sources

Nitrous acid can initiate photochemical air pollution events, but it is not clear where it comes from. Laboratory experiments now suggest that surface-bound nitrite accumulated overnight can release nitrous acid during the daytime.