Electric field effects on photoisomerization process of diphenylpolyenes doped in a polymer film as revealed by a field-induced change in fluorescence spectrum

Absence of far-red emission band in aggregated core antenna complexes

Reported herein is a Stark fluorescence spectroscopy study performed on photosystem II core antenna complexes CP43 and CP47 in their native and aggregated states. The systematic mathematical modeling of the Stark fluorescence spectra with the aid of conventional Liptay formalism revealed that induction of aggregation in both the core antenna complexes via detergent removal results in a single quenched species characterized by a remarkably broad and inhomogenously broadened emission lineshape peaking around 700 nm. The quenched species possesses a fairly large magnitude of charge-transfer character. From the analogy with the results from aggregated peripheral antenna complexes, the quenched species is thought to originate from the enhanced chlorophyll-chlorophyll interaction due to aggregation. However, in contrast, aggregation of both core antenna complexes did not produce a far-red emission band at ∼730 nm, which was identified in most of the aggregated peripheral antenna complexes. The 730-nm emission band of the aggregated peripheral antenna complexes was attributed to the enhanced chlorophyll-carotenoid (lutein1) interaction in the terminal emitter locus. Therefore, it is very likely that the no occurrence of the far-red band in the aggregated core antenna complexes is directly related to the absence of lutein1 in their structures. The absence of the far-red band also suggests the possibility that aggregation-induced conformational change of the core antenna complexes does not yield a chlorophyll-carotenoid interaction associated energy dissipation channel.

A valence bond study of the activation of methyl halides bonds by electric fields

In the present work, the activation of methyl halides bonds under experience of an external electric field (EEF) is explained from the Valence Bond theory perspective. The dissociation mechanism of C–X bonds (X [Formula: see text] Cl, Br, I) influenced by a homogeneous and a heterogeneous field placed parallel to the bond axis is presented. For all examples, an increase in the electric field strength have similar consequences: (i) the decrease of the energy depth along the dissociation path, (ii) an increase of the equilibrium interatomic distance (at high EEFs), and (iii) the transition from a homolytic to a heterolytic dissociation after some field magnitude. These general behaviors are explained through the curve crossing between the ionic and the covalent structure at some field strength.

A new type of radical-pair-based model for magnetoreception

Certain migratory birds can sense the Earth's magnetic field. The nature of this process is not yet properly understood. Here we offer a simple explanation according to which birds literally see the local magnetic field through the impact of a physical rather than a chemical signature of the radical pair: a transient, long-lived electric dipole moment. Based on this premise, our picture can explain recent surprising experimental data indicating long lifetimes for the radical pair. Moreover, there is a clear evolutionary path toward this field-sensing mechanism: it is an enhancement of a weak effect that may be present in many species.

Identification of two emitting sites in the dissipative state of the major light harvesting antenna

In order to cope with the deleterious effects of excess light, photosynthetic organisms have developed remarkable strategies where the excess energy is dissipated as heat by the antenna system. In higher plants one main player in the process is the major light harvesting antenna of Photosystem II (PSII), LHCII. In this paper we applied Stark fluorescence spectroscopy to LHCII in different quenching states to investigate the possible contribution of charge-transfer states to the quenching. We find that in the quenched state the fluorescence displays a remarkable sensitivity to the applied electric field. The resulting field-induced emission spectra reveal the presence of two distinct energy dissipating sites both characterized by a strong but spectrally very different response to the applied electric field. We propose the two states to originate from chlorophyll-chlorophyll and chlorophyll-carotenoid charge transfer interactions coupled to the chlorophyll exciton state in the terminal emitter locus and discuss these findings in the light of the different models proposed to be responsible for energy dissipation in photosynthesis.

External electric field effects on absorption, fluorescence, and phosphorescence spectra of diphenylpolyynes in a polymer film

External electric field effects on absorption, fluorescence, and phosphorescence spectra of a series of unsubstituted diphenylpolyynes have been examined in a PMMA film. The analysis of the electroabsorption spectra indicates that the shorter diphenylpolyynes exhibit only the change in molecular polarizability, whereas the longer ones exhibit the change both in dipole moment and in molecular polarizability following absorption. The finding of the change in dipole moment following absorption of centrosymmetric diphenylpolyynes is interpreted in terms of the symmetry distortion upon doping a polymer film. When the external electric field is applied, the fluorescence yield is reduced and enhanced, respectively, in diphenylacetylene and diphenyloctatetrayne, indicating that the rate of the nonradiative process from the fluorescence state is accelerated in diphenylacetylene and decelerated in diphenyloctatetrayne by an external electric field. All of the diphenylpolyynes used in the present study exhibit the change in molecular polarizability following the phosphorescence process.

Control of delocalization and structural changes by means of an electric field

The strength and, mainly, the direction of a static electric field can be used to control delocalization effects occurring in a non-polar pi-system. The delocalization energy, the weights, and the probabilities of some local electronic structures, the behavior of electron pairs, and the electronic fluctuations are considered and examined in cis-butadiene, used as model system. The effects of the electric field are detected and evaluated in the basis of natural orbital spaces appropriate to investigate the behavior of one- and poly-electron distributions. The consequences of modifying the delocalization effects on structural changes are also investigated. Full geometry optimizations in both Hartree-Fock and MP2 levels show that the changes in bond lengths, guided by the changes of the behavior of the electronic assembly, can be controlled by means of the electric field.

Electrofluorescence of MEH-PPV and Its Oligomers: Evidence for Field-Induced Fluorescence Quenching of Single Chains

Electrofluorescence (Stark) spectroscopy has been used to measure the trace of the change in polarizability (trDeltaalpha) and the absolute value of the change in dipole moment (|Deltamu|) of the electroluminescent polymer poly[2-methoxy,5-(2'-ethyl-hexoxy)-1,4-phenylene vinylene] (MEH-PPV) and several model oligomers in solvent glass matrixes. From electrofluorescence, the measured values of trDeltaalpha increase from 500 +/- 60 A(3) in OPPV-5 to 2000 +/- 200 A(3) in MEH-PPV. The good agreement found between these values and those measured by electroabsorption suggests the electronic properties do not differ strongly between absorption and emission, in contrast to earlier predictions. Evidence of electric-field-induced fluorescence quenching of MEH-PPV in dilute solvent glasses was found. When normalized to the square of the applied electric field, the magnitude of quench is comparable to that reported in the literature for thin films of MEH-PPV. In addition, fluorescence quenching was also observed in the oligomers with a magnitude that increases with increasing chain length. By using the values of trDeltaalpha measured by electrofluorescence, a model is developed to qualitatively explain the chain length dependence to the fluorescence quench observed in the oligomers as a function of exciton delocalization along the oligomer backbone. Various explanations for the origin of this quenching behavior and its chain length dependence are considered.

External Electric Field Effects on State Energy and Photoexcitation Dynamics of Diphenylpolyenes

External electric field effects on state energy and photoexcitation dynamics have been examined for para-substituted and unsubstituted all-trans-diphenylpolyenes doped in a film, based on the steady-state and picosecond time-resolved measurements of the field effects on absorption and fluorescence. The substitution dependence of the electroabsorption spectra shows that the dipole moment of the substituted stilbene in the Franck-Condon excited state becomes larger with increasing difference between the Hammet constants of the substituents. Fluorescence quantum yields of 4-(dimethylamino)-4'-nitrostilbene and 4-(dimethylamino)-4'-nitrodiphenylbutadiene are markedly reduced by an electric field, suggesting that the rates of the intramolecular charge transfer (CT) from the fluorescent state to the nonradiative CT state are accelerated by an external electric field. The magnitude of the field-induced decrease in fluorescence lifetime has been evaluated. The isomerization of the unsubstituted all-trans-diphenylpolyenes to the cis forms is shown to be a significant nonradiative pathway even in a film. Field-induced quenching of their fluorescence as well as field-induced decrease in fluorescence lifetime suggests that the trans to cis photoisomerization is enhanced by an electric field.