Change In Permanent Dipole Moment Research Articles

Coherent exciton coupling in fluorescent protein dimers probed by two-photon polarization ratio

Coherent excitonic coupling of two chromophores in a dimer of a GFP-analogue, VenusA206, was discovered previously by using circular dichroism and a combination of photon antibunching and fluorescence correlation spectroscopy. To further elucidate this effect, here we use a new approach based on the measurements of two-photon polarization ratio (Ω) as a function of excitation wavelength. Theoretically, Ω depends on the angle between the vectors of absorption transition dipole moment and permanent dipole moment change upon excitation. Generally, excitonic coupling of two chromophores should provide a change of this angle. We measured Ω for the tandem dimer of Venus (tdV) with two chromophores per protein chain and, as a control - for the tandem dimer with one of the chromophores knocked out by point mutation (tdVx). Compared to tdVx, we observe in tdV a significant increase of Ω in the red side of the pure electronic transition and a significant decrease of Ω in the blue side. At the same time, the two-photon absorption spectra of the two systems virtually coincide. Our observations are consistent with a model where only a minority of dimers are coupled coherently and the two permanent dipoles are oriented in an oblique head-to-head arrangement with an angle ∼400 between them. For multiphoton microscopy applications, it is interesting to investigate if the coherently coupled GFP dimer can provide a cooperative enhancement of the two-photon cross section, i.e., absorb stronger than two uncoupled monomers. This possibility can come from the “motional narrowing” of excitonic transition and will be discussed.

Illuminating the origins of two‐photon absorption properties in fluorescent protein chromophores

AbstractWe provide here a structural impact on two‐photon absorption cross‐section (σTPA) for 22 distinct fluorescent protein (FP) chromophores. By employing time‐dependent density functional theory, we gain insight into two‐photon absorption (TPA) process by investigating relationship between σTPA and one‐photon electronic transition dipole moment and permanent dipole moment change (Δμ) upon transition. Our results reveal that for the S1 excited state, σTPA is proportional to (Δμ)2 in agreement with two‐state model of TPA process. On the contrary, the TPA spectroscopy of higher excited states (S n, n > 1) is much more complex. We do not find a main driving force of large σTPA that would be common for investigated chromophores. Instead, it seems that channel interference between one‐photon transition dipole moment vectors is responsible for enhancement or diminishment of σTPA. Our in vacuo results may serve as a benchmark to investigate a role of chromophore‐protein interaction in shaping TPA spectra of FPs.

Solute-solvent electronic interaction is responsible for initial charge separation in ruthenium complexes [Ru(bpy)3]2+ and [Ru(phen)3]2+

Origin of the initial charge separation in optically-excited Ruthenium(II) tris(bidentate) complexes of intrinsic D3 symmetry has remained a disputed issue for decades. Here we measure the femtosecond two-photon absorption (2PA) cross section spectra of [Ru(2,2′-bipyridine)3]2 and [Ru(1,10-phenanthroline)3]2 in a series of solvents with varying polarity and show that for vertical transitions to the lower-energy 1MLCT excited state, the permanent electric dipole moment change is nearly solvent-independent, Δμ = 5.1–6.3 D and 5.3–5.9 D, respectively. Comparison of experimental results with quantum-chemical calculations of complexes in the gas phase, in a polarizable dielectric continuum and in solute-solvent clusters containing up to 18 explicit solvent molecules indicate that the non-vanishing permanent dipole moment change in the nominally double-degenerate E-symmetry state is caused by the solute-solvent interaction twisting the two constituent dipoles out of their original opposite orientation, with average angles matching the experimental two-photon polarization ratio.

Femtosecond Two-Photon Absorption Spectroscopy of Poly(fluorene) Derivatives Containing Benzoselenadiazole and Benzothiadiazole.

We have investigated the molecular structure and two-photon absorption (2PA) properties relationship of two push–pull poly(fluorene) derivatives containing benzoselenadiazole and benzothiadiazole units. For that, we have used the femtosecond wavelength-tunable Z-scan technique with a low repetition rate (1 kHz) and an energy per pulse on the order of nJ. Our results show that both 2PA spectra present a strong 2PA (around 600 GM (1 GM = 1 × 10−50 cm4·s·photon−1)) band at around 720 nm (transition energy 3.45 eV) ascribed to the strongly 2PA-allowed 1Ag-like → mAg-like transition, characteristic of poly(fluorene) derivatives. Another 2PA band related to the intramolecular charge transfer was also observed at around 900 nm (transition energy 2.75 eV). In both 2PA bands, we found higher 2PA cross-section values for the poly(fluorene) containing benzothiadiazole unit. This outcome was explained through the higher charge redistribution at the excited state caused by the benzothiadiazole group as compared to the benzoselenadiazole and confirmed by means of solvatochromic Stokes shift measurements. To shed more light on these results, we employed the sum-over-states approach within the two-energy level model to estimate the maximum permanent dipole moment change related to the intramolecular charge transfer transition.

Longitudinal wave function control in single quantum dots with an applied magnetic field.

Controlling single-particle wave functions in single semiconductor quantum dots is in demand to implement solid-state quantum information processing and spintronics. Normally, particle wave functions can be tuned transversely by an perpendicular magnetic field. We report a longitudinal wave function control in single quantum dots with a magnetic field. For a pure InAs quantum dot with a shape of pyramid or truncated pyramid, the hole wave function always occupies the base because of the less confinement at base, which induces a permanent dipole oriented from base to apex. With applying magnetic field along the base-apex direction, the hole wave function shrinks in the base plane. Because of the linear changing of the confinement for hole wave function from base to apex, the center of effective mass moves up during shrinking process. Due to the uniform confine potential for electrons, the center of effective mass of electrons does not move much, which results in a permanent dipole moment change and an inverted electron-hole alignment along the magnetic field direction. Manipulating the wave function longitudinally not only provides an alternative way to control the charge distribution with magnetic field but also a new method to tune electron-hole interaction in single quantum dots.

All-Optical Sensing of the Components of the Internal Local Electric Field in Proteins.

Here, we present a new all-optical method of interrogation of the internal electric field vector inside proteins. The method is based on experimental evaluation of the permanent dipole moment change upon excitation and the pure electronic transition frequency of a fluorophore embedded in a protein matrix. The permanent dipole moment change can be obtained from two-photon absorption measurements. In addition, permanent dipole moment change, tensor of polarizability change, and transition frequency for the free chromophore should be calculated quantum-mechanically. This allows obtaining the components of the electric field by considering the second-order Stark shift. We use the fluorescent protein mCherry as an example to demonstrate the applicability of the method.

High‐Resolution Electronic Spectroscopy Studies of meta‐Aminobenzoic Acid in the Gas Phase Reveal the Origins of its Solvatochromic Behavior

A theoretical and experimental investigation of meta-aminobenzoic acid (MABA) in the gas phase is presented, with the goal of understanding counterintuitive observations on the solvatochromism of this "push-pull" molecule. The adiabatic excitation energies, transition moments, and excited-state structures are examined using the complete active space self-consistent field approach (CASSCF and CASPT2), which shows the first excited electronic state of MABA to be of greater charge transfer character than was found in the para-isomer (PABA). The rotationally resolved electronic spectrum of MABA reveals the existence of two rotamers, owing to asymmetry in the carboxylic acid functional group. Stark measurements in a molecular beam show the change in permanent dipole moment upon excitation to be Δμ≈3.5 D for both rotamers, more than three times larger than that found in PABA. The excited state measurements reported here, along with supporting data from theory, clearly demonstrate how the meta-directing effects of asymmetric substitution in aniline derivatives can drive charge transfer pathways in the isolated molecule.

Stark spectroscopy of charge-transfer transitions in catechol-sensitized TiO 2 nanoparticles

Electronic excited states of catechol bound to titanium dioxide nanoparticles were investigated using electroabsorption (Stark effect) spectroscopy. The electronic transition at about 400 nm, characteristic for catechol bound to TiO 2 is associated with a change in permanent dipole moment by f · |Δ μ| = 15.7 D (where f is the local field correction factor), and a small negative change in the polarizability. Electron transfer distance points to the strong charge-transfer character of this transition. The electroabsorption spectra show also another electronic transition 7000 cm −1 higher energy, partially masked by the TiO 2 absorption.

Light‐Induced Change of Charge Carrier Mobility in Semiconducting Polymers

AbstractSummary: Light‐driven devices based on reversible change of carrier mobility in semiconducting polymers were investigated. The mobility was altered using a photochromic spiropyran capable of a reversible change of permanent dipole moment and ionization potential. While the latter attribute may result in formation of chemical traps and is more important for matrices with similar ionization potential such as PVK, the former phenomenon results in formation of polar traps and is more pronounced in the case of lower‐band‐gap materials.

Resonance Enhancement of Two-Photon Absorption in Fluorescent Proteins

We measure two-photon absorption (2PA) spectra of wild-type green fluorescent protein, cyan fluorescent protein, and monomeric red fluorescent protein in absolute cross section values in a wide spectral range (lambda2PA = 550 - 1300 nm), and find, for the first time to our knowledge, a new S0 --> Sn 2PA transition in all three proteins in the short-wavelength region. This transition is strongly resonantly enhanced, showing 2PA cross section values of approximately 20-160 GM, which are at least 2-4 times higher than those measured in the lowest energy (S0 --> S1) transition of corresponding proteins. We also show that the change of permanent dipole moment upon S0 --> S1 excitation (|Deltamu10|) can be deduced from 2PA cross section, providing a new tool for fast evaluation of |Deltamu10| in physiological conditions.

Nonphotochemical Hole-Burning Study of Selectively Stained Normal and Cancerous Human Ovarian Tissues

Results are presented of nonphotochemical hole-burning (HB) experiments on cancerous ovarian and analogous normal peritoneal in vitro tissues stained with the mitochondrial-selective dye rhodamine 800. A comparison of fluorescence excitation spectra, hole-growth kinetics data, and external electric field (Stark) effects on the shape of spectral holes burned in cancerous and normal tissues stained with rhodamine 800 revealed significant differences only in the dipole moment change (fDeltamu) measured by a combination of HB and Stark spectroscopies. It is shown that the permanent dipole moment change for the S0--> S1 transition of the rhodamine 800 molecules in cancerous tissue is higher than that of normal tissue by a factor of about 1.4. The finding is similar to the HB results obtained earlier for human ovarian surface epithelial cell lines, i.e., OV167 carcinoma and OSE(tsT)-14 normal cells stained with the same mitochondria-specific dye (Walsh et al. Biophys. J. 2003, 84, 1299). We propose that the observed difference in the permanent dipole moment change in cancerous ovarian tissue is related to a modification in mitochondrial membrane potential.

Ionization Potentials of Fluoroindoles and the Origin of Nonexponential Tryptophan Fluorescence Decay in Proteins⊥

This work reports an explanation for the unusual monoexponential fluorescence decay of 5-fluorotryptophan (5FTrp) in single-Trp mutant proteins [Broos, J.; Maddalena, F.; Hesp, B. H. J. Am. Chem. Soc. 2004, 126, 22-23] and substantially clarifies the origin of the ubiquitous nonexponential fluorescence decay of tryptophan in proteins. Our results strongly suggest that the extent of nonexponential fluorescence decay is governed primarily by the efficiency of electron transfer (ET) quenching by a nearby amide group in the peptide bond. Fluoro substitution increases the ionization potential (IP) of indole, thereby suppressing the ET rate, leading to a longer average lifetime and therefore a more homogeneous decay. We report experimental IPs for a number of substituted indoles including 5-fluoroindole, 5-fluoro-3-methylindole, and 6-fluoroindole, along with accurate ab initio calculations of the IPs for these and 20 related molecules. The results predict the IP of 5-fluorotryptophan to be 0.19 eV higher than that of tryptophan. 5-Fluoro substitution does not measurably alter the excitation-induced change in permanent dipole moment nor does it change the fluorescent state from 1La to 1Lb. In combination with electronic structure information this argues that the increased IP and the decreased excitation energy of the 1La state, together 0.3 eV, are solely responsible for the strong reduction of electron transfer quenching. 6-Fluoro substitution is predicted to increase the IP by a mere 0.09 eV. In agreement with our conclusions, the fluorescence decay curves of 6-fluorotryptophan-containing proteins are well fit using only two decay times compared to three required for Trp.

Stark absorption spectroscopy of indole and 3-methylindole.

Indole and 3-methylindole (3-MI) doped into a polymethylmethacrylate (PMMA) film are studied by the Stark absorption (electroabsorption) spectroscopy. The 1La and 1Lb absorption bands are distinguished and the change in permanent dipole moment on 1La excitation is determined by a model fit to the measured absorption and electroabsorption spectra. Analysis of the spectra, measured at normal incidence and magic angle conditions, proved the essential role of the electric-field-induced orientation/alignment effects for polar indole and 3-MI molecules in the PMMA environment at room temperature.

Evidence for Highly Dispersive Primary Charge Separation Kinetics and Gross Heterogeneity in the Isolated PS II Reaction Center of Green Plants

Despite the availability of an X-ray structure and many spectroscopic studies, important issues related to structural heterogeneity, excitonic structure, primary charge separation (CS), and excitation energy transfer dynamics of the isolated reaction center (RC) of photosystem II (PS II) remain unresolved. The issues addressed here include (1) whether the primary CS kinetics at low temperatures are highly dispersive (due to structural heterogeneity), as proposed by Prokhorenko and Holzwarth (J. Phys. Chem. B 2000, 104, 11563), and (2) the nature of the weak lowest-energy Qy absorption band at ∼684 nm that appears as a shoulder on the intense primary electron donor band (P680). Results of low-temperature nonphotochemical hole burning (NPHB) and triplet bottleneck hole burning (TBHB) spectroscopic experiments (including effects of pressure and external electric (Stark) fields) are presented for the RC from spinach with one of the two peripheral chlorophylls removed. Both NPHB and TBHB are observed with excitations within the P680 and 684 nm bands. Both types of hole spectra exhibit a weak dependence on the burn wavelength (λB) between 680 and 686 nm. Furthermore, the permanent dipole moment change (f Δμ), as determined by Stark-NPHB spectroscopy, is identical (0.9 ± 0.1 D) for the two bands, as are the linear electron−phonon coupling parameters (Huang−Rhys factors S17 = 0.7 and S80 = 0.2 for 17 and 80 cm-1 phonons). These similarities, together with published fluorescence line narrowed spectra lead us to favor the gross heterogeneity model in which the 684 nm band is the primary electron donor band (P684) of a subset of RCs that may be more intact than P680-type RCs. It is concluded, based also on the linear pressure shift rates for the P680 and 684 nm bands, that population of either P680* (* ≡ Qy state) or P684* results in both TBHB (due to charge recombination of the primary radical ion pair) and NPHB. It was found that the values of parameters (e.g., electron−phonon coupling, site distribution function) used to simulate the NPHB spectra also provided reasonable fits to the TBHB spectra. Acceptable theoretical simulations of the line-narrowed TBHB spectra were not possible using a single primary CS time. However, satisfactory fits (including λB and burn intensity dependences) were achieved using a distribution of CS times. The observed TBHB is due to P680- and P684-type RCs with the faster CS kinetics since the persistent nonphotochemical holes were saturated prior to measuring the TBHB spectra. (RCs exhibiting the most efficient NPHB have slower CS kinetics as well as higher fluorescence quantum yields.) For the TBHB spectra, the same distribution (Weibull) was used for the P680- and P684-type RCs. The distribution describes quite well the distribution of Prokhorenko and Holzwarth for CS times shorter than 25 ps. Finally, the data indicate that electron exchange contributes only weakly (relative to electrostatics) to the inter-pigment excitonic interactions.

Red Antenna States of PS I of Cyanobacteria: Stark Effect and Interstate Energy Transfer

Previously, Stark hole-burning spectroscopy and effects of pressure at low temperature were used to determine the number of red antenna states of the cyanobacteria Synechococcus elongatus and Synechocystis PCC6803 (Hayes, J. M.; Matsuzaki, S.; Rätsep, M.; Small, G. J. J. Phys. Chem. B 2000 104, 5625. Zazubovich, V.; Matsuzaki, S.; Johnson, T. W.; Hayes, J. M.; Chitnis, P. R.; Small Chem. Phys. 2002, 275, 47). Distinct differences in linear pressure shift rates, the magnitude of the permanent dipole moment change, fΔμ, and electron−phonon coupling strength clearly show that in Synechococcus there are three red states (C708, C715, and C719), whereas in Synechocystis, there are two red states (C708 and C714). In the Stark hole-burning spectra of the lowest states of these two systems, hole splitting was not observed, only hole broadening, for excitation polarization both parallel and perpendicular to the Stark field direction. The theories of Stark hole burning predict that splitting should occur for one of the polarizations unless there is a large, random component to the induced dipole moment change, Δμind, which is not expected to be the case for pigment−protein complexes in which the orientations of pigments relative to the protein matrix are nonrandom. In this paper, Stark hole burning at higher resolution is used to reinvestigate the absence of splitting. However, even at higher resolution, no splitting is detected. This is explained by invoking large variations of the inherent dipole moment change Δμ0 of the dimer (the origin of the red-state absorption) rather than of the induced dipole moment change. These arise from a distribution of the relative orientations and separations between the components of the dimer. This distribution also results in a random component of the polarizability change tensor, Δα. The random components of Δμ0 and Δα not only obscure the Stark splitting but also cause the large inhomogeneous broadening observed for these lowest-energy red states. Temperature-dependent hole widths were also measured for C708 and C714 of Synechocystis. For C714, a T1.3 temperature dependence was observed, consistent with dephasing by the disordered protein matrix. At 708 nm, however, much higher fluences were required to saturate the absorption of the blue edge of the C714 band and then begin to burn C708. The contribution of the C708 component to the broadening was weakly temperature dependent over the range measured, 2 to 14 K. This contribution is due to energy transfer from C708 to C714, and the width measured corresponds to an energy-transfer time of 6 ps. This observation provides further proof of the existence of two red antenna states, C708 and C714.

Carcinoma and SV40-Transfected Normal Ovarian Surface Epithelial Cell Comparison by Nonphotochemical Hole Burning

Results are presented of nonphotochemical-hole-burning experiments on the mitochondrial specific dye rhodamine 800 incubated with two human ovarian surface epithelial cell lines: OSE(tsT)-14 normal cells and OV167 carcinoma cells. This dye is selective for the plasma and inner membranes of the mitochondria, as shown by confocal microscopy images. Dispersive hole-growth kinetics of zero-phonon holes are analyzed with theoretical fits, indicating that subcellular structural heterogeneity of the carcinoma cell line is lower relative to the analogous normal cell line. Broadening of holes in the presence of an applied electric field (Stark effect) was used to determine the permanent dipole moment change for the S 0→S 1 transition in the two cell lines. For the carcinoma cell line, the permanent dipole moment change value is a factor of 1.5 higher than for the normal cell line. It is speculated that this difference may be related to differences in mitochondrial membrane potentials in the two cell lines.

Nonphotochemical hole burning spectroscopy of a mitochondrial selective rhodamine dye molecule in normal and cancerous ovarian surface epithelial cells

Results are presented of nonphotochemical hole burning experiments on the mitochondrial dye rhodamine 800 (MitoFluor Far Red 680, Molecular Probes) incubated with two human ovarian surface epithelial cell lines: OSE(tsT)-14 normal cells and OV167 carcinoma (cancer) cells. This dye is believed to be selective for the plasma and inner membranes of the mitochondria. Importantly, the dispersive (distributed) growth kinetics of zero-phonon holes were found to be significantly different with the OV167 line exhibiting the higher burn efficiency. This may reflect greater structural heterogeneity of that line. Equally interesting is that the permanent dipole moment change (Δ μ) for the S 0→S 1 transition of the dye is markedly different for the two cell lines, the Δ μ value for the carcinoma cell line being a factor of 1.5 higher. Discussion of this finding in terms of membrane potentials is given.

Red antenna states of photosystem I from cyanobacterium Synechococcus elongatus: a spectral hole burning study

The existence of at least three low-energy chlorophyll a (Chl a) antenna states is shown for photosystem I (PS I) of Synechococcus elongatus by the use of nonphotochemical hole burning (NPHB) spectroscopy. In addition to the previously reported states at 708 and 719 nm, it is demonstrated that there is a third state at 715 nm. The responsible Chl a molecules are referred to as C-708, C-715 and C-719. For both Synechococcus and Synechocystis, the lowest energy state is shown to be characterized by strong electron–phonon coupling (large Huang-Rhys factor S), large permanent dipole moment change ( f· Δμ), and large linear pressure shift rates attributable to electron exchange of dimeric Chl a. The lowest energy state of Synechocystis is at 714 nm. The properties of the 714 and 719 nm states are very similar, suggesting that their dimer structures are also similar. Although the other red antenna states of these cyanobacteria have smaller values for S, f· Δμ, and linear pressure shift rate, these are still larger than typically measured for monomeric antenna Chl a. Possible assignments of red absorption bands to particular chlorophyll dimers or trimer are discussed.

Quantum computer hardware based on rare-earth-ion-doped inorganic crystals

We present a scheme for generating multiple, strongly interacting qubits in rare-earth-ion-doped inorganic crystals at cryogenic temperatures. Two ground state hyperfine levels, with hour long lifetimes and ms decoherence times are chosen as qubit states. Controlled logic between the qubits is accomplished using the change in permanent dipole moment induced by an optical transition between the ground and excited state of these ions. The scheme is based on existing material data and established measurement techniques and should therefore be straightforward to realise in practice. The procedure used for creating the qubits can be generalised also to other solid state systems.

New Insights on Persistent Nonphotochemical Hole Burning and Its Application to Photosynthetic Complexes

Nonphotochemical hole burning (NPHB) at low temperatures of the electronic absorption bands of molecular chromophores imbedded in amorphous solids (glasses and polymers) and in proteins is a striking manifestation of configurational tunneling triggered by electronic excitation. The current mechanism of NPHB has it due to a hierarchy of relaxation events that begin in the outer shell and involve the intrinsic two-level systems (TLSint) of the glass and terminate in the inner shell where the rate determining step involving the extrinsic TLS (TLSext) occurs. The TLS correspond to asymmetric intermolecular double well potentials. The TLSext are associated with the chromophore and the inner shell of solvent molecules. The TLSint are intimately associated with the excess free volume of glasses. Their tunneling leads to diffusion of excess free volume. Results for Al-phthalocyanine tetrasulfonate (APT) in hyperquenched glassy water (HGW) and ethanol (HGE) films that provide strong support for the critical role of excess free volume are discussed. Hole spectra of APT/HGW obtained over eight decades of burn fluence reveal that the current mechanism needs to be modified to include multilevel extrinsic systems (MLSext) in order to explain why the antihole (“photoproduct” absorption) lies to the blue of the burn frequency for sufficiently high burn fluences, an intriguing up-conversion process. The spectra also reveal, for the first time, that the zero-phonon hole (ZPH) profile is non-Lorentzian. This is shown to be a natural consequence of the interplay between the three distributions that result in dispersive hole growth kinetics. They are associated with the tunnel parameter λ of the TLSext, the angle α between the laser polarization and transition dipole, and off-resonant absorption of the zero-phonon line (the ω distribution). Theoretical simulations of hole growth data for APT/HGW obtained over six decades of burn fluence show that the λ distribution is of primary importance, describing well the first 80% of the saturated burn. The paper ends with an application of NPHB combined with high pressure and external electric (Stark) fields to the critically important “red” antenna states of photosystem I. The addition of pressure and Stark fields enhances the already impressive selectivity of NPHB. The results show that the linear pressure shift, permanent dipole moment change, and linear electron−phonon coupling are correlated. Of particular importance is that these properties can be used to identify states which involve interacting chlorophyll molecules that possess significant charge transfer character because of electron-exchange coupling. The results also show that the site distribution functions of the antenna states are largely uncorrelated, consistent with the findings for previously studied complexes. This is important because the absence of correlation means that the electronic energy gaps of donor and acceptor states are distributed which, in turn, means that the kinetics can be dispersive under certain conditions.