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.     

Site selection,hole burning,and stark effect on resorufin in poly(methyl methacrylate)

Site-selection spectra of the anionic dye resorufin in poly(methyl methacrylate) have been obtained at pumped helium temperature. Hole burning has been observed in these films upon prolonged irradiation not only at the wavelength of irradiation but in the vibronic sublevels. The Stark spectrum of the hole (i.e., of the “burnt” molecules) yields a value of 0.2 ± 0.05 D for the dipole moment change upon excitation.     

Do bacteria have an electric permanent dipole moment?

In the scientific literature in the last 40 years, some data for the permanent dipole moment and the electric polarizability of Escherichia coli can be found [S.P. Stoylov, Colloid Electro-Optics - Theory, Techniques and Application, Academic Press, London, 1991]. In this paper the data based mainly on electro-optic investigation is considered as much as some dipolophoretic (most often called dielectrophoretic) studies. Serious grounds are found to doubt the conclusions made for the electric dipole moments of bacteria by one of the authors of this paper (SPS) and by some other researchers. This concerns both the permanent dipole moment and the electric charge dependent polarizabilities of E. coli. Here, along with the discussion of the old experimental data, new experimental data are shown for a strain of E. coli HB101. The conclusions from the analysis of the old and the new experimental data is that they do not provide correct evidence for the presence of a permanent dipole moment. It seems that all statements for the existence of electric permanent dipole moment in bacteria [S.P. Stoylov, Colloid Electro-Optics - Theory, Techniques and Application, Academic Press, London, 1991; S.P. Stoylov, S. Sokerov, I. Petkanchin, N. Ibroshev, Dokl. AN URSS 180 (1968) 1165; N.A. Tolstoy, A.A. Spartakov, A.A. Trusov, S.A. Schelkunova, Biofizika 11 (1966) 453; V. Morris, B. Jennings, J. Chem. Soc. Faraday Trans. II 71 (1975) 1948; V. Morris, B. Jennings, J. Colloid Interface Sci. 55 (1978) 313; S.P. Stoylov, V.N. Shilov, S.S. Dukhin, S. Sokerov, I. Petkanchin, in: S.S. Dukhin (Ed.), Electro-optics of Colloids, Naukova Dumka, Kiev, 1977 (in Russian).] based on electro-optic studies are result of incorrect interpretation. Therefore, they should be further ignored.     

Determination of the ground state,excited state and change in dipole moments of magnesium and zinc chlorin

High-resolution Stark effect measurements on the S1 <-- S0 (pi pi*) origin of magnesium chlorin (MgCh) and zinc chlorin (ZnCh) in single crystals of n-octane at 4.2 K are reported. The corresponding change in dipole moment (absolute value(delta mu(ge))) associated with each transition was estimated to be 0.23 +/- 0.04 and 0.27 +/- 0.05 debye, respectively. Each molecule's orientation in the n-octane crystal was also determined. The change in dipole moment of MgCh was also found using solvatochromic shift data (absolute value(delta mu(ge))) = 0.33 +/- 0.08 debye). The ground state dipole moment (mu(g)) of MgCh was determined by dielectric constant measurement of MgCh/benzene solutions (mu(g) = 2.26 +/- 0.08 debye). These were combined to calculate the average excited state dipole moment of MgCh (mu(e) = 2.51 +/- 0.08 debye). The ground state dipole moment of ZnCh was also determined using solvatochromic shift data (mu(g) = 3.17 +/- 0.08 debye). This was combined with its measured absolute value(delta mu(ge)) to calculate the excited state dipole moment of ZnCh (mu(e) = 3.44 +/- 0.08 debye); the S1 <-- S0 (pi pi*) origin band of both complexes was red-shifted at room temperature as the polarity of the solvents was increased, which implies that delta mu(ge) is positive.     

The charge-transfer character of the S0 --> S2 transition in the carotenoid peridinin is revealed by Stark spectroscopy

Peridinin, the carotenoid in the peridinin chlorophyll a protein (PCP), was studied by Stark (electroabsorption) spectroscopy to determine the change in electrostatic properties produced on excitation within the absorption band, in methyl tetrahydrofuran (MeTHF) versus ethylene glycol (EG), at 77 K. Strikingly, a large change in the permanent dipole moment (|Deltamu|) was found between the ground state, S(0) (1(1)A(g)(-)), and the Franck-Condon region of the S(2) (1(1)B(u)(+)) excited state, in both MeTHF (22 D) and EG (approximately 27 D), thus revealing the previously unknown charge transfer (CT) character of this pi-pi transition in peridinin. Such a large |Deltamu| produced on excitation, we suggest, facilitates the bending of the lactone moiety, toward which charge transfer occurs, and the subsequent formation of the previously identified intramolecular CT (ICT) state at lower energy. This unexpectedly large S(2) dipole moment, which has not been predicted even from high-level electronic structure calculations, is supported by calculating the shift of the peridinin absorption band as a function of solvent polarity, using the experimentally derived result. Overall, the photoinduced charge transfer uncovered here is expected to affect the excited-state reactivity of peridinin and, within the protein, be important for efficient energy transfer from the carotenoid S(2) and S(1)/ICT states to the chlorophylls in PCP.     

The Fluorescence Properties of Hypocrellin B and its Amino‐substituted Derivative: Photinduced Intramolecular Proton Transfer and Photoinduced Intramolecular Electron Transfer¶

We report on the emission spectra and emission quantum yields of a newly synthesized hypocrellin dye, Z‐demethoxy‐ 2,3‐ethylenediamino hypocrellin B (EDAHB), and its parent HB in different solvents of varying polarity. Our results demonstrate that EDAHB is one of the few dyes that exhibit highly solvent polarity‐dependent fluorescence in the useful region (680–730 nm). Therefore, it offers some applications in the biomedical field as a fluorescent probe molecule. The solvatochromic effect of EDAHB is proposed to be due to a distinct change in the dipole moment of the dye on excitation. A photoinduced intramolecular proton transfer and a photoinduced intramolecular electron transfer process are considered relevant for the fluorescence properties of HB and EDAHB, respectively.     

The charge-transfer properties of the S2 state of fucoxanthin in solution and in fucoxanthin chlorophyll-a/c2 protein (FCP) based on stark spectroscopy and molecular-orbital theory

Fucoxanthin chlorophyll-a/c 2 protein (FCP), the membrane-intrinsic light harvesting complex from the diatom Cyclotella meneghiniana, is characterized by Stark spectroscopy to obtain a quantitative measure of the excited-state dipolar properties of the constituent pigments. The electro-optical properties of the carotenoid fucoxanthin (Fx), the primary light harvester in FCP, were determined from the Stark spectrum measured in a MeTHF glass (77 K) and compared to the results from electronic-structure calculations. On photon absorption by Fx, a 17 D change in the static dipole moment (|Delta mu|exp), and a somewhat larger |Delta mu|exp at the red edge, are measured for the S 0 --> S 2 (1 (1)A g (-)-like -->1 (1)B u *+-like) transition. The large change in dipole moment indicates that Fx undergoes photoinduced charge transfer (CT), and underscores the influence of the S 2 state on the polarity-dependent excited-state dynamics of Fx that has so far been attributed to, and discussed in terms of, the S 0 and the S 1/ICT states. MNDO-PSDCI and SACCI-CISD calculations indicate that the 1 (1)B u (*+)-like state intrinsically possesses a dipole moment much smaller than the 2 (1)A g (*-)-like state, suggesting that solvent fields promote the mixing of these two states and could account for the large dipole moments measured here for the S 0 --> S 2 transition. These CT properties of the 1 (1)B u (*+)-like state of Fx are further enhanced in the protein and underpin its photosynthetic capabilities for light harvesting and energy transfer (ET). In FCP, the CT properties of the Fx's vary according to the energetic position: between 450 and 500 nm there appear to be two sets of Fx's that exhibit |Delta mu| exp values on the order of 5 and 15 D, whereas the red-most Fx's, that are very efficient in ET to chlorophyll-a (Chl-a), exhibit strikingly large |Delta mu| exp values on the order of 40 D. Such magnitudes of |Delta mu| exp suggest a mechanism that enhances Coulombic coupling to promote ET from the S 2 state of Fx to Chl-a. These three sets of Fx's, including a fourth red Fx, are identified by fitting the Stark spectrum of FCP with the Stark spectrum of Fx in MeTHF. In contrast to the Fx's in the protein, the electrostatic properties of the Chl's in FCP are comparatively much smaller. Notably, for the Q y band of Chl-a, a |Delta mu| exp of 0.92 D and a change in polarizability ( Delta alpha exp) of 20 A (3), indicate that the Chl-a's are monomeric in nature and decoupled from each other.     

Stark shifts of the visible absorption systems of the NO−2 ion in NaNO2

Stark effect measurements on the lowest triplet and singlet transitions of the nitrite ion show that the change of dipole moment on excitation is very small (ca. 0.30 D) and of opposite sign for the two states. The dipole moment change in the triplet manifold is also found to distinctly depend on vibrational (ν₂) excitation. This is explained as a result of vibronic coupling among B₁ triplet states.     

Quantum-chemical investigation of the electroabsorption spectra of directly meso-meso-linked porphyrin arrays: essential role of charge-transfer excited states accidentally overlapping with soret bands

The electroabsorption (EA) spectra of directly meso-meso-linked porphyrin arrays (Zn, n = 1-3) have been investigated by means of the sum-over-states (SOS) approach at the INDO/S-SCI level theory. The experimental EA spectra of Zn (n > or = 2) exhibit an unusual second-derivative line shape at the exciton split low-energy B(x) band in contrast to the first-derivative spectrum of Z1, which is readily ascribed to a quadratic Stark shift of the B (Soret) band. Although the second-derivative line shape is usually attributed to a difference in the permanent dipole moment (Deltamu) between the ground and excited states, it should be vanishing for Zn due to their essentially D(2)(d) or D(2)(h) symmetry. As pointed out in our previous studies, the interporphyrinic charge-transfer (CT) excited states are accidentally overlapping with the excitonic B bands and the present calculations reveal that the B(x) state is strongly coupled via a transition dipole moment with two such CT states. These situations give rise to a quadratic Stark effect on the B(x) band that is intermediate between Stark shift (first derivative) and Stark broadening (second derivative), and play a central role in establishing the anomalous second derivative nature of the EA spectrum. Moreover, based on the comparison between the theoretical and experimental spectra, there must be an additional factor that further enhances the second derivative nature of the EA spectrum of porphyrin arrays. Discussions on this issue including the preliminary investigations on the role of solvent (PMMA)-induced asymmetry are also presented.     

Triangular Rhodamine Triads and Their Intrinsic Photophysics Revealed from Gas-Phase Ion Fluorescence Experiments

When ionic dyes are close together, the internal Coulomb interaction may affect their photophysics and the energy-transfer efficiency. To explore this, we have prepared triangular architectures of three rhodamines connected to a central triethynylbenzene unit (1,3,5-tris(buta-1,3-diyn-1-yl)benzene) based on acetylenic coupling reactions and measured fluorescence spectra of the isolated, triply charged ions in vacuo. We find from comparisons with previously reported monomer and dimer spectra that while polarization of the π-system causes redshifted emission, the separation between the rhodamines is too large for a Stark shift. This picture is supported by electrostatic calculations on model systems composed of three linear and polarizable ionic dyes in D_3h configuration: The electric field that each dye experiences from the other two is too small to induce a dipole moment, both in the ground and the excited state. In the case of heterotrimers that contain either two rhodamine 575 (R575) and one R640 or one R575 and two R640, emission is almost purely from R640 although the polarization of the π-system expectedly diminishes the dipole-dipole interaction.     

Optical Stark spectroscopy of the B̃(1)A(')(000)←X̃(1)A(')(000) system of copper hydroxide

The B?(1)A(')(000)←X?(1)A(')(000) band system of a cold beam of CuOH has been studied field-free and in the presence of a static electric field. The Stark tuning of the low-J levels of the X?(1)A(')(000) state were analyzed to give a value of 3.968(32) D for the a-component of the permanent electric dipole moment, μ(a). An upper limit of 0.3 D for μ(a)(B?(1)A(')) is established from the lack of observable Stark tuning for the low-J levels of the B?(1)A(')(000) state. The experimental value for μ(a)(X?(1)A(')) is compared to theoretical predictions and other Cu-containing molecules. A molecular orbital correlation diagram is used to rationalize the large change in μ(a) upon excitation. The electronegativity of OH was determined to be 2.81 from a comparison of the determined μ(a) with the experimental μ values for CuF, CuO, and CuS.     

Experimental measurement of the induced dipole moment of an isolated molecule in its ground and electronically excited states: indole and indole-H2O

Reported here are measurements of the magnitude and orientation of the induced dipole moment that is produced when an indole molecule in its ground S(0) and electronically excited S(1) states is polarized by the attachment of a hydrogen bonded water molecule in the gas phase complex indole-H(2)O. For the complex, we find the permanent dipole moment values mu(IW)(S(0)) = 4.4 D and mu(IW)(S(1)) = 4.0 D, values that are substantially different from calculated values based on vector sums of the dipole moments of the component parts. From this result, we derive the induced dipole moment values mu(I) (*)(S(0)) = 0.7 D and mu(I) (*)(S(1)) = 0.5 D. The orientation of the induced moment also is significantly different in the two electronic states. These results are quantitatively reproduced by a purely electrostatic calculation based on ab initio values of multipole moments.     

Matrix-induced Linear Stark Effect of Single Dibenzoterrylene Molecules in 2,3-Dibromonaphthalene Crystal

Absorption and fluorescence from single molecules can be tuned by applying an external electric field – a phenomenon known as the Stark effect. A linear Stark effect is associated to a lack of centrosymmetry of the guest in the host matrix. Centrosymmetric guests can display a linear Stark effect in disordered matrices, but the response of individual guest molecules is often relatively weak and non-uniform, with a broad distribution of the Stark coefficients. Here we introduce a novel single-molecule host-guest system, dibenzoterrylene (DBT) in 2,3-dibromonaphthalene (DBN) crystal. Fluorescent DBT molecules show excellent spectral stability with a large linear Stark effect, of the order of 1.5 GHz/kVcm⁻¹, corresponding to an electric dipole moment change of around 2 D. Remarkably, when the electric field is aligned with the a crystal axis, nearly all DBT molecules show either positive or negative Stark shifts with similar absolute values. These results are consistent with quantum chemistry calculations. Those indicate that DBT substitutes three DBN molecules along the a-axis, giving rise to eight equivalent embedding sites, related by the three glide planes of the orthorhombic crystal. The static dipole moment of DBT molecules is created by host-induced breaking of the inversion symmetry. This new host–guest system is promising for applications that require a high sensitivity of fluorescent emitters to electric fields, for example to probe weak electric fields.     

A molecular beam optical Stark study of the [15.8] and [16.0] 2Pi1/2-X 4Sigma- (0,0) band systems of rhodium monoxide, RhO

The SR11(0) and SR11(1) branch features of the [15.8] and [16.0]2Pi1/2-X 4Sigma- (0,0) subband systems of rhodium monoxide, RhO, have been studied at near the natural linewidth limit of resolution by optical Stark spectroscopy using laser induced fluorescence detection. The Stark shifts and splittings were analyzed to produce the magnitude of the permanent electric dipole moment, |mu|, of 3.81(2) D for the X 4Sigma3/2- (v=0) state. The results are compared to density functional theory calculations. Trends in observed values of |mu| across the 4d series of transition metal monoxides are interpreted in terms of simple single configuration molecular orbital correlation diagrams.     

Stark Absorption Spectroscopy of Peridinin and Allene-Modified Analogues

Stark absorption spectra of peridinin (Per) and five allene-modified analogues and their angular dependence as a function of an externally applied electric field were measured in methyl methacrylate polymer at 77K. In all cases, the energetically lowest absorption band has a significant change of static dipole moment upon photoexcitation (Δμ). In particular, Per has the largest value of |Δμ|. The angles between Δμ and the transition dipole moment of all the analogues were determined. It is suggested that the allene group in Per plays a key role as the electron donor in the charge transfer process following photoexcitation. The results of MNDO-PSDCI calculations support this idea.     

Experimental studies of light-induced charge transfer and charge redistribution in (X(2)-bipyridine)Re(I)(CO)(3)Cl complexes

Stark emission spectroscopy, transient DC photoconductivity (TDCP), and ground-state dipole moment measurements have been used to evaluate charge transfer (CT) within various (X(2)-bipyridine)Re(I)(CO)(3)Cl complexes following (3)MLCT excited-state formation. The Stark technique reports on vector differences between ground-state (mu(g)) and excited-state (mu(e)) dipole moments, while TDCP, when combined with independently obtained mu(g) information, reports on scalar differences. For systems featuring collinear, same-signed ground- and excited-state dipole moments, the scalar and vector differences are equivalent. However, for the low symmetry systems studied here, they are distinctly different. The vector difference yields the effective adiabatic one-electron-transfer distance (R(12)), while the combined vector and scalar data yield information about dipole rotation upon ground-state/excited-state interconversion. For the systems examined, charge transfer distances are substantially smaller than geometric electron-donor/electron-acceptor site separation distances. The measured distances are significantly affected by changes in acceptor ligand substituent composition. Electron-donating substituents decrease CT distances, while electron-withdrawing substituents increase CT distances, as do aromatic substituents that are capable of expanding the bipyridyl ligand (acceptor ligand) pi system. The Stark measurements additionally indicate that the CT vector and the transition dipole moment are significantly orthogonal, a consequence of strong polarization of the Re-Cl bond (orthogonal to the metal/acceptor-ligand plane) in the ground electronic state and relaxation of the polarization in the upper state. The ground-state Re-Cl bond polarization is sufficiently large that the overall ground-state scalar dipole moment exceeds the overall excited-state scalar dipole moment, despite transfer of an electron from the metal center to the diimine ligand. This finding provides an explanation for the otherwise puzzling negative solvatochromism exhibited in this family of compounds. Combining TDCP and Stark results, we find that the dipole moment can be rotated in some instances by more than 90 degrees upon (3)MLCT excited-state formation. The degree of rotation or reorientation can be modulated by changing the identity of the acceptor ligand substituents. Reorientational effects are smallest when the compounds feature aromatic substituents capable of spatially extending the pi system of the acceptor ligand.     

Deflection and deceleration of hydrogen Rydberg molecules in inhomogeneous electric fields

Hydrogen molecules are excited in a molecular beam to Rydberg states around n=17-18 and are exposed to the inhomogeneous electric field of an electric dipole. The large dipole moment produced in the selected Stark eigenstates leads to strong forces on the H2 molecules in the inhomogeneous electric field. The trajectories of the molecules are monitored using ion-imaging and time of flight measurements. With the dipole rods mounted parallel to the beam direction, the high-field-seeking and low-field-seeking Stark states are deflected towards and away from the dipole, respectively. The magnitude of the deflection is measured as a function of the parabolic quantum number k and of the duration of the applied field. It is also shown that a large deflection is observed when populating the (17d2)1 state at zero field and switching the dipole field on after a delay. With the dipole mounted perpendicular to the beam direction, the molecules are either accelerated or decelerated as they move towards the dipole. The Rydberg states are found to survive for over 100 micros after the dipole field is switched off before being ionized at the detector and the time of flight is measured. A greater percentage change in kinetic energy is achieved by initial seeding of the beam in helium or neon followed by inhomogeneous field deceleration/acceleration. Molecular dynamics trajectory simulations are presented highlighting the extent to which the trajectories can be predicted based on the known Stark map. The spectroscopy of the populated states is discussed in detail and it is established that the N+=2, J=1, MJ=0 states populated here have a special stability with respect to decay by predissociation.     

Effect of temperature on surface properties of cervical tissue homogenate and organic phase monolayers

The temperature dependence of Langmuir monolayers of normal and cancerous human cervical tissues and their organic phases between temperatures of 37 and 45 degrees C was evaluated. Analysis of the surface pressure-area isotherms revealed significantly different increase in fluidity of the cancerous cervical tissue monolayer at 42 degrees C as opposed to the normal cervical tissue monolayers (p<0.05). Similarly, in the case of cervical cancerous organic phase monolayers significant increase of fluidity was observed at 40 degrees C whereas no such change was observed in the normal cervical organic phase monolayers. The effect of temperature was found to be different in cancerous and normal cervical tissues and this may be due to the different lipid profiles in them. Cancerous cervical tissues had 1.8-fold higher total lipids as compared to the normals. Similarly, the PC, PE, PI, PG, SM and PS levels in cancerous cervical tissues were 3.6, 2.0, 2.3, 4.7, 1.7 and 2.2 times higher than those of normal cervical tissues, respectively. Significant cancer-normal difference in minimum surface tension and hysteresis area was found at all temperatures studied for both tissue homogenates and organic phases. For example, cancerous tissue homogenates showed minimum surface tensions of 51.9+/-4.6, 54.4+/-5.9, 57.6+/-6.0 and 51.9+/-5.6mN/m at temperatures 37, 40, 42 and 45 degrees C whereas the corresponding values for normal cervical tissue homogenates were 39.3+/-3.6, 39.2+/-3.7, 39.2+/-3.8 and 39.1+/-3.6, respectively. The fluidity change at hyperthermic range of temperature can be correlated to the increased efficiency of drug on combination therapy with hyperthermia. These results may have implications in manipulating the fluidity of cervical cancer tissue membranes for better permeability thereby leading to better therapeutic strategies for cervical cancer.     

Stark effects in gas-phase electronic spectra. Dipole moment of aniline in its excited S(1) state

Measurements of the Stark effect on the rotationally resolved S(1)<--S(0) fluorescence excitation spectrum of aniline are reported, providing quantitative information about the degree of charge transfer in the electronic transition. We find that mu(a)(S(1)) = 2.801 +/- 0.007 D, a value that is approximately 150% larger than the ground state, mu(a)(S(0)) = 1.129 +/- 0.005 D. The enhanced value of the dipole moment in the S(1) state is attributed to more efficient electron donation by the quasi-planar amino group to the aromatic ring.     

Determination of the dipole moment of nitrosyl fluoride in the (001) and (101) states by combination of microwave and laser stark data.

The combination of microwave measurements of the Stark effect in the (001) excited vibrational state with fourteen laser Stark transitions of the hot band (101) ← (001), previously reported in the literature, yielded the values of the dipole moment of ONF both in the (001) and (101) states.