Permanent Dipole Moment Difference Research Articles

Photophysical properties of 3-arylthioimidazo[1,2-a]pyridine derivatives: The role of peripheral electron-donating and electron-withdrawing groups in the advance of organic materials engineering

3-Arylthioimidazo[1,2-a]pyridine derivatives may present potential use in fields such as optoelectronics, organic light-emitting diodes (OLEDs), bioimaging, and may be employed as bioprobes in DNA and RNA. However, before these materials can be used for such applications, it is necessary to assess their photophysical properties. Here, we investigated the photophysical properties of five novel 3-arylthioimidazo[1,2-a]pyridine derivatives with different peripheral electron-withdrawing and electron-donating groups. The use of spectroscopic techniques, including UV–vis absorption, fluorescence, time-resolved fluorescence spectroscopy, combined with quantum chemical calculations, allowed us to gain a comprehensive understanding of the compounds' photophysical properties. The results revealed that the 3-arylthioimidazo[1,2-a]pyridine derivatives possess remarkable emissive properties, with a 4.5-fold increase when substituting the trifluoromethyl group for a methoxy group. Furthermore, it has been observed that the compounds containing peripheral groups with halogen atoms exhibit fluorescence quenching. The electron-withdrawing character of the imidazo[1,2-a]pyridine and the position of the lateral substituent hindered the impact of electron-withdrawing groups, while electron-donating groups enhanced the photophysical properties, resulting in increased Stokes shift, larger permanent dipole moment difference, and charge displacement. In conclusion, this study provides key insights into the photophysics of 3-arylthioimidazo[1,2-a]pyridine derivatives and the role of peripheral electron-donating and electron-withdrawing groups. These findings hold great potential for the advancement of optical materials and set foundation for future technological applications.

Influence of peripherical modifications in linear and nonlinear photophysical properties of imidazo[4,5-b]pyridine derivatives

This study reports a wide study on linear and nonlinear optical properties of six imidazo[4,5-b]pyridines derivates. The compounds are coupled with a series of peripheral groups forming different dipolar structures (D-π-A and A-π-A). Photophysical parameters such as the anisotropy coefficient, transition dipole moment, and permanent dipole moment difference were determined using spectroscopic techniques. The two-photon absorption (2PA) cross-section spectra were obtained by the tunable femtosecond Z-scan technique. In addition, quantum chemical calculations and the Sum Over States (SOS) model for two energy levels were applied to support the experimental data. A single 2PA band, with 2PA cross-section varying from 70 to 110 GM, was observed in the 2PA cross-section spectra in the molecules with only acceptor groups. In addition, the presence of a second band around 660 nm and an increase in 2PA cross-section values (around 110 to 130 GM in the principal band) was observed for the molecules with a combination of the methoxy radical (OMe) with halogen groups (D-π-A structure). These results indicate that the non-centrosymetric A-Imp-D structures facilitate the polarizability and internal charge transfer processes, resulting in photophysical and two-photon absorption properties enhancement compared to A-Imp-A structures.

Solvatochromic study of triphenylamine derivatives: Estimation of the permanent dipole moment difference

Solvatochromic study of triphenylamine derivatives: Estimation of the permanent dipole moment difference

Measurement of linewidths and permanent electric dipole moment change of the Ce4f-5dtransition in Y2SiO5for qubit readout scheme in rare-earth ion based quantum computing

In this work the inhomogeneous (zero-phonon line) and homogeneous line widths, and one projection of the permanent electric dipole moment change for the Ce 4f-5d transition in Y_2SiO_5 were measured in order to investigate the possibility for using Ce as a sensor to detect the hyperfine state of a spatially close-lying Pr or Eu ion. The experiments were carried out on Ce doped or Ce-Pr co-doped single Y_2SiO_5 crystals. The homogeneous line width was measured to be about 3 MHz, which is essentially limited by the excited state lifetime. Based on the line width measurements, the oscillator strength, absorption cross section and saturation intensity were calculated to be about 9*10^-7, 5*10^-19 m^2 and 1*10^7 W/m^2, respectively. One projection of the difference in permanent dipole moment, Delt_miu_Ce, between the ground and excited states of the Ce ion was measured as 6.3 * 10^-30 C*m, which is about 26 times as large as that of Pr ions. The measurements done on Ce ions indicate that the Ce ion is a promising candidate to be used as a probe to read out a single qubit ion state for the quantum computing using rare-earth ions.

Hole drift mobility in anthrone and antrachinone layers

Drift mobility of holes in antrachinone and anthrone polycrystalline thin films evaporated in the vacuum of the order of 10-5 Torr was measured with time-of-flight- method. The one order difference in mobility values for both –four ring acenes purified with zone melting before vaporization and identical in crystallization structure but with different molecule symmetry may have the origin in the presence of the difference in permanent dipole moment for both molecules.

Modeling non-Lorentzian two-photon absorption line shape in dipolar chromophores

We present a new approach to modeling of homogeneous line shape in two-photon absorption (2PA) spectra of chromophores with large permanent dipole moment difference between the ground- and excited electronic states using numerical solution of stochastic two-level density matrix equation of motion. Good agreement with experimental 2PA line shapes is obtained for S 1←S 0 transition of Styryl 9 M, which allows us to estimate that the permanent dipole moment difference varies in this chromophore within the S 1←S 0 band in the range, Δ μ=12–25 D.

New all-optical method for measuring molecular permanent dipole moment difference using two-photon absorption spectroscopy

Stark effect, in combination with spectral hole burning and single-molecule spectroscopy, has been a fruitful technique to study permanent electric dipole moment of molecules in condensed phase. However, because measuring Stark shifts relies on external fields and narrow line- or hole-widths, the applicability of this method at ambient conditions required by most biological systems has remained limited. Here we demonstrate a new all-optical method for measuring the molecular dipole moment difference between ground and excited states using two-photon absorption (2PA) spectroscopy. We show that the value and orientation of the static dipole moment difference can be determined from the corresponding absolute 2PA cross-section. We use this new method to determine for the first time the strength of local electric field E loc =0.1–1.0×10 8 V/cm inside beta-barrel of Fruit series of red fluorescent proteins. Because our method does not rely on external field and is applicable in liquid solutions, it is well suited for the study of biological systems.

Modulation of supercontinuum generation and formation of an attosecond pulse from a generalized two-level medium

The propagation of ultrashort pulses in a generalized two-level system, consisting of permanent dipole moments, is simulated by solving the full Maxwell–Bloch equations. Special attention has been paid to the supercontinuum generation of spectra and the formation of attosecond (as) pulses. It is found that the supercontinuum generation is strongly modulated by both area and width of the pulse, resulting from the interference between the splitting pulses in the time domain and the implication of the time–energy uncertainty relation. The effect of the permanent dipole moment on the supercontinuum generation is discussed in detail. Calculations show that a well-shaped 132 as pulse can be generated from a 2 fs incoming pulse under the condition where the permanent dipole moment difference between two levels is equal to the transition dipole moment between them. Influences of carrier-envelope phase and time-dependent ionization on the spectral and temporal evolution of the ultrashort pulses are also discussed at length.

Strong Two-Photon Absorption in New Asymmetrically Substituted Porphyrins: Interference between Charge-Transfer and Intermediate-Resonance Pathways

We study two-photon absorption (2PA) in two series of new free-base porphyrins with 4-(diphenylamino)stilbene or 4,4'-bis-(diphenylamino)stilbene (BDPAS) attached via pi-conjugating linkers at the porphyrin meso-position. We show that this new substitution modality increases the 2PA cross section in the Soret band region (excitation wavelength 750-900 nm) of the core porphyrin by nearly 2 orders of magnitude, from sigma(2) approximately 10 GM for the meso-phenyl-substituted analogue to sigma(2) approximately 10(3) GM for the ethynyl-linked BDPAS-porphyrin dyad. The 2PA properties are quantitatively described by considering two different and interfering 2PA quantum transition pathways. The first path involves virtual transition via intermediate one-photon resonance. The second path bypasses the intermediate resonance and occurs due to a large permanent dipole moment difference between the ground and the final electronic states. To our best knowledge, this is the first experimental observation of the combined effect of these two pathways on one particular two-photon transition, resulting in quantum-interference-modulated 2PA strength.

Hystons, new quasi-particles, and electron transfer in bacterial photosynthesis

For photosynthetic purple bacteria the collective vibrations of the linked polar fragments and polar elements both in the LH2 antenna complex and reaction center (RC) with their protein environment are described as a motion of hystons, new quasi-particles. Taking into account the fact that hystons modulate the frequency of exciton transitions, the half-width of absorption spectra had been defined as a function of the difference in permanent dipole moment between excited and ground states. Obtained equations for the temperature dependence of full half-width provide an excellent fit to known experimental data for B800 spectra of Rb. sphaeroides and Rps. acidophila as well as for P870 primary electron donor band of Rb. sphaeroides with 63, 52 and 142 cm −1 hyston modes, respectively. The theory has been extended and applied to the calculation of temperature dependence at the rate of electron transfer (ET). After fitting to known data, it has been shown that the 304(±10) cm −1 hyston mode, localized mainly on the bacterioheophytin, governs the secondary ET (from the bacterioheophytin to ubiquinone) in the Rps. sphaeroides RC. It is also shown that the primary ET in the RCs of Rps. viridis and Rb. sphaeroides are controlled by special pair pseudo-hyston modes with frequencies 139(±11) and 166(±28) cm −1 , respectively.

Resonant coherent hyper-Raman scattering (CHRS). II. A theory on the general relationship between CHRS and coherent Raman scattering (CRS) processes

A theoretical description of the resonant coherent hyper-Raman scattering for an arbitrary system–bath interaction is presented. Particularly the relationship between the coherent hyper-Raman scattering (CHRS) and coherent Raman scattering (CRS) signals is established without alluding to any specific model of optical broadening. The permanent dipole moment difference between the excited and ground states of a given chromophore is shown to be proportional to the ratio of the CHRS to CRS signals, and it is proved that this result is valid regardless of the model for the chromophore–bath interaction.

Spectroscopy and Photochemistry of meso-Diphenyltribenzonaphthoporphyrin at Low Temperatures: A Novel System for Hole-Burning Applications

We present a custom-tailored material for the application of spectral hole-burning in optical storage and imaging spectroscopy. The photoactive molecule meso-diphenyltribenzonaphthoporphyrin (P2TriBNP) combines the advantages of chlorins and simple porphyrins used so far, namely, the spectral separation between photoeduct and photoproduct states and a high photochemical hole-burning yield. Basic characteristics of the material were measured. We obtained the values of 3.2 ± 1.5% for the hole-burning yield, of ∼0.6 for the Debye−Waller factor in poly(vinyl butyral) (PVB), and a spectral separation between the educt and product absorption of 120 cm-1. The difference in the dipole moment between the ground and the excited state shows two contributions. The permanent dipole moment difference has a value of 0.29 ± 0.03 D. A root mean square value of 0.15 ± 0.02 D for a distributed, matrix-induced component was measured. Because of their high photochemical yields and the large absorption cross sections, P2TriBNP and similar compounds open the way for a new class of experiments dealing with lower light intensities and shorter exposure times than before. However, we observed the lifetime of narrow spectral holes to be limited to several hours. Nevertheless, the material exhibits a high potential for the application in imaging spectroscopy, e.g., the simultaneous recording of spatial and spectral information of astronomical objects.

Stark effect on spectral holes of chlorophyll a at 2 K

We studied the electric field effect on spectral holes burned in the S 1←S 0 absorption band of in poly(vinyl butyral) film doped chlorophyll (Chl) a at a temperature of T=2.0 K. With the laser light beam collinearly applied, the external electric field produced a symmetric broadening of the holes which showed a linear dependence on the field strength. This effect was ascribed to the matrix-induced dipole moment difference ( δμ ind) between the S 1 and S 0 states of Chl a. The relatively small value of Δμ ind (0.217 is in accord with the change of the molecular polarizability upon electronic excitation. The low-temperature permanent dipole moment difference of Chl a was estimated to be less than 0.1 D.

Electric field modulation of the fluorescence from Rhodobacter sphaeroides reaction centers

The fluorescence intensity from randomly oriented Rhodobacter ( Rb.) sphaeroides photosynthetic reaction centers at 77 K increases in an applied electric field. This is ascribed to a net reduction in the rate of the initial electron transfer reaction which competes with fluorescence due to the effect of the field on the free energy of charge separation. Quantitative analysis of the fluorescence change in an electric field indicates that the difference in permanent dipole moment between the ground and initially formed excited state is greater than between the ground and emitting state.

Stark effect spectroscopy of Rhodobacter sphaeroides and Rhodopseudomonas viridis reaction centers

The nature of the initially excited state of the primary electron donor or special pair has been investigated by Stark effect spectroscopy for reaction centers from the photosynthetic bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides at 77 K. The data provide values for the magnitude of the difference in permanent dipole moment between the ground and excited state, [unk]Deltamu[unk], and the angle [unk] between Deltamu and the transition dipole moment for the electronic transition. [unk]Deltamu[unk] and [unk] for the lowest-energy singlet electronic transition associated with the special pair primary electron donor were found to be very similar for the two species. [unk]Deltamu[unk] for this transition is substantially larger than for the Q(y) transitions of the monomeric pigments in the reaction center or for pure monomeric bacteriochlorophylls, for which Stark data are also reported. We conclude that the excited state of the special pair has substantial charge-transfer character, and we suggest that charge separation in bacterial photosynthesis is initiated immediately upon photoexcitation of the special pair. Data for Rhodobacter sphaeroides between 340 and 1340 nm are presented and discussed in the context of the detection of charge-transfer states by Stark effect spectroscopy.

Single beam two-photon optical bistability in a submicron size Fabry-Perot cavity

We describe a semiclassical theoretical model on two-photon optical bistability in a nonlinear Fabry-Perot cavity filled with molecules having large permanent dipole moments under the plane wave and mean field approximations. The mathematical analysis follows with the assumption of a two-level system with large permanent dipoles. In the steady state situation, field equation is shown to provide the bistable behavior of the nonlinear cavity when resonant with two-photon excitation under certain conditions. It is shown that for a large difference in permanent dipole moment \mu_{0} = (\mu_{aa}-\mu_{bb})/2 between levels |a\rangle and |b\rangle, (\mu_{0}^{2}/|\mu_{ba}|^{2}\gg1 , where μ ba is the transition dipole moment), the two-photon optical bistability can result in a submicron size Fabry-Perot cavity.

THE EFFECT OF MOLECULAR POLARIZATION ON THE ELECTROCHROMISM OF CAROTENOIDS—I. THE INFLUENCE OF A CARBOXYLIC GROUP

Abstract— Electrochromism of oriented all‐trans‐β‐apo‐8′‐carotenoic acid is studied in thin capacitors. The linear electrochromism is very strong, in contrast to that of symmetrical carotenoids. It is proportional to the first derivative of the absorption spectrum. The quadratic electrochromism can be described as a superposition of fractions proportional to the first and second derivatives of the absorption spectrum. The permanent dipole moment difference between the ground state and the excited state of the carotenoic acid molecule is Δμ= 3.6 × 10‐29 C·m (±20%) (10.7 Debyes). The polarizability difference parallel to the long axis of the molecule is Δα|| = 1.17 × 10‐37 C·m2·V‐1 (±20%) (1050 Å3). Furthermore, the relative permittivity of the solid carotenoic ethyl ester is r= 3.5 ± 0.2.Δμ is due to the polarizing force of the carboxylic group. This force is equivalent to a mean local electric field of Ft≅3 × 106V/cm. Such a “local field” may also be exerted on a symmetrical carotenoid in the membrane of photosynthesis, e.g. by asymmetrical complex formation with a polarizing molecule. To obtain an effective permanent field of Fp≅ 2 × 106V/cm across the membrane, as postulated in photosynthesis, a local field of Fl≅ 5.5 × 105 V/cm would be sufficient. Fp is shown to be directed from inside to outside of the thylakoid. To realize this, e.g. a positive polar (i.e. electron‐attracting) complex partner of the carotenoid, located more to the inside of the thylakoid, can be postulated.