Effects of Matrix Temperature and Rigidity on the Electronic Properties of Solvatochromic Molecules: Electroabsorption of Coumarin 153

Poly(dimethylsiloxane) as a room-temperature solid solvent for photophysics and photochemistry

Medium viscosity strongly affects the dynamics of solvated species and can drastically alter the deactivation pathways of their excited states. This study demonstrates the utility of poly(dimethylsiloxane) (PDMS) as a room-temperature solid-state medium for optical spectroscopy. As a thermoset elastic polymer, PDMS is transparent in the near ultraviolet, visible, and near infrared spectral regions. It is easy to mould into any shape, forming surfaces with a pronounced smoothness. While PDMS is broadly used for the fabrication of microfluidic devices, it swells in organic solvents, presenting severe limitations for the utility of such devices for applications employing non-aqueous fluids. Nevertheless, this swelling is reversible, which proves immensely beneficial for loading samples into the PDMS solid matrix. Transferring molecular-rotor dyes (used for staining prokaryotic cells and amyloid proteins) from non-viscous solvents into PDMS induces orders-of-magnitude enhancement of their fluorescence quantum yield and excited-state lifetimes, providing mechanistic insights about their deactivation pathways. These findings demonstrate the unexplored potential of PDMS as a solid solvent for optical applications.

Solvation dynamics: improved reproduction of the time-dependent Stokes shift with polarizable empirical force field chromophore models

The inclusion of explicit polarization in molecular dynamics simulation has gained increasing interest during the last several years. An understudied area is the role of polarizability in computer simulations of solvation dynamics around chromophores, particularly for the large solutes used in experimental studies. In this work, we present fully polarizable ground and excited state force fields for the common fluorophores N-methyl-6-oxyquinolium betaine and Coumarin 153. While analyzing the solvation responses in water, methanol, and the highly viscous ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate we found that the inclusion of solute polarizability considerably increases the agreement of the obtained Stokes shift relaxation functions with experimental data. Solute polarizability slows down the inertial solvation response in the femtosecond time regime and enables the chromophore to adapt its dipole moment to the environment. Furthermore, the developed chromophore force field reproduces the solute dipole moments in both the electronic ground and excited state in environments ranging from gas phase to highly polar media correctly. Based on these studies it is anticipated that polarizable models of chromophores will lead to an improved understanding of the relationship of their environment to their spectroscopic properties.

Mechanism of Ligand-Controlled Emission in Silicon Nanoparticles

Although bulk silicon (Si) is known to be a poor emitter, Si nanoparticles (NPs) exhibit size-dependent photoluminescence in the red or near-infrared due to quantum confinement. Recently, it has been shown that surface modification of Si NPs with nitrogen-capped ligands results in bluer emission wavelengths and quantum yields of up to 90%. However, the emission mechanism operating in these surface-modified Si NPs and the factors that determine their emission maxima are still unclear. Here, the emission in these species is shown to arise from a charge-transfer state between the Si surface and the ligand. The energy of this state is linearly correlated to the calculated ground-state dipole moment of the free ligand. This trend can be used in a predictive fashion for the design and synthesis of Si NPs with a broader range of emission wavelengths.

Solvent effects on excitation energies obtained using the state-specific TD-DFT method with a polarizable continuum model based on constrained equilibrium thermodynamics

Nonequilibrium solvation effects need to be treated properly in the study of electronic absorption processes of solutes since solvent polarization is not in equilibrium with the excited-state charge density of the solute. In this work, we developed a state specific (SS) method based on the novel nonequilibrium solvation model with constrained equilibrium manipulation to account for solvation effects in electronic absorption processes. Time-dependent density functional theory (TD-DFT) is adopted to calculate electronic excitation energies and a polarizable continuum model is employed in the treatment of bulk solvent effects on both the ground and excited electronic states. The equations based on this novel nonequilibrium solvation model in the framework of TDDFT to calculate vertical excitation energy are presented and implemented in the Q-Chem package. The implementation is validated by comparing reorganization energies for charge transfer excitations between two atoms obtained from Q-Chem and those obtained using a two-sphere model. Solvent effects on electronic transitions of coumarin 153 (C153), acetone, pyridine, (2E)-3-(3,4-dimethoxyphenyl)-1-(2-hydroxyphenyl)prop-2-en-1-one (DMHP), and uracil in different solvents are investigated using the newly developed code. Our results show that the obtained vertical excitation energies as well as spectral shifts generally agree better with the available experimental values than those obtained using the traditional nonequlibrium solvation model. This new model is thus appropriate to study nonequilibrium excitation processes in solution.

Two-photon voltmeter for measuring a molecular electric field

We present a new approach for determining the strength of the dipolar solute-induced reaction field, along with the ground- and excited-state electrostatic dipole moments and polarizability of a solvated chromophore, using exclusively one-photon and two-photon absorption measurements. We verify the approach on two benchmark chromophores N,N-dimethyl-6-propionyl-2-naphthylamine (prodan) and coumarin 153 (C153) in a series of toluene/dimethyl sulfoxide (DMSO) mixtures and find that the experimental values show good quantitative agreement with literature and our quantum-chemical calculations. Our results indicate that the reaction field varies in a surprisingly broad range, 0-10(7)  V cm(-1) , and that at close proximity, on the order of the chromophore radius, the effective dielectric constant of the solute-solvent system displays a unique functional dependence on the bulk dielectric constant, offering new insight into the close-range molecular interaction.

Plasticization of poly(vinylpyrrolidone) thin films under ambient humidity: insight from single-molecule tracer diffusion dynamics

Studies on diffusion dynamics of single molecules (SMs) have been useful in revealing inhomogeneity of polymer thin films near and above the glass-transition temperature (T(g)). However, despite several applications of polymer thin films where exposure to solvent (or vapor) is common, the effect of absorbed solvent molecules on local morphology and rigidity of polymer matrices is yet to be explored in detail. High-T(g) hydrophilic polymers such as poly(vinylpyrrolidone) (PVP) are used as pharmaceutical coatings for drug release in aqueous medium, as they readily absorb moisture, which results in effective lowering of the T(g) and thereby leads to plasticization. The effect of moisture absorption on swelling and softening of PVP thin films was investigated by visualizing the diffusion dynamics of rhodamine 6G (Rh6G) tracer molecules at various ambient relative humidities (RH). Wide-field epifluorescence microscopy, in conjunction with high-resolution SM tracking, was used to monitor the spatiotemporal evolution of individual tracers under varied moisture contents of the matrix. In the absence of atmospheric moisture, Rh6G molecules in dry PVP films are translationally inactive, suggestive of rigid local environments. Under low moisture contents (RH 30-50%), translational mobility remains arrested but rotational motion is augmented, indicating slight swelling of the polymer network which marks the onset of plasticization. The translational mobility of Rh6G was found to be triggered only at a threshold ambient RH, beyond which a large proportion of tracers exhibit extensive diffusion dynamics. Interestingly, SM tracking data at higher moisture contents of the film (RH ≥ 60%) reveal that the distributions of dynamic parameters (such as diffusivity) are remarkably broad, spanning several orders of magnitude. Furthermore, Rh6G molecules display a wide variety of translational motion even at a fixed ambient RH, clearly pointing out the extremely inhomogeneous environment of plasticized PVP network. Intriguingly, it is observed that a majority of tracers undergo anomalous subdiffusion even under high moisture contents of the matrix. Analyses of SM trajectories using velocity autocorrelation function reveal that subdiffusive behaviors of Rh6G are likely to originate from fractional Brownian motion, a signature of tracer dynamics in viscoelastic medium.

Dynamics of water and ions near DNA: comparison of simulation to time-resolved stokes-shift experiments

Time-resolved Stokes-shift experiments measure the dynamics of biomolecules and of the perturbed solvent near them on subnanosecond time scales, but molecular dynamics simulations are needed to provide a clear interpretation of the results. Here we show that simulations using standard methods quantitatively reproduce the main features of TRSS experiments in DNA and provide a molecular assignment for the dynamics. The simulations reproduce the magnitude and unusual power-law dynamics of the Stokes shift seen in recent experiments [ Andreatta, D., et al. J. Am. Chem. Soc. 2005, 127, 7270 ]. A polarization model is introduced to eliminate cross-correlations between the different components contributing to the signal. Using this model, well-defined contributions of the DNA, water, and counterion to the experimental signal are extracted. Water is found to have the largest contribution and to be responsible for the power-law dynamics. The counterions have a smaller, but non-negligible, contribution with a time constant of 220 ps. The contribution to the signal of the DNA itself is minor and fits a 30 ps stretched exponential. Both time-averaged and dynamic distributions are calculated. They show a small subset of ions with a different coupling but no other evidence of substates or rate heterogeneity.

Effects of solvation on one- and two-photon spectra of coumarin derivatives: A time-dependent density functional theory study

We report one- and two-photon absorption excitation energies and cross sections for a series of 7-aminocoumarins using time-dependent density functional theory with various basis sets and functionals, including exchange-correlation functionals using the Coulomb-attenuating method, to evaluate their performance in the gas phase and in solvents. Except for the results of one functional, the computed one-photon excitation energies and transition dipole moments are in good agreement with experiment. The range of errors obtained from various functionals is discussed in detail. The relationship of donor and acceptor groups with the one- and two-photon resonances and intensities is also discussed.

Effects of Solute Electronic Polarizability on Solvation in a Room-Temperature Ionic Liquid

The effects of solute polarizability on solvation and solute transport in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI+PF(6)-) are investigated via molecular dynamics simulations. A valence-bond description is employed to account for the instantaneous adjustment of the solute electronic charge distribution to the fluctuating solvent environment. It is found that the ultrafast inertial component of solvation dynamics becomes slower as the solute polarizability grows. Moreover, its contribution to overall solvent relaxation becomes reduced with increasing polarizability, especially in the case of nonequilibrium solvation dynamics. Overall, the inclusion of the solute electronic polarizability in the simulations improves the agreement with time-dependent Stokes shift measurements.

Surface-assisted transient displacement charge technique. I. Photoinduced charge transfer in self-assembled monolayers

Surface-assisted photoinduced transient displacement charge (SPTDC) technique was developed in order to study light-induced charge transfer in surface-bound molecules and applied to investigation of self-assembled monolayers of 7-diethylaminocoumarin and 2,4-dinitrophenylamine. The dipole moment change measured by SPTDC correlates reasonably well with that measured in solution by standard PTDC technique and with semiempirical calculations. Shortening of the excited-state lifetime of surface-immobilized coumarin due to stimulated emission was observed in both fluorescence and dipole measurements. The dipole signal decline in low-polarity solvents indicates the importance of dipole-dipole interaction that causes reorientation of molecules upon photoexcitation.

Electroabsorption Spectroscopy of 6-Hydroxyquinoline Doped in Polymer Films: Stark Shifts and Orientational Effects

Stark absorption spectroscopy was applied to 6-hydroxyquinoline (6-HQ) doped in polymer films of poly(methyl methacrylate) (PMMA) and poly(vinyl alcohol) (PVA) at temperatures of 50-300 K. The electroabsorption (E-A) spectrum of 6-HQ markedly depends on temperature in a PMMA film. The polarization dependence as well as the temperature dependence of the E-A spectra reveals that 6-HQ is oriented along the direction of the applied electric field at room temperature in a PMMA film. As the temperature becomes lower, the field-induced orientation of 6-HQ is restricted, and only the Stark shift induced by a change in electric dipole moment and in molecular polarizability is observed. On the other hand, E-A spectra of 6-HQ doped in a PVA film are essentially independent of temperature, suggesting that 6-HQ is not oriented along the electric field even at room temperature in PVA. These results show that the molecular motion of 6-HQ in a polymer film is very sensitive to the microenvironment of the surrounding matrix.

Solvation of Coumarin 153 in Supercritical Fluoroform

We present a study of local density augmentation around an attractive solute (i.e., giving rise to more attractive interaction with the solvent than solvent-solvent interactions) in supercritical fluoroform. This work is based on molecular dynamics simulations of coumarin 153 in supercritical fluoroform at densities both above and below the critical density, ranging from dilute gas-like to liquid-like, at a reduced temperature (T/T(c)) of 1.03. We focused on studying the structure of the solvation shell and the variation of the solute electronic absorption and emission shifts with density. Quantum calculations at the density functional theory (DFT) level were run on the solute in the ground state, and time-dependent DFT calculations were performed in the solute excited state in order to determine the solute-solvent potential parameters. The results obtained for the Stokes shift are in agreement with the experimental measurements. To evaluate local density augmentation from simulations, we used two different definitions, one based on the solvation number and the other derived from solvatochromic shifts. In the former case, the agreement with experimental results is good, while, in the latter case, better agreement is achieved by perturbatively including the induced-dipole contribution to the solvation energy.

Polar Solvation and Solvation Dynamics in Supercritical CHF3: Results from Experiment and Simulation

Solvation dynamics of the probe trans-4-(dimethylamino)-4'-cyanostilbene (DCS) have been measured in supercritical fluoroform at 310 K (1.04 Tc) and solvent densities over the range 1.4-2.0 rho(c) using optical Kerr-gated emission spectroscopy. Steady-state measurements and computer simulations of this and the related system coumarin 153 (C153) in fluoroform are used to help interpret the observed dynamics. The solvent contribution to the Stokes shift of DCS is estimated to be 2300 +/- 400 cm(-1) and nearly density independent over the range (0.7-2.0)rho(c). Spectral response functions are bimodal and can be fit to biexponential functions having time constants of approximately 0.5 ps (85%) and 3-10 ps (15%) over the observable range ((1.4-2.0)rho(c)). Computer simulations based on a 2-site model of fluoroform and assuming an electrostatic solvation mechanism appear to properly account for the magnitude and weak density dependence of the Stokes shifts but predict much faster solvation than is observed. Possible reasons for the discrepancy are discussed.

Microviscosity in Multiple Regions of Complex Aqueous Solutions of Poly(ethylene oxide)−Poly(propylene oxide)−Poly(ethylene oxide)

Aqueous poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO109-PPO41-PEO109) copolymers are nonionic surfactants that self-organize to form aggregate structures with increasing temperature or concentration. We have studied two concentrations over a range of temperatures so that the copolymers are in one of three microphases: unimers, micelles, or hydrogels formed from body centered cubic aggregates of micelles. Three different coumarin dyes were chosen based on their hydrophobicity so that different aggregate regions could be probed independently-water insoluble coumarin 153 (C153), hydrophobic coumarin 102 (C102), and the hydrophilic sodium carboxylate form of coumarin 343 (C343-). Fluorescence anisotropy experiments provide detailed information on the local microviscosity. C153 experiences a fourfold increase in reorientation time and hence microviscosity with increasing temperature through the microphase transition from unimers to micelles. C102 also shows an increase in microviscosity with temperature but smaller in magnitude and with the microphase transition shifted to higher temperature relative to C153. C343- shows only a slight sensitivity to the microphase transition. For any of the three coumarin probes, fluorescence anisotropies do not show any correlation with the microphase transition to form cubic hydrogels.

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.

Fluorescence probing of interior, interfacial, and exterior regions in solution aggregates of poly(ethylene oxide)- poly(propylene oxide)-poly(ethylene oxide) triblock copolymers

Fluorescence spectroscopy is used to probe local environments within regions of different polarity and hydrophobicity in aqueous aggregates of PEO(109)-PPO(41)-PEO(109) triblock copolymers. These copolymer aggregates have well characterized microphases in aqueous solution. Concentrations and temperatures for our studies are chosen such that the copolymers are in unimer, micellar, or micellar hydrogel forms. The observed fluorescence spectra and lifetimes from solutions individually labeled with each of the three coumarin probes report on the changes in the local polarity of the core, exterior, interfacial, and corona regions of these copolymer aggregates. This multiple fluorescence probe methodology will be straightforward to apply in general to problems in polymer and biopolymer aggregates, especially those that display strong hydrophobic effects.

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.

Stark spectroscopy on photoactive yellow protein, E46Q, and a nonisomerizing derivative, probes photo-induced charge motion

The change in the electrostatic properties on excitation of the cofactor of wild-type photoactive yellow protein (WT-PYP) have been directly determined using Stark-effect spectroscopy. We find that, instantaneously on photon absorption, there is a large change in the permanent dipole moment, /Delta(-->)mu/, (26 Debye) and in the polarizability, (-)Deltaalpha, (1000 A(3)). We expect such a large degree of charge motion to have a significant impact on the photocycle that is associated with the important blue-light negative phototactic response of Halorhodospira halophila. Furthermore, changing E46 to Q in WT-PYP does not significantly alter its electrostatic properties, whereas, altering the chromophore to prevent it from undergoing trans-cis isomerization results in a significant diminution of /Delta(-->)mu/ and (-)Deltaalpha. We propose that the enormous charge motion that occurs on excitation of 4-hydroxycinnamyl thioester, the chromophore in WT-PYP, plays a crucial role in initiating the photocycle by translocation of the negative charge, localized on the phenolate oxygen in the ground state, across the chromophore. We hypothesize that this charge motion would consequently increase the flexibility of the thioester tail thereby decreasing the activation barrier for the rotation of this moiety in the excited state.

Measurement of the electronic properties of the flavoprotein old yellow enzyme (OYE) and the OYE:p-Cl phenol charge-transfer complex using Stark spectroscopy

Low-temperature absorption and Stark spectroscopy have been used to study the electronic properties of oxidized flavin mononucleotide (FMN) in old yellow enzyme (OYE) and OYE complexed with p-chlorophenol (p-Cl phenol). The low-temperature absorbance spectrum of OYE showed splittings of the blue and near-UV vibronic bands, which appears to be due to hydrogen bonding between the isoalloxazine moiety and the protein. A Stark spectroscopic analysis showed that the electronic structure of the FMN cofactor in OYE is not significantly perturbed relative to flavins in simple solvents. However, the charge-transfer band in the OYE:p-Cl phenol complex showed a large Stark effect indicative of substantial charge displacement. The magnitude and direction of this charge displacement are consistent with significant charge transfer along the charge-transfer transition dipole moment direction. In addition, the Stark spectrum of the CT band showed unexpected fine structure that could correlate with vibrational progressions in either the p-Cl phenol donor or the flavin acceptor.