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Bromination and iodination of title double decker (DD) phenylsilsesquioxane macromonomers occurs at ortho and para positions, respectively, as in PhT8,10,12 cages. Heck cross-coupling with 4-Me/CNs...

Macromonomers [RPhSiO1.5]8,10,12 and [RCH═CHSiO1.5]8,10,12, where R is a conjugated group, have previously been shown to offer photophysical properties wherein excitation promotes an electron from ...

The subject of electromagnetism has often been called electrodynamics to emphasize the dominance of the electric field in dynamic light–matter interactions that take place under non-relativistic conditions. Here we show experimentally that the often neglected optical magnetic field can nevertheless play an important role in a class of optical nonlinearities driven by both the electric and magnetic components of light at modest (non-relativistic) intensities. We specifically report the observation of magneto-electric rectification, a previously unexplored nonlinearity at the molecular level which has important potential for energy conversion, ultrafast switching, nano-photonics, and nonlinear optics. Our experiments were carried out in nanocrystalline pentacene thin films possessing spatial inversion symmetry that prohibited second-order, all-electric nonlinearities but allowed magneto-electric rectification., The role of the optical magnetic field is generally not considered at nonrelativistic light intensities. Here the authors show magneto-electric rectification, an optical nonlinear effect due to electric and magnetic field coupling, in a thin film of the organic semiconductor pentacene at non-relativistic intensities.

Probing transient states in molecules having vibronic transitions with femtosecond (fs) laser pulses often results in coherent oscillations either in the ground state, the excited states, or both. We find such coherent oscillations are highly solvent-dependent and provide a holistic overview of the pump-probe experiments for ultrafast dye dynamics at interfaces. For molecules dissolved in single solvents, modulations in oscillations occur due to transitions in the sub-vibrational levels of the electronic state. For binary solvents, in particular, these modulations are strongly sensitive to solvent compositions. The changes induced by various solvent compositions are drastic enough to act as a control parameter for dynamical control processes. We demonstrate an end-to-end understanding of ground-state coherent oscillations, vibrational cooling, ground-state recovery processes, and excited-state dynamics through a series of experiments. We further present a methodology for establishing such control using near-infrared dyes to measure the oscillations with fs pump-probe techniques. In the case of immiscible binary solvents, the same method allows us to investigate the liquid–liquid interface. Our control methodology is validated by an experiment using a cyanine dye dissolved in dimethyl sulfoxide, interfaced with neat diethyl-ether. The dye dynamics are retarded on moving from the bulk dye solution towards the interface with the neat diethyl-ether. When sampled along the direction of the vector pointing from the bulk towards the near interface, monotonically decreasing time constants are obtained. This result strongly suggests the importance of microheterogeneity in interfacial dynamics.

We present ultrafast-depopulation-dynamics of higher-lying-excited-states using femtosecond fluorescence up-conversion techniques for two near-infrared (NIR) tricarbocyanine dyes (IR144 and IR140) in primary alcohols. With visible excitation wavelengths, such dyes show two distinct emission-bands with large peak wavelength difference: one at the visible region: S2 → S0, and the other at NIR region: S1 → S0. We show that exact band-positions, intensities, and fluorescence-decay-timescales (τ) depend strongly on viscosity and polarity of solvents. Interestingly, though the faster component of τ increased for IR144 with increasing viscosity and chain-length of alcohols, the reverse was seen for IR140, indicating the possible formation of ion-pair of IR140 with alcohols.

We report induced second harmonic generation mediated by a new rectification nonlinearity in polycrystalline pentacene. Results with tilted pulse fronts confirm its magnetoelectric origin, ultrafast character and capability of supporting all-optical switching at right angles.

Effect of solution pH on two-photon absorption cross-section of highly fluorescent Coumarin and Rhodamine dyes with high repetition rate femtosecond laser pulses at 780 nm is presented using two-photon induced fluorescence technique. A correspondence in the measured two-photon and single-photon cross-section values is seen when the pH changes from acidic to basic conditions (pH = 2–10) for solutions in 1:1 water-ethanol binary mixture. By plotting changes in the single-photon and two-photon fluorescence in this pH range, the excited state pKa values are found. The ground state pKa values are also affected by the protonation deprotonation equilibrium as a result of variation in pH from acidic to basic, which are characterized by changes in absorbance spectra. Most of these single-photon and two-photon induced fluorescence spectra show characteristic blue shifts. Different fluorescence quantum yields calculated at each pH reflect a change in structure corresponding to their associated properties as a result of acid base equilibrium.

Cross-polarized light scattering in silsesquioxane (SSQ) molecules probes the azimuthal variation of intramolecular potentials. SSQ molecules with small and large cage interiors reveal variations in the sphericity of orbital shapes for the first time.

We report harmonic generation mediated by magneto-electric charge separation in pentacene and optically-induced magnetization in a series of liquid tetrahalides for the first time.

Using a degenerate pump probe technique at 800 nm, Ground State Vibrational Coherence (GSVC) of a cyanine dye (IR780) is explored in various solvents. Five representative solvents were studied, of which GSVC is present in three: methanol (MeOH), ethanol (EtOH) and dichloromethane (DCM). The maximum number of damped oscillations was observed in DCM solvent and the oscillations persisted for 1.2 ps. However, GSVC is absent in two solvents: acetonitrile (ACN) and dimethyl sulfoxide (DMSO). For nondegenerate pump probe studies with a 400 nm pump and a supercontinuum probe, GSVC was absent for IR780 in all of the five solvents studied. Instead these studies showed excited state absorption corresponding to S1 → Sn absorption bands in the 450 to 650 nm range. Decay profiles for these correspond to the higher electronic states (Sn, n ≥ 2) in contrast to the degenerate case involving S1. Complete dynamics of IR780 with 30 fs pulses are reported by invoking three time-constants to fit the kinetic traces that can, in turn, be assigned as: vibrational cooling, intermediate state time, and ground state recovery (GSR). All these dynamics exhibit noticeable solvent dependence. A plateau region is also observed in the early part of the GSR dynamics, strongly suggesting the existence of an intermediate state between the ground (S0) and excited singlet (S1) states.

The nonlinear optical absorption properties of three meso-substituted aromatic, neutral tetrathia[22]porphyrin(2.1.2.1) compounds were investigated using an open aperture Z-scan technique at 527 nm with a pulse width of 100 ns in dichloromethane. The responses obtained were accounted for by invoking reverse saturable absorption arising due to strong excited state absorption. Both ground and excited state absorption cross-sections are reported, which follow the same trend as the respective structural parameters.

Ultrafast dynamics and third-order nonlinear optical studies of tetraoxa[22]porphyrin(2.1.2.1)s are reported using a single beam Z-scan technique with femtosecond (fs) and nanosecond (ns) pulses at respective near-IR and visible wavelengths. Excited state absorption mediated reverse saturable absorption was observed at 527 nm while two-photon absorption was observed at 800 nm. High nonlinear refractive index coefficient (γ) and nonlinear absorption coefficient (β) values and the fast response times were attributed to the availability of polarizable π-electrons of the cyclic conjugated aromatic porphyrin framework. Our proposed mechanism of third-order nonlinearity was verified using ultrafast transient absorption spectroscopy. With the observed fast electronic nonlinearities the studied molecules are potential candidates for use as optical limiters.

Optical interactions involving both electric and magnetic fields in two-photon scattering can stimulate molecular librations/rotations, via torque-enhanced magnetic transitions. Here we report ultrafast observations of magneto-electrically-induced vibrations and rotations.

Rotational assignments and polarization states of inelastic components in the cross-polarized spectrum of light scattered by liquid samples of tetrahedral molecules provide direct evidence of a theorized enhancement mechanism for induced magnetism.

Ultrafast depopulation dynamics of highly excited states of near-infrared (NIR) tri-carbocyanine dyes is presented in primary alcohols using the femtosecond fluorescence upconversion technique. Pronounced solvent dependence is observed in the dynamics of the highly excited states. Typically, the tri-carbocyanine dyes have strong absorption and fluorescence with a single peak in the NIR region, however, on tuning the excitation wavelength to the visible region, two distinct emission bands are seen with a large peak wavelength difference. Such dual fluorescent peaks correspond to the respective transitions: S2 → S0 for the visible region and S1 → S0 for the NIR region. The exact fluorescent band positions or their intensities strongly depend on the viscosity and polarity of the solvents. The fluorescence decay timescales measured using fluorescence upconversion techniques with femtosecond time resolution also vary significantly as a function of solvent polarity. The faster decay on the ~250 fs and slower decay on the order of picosecond timescale was found which are strongly depending with increasing the chain length of alcohol. Specifically, for example, in the IR144 dye, the faster decay time constant (τ1) increase with the increase in the viscosity and chain length of the alcohol while, in the case of the IR140 dye, we noticed that the τ1 value decreases with increasing viscosity and chain length of alcohols. We invoke the ion-pair formation of IR140 dye with alcohols to explain such behavior.

We report experimental observations of a mechanism that potentially supports and intensifies induced magnetization at optical frequencies without the intervention of spin-orbit or spin-spin interactions. Energy-resolved spectra of scattered light, recorded at moderate intensities (108 W/cm2) and short timescales (

We show novel dual fluorescent behavior of a popular indocyanine dye molecule, IR125, in dimethyl-sulfoxide solvent when excited at 527 nm instead of the conventional near-infrared excitation. We argue that the dual fluorescence at 571 nm and at 851 nm are from two excited electronic states of IR125. Intensity of dual fluorescence changes as a function of incident laser polarization. All fluorescence intensities are higher for circular polarization as compared to linear or elliptical polarizations. Such enhanced fluorescence for circular polarization component of incident laser at different laser powers indicates involvement of higher excited states in the fluorescence of IR125.

Anomalous SA (saturable absorption) in presence of reverse saturable absorption (RSA) in Rhodamine-700 is reported employing Z-scan with 100 ns, 250 Hz pulses in methanol. SA is observed only at a particular intensity range otherwise RSA survives.

Ultrafast pump-probe technique has been invoked to understand the liquid:liquid interface by using a cyanine dye dissolved in dimethyl sulfoxide (DMSO), interfaced with neat diethyl-ether (Et2O). We show that the dynamics of the dye gets slow down on moving from the bulk dye solution in DMSO towards the interface with neat-diethyl ether. Hence we get an alternate picture of interface by looking at the time constants. Four time constants were used for fitting the experimental data which are assigned to be coherent artifact, vibrational cooling, intermediate state time and finally ground state recovery time. The drastic change in signal was observed at a distance of 0.1 mm from the interface. The decreasing time constants from bulk to near interfaced solution strongly suggests involvement of heterogeneity in the system because of the penetration of diethyl ether through the interface into the DMSO layer where the dye is dissolved, which strongly affects the dynamics at near the interface of the DMSO with neat-diethyl ether layer (at 0.1 mm distance from the interface).

Third order nonlinearities of Mn(III)-Phthalocyanine chloride in dimethyl-sulphoxide under 50 fs pulses, operating at 94 MHz, by eliminating cumulative thermal effects have been investigated and reported by us. Modifications were done in data acquisition during Z-scan experiment, which included recording of time evolution waveform traces in an oscilloscope and not collection of Z versus transmission and utilization of a chopper of a suitable duty cycle. Time evolution traces were further processed analytically through MatLab® programming, which yielded Z-scan traces similar to what was obtained with single shot 50 fs pulse. We observed reverse saturable absorption at 800 nm owing to excited state absorption. We show that the nonlinear refractive index (γ) and nonlinear absorption coefficient (β) are over estimated almost 100 times, when MHz pulses are used compared to a situation, where thermo-optical nonlinearities are accounted. Illumination and dark periods are carefully set in a way, so that the sampl...