Solvents Of Different Viscosity Research Articles

Photocyclization reaction and related photodynamics in the photoproducts of a tetraphenylethylene derivative with bulky substituents: unexpected solvent viscosity effect.

Tetraphenylethylene (TPE) derivatives are ones of the most versatile building blocks showing aggregation-induced emission (AIE). However, their applications are limited by the photophysical and photochemical processes that occur in their excited state. Herein, we report a detailed study of the photochemical behaviour of a new TPE derivative with bulky terphenyl groups (TTECOOBu) in solvents of different viscosities and in a PMMA film. UV light irradiation shows an efficient photocyclization reaction, which produces a 9,10-diphenylphenanthrene (DPP) derivative photoproduct. The emission spectra of the irradiated samples show intermediate (∼420 nm) and final (∼380 nm) species. The photocyclization events are more efficient in environments of higher viscosities or rigidity. We show that in a photoirradiated PMMA film containing TTECOOBu, it is possible to etch a message for more than 1 year. The kinetics is dictated by the motions of the phenyl rings and is faster when their motions are precluded or inhibited. We also elucidated the femto- to millisecond photodynamics of the intermediate and final photoproducts and provide a full picture of their relaxation, with the latter in ∼1 ns at S1 and ∼1 μs at T1. We also demonstrate that the kinetics of the bulky TTECOOBu is much slower than that of the TPE core. Our results also show that both photoevents are not reversible contrary to the case of TPE kinetics. We believe that these results will shed more light on the photochemical behaviour of TPE derivatives and should help in the development of novel TPE-based materials with improved photostability and photo-properties.

Torsional disorder and planarization dynamics: 9,10-bis(phenylethynyl)anthracene as a case study.

Conjugated molecules with phenylethynyl building blocks are usually characterised by torsional disorder at room temperature. They are much more rigid in the electronic excited state due to conjugation. As a consequence, the electronic absorption and emission spectra do not present a mirror-image relationship. Here, we investigate how torsional disorder affects the excited state dynamics of 9,10-bis(phenylethynyl)anthracene in solvents of different viscosities and in polymers, using both stationary and ultrafast electronic spectroscopies. Temperature-dependent measurements reveal inhomogeneous broadening of the absorption spectrum at room temperature. This is confirmed by ultrafast spectroscopic measurements at different excitation wavelengths. Red-edge irradiation excites planar molecules that return to the ground state without significant structural dynamics. In this case, however, re-equilibration of the torsional disorder in the ground state can be observed. Higher-energy irradiation excites torsionally disordered molecules, which then planarise, leading to important spectral dynamics. The latter is found to occur partially via viscosity-independent inertial motion, whereas it is purely diffusive in the ground state. This dissimilarity is explained in terms of the steepness of the potential along the torsional coordinate.

Binding Modes of a Phenylpyridinium Styryl Fluorescent Dye with Cucurbiturils.

In order to explore how cucurbituril hosts accommodate an N-phenyl-pyridinium derivative guest, the complexation of the solvatochromic dye, 4-(4-(dimethylamino)styryl)-1-phenylpyridinium iodide (PhSt) with α,α′,δ,δ′-tetramethyl-cucurbit[6]uril (Me4CB6) and cucurbit[7]uril (CB7) was investigated by absorption spectroscopic, fluorescence and NMR experiments. In aqueous solutions, PhSt forms 1:1 complexes with both cucurbiturils, the complex with CB7 has a higher stability constant (Ka = 6.0 × 106 M−1) than the complex with Me4CB6 (Ka = 1.1 × 106 M−1). As revealed by NMR experiments and confirmed by theoretical calculations, CB7 encapsulates the whole phenylpyridinium entity of the PhSt cation guest, whereas the cavity of Me4CB6 includes only the phenyl ring, the pyridinium ring is bound to the carbonyl rim of the host. The binding of PhSt to cucurbiturils is accompanied by a strong enhancement of the fluorescence quantum yield due to the blocking of the deactivation through a twisted intramolecular charge transfer (TICT) state. The TICT mechanism in PhSt was characterized by fluorescence experiments in polyethylene glycol (PEG) solvents of different viscosities. The PhSt-CB7 system was tested as a fluorescence indicator displacement (FID) assay, and it recognized trimethyl-lysine selectively over other lysine derivatives.

Effect of Temperature on the Spectral Fluorescent Properties of Positively Solvatochromic Merocyanines

Fluorescence quenching of positively solvatochromic merocyanines with a bridge group in the polymethine chain increases linearly with increasing temperature and is enhanced with increased chain length. Analysis of the kinetics of the fluorescence quenching gave the activation energy of this process. The absorption and fluorescence spectra in solvents of different viscosity and polarity at room temperature and 77 K showed that the major channel for fluorescence quenching is photoisomerization of the bonds in the polymethine chain. This conclusion was supported by TDDFT quantum-chemical calculations for the bond lengths of the merocyanines in the ground and excited states.

"Where does the fluorescing moiety reside in a carbon dot?" - Investigations based on fluorescence anisotropy decay and resonance energy transfer dynamics.

It has been shown recently that aggregated dyes are responsible for very high fluorescence in a carbon dot (CD). However, what is the location of the fluorescing moiety in CD? Is it inside the CD or attached to the CD's surface? In order to answer these intriguing questions regarding the location of the fluorescing moiety in a CD, we performed rotational anisotropy decay dynamics and resonance energy transfer (RET) dynamics. Rotational correlation time of ∼120 picoseconds nullifies the fact that the whole CD is fluorescing. Instead, we can say that the fluorescing moiety is either embedded inside the CD or attached to the surface of the CD or linked to the CD through covalent bonds. From the fluorescence anisotropy decay dynamics in solvents of different viscosities, we could show that the fluorescing moiety is not attached to the surface of the CD or for that matter, the fluorescing moiety is not in a rigid environment inside the CD. RET dynamical analysis has shown that the time for RET (from CD to acceptor Rh123) is about 5.4 ns and the RET dynamics are independent of the acceptor concentration. Using RET dynamics, we could prove that the fluorescing moiety is not outside the CD; rather, it is inside the CD, but not in a rigid environment. The geometric distance between the fluorescing moiety of the CD and the acceptor (Rh123) has been obtained to be 4.55 nm. Using Förster formulation, the distance between the fluorescing moiety inside the CD and the acceptor Rh123 has been calculated to be 4.24 nm. Thus, we could not only reveal the exact location of the fluorescing moiety in a CD, but we could also demonstrate that unlike for many other nanomaterials, Förster formulation could explain the experimental observables regarding RET involving CD reasonably well.

Diffusion behavior of the fluorescent proteins eGFP and Dreiklang in solvents of different viscosity monitored by fluorescence correlation spectroscopy

AbstractFluorescence correlation spectroscopy relies on temporal autocorrelation analysis of fluorescence intensity fluctuations that spontaneously arise in systems at equilibrium due to molecular motion and changes of state that cause changes in fluorescence, such as triplet state transition, photoisomerization and other photophysical transformations, to determine the rates of these processes. The stability of a fluorescent molecule against dark state conversion is of particular concern for chromophores intended to be used as reference tags for comparing diffusion processes on multiple time scales. In this work, we analyzed properties of two fluorescent proteins, the photoswitchable Dreiklang and its parental eGFP, in solvents of different viscosity to vary the diffusion time through the observation volume element by several orders of magnitude. In contrast to eGFP, Dreiklang undergoes a dark-state conversion on the time scale of tens to hundreds of microseconds under conditions of intense fluorescence excitation, which results in artificially shortened diffusion times if the diffusional motion through the observation volume is sufficiently slowed down. Such photophysical quenching processes have also been observed in FCS studies on other photoswitchable fluorescent proteins including Citrine, from which Dreiklang was derived by genetic engineering. This property readily explains the discrepancies observed previously between the diffusion times of eGFP- and Dreiklang-labeled plasma membrane protein complexes.

Simulation of the [formula omitted] photodynamics of azobenzene: Decoherence and solvent effects

We present surface hopping simulations of the photodynamics of azobenzene excited in the π→π∗ band, in vacuo and in two solvents of different viscosity. We apply the overlap based quantum decoherence correction (ODC) previously devised by our group, and we propose a criterium to calibrate the ODC when no experimental or theoretical benchmarks are available. We show that decoherence has an important effect on the excited state decay. Solvation also affects the lifetimes, as well as the quantum yields and important details of the reaction mechanism.

Photophysics, RET Probes and Imaging Applications for Fluorescent BSA Au Nanoclusters

Protein protected fluorescent metal nanoclusters are attracting great deal of attention from fluorescence community. This work explores photo-physics, fluorescence polarization properties; resonance energy transfer (RET) probes, and cellular imaging applications of novel fluorescent probe bovine serum albumin protected gold (BSA Au) nanoclusters. Important findings were long fluorescence lifetime (> 1µs) and polarized fluorescence emission studied in solvents of different viscosity. The BSA Au cluster, due to long lifetime and high polarization, can potentially be used in developing fluorescence polarization immunoassays. Moreover, we describe an approach by developing RET of BSA Au clusters with the near infra-red (NIR) dye Dylight 750 (Dy750), where BSA Au cluster act as a donor to Dy750, showing 46% energy transfer efficiency to the NIR dye Dy750 with long lifetime component in acceptor decay through RET. Such RET probe with narrow emission spectrum and long lifetime component can be explored in microscopy imaging applications. Furthermore, we describe the use of these long lived BSA Au clusters in cellular and tissue imaging applications. We have shown using multiple pulse excitation (MPE) scheme that one can increase the intensity of long lived BSA Au cluster in cells without increasing the short lived auto-fluorescence background thereby increasing signal to background ratio at least by 15 times. Moreover, by applying time gated detection strategy to MPE imaging experiment we increased the signal to noise ratio to 30, a dramatic improvement in contrast for low fluorescence quantum yield dye. In summary, BSA Au clusters can be used as a fluorescent cellular stain advancing the bioimaging technology via their long fluorescence lifetime.

Excited State Characterization and Energy Transfer in Hyperbranched Polytruxenes and Polytruxene-block-Polythiophene Multiblock Copolymers

A comprehensive investigation of the excited state characteristics of two hyperbranched truxene polymers (one end-terminated with poly(3-hexylth- iophene) blocks, P3HT) and a bistruxene model compound has been undertaken aiming to rationalize its inherent photophysical properties, including the energy transfer processes between the truxene (donor) and P3HT (acceptor) moieties. The study comprises qualitative absorption, emission, and triplet-singlet difference spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion and singlet oxygen formation) and fluorescence decay times. From the time-resolved data in solvents of different viscosity and as a function of temperature, it was established that with the P3HT- terminated hyperbranched polytruxene, the excited state deactivation mainly results from energy transfer and that conformational relaxation is absent in these systems, which gives further support for the rigidity of these polymers both in the ground and excited state. An energy transfer efficiency of 91% was obtained at room temperature. From a qualitative analysis of the data, it was also seen that radiationless processes (particularly the S1∼∼→S0 internal conversion channel) mainly contribute to the excited state deactivation of the hyperbranched polytruxenes, a behavior that is in contrast to what was observed for the bistruxene model compound. Spectral and fluorescence time-resolved data in thin films was also obtained and compared with the solution data.

Structural Spectroscopy and Dynamics of Inter- and Intramolecular H-Bonding Interactions of Topotecan, a Potent Anticancer Drug, in Organic Solvents and in Aqueous Solution

We report on the role of H-bonding interactions on the UV-visible absorption and emission (steady-state and time-resolved) spectroscopy of topotecan (TPT) in solution. In aprotic solvents, a very fast (less than 10 ps) excited-state intramolecular proton-transfer reaction occurs in the absorbing enol (E) form to give a zwitterion (Z) form, emitting with a large Stokes shift. In protic solvents like methanol, the time constant of Z* formation is longer (32 ps) due to the participation of solvent molecules in the proton-transfer reaction. In aqueous solution at near-neutral pH (6.24), a ground-state equilibrium is established between E, cation (C), and Z forms. Direct excitation of E leads to Z* through two channels: a very fast one (less than 10 ps) involving an intramolecular proton-tranfer and a slower one (680 ps) with the C* intermediate formation and reaction. A fast (42 ps) deprotonation of E* to give the excited anion (A*) also competes with the photoformation of Z* at the S(1) state. At pH =12.15, the A structures are the principal emitting species (τ(A) ~ 0.41 ns), showing the largest Stokes shift. In aqueous solutions, we cannot exclude the existence of an equilibrium between the lactone and carboxylate forms of TPT, whose spectroscopic (absorption and emission spectra) and dynamical behaviors should not be very different. Time-resolved emission anisotropy measurements in solvents of different viscosities suggest that the rotational relaxation time (φ) of TPT is mainly governed by the viscosity of the medium, increasing from 104 ps (in tetrahydrofuran, THF) to 156 ps (in water) and 338 ps (in dimethyl sulfoxide, DMSO). These results give spectroscopic and dynamical information on the structures, stability, and dynamics (picosecond to nanosecond time scale) of TPT in solution. They provide insights on the role of the intermolecular H-bonding surrounding medium on the ground- and excited-state structure and reaction of TPT. The finding should contribute to a better understanding of the relationship between the structures of the drug and its surroundings.

Development and validation of a 10 kHz–1 MHz magnetic susceptometer with constant excitation field

The design and validation of a mutual inductance AC susceptometer with constant excitation field of up to 4.25 Oe, operating at frequencies from 10 kHz to 1 MHz, is presented. Considerations such as parasitic capacitances between wire turns and sensing bridge electronics were taken into account in order to extend the operating frequency range. An 18AWG wire with considerable insulator thickness was used for coil construction to keep parasitic capacitive reactance negligible relative to coil inductive reactance, and to obtain controlled field operation. A high speed instrumentation amplifier (slew rate over 33 V/μs) was designed and constructed using voltage feedback LM7171 operational amplifiers. The system was calibrated with Dy2O3 to account for mismatches in signal amplitude and phase shifts due to the electronics, coil coupling and imperfections, and external disturbances. AC susceptometer operation in the 10 kHz–1 MHz frequency range was validated by measuring the complex susceptibility of cobalt ferrite nanoparticles suspended in solvents of different viscosities. Good agreement was found between the experimental Brownian relaxation times and those predicted theoretically from the viscosity of the suspending media and the hydrodynamic diameter of the nanoparticles.

Probing Local Environments by Time-Resolved Stimulated Emission Spectroscopy

Time-resolved stimulated emission spectroscopy was employed to probe the local environment of DASPMI (4-(4-(dimethylamino)styryl)-N-methyl-pyridinium iodide) in binary solvents of different viscosity and in a sol-gel matrix. DASPMI is one of the molecules of choice to probe local environments, and the dependence of its fluorescence emission decay on viscosity has been previously used for this purpose in biological samples, solid matrices as well as in solution. The results presented in this paper show that time-resolved stimulated emission of DASPMI is a suitable means to probe the viscosity of local environments. Having the advantage of a higher time resolution, stimulated emission can provide information that is complementary to that obtained from fluorescence decay measurements, making it feasible to probe systems with lower viscosity.

The Excited Triplet State of Azoalkanes: Electron Spin Polarization and Magnetic Field Effects During Triplet-Sensitized Photolysis of trans-Azocumene in Solution

The triplet-sensitized photodecomposition of azocumene into nitrogen and cumyl radicals is investigated by time-resolved electron paramagnetic resonance and absorption spectroscopy. The radicals are found to be created spin polarized with a yield depending on the strength of the applied magnetic field. The phenomenon arises because in triplet azocumene, the decay into radicals competes with a fast triplet-sublevel selective intersystem crossing back to the azocumene ground state. The size of the initial spin polarization of the radicals and the magnetic field effect on their yield are determined in solvents of different viscosities. Data analysis yields rate constants for the intersystem crossing and the cleavage reaction of triplet azocumene as well as its zero-field splitting DZFS. At room temperature in nonpolar solvents, the most probable values are: kx = ky = 1.2 × 1011 s−1 and kz = 1.9 × 1010 s−1 for the intersystem crossing from the energetically lower and upper triplet substates, respectively, kp = 1.6 × 109 s−1 for the cleavage reaction and for the zero-field splitting DZFS = −3.4 × 1010 s−1 (0.18 cm−1).

What is the difference between the dynamics of anion- and keto-type of photochromic salicylaldehyde azine?

The normal and anion structures of salicylaldehyde azine (SAA) in solvents of different viscosities and polarities have been studied by means of femto- to nanosecond time-resolved emission techniques. In the normal form, an excited-state intramolecular proton-transfer (ESIPT) reaction takes place with a time constant shorter than 80 fs to produce an excited keto-type tautomer in which intramolecular-vibrational energy redistribution and vibrational cooling occur in 100 fs to 2 ps. The viscosity-dependent emission decay in the red part of the spectrum with 5-11 ps reflects a twisting motion leading to rotamers of these keto-type structures, most probably of (n,pi*) nature. For the anion type, the viscosity dependent rise-times (3 to 400 ps) at the red part of the emission, and the wavelength-dependent fluorescence lifetimes (20 to 1100 ps) indicate a stepwise formation of different conformers of the anions. The results reported here should be relevant to a better understanding of the photobehaviour of photochromic compounds and charged chromophores in biological systems.

Viscosity dependence of rubrene fluorescence quenching by organic radicals via energy transfer

The kinetics of rubrene (5,6,11,12-tetraphenylnaphthacene) fluorescence quenching by energy transfer to the radicals of N,N,N',N'-tetramethyl-para-phenylenediamine (TMPD+*), tetracyanoquinonedimethane (TCNQ-*) and galvinoxyl (2,6-di-tert-butyl-alpha-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy; GAL*), have been measured in solvents of different viscosities. At room temperature the observed quenching rate constants vary between 0.12 and 7.8 x 10(10) M(-1) s(-1), always exceeding the diffusion rate constants given by the simplest version of Smoluchowski theory combined with the rate constants for dipole-dipole energy transfer. This discrepancy is explained by accounting for the non-stationary stage in the Förster kinetics, even though the experimental fluorescence decays are apparently mono-exponential.

Free Volume Dependence of the Internal Rotation of a Molecular Rotor Probe in Room Temperature Ionic Liquids

The fluorescence efficiency of a well-known microviscosity probe, 9-(dicyanovinyl)julolidine (DCVJ), which is highly sensitive to the viscosity of the medium, has been studied in seven imidazolium ionic liquids (ILs) of varying viscosities over a temperature range of 10-60 degrees C. The microviscosities around the probe in different ILs have been estimated from the linear dependence of the logarithm of fluorescence quantum yield (log phi(f)) on the logarithm of the bulk viscosity (log eta) in various conventional solvents of different viscosities at room temperature. These microviscosities, which represent the local environments around the probe, are found to be significantly different from the directly measured bulk viscosities of these ILs. The log phi(f) vs log (eta/T) plots, which are also expected to be linear, interestingly show a bilinear behavior in more viscous ILs with a break around 28-30 degrees C. The observation of a similar break in the Arrhenius plots of the rate constant of the internal rotation in DCVJ and absence of any such break in the temperature dependence of the mobility of the ILs allow us to determine the important role of the free volume around the probe in dictating the nonradiative deactivation rate or the fluorescence efficiency of DCVJ. The break in the plots, which implies a change in the available free volume around the probe at approximately 28-30 degrees C, presumably arises from the repositioning of the probe from one environment to a different one of these microheterogeneous ILs with change of temperature.

Femtosecond dynamics of a non-steroidal anti-inflammatory drug (piroxicam) in solution: The involvement of twisting motion

In this contribution, we report on fast and ultrafast dynamics of a non-steroidal anti-inflammatory drug, piroxicam (PX), in methyl acetate (MAC) and triacetin (TAC), two solvents of different viscosities. The enol form of PX undergoes a femtosecond (shorter than 100 fs) electronically excited state intramolecular proton-transfer reaction to produce keto tautomers. These structures exhibit an internal twisting motion to generate keto rotamers in ∼2–5 ps, a time being longer in TAC. The transient absorption/emission spectrum is very broad indicating that the potential-energy surface at the electronically excited state is very flat, and reflecting the involvement of several coordinates along which the wavepacket of the fs-produced structures evolve.

Luminescence Lifetime Standards for the Nanosecond to Microsecond Range and Oxygen Quenching of Ruthenium(II) Complexes

A rapid and reproducible method for determining the temperature dependence of luminescence lifetimes has been developed. With the use of this method, a set of standards for the excited-state lifetime oxygen quenching of several ruthenium(II) transition metal complexes was established. With the use of three solvents of different viscosities and two metal complexes with widely different lifetimes, an overlapping range of ca. 100 ns to 6 micros was obtained. The decays are pure single exponentials, which means that they can be used reliably with both phase and pulsed lifetime instruments. For a pure single-exponential decay, a properly operating phase shift instrument will give the same lifetime as a time domain instrument. With the use of a thermal deactivation model and a three-parameter temperature-dependent oxygen quenching constant, the lifetime temperature-dependent data was well fit by a simple six-parameter equation that covers the temperature range of 10-50 degrees C and oxygen pressures from 0 to 1 atm of oxygen with excellent precision (ca. <1%). This permits both laboratory and field calibration of instruments.

Photoionization affected by chemical anisotropy

The kinetic constants of rhodamine 3B quenching by N,N-dimethyl aniline were extracted from the very beginning of the quenching kinetics, recently studied in a few solvents of different viscosities. They were well fitted with the conventional kinetic constant definition, provided the radial distribution function of simple liquids was ascribed to the reactant pair distribution and the contact electron transfer rate was different in all the cases. This difference was attributed to the chemical anisotropy averaging by the rotation of reactants, which is the faster in solvents of lower viscosity. With the proper choice of a space dependent encounter diffusion, the whole quenching kinetics was well fitted with an encounter theory, using the Marcus [J. Chem. Phys. 24, 966 (1956); 43, 679 (1965)] transfer rate instead of the contact Collins-Kimball [J. Colloid. Sci. 4, 425 (1949)] approximation. Not only the beginning and middle part of the quenching were equally well fitted, but the long time (Markovian) rate constant was also found to be the same as previously obtained. Moreover, the concentration dependencies of the fluorescence quantum yield and the Stern-Volmer constant were specified and await their experimental verification.

Primary and Secondary Structure Dependence of Peptide Flexibility Assessed by Fluorescence-Based Measurement of End-to-End Collision Rates

The intrachain fluorescence quenching of the fluorophore 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) is measured in short peptide fragments, namely the two strands and the turn of the N-terminal beta-hairpin of ubiquitin. The investigated peptides adopt a random-coil conformation in aqueous solution according to CD and NMR experiments. The combination of quenchers with different quenching efficiencies, namely tryptophan and tyrosine, allows the extrapolation of the rate constants for end-to-end collision rates as well as the dissociation of the end-to-end encounter complex. The measured activation energies for fluorescence quenching demonstrate that the end-to-end collision process in peptides is partially controlled by internal friction within the backbone, while measurements in solvents of different viscosities (H2O, D2O, and 7.0 M guanidinium chloride) suggest that solvent friction is an additional important factor in determining the collision rate. The extrapolated end-to-end collision rates, which are only slightly larger than the experimental rates for the DBO/Trp probe/quencher system, provide a measure of the conformational flexibility of the peptide backbone. The chain flexibility is found to be strongly dependent on the type of secondary structure that the peptides represent. The collision rates for peptides derived from the beta-strand motifs (ca. 1 x 10(7) s(-1)) are ca. 4 times slower than that derived from the beta-turn. The results provide further support for the hypothesis that chain flexibility is an important factor in the preorganization of protein fragments during protein folding. Mutations to the beta-turn peptide show that subtle sequence changes strongly affect the flexibility of peptides as well. The protonation and charge status of the peptides, however, are shown to have no significant effect on the flexibility of the investigated peptides. The meaning and definition of end-to-end collision rates in the context of protein folding are critically discussed.