Short-wavelength fluorescence caused by sequential two-photon excitation of some cyanine dyes: Effect of solvent viscosity on the quantum yields

Short-wavelength (SW) fluorescence of some cyanine dyes caused by sequential two-photon excitation was studied. The fluorescence quantum yield of the SW fluorescence shows a significant dependence on the solvent viscosity, but only a small dependence on temperature. This reveals the dynamic character of the emitting state: much lower intramolecular barrier and larger solute-solvent viscous drag compared to the S₁ state in the molecular conformational change which is important as a radiationless decay channel.     

Picosecond kinetics of radiationless relaxations of triphenyl methane dyes. Evidence for a rapid excited-state equilibrium between states of differing geometry

The photophysics of several triphenylmethane (TPM) dyes has been studied in n-alcohols and glycerol/water solutions. We have been able to measure the rate and characterize the viscosity dependence of several radiationless decay channels. We propose a kinetic scheme to account for our observations. Important points in this model are, the existence of more than one ground-state species (in some dyes), a rapid excited-state equilibrium between states of differing geometry and solvent-induced spectral shifts.     

Time-resolved fluorescence depolarization studies of rotational relaxation in viscous media

Rotational relaxation times (τ_roi) were measured for xanthene dyes in several n-alcohols, ethylene glycol and glycerol by following the fluorescence depolarization using a single photon counting technique. Additional studies were made of the fluidity of the hydrocarbon micellar interior using pyrene derivatives as fluorescent probes. While the correlation between τ_roi and viscosity for the smaller n-alcohols is good, as anticipated by the models of Stokes—Einstein—Debye and later Perrin, a saturation in this correlation is observed in more viscous systems in accordance with recent molecular dynamics calculations. The influence of molecular structure on both viscosity and rotational relaxation times is also discussed. Pyrene, 1-pyrene suphonic acid and 1-pyrene butyeric acid exhibit rapid reorientation τ_roi ? 10^?9 s, in glycerol and micellar systems.     

The synthesis and characterization of novel coumarin-containing cyanine dyes via “Click” chemistry

Four novel cyanine dye derivatives (C-Cy(1–4)) in which a coumarin moiety is attached to benzoindole ring of asymmetric trimethine cyanine dyes or hemicyanine dye by 1,2,3-triazole have been successfully synthesized via “Click” reaction between the corresponding alkyl-end cyanine dyes and 3-azidocoumarin. Six reaction systems were employed to find the suitable “Click” conditions of cyanine dyes. It was found that the combination of DMF, CuI and di-i-propylethylamine (DIPEA) was the most excellent system for “Click” reaction. All of the compounds were characterized by ¹H-NMR, MS, UV and FL.     

Effect of viscosity on the singlet-excited state dynamics of some hemicyanine dyes

Photophysical properties of hemicyanine dyes (1 - 3) were investigated in solvents of varying polarity and viscosity. Hemicyanines possess relatively low fluorescence quantum yields (1%) in polar solvents. A significant increase in fluorescence quantum yield and lifetimes was observed with increase in viscosity of the solvent medium. The radiative, as well as nonradiative decay channels from the singlet-excited state were investigated by varying the viscosity of the medium. The viscosity-dependent radiationless relaxation observed in hemicyanine dyes is suggestive of a restricted rotor motion in the singlet excited state.     

Photoisomerization through a “funnel” in polar and non-polar solvents

Fast radiationless deactivation S_e ∼→ S₀ of a photoexcited state through a “funnel” is discussed. The experimental rates of such transitions cannot be explained in general by a one-dimensional model considering only the angle of isomerization. Inclusion of the full number of degrees of freedom leads to a multi-dimensional intersection of the adiabatic potential energy surfaces U_e and U₀ in the region of the “funnel” where non-adiabatic interactions are strong and cause a fast electronic transition S_e ∼→ S₀. The rate constant of the isomerization reaction is found to be strongly related to the rate of rotational relaxation in the solvent and relaxation of the solvent orientational polarization.     

Fluorescence polarization anisotropy and kinetics of malachite green measured as a function of solvent viscosity

The fluorescence kinetics and polarization anisotropy of the triphenylmethane dye malachite green were measured as a function of solvent viscosity. The relationshp between the relaxation kinetics and the solvent viscosity was investigated in order to obtain information on the effect of enviromnental changes on the orientational order of dye molecules in solution. It was found that the fluorescence lifetimes follow an η^{$\text{2}\text{3}$} dependence for 1 < η < 60 P, η^{$\text{1}\text{2}$} dependence for 60 P < η < 1000 P and approach a constant for η > 1000 P. The dependence of the fluorescence decay rates on the solvent viscosity was fit to k = 5 × 10¹⁰η^{?$\text{2}\text{3}$} + 5 × 10⁸ s^?1. The fluorescence polarization anisotropy term, R(0), was also measured as a function of solvent viscosity. A marked decrease in R(0) was observed at a viscosity of 1000 P. For η < 1000 P, R(0) was found to be close to the expected value for a random distribution of molecules, 0.4; and for η > 3000 P, R(0) was measured to be ≈0.11. This small value of R(0) may indicate a nonrand The observed change of the polarization anisotropy with increasing viscosity indicates that the dye molecules become ordered at higher viscosities. This may arise through the formation of a long range order due to lack of rotational deexcitation of the malachite green dye molecules at high viscosities.     

Lösungsmittel-, Viskositäts-, Säure- und Substituenteneinflüsse auf die Photoisomerisierung und Relaxation symmetrischer Triazacarbocyanine bei 283 bis 323 K

Influence of Solvents, Viscosity, Acid, and Substituents on the Photoisomerization and the Relaxation of Symmetrical Triazacarbocyanine Dyes at 283 to 323 K Photobleaching and the reverse dark reaction of seven symmetrical triazacarbocyanine dyes with different heterocycles were studied dependent on solvent effects (protic and aprotic solvents), on effects of viscosity (glycerol/EtOH mixtures), and on effects of added acid. No effects of dissolved O₂ or added I₂ has been observed. Kinetic data (ΔH^≠, ΔS^≠, and ΔG) of the reverse reaction are given. The effect of the substituents has been studied on two series of substituted triazacarbocyanine dyes. The mechanism of the reverse reaction is discussed.     

Spectral,lifetime and photochemical characteristics of 4-acetoxy-chalcone in viscous media

Both fluorescence and excitation spectra of 4-acetoxy-chalcone (4-AC) are bathochromically shifted as the medium polarity increases. The fluorescence quantum yields are sensitive to medium viscosity and increase sharply as the medium viscosity increases due to prohibition of radiationless deactivation pathways. The fluorescence lifetime of 4-AC in glycerol has been measured as τ = 1.9 ns. The photochemical quantum yields (thought to be a trans-cis photo-isomerization) are also sensitive to medium viscosity with minimum φ_c values obtained in highly viscous solvents. The effect of medium viscosity on the quenching of 4-AC fluorescence using picric acid as a quencher has been studied. A static-type quenching mechanism is proposed to account for the quenching efficiency as the medium viscosity increases.     

Fractional viscosity dependence of relaxation rates and non-steady-state dynamics in barrierless reactions in solution

A finite decay model of barrierless electronic relaxation in solution is studied both analytically and numerically. The fractional viscosity (η) dependence of the rate (k∝η^−α with 1 > α> 0) is shown to be strongly correlated with non-steady-state dynamics on the reaction potential surface. We also find that the exponent α may depend strongly on the wavelength of the exciting light.     

Experimental verification of the theory of J-diffusion in nitrogen gas

The measured linewidths of NMR spectra for ¹⁵N₂ and ¹⁴N₂ have been used to calculate rotational (τJ) and orientational (τθ₂) relaxation times. Within the density range studied (0–100 amagat) τθ₂ is proportional to τ_J in line with the impact theory of orientational and rotational relaxation.     

Electronic excited state energy transfer,trapping by dimers and fluorescence quenching in concentrated dye solutions: Picosecond transient grating exp

Picosecond transient grating experiments are used to examine electronic excited state dynamics in concentrated dye solutions. A model based on radiationless excited state transport and trapping by dimers describes the phenomena responsible for fluorescence quenching. The trapping rate constant is found to have a cubic concentration dependence. Rhodamine 6G dimer lifetimes in glycerol and ethanol are 830 ps and <50 ps respectively. The difference arises due to the viscosity dependence of the dimer radiationless relaxation rate.     

Viscosity dependence of internal conversion rates of polyatomic molecules

A microscopic theory of the dependence of internal conversion rates (R) on viscosity (η) is developed by extending usual theories of vibronic energy relaxation. The viscosity dependence of R is assumed to arise from the viscosity dependence of the damping rates γ of soft (torsional) modes of intramolecular vibration (frequencies ω). From the theory developed R is found to increase with η when γ « ω. When γ ? ω the rate R is found to decrease with η. In this case the dependence of R on η can often be approximated in a wide range of η according to R ∝ η^?α. The value of α depends on certain microscopic parameters, and 0 ? α ? 1 and α ? 1 might be found. In particular, the Oster-Nishijima result (α = 1) is included as a limiting case. A comparison between theory and experiment is given, which shows the theoretical results to be in full agreement with the experimental ones.     

Solution properties of styrene-p-chlorostyrene diblock copolymers—II. Light scattering studies in a selective solvent

Conformational behaviour of styrene-p-chlorostyrene diblock copolymers in cumene (a good solvent for polystyrene but a θ solvent for poly-p-chlorostyrene) was studied over the temperature range 15–60° by light scattering, osmotic pressure and intrinsic viscosity measurements. Two samples of copolymer (AB-4 and AB-2) were used. The composition of the samples was c. 50 mol% of styrene and the number-average molecular weights were 27.7 × 10⁴ for the AB-4 and 19.5 × 10⁴ for the AB-2. It was found that below 40° the number-average molecular weight of the AB-4 sample seemed to increase gradually with decreasing temperature and around 40°, marked decreases in the osmotic second virial coefficient and intrinsic viscosity were observed. The Zimm plot for the AB-2 sample was fairly normal at 40°. It seems that the temperature where an anomaly becomes evident in Zimm plots is dependent on the molecular weight of the sample. The experimental results for the diblock copolymers could be understood from the view that intermolecular association took place to some extent in the solutions on lowering the temperature giving rise to multi-molecular micelles.     

Photovoltaic properties of new cyanine-naphthalimide dyads synthesized by ‘Click’ chemistry

Two novel cyanine dyads, in which a naphthalimide unit is attached to benzoindole ring of unsymmetric trimethine cyanine, have been synthesized via ‘Click’ reaction and characterized by ¹H NMR, ¹³C NMR, and MS-ESI. Under the illumination of AM 1.5 (75 mW cm⁻²), the power conversion efficiency of cyanine I reached 4.8% (J_sc = 14.5 mA cm⁻², V_oc = 0.50, FF = 0.49). The results show that the two cyanine dyes are promising sensitizers for nanocrystalline dye-sensitized solar cell.     

Simulated glass transition in free‐standing thin polystyrene films

In this article, we investigate the glass transition in polystyrene melts and free‐standing ultra‐thin films by means of large‐scale computer simulations. The transition temperatures are obtained from static (density) and dynamic (diffusion and orientational relaxation) measurements. As it turns out, the glass transition temperature of a 3 nm thin film is ～60 °K lower than that of the bulk. Local orientational mobility of the phenyl bonds is studied with the help of Legendre polynomials of the second‐order P₂(t). The α and β relaxation times are obtained from the spectral density of P₂(t). Our simulations reveal that interfaces affect α and β‐relaxation processes differently. The β relaxation rate is faster in the center of the film than near a free surface; for the α relaxation rate, an opposite trend is observed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1160–1167, 2010     

Pyrazole‐substituted Near‐infrared Cyanine Dyes Exhibit pH‐dependent Fluorescence Lifetime Properties

Near‐infrared heptamethine cyanine dye is functionalized with pyrazole derivatives at the meso‐position to induce pH‐dependent photophysical properties. The presence of pyrazole unsubstituted at ¹N‐position is essential to induce pH‐dependent fluorescence intensity and lifetime changes in these dyes. Replacement of meso‐chloro group of cyanine dye IR820 with ¹N‐unsubstituted pyrazole resulted in the pH‐dependent fluorescence lifetime changes from 0.93 ns in neutral media to 1.27 ns in acidic media in DMSO. Time‐resolved emission spectra (TRES) revealed that at lower pH, the pyrazole consists of fluorophores with two distinct lifetimes, which cor‐responds to pH‐sensitive and non‐pH‐sensitive species. In contrast, ¹N‐substituted pyrazoles do not exhibit pH response, suggesting excited state electron transfer as the mechanism of pH‐dependent fluorescence lifetime sensitivity for this class of compounds.     

Resonant cars spectroscopy of s-tetrazine vapour

CARS experiments on s-tetrazine vapour show that the electronic resonance enhancement in the Q-branch is most effective for the lower J values. This suggests that the radiationless relaxation rate in the excited state increases with rotational quantum number. Delayed picosecond CARS experiments indicate that the rotation-vibration coupling for the 1008 cm^?1 ground-state vibrational mode is approximately 2 MHz.     

Studies of the micro-brownian motion of a polymer chain by the fluorescence polarization method. III. Fluorescent conjugates of polyethyleneimine

Fluorescent conjugates of polyethyleneimine (PEI) were prepared by conjugation of fluorescent dyes, fluorescein isocyanate (FIC), and 1-dimethylaminonaphthalene-5-sulfonyl chloride (DNS), to PEI. The degree of polarization of the fluorescence was measured as a function of temperature and solvent viscosity on aqueous solutions of the conjugates and the data thus obtained were analyzed in terms of an equation of the Perrin type to calculate the mean relaxation time of the conjugate. The mean relaxation times obtained for the two types of the conjugates, which differ in the excited lifetime by a factor of about three, practically agree with one another and are about 2.5 X 10^?8 sec. The relaxation time of the DNS conjugate increases with increasing molecular weight of the conjugate from 2 X 10^?8 to 4 X 10^?8 sec. These values are much larger than those of the PAA conjugates reported in Part I of this series. The relaxation time of this order may correspond to that for the cooperative rotary motion of about ten monomeric residues on the PEI chain, that is, for the motion of an intermediate segment of the PEI molecule in solution. Finally, relaxation time–molecular weight relationships for various types of fluorescent conjugates are compared. It is suggested that these data may serve as a basis for elucidating the mode of motion of a given molecule in solution from the polarization data.     

Studies of 2‐Azaazulenium Derivatives: Unsymmetrical Trimethine Cyanine Dyes Bearing a 2‐Azaazulenium Moiety as One of the Terminal Groups

We report here the synthesis of a series of symmetrical and unsymmetrical trimethine cyanine dyes derived from 2‐azaazulene, combined spectral and quantum‐chemical investigations of their molecular geometry and electron structure, as well as the nature of the lowest electron transitions. Based on the analysis of both calculations and experimental data obtained from absorption and ¹³C NMR spectra, it was concluded that the 2‐azaazulene residue can be treated as a weakly basic terminal group; its donor properties are provided with the participation of the HOMO?1, in contrast to the typical Brooker’s terminal residues with their donor HOMOs. The new classification of the terminal groups of cyanine dyes, and hence the classification of types of unsymmetrical cyanines, is proposed. It is shown that the nature of the higher electron transitions (delocalized or local) in the cyanine dyes depends on their type. In the unsymmetrical trimethine cyanine of the mixed type, negative deviations are observed in their absorption spectra.