Chellappan Selvaraju

Excited state reactions of acridinedione dyes with onium salts: mechanistic details ☆

Abstract Diaryliodonium and triarylsulphonium salts are thermally stable UV photoinitiators for cationic polymerization. Diaryliodonium salts (DPI-1–7) and triarylsulphonium (type-I, type-II and type-III) salts with different substituent have been prepared. Excited state reactions of these onium salts with decahydroacridinedione dyes (ADD-1–6) have been studied. ADD acts as an efficient sensitizer for the decomposition of these onium salts. Photosensitization occurs through electron transfer, which is confirmed by the observation of enol radical cation of ADD and diarylsulphinium radical cation in the laser flash photolysis. The mechanism of photosensitization and the mechanism of photoacid release have been discussed. Both singlet and triplet state of the ADD are involved in the photosensitization of diaryliodonium salts, whereas singlet state of ADD is alone involved in the photosensitization of triarylsulphonium salts. The quantum yield of photoacid generated in direct and the sensitized process is determined and this allows to gauge the practical efficiency of onium salts/ADD combinations as the photoinitiator for the cationic polymerization applications.

Excited state behaviour of acridinedione dyes in PMMA matrix: inhomogeneous broadening and enhancement of triplet

Photophysics and photochemistry of acridinedione (ADD) dyes doped in PMMA matrix have been studied using fluorescence and flash photolysis technique. The absorption and emission spectra of acridinedione dyes in PMMA matrix are broad. A pronounced influence of λexc on the red shift of ADD fluorescence spectra was found which is independent of the various substituents in the ADD dyes. This observed shift was explained by the inhomogeneity in the PMMA matrix site results in the migration of the electronic excitation energy from the blue centres to red ones. The inhomogeneous distribution of ADD dyes in PMMA matrix causes the non-exponential fluorescence decay. The triplet lifetimes of these dyes in polymer matrix is found to be in millisecond time domain, whereas in solution the triplet lifetime is in microseconds. Laser flash photolysis shows that the photoionisation of ADD dyes in PMMA matrix results in the formation of an electron and a cation radical. Steady state photolysis of ADD dyes in PMMA matrix was carried out. ADD dyes have higher photostability in PMMA matrix than in solution.

Role of Photoionization on the Dynamics and Mechanism of Photoinduced Electron Transfer Reaction of Coumarin 307 in Micelles

The dynamics and mechanism of the photoinduced electron transfer (PET) reaction between coumarin 307 (C307) and aromatic amines in micelles have been studied by using steady-state (S-S) and time-resolved (T-R) absorption and fluorescence spectroscopy. Based on the fluorescence quenching time scale, PET in micelles is grouped into two types: (i) ultrafast electron transfer (ET) due to the close contact of the donor and acceptor in micelles and (ii) diffusion averaged dynamic electron transfer (DADET) which is controlled by the diffusion of the reactants in micellar Stern layer and diffusion of the micelles. The DADET does not affect the photoionization and solvation processes whereas ultrafast ET competes with the photoionization and faster than the solvation process. Both ultrafast and DADET shows Marcus inversion in the ET rates at the similar exergonicity and indicates that the role of diffusion and solvent reorganization is negligible toward the activation barrier for the ET reaction in micelles. The activation barrier for the ET reactions in micelles is mainly due to intramolecular reorganization energy. The intramolecular reorganization energy must be higher in CTAB due to the photoionization and subsequent recombination and also involvement of triplet state in the PET. The ET reaction between coumarin radical cation and amine is reported for the first time in the C307-amine systems in micelles which are confirmed by the effect on amine concentration of the decay of coumarin radical cation and the dynamics of the ground-state recovery of C307. A mechanism for the PET reaction between C307-amine systems is proposed in micelles including photoionization, ultrafast and dynamic ET, and solvation dynamics.

Existence of a New Emitting Singlet State of Proflavine: Femtosecond Dynamics of the Excited State Processes and Quantum Chemical Studies in Different Solvents

Proflavine (3,6-diaminoacridine) shows fluorescence emission with lifetime, 4.6 ± 0.2 ns, in all the solvents irrespective of the solvent polarity. To understand this unusual photophysical property, investigations were carried out using steady state and time-resolved fluorescence spectroscopy in the pico- and femtosecond time domain. Molecular geometries in the ground and low-lying excited states of proflavine were examined by complete structural optimization using ab initio quantum chemical computations at HF/6-311++G** and CIS/6-311++G** levels. Time dependent density functional theory (TDDFT) calculations were performed to study the excitation energies in the low-lying excited states. The steady state absorption and emission spectral details of proflavine are found to be influenced by solvents. The femtosecond fluorescence decay of the proflavine in all the solvents follows triexponential function with two ultrafast decay components (τ(1) and τ(2)) in addition to the nanosecond component. The ultrafast decay component, τ(1), is attributed to the solvation dynamics of the particular solvent used. The second ultrafast decay component, τ(2), is found to vary from 50 to 215 ps depending upon the solvent. The amplitudes of the ultrafast decay components vary with the wavelength and show time dependent spectral shift in the emission maximum. The observation is interpreted that the time dependent spectral shift is not only due to solvation dynamics but also due to the existence of more than one emitting state of proflavine in the solvent used. Time resolved area normalized emission spectral (TRANES) analysis shows an isoemissive point, indicating the presence of two emitting states in homogeneous solution. Detailed femtosecond fluorescence decay analysis allows us to isolate the two independent emitting components of the close lying singlet states. The CIS and TDDFT calculations also support the existence of the close lying emitting states. The near constant lifetime observed for proflavine in different solvents is suggested to be due to the similar dipole moments of the ground and the evolved emitting singlet state of the dye from the Franck-Condon excited state.

Photoionization and time-dependent stokes shift of coumarin 307 in soft matter: solvation and radical-ion pair recombination dynamics.

Photoionization, fluorescence time-dependent Stokes shift (TDSS), and rotational dynamics of coumarin 307 (C307) have been investigated in soft matter system such as micelles using time-resolved transient absorption and fluorescence spectroscopy. Photoionization of C307 leads to the formation of coumarin radical cation and hydrated electron, which were characterized by their respective transient absorption. The photoionization yields are significantly higher in anionic sodium dodecyl sulfate (SDS) micelle than in cationic cetyltrimethylammonium bromide (CTAB) and neutral Triton X-100 (TX-100) micelles, indicating the influence of micellar surface charge on the efficient separation of radical cation-hydrated electron pair. The CTAB micelle favors the recombination of radical cation and hydrated electron leading to the formation of triplet state of C307, which causes a decrease in the photoionization yield. C307 exhibits TDSS in all micelles; the time evolution and the magnitude of the TDSS depend on nature of the micelle. In TX-100 micelles, the decay of the TDSS exhibits ultraslow component (165 ns) and is affected by the presence of electron scavengers. The ultraslow component in TX-100 micelle originates from the recombination of radical cation-hydrated electron, which results in the formation of twisted intramolecular charge transfer (TICT) state; such formation of TICT state was not observed in SDS and CTAB micelles. To the best of our knowledge, this is the first report where the radical-ion pair recombination dynamics is probed using TDSS in combination with time-resolved transient absorption studies. The activation energy for the solvent relaxation and radical-ion pair (solvent separated) recombination process was found to be 6.1 and 3.0 kcal mol(-1), respectively. Temperature effect on TDSS in TX-100 micelles confirmed the increase in the water hydration, and size of the micelle influences the relative contribution of the solvation and radical-ion pair recombination dynamics toward the total TDSS. We propose that TDSS observed in neutral micelles and reported in other biomolecules such as proteins by the 7-amino coumarin probe is not only due to the solvation dynamics alone but also due to the radical-ion pair recombination dynamics.

A3-Type star stilbene and cyanostilbene molecules: synthesis, fluorescence properties and bio-imaging application

Stilbene and cyanostilbene based new A3-type star molecules containing blocked isocyanate functionalities were synthesized and structures of the compounds were confirmed. The photophysical properties of the molecules were studied using absorption and fluorescence spectroscopic techniques. The intramolecular charge transfer absorption of star stilbene and star cyanostilbene was found at 370 nm and 430 nm, respectively. Upon excitation at ICT absorption, the stilbene and cyanostilbene star molecules showed fluorescence with a maximum in the region of 443–470 nm and 498–540 nm, respectively. Upon increasing the solvent polarity, the fluorescence maximum of both the star molecules showed a bathochromic shift with an increase in the Stokes shift. The change in the dipole moment upon excitation was calculated from the Lippert–Mataga plot and it was found that the molecules were more polar in their excited state. The fluorescence quantum yields and fluorescence lifetimes of these molecules were determined; the average lifetime value for the stilbene based molecule was found to be 2.05 ns and this value for the cyanostilbene based molecule was found to be 0.66 ns. Star cyanostilbene showed aggregation-induced emission; it displayed lower wavelength emission in pure THF but it exhibited emission with a red shift in a water/THF binary mixture. These molecules also showed considerably low band gap energies between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO); the values were in the range of 2.55–2.87 eV. Based on the photophysical studies, the molecule containing the cyanostilbene unit was used as a fluorophore for bio-imaging application in zebrafish eggs as the model and the results obtained were found to be excellent.

Jones-Ray effect on the organization of lysozyme in the presence of NaNO3 at an air/water interface: is it a cause or consequence?

The study reports on the anomalous aggregation and enhanced viscosity of lysozyme (Lyz) in the presence of low concentrations of sodium nitrate at an air/buffer interface. For salt concentrations of about 10 mM of NaNO3, the interactions seem to be electrostatic in origin possibly due to the anisotropy of charge distribution on the protein and its correlation with high-complementarity non-electrostatic interactions resulting in a sudden increase in viscosity values. In the presence of low concentrations of the electrolyte, a thick viscoelastic protein film is created due to local amorphous aggregation, while native Lyz adsorbs in a fragile monolayer film without changing its secondary structural features. On increasing concentration beyond 10 mM, the protein behaves almost like a pure buffer without showing any surface activity or aggregation and is highly stable at the interface. For the first time, the rheological changes here confirm the Jones-Ray effect due to the synergy between NaNO3 (∼10 mM) and the protein whereas earlier reports on this effect have dealt only with pure electrolyte–water interactions. Our experimental studies indicate that with the appropriate choice of solution conditions and specific electrolyte concentration, one can either drive the protein to form amorphous aggregates which can result in protein crystallization or enhance protein stability for long time periods by preventing aggregation through self-association.

High yield synthesis of branched gold nanoparticles as excellent catalysts for the reduction of nitroarenes

A one-pot preparation of branched gold nanoparticles (NPs) in aqueous solution using acetanilide as both a reducing and shape-directing agent has been achieved. The structure, morphology and optical properties of the branched gold NPs can be tuned by controlling the concentration of acetanilide. In contrast to reported protocols, high quality and stable branched gold NPs can be produced exclusively without any sophisticated purification techniques. More interestingly, the gold NPs thus obtained are predominantly enriched with a uniform facet of {111}, as confirmed by HRTEM and XRD methods. When compared to reported gold NPs, the branched gold NP catalysts exhibited two to three orders of magnitude higher activity for the reduction of 4-nitrophenol into 4-aminophenol. Furthermore, the average turnover frequency of the branched gold NPs was found to be 19 molecules per min, which is significantly higher than that of spherical gold NPs and other reported branched gold NP catalyst systems.