The ultrafast photophysical characterization of 5,10,15,20-meso-tetrakis pentafluorophenyl porphyrin (H2F20TPP) in 4:1 dichloromethane (DCM) and tetrahydrofuran (THF) solution has been done in the femtosecond-picosecond time domain, by combining fluorescence up-conversion and femtosecond transient absorption spectroscopy. Fluorescence up-conversion studies on H2F20TPP were done demonstrating fluorescence dynamics over the whole spectral range from 440 to 650 nm when excited at 405 nm, 360.5 cm(-1) excess vibrational energy of Soret band (411 nm). Single-exponential decay with ∼160 ± 50 fs lifetime of Soret fluorescence (also called S2 fluorescence or B band fluorescence) at around 440 nm was observed. On going from 440 nm, S2 fluorescence to S1 fluorescence, (Q-band) around 640 nm (wavelength of 0-0 transition in the stationary spectrum), single-exponential fluorescence time profile turns into a multiexponential time profile and it could be resolved critically into five-exponential components. An ultrafast rise component with ∼160 ± 50 fs followed by two decay components: a very fast decay component with 200 ± 50 fs time constant and another relatively slower 1.8 ± 0.5 ps decay component. Next, a very prominent rise component with 105 ± 30 ps lifetime followed by long-lived 10 ns decay component. The initial rise of S1 (Q-band) fluorescence around 640 nm agreed with the decay time of S2 (Soret or B band) fluorescence indicates that internal conversion (IC) from relaxed S2 to vibrationally excited S1 occurs in the ∼160 fs time scale and subsequent very fast decay with 200 fs time constant, which is assigned to be intramolecular vibrational dephasing or redistribution. The 1.8 ps decay component of S1 fluorescence is attributed to be "hot" fluorescence from vibrationally excited S1 state, and it reveals the vibrational relaxation time induced by elastic or quasi-elastic collision with solvent molecules. The 105 ps rise component is the creation time of the thermally equilibrated S1 state population, and it could be attributed either to an excited state conformational relaxation/intramolecular charge transfer or a molecular cooling process by dissipation of excess energy within the solvent by inelastic collision. Finally, the decay of equilibrated S1(Qx state) occurs on 10 ns to S0 by fluorescence. Femtosecond resolved transient absorption studies on H2F20TPP in the spectral range 390-620 nm following both S2 (Soret band) and S1 (Qx) band excitation have been done and they complement the observations found in fluorescence up-conversion studies. The stimulated emission (SE) kinetics observed at 640 nm, S1 emission peak, in 2-10 ps time domain rebuilds a dynamic similar to that observed by fluorescence up-conversion study. The transient absorption kinetics upon S1 excitation were observed mainly to be biexponential with decay constants 105 ps and 10 ns, respectively. At a long time window (6 ns), a long-lived rise component could be predicted followed by two long-lived decay components for both the excitations in betw...

Steady state and time resolved fluorescence quenching behaviors of meso-Tetrakis (pentafluorophenyl) porphyrin (H(2)F(20)TPP) in presence of different aliphatic and aromatic amines have been executed in homogeneous dichloromethane (DCM) solution. At room temperature in DCM, free base (H(2)F(20)TPP) shows fluorescence with two distinct peaks at 640 and 711 nm and natural lifetime tauf=9.8 ns which are very similar to that of meso-tetraphenyl porphyrin (TPP). Unlike TPP, addition of both aliphatic and aromatic amines to a solution containing H(2)F(20)TPP results in an efficient decrease in fluorescence intensity without altering the shape and peak position of fluorescence emission. Upon addition of amines there was no change in optical absorption spectra of H(2)F(20)TPP. The fluorescence quenching rate constants ranged from 1 x 10(9) to 4 x 10(9) s(-1), which are one order below to the diffusion control limit, and temperature dependent quenching rate constants yield the activation energies which are found to be order of 0.1 eV. Femto second transient absorption studies reveal the existence of amine cation radical and porphyrin anion radicals with very short decay time (15 ps). The fluorescence quenching reaction follows Stern-Volmer kinetics. Steady state and time-resolved data are interpreted within general kinetic scheme of Marcus semi-classical model which attributes bimolecular electron transfer process between amines and the lowest excited singlet state of H(2)F(20)TPP. Calculated internal reorganization energies are found to be in between 0.04 and 0.22 ev. Variation of electron transfer rate as function of free energy change (DeltaG(0)) points the ET reactions in the present systems are in Marcus normal region. This is the first example of reductive fluorescence quenching of free base neutral porphyrins in homogeneous organic solvent ever known.

A new approach, involving the anchoring-unanchoring of a fluorophore, has been developed for the detection of Au-species. The fluorescent probe was found to be highly selective for sensing gold species in the presence of several other metal ions. A successful application to bioimaging has also been demonstrated with A549 lung cancer cells.

Enhanced reductive fluorescence quenching of meso-tetrakis-5,10,15,20-pentafluorophenyl porphyrin (H2F20TPP) by two different phenols, 4-methoxy phenol (4-MeOPhOH) and 2,6-dimethoxy phenol (2,6-DiMeOPhOH) in the presence of various pyridine bases in dichloromethane solution is studied using steady state and time resolved fluorescence spectroscopic methods by employing time correlated single photon counting (TCSPC) and fluorescence up-conversion techniques. An enhanced quenching behaviour of H2F20TPP is observed when phenols are hydrogen bonded to various pyridine bases. Quenching observed in the steady state and time resolved studies in the nanosecond time domain follows second order kinetics and generates quenching rate constants and hydrogen bond equilibrium constants, the latter of which agree quite closely with those obtained from independent spectroscopic measurements. A significant kinetic deuterium isotope effect is observed, indicating the importance of proton movement in the quenching processes. This quenching effect is attributed to be due to a tri-molecular transition state involving H2F20TPP and a hydrogen bonded phenol complex, in which electron transfer from phenol to excited H2F20TPP is concerted with proton movement from the phenol to hydrogen bonded base. Observed quenching behaviours are rationalized by invoking diffusion controlled proton coupled electron transfer. Fluorescence up-conversion studies in the 100 ps time domain confirm ultrafast PCET for 4-MeOPhOH and base pairs which fall in a non-diffusive regime.

Noble-metal nanoparticles labeled with fluorescent molecules are used in a variety of applications requiring the measurement of size and diffusion properties of single nanoprobes. We have successfully used intrinsic surface-plasmon-induced photoluminescence (SPPL) signatures of monodispersed bare gold and silver nanoparticles in water to detect and measure their precise diffusion coefficient, concentration and hydrodynamic radius by fluorescence correlation spectroscopy (FCS). Measurement of the effective hydrodynamic radius confirms particle size to be 80 ± 8 and 64 ± 14 nm for gold and silver, respectively, which is in excellent agreement with scanning electron microscopic measurements made on the same particles. Detection of bare gold and silver nanoparticles at the single-molecule level with moderately high value of "per particle brightness" (PPB) confirms those particles to be used as fluorescent probes in biological research and in different medical and biotechnology applications where fluorescence detection plays a vital role. Additionally, these results demonstrate an alternative method for measuring hydrodynamic properties, particularly the size-distribution of bare noble-metal nanoparticles in solution using data-fitting algorithm for FCS based on the maximum entropy method (MEMFCS).