Time-resolved Fluorescence Up-conversion Spectroscopy Research Articles

Photo-physical properties of substituted 2,3-distyryl indoles: Spectroscopic, computational and biological insights

The structural dependence of the photo-physical properties of substituted 2,3-distyryl (23DSI) indoles were studied using several spectroscopic techniques including steady-state UV-VIS spectroscopy, steady-state fluorescence spectroscopy, steady-state excitation spectroscopy, time correlated single photon counting (TCSPC) spectroscopy, and time-resolved fluorescence upconversion spectroscopy (TRFLS). Each of 23DSI derivatives investigated showed distinct fluorescence emission and UV-VIS spectra, indicating strong structural dependence of the emission and the excitation. The UV-VIS spectra of the 23DSI derivatives showed three main identical absorption bands with minor deviations in the absorbance caused by substituent groups on the distyryl rings. The time-resolved fluorescence up-conversion studies indicated that the fluorescence undergoes a mono-exponential decay whereas the calculated fluorescence lifetime showed relatively short fluorescence lifetimes of approximately 1 ns. All of the 23DSI derivatives showed two-photon absorption upon direct excitation of 1.6 W laser pulses at 800 nm. These studies suggest that the substituents, attached to distyryl core, are capable of boosting or hindering fluorescence intensities by distorting the π-conjugation of the 23DSI molecule. Our studies showed that 23DSI (p-F) has the highest fluorescence emission quantum yield. Theoretical calculations for the ground state of 23DSI derivatives confirmed differences in electron densities in 23DSI derivatives in the presence of different substituent attachments. The excellent fluorescence emission, high fluorescence quantum yield and two-photon absorption properties of these 23DSI molecules make them attractive candidates for potential applications in the fields of biological imaging, biomedicine, fluorescent probes, and photodynamic inactivation (PDI). B. subtilis samples, treated with micro molar solutions of 23DSI (p-OCH3) and 23DSI (p-CH3), showed very effective photodynamic inactivation (PDI) upon irradiation with white light.

Spectroscopic Study of a Photoactive Antibacterial Agent: 2,3-Distyrylindole.

Optical properties and fluorescence decay dynamics of a photoactive indole based antibacterial chromophore system, 2,3-distyrylindole (23DSI), were investigated using various spectroscopic characterization techniques. Experimental studies were done by utilizing steady-state UV-vis spectroscopy, steady-state fluorescence spectroscopy, time-resolved fluorescence upconversion spectroscopy, and time-correlated single-photon counting spectroscopy. Our studies show that the 23DSI molecule has a multiphoton absorption property as indicated by two- and three-photon absorption in the both the solution and the solid phases. The ultrafast time-resolved fluorescence upconversion studies show that this molecule undergoes a fast decay process with an average time constant of 34 ps, a single exponential decay, and an average fluorescence lifetime of 1 ns. The compound 23DSI did not show any signs of singlet oxygen production. The density functional theory (DFT) calculations showed that the 23DSI molecule has conjugated electron densities that are responsible for multiphoton absorption. The chlorine-substituted styryl groups, attached to the central indole ring facilitate the excellent electron delocalization within the molecule. This optimal electron delocalization, combined with the good electron conjugation in the 23DSI molecule is important for efficient multiphoton absorption and is in excellent agreement with experimental observations. Both the optical spectrum and emission spectrum using DFT calculations are also surprisingly well matched with the experimentally measured UV-vis spectrum and the emission spectrum, respectively. Combined experimental and theoretical studies suggest that excited electrons initially relax to the singlet state (S1) by internal conversion (IC) and subsequently relax back to their ground state by emitting absorbed energy as fluorescence emission. The outstanding multiphoton absorption capabilities of this 23DSI molecule support its potential application in both biological imaging and photodynamic inactivation (PDI).

A Modified Four-State Model for the “Dual Fluorescence” of N6,N6-Dimethyladenine Derived from Femtosecond Fluorescence Spectroscopy

The radiationless deactivation of the excited electronic states of the dual fluorescence molecule N(6),N(6)-dimethyladenine (DMAde) was investigated using femtosecond time-resolved fluorescence up-conversion spectroscopy. The molecules were studied in solution in water and in dioxane. Fluorescence-time profiles were recorded in the wide wavelength range of 290 <or= lambda(fl) <or= 650 nm. The excitation wavelengths in the region of the first UV absorption band were tuned from close to the electronic origin (lambda(pump) = 294 nm) to excess energies of approximately 5400 cm(-1) above (lambda(pump) = 258 nm). Global fits to the measured curves turned out to reflect distinctive molecular relaxation processes on five well-defined time scales. Sub-100 fs and 0.52(3) ps lifetimes were found to predominate at the shortest UV and blue emission wavelengths in water, 1.5(1) and 3.0(2) ps components at intermediate wavelengths and a 62(1) ps value in the red region of the spectrum (2sigma error limits of the last digits in parentheses). In dioxane, these lifetimes changed to <or=0.27 and 0.63(4) ps in the UV, 1.5(1) and 10.9(10) ps in a wide range of intermediate, and 1.40(4) ns at the longest wavelengths. However, little dependence of the respective time constants on lambda(pump) was observed, indicating that the ensuing relaxation processes proceed via practically barrierless pathways through conical intersections. Building on the knowledge for the parent molecule adenine (Ade), the observations were rationalized with the help of a modified four-state model for the electronic dynamics in DMAde with the pipi*(L(a)), pipi*(L(b)), and npi* states similar to those in Ade and an intramolecular charge-transfer (ICT) state, which has no counterpart in Ade, responsible for the long-wavelength fluorescence.

Strong Two Photon Absorption and Photophysical Properties of Symmetrical Chromophores with Electron Accepting Edge Substituents

Two photon absorption (TPA) and photophysical properties of three new symmetrical chromophores with electron accepting phthalimide edge substituents have been studied. The three chromophores contain fluorene, alcoxy-substituted divinyl benzene, and carbazole moieties as central cores, respectively. The femtosecond time-resolved fluorescence upconversion spectroscopy and two photon excited fluorescence technique have been carried out. The effect of solvent polarity on TPA and on photophysics has also been determined. Ultrafast fluorescence dynamics, with decay times ranging from 1 to 13 ps, are revealed in polar solvents. This is attributed to the relaxation of the chromophores to the intramolecular charge transfer state. The chromophore bearing fluorene central core, being of the type A-pi-A, is the most efficient concerning TPA. Strong TPA, with a cross section value as high as 2100 GM at an excitation wavelength of 770 nm is found in acetophenone which is a solvent of intermediate polarity. The TPA spectra were also reproduced using a sum over states three-state model. A study of the TPA induced photobleaching of the fluorene molecule, doped in a solid poly(methyl-methacrylate) film, has shown that this material is very promising for efficient TPA optical data storage.

Femtosecond excitation dynamics in gold nanospheres and nanorods

Femtosecond visible photoluminescence is detected from gold nanoparticles using time-resolved fluorescence upconversion spectroscopy. We directly compared this fast luminescence from gold nanospheres $(\ensuremath{\emptyset}25\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ with that obtained from nanorods ($\ensuremath{\emptyset}15\ifmmode\times\else\texttimes\fi{}40\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $\ensuremath{\emptyset}15\ifmmode\times\else\texttimes\fi{}27\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) under vis $(3.02\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ and $\mathrm{UV}(4.65\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ excitation. A fast $(\ensuremath{\sim}50\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ decay was obtained for the nanoparticles and the emission was depolarized. Degenerate femtosecond pump-probe experiments in the low excitation intensity regime demonstrated much slower electron thermalization and/or equilibration dynamics on the time scale of a few hundred femtoseconds. These features strongly indicate a $d$-hole-conduction electron recombination process as the origin of this photoluminescence. A direct comparison of the fast emission spectra from nanorods and nanospheres is used to discuss the emission enhancement mechanism. These results suggest that the classical local field enhancement theory describes quantitatively well many of the emission features of nanorods with respect to those for nanospheres without invoking more complex models.

Femtosecond fluorescence up-conversion spectroscopy of adenine and adenosine: experimental evidence for the πσ* state?

Femtosecond time-resolved fluorescence up-conversion spectroscopy has been used in a study of the excited electronic state lifetimes of the purine base adenine (Ade) and its ribonucleoside adenosine (Ado) in aqueous solution. The molecules were excited at wavelengths in the range 245 ⩽ λ exc ⩽ 280 nm using tunable UV pulses from a frequency doubled non-collinear optical parametric amplifier (NOPA). The observed biexponential temporal fluorescence profiles of Ade could be fitted using a fast decay time between τ 1 = (0.34 ± 0.07) ps at the shorter excitation wavelengths and τ 1 = (0.67 ± 0.14) ps at the longer excitation wavelengths and, independent of the excitation wavelength, a slow decay time of τ 2 = (8.4 ± 0.8) ps. The two values were assigned to the “canonical” 9H-Ade tautomer ( τ 1) and the less stable 7H-Ade tautomer ( τ 2) which are known to be present in aqueous solution. The excited state lifetime of 9H-Ade in H 2O is thus sub-picosecond even around the electronic origin of the first excited ππ* state, in contrast to a report for the 0–0 level in the gas phase (≳9 ps). The fluorescence decay profiles of Ado, in which the 9H atom is substituted by the ribose, could be described assuming monoexponential behavior with a lifetime τ = (0.31 ± 0.05) ps. The results are consistent with fast radiationless electronic relaxation from the excited ππ* to the S 0 ground state. The apparent step in the lifetime of 9H-Ade centered at λ exc ≈ 265 nm might be interpreted as evidence for the opening of an additional radiationless electronic relaxation pathway, which could arise from the πσ* state predicted at about that energy.

Femtosecond fluorescence up-conversion spectroscopy of a rotation-restricted azobenzene after excitation to the S1state

Femtosecond time-resolved fluorescence up-conversion spectroscopy has been used in a study of the photoinduced isomerization reactions of a rotation-restricted trans-azobenzene (trans-AB) derivative capped by a crown ether (1), a chemically similar open derivative (2), and unsubstituted trans-AB (3) after excitation to the S1 (npi*) state at lamda=475 nm in dioxane solution. The observed biexponential temporal fluorescence profiles for 1 and 2 were almost indistinguishable within experimental error. The fitted fast fluorescence decay times (+/-2sigma) for the two compounds were tau1 (1) = (0.79 +/- 0.20) and tau1 (2) = (1.05 +/- 0.20) ps, compared to tau1 (3) = (0.37 +/- 0.06) ps. The second decay components could be described with tau2 (1) = (20.3 +/- 9.5) resp. tau2 (2) = (19.0 +/- 6.0) ps, vs. tau2 (3) = (3.26 +/- 0.85) ps. The very similar lifetimes strongly suggest that trans-cis isomerization of 1 and 2 after S1 excitation is governed by the same mechanism. Since 1 cannot isomerize by a simple large-amplitude rotation of one of the phenyl rings about the central NN bond, the isomerization dynamics of both ABs should be better described as "inversion" at the N atom(s) rather than large-amplitude "rotation".

Polar Solvation Dynamics of H2O and D2O at the Surface of Zirconia Nanoparticles

Time-resolved fluorescence-upconversion spectroscopy has been used to study the polar solvation dynamics of H{sub 2}O and D{sub 2}O at the surface of zirconia (ZrO{sub 2}) nanoparticles. While an isotope effect is observed for the solvation dynamics of bulk D{sub 2}O, there is no isotope effect on the interfacial solvation dynamics. The interfacial solvation dynamics are the same for H{sub 2}O and D{sub 2}O and are faster than the bulk solutions. The bulk isotope effect is due to stronger hydrogen bonding in D{sub 2}O compared to H{sub 2}O, slowing the reorientation of the excited-state dipoles in the bulk D{sub 2}O. The lack of isotope effect for the interfacial dynamics is explained in terms of the solvent interacting with the ZrO{sub 2} surface.

Water Immobilization at Surfactant Interfaces in Reverse Micelles

The mobility of water in water/Aerosol OT (AOT)/isooctane reverse micelles has been investigated via polar solvation dynamics measurements using ultrafast, time-resolved fluorescence-upconversion spectroscopy. For the smallest reverse micelles studied, w0 = 1.1, no solvation dynamics are observed on the time scale of the instrument, suggesting that the water is essentially frozen. As w0 increases, the mobility of water increases, and the time-correlation functions exhibit multiexponential decays. A subpicosecond component is observed on a time scale similar to that of diffusive bulk-water motion.1 The other components decay on time scales of a few picoseconds to hundreds of picoseconds. The amplitude of the solvent response increases as w0 increases, which is attributed to increased mobility of the water inside the reverse micelles. Because the micellar interior is highly ionic, the solvation dynamics of coumarin 343 in a 1 M Na+ (Na2SO4) solution were also investigated to compare the dynamics of water in...