Femtosecond Fluorescence Upconversion Study Research Articles

The role of viscosity in various dynamical processes of different fluorophores in ionic liquid— cosolvent mixtures: a femtosecond fluorescence upconversion study

Literature reports provide ample evidence of the dynamical studies of various fluorophores in different room-temperature ionic liquid (RTIL)-cosolvent mixtures. However, most of the experimental and simulation studies reveal that ∼50% of the spectral relaxation dynamics is fast and cannot be resolved using traditional time correlated single photon counting (TCSPC) measurements. Our group has also investigated the dynamics of a solvatochromic probe coumarin 153 (C153) in a RTIL-cosolvent mixture using a TCSPC setup (S. Sarkar, R. Pramanik, C. Ghatak, P. Setua and N. Sarkar, J. Phys. Chem. B, 2010, 114, 2779-2789). Consequently, a major portion of the solvation dynamics remained undetected and moreover we could not monitor the dynamics beyond 0.4 mole fraction of the cosolvents. Thus in this study, we have rekindled our interest to sufficiently capture the rotational anisotropy and solvation dynamics of C153 beyond 0.4 mole fraction of the cosolvents in the RTIL-cosolvent mixture employing femtosecond fluorescence upconversion measurements. Additionally, we have utilized another RTIL with a higher alkyl chain length and viscosity to obtain a comprehensive and quantitative picture of the role of viscosity on the dynamics of the probe molecule. The most interesting observation of the present work is that the viscosities of different RTIL-cosolvent mixtures can efficiently control the cis-trans isomerization kinetics of the anionic fluorophore merocyanine 540 (MC 540) and the translational diffusion of a hydrophobic probe. The optimization of geometrical structures of [EmimOs]- and [EmimOs]-cosolvent mixtures followed by frequency analyses in both gas and solution phases have been carried out using quantum chemical calculations with the aid of density functional theory (DFT) methods. The computation based on the bond distances, electron densities and non-covalent interactions (NCI) has also been used to investigate the existence of the hydrogen-bond (H-bond). Again to comprehend van der Waals interactions and the conventional hydrogen-bond, the evolution of NCI plots are simulated. Therefore, the detailed experimental and theoretical studies presented in this manuscript lead to the inference that addition of the conventional solvents finely tunes the physicochemical properties of RTILs and broadens their scope of applications in the fields of chemistry and biology.

Femtosecond Fluorescence Upconversion Study of a Naphthalimide-Bithiophene-Triphenylamine Push-Pull Dye in Solution.

There is a high interest in the development of new push-pull dyes for the use in dye sensitized solar cells. The pronounced charge transfer character of the directly photoexcited state is in principle favorable for a charge injection. Here, we report a time-resolved fluorescence study of a triphenylamine-bithiophene-naphthalimide dye in four solvents of varying polarity using fluorescence upconversion. The recording of femtosecond time-resolved fluorescence spectra corrected for the group velocity dispersion allows for a detailed analysis discriminating between spectral shifts and total intensity decays. After photoexcitation, the directly populated state (S1/FC) evolves toward a relaxed charge transfer state (S1/CT). This S1/CT state is characterized by a lower radiative transition moment and a higher nonradiative quenching. The fast dynamic shift of the fluorescence band is well described by solvation dynamics in polar solvents, but less so in nonpolar solvents, hinting that the excited-state relaxation process occurs on a free energy surface whose topology is strongly governed by the solvent polarity. This study underlines the influence of the environment on the intramolecular charge transfer (ICT) process, and the necessity to analyze time-resolved data in detail when solvation and ICT occur simultaneously.

Size of CdTe Quantum Dots Controls the Hole Transfer Rate in CdTe Quantum Dots–MEHPPV Polymer Nanoparticle Hybrid

Design of light harvesting systems using inorganic–organic hybrid nanostructures is an emerging field of research. Here, we design hybrid nanostructures by using electrostatic attraction of negatively charged CdTe QDs with positively surface functionalized poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) polymer nanoparticles for developing an efficient light harvesting system. Interfacial charge transfer between QDs and polymer nanoparticles has been investigated by ultrafast spectroscopy which is crucial for designing an efficient light harvesting system. Interfacial charge transfer dynamics are being controlled by tuning the size of QDs which is eventually controlled by highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) levels of QDs. Femtosecond fluorescence upconversion and transient absorption spectroscopic studies reveal that the hole transfer process takes place from QDs to polymer nanoparticles and the rate of hole transfer process is size depen...

An ultrafast spectroscopic and quantum mechanical investigation of multiple emissions in push-pull pyridinium derivatives bearing different electron donors.

A joint experimental and theoretical approach, involving state-of-the-art femtosecond fluorescence up-conversion measurements and quantum mechanical computations including vibronic effects, was employed to get a deep insight into the excited state dynamics of two cationic dipolar chromophores (Donor-π-Acceptor(+)) where the electron deficient portion is a N-methyl pyridinium and the electron donor a trimethoxyphenyl or a pyrene, respectively. The ultrafast spectroscopic investigation, and the time resolved area normalised emission spectra in particular, revealed a peculiar multiple emissive behaviour and allowed the distinct emitting states to be remarkably distinguished from solvation dynamics, occurring in water in a similar timescale. The two and three emissions experimentally detected for the trimethoxyphenyl and pyrene derivatives, respectively, were associated with specific local emissive minima in the potential energy surface of S1 on the ground of quantum-mechanical calculations. A low polar and planar Locally Excited (LE) state together with a highly polar and Twisted Intramolecular Charge Transfer (TICT) state is identified to be responsible for the dual emission of the trimethoxyphenyl compound. Interestingly, the more complex photobehaviour of the pyrenyl derivative was explained considering the contribution to the fluorescence coming not only from the LE and TICT states but also from a nearly Planar Intramolecular Charge Transfer (PICT) state, with both the TICT and the PICT generated from LE by progressive torsion around the quasi-single bond between the methylpyridinium and the ethene bridge. These findings point to an interconversion between rotamers for the pyrene compound taking place in its excited state against the Non-equilibrated Excited Rotamers (NEER) principle.

Spectroscopic evidence of the presence of an activation barrier in the otherwise barrierless excited state potential energy surface of auramine-O: A femtosecond fluorescence up-conversion study

A widely acclaimed model for the excited state relaxation dynamics of auramine-O involves orientational relaxation of dimethylanilino moieties along the barrierless excited state potential energy surface (PES). Such a model would necessitate similar excited state dynamics in media offering similar viscous drag. However, we have noticed an interesting experimental observation showing auramine-O to have ~8 times larger fluorescence quantum yield in chloroform than in methanol, though both the solvents have the same viscosity. The femtosecond fluorescence transients of auramine-O in chloroform surprisingly depict a rise in fluorescence transients, which has not been observed before. This, along with the simultaneous observation of unexpectedly large fluorescence lifetime and multi-exponential transients (in chloroform) questions the thoroughly accepted barrierless model of auramine-O relaxation dynamics, as the barrierless model would demand a short lifetime and single-exponential decay. Temperature dependent quantum yield measurements along with solvent dependent excited state multi-coordinate time-dependent density functional theory calculations further unveil the exact nature of PES. All these results concomitantly conclude that, at-least in chloroform, upon photo-excitation auramine-O must pass over an activation barrier before damping the excited state population into ground state via a sink function through adiabatic coupling of the electronic states.

A femtosecond fluorescence study of vibrational relaxation and cooling dynamics of UV dyes

We present a femtosecond broad-band fluorescence up-conversion study of the vibrational relaxation dynamics of two UV chromophores, 2,5-diphenyloxazole (PPO) and para-terphenyl (pTP), pumped with a large excess of vibrational energy (>2000 cm(-1)). The band narrowing of the transient fluorescence spectrum reflects a biphasic cooling process in a few hundreds of fs and a few ps. In the sub-ps regime, our data suggest a structural rearrangement in the excited state, followed by thermalization of the excess energy. These dynamics affect the fluorescence spectra of PPO and pTP in different ways. In PPO, the damping of a low frequency vibrational wavepacket and a significant sub-ps narrowing of the band characterize the vibrational relaxation. In pTP, the latter is faster and appears as a red shift with distortion of the band in <200 fs.

Assessing solvent effects on the singlet excited state lifetime of uracil derivatives: A femtosecond fluorescence upconversion study in alcohols and D 2O

The excited state lifetimes of uracil, thymine and 5-fluorouracil have been measured using femtosecond UV fluorescence upconversion in various protic and aprotic polar solvents. The fastest decays are observed in acetonitrile and the slowest in aqueous solution while those observed in alcohols are intermediate. No direct correlation with macroscopic solvent parameters such as polarity or viscosity is found, but hydrogen bonding is one key factor affecting the fluorescence decay. It is proposed that the solvent modulates the relative energy of two close-lying electronically excited states, the bright ππ ∗ and the dark nπ ∗ states. This relative energy gap controls the non-radiative relaxation of the ππ ∗ state through a conical intersection close to the Franck–Condon region competing with the ultrafast internal conversion to the ground state. In addition, an inverse isotope effect is observed in D 2O where the decays are faster than in H 2O.

Ultrafast UV photon echo peak shift and fluorescence up conversion studies of non-polar solvation dynamics

We present photon echo peak shift and femtosecond fluorescence up-conversion studies of non-polar solvation dynamics of a simple non-polar dye p-terphenyl in ethanol and cyclohexane, using excitation in the UV range at 290 nm. The UV fluorescence up-conversion experiments were combined with a polychromatic detection and the results highlight the high sensitivity of this approach to fully characterize the excited state dynamics of the dye. We also demonstrate the feasibility of UV photon echo and transient grating and its sensitivity for the detection of non-polar solvation dynamics by measuring the frequency correlation function of the dye in the ground state. While solvation dynamics in the picosecond regime is observed in ethanol, electronic coherence dephasing occurs on timescales faster than 100 fs in ethanol as well as in the non-polar solvent cyclohexane.

Excited State Intramolecular Proton Transfer in Schiff Bases. Decay of the Locally Excited Enol State Observed by Femtosecond Resolved Fluorescence

Although the late (t>1 ps) photoisomerization steps in Schiff bases have been described in good detail, some aspects of the ultrafast (sub-100 fs) proton transfer process, including the possible existence of an energy barrier, still require experimental assessment. In this contribution we present femtosecond fluorescence up-conversion studies to characterize the excited state enol to cis-keto tautomerization through measurements of the transient molecular emission. Salicylideneaniline and salicylidene-1-naphthylamine were examined in acetonitrile solutions. We have resolved sub-100 fs and sub-0.5 ps emission components which are attributed to the decay of the locally excited enol form and to vibrationally excited states as they transit to the relaxed cis-keto species in the first electronically excited state. From the early spectral evolution, the lack of a deuterium isotope effect, and the kinetics measured with different amounts of excess vibrational energy, it is concluded that the intramolecular proton transfer in the S1 surface occurs as a barrierless process where the initial wave packet evolves in a repulsive potential toward the cis-keto form in a time scale of about 50 fs. The absence of an energy barrier suggests the participation of normal modes which modulate the donor to acceptor distance, thus reducing the potential energy during the intramolecular proton transfer.

Ultrafast Nonadiabatic Dynamics of [FeII(bpy)3]2+ in Solution

The ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band (1MLCT) has been characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad 1MLCT emission band at approximately 600 nm, which decays in < or =20 fs, and a weak emission at approximately 660 nm, which we attribute to the 3MLCT, populated by intersystem crossing (ISC) from the 1MLCT state. The TA studies show a short-lived (<150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm, along with the ground-state bleach (GSB). We identify the short-lived ESA as being due to the 3MLCT state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state 5T2 lifetime. A singular value decomposition and a global analysis of the TA data, based on a sequential relaxation model, reveal three characteristic time scales: 120 fs, 960 fs, and 665 ps. The first is the decay of the 3MLCT, the second is identified as the population time of the 5T2 state, while the third is its decay time to the ground state. The anomalously high ISC rate is identical in [RuII(bpy)3]2+ and is therefore independent of the spin-orbit constant of the metal atom. To reconcile these rates with the regular quasi-harmonic vibrational progression of the 1MLCT absorption, we propose a simple model of avoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction. The subsequent relaxation steps down to the 5T2 state dissipate approximately 2000 cm-1/100 fs. This rate is discussed, and we conclude that it nevertheless can be described by the Fermi golden rule, despite its high value.

Singlet excited state dynamics of uracil and thymine derivatives: A femtosecond fluorescence upconversion study in acetonitrile

The excited state properties of uracil, thymine and four analogous uracil compounds have been studied in acetonitrile by steady-state and time-resolved spectroscopy. The excited state lifetimes were measured using femtosecond UV fluorescence upconversion. The excited state lifetimes of uracil and its 1- and 3-methyl substituted derivatives are well described by one ultrafast (⩽100 fs) component. Five substituted compounds show a more complex behavior, exhibiting longer excited state lifetimes and bi-exponential fluorescence decays. These longer decays are substantially faster in acetonitrile than in aqueous solution showing that the excited state deactivation mechanism is in part governed by the solvent.

Ultrafast excited-state intramolecular double proton transfer dynamics of [2,2′-bipyridyl]-3,3′-diamine

A femtosecond fluorescence upconversion study of [2,2′-bipyridyl]-3,3′-diamine (BP(NH 2) 2), in liquid solution, is reported. It is concluded that photo-excited BP(NH 2) 2 undergoes a branched intramolecular double proton transfer reaction comprising two trajectories: (a) ultrafast double proton transfer (<100 fs) followed by twisting (∼250 fs); (b) a combined process of double proton transfer and twisting, with an overall reaction time of ∼250 fs. Picosecond transient fluorescence is attributed to vibrational cooling in the excited product state. The lifetime of ∼10 ps of the tautomer product state is indicative of conical intersection of the product- and ground-state potential energy surfaces.

Femtosecond Fluorescence Upconversion Studies of Light Harvesting by β-Carotene in Oxygenic Photosynthetic Core Proteins

Energy transfer from β-carotene to chlorophyll in the photosystem (PS) I core complex and the CP43, CP47, and reaction center (RC) proteins of PSII was studied by the femtosecond fluorescence upconversion technique. The carotenoid S2 lifetimes/transfer efficiencies, the latter obtained by comparison with β-carotene in solution, are ∼97 fs/33% (CP43), ∼100 fs/31% (CP47), ∼106 fs/26% (PSII−RC), and ∼62 fs/57% (PSI). By combining previous steady-state fluorescence excitation measurements with our ultrafast results on the carotenoid S2 lifetime and chlorophyll rise kinetics, we conclude that the β-carotene S1 state is insignificant for light harvesting in the PSII core proteins. Measurement of the steady-state fluorescence excitation spectrum of PSI at 77 K in this work yielded a β-carotene to chlorophyll energy transfer efficiency of 62 ± 3%, suggesting dominant transfer from the carotenoid S2 state. Interestingly, while both PSI and PSII almost exclusively utilize the carotenoid S2 state for light harvestin...

Femtosecond fluorescence up-conversion and transient absorption studies of the S 2 state in the hydrogen-bonded benzopyranthione–water complex

We have studied for the first time the photophysics of the thioketone, 4H-1-benzopyrane-4-thione (BPT), in water and deuterated water, by using femtosecond fluorescence up-conversion and transient absorption spectroscopies. We propose that the S 2-state deactivation occurs in the hydrogen-bonded BPT–water complex. The lifetime of the complex in the S 2 state is found to be as short as 1.0 ± 0.1 ps in H 2O, and 2.2 ± 0.1 ps in D 2O.

Femtosecond Fluorescence Upconversion Study of the Rotations of Perylene and Tetracene in Hexadecane

The rotational relaxation of perylene and tetracene in hexadecane has been studied at several temperatures by femtosecond fluorescence upconversion. The initial emission anisotropy values of both D2h symmetric fluorophores are now found to be very close to the theoretical value of 0.4, and the anisotropy decay observed with subpicosecond time resolution can no longer be described as a biexponential. The traditional in-plane and out-of-plane rotational diffusion time constants of perylene in hexadecane are verified to be linearly dependent on viscosity/temperature. Moreover, a third, ultrafast rotation, empirically fit to a time constant of ∼ 450 fs for perylene and ∼600 fs for tetracene, has been found. It is independent of temperature and viscosity, and the amplitude (“βfast”) explains the previously described “r0 defect ”. Since βfast is ∼0.03 for tetracene vs ∼0.05 for perylene, we presently suggest that “in-plane” femtosecond libration of the molecules within a solvent pocket yields the observed anisotropy.

Femtosecond fluorescence upconversion study of a boron dipyrromethene dye in solution

A femtosecond fluorescence upconversion study is reported for the boron-dipyrromethene (BDP) dye, 2,6-diethyl-1,3,5,7-tetramethyl-8-phenyl-4-difluorobora-3 a,4 a-diaza-( s)-indacene ( BDP-1), dissolved in cyclohexane, chloroform and acetonitrile. After pulsed photoexcitation into the higher excited states, S 2 or S 3 (at energies of ∼ 26 300 cm −1 and ∼ 30 000 cm −1 above the S 0 state, respectively), the fluorescence from the S 1 state shows an ultrafast solvent-dependent rise, with characteristic times between 100 and 230 fs. The rise is attributed to S n →S 1 ( n=2,3) internal conversion followed by vibrational relaxation. In addition, the BDP-1 fluorescence shows picosecond transient behavior that is attributed to vibrational cooling in the excited S 1 state.

Femtosecond fluorescence upconversion studies of excited-state proton-transfer dynamics in 2-(2 ′-hydroxyphenyl)benzoxazole (HBO) in liquid solution and DNA

A femtosecond fluorescence upconversion study is reported for HBO in solution, as well as for HBO incorporated in DNA. The typical time for the excited-state intramolecular proton-transfer reaction of the syn-enol tautomer in solution and in DNA has been determined to be 150 fs. In addition, the lifetimes of the keto, the anti-enol and the ‘solvated enol’ tautomer forms were determined in protic solvents, aprotic solvents and DNA. Picosecond rise and decay components in the fluorescence transients with characteristic times between 3 and 25 ps are also observed and attributed to the effects of vibrational cooling.

Femtosecond fluorescence upconversion studies of barrierless bond twisting of auramine in solution

Femtosecond fluorescence upconversion studies have been performed for auramine (a diphenylmethane dye), dissolved in ethanol, as a function of temperature. It is found that the (sub)picosecond decay components in the fluorescence slow down as the temperature is lowered from 293 K to 173 K. From the observation of a residual fluorescence, with a viscosity-dependent lifetime of about 30 ps (or longer at higher viscosity), and transient absorption results it is concluded that the two-state sink function model [B. Bagchi, G. R. Fleming, and D. W. Oxtoby, J. Chem. Phys. 78, 7375 (1983)] does not apply in the case of auramine. Comparison of the auramine fluorescence kinetics in ethanol and decanol shows that diffusional twisting and not solvation is the main cause for the (sub)picosecond excited state relaxation. To explain the experimental results, adiabatic coupling between a locally excited emissive state (F) and a nonemissive excited state (D) is considered. Torsional diffusion motions of the phenyl groups in the auramine molecule are held responsible for the population relaxation along the adiabatic potential of the mixed state, S1 (comprised of the F and D states). Simulation of the excited state dynamics is feasible assuming a barrierless-shaped potential energy for S1 and applying the Smoluchowski diffusion equation. The temporal behavior of the auramine band emission was simulated for the temperature range 293 K &amp;gt;T&amp;gt;173 K, with the temperature, T, and the viscosity coefficient, η, being the only variable parameters. The simulated temporal behavior of the emission in the investigated temperature range is compatible with that obtained experimentally. The rotational diffusion coefficient for the auramine phenyl groups as extracted from the simulations is found to follow the Einstein–Stokes relation. From the numerical calculations the effective radius of the twisting phenyl groups is determined as 1.0 Å which compares well with the actual value of 1.2 Å.

Femtosecond studies of double proton transfer in [2,2′-Bipyridyl]-3,3′-Diol in Sol-Gel glasses

From a femtosecond fluorescence up-conversion study of [2,2′-bipyridyl]-3,3′-diol in the TMOS sol-gel glass, it is shown that the double proton transfer dynamics is significantly influenced when the wavelength of the exciting laser pulses is varied. From the data an energy barrier in the excited-state energy surface of about 300 cm−1 is concluded in the dienol to monoketo reaction.

Intramolecular evolution from a locally excited state to an excimer-like state in a multichromophoric dendrimer evidenced by a femtosecond fluorescence upconversion study

Abstract A time-resolved fluorescence upconversion study on a polyphenylene dendrimer with eight peryleneimide chromophores on the surface (1) and on a monochromophoric model compound (2) is reported. The time-dependent fluorescence spectra of the dendrimer show that the initial excitation is into a locally excited chromophore. They further indicate the existence of a decay channel that leads to excited state interaction between chromophores in one dendrimer which takes place on a 5 ps timescale.