Broadband Fluorescence Up-conversion Research Articles

Modulation of Symmetry-Breaking Intramolecular Charge-Transfer Dynamics Assisted by Pendant Side Chains in π-Linkers in Quadrupolar Diketopyrrolopyrrole Derivatives.

The effect of the length of pendant side chains in centrosymmetric quadrupolar molecules on dynamics of their most perplexing photophysical phenomenon, i.e., symmetry-breaking intramolecular charge transfer, has been discovered. Unexpectedly, considerable influence of length of these pendant side chains in π-linkers arose as a structural factor enabling the control of the degree of fluorescence solvatochromism. The symmetry-breaking intramolecular charge-transfer dynamics has been described on quadrupolar diketopyrrolopyrrole derivatives possessing fluorene moieties as π-linkers and diarylamino groups as electron donors. On the basis of the evolution of transient fluorescence spectra obtained by a femtosecond broadband fluorescence up-conversion spectroscopy, it was found that the relative contribution of diffusive solvation and torsional relaxation in overall spectral relaxation can be modulated by the length of pendant side chain in π-linkers. Consequently, we demonstrated that this modulation plays a significant role in determining the photophysical properties of diketopyrrolopyrroles in a polar medium.

Femtosecond broadband fluorescence upconversion spectroscopy: Spectral coverage versus efficiency.

Sum frequency mixing of fluorescence and ∼1300 nm gate pulses, in a thin β-barium borate crystal and non-collinear type II geometry, is quantified as part of a femtosecond fluorimeter [X.-X. Zhang et al., Rev. Sci. Instrum. 82, 063108 (2011)]. For a series of fixed phasematching angles, the upconversion efficiency is measured depending on fluorescence wavelength. Two useful orientations of the crystal are related by rotation around the surface normal. Orientation A has higher efficiency (factor ∼3) compared to B at the cost of some loss of spectral coverage for a given crystal angle. It should be used when subtle changes of an otherwise stationary emission band are to be monitored. With orientation B, the fluorescence range λF > 420-750 nm is covered with a single setting of the crystal and less gate scatter around time zero. The accuracy of determining an instantaneous emission band shape is demonstrated by comparing results from two laboratories.

Solvent-dependent excited-state hydrogen transfer and intersystem crossing in 2-(2'-hydroxyphenyl)-benzothiazole.

The excited-state intramolecular hydrogen transfer (ESIHT) of 2-(2'-hydroxyphenyl) benzothiazole (HBT) has been investigated in a series of nonpolar, polar aprotic, and polar protic solvents. A variety of state-of-the-art experimental methods were employed, including femto- and nanosecond transient absorption and fluorescence upconversion spectroscopy with broadband capabilities. We show that the dynamics and mechanism of ESIHT of the singlet excited HBT are strongly solvent-dependent. In nonpolar solvents, the data demonstrate that HBT molecules adopt a closed form stabilized by O-H···N chelated hydrogen bonds with no twisting angle, and the photoinduced H transfer occurs within 120 fs, leading to the formation of a keto tautomer. In polar solvents, owing to dipole-dipole cross talk and hydrogen bonding interactions, the H transfer process is followed by ultrafast nonradiative deactivation channels, including ultrafast internal conversion (IC) and intersystem crossing (ISC). This is likely to be driven by the twisting motion around the C-C bond between the hydroxyphenyl and thiazole moieties, facilitating the IC back to the enol ground state or to the keto triplet state. In addition, our femtosecond time-resolved fluorescence experiments indicate, for the first time, that the lifetime of the enol form in ACN is approximately 280 fs. This observation indicates that the solvent plays a crucial role in breaking the H bond and deactivating the excited state of the HBT. Interestingly, the broadband transient absorption and fluorescence up-conversion data clearly demonstrate that the intermolecular proton transfer from the excited HBT to the DMSO solvent is about 190 fs, forming the HBT anion excited state.

Composition Dependent Stokes Shift Dynamics in Binary Mixtures of 1-Butyl-3-methylimidazolium Tetrafluoroborate with Water and Acetonitrile: Quantitative Comparison between Theory and Complete Measurements

Here we predict, using a semimolecular theory, the Stokes shift dynamics of a dipolar solute in binary mixtures of an ionic liquid (IL), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]), with water (H2O) and acetonitrile (CH3CN), and compare with the experimental results. The latter are from the recent measurements that combined broad-band fluorescence up-conversion (FLUPS) with time-correlated single photon counting (TCSPC) techniques and used coumarin 153 (C153) as a solute probe. Nine different compositions of ([Bmim][BF4] + H2O) and ([Bmim][BF4] + CH3CN) binary mixtures are considered for the extensive comparison between theory and experiments. Two separate model calculations have been performed using the available experimental frequency dependent dielectric function, ε(ω). These calculations semiquantitatively reproduce the experimentally observed (i) IL mole fraction dependence of dynamic Stokes shifts in these mixtures, (ii) composition dependence of average fast, slow, and solvation times, (iii) viscosity dependence of slow times, and (iv) the nonlinear dependence of average solvation times on experimental inverse conductivity. Variations of the calculated dynamics on water dipole moment values (gas phase or liquid phase) and sensitivity to different measurements of ε(ω) for ([Bmim][BF4] + H2O) mixtures are examined. In addition, the importance of the missing contribution to experimental ε(ω) from high frequency collective solvent intermolecular modes for generating the experimentally observed sub-picosecond solvation response in these (IL + polar solvent) binary mixtures has been explored.

Excess Dynamic Stokes Shift of Molecular Probes in Solution

The solvation dynamics of molecular probes is studied by broad-band fluorescence upconversion. The time-dependent position of the S1 → S0 emission band or of a vibronic line shape is measured with ~80 fs, 10 cm(-1) resolution. Polar solutes in acetonitrile and acetone, when excited into S1 with excess vibrational energy, show a dynamic Stokes shift which extends to the red beyond the quasistationary state. Equilibrium is then reached by a slower blue shift on a 10 ps time scale. In methanol, excess vibrational energy as large as ~14,000 cm(-1) shows no such effect. Nonpolar solutes exhibit an excess red shift of the emission band in both polar and nonpolar solvents even upon excitation near the vibronic origin. The observed dynamics are discussed in terms of transient heating of the excited chromophore, conformational change, and changes of the molecular cavity size. For solvation studies the optical excitation should be chosen close to the band origin.

Fluorescence following Excited-State Protonation of Riboflavin at N(5)

Excited-state protonation of riboflavin in the oxidized form is studied in water. In the -1 < pH < 2 range, neutral and N(1)-protonated riboflavin coexist in the electronic ground state. Transient absorption shows that the protonated form converts to the ground state in <40 fs after optical excitation. Broadband fluorescence upconversion is therefore used to monitor solvation and protonation of the neutral species in the excited singlet state exclusively. A weak fluorescence band around 660 nm is assigned to the product of protonation at N(5). Its radiative rate and quantum yield relative to neutral riboflavin are estimated. Protonation rates agree with proton diffusion times for H(+) concentrations below 5 M but increase at higher acidities, where the average proton distance is below the diameter of the riboflavin molecule.

Complete Solvation Response of Coumarin 153 in Ionic Liquids

The dynamic Stokes shift of coumarin 153, measured with a combination of broad-band fluorescence upconversion (80 fs resolution) and time-correlated single photon counting (to 20 ns), is used to determine the complete solvation response of 21 imidazolium, pyrrolidinium, and assorted other ionic liquids. The response functions so obtained show a clearly bimodal character consisting of a subpicosecond component, which accounts for 10-40% of the response, and a much slower component relaxing over a broad range of times. The times associated with the fast component correlate with ion mass, confirming its origins in inertial solvent motions. Consistent with many previous studies, the slower component is correlated to solvent viscosity, indicating that its origins lie in diffusive, structural reorganization of the solvent. Comparisons of observed response functions to the predictions of a simple dielectric continuum model show that, as in dipolar solvents, solvation and dielectric relaxation involve closely related molecular dynamics. However, in contrast to dipolar solvents, dielectric continuum predictions systematically underestimate solvation times by factors of at least 2-4.

Sequential photoisomerisation dynamics of the push-pull azobenzene Disperse Red 1

The ultrafast dynamics of the push-pull azobenzene Disperse Red 1 following photoexcitation at λ(pump) = 475 nm in solution in 2-fluorotoluene have been probed by broadband transient absorption spectroscopy and fluorescence up-conversion spectroscopy. The measured two-dimensional spectro-temporal absorption map features a remarkable "fast" excited-state absorption (ESA) band at λ ≈ 570 nm appearing directly with the excitation laser pulse and showing a sub-100 fs lifetime with a rapid spectral blue-shift. Moreover, its ultrafast decay is paralleled by rising distinctive ESA at other wavelengths. Global fits to the absorption-time profiles using a consecutive kinetic model yielded three time constants, τ(1) = 0.08 ± 0.03 ps, τ(2) = 0.99 ± 0.02 ps, and τ(3) = 6.0 ± 0.1 ps. Fluorescence-time profiles were biexponential with time constants τ(1)' = 0.12 ± 0.06 ps and τ(2)' = 0.70 ± 0.10 ps, close to the absorption results. Based on the temporal evolution of the transient spectra, especially the "fast" excited-state absorption band at λ ≈ 570 nm, and on the global kinetic analysis of the time profiles, τ(1) is assigned to an ultrafast transformation of the optically excited ππ* state to an intermediate state, which may be the nπ* state, τ(2) to the subsequent isomerisation and radiationless deactivation time to the S(0) electronic ground state, and τ(3) to the eventual vibrational cooling of the internally "hot" S(0) molecules.

Measurements of the complete solvation response of coumarin 153 in ionic liquids and the accuracy of simple dielectric continuum predictions

The complete solvation response of coumarin 153 (C153) has been determined over the range 10(-13)-10(-8) s in a variety of ionic liquids by combining femtosecond broad-band fluorescence upconversion and picosecond time-correlated single photon counting measurements. These data are used together with recently reported dielectric data in eight ionic liquids to test the accuracy of a simple continuum model for predicting solvation dynamics. In most cases the features of the solvation response functions predicted by the dielectric continuum model are similar to the measured dynamics of C153. The predicted dynamics are, however, systematically faster than those observed, on average by a factor of 3-5. Computer simulations of a model solute/ionic liquid system also exhibit the same relationship between dielectric predictions and observed dynamics. The simulations point to spatial dispersion of the polarization response as an important contributor to the over-prediction of solvation rates in ionic liquids.

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.

Femtosecond broadband fluorescence upconversion spectroscopy: Improved setup and photometric correction

A setup for fluorescence upconversion spectroscopy (FLUPS) is described which has 80 fs temporal response (fwhm) for emission in the spectral range 425-750 nm. Broadband phase matching is achieved with tilted gate pulses at 1340 nm. Background from harmonics of the gate pulse is removed and sensitivity increased compared to previous designs. Photometric calibration of the upconversion process is performed with a set of fluorescent dyes. For Coumarin 153 in methanol the peak position, bandwidth, and asymmetry depending on delay time are reported.

Ultrafast Excited State Dynamics of the Protonated Schiff Base of All- trans Retinal in Solvents

We present a comparative study of the ultrafast photophysics of all- trans retinal in the protonated Schiff base form in solvents with different polarities and viscosities. Steady-state spectra of retinal in the protonated Schiff base form show large absorption-emission Stokes shifts (6500–8100 cm −1) for both polar and nonpolar solvents. Using a broadband fluorescence up-conversion experiment, the relaxation kinetics of fluorescence is investigated with 120 fs time resolution. The time-zero spectra already exhibit a Stokes-shift of ∼6000 cm −1, indicating depopulation of the Franck-Condon region in ≤100 fs. We attribute it to relaxation along skeletal stretching. A dramatic spectral narrowing is observed on a 150 fs timescale, which we assign to relaxation from the S 2 to the S 1 state. Along with the direct excitation of S 1, this relaxation populates different quasistationary states in S 1, as suggested from the existence of three distinct fluorescence decay times with different decay associated spectra. A 0.5–0.65 ps decay component is observed, which may reflect the direct repopulation of the ground state, in line with the small isomerization yield in solvents. Two longer decay components are observed and are attributed to torsional motion leading to photo-isomerization. The various decay channels show little or no dependence with respect to the viscosity or dielectric constant of the solvents. This suggests that in the protein, the bond selectivity of isomerization is mainly governed by steric effects.