Ultrafast Dynamics of Dye Molecules in Solution as a Function of Temperature

THEORETICAL INVESTIGATION OF ULTRAFAST DYNAMICS OF THE RHODAMINE-700 MOLECULE IN SOLVENTS

The density matrix theory is used to calculate the fluorescence depletion spectra and the internal conversion (IC) times of rhodamine-700 (R-700) in methanol, ethanol, and DMSO solvents. The calculated IC times from Sx to S1 states of R-700 in methanol, ethanol, and DMSO solvents are about 20, 33, and 70 fs, respectively. The times of the excited solvation processes for R-700 in methanol, ethanol, and DMSO solvents are about 8.0, 7.0, and 3.0 ps, respectively.

Experimental and theoretical study of the exited state of aminostyryl terpyridine derivatives: hydrogen-bonding effects

The synthesis, structure and photophysical behavior of the aminostyryl terpyridine derivatives, named 4'-(4-{2-[4-(N,N-dimethylaniline)]vinyl}phenyl)-2,2':6',2″-terpyridine (M(1)) and the related model compounds 4'-(4-{2-[4-(N,N-diphenylammino)phenyl]vinyl}phenyl)-2,2':6',2″-terpyridine (M(2)), respectively, are reported. Large solvatochromic shifts of the first excited-state fluorescence maximum suggest the intramolecular charge transfer characters for both compounds. In addition, with N,N-dimethyl substituents, its fluorescence is quenched a lot in protic solvents. This is consisted with the decay of its S(1) state, through nonradiative internal conversion, to the ground state, which is facilitated by the formation of the hydrogen bond between M(1) and alcohols. Whereas, the introduction of N,N-diphenyl substituents has been proved to be a hydrogen-bond-free case with the unchanged Φ(f). Furthermore, the formation of TICT state via diffusive twisting motion of the dimethyl/phenylamino group is the major relaxation process, which is proved by the ultrafast relaxation dynamics experiment and theoretically conformational optimization of the first excited-state of both the compounds.

High pressure effect on the ultrafast energy relaxation rate of LDS698 (C19H23N2O4Cl) in a solution

Effects of high pressure in a range of up to 1.7 GPa on ultrafast energy relaxation of LDS698 (C(19)H(23)N(2)O(4)Cl) molecules in solution have experimentally been illustrated by a method of femtosecond time-resolved absorption spectroscopy. The rates of the intramolecular and intermolecular energy relaxations show quite different pressure dependences. The observed results are in good agreement with the theoretical interpretation based on the pressure influences on the molecular energy gaps, the intermolecular H-bond interaction, and the solution viscosity.

Theoretical description of femtosecond fluorescence depletion spectrum of molecules in solution

A theoretical model used for calculating the fluorescence depletion spectrum (FDS) of molecules in liquids induced by femtosecond pump-probe laser pulses is proposed based on the reduced density matrix theory. The FDS intensity is obtained by calculating the stimulated emission of the excited electronic state. As an application of the theoretical model, the FDS of oxazine 750 (OX-750) molecule in acetone solution is calculated. The simulated FDS agrees with the experimental result of Liu et al. [J. Y. Liu et al., J. Phys. Chem. A 107, 10857 (2003)]. The calculated vibrational relaxation rate is 2.5 ps(-1) for the OX-750 molecule. Vibrational population dynamics and wave packet evolution in the excited state are described in detail. The effect of the probe pulse parameter on the FDS is also discussed.

Novel infrared spectra for intermolecular dihydrogen bonding of the phenol-borane-trimethylamine complex in electronically excited state

The intermolecular dihydrogen bonding in the electronically excited states of the dihydrogen-bonded phenol-BTMA complex in gas phase was theoretically investigated using the time-dependent density functional theory method for the first time. It was theoretically demonstrated that the S(1) state of the dihydrogen-bonded phenol-BTMA complex is a locally excited state, in which only the phenol moiety is electronically excited. The infrared spectra of the dihydrogen-bonded phenol-BTMA complex in ground state and the S(1) state were calculated at both the O-H and B-H stretching vibrational regions. A novel infrared spectrum of the dihydrogen-bonded phenol-BTMA complex in the electronically excited state was found. The stretching vibrational absorption bands of the dihydrogen-bonded O-H and B-H groups are very strong in the ground state, while they are disappeared in the S(1) state. At the same time, a new strong absorption band appears at the C[Double Bond]O stretching region. From the calculated bond lengths, it was found that both the O-H and B-H bonds in the dihydrogen bond O-H...H-B are significantly lengthened in the S(1) state of the dihydrogen-bonded phenol-BTMA complex. However, the C-O bond in the phenol moiety is markedly shortened in the excited state, and then has the characteristics of C[Double Bond]O group. Furthermore, it was demonstrated that the intermolecular dihydrogen bonds in the electronically excited state of the dihydrogen-bonded phenol-BTMA complex are strengthened, since calculated H...H distance is drastically shortened in the S(1) state.

Site-selective photoinduced electron transfer from alcoholic solvents to the chromophore facilitated by hydrogen bonding: a new fluorescence quenching mechanism

Solute-solvent intermolecular photoinduced electron transfer (ET) reaction was proposed to account for the drastic fluorescence quenching behaviors of oxazine 750 (OX750) chromophore in protic alcoholic solvents. According to our theoretical calculations for the hydrogen-bonded OX750-(alcohol)(n) complexes using the time-dependent density functional theory (TDDFT) method, we demonstrated that the ET reaction takes place from the alcoholic solvents to the chromophore and the intermolecular ET passing through the site-specific intermolecular hydrogen bonds exhibits an unambiguous site selectivity. In our motivated experiments of femtosecond time-resolved stimulated emission pumping fluorescence depletion spectroscopy (FS TR SEP FD), it could be noted that the ultrafast ET reaction takes place as fast as 200 fs. This ultrafast intermolecular photoinduced ET is much faster than the diffusive solvation process, and even significantly faster than the intramolecular vibrational redistribution (IVR) process of the OX750 chromophore. Therefore, the ultrafast intermolecular ET should be coupled with the hydrogen-bonding dynamics occurring in the sub-picosecond time domain. We theoretically demonstrated for the first time that the selected hydrogen bonds are transiently strengthened in the excited states for facilitating the ultrafast solute-solvent intermolecular ET reaction.

Ultrafast Hydrogen Bond Strengthening of the Photoexcited Fluorenone in Alcohols for Facilitating the Fluorescence Quenching

The time-dependent density functional theory (TDDFT) method was performed to investigate the excited-state hydrogen-bonding dynamics of fluorenone (FN) in hydrogen donating methanol (MeOH) solvent. The infrared spectra of the hydrogen-bonded FN-MeOH complex in both the ground state and the electronically excited states are calculated using the TDDFT method, since the ultrafast hydrogen-bonding dynamics can be investigated by monitoring the vibrational absorption spectra of some hydrogen-bonded groups in different electronic states. We demonstrated that the intermolecular hydrogen bond C=O...H-O between fluorenone and methanol molecules is significantly strengthened in the electronically excited-state upon photoexcitation of the hydrogen-bonded FM-MeOH complex. The hydrogen bond strengthening in electronically excited states can be used to explain well all the spectral features of fluorenone chromophore in alcoholic solvents. Furthermore, the radiationless deactivation via internal conversion (IC) can be facilitated by the hydrogen bond strengthening in the excited state. At the same time, quantum yields of the excited-state deactivation via fluorescence are correspondingly decreased. Therefore, the total fluorescence of fluorenone in polar protic solvents can be drastically quenched by hydrogen bonding.

A Bioassay Based on the Ultrafast Response of a Reporter Molecule

The capability of using ultrafast detection technologies for a fast analysis of biomolecular reactions has been explored. As an example, the ultrafast response of tetramethylrhodamine (TMR)—labeled bovine serum albumin (BSA) as a function of different extents in proteolytic cleavage was investigated. The authors compared 4 samples of masses differing over several orders of magnitude: untreated, TMR-labeled BSA (66 kDa), TMR-labeled BSA treated with elastase (6-33 kDa) and with subtilisin (< 3 kDa), and the pure label TMR (0.4 kDa). A direct comparison with gel electrophoresis revealed that various ultrafast parameters give robust information about the progress of the proteolytic cleavage. The authors found the ratio of the transient absorption signal observed at 0 psec and 50 psec after excitation (λPump = 540 nm, λProbe = 570 nm) to be the most precise parameter for determining the cleavage. This parameter allowed determining the mass accurately within 1 sec (Z' factor of 0.83) or 600 msec (Z' factor of 0.64), measuring time per sample. This indicates that many of the known ultrafast detection technologies might be used for monitoring biochemical reactions, probably even without any labeling procedure. The authors also discuss briefly which ultrafast processes contribute to the signals and how they are affected by changes in the biomolecular environment. ( Journal of Biomolecular Screening 2007:341-350)

Ultrafast Dynamics of the Azobenzene−Coumarin Complex: Investigation of Cooling Dynamics Measured by an Integrated Molecular Thermometer

The energy dissipation mechanism from photoexcited azobenzene (Az) was studied by femtosecond time-resolved UV absorption spectroscopy using 7-amino-4-trifluoromethylcoumarin (ATC) as a probe. The distance between the probe molecule and Az was fixed by covalently linking them together through a rigid proline spacer. Picosecond dynamics in THF solutions were studied upon excitation into the S1 state by a 100 fs laser pulse at 480 nm. Transient absorption spectra obtained for Az-Pro-ATC combined the S1 state absorption and vibrationally excited ground-state absorption of ATC. Correction of the transient spectrum of Az-Pro-ATC for the S1 absorption provided the time-resolved absorption spectrum of the ATC hot band. Three major components were observed in the transient kinetics of Az-Pro-ATC vibrational cooling. It is proposed that in ca. 0.25 ps after the excitation, the S1 state of azobenzene decays to form an initial vibrationally excited nonthermalized ground state of Az-Pro-ATC that involves vibrational modes of both azobenzene and coumarin. This hot ground state decays in ca. 0.32 ps to the next, vibrationally equilibrated, transient state by redistributing the energy within the molecule. Subsequently, the latter state cools by transferring its energy to the closest solvent molecules in ca. 5 ps; then, the energy diffuses to the bulk solvent in 13 ps.

Single-molecule pump-probe experiments reveal variations in ultrafast energy redistribution

Single-molecule pump probe (SM2P) is a novel, fluorescence-based technique that allows the study of ultrafast processes on the single-molecule level. Exploiting SM2P we have observed large variations (from 1 ps to below 100 fs) in the energy redistribution times of chemically identical molecules in the same sample. Embedding the molecules in a different matrix or changing the excitation wavelength does not lead to significant changes in the average redistribution time. However, chemically different molecules exhibit different characteristic redistribution times. We therefore conclude that the process measured with the SM2P technique is dominated by intramolecular energy redistribution and not intermolecular transfer to the surrounding matrix. The matrix though is responsible for inducing conformational changes in the molecule, which affect the coupling between electronic and vibrational modes. These conformational changes are the main origin of the observed broad distribution of redistribution times.