Dynamics of Solvent Response in Methanol–Chloroform Binary Solvent Mixture: A Case of Synergistic Solvation

Steady-state absorption, emission, and femtosecond transient absorption spectroscopies were used to ascertain the static and dynamic nature of the solvent response of methanol–chloroform binary solvent mixtures of different stoichiometric ratios using 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4<i>H</i>-pyran (DCM) as the probe molecule. The appearance of synergistic solvation behavior in the steady-state absorption measurements can be explained in terms of solvent–solvent interactions through an extended hydrogen-bonding network. The disappearance of such synergistic behavior in the excited state of the DCM dye was recently proposed by us to be due to the weak nature of the intermolecular interactions present in binary solvent mixtures (J. Phys. Chem. B 2012, 116, 1345). It was anticipated and subsequently confirmed by the dynamics of the solvent response that the disruption of the weak interactive solvent network is the main reason for the absence of the synergism in the excited state. As expected, we observed the slowest dynamics for the mixture with <i>X</i>_MeOH = 0.45, with an average solvation time of 12.03 ps, which is much higher than the values for the pure bulk counterparts (⟨τ_s⟩_Methanol = 4.32 ps and ⟨τ_s⟩_Chloroform = 1.32 ps). The unprecedented slowing of solvation for DCM is probably due to the rigid interactive methanol-chloroform solvent system in the first solvation shell, followed by solvent rearrangements around the solute dipole. Overall interactions present within the methanol-chloroform binary solvent mixture furnish clear evidence of solvent association through weak hydrogen bonding

Observing Vibrational Wavepackets during an Ultrafast Electron Transfer Reaction

Recent work has proposed that coherent effects impact ultrafast electron transfer reactions. Here we report studies using broadband pump–probe and two-dimensional electronic spectroscopy of intramolecular nuclear motion on the time scale of the electron transfer between oxazine 1 (Ox1) and dimethylaniline (DMA). We performed time–frequency analysis on the time domain data to assign signal amplitude modulations to ground or excited electronic states in the reactive system (Ox1 in DMA) relative to the control system (Ox1 in chloronaphthalene). It was found that our ability to detect vibrational coherence via the excited electronic state of Ox1 diminishes on the time scale that population is lost by electron transfer. However, the vibrational wavepacket is not damped by the electron transfer process and has been observed previously by detecting the Ox1 radical transient absorption. The analysis presented here indicates that the “addition” of an electron to the photoexcited electron acceptor does not significantly perturb the vibrational coherence, suggesting its presence as a spectator, consistent with the Born–Oppenheimer separation of electronic and nuclear degrees of freedom

Origin of Strong Synergism in Weakly Perturbed Binary Solvent System: A Case Study of Primary Alcohols and Chlorinated Methanes

A strong synergistic solvation was observed for the mixtures of hydrogen bond donating and accepting solvent pairs. The nature of the interactions between two solvent pairs was investigated with different dye molecules viz. coumarin 480, coumarin 153, 4-aminophthalimide, and <i>p</i>-nitroaniline. Coumarin 480 in differenet alcohols–CHCl₃ (alcohols: MeOH, EtOH, BuOH) binary mixture shows a strong synergism, which is explained in the backdrop of solvent–solvent interactions. Fluorescence quenching of C480 by 1,2-phenylenediamine in the binary solvent mixture exhibited the maximum deviation in quenching constant corresponding to ∼0.45 mol fraction of MeOH in MeOH–CHCl₃ binary mixture and hence suggested the maximum extent of hydrogen-bonding interactions prevailing at this proportion of mixture. The solvation behavior of MeOH–CHCl₃ mixture shows strong probe dependence with no synergism observed in <i>p</i>-nitroaniline, which is ascribed to its higher ground state dipole moment (8.8 D) relative to C480 (6.3 D). Interestingly, the strong synergistic signature observed through spectrophotometric measurement of C480 in alcohol–CHCl₃ binary mixture is absent when studied by fluorescence measurement. The higher excited state dipole moment of coumarin 480 (13.1 D) is considered to be the driving force for the absence of synergism in the excited state. In such strongly perturbed systems (due to high dipole moment values) the dominant phenomenon is preferential solvation. Analysis of proton NMR of MeOH–CHCl₃ binary solvent mixture indicates the existence of MeOH–CHCl₃ clusters in the stoichiometric ratio of 1:2.15. Refractive index measurement also infers the existence of hydrogen bonded network structure between MeOH and CHCl₃. A modified Bosch solvent exchange model has been used to determine the feasibility of synergistic behavior and polarity parameter of the mixed solvent structure of MeOH–CHCl₃ binary solvent mixture

Methylene Blue Exciton States Steer Nonradiative Relaxation: Ultrafast Spectroscopy of Methylene Blue Dimer

The photochemistry and aggregation properties of methylene blue (MB) lead to its popular use in photodynamic therapy. The facile formation of strongly coupled “face-to-face” H-aggregates in concentrated aqueous solution, however, significantly changes its spectroscopic properties and photophysics. The photoinitiated dynamics of the simplest MB aggregate, MB₂, was investigated over femtosecond to nanosecond time scales revealing sequential internal conversion events that fully relax the excited population. MB monomer dynamics were analyzed in tandem for a direct comparison. First, ultrafast internal conversion from the electric-dipole allowed upper exciton state to the lower forbidden exciton state was evaluated by use of broadband transient absorption (BBTA) and two-dimensional electronic spectroscopy (2DES) with a time resolution of ∼10 fs. Lineshape analysis of MB and MB₂ 2DES bands at 298 and 77 K show effectively no difference in the diagonal/antidiagonal line width ratio for the dimer, in marked contrast to the distinct reduction of the homogeneous line width for MB. This result is interpreted as ultrafast population relaxation imposing a limitation to the homogeneous line width, instead of pure dephasing as in the case of the monomer. Narrowband transient absorption was performed with the aid of target analysis, to model the dynamics at longer times. The MB dynamics were described by a sequential model featuring vibrational relaxation (1–10 ps) followed by intersystem crossing and internal conversion (τ ∼ 370 ps) leaving behind MB triplet species. Alternatively, the dimer dynamics were entirely quenched within ∼10 ps, yielding a ground state recovery time of 3–4 ps. Such fast and complete relaxation to the ground state demonstrates the effect of concentration quenching when monomers are brought into close proximity. The formation of exciton states introduces an initial energy funnel that eventually leads to population relaxation to the ground state, preventing even the dissociation of dimers despite having internal energies well above its binding energy

Excited State Relaxation Dynamics of Model Green Fluorescent Protein Chromophore Analogs: Evidence for <i>Cis–Trans</i> Isomerism

Two green fluorescent protein (GFP) chromophore analogs (4<i>Z</i>)-4-(<i>N</i>,<i>N</i>-dimethylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5<i>H</i>-imidazolin-5-one (DMPI) and (4<i>Z</i>)-4-(<i>N</i>,<i>N</i>-diphenylaminobenzylidene)-1-methyl-2-phenyl-1,4-dihydro-5<i>H</i>-imidazolin-5-one (DPMPI) were investigated using femtosecond fluorescence up-conversion spectroscopy and quantum chemical calculations with the results being substantiated by HPLC and NMR measurements. The femtosecond fluorescence transients are found to be biexponential in nature and the time constants exhibit a significant dependence on solvent viscosity and polarity. A multicoordinate relaxation mechanism is proposed for the excited state relaxation behavior of the model GFP analogs. The first time component (τ₁) was assigned to the formation of twisted intramolecular charge transfer (TICT) state along the rotational coordinate of N-substituted amine group. Time resolved intensity normalized and area normalized emission spectra (TRES and TRANES) were constructed to authenticate the occurrence of TICT state in subpicosecond time scale. Another picosecond time component (τ₂) was attributed to internal conversion via large amplitude motion along the exomethylenic double bond which has been enunciated by quantum chemical calculations. Quantum chemical calculation also forbids the involvement of hula-twist because of high activation barrier of twisting. HPLC profiles and proton-NMR measurements of the irradiated analogs confirm the presence of <i>Z</i> and <i>E</i> isomers, whose possibility of formation can be accomplished only by the rotation along the exomethylenic double bond. The present observations can be extended to <i>p</i>-HBDI in order to understand the role of protein scaffold in reducing the nonradiative pathways, leading to highly luminescent nature of GFP

Ultrafast Photophysics of a Dinitrogen-Bridged Molybdenum Complex

Among the many metal–dinitrogen complexes synthesized, the end-on bridging (μ₂, η¹, η¹N₂) coordination mode is notoriously unreactive for nitrogen fixation. This is principally due to the large activation energy for ground-state nitrogen–element bond formation and motivates exploration of the photoexcited reactivity of this coordination mode. To provide the foundation for this concept, the photophysics of a dinitrogen-bridged molybdenum complex was explored by ultrafast electronic spectroscopies. The complex absorbs light from the UV to near-IR, and the transitions are predominantly of metal-to-ligand charge transfer (MLCT) character. Five excitation wavelengths (440, 520, 610, 730, and 1150 nm) were employed to access MLCT bands, and the dynamics were probed between 430 and 1600 nm. Despite the large energy space occupied by electronic states (ca. 1.2 eV), the dynamics were independent of the excitation wavelength. In the proposed kinetic model, photoexcitation from a Mo–NN–Mo centered ground state populates the π*-state delocalized over two terpyridine ligands. Due to a large terpyridine–terpyridine spatial separation, electronic localization occurs within 100 fs, augmented by symmetry breaking. The subsequent interplay of internal conversion and intersystem crossing (ISC) populates the lowest ³MLCT state in 2–3 ps. Decay to the ground state occurs either directly or via a thermally activated metal-centered (³MC) trap state having two time constants (10–15 ps, 23–26 ps [298 K]; 103 ps, 612 ps [77 K]). ISC between ¹MLCT and ³MLCT involves migration of energized electron density from the terpyridine π* orbitals to the Mo–NN–Mo core. Implication of the observed dynamics for the potential N–H bond forming reactivity are discussed

Direct Synthesis of CdSe Nanocrystals with Electroactive Ligands

We report the synthesis and characterization of cadmium selenide nanocrystals with electroactive ligands directly attached to the surface. The conventional surfactant-assisted synthesis yields nanocrystals with surfaces functionalized with insulating organic ligands. These insulating ligands act as a barrier for charge transport between nanocrystals. Electroactive (reducing/oxidizing) ligands like ferrocene and cobaltocene have potential for applications as photoexcited hole conductors and photoredox systems. Although ferrocene ligands anchored to the nanocrystal surface through insulating long-chain hydrocarbon spacers have previously been reported, this approach is limited because the charge transfer between nanocrystal and ferrocene is highly sensitive to their separation. We report here ferrocene directly bound to the inorganic core of the nanocrystal, and as a result the distance between the nanocrystals and the electroactive moiety is minimized

Broad-Band Pump–Probe Spectroscopy Quantifies Ultrafast Solvation Dynamics of Proteins and Molecules

In this work, we demonstrate the use of broad-band pump–probe spectroscopy to measure femtosecond solvation dynamics. We report studies of a rhodamine dye in methanol and cryptophyte algae light-harvesting proteins in aqueous suspension. Broad-band impulsive excitation generates a vibrational wavepacket that oscillates on the excited-state potential energy surface, destructively interfering with itself at the minimum of the surface. This destructive interference gives rise to a node at a certain probe wavelength that varies with time. This reveals the Gibbs free-energy changes of the excited-state potential energy surface, which equates to the solvation time correlation function. This method captures the inertial solvent response of water (∼40 fs) and the bimodal inertial response of methanol (∼40 and ∼150 fs) and reveals how protein-buried chromophores are sensitive to the solvent dynamics inside and outside of the protein environment