An Ab Initio Microscope: Molecular Contributions to the Femtosecond Time‐Dependent Fluorescence Shift of a Reichardt‐Type Dye

Abstract

Despite decades of effort, the properties of liquid water are not fully understood. In recent years it was found that geometric confinement has a strong effect on water, making it quite different from the bulk. [1] To understand protein functionality, agglomeration, and folding, as well as DNA stability, it appears to be essential to explicitly consider the presence of water. [2] With current IR-based spectroscopic techniques, it is possible to monitor rapid photochemical reactions. [3] These techniques are, however, severely limited by strong absorption due to water, in addition to spectral crowding. To overcome these limitations, compounds with isotopically substituted carbonyl groups have been used. [4] IR spectroscopy observes the hydrogen-bond network indirectly by its coupling to the vibrational modes. Fluctuations of the hydrogen-bond network can be directly measured by THz spectroscopy which probes the dielectric bulk response. [5] Inside biomolecules or confinements, the absorption background of water can be entirely avoided when the fluorescence of a suitably placed chromophore is monitored. For this purpose, dyes are employed whose fluorescence depends strongly on solvent polarity. [6] In this context, the molecular probe Nmethyl-6-oxyquinolinium betaine (MQ) is especially attractive because of its small size and water solubility, allowing insertion into DNA or proteins. Structurally, it resembles the polarity indicator dye introduced by Reichardt and coworkers. [7] The local THz spectrum, up to the far-IR intramolecular modes, can then be extracted almost quantitatively from the time-dependent Stokes shift (TDSS) of its fluorescence as measured by femtosecond spectroscopy. The connection between TDSS data of MQ and the THz spectrum of its surroundings has been established by simple dipolar continuum theory. [8] It should be noted that the Stokes shift of the chromophore represents only an indirect measurement of the water dynamics. In addition, the presence of the chromophore has the potential of affecting this very water dynamics to a certain extent.However, the time-dependent dielectric response can be traced back to the structure and dynamics around the molecular probe in atomistic detail with the help of molecular dynamics simulations in combination with the experimentally observed evolution of the Stokes shift.Published attempts at describing time-dependent solvation with molecular dynamics used either a quantum mechanics/molecular dynamics (QM/MM) approach, including only the solute and a few water molecules in a quantum mechanical treatment, [9] or a model potential derived from quantum mechanical calculations of the chromophore. [10] In our approach, we use density functional theory (DFT) for solute and solvent, which is known to yield an accurate picture of solvent effects and the dynamics of hydrogen-bond networks, also for excited-state solutes. [11] Instead of the dielectric linear response of the solvent or the time-dependent solvation energy, we compute the time-dependent fluoresce...

Solvation dynamics: improved reproduction of the time-dependent Stokes shift with polarizable empirical force field chromophore models

“…2,14,22,[34][35][36][37][38] In contrast, MQ lowers its dipole moment upon excitation, which makes it an interesting test case, and has gained increasing interest due to its small size and rigid structure. 14,19,20,[39][40][41] As solvents, methanol (MeOH) and the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EmimOTf) were chosen. Water was also examined, but only with MQ since C153 is not soluble in water.…”

“…The investigation of solvent properties via calculation or measurement of the time-dependent Stokes shift (TDSS) has been of high interest during the last decades. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The TDSS probes the timescale of solvent rearrangement by excitation of a dissolved chromophore, which causes the solvent to reorganize and the wavelength of the emitted fluorescence light of the chromophore to change. Upon excitation, both the dipole moment and the polarizability of the chromophore change, so that both the electrostatic and dispersion interactions change.…”

“…In fact, polarizable computer simulations of the TDSS to date comprise only artificial mono-or diatomic probe molecules, [28][29][30] theoretical models, 31,32 or combined approaches where the chromophore is treated quantum-mechanically or semi-empirically. 19,20,33,34 Stratt and coworkers 35 recently used a polarizable model of Coumarin 153 to calculate solute-pump/solvent probe spectra, where the polarizabilities of each atom were obtained by regression of the molecular quantum-mechanical value. Although the mentioned studies confirm that solute polarizability has profound impacts on the TDSS, namely slowing down the observed response and broadening the distribution of the energy gap, the computational TDSS of polarizable, experimentally-relevant chromophores has yet to be carefully studied.…”

Large Stokes Shift Ionic‐Liquid Dye

“…Stabilized by solvation effects in water the OH proton is released in the excited state and the fluorescence gives the characteristic response of the non protonated form of the dye [6MQz].This finding underlines that the embedded [6MQc] + in the IL dye can be used as photo acid as reported by other groups. [14][15][16][17][18][19][20] In conclusion, we have presented an ew ionic-liquid dye showing al arge Stokes shift, rather high emission quantum yields,p hotostability,a nd good solubility in diverse solvents. In aprotic solvents and ionic liquids including the same weakly interacting anion the solubility of the ionic-liquid dye can increase up to an order of magnitude compared to the initial molecular dye [6MQz] (see SI8).…”

“…Thec olor of this solvatochromatic dye molecule depends strongly on the polarity of the surrounding,a st he permanent dipole moments strongly change upon excitation from the ground to the excited state.T hus [6MQz] has been used as dye for understanding solvation processes and proton transfer to solvents. [14][15][16][17][18][19][20][21][22] In some cases the polarity probe was incorporated into biomolecules such as sugars or enzymes for probing the chemical environment and providing biocompatibility. [10,11,18,19] Whereas other dyes have been extensively applied to probe the polarity of ionic liquids,t he protonated form of [6MQz] has never been used as moiety of an ionic liquid itself.…”