Femto- and picosecond fluorescence studies of solvation and non-radiative deactivation of ionic styryl dyes in liquid solution

Intramolecular photo-driven charge transfer in a series of pyridyl substituted phenyloxazoles. Structural relaxation in meta-substituted ethylpyridinium derivative of phenyloxazole

A series of pyridyl (pyridinium) substituted benzoxazoles were studied by steady state absorption, fluorescence spectroscopy, time-resolved fluorescence spectroscopy, fs pulse absorption and polarization spectroscopy, and quantum-chemical calculations. The spectral and kinetic parameters of the fluorophores in MeCN and EtOAc were obtained experimentally and were calculated by means of DFT and TDDFT methods. A scheme including four transient excited states was proposed for the interpretation of differential absorption kinetics of the charged fluorophores. Expressions describing the actual kinetics graphs, the decay associated spectra, and the species-associated spectra were derived. The charge shift step was found to be dependent on average solvation times. A charge shift followed by the formation of the twisted conformer was found for the excited 1-ethyl-3-(5-phenyloxazol-2-yl)pyridinium 4-methyl-1-benzenesulfonate in MeCN and EtOAc. Conformational analysis confirms a large amplitude motion of the meta-substituted ethylpyridinium group as an additional structural relaxation path producing an abnormally large fluorescence Stokes shift.

Enhancement of NIR emission by a tight confinement of a hemicyanine dye within zeolitic MgAPO-5 nanochannels

The encapsulation of a hemicyanine dye, LDS 730, into the 1D nanochannels of MgAPO-5 aluminophosphate by "one-pot" synthesis, based on "in situ" occlusion via a crystallization inclusion method, has led to a hybrid material with emission in the NIR region. The tight fitting between the molecular size of the guest dye and the pore dimensions of the host has enabled a rigid conformation of the LDS 730 dye within the nanochannels. Consequently, fluorescence in the NIR range of the spectra is enhanced with respect to the dye in solution. The synthesis of the hybrid material was optimized through a systematic variation of the gel composition via MW in order to obtain a pure phase.

Long-living optical gain induced by solvent viscosity in a push–pull molecule

The combination of continuum and ultrafast pump–probe spectroscopy, in viscous and non-viscous environments, with DFT and TDDFT calculations, is effective in unraveling important features of the twisted intramolecular charge transfer mechanism in a new push–pull molecule that possesses aggregation induced emission properties.

Photoexcitation and relaxation kinetics of molecular systems in solution: towards a complete in silico model

A theoretical–computational method is proposed for modelling the complete kinetics – from photo-excitation to relaxation – of a chromophore in solution.

Probing Local Environments by Time-Resolved Stimulated Emission Spectroscopy

Time-resolved stimulated emission spectroscopy was employed to probe the local environment of DASPMI (4-(4-(dimethylamino)styryl)-N-methyl-pyridinium iodide) in binary solvents of different viscosity and in a sol-gel matrix. DASPMI is one of the molecules of choice to probe local environments, and the dependence of its fluorescence emission decay on viscosity has been previously used for this purpose in biological samples, solid matrices as well as in solution. The results presented in this paper show that time-resolved stimulated emission of DASPMI is a suitable means to probe the viscosity of local environments. Having the advantage of a higher time resolution, stimulated emission can provide information that is complementary to that obtained from fluorescence decay measurements, making it feasible to probe systems with lower viscosity.

Solvent and pH dependent fluorescent properties of a dimethylaminostyryl borondipyrromethene dye in solution

Steady-state and time-resolved fluorescence techniques have been used to study the photophysical properties of the fluorescent BODIPY-derived dye 3-{2-[4-(dimethylamino)phenyl]ethenyl}-4,4-difluoro-8-(4-methoxyphenyl)-1,5,7-trimethyl-3a,4a-diaza-4-bora-s-indacene. This compound has been synthesized via a microwave-assisted condensation of p-N,N-dimethylaminobenzaldehyde with the appropriate 1,3,5,7-tetramethyl substituted borondipyrromethene unit. The fluorescence properties of the dye are strongly solvent dependent: increasing the solvent polarity leads to lower fluorescence quantum yields and lifetimes, and the wavelength of maximum fluorescence emission shifts to the red. The Catalán solvent scales are found to be the most suitable for describing the solvatochromic shifts of the fluorescence emission. These are dominated by polarity/polarizability effects, as confirmed by quantum-chemical calculations performed in the dielectric continuum approximation. Fluorescence decay profiles of the dye can be described by a single-exponential fit in most solvents investigated, while two decay times are found in alcohols. The dye undergoes a reversible protonation-deprotonation reaction in the acidic pH range with a pK(a) of 2.25 in acetonitrile solution. Fluorimetric titrations as a function of pH produce fluorescence emission enhancements at lower pH. The fluorescence excitation spectra show a hypsochromic shift from 600 nm for the neutral amine to 553 nm for the ammonium form, so that ratiometric measurements can be used to determine pK(a).

Charge-Transfer Transitions in Twisted Stilbenoids: Interchangeable Features and Generic Distinctions of Single- and Double-Bond Twists

The charge-transfer transitions relevant to single- and double-bond photochemical twisting have been studied in the framework of the biradicaloid state theory using the AM1 method for a family of donor-acceptor-substituted stilbenoids and a series of sparkle-simulated model stilbenes. Features in common and mutually interchangeable properties for the two transition types as well as their peculiarities are revealed; they are considered in relation to the varied donor-acceptor strength of the substituents, with particular attention given to the occurrence of S0-S1 state conical intersections. The difference in critical points at which the conical intersections occur for double-bond and single-bond twisted stilbenoids is shown to be related to the splitting of the cyanine limit of their planar counterparts.

Dipolar relaxation in a lipid bilayer detected by a fluorescent probe, 4″-dimethylaminochalcone

The dynamic behavior of polar molecules in egg phosphatidylcholine (PC) bilayers has been studied using a membrane fluorescent probe, 4''-dimethylaminochalcone (DMAC). Time and spectrally resolved fluorescence spectroscopy of DMAC incorporated in PC liposomes, as compared to studies of the probe in organic solvents, shows the existence of two independent populations, associated with different extent and speed of dipolar solvent relaxation. The first DMAC population represents approximately 69% of the fluorescence-emitting molecules, has a short fluorescence decay time (0.32 ns) and undergoes Stokes shift of 80 nm. The remaining 31% fraction of DMAC molecules has a decay time of 0.74 ns and undergoes a high (106 nm) Stokes shift. A fraction of the shift, ca. 24 nm for the first and 46 nm for the second population, is attributed to the fast (<0.1 ns) rotational relaxation of nearby dipolar molecules, which might be water. This two-state model accounts well for the detailed fluorescence properties of DMAC in egg PC, i.e. its broadened steady-state spectrum, its average fluorescence quantum yield and its complex wavelength-dependent fluorescence decays.

Introduction to the symposium-in-print: photoisomerization pathways, torsional relaxation and the hula twists

A review of recent literature on volume-conserving cis-trans photoisomerization reaction mechanism, including hula twist, is presented. Differences between substrates trapped in amorphous solids and chromophores that are protein bound are discussed.

Examples of hula-twist in photochemical cis- trans isomerization

ALiu and Hammond recently reasoned that the hula-twist (HT), a volume-conserving cis-trans isomerization mechanism, is involved in reactions of confined systems. We now show that HT can be applied to various reported photochemical isomerization of chromophores (small organic systems as well as photoactive bio-pigments). The results, when taken as a whole, argue powerfully that HT is a common supramolecular photoisomerization reaction mechanism.