Jacek Dobkowski

Dual and multiple fluorescence mechanisms of p-dimethylaminobenzaldehyde and its trimethylene-bridged double molecule

Abstract p-Dimethylaminobenzaldehyde (DMABA, I) and its N-to-N trimethylene-bridged double molecule (III) exhibit dual or multiple fluorescences. Several mechanisms were proved to be responsible for the long-wave fluorescence bands: intramolecular solvent-assisted relaxation in the excited state (I and III); ground state aggregation at low temperatures (I in nonpolar solvents); excimer formation (III; and I only at high concentrations in some solvents). Intramolecular interaction in the ground state of III prepares an excimer-like structure. The dimer of DMBA aside of its own emission, may relax and emit an excimer-like fluorescence.

Unusual, Solvent Viscosity-Controlled Tautomerism and Photophysics: Meso-Alkylated Porphycenes

Stationary and time-resolved studies of 9,10,19,20-tetramethylporphycene and 9,10,19,20-tetra-n-propylporphycene in condensed phases reveal the coexistence of trans and cis tautomeric forms. Two cis configurations, cis-1 and cis-2, play a crucial role in understanding the excited-state deactivation and tautomer conversion dynamics. The trans-trans tautomerization, involving intramolecular transfer of two hydrogen atoms, is extremely rapid (k ≥ 10(13) s(-1)), both in the ground and lowest electronically excited states. The cis-1-trans conversion rate, even though the process is thermodynamically more favorable, is much slower and solvent-dependent. This is explained by the coupling of alkyl group rotation with the hydrogen motion. Excited-state deactivation is controlled by solvent viscosity: the S(1) depopulation rate decreases by more than 2 orders of magnitude when the chromophore is transferred from a low-viscosity solution to a polymer film. Such behavior confirms a model for excited state deactivation in porphycene, which postulates that a conical intersection exists along the single hydrogen transfer path leading from the trans to a high energy cis-2 tautomeric form. For this process, the tautomerization coordinate includes not only hydrogen translocation but also large-amplitude twisting of the two protonated pyrrole moieties attached to the opposite sides of the ethylene bridge.

CELL ADHESION TO POLYMERIC SURFACES: EXPERIMENTAL STUDY AND SIMPLE THEORETICAL APPROACH

In a medium without serum, the initial adhesion of L1210 cells to nonsulfonated and sulfonated polymer surfaces was investigated. In the case of sulfonated polymer surfaces, the relative number of adhering cells strongly increases with an increase of the interfacial surface tension; that is, adhesion strongly depends on the surface density of sulfonic groups. However, in the case of nonsulfonated polymer surfaces, the relative number of adhering cells is high and independent of the interfacial surface tension. To extend the basic knowledge of these phenomena, a semi-empirical quantum chemical computational study was undertaken. Simple probe molecules were chosen that mimic the chemical properties of functional groups present on polymeric surfaces. The energies of interaction between these molecules and ones representing the midchain polypeptide building blocks were calculated. To discuss the steric effects involved in similar interactions on real surfaces, a simple model of polymeric surfaces was proposed. Also the interactions among such surfaces and the short hydrated polypeptide chain were studied at the molecular mechanics level of theory. The derived intermolecular energy parameter was found to change in parallel to the number of adhered cells within the two groups of substrata under study: nonsulfonated and sulfonated. The computational results suggest the possible existence of differently arranged cell membrane protein centers responsible for docking to these two types of surfaces.

On the Origin of Radiationless Transitions in Porphycenes

A model for the radiationless deactivation of porphycenes has been proposed based on the theoretical prediction that the lowest electronic state of one of the cis tautomeric forms corresponds to an open-shell nonplanar structure. The radiationless channel is provided by crossing of the potential energy surfaces of the ground and lowest excited states along the hydrogen-transfer coordinate. The model explains the large dependence of fluorescence of several porphycenes on viscosity. It also allows prediction of the emissive properties for differently substituted porphycenes.

Substituent and Solvent Effects on the Excited State Deactivation Channels in Anils and Boranils

Differently substituted anils (Schiff bases) and their boranil counterparts lacking the proton-transfer functionality have been studied using stationary and femtosecond time-resolved absorption, fluorescence, and IR techniques, combined with quantum mechanical modelling. Dual fluorescence observed in anils was attributed to excited state intramolecular proton transfer. The rate of this process varies upon changing solvent polarity. In the nitro-substituted anil, proton translocation is accompanied by intramolecular electron transfer coupled with twisting of the nitrophenyl group. The same type of structure is responsible for the emission of the corresponding boranil. A general model was proposed to explain different photophysical responses to different substitution patterns in anils and boranils. It is based on the analysis of changes in the lengths of CN and CC bonds linking the phenyl moieties. The model allows predicting the contributions of different channels that involve torsional dynamics to excited state depopulation.

Albumin adsorption on unmodified and sulfonated polystyrene surfaces, in relation to cell-substratum adhesion.

Albumin is commonly applied for blocking the adsorption of other proteins and to prevent the nonspecific adhesion of cells to diverse artificial substrata. Here we address the question of how effective these albumin properties are--by investigating unmodified and sulfonated polystyrene substrata with distinctly different wettabilities. As clearly shown with (125)I-radioisotopic assays, above a concentration of 10-20 μg/mL, the efficiency of bovine serum albumin (BSA) adsorption became markedly higher on the sulfonated surface than on the unmodified one. This study was assisted with the atomic force microscopy. On the unmodified surface, BSA, adsorbed from sufficiently concentrated solutions, formed a monolayer, with occasional intrusions of multilayered patches. Conversely, the arrangement of BSA on the sulfonated surface was chaotic; the height of individual molecules was lower than on the unmodified polystyrene. Importantly, the adhesion study of LNCaP and DU145 cells indicated that both surfaces, subjected to the prior BSA adsorption, did not completely loose their cell-adhesive properties. However, the level of adhesion and the pattern of F-actin organization in adhering cells have shown that cells interacted with unmodified and sulfonated surfaces differently, depending on the arrangement of adsorbed albumin. These results suggest the presence of some bare substratum area accessible for cells after the albumin adsorption to both types of investigated surfaces.

Atomic force microscopy evidence for conformational changes of fibronectin adsorbed on unmodified and sulfonated polystyrene surfaces

The effect of polystyrene surface polarity on the conformation of adsorbed fibronectin (FN) has been studied with atomic force microscopy. We demonstrated that bare sulfonated and nonsulfonated polystyrene surfaces featured similar topographies. After the FN adsorption, direct comparison of both types of substrata revealed drastically different topographies, roughness values, and also cell-adhesive properties. This was interpreted in terms of FN conformational changes induced by the surface polarity. At high-solute FN concentrations the multilayer FN adsorption took place resulting, for the sulfonated substratum, in an increase of surface roughness, whereas for the nonsulfonated one the roughness was approximately stable. Conversely, the FN conformation characteristic for the first saturative layer tended to be conserved in the consecutive layers, as evidenced by height histograms. The height of individual FN molecules indicated, consonantly with the derived thickness of the adsorbed protein layer (the latter value being 1.4 nm and 0.6 nm, respectively, for an unmodified and sulfonated polystyrene surface), that molecules are flattened on polar surfaces and more compact on nonsulfonated ones. It was also demonstrated that the FN adsorption and conformation on polymeric substrata, and hence the resultant cell-adhesive properties, depended on the chemistry of the original surface rather than on its topography. Our results also demonstrated the ability of surface polarity to influence the protein conformation and its associated biological activity.

Intramolecular charge-transfer properties of a molecule with a large donor group: the case of 4′-(pyren-1-yl)benzonitrile

Picosecond transient absorption, nanosecond fluorescence decays, stationary absorption, and fluorescence spectra have been recorded in different solvents at various temperatures for 4′-(pyren-1-yl)benzonitrile (Py-BN) and compared with corresponding data for two structurally similar compounds: 4′-(pyren-1-yl)-N,N-dimethylaniline (Py-DMA) and 4′-(pyren-1-yl)acetophenone (Py-BK). Quantum chemical and molecular mechanics calculations were performed for Py-BN in order to monitor variations in the electronic transitions, energies, oscillator strengths, and dipole moments upon changing the molecular geometry by twisting the pyrenyl group with respect to the benzonitrile subunit. Both experimental results and quantum chemical calculations indicate that after excitation Py-BN relaxes by mutual twisting of the two subunits towards a more planar geometry.

Electronic spectroscopy and photophysics of 2-(N-methyl-N-isopropylamino)-5-cyanopyridine and related compounds

In polar solvents, dual fluorescence is observed for three electron donor–acceptor molecules, consisting of the m-cyanopyridine electron acceptor and one of three slightly different N,N-dialkylamino donor groups: N-methyl-N-isopropyl, N,N-diethyl, and N,N-dimethyl. A low energy “anomalous” emission is assigned to a twisted intramolecular charge transfer (TICT) state. Kinetic and steric factors account for variations of the TICT vs. primary emission intensity ratio in the three compounds. Comparison with the “TICT paradigm”, N,N-dimethylaminobenzonitrile is also provided. In alcohol solutions, relative enhancement of the TICT fluorescence, as well as a fast radiationless process from the initially excited state are observed. Possible sources of these phenomena are discussed.

Low-temperature dual fluorescence in 9-morpholinoacridine: Picosecond TICT state formation?

Abstract Dual fluorescence a and b of 9-morpholinoacridine (9-MAc) in butyronitrile between 140 K and 200 K is observed. The excitation spectra of a and b differ: the high-energy emission b is preferentially excited within the first excited state S1, whereas the low-energy emission a within the second, S2. The subtraction of the fast component from the total a emission indicates that both fluorescences are kinetically coupled. The kinetics of the transient absorption band correspond to those of secondary fluorescence. Luminescence properties of 9-MAc can be explained in terms of the TICT state model.