Jacek Waluk

Controlling intramolecular hydrogen transfer in a porphycene molecule with single atoms or molecules located nearby

Although the local environment of a molecule can play an important role in its chemistry, rarely has it been examined experimentally at the level of individual molecules. Here we report the precise control of intramolecular hydrogen-transfer (tautomerization) reactions in single molecules using scanning tunnelling microscopy. By placing, with atomic precision, a copper adatom close to a porphycene molecule, we found that the tautomerization rates could be tuned up and down in a controlled fashion, surprisingly also at rather large separations. Furthermore, we extended our study to molecular assemblies in which even the arrangement of the pyrrolic hydrogen atoms in the neighbouring molecule influences the tautomerization reaction in a given porphycene, with positive and negative cooperativity effects. Our results highlight the importance of controlling the environment of molecules with atomic precision and demonstrate the potential to regulate processes that occur in a single molecule.

Nanostructured silver–gold bimetallic SERS substrates for selective identification of bacteria in human blood

Surface-enhanced Raman spectroscopy (SERS) is a potentially important tool in the rapid and accurate detection of pathogenic bacteria in biological fluids. However, for diagnostic application of this technique, it is necessary to develop a highly sensitive, stable, biocompatible and reproducible SERS-active substrate. In this work, we have developed a silver-gold bimetallic SERS surface by a simple potentiostatic electrodeposition of a thin gold layer on an electrochemically roughened nanoscopic silver substrate. The resultant substrate was very stable under atmospheric conditions and exhibited the strong Raman enhancement with the high reproducibility of the recorded SERS spectra of bacteria (E. coli, S. enterica, S. epidermidis, and B. megaterium). The coating of the antibiotic over the SERS substrate selectively captured bacteria from blood samples and also increased the Raman signal in contrast to the bare surface. Finally, we have utilized the antibiotic-coated hybrid surface to selectively identify different pathogenic bacteria, namely E. coli, S. enterica and S. epidermidis from blood samples.

Electronic states of the phenoxyl radical

The phenoxyl radical and two of its isotopomers were investigated by UV-VIS and IR polarization spectroscopy of molecular samples immobilized in cryogenic argon matrices. Analysis of the combined electronic and infrared linear dichroism data led to determination of absolute transition moment directions and symmetry assignments for four low-lying excited electronic states. The bands observed at 16 000, 25 200, 33 900, and 41 800 cm−1 were assigned to 2A1, 2B1, 2A1, and 2B1 π–π* states, respectively. A very weak transition observed in the near-infrared close to 8900 cm−1 was assigned to an optically forbidden 2B2 n–π* state. The electronic transitions predicted by time dependent density functional theory (TD-UB3LYP/cc-pVTZ) were in good agreement with the observed transitions.

Excited charge transfer states in 4-aminopyrimidines, 4-(dimethylanilino)pyrimidine and 4-(dimethylamino)pyridne

Abstract This paper presents a comparative study of the photoinduced electron transfer in a series of donor-acceptor compounds and their hydrogen-bonded complexes in solution. 4-(Dimethylamino) pyridine III, similarly to 4-(dimethylamino) pyrimidine I and 4-(dimethylamino)-5-methylpyrimidine II and contrary to 4-(N,N-dimethylanilino)pyrimidine IV, shows dual luminescence in a sufficiently polar and mobile environment. The results of steady-state and kinetic investigations as well as quantum chemical calculations of I–III fit well into the twisted intramolecular charge transfer (TICT) state model. On the other hand, the results suggest an enhanced planarity of the ICT fluorescent state of IV. INDO/S calculations confirm the large probability of the allowed radiative transitions in the latter compound. Photophysics of all the compounds under study is modified by hydrogen bonding: fluorescence quantum yields are strongly reduced in the presence of alcohols.

Detection of Hepatitis B virus antigen from human blood: SERS immunoassay in a microfluidic system

A highly sensitive immunoassay utilizing surface-enhanced Raman scattering (SERS) has been developed with a new Raman reporter and a unique SERS-active substrate incorporated into a microfluidic device. An appropriately designed Raman reporter, basic fuchsin (FC), gives strong SERS enhancement and has the ability to bind both the antibody and gold nanostructures. The fuchsin-labeled immuno-Au nanoflowers can form a sandwich structure with the antigen and the antibody immobilized on the SERS-active substrate based on Au-Ag coated GaN. Our experimental results indicate that this SERS-active substrate with its strong surface-enhancement factor, high stability and reproducibility plays a crucial role in improving the efficiency of SERS immunoassay. This SERS assay was applied to the detection of Hepatitis B virus antigen (HBsAg) in human blood plasma. A calibration curve was obtained by plotting the intensity of SERS signal of FC band at 1178cm(-1) versus the concentration of antigen. The low detection limit for Hepatitis B virus antigen was estimated to be 0.01IU/mL. The average relative standard deviation (RSD) of this method is less than 10%. This SERS immunoassay gives exact results over a broad linear range, reflecting clinically relevant HBsAg concentrations. It also exhibits high biological specificity for the detection of Hepatitis B virus antigen.

Spectroscopy of doubly hydrogen-bonded 7-azaindole. Reinvestigation of the excited state reaction

Luminescence of 7-azaindole (7AI) in non-polar solvents and alcohols was investigated with respect to its dependence upon temperature, concentration, and excitation energy. It was found that at low temperature oligomeric structures of 7AI are formed in the ground state. High-energy fluorescence (F1) was shown to consist of dimeric and oligomeric contributions. The phosphorescence was assigned to oligomers. The mechanism of excited state cooperative double proton transfer is discussed along with the results of INDO/S calculations of transition energies and charge densities.

Proton tunnelling in porphycene seeded in a supersonic jet

Abstract Porphycene, a constitutional isomer of porphyrin, was investigated in a supersonic jet expansion using laser-induced fluorescence and hole burning techniques. The lines recorded in the fluorescence excitation spectrum were split into doublets. No splitting was detected if one or both inner hydrogen atoms were replaced by deuterium. The doublet structure also disappeared upon formation of a complex between porphycene and water or alcohol. These findings indicate that the origin of the doublet structure in bare porphycene is the tunneling splitting due to the exchange of two inner protons between the nitrogen atoms. From the spectral response to deuteration, and to a change in cooling conditions, it is deduced that the tunneling splitting is higher in the ground state than in the first excited singlet state.

Proton transfer with a twist? Femtosecond Dynamics of 7‐(2‐pyridyl)indole in Condensed Phase and in Supersonic Jets

The results of spectral and photophysical studies strongly suggest that photoinduced proton transfer in 7-(2-pyridyl)indole (1) is accompanied by mutual twisting of the pyridyl and indole moieties. This conclusion is supported by the unusual finding that the photoreaction is faster in a cold, supersonicjet-isolated molecule than in solutions at room temperature, and by the ultrafast repopulation of the ground state substrate. The twisting and the presence of S1–S0 conical intersection (CI) are also predicted by calculations. The phenomenon may be quite general for several classes of organic molecules with intramolecular hydrogen bonds. Recent experimental and theoretical developments demonstrate that proton transfer—a fundamental chemical reaction—is by no means a one-dimensional process. [1] In particular, for tautomerization occurring along a hydrogen bond, coupling with modes that modulate the hydrogen bond strength may be crucial. [2] Much less explored is the possibility of large conformational changes accompanying proton/hydrogen transfer, although the role of torsional motion in the deactivation of phototautomerization products has been discussed in some depth. [3–9] Compound 1 is a member of a series of three isomeric 7pyridylindoles (Scheme 1), which were studied previously in solution. [10] Compounds 1 and 2 can exist in syn and anti

Ground-and Excited-State Tautomerization Rates in Porphycenes

The rates of double hydrogen transfer in the ground and excited electronic states have been measured for porphycene and its derivatives by using a new method based on pump-probe polarization spectroscopy. Changing the strength of two intramolecular hydrogen bonds by altering the NHN distance leads to differences in the tautomerization rate exceeding three orders of magnitude. The reaction is considerably slower in the lowest electronically excited state. A correlation was found between the tautomerization rates and (1)H chemical shifts of the internal protons.

Site-population conserving and site-population altering photo-orientation of matrix-isolated free-base porphine by double proton transfer: IR dichroism and vibrational symmetry assignments

Abstract Free-base porphine isolated in rare-gas matrices has been photo-oriented by double-proton transfer upon irradiation with linearly polarized light into either the x - or the y -polarized vibronic peaks in its Q x band. Evidence for two limiting situations has been obtained, depending on the conditions of irradiation: site-population conserving and site-population altering. Quantitative agreement with the theory of photoselection has been found. The dichroism in the polarized FT IR spectra of the oriented samples directly yields the absolute symmetry assignments of over 150 vibrations. Site structure has been observed both in the visible and in the IR absorption; the two are correlated. This permitted the recording of separate IR spectra of porphine in the two major different sites. These spectra are quite distinct, with the vibrational frequencies differing by as much as 5 cm −1 .