Sylwester Gawinkowski

Highly reproducible, stable and multiply regenerated surface-enhanced Raman scattering substrate for biomedical applications

We fabricated a Surface Enhanced Raman Scattering (SERS)-active surface based on photo-etched and Au-coated GaN. The highest enhancement factor (EF) in SERS and high reproducibility of spectra were obtained from surfaces covered with bunched nanopillars which were produced by relatively long defect-selective photo-etching. The surfaces exhibited SERS enhancements of the order of 2.8 × 106 for malachite green isothiocyanate (MGITC) and 2 × 106 for p-mercaptobenzoic acid (PMBA). These SERS enhancement factors were comparable to those of conventional SERS substrates, while the EF for MGITC was two orders of magnitude larger than the corresponding one reported for the SERS platform made on porous GaN. The standard deviation of the relative intensity of the 1180 cm−1 mode of MGITC was less than 5% for 100 randomly distributed locations across a single platform and less than 10% between different platforms. The SERS signal of MGITC at our GaN/Au surface (kept under ambient conditions) was extremely stable. We could not detect any peak shift or appreciable change of intensity even after three months. We used these surfaces to detect biological molecules such as amino acids and bovine serum albumin (BSA) at low concentration and with short detection time. We developed simple and effective cleaning procedures for our substrates. After cleaning, the same substrate could be used multiple times retaining the SERS activity. We are not aware of any other multiply regenerated SERS substrate which provides simultaneously such high stability with high enhancement, good uniformity, and high reproducibility.

Hot Carrier-Induced Tautomerization within a Single Porphycene Molecule on Cu(111).

Here, we report the study of tautomerization within a single porphycene molecule adsorbed on a Cu(111) surface using low-temperature scanning tunneling microscopy (STM) at 5 K. While molecules are adsorbed on the surface exclusively in the thermodynamically stable trans tautomer after deposition, a voltage pulse from the STM can induce the unidirectional trans → cis and reversible cis ↔ cis tautomerization. From the voltage and current dependence of the tautomerization yield (rate), it is revealed that the process is induced by vibrational excitation via inelastic electron tunneling. However, the metastable cis molecules are thermally switched back to the trans tautomer by heating the surface up to 30 K. Furthermore, we have found that the unidirectional tautomerization can be remotely controlled at a distance from the STM tip. By analyzing the nonlocal process in dependence on various experimental parameters, a hot carrier-mediated mechanism is identified, in which hot electrons (holes) generated by the STM travel along the surface and induce the tautomerization through inelastic scattering with a molecule. The bias voltage and coverage dependent rate of the nonlocal tautomerization clearly show a significant contribution of the Cu(111) surface state to the hot carrier-induced process.

Direct Observation of Photoinduced Tautomerization in Single Molecules at a Metal Surface

Molecular switches are of fundamental importance in nature, and light is an important stimulus to selectively drive the switching process. However, the local dynamics of a conformational change in these molecules remain far from being completely understood at the single-molecule level. Here, we report the direct observation of photoinduced tautomerization in single porphycene molecules on a Cu(111) surface by using a combination of low-temperature scanning tunneling microscopy and laser excitation in the near-infrared to ultraviolet regime. It is found that the thermodynamically stable trans configuration of porphycene can be converted to the metastable cis configuration in a unidirectional fashion by photoirradiation. The wavelength dependence of the tautomerization cross section exhibits a steep increase around 2 eV and demonstrates that excitation of the Cu d-band electrons and the resulting hot carriers play a dominant role in the photochemical process. Additionally, a pronounced isotope effect in the cross section (∼100) is observed when the transferred hydrogen atoms are substituted with deuterium, indicating a significant contribution of zero-point energy in the reaction. Combined with the study of inelastic tunneling electron-induced tautomerization with the STM, we propose that tautomerization occurs via excitation of molecular vibrations after photoexcitation. Interestingly, the observed cross section of ∼10(-19) cm(2) in the visible-ultraviolet region is much higher than that of previously studied molecular switches on a metal surface, for example, azobenzene derivatives (10(-23)-10(-22) cm(2)). Furthermore, we examined a local environmental impact on the photoinduced tautomerization by varying molecular density on the surface and find substantial changes in the cross section and quenching of the process due to the intermolecular interaction at high density.

Bridging the Gap between Porphyrins and Porphycenes: Substituent‐Position‐Sensitive Tautomerism and Photophysics in meso‐Diphenyloctaethylporphyrins

2,3,7,8,12,13,17,18-Octaethyl-5,15-diphenylporphyrin (1) is characterized by an inner cavity with a rectangular shape and small NH⋅⋅⋅N distances. It resembles porphycene, which is a constitutional isomer of porphyrin known for its strong intramolecular hydrogen bonds and rapid tautomerization. Such distortion of the porphyrin cavity leads to tautomeric properties of 1 that are intermediate between those of porphyrin and porphycene. In particular, a tautomerization in the lowest excited singlet state of 1 has been discovered, occurring with a rate three orders of magnitude lower than that in porphycene, but three to four orders of magnitude higher than that in porphyrin. An isomer of 1, 2,3,7,8,12,13,17,18-octaethyl-5,10-diphenylporphyrin (2), exhibits a different kind of geometry distortion. This molecule is nonplanar, but the inner cavity shape and dimensions are similar to those of the parent porphyrin. The same hydrogen-bonding strength as that in porphyrin is observed for 2. In contrast, the nonplanarity of 2 significantly influences the photophysics, leading to a decrease in fluorescence quantum yield and lifetime. Absorption, magnetic circular dichroism, and fluorescence spectra are similar for 1 and 2 and resemble those of parent porphyrin. This is a consequence of comparable energy splittings of the frontier orbitals, ΔHOMO≈ΔLUMO. The results demonstrate that judicious selection of substituents and their position enables a controlled modification of geometry, hydrogen-bonding strength, tautomerization rate, and photophysical and spectral parameters of porphyrinoids.

Direct observation of double hydrogen transfer via quantum tunneling in a single porphycene molecule on a Ag(110) surface

Quantum tunneling of hydrogen atoms (or protons) plays a crucial role in many chemical and biological reactions. Although tunneling of a single particle has been examined extensively in various one-dimensional potentials, many-particle tunneling in high-dimensional potential energy surfaces remains poorly understood. Here we present a direct observation of a double hydrogen atom transfer (tautomerization) within a single porphycene molecule on a Ag(110) surface using a cryogenic scanning tunneling microscope (STM). The tautomerization rates are temperature independent below ∼10 K, and a large kinetic isotope effect (KIE) is observed upon substituting the transferred hydrogen atoms by deuterium, indicating that the process is governed by tunneling. The observed KIE for three isotopologues and density functional theory calculations reveal that a stepwise transfer mechanism is dominant in the tautomerization. It is also found that the tautomerization rate is increased by vibrational excitation via an inelastic electron tunneling process. Moreover, the STM tip can be used to manipulate the tunneling dynamics through modification of the potential landscape.

Structure, electronic states, and anion-binding properties of cyclo[4]naphthobipyrroles.

Three octaalkyl-substituted cyclo[4]naphthobipyrroles, studied in solution in the form of their sulfates, reveal absorption and magnetic circular dichroism (MCD) spectra very similar to those of the parent cyclo[8]pyrrole. A unique feature of these systems is a strong absorption in the near IR region. The analysis of MCD patterns based on a perimeter model reveals a hard-chromophore character of cyclo[4]naphthobipyrroles, i.e., ΔHOMO ≪ ΔLUMO. Comparison of Raman spectra obtained for crystalline samples and solutions, combined with the analysis of absorption and MCD spectra based on quantum chemical calculations reveals that cyclo[4]naphthobipyrroles exist in solutions as undissociated sulfates of the doubly protonated forms.

1H-pyrrolo[3,2-h]quinoline: a benchmark molecule for reliable calculations of vibrational frequencies, IR intensities, and Raman activities.

Reliable assignment of 55 out of 57 vibrational modes has been achieved for 1H-pyrrolo[3,2-h]quinoline using IR, Raman, and fluorescence spectroscopy combined with quantum chemical calculations. The experiments provided a data set for assessing the performance of different models/basis sets for predicting the vibrational frequencies, as well as IR and Raman intensities for a molecule with 13 heavy atoms. Among six different tested DFT functionals, the hybrid B3LYP used with Poples split-valence basis sets is suggested as the best choice for accurate and cost-effective IR/Raman spectral simulations. Neither HF nor MP2 methods can satisfactorily describe the vibrational structure. Increasing the basis set size from double to triple-ζ and by adding polarization and diffuse functions does not necessarily improve the results, especially regarding the predictions of vibrational frequencies. With respect to the intensities, extending the basis set helps, with the accuracy increasing systematically for the Raman spectra, and in a less regular fashion for the IR. A large difference in accuracy is observed while comparing the spectral parameters predicted for in-plane and out-of-plane normal modes. The former are reliably computed with modest basis sets, whereas for the out-of-plane vibrations, larger basis sets are necessary, but even in this case, the out-of-plane vibrations are reproduced with much less accuracy than in-plane modes. This effect is general, as it has been observed using different functionals and basis sets.

1,4-Bis(1,3-dioxo-2-indenylidene)cyclohexane: polymorphism, gas phase oxidation and enol form mediated radical formation in the solid state

Crystallization of 1,4-bis(1,3-dioxo-2-indenylidene)cyclohexane from toluene and dichloromethane solutions afforded yellow photochromic and orange non-photochromic crystals, respectively. In situRaman spectroscopy of the photochromic form revealed the presence of an enol form. Both forms undergo sublimation assisted dehydrogenation under aerobic conditions.

Quantum tunneling in real space: Tautomerization of single porphycene molecules on the (111) surface of Cu, Ag, and Au

Tautomerization in single porphycene molecules is investigated on Cu(111), Ag(111), and Au(111) surfaces by a combination of low-temperature scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations. It is revealed that the trans configuration is the thermodynamically stable form of porphycene on Cu(111) and Ag(111), whereas the cis configuration occurs as a meta-stable form. The trans → cis or cis → trans conversion on Cu(111) can be induced in an unidirectional fashion by injecting tunneling electrons from the STM tip or heating the surface, respectively. We find that the cis ↔ cis tautomerization on Cu(111) occurs spontaneously via tunneling, verified by the negligible temperature dependence of the tautomerization rate below ∼23 K. Van der Waals corrected DFT calculations are used to characterize the adsorption structures of porphycene and to map the potential energy surface of the tautomerization on Cu(111). The calculated barriers are too high to be thermally overcome at cryogenic temperatures used in the experiment and zero-point energy corrections do not change this picture, leaving tunneling as the most likely mechanism. On Ag(111), the reversible trans ↔ cis conversion occurs spontaneously at 5 K and the cis ↔ cis tautomerization rate is much higher than on Cu(111), indicating a significantly smaller tautomerization barrier on Ag(111) due to the weaker interaction between porphycene and the surface compared to Cu(111). Additionally, the STM experiments and DFT calculations reveal that tautomerization on Cu(111) and Ag(111) occurs with migration of porphycene along the surface; thus, the translational motion couples with the tautomerization coordinate. On the other hand, the trans and cis configurations are not discernible in the STM image and no tautomerization is observed for porphycene on Au(111). The weak interaction of porphycene with Au(111) is closest to the gas-phase limit and therefore the absence of trans and cis configurations in the STM images is explained either by the rapid tautomerization rate or the similar character of the molecular frontier orbitals of the trans and cis configurations.

Resonance Raman spectroscopy study of protonated porphyrin

Resonance Raman microscopy was used to study the resonance Raman scattering of the diacid (diprotonated form) of free-base porphyrin (21H,23H-porphine) in a crystal powder and KBr pellets. Intensive lines in the spectral range between 100÷1000cm-1 have been detected and assigned as spectral manifestation of out-of-plane modes. The Raman spectra were simulated by means of DFT methods and compared with the experimental data. It is evident from experimental and theoretical results that the activation of out-of-plane modes arises from saddle distortion of the porphyrin macrocycle upon formation of its diprotonated form.