Deuterium Isotope Effect in Single Molecule Photophysics and Photochemistry of Hypericin

The peripherical protons of the dye molecule hypericin can undergo structural interconversion (tautomerization) between different isomers separated by a low energy barrier with rates that depends sensitively on the interaction with local chemical environment defined by the nature of host material. We investigate the deuterium (D) isotope effect of hypericin tautomerism at the single-molecule level to avoid ensemble averaging in different polymer matrices by a combined spectroscopic and computational approach. In the 'innocent' PMMA matrix only intramolecular isotope effects on the internal conversion channel and tautomerization are observed; while PVA specifically interacts with the probe via H- and D-bonding. This establishes a single molecular picture on intra- and intermolecular nano-environment effects to control chromophore photophysics and -chemistry.

Surface amplification of tetraphenylporphyrin overtone and combination Raman bands in drop coating deposition Raman (DCDR) on electrically conductive surfaces

It is essential to realize a Raman measurement technique without artifact or fluorescence signals for high-quality and reliable data in a valid molecular-level analysis and interpretation. This requirement applies especially to a molecule with strong fluorescence like porphyrin. Here, the surface of a gold substrate performs better as a DCDR substrate for tetraphenylporphyrin than other surfaces, such as tantalum, indium tin oxide glass, or aluminium. Polarized Raman spectra of tetraphenylporphyrin demonstrated the oriented deposition of porphyrin crystallites on the Au substrate using the drop coating technique. The emission anisotropy suggests that the deposited crystallites are arranged outward radially with the porphyrin ring orientation. The orientation is signed by the NHHN axis that is parallel to the radial vector along the X-axis. Moreover, it also demonstrates high chemical stability after preservation and repeated measurements. The Raman signal on a gold substrate is enhanced more than on other substrates beyond mere preconcentration of analytes or the coffee-ring effect only, which might be due to the contribution of the SERRS effect. This effect will be discussed based on the interactions among localized surface plasmons, vibronic transitions, and Raman active vibrational modes.

Direct Observation of Structural Heterogeneity and Tautomerization of Single Hypericin Molecules

Tautomerization is a fundamental chemical reaction which involves the relocation of a proton in the reactants. Studying the optical properties of tautomeric species is challenging because of ensemble averaging. Many molecules, such as porphines, porphycenes, or phenanthroperylene quinones, exhibit a reorientation of the transition dipole moment (TDM) during tautomerization, which can be directly observed in single-molecule experiments. Here, we study single hypericin molecules, which is a prominent phenanthroperylene quinone showing antiviral, antidepressive, and photodynamical properties. Observing abrupt flipping of the image pattern combined with time-dependent density functional theory calculations allows drawing conclusions about the coexistence of four tautomers and their conversion path. This approach allows the unambiguous assignment of a TDM orientation to a specific tautomer and enables the determination of the chemical structure in situ. Our approach can be applied to other molecules showing TDM reorientation during tautomerization, helping to gain a deeper understanding of this important process.

What can single-molecule Fano resonance tell?

In this work, we showcase applications of single-molecule Fano resonance (SMFR) measurements beyond the determination of molecular excitonic energy and associated dipole orientation. We use the SMFR measurement to probe the local influence of a man-made single chlorine vacancy on the molecular transition of a single zinc phthalocyanine, which clearly reveals the lifting-up of the double degeneracy of the excited states due to defect-induced configurational changes. Furthermore, time-trace SMFR measurements at different excitation voltages are used to track the tautomerization process in a free-base phthalocyanine. Different behaviors in switching between two inner-hydrogen configurations are observed with decreasing voltages, which helps to reveal the underlying tautomerization mechanism involving both the molecular electronic excited states and vibrational excited states in the ground state.

Confocal microscopy with a microlens array

Confocal laser scanning microscopy (CLSM) is a preferred method for obtaining optical images with submicrometer resolution. Replacing the pinhole and detector of a CLSM with a digital camera [charge-coupled device (CCD) or complementary metal oxide semiconductor (CMOS)] has the potential to simplify the design and reduce cost. However, the relatively slow speed of a typical camera results in long scans. To address this issue, in the present investigation a microlens array was used to split the laser beam into 48 beamlets that are focused onto the sample. In essence, 48 pinhole-detector measurements were performed in parallel. Images obtained from the 48 laser spots were stitched together into a final image.

Hypericin: Single Molecule Spectroscopy of an Active Natural Drug

Hypericin is one of the most efficient photosensitizers used in photodynamic tumor therapy (PDT). The reported treatments of this drug reach from antidepressive, antineoplastic, antitumor and antiviral activity. We show that hypericin can be optically detected down to a single molecule at ambient conditions. Hypericin can even be observed inside of a cancer cell, which implies that this drug can be directly used for advanced microscopy techniques (PALM, spt-PALM, or FLIM). Its photostability is large enough to obtain single molecule fluorescence, surface enhanced Raman spectra (SERS), fluorescence lifetime, antibunching, and blinking dynamics. Sudden spectral changes can be associated with a reorientation of the molecule on the particle surface. These properties of hypericin are very sensitive to the local environment. Comparison of DFT calculations with SERS spectra show that both the neutral and deprotonated form of hypericin can be observed on the single molecule and ensemble level.

Single-molecule tautomerization tracking through space- and time-resolved fluorescence spectroscopy

Tautomerization, the interconversion between two constitutional molecular isomers, is ubiquitous in nature¹, plays a major role in chemistry² and is perceived as an ideal switch function for emerging molecular-scale devices³. Within free-base porphyrin⁴, porphycene⁵ or phthalocyanine⁶, this process involves the concerted or sequential hopping of the two inner hydrogen atoms between equivalent nitrogen sites of the molecular cavity. Electronic and vibronic changes⁶ that result from this NH tautomerization, as well as details of the switching mechanism, were extensively studied with optical spectroscopies, even with single-molecule sensitivity⁷. The influence of atomic-scale variations of the molecular environment and submolecular spatial resolution of the tautomerization could only be investigated using scanning probe microscopes^3,8-11, at the expense of detailed information provided by optical spectroscopies. Here, we combine these two approaches, scanning tunnelling microscopy (STM) and fluorescence spectroscopy^12-15, to study the tautomerization within individual free-base phthalocyanine (H₂Pc) molecules deposited on a NaCl-covered Ag(111) single-crystal surface. STM-induced fluorescence (STM-F) spectra exhibit duplicate features that we assign to the emission of the two molecular tautomers. We support this interpretation by comparing hyper-resolved fluorescence maps^15-18(HRFMs) of the different spectral contributions with simulations that account for the interaction between molecular excitons and picocavity plasmons¹⁹. We identify the orientation of the molecular optical dipoles, determine the vibronic fingerprint of the tautomers and probe the influence of minute changes in their atomic-scale environment. Time-correlated fluorescence measurements allow us to monitor the tautomerization events and to associate the proton dynamics to a switching two-level system. Finally, optical spectra acquired with the tip located at a nanometre-scale distance from the molecule show that the tautomerization reaction occurs even when the tunnelling current does not pass through the molecule. Together with other observations, this remote excitation indicates that the excited state of the molecule is involved in the tautomerization reaction path.

Mapping the electric field distribution of tightly focused cylindrical vector beams with gold nanorings

In this paper gold nanorings (NRs) are applied as particularly well-suited sensing elements for mapping the radially symmetric electric fields in the high numerical aperture focus of cylindrical vector beams. The optical properties of gold nanorings are analyzed by a combination of extinction and single particle dark field spectroscopy as well as confocal photoluminescence (PL) imaging. The results are compared to numerical calculations. The in-plane components in the focus of the cylindrical vector beams are estimated through the PL intensity distributions of the NRs. The optimum overlap between the structure and excitation is visualized by a narrow centre spot in the far-field PL scan.

Nature of Large Temporal Fluctuations of Hydrogen Transfer Rates in Single Molecules

Double hydrogen transfer was monitored in single molecules of parent porphycene and its tetra- t-butyl derivative using confocal fluorescence microscopy. The molecules have been embedded in a polymer matrix. Under such conditions, a significant fraction of the population reveals a huge decrease of the tautomerization rate with respect to the value obtained from ensemble studies in solution. This effect is explained by a model that assumes that the rate is determined by the reorganization coordinate that involves slow relaxation of the polymer matrix. The model provides indirect evidence for the dominant role of tunneling. It is proposed that tautomerization in single molecules of the porphycene family can be used to probe polymer relaxation dynamics on the time scale ranging from picoseconds to minutes.

Unusual effects in single molecule tautomerization: hemiporphycene

Temporal evolution of the fluorescence spectra of individual hemiporphycene molecules reveals unusual double hydrogen transfer reaction dynamics.

Non-typical fluorescence studies of excited and ground state proton and hydrogen transfer

Fluorescence studies of tautomerization have been carried out for various systems that exhibit single and double proton or hydrogen translocation in various environments, such as liquid and solid condensed phases, ultracold supersonic jets, and finally, polymer matrices with single emitters. We focus on less explored areas of application of fluorescence for tautomerization studies, using porphycene, a porphyrin isomer, as an example. Fluorescence anisotropy techniques allow investigations of self-exchange reactions, where the reactant and product are formally identical. Excitation with polarized light makes it possible to monitor tautomerization in single molecules and to detect their three-dimensional orientation. Analysis of fluorescence from single vibronic levels of jet-isolated porphycene not only demonstrates coherent tunneling of two internal protons, but also indicates that the process is vibrational mode-specific. Next, we present bifunctional proton donor-acceptor systems, molecules that are able, depending on the environment, to undergo excited state single intramolecular or double intermolecular proton transfer. For molecules that have donor and acceptor groups located in separate moieties linked by a single bond, excited state tautomerization can be coupled to mutual twisting of the two subunits.

Spectroscopic and microscopic investigations of tautomerization in porphycenes: condensed phases, supersonic jets, and single molecule studies

Tautomerization of porphycene, coherent in supersonic jets and a rate process in solutions, can be controlled for single molecules on surfaces.

Single molecule optical measurements of orientation and rotations of biological macromolecules

Subdomains of macromolecules often undergo large orientation changes during their catalytic cycles that are essential for their activity. Tracking these rearrangements in real time opens a powerful window into the link between protein structure and functional output. Site-specific labeling of individual molecules with polarized optical probes and measurement of their spatial orientation can give insight into the crucial conformational changes, dynamics, and fluctuations of macromolecules. Here we describe the range of single molecule optical technologies that can extract orientation information from these probes, review the relevant types of probes and labeling techniques, and highlight the advantages and disadvantages of these technologies for addressing specific inquiries.

Non-classical effects in proton or hydrogen transfer

Results of thorough investigations of tautomerism in the ground and excited electronic states performed for various organic molecules under different regimes: condensed phases, isolated and single molecules, with a special emphasis on porphycene, a porphyrin isomer, demonstrate that, in order to precisely describe the proton/hydrogen transfer path, one has to explicitly consider tunneling. Tautomerization is a multidimensional process, controlled by excitation of specific vibrational modes. Vibrational excitation can both enhance or hinder the reaction. The role of specific vibrational modes can now be assessed, even at the level of single molecules.

Nanostar probes for tip-enhanced spectroscopy

To overcome the current limit of tip-enhanced spectroscopy that is based on metallic nano-probes, we developed a new scanning probe with a metallic nanostar, a nanoparticle with sharp spikes. A Au nanoparticle of 5 nm was first attached to the end of a tip through DNA-DNA hybridization and mechanical pick-up. The nanoparticle was converted to a nanostar with a core diameter of ∼70 nm and spike lengths between 50 nm and 80 nm through the reduction of Au(3+) with ascorbic acid in the presence of Ag(+). Fabrication yields of such tips exceeded 60%, and more than 80% of such tips showed a mechanical durability sufficient for use in scanning microscopy. Effectiveness of the new probes for tip-enhanced Raman scattering (TERS) and tip-enhanced fluorescence (TEF) was confirmed. The probes exhibited the necessary enhancement for TEF, and the tip-on and tip-off ratios varied between 5 and 100. This large tip-to-tip variability may arise from the uncontrolled orientation of the apexes of the spike with respect to the sample surface, which calls for further fabrication improvement. The result overall supports a new fabrication approach for the probe that is effective for tip-enhanced spectroscopy.

A tunable Au core–Ag shell nanoparticle tip for tip-enhanced spectroscopy

A single Au nanoparticle (NP) with a diameter of 5 nm was transferred to the end of a Si-tip through a picking process, and an Ag shell with a controlled thickness was formed on the Au core.

Simultaneous Measurement of the Three-Dimensional Orientation of Excitation and Emission Dipoles

The emission properties of most fluorescent emitters, such as dye molecules or solid-state color centers, can be well described by the model of an oscillating electric dipole. However, the orientations of their excitation and emission dipoles are, in most cases, not parallel. Although single molecule excitation and emission dipole orientation measurements have been performed in the past, no experimental method has so far looked at the three-dimensional excitation and emission dipole geometry of individual emitters simultaneously. We present the first experimental study, using defocused imaging in conjunction with radially polarized excitation scanning, to measure both the excitation as well as emission dipole orientations of single molecules, which allows us to sample the distribution of their mutual orientation. We find an unexpectedly broad distribution of the angle between both dipoles which we attribute to the interaction between the observed molecules and the substrate they are immobilized on.

Self-aligned placement and detection of quantum dots on the tips of individual conical plasmonic nanostructures

Hybrid structures of few or single quantum dots (QDs) coupled to single optical antennas are of prime interest for nano-optical research. The photoluminescence (PL) signal from single nanoemitters, such as QDs, can be enhanced, and their emission characteristics modified, by coupling them to plasmonic nanostructures. Here, a self-aligned technique for placing nanoscale QDs with about 10 nm lateral accuracy and well-defined molecular distances to the tips of individual nanocones is reported. This way the QDs are positioned exactly in the high near-field region that can be created near the cone apex. The cones are excited in the focus of a radially polarized laser beam and the PL signal of few or single QDs on the cone tips is spectrally detected.

The role of molecular dipole orientation in single-molecule fluorescence microscopy and implications for super-resolution imaging

Numerous methods for determining the orientation of single-molecule transition dipole moments from microscopic images of the molecular fluorescence have been developed in recent years. At the same time, techniques that rely on nanometer-level accuracy in the determination of molecular position, such as single-molecule super-resolution imaging, have proven immensely successful in their ability to access unprecedented levels of detail and resolution previously hidden by the optical diffraction limit. However, the level of accuracy in the determination of position is threatened by insufficient treatment of molecular orientation. Here we review a number of methods for measuring molecular orientation using fluorescence microscopy, focusing on approaches that are most compatible with position estimation and single-molecule super-resolution imaging. We highlight recent methods based on quadrated pupil imaging and on double-helix point spread function microscopy and apply them to the study of fluorophore mobility on immunolabeled microtubules.

Double Hydrogen Transfer in Low Symmetry Porphycenes

The rate constants of intramolecular double hydrogen transfer have been determined for of tert-butyl-substituted porphycenes: 2-tert-butyl-, 2,7-di-tert-butyl-, 2,7,12-tri-tert-butyl-, and 2,7,12,17-tetra-tert-butylporphycene using femtosecond pump-probe polarization spectroscopy. The rates increase monotonically with the number of substituents. As is the case for other porphycenes studied so far, the tautomerization becomes slower after excitation to the lowest excited singlet state. The potential for the trans-trans self-exchange reaction is symmetrical in di- and tetra-tert-butyl substituted derivatives, but not for the singly and triply substituted ones. Our studies enabled determination of the relative populations of nonequivalent tautomers and thus of the equilibrium constants in S0 and S1 states, as well as estimation of ground and excited state activation energies for the tautomerization process.

Excitonic luminescence of hemiporphyrazines

Metal-free hemiporphyrazine (HpH(2)) is a notoriously insoluble material possessing interesting photophysical properties. Here we report the synthesis, structure, and photophysical properties of an octahedral zinc trans-ditriflate hemiporphyrazine complex "HpH(2)Zn(OTf)(2)" that contains a neutral hemiporphyrazine ligand. The photophysical properties of hemiporphyrazine are largely unaffected by introduction of zinc(II) triflate, but a dramatic increase in solubility is observed. HpH(2)Zn(OTf)(2) therefore provides a convenient model system to evaluate the impact of aggregation on the photophysical properties of hemiporphyrazine. Soluble aggregates and crystalline materials containing planar hemiporphyrazines exhibit relatively strong absorbance of visible light (450-600 nm) and red luminescence (600-700 nm). Hemiporphyrazine monohydrate (HpH(2)·H(2)O), in contrast, has a nonplanar "saddle-shaped" conformation that exhibits very little absorbance of visible light in solution or in the solid state. Upon photoexcitation at 380 nm, HpH(2)Zn(OTf)(2) and HpH(2) exhibit multiwavelength emissions centered at 450 and 650 nm. Emissions at 450 nm are highly anisotropic, while emissions at 650 nm are fully depolarized with respect to a plane-polarized excitation source. Taken together, our results suggest that excitonic coupling of aggregated and crystalline hemiporphyrazines results in increased absorbance and emission of visible light from S(0) ↔ S(1) transitions that are usually symmetry forbidden in isolated species. In contrast to previously proposed theories involving excited-state intramolecular proton transfer, we propose that the multiple-wavelength luminescent emissions of HpH(2)Zn(OTf)(2) and HpH(2) are due to emissive S(1) and S(2) states in aggregated and crystalline hemiporphyrazines. These results may provide a better understanding of the nonlinear optical properties of these materials in solution and in the solid state.

The long and winding road to new porphycenes

We describe attempts — not always successful — made over the years to improve the efficiency of porphycene synthesis and to produce novel compounds, custom-designed for specific purposes. New porphycenes are reported, some of them obtained rather unexpectedly as by-products of the planned reactions. Structure and energy computations of possible tautomeric forms in porphycenes substituted by one, two, three, and four tert-butyl groups lead to predictions regarding the kinetics and mechanisms of intramolecular double hydrogen transfer. The occurrence of tautomerization in single molecules of tert-butylsubstituted porphycenes is demonstrated by using fluorescence polarization techniques.

Dual fluorescence in 9-amino-2,7,12,17-tetraphenylporphycene

The absorption spectrum of the asymmetric 9-amino-2,7,12,17-tetraphenylporphycene shows new, strongly red-shifted bands compared to the symmetric parental 2,7,12,17-tetraphenylporphycene and to the also asymmetric 9-acetoxy-2,7,12,17-tetraphenylporphycene. Dual emission is also observed with relative contributions that depend strongly on the excitation wavelength and temperature. The gap between the two fluorescence bands is 84 nm. Tautomerization in both the ground and excited states is shown to account for these observations, the 9-amino group being particularly able to selectively lower the energy of the first excited singlet state of just one of the trans tautomers. Introduction of amino groups in porphycenes may be a convenient way to gain a deeper insight into the tautomerization mechanisms in this macrocyclic core.

Probing the radiative transition of single molecules with a tunable microresonator

Using a tunable optical microresonator with subwavelength spacing, we demonstrate controlled modulation of the radiative transition rate of a single molecule, which is measured by monitoring its fluorescence lifetime. Variation of the cavity length changes the local mode structure of the electromagnetic field, which modifies the radiative coupling of an emitting molecule to that field. By comparing the experimental data with a theoretical model, we extract both the pure radiative transition rate as well as the quantum yield of individual molecules. We observe a broad scattering of quantum yield values from molecule to molecule, which reflects the strong variation of the local interaction of the observed molecules with their host environment.

Excitation isotropy of single CdSe/ZnS nanocrystals

We study the dimensionality of the excitation transition dipole moment for single CdSe/ZnS core-shell nanocrystals using azimuthally and radially polarized laser modes. The comparison of measured and simulated single nanocrystal excitation patterns shows that single CdSe/ZnS quantum dots possess a spherically degenerated excitation transition dipole. We show that the dimensionality of the excitation transition dipole moment distribution is the same for all individual CdSe/ZnS nanocrystals, disregarding the difference in core size and irrespective of variations in the local environment. In contrast to the emission transition dipole moment, which is oriented in one plane, the excitation transition dipole moment of a single CdSe/ZnS quantum dots possesses an isotropy in three dimensions.