Spectroscopy and Tautomerization Studies of Porphycenes

Tautomerization in porphycenes, constitutional isomers of porphyrins, is strongly entangled with spectral and photophysical parameters. The intramolecular double hydrogen transfer occurring in the ground and electronically excited states leads to uncommon spectroscopic characteristics, such as depolarized emission, viscosity-dependent radiationless depopulation, and vibrational-mode-specific tunneling splittings. This review starts with documentation of the electronic spectra of porphycenes: Absorption and magnetic circular dichroism are discussed, together with their analysis based on the perimeter model. Next, photophysical characteristics are presented, setting the stage for the final part, which discusses the developments in research on tautomerism. Porphycenes have been studied in different experimental regimes: molecules in condensed phases, isolated in supersonic jets and helium nanodroplets, and, recently also on the level of single molecules investigated by optical and scanning probe microscopies. Because of the rich and detailed information obtained from these diverse investigations, porphycenes emerge as very good models for studying the complex, multidimensional phenomena involved in the process of intramolecular double hydrogen transfer

Improved Method of Fluorescence Quantum Yield Determination

In the most widely used procedure for luminescence quantum yield determination, absorption and emission spectra are measured on two different instruments. This leads to errors caused by wavelength misalignment and different monochromator characteristics of the spectrophotometer and the spectrofluorometer. These errors can be avoided using a method for fluorescence quantum yield determination that relies on simultaneous absorption and fluorescence emission (SAFE) measurement using a single commercial spectrofluorometer. The method’s performance is compared with the standard routinely used procedure for the relative quantum yield determination. The advantages of SAFE measurement are discussed. The proposed novel approach eliminates a number of potential errors in quantum yield determination protocol and provides higher accuracy

Evidence for Dominant Role of Tunneling in Condensed Phases and at High Temperatures: Double Hydrogen Transfer in Porphycenes

Investigation of the double hydrogen transfer in porphycene, its 2,7,12,17-tetra-<i>tert</i>-butyl derivative, and their N-deuterated isotopologues revealed the dominant role of tunneling, even at room temperature in condensed phase. Ultrafast optical spectroscopy with polarized light employed in a wide range of temperatures allowed the identification and evaluation of contributions of two tunneling modes: vibrational ground-state tunneling, occurring from the zero vibrational level, and vibrationally activated, via a large amplitude, low-frequency mode. Good correspondence was found between the rates of incoherent tunneling occurring in condensed phase and the values estimated on the basis of tunneling splittings observed in molecules isolated in supersonic jets or helium nanodroplets. The results provide solid experimental insight into widely proposed quantum facets of ubiquitous hydrogen-transfer phenomena

Arresting Tautomerization in a Single Molecule by the Surrounding Polymer: 2,7,12,17-Tetraphenyl Porphycene

2,7,12,17-Tetraphenyl porphycene (TPPo) is known to undergo ultrafast (<1 ps) double-hydrogen transfer in solution. Fluorescence studies of single TPPo molecules embedded in a polymer matrix and excited with an azimuthally polarized laser beam reveal, for a considerable fraction of the population, double-lobe spatial emission patterns attributed to the absence of tautomerism. In consecutive image scans, these patterns change their orientation to nearly orthogonal. In some cases, switching of the orientation direction occurs through an intermediate case with a doughnut-shaped pattern, indicating the presence of fast tautomerism. These findings demonstrate that local polymer environment can drastically change the thermodynamics or kinetics of tautomerization. They also point out the contribution of the motion of peripheral substituents in the hydrogen-transfer coordinate. TPPo is proposed as a good probe for studying relaxation dynamics in thin polymer films

Evidence for Dominant Role of Tunneling in Condensed Phases and at High Temperatures: Double Hydrogen Transfer in Porphycenes

Investigation of the double hydrogen transfer in porphycene, its 2,7,12,17-tetra-<i>tert</i>-butyl derivative, and their N-deuterated isotopologues revealed the dominant role of tunneling, even at room temperature in condensed phase. Ultrafast optical spectroscopy with polarized light employed in a wide range of temperatures allowed the identification and evaluation of contributions of two tunneling modes: vibrational ground-state tunneling, occurring from the zero vibrational level, and vibrationally activated, via a large amplitude, low-frequency mode. Good correspondence was found between the rates of incoherent tunneling occurring in condensed phase and the values estimated on the basis of tunneling splittings observed in molecules isolated in supersonic jets or helium nanodroplets. The results provide solid experimental insight into widely proposed quantum facets of ubiquitous hydrogen-transfer phenomena

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 <i>trans</i> tautomer after deposition, a voltage pulse from the STM can induce the unidirectional <i>trans</i> → <i>cis</i> and reversible <i>cis</i> ↔ <i>cis</i> tautomerization. From the voltage and current dependence of the tautomerization yield (rate), it is revealed that the process is induced by vibrational excitation <i>via</i> inelastic electron tunneling. However, the metastable <i>cis</i> molecules are thermally switched back to the <i>trans</i> 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

Near-Field Enhanced Photochemistry of Single Molecules in a Scanning Tunneling Microscope Junction

Optical near-field excitation of metallic nanostructures can be used to enhance photochemical reactions. The enhancement under visible light illumination is of particular interest because it can facilitate the use of sunlight to promote photocatalytic chemical and energy conversion. However, few studies have yet addressed optical near-field induced chemistry, in particular at the single-molecule level. In this Letter, we report the near-field enhanced tautomerization of porphycene on a Cu(111) surface in a scanning tunneling microscope (STM) junction. The light-induced tautomerization is mediated by photogenerated carriers in the Cu substrate. It is revealed that the reaction cross section is significantly enhanced in the presence of a Au tip compared to the far-field induced process. The strong enhancement occurs in the red and near-infrared spectral range for Au tips, whereas a W tip shows a much weaker enhancement, suggesting that excitation of the localized plasmon resonance contributes to the process. Additionally, using the precise tip–surface distance control of the STM, the near-field enhanced tautomerization is examined in and out of the tunneling regime. Our results suggest that the enhancement is attributed to the increased carrier generation rate via decay of the excited near-field in the STM junction. Additionally, optically excited tunneling electrons also contribute to the process in the tunneling regime

7‑Hydroxyquinoline-8-carbaldehydes. 1. Ground- and Excited-State Long-Range Prototropic Tautomerization

Ground- and excited-state long-range prototropic tautomerization were studied for a series of 7-hydroxyquinoline-8-carbaldehydes (<b>7-HQCs</b>) by ¹H and ¹³C NMR spectroscopy, photostationary and time-resolved UV–vis spectroscopic methods, and quantum chemical computations. These molecules represent trifunctional proton-donating/accepting systems that have been proposed to serve as models of a reversible optically driven molecular switch composed of two moieties: a molecular “frame” (7-hydroquinolines, <b>7-HQs</b>) and a proton “crane” (carbaldehyde group). The NMR and electronic absorption spectra indicate a solvent-dependent equilibrium between two tautomeric forms, <b>OH</b> (7-quinolinol)) and <b>NH</b> (7­(1<i>H</i>)-quinolinone), already in the ground state of all the compounds under study (7-hydroxy-2-methoxy-4-methylquinoline-8-carbaldehyde, <b>HMMQC</b>, shows only a trace of the <b>NH</b> form in highly polar and/or protic media). Electronic absorption and fluorescence of <b>7-HQCs</b> are rationalized in terms of the ground- and excited-state hydrogen atom transfer (HAT). This process was identified by comparing the UV–vis spectroscopic properties of <b>7-HQCs</b> with those of <b>7-HQs</b>, synthetic precursors of the former, as well as with the characteristics of corresponding protonated cations and deprotonated anions (part 2). The experimental results are corroborated by the density functional theory (DFT) and ab initio computations, which shed some light on the differences in photophysics between variously substituted <b>7-HQCs</b>

7‑Hydroxyquinoline-8-carbaldehydes. 2. Prototropic Equilibria

Prototropic equilibria were studied for a series of 7-hydroxyquinoline-8-carbaldehydes (<b>7-HQCs</b>) by ¹H NMR spectroscopy, photostationary and time-resolved UV–vis spectroscopic methods, and quantum chemical computations. These molecules represent trifunctional proton-donating/accepting systems that in aqueous solutions may assume four main neutral and ionic structures: 7-quinolinol (<b>OH</b>), 7­(1<i>H</i>)-quinolinone (<b>NH</b>), deprotonated anion (<b>A</b>), and protonated cation (<b>C</b>). Electronic absorption and fluorescence of <b>7-HQCs</b> are rationalized in terms of the ground and excited-state long-range tautomerization (part 1) as well as protonation and deprotonation processes. The photophysical properties of neutral and ionic forms of <b>7-HQCs</b> are compared with those of 7-hydroxyquinolines (<b>7-HQs</b>), synthetic precursors of the former. The experimental results are corroborated by ab initio computations

Excited-State Proton Transfer in <i>syn</i>-2-(2′-Pyridyl)pyrrole Occurs on the Nanosecond Time Scale in the Gas Phase

Microwave and UV excitation spectra of 2-(2′-pyridyl)pyrrole (2PP) have been recorded at high resolution in the gas phase. Analyses of these data show that the <i>syn</i> conformer of 2PP is a planar molecule in both the ground (S₀) and first excited (S₁) electronic states, and that the S₁ state undergoes a relatively slow excited-state proton transfer (ESPT) reaction when excited by light, as measured by the homogeneous line broadening that is observed in its UV spectrum. Apparently, excitation of the S₁ state moves electronic charge from the pyrrole ring to the pyridine ring, but the simultaneous transfer of the proton is inhibited by an unfavorably oriented dipole under solvent-free conditions. The rate of the ESPT reaction is enhanced by more than an order of magnitude with simultaneous excitation of a 144 cm^–1 in-plane vibrational mode