Ultrafast capture of electrons ejected by photoionization leading to the formation of a charge-separated state at a high energy level

Electron transfer reactions driven by two-photon ionization in the higher excited state were investigated via transient absorption spectroscopy, with the aim to develop a method for creating the charge-separated (CS) state with a large formation rate, high energy level, and long lifetime. In the proof-in-principle experiments using pyrene and biphenyl as a model system, femtosecond transient absorption spectroscopy revealed that intense irradiation of an ultraviolet laser pulse at 355 nm efficiently pumps up pyrene into a higher excited state via a stepwise two-photon absorption, and then an ionization process takes place. An electron ejected from pyrene is directly captured by biphenyl with a time constant of 200 fs without the diffusion process of the electron in solution. The energy level of the CS state (Py⁺-Bp^-) thus formed was estimated to be higher than that of the S₁ state of pyrene by 0.53 eV. In addition, the subsequent ionic dissociation without a remarkable geminate recombination in the sub-nanosecond to nanosecond time region effectively avoids the quantity loss of the CS state. By applying the two-photon excitation method, we experimentally achieved ultrafast formation of the long-lived CS state at a high energy beyond the traditional framework of electron transfer reactions.

Geometrical Evolution and Formation of the Photoproduct in the Cycloreversion Reaction of a Diarylethene Derivative Probed by Vibrational Spectroscopy

The front cover artwork is provided by the groups of Prof. Hiroshi Miyasaka (Osaka University, Japan), Prof. Masahiro Irie (Rikkyo University, Japan), Prof. Seiya Kobatake (Osaka City University, Japan) and Prof. Akira Sakamoto (Aoyama Gakuin University, Japan). The image shows the coherently vibrating closed form of a photochromic diarylethene derivative in the excited state, and subsequent structural evolution into the open form in the cycloreversion reaction. Read the full text of the Article at 10.1002/cphc.202000315.

Geometrical Evolution and Formation of the Photoproduct in the Cycloreversion Reaction of a Diarylethene Derivative Probed by Vibrational Spectroscopy

The geometrical evolution of the reactant and formation of the photoproduct in the cycloreversion reaction of a diarylethene derivative were probed using time-resolved absorption spectroscopies in the visible to near-infrared and mid-infrared regions. The time-domain vibrational data in the visible region show that the initially formed Franck-Condon state is geometrically relaxed into the minimum in the excited state potential energy surface, concomitantly with the low-frequency coherent vibrations. Theoretical calculations indicate that the nuclear displacement in this coherent vibration is nearly parallel to that in the geometrical relaxation. Time-resolved mid-infrared spectroscopy directly detected the formation of the open-ring isomer with the same time constant as the decrease of the closed-ring isomer in the excited state minimum. This observation reveals that no detectable intermediate, in which the population is accumulated, is present between the excited closed-ring isomer and the open-ring isomer in the ground state.

Restriction of the conrotatory motion in photo-induced 6π electrocyclic reaction: formation of the excited state of the closed-ring isomer in the cyclization

The electrocyclic reaction dynamics of a photochromic dithiazolylarylene derivative, 2,3-dithiazolylbenzothiophene (DTA) was investigated by using time-resolved transient absorption and fluorescence spectroscopies. The closed-ring isomer of DTA undergoes cycloreversion through the conical intersection mediating the potential energy surfaces of the excited and ground states, which is in agreement with the Woodward–Hoffmann rules for the electrocyclic reactions of 6π electron systems. On the other hand, a large portion of the open-ring isomer undergoes cyclization along the distinct reaction scheme, in which the cyclization takes place in the excited state manifold leading to the formation of the excited state of the closed-ring isomer. The suppression of the geometrical motion of DTA due to the intramolecular interaction could open a new efficient reaction pathway resulting in the formation of the electronically excited state of the product.

Restriction of the molecular geometry opens up a novel pathway in the cyclization reaction of a photochromic dithiazolylarylene derivative.

A dominant factor of the cycloreversion reactivity of diarylethene derivatives as revealed by femtosecond time-resolved absorption spectroscopy

Dynamics of the cycloreversion reaction of a photochromic diarylethene derivative with a small ring-opening reaction yield (∼1%) was investigated by using femtosecond transient absorption spectroscopy. The reaction rate constant and activation barrier on the reaction coordinate were quantitatively analyzed on the basis of the temperature and excitation wavelength dependencies of the reaction yield and excited state dynamics. From the comparison of the present results with those in a more reactive derivative, we concluded that a key factor regulating the overall reaction yield is the branching ratio at the conical intersection where the excited state population is split into the product and the initial reactant. The excitation wavelength dependence of the dynamics indicated that the geometrical relaxation and vibrational cooling proceed in a few picosecond time scale behind the cycloreversion process, and the vibrational excess energy assists the molecule to climb up the energy barrier.

Excited state dynamics for visible-light sensitization of a photochromic benzil-subsituted phenoxyl-imidazolyl radical

Visible-light sensitized photoswitches have been paid particular attention in the fields of life sciences and materials science because long-wavelength light reduces photodegradation, transmits deep inside of matters, and achieves the selective excitation in condensed systems. Among various photoswitch molecules, the phenoxyl-imidazolyl radical complex (PIC) is a recently developed thermally reversible photochromic molecule whose thermal back reaction can be tuned from tens of nanoseconds to tens of seconds by rational design of the molecular structure. While the wide range of tunability of the switching speed of PIC opened up various potential applications, no photosensitivity to visible light limits its applications. In this study, we synthesized a visible-light sensitized PIC derivative conjugated with a benzil unit. Femtosecond transient absorption spectroscopy revealed that the benzil unit acts as a singlet photosensitizer for PIC by the Dexter-type energy transfer. Visible-light sensitized photochromic reactions of PIC are important for expanding the versatility of potential applications to life sciences and materials science.

Solid-State, Near-Infrared to Visible Photon Upconversion via Triplet-Triplet Annihilation of a Binary System Fabricated by Solution Casting

Herein, triplet-triplet annihilation upconversion (TTA-UC) from near-infrared (NIR, 785 nm) to visible (yellow, centered at 570 nm) regions has been demonstrated in the binary solid of condensed chromophores. Microparticles of the binary solid comprising rubrene as a matrix (emitter) and π-extended Pd-porphyrin as a dopant (sensitizer) in a mole ratio of 1000:1 were obtained by solution casting. Excitation intensity dependence and quantum yield (QY) of the upconverted emission were characterized for individual particles under a microscope and revealed a low threshold intensity (∼100 mW/cm²) as compared to the solution and moderate UC-QY (∼0.5%) in the NIR range. The factors contributing to the UC-QY were investigated by time-resolved and steady-state spectroscopies. It was found that the intersystem crossing of the sensitizer, triplet energy transfer, and TTA occurred efficiently in the binary solid, and the fluorescence QY of the emitter governed the UC-QY.

Ionization dynamics of a phenylenediamine derivative in solutions as revealed by femtosecond simultaneous and stepwise two-photon excitation

Femtosecond transient absorption spectroscopy with off-resonant simultaneous and resonant stepwise two-photon excitation methods were applied to the direct observation of photoionization dynamics of a phenylenediamine derivative in n-hexane, ethanol and acetonitrile solutions. Upon the selective excitation of the solute via the off-resonant two-photon excitation to the energy level almost equivalent with the ionization potential in the gas phase, rapid appearance of the radical cation (within ca. 100-200 fs) was observed in polar and nonpolar solutions. On the other hand, in the case where the excited energy level from the ground state is 0.8 eV lower than the ionization potential in the gas phase, the radical cation appears only in polar solutions in sub-ps to ps time scales, indicating that the photoionization does not occur directly from the highly electronically excited state even in the polar solution. Comparison of the dynamics between ethanol and acetonitrile solutions strongly suggested that the solvation process of the precursor species leading to the ionization took a crucial role in the electron ejection process with lower energy in polar solutions.

Direct observation of photoionization dynamics in solution phase induced by femtosecond two-photon excitation

In solution phase, the solute can be photo-ionized in the lower excitation energy than its ionization potential in gas phase. Therefore, the specific interaction is expected to be exist between the surrounding media and higher excited (Sn) state of the solute. In order to elucidate such polarization effect of solvent on the photoionization process, femtosecond double-pulse excitation was applied to direct detection of low-energy photoionization dynamics of a phenylenediamine derivative in solution phase. From the results of the transient absorption change, in polar solvent, it is clearly indicated that photoionization does not proceed directly from the Sn state, but through specific intermediate state. Moreover,

Cyclization reaction dynamics of an inverse type diarylethene derivative as revealed by time-resolved absorption and fluorescence spectroscopies

Photocyclization reaction dynamics of an inverse type diarylethene derivative was investigated in alkane solutions by means of ultrafast laser spectroscopies. Femtosecond transient absorption spectroscopy showed that the Franck-Condon state formed by photoexcitation is geometrically relaxed to a transient species within 100 fs and subsequently the cyclization process takes place with a time constant of 36 ps. This time constant is much longer than those in normal type derivatives. Steady-state and time-resolved fluorescence measurements with the aid of quantum chemical calculations revealed that there exist three kinds of conformers, one parallel and two anti-parallel forms, in the ground state. One of the anti-parallel conformers undergoes the cyclization reaction, while the other two conformers are nonreactive species and their major relaxation processes are radiative decay and intersystem crossing into the triplet states. The triplet states thus formed no longer undergo the cyclization reaction in the late time region.

Stepwise Two-Photon-Induced Electron Transfer from Higher Excited States of Noncovalently Bound Porphyrin-CdS/ZnS Core/Shell Nanocrystals

There has been an increasing amount of interest in stepwise two-photon-absorption (2PA)-induced photochemical reactions because of their extremely lower power thresholds compared to that of the simultaneous process and drastic reaction enhancements in some cases. However, stepwise 2PA-induced photochemical reactions were reported only in single chromophores and covalently bound bichromophores and there are few reports on these reactions in noncovalently bound systems because of weak electronic interactions among chromophores. This study demonstrated the stepwise 2PA-induced electron transfer from higher excited states in noncovalently bound protoporphyrin IX·CdS/ZnS core/shell nanocrystals (NCs). The electron transfer from higher excited states of porphyrin to CdS NCs successfully overcomes the activation barrier associated with the wide bandgap ZnS shell, indicating that a high reduction potential can be obtained with the stepwise 2PA process. The concept presented in this study can be applied to various noncovalently bound multichromophore systems to explore nonlinear photoresponses.

Multiphoton-gated cycloreversion reaction of a fluorescent diarylethene derivative as revealed by transient absorption spectroscopy

The one- and two-photon cycloreversion reactions of a fluorescent diarylethene derivative with oxidized benzothiophene moieties were investigated by means of ultrafast laser spectroscopy. Femtosecond transient absorption spectroscopy under the one-photon excitation condition revealed that the excited closed-ring isomer is simply deactivated into the initial ground state with a time constant of 2.6 ns without remarkable cycloreversion, the results of which are consistent with the very low cycloreversion reaction yield (<10-5) under steady-state light irradiation. On the other hand, an efficient cycloreversion reaction was observed under irradiation with a picosecond laser pulse at 532 nm. The excitation intensity dependence of the cycloreversion reaction indicates that a highly excited state attained by the stepwise two-photon absorption is responsible for the marked increase of the cycloreversion reaction, and the quantum yield at the highly excited state was estimated to be 0.018 from quantitative analysis, indicating that the reaction is enhanced by a factor of >1800.

Fluorescence On/Off Switching in Nanoparticles Consisting of Two Types of Diarylethenes

Single- and double-component nanoparticles consisting of two types of diarylethenes, 1,2-bis(3-methyl-5-phenyl-2-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene (2a), were fabricated by a reprecipitation method. Nanoparticles consisting of 1a exhibited orange or green fluorescence depending on the fabrication condition and did not undergo any photocyclization reaction. On the other hand, nanoparticles consisting of 2a underwent photoreversible photochromic reactions upon alternating irradiation with ultraviolet and visible light. Nanoparticles consisting of 1a and 2a exhibited fluorescence on/off switching with rapid switching speed and high on/off contrast, accompanying the photochromic reactions of 2a. The dependence of fluorescence on/off switching properties on Förster distance and molar fraction was observed and quantitatively evaluated by a simplified model.

Efficient Cycloreversion Reaction of a Diarylethene Derivative in Higher Excited States Attained by Off-Resonant Simultaneous Two-Photon Absorption

Off-resonant excitation of the closed-ring isomer of a photochromic diarylethene derivative at 730 nm induced the efficient cycloreversion reaction with a yield of ∼20%, while the reaction yield was only 2% under one-photon excitation at 365 nm. Excitation wavelength dependence of the one-photon cycloreversion reaction yield under steady-state irradiation in a wide wavelength range showed that the specific electronic state leading to the large cycloreversion reaction yield, which is originally forbidden in the optical transition but partially allowed owing to the low symmetry of the molecule, is spectrally overlapped with the electronic state accessible by the allowed one-photon optical transition in the UV region. Femtosecond transient absorption spectroscopy also revealed that the off-resonant two-photon excitation preferentially pumped the molecule into the specific state, leading to the 10-fold enhancement of the cycloreversion reaction.

Optical properties and solvatofluorochromism of fluorene derivatives bearing S,S-dioxidized thiophene

Fluorene derivatives having phenylthiophene or benzothiophene showed strong solvatofluorochromism by the oxidation of the thiophene ring.