Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene derivative

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene (fDAE) derivative were investigated by time-resolved absorption and fluorescence spectroscopies. Prescreening with quantum chemical calculation predicted that a derivative with methylthienyl groups (mt-fDAE) in the closed-ring isomer has a two-photon absorption cross-section larger than 1000 GM, which was experimentally verified by Z-scan measurements and excitation power dependence in transient absorption. Comparison of transient absorption spectra under one-photon and simultaneous two-photon excitation conditions revealed that the closed-ring isomer of mt-fDAE populated into higher excited states deactivates following three pathways on a timescale of ca. 200 fs: (i) the cycloreversion reaction more efficient than that by the one-photon process, (ii) internal conversion into the S1 state, and (iii) relaxation into a lower state (S1' state) different from the S1 state. Time-resolved fluorescence measurements demonstrated that this S1' state is relaxed to the S1 state with the large emission probability. These findings obtained in the present work contribute to extension of the ON-OFF switching capability of fDAE to the biological window and application to super-resolution fluorescence imaging in a two-photon manner.

Optically Distinguishable Electronic Spin-isomers of a Stable Organic Diradical

Herein, we introduce a model of electronic spin isomers, the electronic counterpart of nuclear spin isomers, by using a stable organic diradical. The diradical, composed of two benzotriazinyl radicals connected by a rigid triptycene skeleton, exhibits a small singlet-triplet energy gap of -3.0 kJ/mol, indicating ca. 1:1 coexistence of the two spin states at room temperature. The diradical shows characteristic near-IR absorption bands, which are absent in the corresponding monoradical subunit. Variable temperature measurements revealed that the absorbance of the NIR band depends on the abundance of the singlet state, allowing us to identify the NIR band as the singlet-specific absorption band. It enables photoexcitation of one of the two spin states coexisting in thermal equilibrium. Transient absorption spectroscopy disclosed that the two spin states independently follow qualitatively different excited-state dynamics. These results demonstrate a novel approach to the design and study of electronic spin isomers based on organic diradicals.

Site-Selective Photo-Crosslinking of Stilbene Pairs in a DNA Duplex Mediated by Ruthenium Photocatalyst

We herein report a method for site-selective photo-crosslinking of a DNA duplex. A stilbene pair was introduced into a DNA duplex and a ruthenium complex was conjugated with a triplex-forming oligonucleotide. We demonstrated that [2+2] photocycloaddition of the stilbene pair occurred upon irradiation with visible light when the ruthenium complex was in close proximity due to triplex formation. No reaction occurred when the ruthenium complex was not in proximity to the stilbene pair. The wavelength of visible light used was of lower energy than the wavelength of UV light necessary for direct excitation of stilbene. Quantum chemical calculation indicated that ruthenium complex catalyzed the photocycloaddition via triplet-triplet energy transfer. Site selectivity of this photo-crosslinking system was evaluated using a DNA duplex bearing two stilbene pairs as a substrate; we showed that the site of crosslinking was precisely regulated by the sequence of the oligonucleotide linked to the ruthenium complex. Since this method does not require orthogonal photoresponsive molecules, it will be useful in construction of complex photoresponsive DNA circuits, nanodevices and biological tools.

Impact of Ring-Closing on the Photophysical Properties of One-Dimensional π-Conjugated Molecular Aggregate

The self-assembled state of molecules plays a pivotal role in determining how inherent molecular properties transform and give rise to supramolecular functionalities and has long attracted attention. However, understanding the influence of morphologies spanning the nano- to mesoscopic scales of supramolecular assemblies derived from identical intermolecular interactions has been notoriously challenging due to dynamic structural change and monomer exchange of assemblies in solution. In this study, we demonstrate that curved one-dimensional molecular assemblies (supramolecular polymers) of lengths of around 70-200 nm, originating from the same luminescent molecule, exhibit distinct photoluminescent properties when they form closed circular structures (toroids) versus when they possess chain termini in solution (random coils). By exploiting the difference in kinetic stability between the toroids and random coils, we developed a dialysis protocol to selectively purify the former. It was revealed that these terminus-free closed structures manifest higher energy and more efficient luminescence compared with their mixed state with random coils. Time-resolved fluorescence measurements unveiled that random coils, due to their dynamic structural fluctuation in solution, generate local defects throughout the main chain, leading to luminescence from lower energy levels. In mixtures of the two assemblies, luminescence was exclusively observed from such a lower energy level of random coils, a result attributed to energy transfer between the assemblies. This work emphasizes that for identical supramolecular assemblies, only averaged properties have traditionally been considered, but their structures at the nano- to mesoscopic scale are important especially if they have a certain degree of shape persistency even in solution.

Gaining control on optical force by the stimulated-emission resonance effect

The resonance between an electronic transition of a micro/nanoscale object and an incident photon flux can modify the radiation force exerted on that object, especially at an interface. It has been theoretically proposed that a non-linear stimulated emission process can also induce an optical force, however its direction will be opposite to conventional photon scattering/absorption processes. In this work, we experimentally and theoretically demonstrate that a stimulated emission process can induce a repulsive pulling optical force on a single trapped dye-doped particle. Moreover, we successfully integrate both attractive pushing (excited state absorption) and repulsive pulling (stimulated emission) resonance forces to control the overall exerted optical force on an object, validating the proposed non-linear optical resonance theory. Indeed, the results presented here will enable the optical manipulation of the exerted optical force with exquisite control and ultimately enable single particle manipulation.

Cage-Like Sodalite-Type Porous Organic Salts Enabling Luminescent Molecule's Incorporation and Room-temperature Phosphorescence Induction in Air

Expression of room-temperature phosphorescence (RTP) in organic materials requires complicated molecular design and specific intermolecular interactions, and therefore types of RTP materials are restricted. This work presents cage-like sodalite-type porous organic salts (s-POSs) as host materials for luminescent molecules to induce RTP, using tetrasulfonic acid with an adamantane core and triphenylmethylamines that are modified with substituents in the para-positions of benzene rings (TPMA-X). By adding a representative luminescent molecule (pyrene) to a reaction solution during construction of s-POSs, the molecule is incorporated in a facile manner. s-POSs with a heavy halogen atom (X: Iodine) on the pore surface give heavy atom effects, suppression of thermal vibration, and protection from oxygen, for the incorporated molecule, which induce its RTP even in air. This strategy can be applied to various luminescent molecules, which may lead to the achievement of RTP of various colors.

Photoinduced crystal melting with luminescence evolution based on conformational isomerisation

The phenomenon of crystal melting by light irradiation, known as photo-induced crystal-to-liquid transition (PCLT), can dramatically change material properties with high spatiotemporal resolution. However, the diversity of compounds exhibiting PCLT is severely limited, which hampers further functionalisation of PCLT-active materials and the fundamental understandings of PCLT. Here, we report on heteroaromatic 1,2-diketones as the new class of PCLT-active compounds, whose PCLT is based on conformational isomerisation. In particular, one of the diketones demonstrates luminescence evolution prior to crystal melting. Thus, the diketone crystal exhibits dynamic multistep changes in the luminescence colour and intensity during continuous ultraviolet irradiation. This luminescence evolution can be ascribed to the sequential PCLT processes of crystal loosening and conformational isomerisation before macroscopic melting. Single-crystal X-ray structural analysis, thermal analysis, and theoretical calculations of two PCLT-active and one inactive diketones revealed weaker intermolecular interactions for the PCLT-active crystals. In particular, we observed a characteristic packing motif for the PCLT-active crystals, consisting of an ordered layer of diketone core and a disordered layer of triisopropylsilyl moieties. Our results demonstrate the integration of photofunction with PCLT, provide fundamental insights into the melting process of molecular crystals, and will diversify the molecular design of PCLT-active materials beyond classical photochromic scaffolds such as azobenzenes.

A Kinetically Stabilized Nitrogen-Doped Triangulene Cation: Stable and NIR Fluorescent Diradical Cation with Triplet Ground State

A kinetically-stabilized nitrogen-doped triangulene cation derivative has been synthesized and isolated as the stable diradical with a triplet ground state that exhibits near-infrared emission. As was the case for a triangulene derivative we previously synthesized, the triplet ground state with a large singlet-triplet energy gap was experimentally confirmed by magnetic measurements. In contrast to the triangulene derivative, the nitrogen-doped triangulene cation derivative is highly stable even in solution under air and exhibits near-infrared absorption and emission because the alternancy symmetry of triangulene is broken by the nitrogen cation. Breaking the alternancy symmetry of triplet alternant hydrocarbon diradicals by a nitrogen cation would therefore be an effective strategy to create stable diradicals possessing magnetic properties similar to the parent hydrocarbons but with different electrochemical and photophysical properties.

s-Indacene Revisited: Modular Synthesis and Modulation of Structures and Molecular Orbitals of Hexaaryl Derivatives

Though s-indacene is an intriguing antiaromatic hydrocarbon of 12 π-electrons, it has been underrepresented due to the lack of efficient and versatile methods to prepare stable derivatives. Herein we report a concise and modular synthetic method for hexaaryl-s-indacene derivatives bearing electron-donating/-accepting groups at specific positions to furnish C_2h-, D_2h-, and C_2v-symmetric substitution patterns. We also report the effects of substituents on their molecular structures, frontier molecular orbital (MO) levels, and magnetically induced ring current tropicities. Both theoretical calculations and X-ray structure analyses indicate that the derivatives of the C_2h-substitution pattern adopt different C_2h structures with significant bond length alternation depending on the electronic property of the substituents. Due to the nonuniform distribution of the frontier MOs, their energy levels are selectively modulated by the electron-donating substituents. This leads to the inversion of the HOMO and HOMO-1 sequences with respect to those of the intrinsic s-indacene as theoretically predicted and experimentally proven by the absorption spectra at visible and near-infrared regions. The NICS values and the ¹H NMR chemical shifts of the s-indacene derivatives indicate their weak antiaromaticity. The different tropicities are explained by the modulation of the HOMO and HOMO-1 levels. In addition, for the hexaxylyl derivative, weak fluorescence from the S₂ excited state was detected due to the large energy gap between the S₁ and S₂ states. Notably, an organic field-effect transistor (OFET) fabricated using the hexaxylyl derivative exhibited moderate hole carrier mobility, a result which opens the door for optoelectronic applications of s-indacene derivatives.

Edge-to-Center Propagation of Photochemical Reaction during Single-Crystal-to-Single-Crystal Photomechanical Transformation of 2,5-Distyrylpyrazine Crystals

Photomechanical molecular crystals are promising materials for photon-powered artificial actuators. To interpret the photomechanical responses, the spatiotemporal distribution of photoproducts in crystals could be an important role in addition to molecular structures, molecular packings, illumination conditions, crystal morphology, crystal size, and so on. In this study, we have found that single crystals of 2,5-distyrylpyrazine show a smooth single-crystal-to-single-crystal photomechanical expansion, and the photochemical reaction propagates from the edge to the center of the single crystal. We revealed that the surface effect (special reactivity at the crystal surface) in addition to the cooperative effect (the reaction is facilitated by neighboring molecules) is essential for the edge-to-center propagation of the photochemical reaction. Our results would provide a foundation for future studies of the photochemical reaction dynamics in photomechanical molecular crystals.

Impact of Hydrophobic/Hydrophilic Balance on Aggregation Pathways, Morphologies, and Excited-State Dynamics of Amphiphilic Diketopyrrolopyrrole Dyes in Aqueous Media

We report the thermodynamic and kinetic aqueous self-assembly of a series of amide-functionalized dithienyldiketopyrrolopyrroles (TDPPs) that bear various hydrophilic oligoethylene glycol (OEG) and hydrophobic alkyl chains. Spectroscopic and microscopic studies showed that the TDPP-based amphiphiles with an octyl group form sheet-like aggregates with J-type exciton coupling. The effect of the alkyl chains on the aggregated structure and the internal molecular orientation was examined via computational studies combining MD simulations and TD-DFT calculations. Furthermore, solvent and thermal denaturation experiments provided a state diagram that indicates the formation of unexpected nanoparticles during the self-assembly into nanosheets when longer OEG side chains are introduced. A kinetic analysis revealed that the nanoparticles were obtained selectively as an on-pathway intermediate state toward the formation of thermodynamically controlled nanosheets. The metastable aggregates were used for seed-initiated supramolecular assembly, which allowed establishing control over the assembly kinetics and the aggregate size. The sheet-like aggregates prepared using the seeding method exhibited coherent vibration in the excited state, indicating a well-ordered orientation of the TDPP units. These results underline the significance of fine tuning of the hydrophobic/hydrophilic balance in the molecular design to kinetically control the assembly of amphiphilic π-conjugated molecules into two-dimensional nanostructures in aqueous media.

All-Visible (>500 nm)-Light-Induced Diarylethene Photochromism Based on Multiplicity Conversion via Intramolecular Energy Transfer

Photoswitching molecules that reversibly switch upon visible-light irradiation are some of the most attractive targets for biological and imaging applications. In this study, we found a diarylethene (DAE) derivative having a covalently attached perylenebisimide (PBI) unit (DAE-PBI dyad) underwent an unexpected cyclization reaction upon irradiation with green (500-550 nm) light, where the DAE unit has no absorbance. The photoreactivity was enhanced in solvents containing heavy atoms and in the presence of oxygen. As inferred from the solvent dependence and the calculated excited-state energies of DAE and PBI units, it was suggested that the probable mechanism for this unique visible-light-induced cyclization reaction is multiplicity conversion based on intramolecular energy transfer from the excited singlet state of the PBI unit to the triplet state of DAE units (i.e., DAE-¹[PBI]* → ³[DAE]*-PBI). Such a unique photoreaction mechanism with the assistance of oxygen will pave the way for new molecular design for the development of visible-light switching molecules.

Doubly linked chiral phenanthrene oligomers for homogeneously π-extended helicenes with large effective conjugation length

Helically twisted conductive nanocarbon materials are applicable to optoelectronic and electromagnetic molecular devices working on the nanometer scale. Herein, we report the synthesis of per-peri-perbenzo[5]- and [9]helicenes in addition to previously reported π-extended [7]helicene. The homogeneously π-extended helicenes can be regarded as helically fused oligo-phenanthrenes. The HOMO−LUMO gap decreased significantly from 2.14 to 1.15 eV with increasing helical length, suggesting the large effective conjugation length (ECL) of the π-extended helical framework. The large ECL of π-extended helicenes is attributed to the large orbital interactions between the phenanthrene subunits at the 9- and 10-positions, which form a polyene-like electronic structure. Based on the experimental results and DFT calculations, the ultrafast decay dynamics on the sub-picosecond timescale were attributed to the low-lying conical intersection.

Helically twisted conductive nanocarbon materials are applicable to optoelectronic and electromagnetic molecular devices but the design of nanocarbons with an absorption edge in the low energy region is challenging. Here, the authors report the synthesis of a helically fused oligophenanthrenes and demonstrate an increased effective conjugation length leading to an absorption edge in the NIR region.

Fluorescent supramolecular polymers of barbiturate dyes with thiophene-cored twisted π-systems

Because supramolecular polymerization of emissive π-conjugated molecules depends strongly on π-π stacking interaction, the formation of well-defined one-dimensional nanostructures often results in a decrease or only a small increase of emission efficiency. This is also true for our barbiturate-based supramolecular polymers wherein hydrogen-bonded rosettes of barbiturates stack quasi-one-dimensionally through π-π stacking interaction. Herein we report supramolecular polymerization-induced emission of two regioisomeric 2,3-diphenylthiophene derivatives functionalized with barbituric acid and tri(dodecyloxy)benzyl wedge units. In CHCl3, both compounds are molecularly dissolved and accordingly poorly emissive due to a torsion-induced non-radiative decay. In methylcyclohexane-rich conditions, these barbiturates self-assemble to form crystalline nanofibers and exhibit strongly enhanced emission through supramolecular polymerization driven by hydrogen-bonding. Our structural analysis suggests that the barbiturates form a tape-like hydrogen-bonding motif, which is rationalized by considering that the twisted geometries of 2,3-diphenylthiophene cores prevend the competing rosettes from stacking into columnar supramolecular polymers. We also found that a small difference in the molecular polarity originating from the substitutional position of the thiophene core influences interchain association of the supramolecular polymers, affording different luminescent soft materials, gel and nanosheet.

Femtosecond Dynamics of Stepwise Two-Photon Ionization in Solutions as Revealed by Pump-Repump-Probe Detection with Burst Mode of Photoexcitation

Pump-repump-probe spectroscopy with a burst mode of photoexcitation was applied to the direct observation of photoionization dynamics of perylene in the solution phase. The irradiation of the pump pulse train...

1,2,3-Tri(9-anthryl)benzene: Photophysical Properties and Solid State Intermolecular Interactions of Radially Arranged, Congested Aromatic π-Planes

We report the Negishi coupling based synthesis of 1,2,3-tri(9-anthryl)benzene derivatives, containing three radially arranged anthracenes in a π-cluster. In the crystalline state of the unsubstituted derivative, intermolecular π-π and CH-π interactions between the anthracene units drive the formation of a two-dimensional packing structure. Owing to though-space π-conjugation between anthracene units, the substances have unique electronic properties. The excited state dynamic behavior occurring between the three radially arranged anthracene moieties, such as exciton localization/delocalization, was elucidated by means of transient absorption measurements and quantum chemical calculations. Interestingly, even though the three anthracenes are closely oriented with a ca. 3.0 Å distances between their C-9 positions, exciton localization on two anthracene units is energetically favorable because of the flexible nature of the radially arranged aromatic rings.

Direct determination of molar absorption coefficients of several molecules in the lowest excited singlet states

Molar absorption coefficient of the lowest excited state is an indispensable information for the quantitative investigation of photochemical reactions by means of transient absorption spectroscopy. In the present work, we quantitatively estimated the molar absorption coefficients of the S₁ state of the solute in three solution systems, Rhodamine B in ethanol, ZnTPP in DMF and N,N'-bis(2,6-diisopropylphenyl)terrylene-3,4,11,12-tetracarboxydiimide (TDI) in chloroform, by perfectly bleaching the ground state molecules using the picosecond 532-nm laser pulse with a large number of photons. These solution systems were selected because no obvious photodegradation was detected in the present range of the excitation intensity. The molar absorption coefficient obtained by this method was verified by the numerical analysis of the excitation intensity dependence of the transient absorbance by taking into account the inner filter effect (absorption of the excitation light by the S₁ state produced by the leading part of the pump pulse) and the decrease of the ground state molecules by the pump process (depletion). In addition, these molar absorption coefficients were confirmed by the comparison of relations between the excitation intensity and the transient absorbance of the S₁ state under the condition where the fraction of the excited solute is ≪ 10% by the femtosecond pulsed laser excitation. From these results, the error of the molar absorption coefficients was estimated to be < 5%. These values can be used as reference ones for the estimation of molar absorption coefficients of other systems leading to the quantitative elucidation of the photochemical reactions detected by the transient absorption spectroscopy.

Molecular crystalline capsules that release their contents by light

Here, we present single crystalline capsules of a photoresponsive molecule produced by simple recrystallization from organic solutions without direct human processing. During the crystal growth process, a movie was taken of the capsule taking in the organic solution. The capsules responded rapidly (<1 s) to the UV light stimuli and released the captured solution or solute. In principle, they can take in any substance dissolved in organic solvents, and their size can be controlled. Moreover, the capsule can be broken by multi-photon excitation using a near-infrared laser within the biological window. Furthermore, because the molecular packing in the crystal is unidirectional, the response can be controlled by the polarization of the light. This study shows the new potential of photoresponsive molecules.

Slow photoionization via higher excited states of N,N-dimethylaniline in ethanol solution probed by femtosecond transient absorption spectroscopy under two-pulse two-photon excitation

Photoionization dynamics of N,N-dimethylaniline (DMA) from highly electronically excited states in ethanol solution was investigated by means of femtosecond two-pulse two-photon excitation transient absorption (2PE-TA) spectroscopy. The first pump pulse prepares the lowest singlet excited state (S₁ state) of DMA, and the second one excites the S₁ state into higher excited states. In the case with the second pulse at 500 nm, the ionization took place via a rapid channel (<100 fs) and a slow one with the time constant of ∼10 ps. The excitation wavelength effect of the second pulse indicated that a specific electronic state produced directly from higher excited states was responsible for the slow ionization. By integrating these results with the time evolution of the transient absorption spectra of the solvated electron in neat ethanol detected by the simultaneous two-photon excitation, it was revealed that the slow ionization of DMA in ethanol was regulated by the formation of the anionic species just before the completion of the solvation of the electron, leading to the solvated electron in the relaxed state. From these results, it was strongly suggested that the capture of the electron of the Rydberg-like state by the solvent or solvent cluster regulates the appearance of the cation radical.

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.