Dynamics and Mechanisms of the Multiphoton Gated Photochromic Reaction of Diarylethene Derivatives

Relaxation dynamics of higher excited states of perylene-substituted perylene bisimide derivatives

Stepwise two-photon absorption processes have received considerable attention, especially in photocatalysis, due to their relatively lower power threshold, characteristic spatial selectivity, amplification of chemical reactions, and so on. Meanwhile, studies on the relaxation dynamics of higher excited states in condensed systems have been limited for several molecular systems due to the short-lived nature of these states. In this study, we synthesized perylene-substituted perylene bisimide (PBI) and its derivate as model compounds and investigated their excited-state dynamics, including higher excited states, using pump-repump-probe spectroscopy. We revealed that these molecules form charge-transfer (CT) states instantaneously after the excitation, regardless of whether it is the perylene moiety or the PBI moiety that is excited. The lifetime of the CT state was shorter when the distance between the donor (perylene) and the acceptor (PBI) was shorter. Moreover, we also revealed that a higher-lying CT state generated by the stepwise excitation of the CT state using a 740-nm pulse induced Stark effect to the neighboring perylene moiety. The Stark effect not only gives more detailed information about the CT state, but also presents the possibility of new photofunctions, such as instantaneous modulation of the electronic state to achieve optimal electronic properties. These insights contribute to understanding advanced photochemical reactions and would be important for exploring photocatalytic reactions involving higher excited states.

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.

Bridged-Imidazole Dimer Exhibiting Three-State Negative Photochromism with a Single Photochromic Unit

Photochromic molecules that can exhibit multiple states of photochromism in a single photochromic unit are considered more attractive than traditional bistable photochromic molecules because they can offer more versatility and control in photoresponsive systems. We have synthesized a negative photochromic 1-(1-naphthyl)pyrenyl-bridged imidazole dimer (NPy-ImD) that has three different isomers: a colorless isomer, 6MR, a blue-colored isomer, 5MR-B, and a red-colored isomer, 5MR-R. NPy-ImD can interconvert between these isomers via a short-lived transient biradical, BR, upon photoirradiation. 5MR-R is the most stable isomer, and the energy levels of 6MR, 5MR-B, and BR are relatively close to each other. The colored isomers 5MR-R and 5MR-B are photochemically isomerized to 6MR via the short-lived BR upon irradiation with blue light and red light, respectively. The absorption bands of 5MR-R and 5MR-B are well separated by more than 150 nm, with a small overlap, which means they can be selectively excited with different light sources, visible light for 5MR-R and NIR light for 5MR-B. The colorless isomer 6MR is formed from the short-lived BR through a kinetically controlled reaction. 6MR and 5MR-B can then be converted to the more stable isomer 5MR-R through a thermodynamically controlled reaction, which is facilitated by the thermally accessible intermediate, BR. Notably, 5MR-R photoisomerizes to 6MR when irradiated with CW-UV light, whereas it photoisomerizes to 5MR-B by a two-photon process when irradiated with nanosecond UV laser pulses.

Multiphoton Control of 6π Photocyclization via State-Dependent Reactant-Product Correlations

Multiphoton excitation promises opportunities for opening new photochemical reaction pathways and controlling photoproduct distributions. We demonstrate photonic control of the 6π photocyclization of ortho-terphenyl to make 4a,4b-dihydrotriphenylene (DHT). Using pump-repump-probe spectroscopy we show that 1 + 1' excitation to a high-lying reactant electronic state generates a metastable species characterized by a red absorption feature that accompanies a repump-induced depletion in the one-photon trans-dihydro product (trans-DHT); signatures of the new photoproduct are clearer for a structural analogue of the reactant that is sterically inhibited against one-photon cyclization. Quantum-chemical computations support assignment of this species to cis-DHT, which is accessible photochemically along a disrotatory coordinate from high-lying electronic states reached by 1 + 1' excitation. We use time-resolved spectroscopy to track photochemical dynamics producing cis-DHT. In total, we demonstrate that selective multiphoton excitation opens a new photoreaction channel in these photocyclizing reactants by taking advantage of state-dependent correlations between reactant and product electronic states.

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.

Green and far-red-light induced electron injection from perylene bisimide to wide bandgap semiconductor nanocrystals with stepwise two-photon absorption process

Stepwise two-photon absorption (2PA) processes are becoming an important technique because it can achieve high reductive photochemical reactions with visible and near infrared light and intensity-gated high spatiotemporal selectivity with much lower power thresholds than those of the simultaneous 2PA. However, excited states generated by stepwise 2PA (higher excited states and excited states of transient species) are so short-lived that the efficiency for the stepwise 2PA induced photochemical reactions is usually quite low, which limits the versatility for this technique. Here, we demonstrated that the electron of the higher excited state can be efficiently extracted in a nanohybrid of organic molecules and wide bandgap semiconductor nanocrystals (NCs). Using perylene bisimide (PBI)-coordinated CdS NCs as a model compound, we demonstrated that the electron of the higher excited state of PBI generated by stepwise 2PA can be extracted to the conduction band of CdS NCs with a quantum yield of ∼0.5-0.7. Moreover, the extracted electron survives at the conduction band of CdS NCs over nanoseconds, which is more than hundred times longer than the lifetime of the S2 state of PBI. This method can be applied to other organic molecules and larger wide bandgap semiconductors, and therefore, will expand the versatility for the photochemical reactions utilizing the short-lived excited states.

A monomolecular platform with varying gated photochromism

In the development of modern high-performance photoelectric materials, the gated photochromic materials have attracted wide attention. However, the integration of varying signal regulations into gated photochromism to construct efficient photochromic materials is still an urgent necessity. Herein, we designed and synthesized a new gated photoswitching DTEP based on a Schiff base with a diarylethene core. The photochromic properties of compound DTEP can be regulated to different degrees by multiple stimuli, including UV/visible light, Cu²⁺ and Ni²⁺. The compound DTEP showed different response abilities to Cu²⁺ and Ni²⁺, due to the diverse complexation modes between DTEP and Cu²⁺ as well as Ni²⁺. The photochromic properties of compound DTEP could be inhibited completely by the introduction of Cu²⁺ to form a 1 : 1 complexation, while the weak gated photochromism could be found from the DTEP–Ni²⁺ complex in a 1 : 2 stoichiometry. Relying on such varying degrees of gated photochromic properties, a new molecular logic circuit was constructed to undertake complicated logical operations.

A strategy to realize varying degrees of gated photochromic properties by coordinating with different metal ions within one unimolecular system was devised to achieve the construction of a logic circuit for multi-functional molecular switching.

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.

Nanosecond laser photothermal effect-triggered amplification of photochromic reactions in diarylethene nanoparticles

Intense ns pulse laser excitation to nanoparticle colloids of a photochromic diarylethene induced an amplified cycloreversion reaction. The mechanism was explained as a 'photosynergetic response' coupled with nanoscale laser heating and the photochemical reaction in nanoparticles.

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.

Core structure dependence of cycloreversion dynamics in diarylethene analogs

Increased core rigidity in diarylethene-type photoswitches results in shallower excited-state potential energy surfaces and faster funneling towards the conical intersections from which cycloreversion and nonreactive deactivation occur.

Tailoring the Mechanical Properties of 3D Microstructures Using Visible Light Post-Manufacturing

The photochemistry of anthracene, a new class of photoresist for direct laser writing, is used to enable visible-light-gated control over the mechanical properties of 3D microstructures post-manufacturing. The mechanical and viscoelastic properties (hardness, complex elastic modulus, and loss factor) of the microstructures are measured over the course of irradiation via dynamic mechanical analysis on the nanoscale. Irradiation of the microstructures leads to a strong hardening and stiffening effect due to the generation of additional crosslinks through the photodimerization of the anthracene functionalities. A relationship between the loss of fluorescence-a consequence of the photodimerization-and changes in the mechanical properties is established. The fluorescence thus serves as a proxy read-out for the mechanical properties. These photoresponsive microstructures can potentially be used as "mechanical blank slates": their mechanical properties can be readily adjusted using visible light to serve the demands of different applications and read out using their fluorescence.

Exploring the ultrafast dynamics of a diarylethene derivative using sub-10 fs laser pulses

The fast internal conversion S₁ → S₀ of a diarylethenes photoswitch, facilitated by two vibrational stretching modes, results in a low quantum yield of the ring-opening reaction.

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.

New insights into the photoswitching mechanisms of normal dithienylethenes

The photoswitching and competitive processes of the referent photochromic diarylethene derivative 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (DTE) and a novel bridged analog DTE-m5 have been investigated by state-of-the-art TD-DFT calculations and ultrafast spectroscopy supported by advanced chemometric data treatments. Focusing on DTE, the overall deactivation pathway of both antiparallel (AP) and parallel (P) conformers of the open form (OF) (1 : 1 in solution) has been resolved and rationalized starting from the Franck-Condon (FC) region to the ground state recovery. For the photo-excited P conformer, after ultrafast relaxation (∼200 fs) towards the S₁ relaxed state, an expected ISC occurred (55 ps) to produce a triplet state, ³P, the latter relaxing within 2.5 μs. Concerning the AP conformer, the photocyclization reaction is reported to proceed immediately (100 fs) starting from the FC region while the relaxed singlet state is populated in parallel. For the first time, we discovered that the latter state evolves through an unexpected ISC process (1 ps) giving rise to a second triplet state,³AP. For DTE-m5, by slightly constraining the molecule with the bridge, this triplet becomes reactive and participates in the formation of 10% of closed form (CF) probably through an adiabatic mechanism. Concerning the photoreversion, in accordance with the literature, we report on a two-step process, a 190 fs vibrational relaxation followed by a 6 ps ring-opening reaction. For the overall species at the singlet or triplet manifold, the use of advanced MCR-ALS allows us to obtain specific spectral signatures. This study is therefore a new step within the comprehension of DTE photochemistry.

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.

Light penetration-coupled photoisomerization modeling for photodeformation of diarylethene single crystal: upscaling isomerization to macroscopic deformation

Diarylethene is one of the photo-responsive materials that show rapid and reversible changes in their color/electrochemical properties and macroscopic deformations in the crystalline phase by light irradiation. Photoisomerization is the main cause of the photo reactivity of diarylethene, and we established a statistical model based on the density matrix formalism, which predicts quantitative isomerization progress as a population term. The model reflects photo-switching properties of the target molecule, which were characterized by first principle calculations, and external stimulus factors (light irradiation conditions and temperature). By merging light penetration physics with the model, we derived light penetration depth dependent isomerization progress to theoretically investigate photodeformation of single crystal. The model well reproduced in-plane shear deformation under ultraviolet light irradiation which would provide guideline for photoactuator design. In addition, the statistical model addressed crucial findings (primary stimuli and molecular design parameter for increasing the isomerization rate, external stimuli enhancing fluorescence performance) itself.

Photochromic Mechanism of a Bridged Diarylethene: Combined Electronic Structure Calculations and Nonadiabatic Dynamics Simulations

Intramolecularly bridged diarylethenes exhibit improved photocyclization quantum yields because the anti-syn isomerization that originally suppresses photocyclization in classical diarylethenes is blocked. Experimentally, three possible channels have been proposed to interpret experimental observation, but many details of photochromic mechanism remain ambiguous. In this work we have employed a series of electronic structure methods (OM2/MRCI, DFT, TDDFT, RI-CC2, DFT/MRCI, and CASPT2) to comprehensively study excited state properties, photocyclization, and photoreversion dynamics of 1,2-dicyano[2,2]metacyclophan-1-ene. On the basis of optimized stationary points and minimum-energy conical intersections, we have refined experimentally proposed photochromic mechanism. Only an S₁/S₀ minimum-energy conical intersection is located; thus, we can exclude the third channel experimentally proposed. In addition, we find that both photocyclization and photoreversion processes use the same S₁/S₀ conical intersection to decay the S₁ system to the S₀ state, so we can unify the remaining two channels into one. These new insights are verified by our OM2/MRCI nonadiabatic dynamics simulations. The S₁ excited-state lifetimes of photocyclization and photoreversion are estimated to be 349 and 453 fs, respectively, which are close to experimentally measured values: 240 ± 60 and 250 fs in acetonitrile solution. The present study not only interprets experimental observations and refines previously proposed mechanism but also provides new physical insights that are valuable for future experiments.

Fast Photochromic Molecules toward Realization of Photosynergetic Effects

There has been a growing interest toward the development of advanced photofunctional materials whose photoresponses involve multiple photons and molecules because these materials show the photoresponses which cannot be achieved by a one-photon reaction of a single chromophore. These cooperative interactions of multiple photons and molecules are recently termed as the "photosynergetic" effects, and the understanding and utilization of these effects are becoming important research topics. In this Perspective, we overview the recent progress of the fast T-type photochromic molecules involving the stepwise two-photon absorption processes. Although high power pulse lasers were necessary to induce conventional simultaneous and stepwise two-photon absorption processes, the stepwise two-photon absorption process with the fast photochromic compound can be initiated by extremely weak continuous wave (CW) LEDs. The basic concept and future outlook of the fast photochromism involving the stepwise two-photon absorption process will be discussed.

Linkage Isomerization via Geminate Cage or Bimolecular Mechanisms: Time-Resolved Investigations of an Organometallic Photochrome

The extent of the photoinitiated linkage isomerization of dicarbonyl(3-cyanomethylpyridine-κN)(η(5)-methylcyclopentadienyl)manganese (4) to dicarbonyl(3-cyano-κN-methylpyridine)(η(5)-methylcyclopentadienyl)manganese (5) was examined by time-resolved infrared spectroscopy on picosecond to microsecond time scales in room temperature isooctane to determine the extent the isomerization occurs as a geminate cage rearrangement. We previously reported that a substantial part of the conversion between 4 and 5 must be a bimolecular reaction between a solvent coordinated dicarbonyl(η(5)-methylcyclopentadienyl)manganese (3) and uncoordinated 3-cyanomethylpyridine. For the purpose of designing a molecular device, it would be desirable for the photoisomerization to occur in a geminate cage reaction, because the faster the isomerization, the less opportunity for side reactions to occur. In this study, assignments of transients are identified by comparison with transients observed for model reactions. Within 100 μs after photolysis of 4 in isooctane, no 5 is observed. Instead, the solvent coordinated 3 is observed within 25 ps after irradiation. The formation of 5 is observed only in the presence of 9 mM 3-cyanomethylpyridine but not until 10-50 μs after irradiation of 4. Within the limits of detection, these results indicate the conversion of 4 to 5 occurs exclusively via a bimolecular reaction of 3-cyanomethylpyridine with solvent coordinated 3 and not a geminate cage reaction between 3-cyanomethylpyridine and the dicarbonyl(η(5)-methylcyclopentadienyl)manganese fragment.

Cycloreversion dynamics of a photochromic molecular switch via one-photon and sequential two-photon excitation

Ultrafast pump-probe (PP) and pump-repump-probe (PReP) measurements examine the ring-opening reaction of a photochromic molecular switch following excitation to the first and higher excited states. Sequential two-photon excitation is a sensitive probe of the excited-state dynamics, because the secondary excitation maps the progress along the S1 reaction coordinate onto the higher excited states of the molecule. In this contribution, secondary excitation at 800 nm accesses more reactive regions of the excited-state potential energy surfaces than are accessible with direct vertical excitation in the visible or UV. The quantum yield for cycloreversion increases by a factor of 3.5 ± 0.9 compared with one-photon excitation when the delay between the 500 nm pump and 800 nm repump laser pulses increases beyond ~100 fs, in contrast with a slower ~3 ps increase that was previously observed for one-color sequential excitation at 500 nm. The evolution of an excited-state absorption band reveals the dynamics of the higher-lying excited state for both one-photon excitation in the UV and sequential two-photon excitation. The spectroscopy and dynamics of the higher-lying excited state are similar for both excitation pathways, including a lifetime of ~100 fs. The complementary PP and PReP measurements provide a detailed picture of the ultrafast excited-state dynamics, including new insight on the role of excited states above S1 in controlling the photochemical cycloreversion reaction.

Competitive direct vs. indirect photochromism dynamics of constrained inverse dithienylethene molecules

Relaxation observed through several parallel pathways from the first excited state to the ground-state in inverse dithienylethene molecules.

Chemical control of photochromism and a multiresponsive molecular switch based on a diarylethene derivative containing naphthol

A diarylethene derivative with a naphthol group was synthesized, and its photochromic reaction could be regulated by chemical inputs.

Potential energy surfaces and quantum yields for photochromic diarylethene reactions

Photochromic diarylethenes (DAEs) are among the most promising molecular switching systems for future molecular electronics. Numerous derivatives have been synthesized recently, and experimental quantum yields (QYs) have been reported for two categories of them. Although the QY is one of the most important properties in various applications, it is also the most difficult property to predict before a molecule is actually synthesized. We have previously reported preliminary theoretical studies on what determines the QYs in both categories of DAE derivatives. Here, reflecting theoretical analyses of potential energy surfaces and recent experimental results, a rational explanation of the general guiding principle for QY design is presented for future molecular design.

Cross-term selective, two-pulse correlation measurements by phase-shifted parallel modulation for analysis of a multi-photon process

We propose a new modulation technique for phase-sensitive detection to extract a cross-term signal appearing in multi-pulse experiments and demonstrate the selective detection of cross-term signal on two-pulse correlation measurements. This modulation technique can be widely applied to the selective detection in multi-beam laser experiments such as two-pulse correlation, auto-correlation, and double-resonance experiments, because the selectivity can be realized by using a conventional single-frequency mechanical chopper and 2f phase-sensitive detection. The effectiveness of the proposed technique was confirmed by two-pulse correlation measurements of two-photon-excited fluorescence from rhodamine 6G. In addition to the selective observation of a correlation peak, the technique succeeded in observing a weak correlation background. By combining asymmetric, two-pulse correlation measurements with the proposed technique, it was clarified that the background was produced by a three-photon process and assigned to fluorescence depletion caused by an up-conversion process. The results indicate that the cross-term selective, two-pulse correlation method that is based on the phase-shifted parallel modulation technique is powerful tool to find and analyze a high-order optical event buried in a low-order optical event.

Controlling the Excited-State Reaction Dynamics of a Photochromic Molecular Switch with Sequential Two-Photon Excitation

Sequential two-photon excitation increases the cycloreversion yield of a diarylethene-type photochromic molecular switch compared with one-photon excitation. This letter shows for the first time that an optimal delay of ∼5 ps between primary and secondary excitation events gives the largest enhancement of the ring-closing reaction. Pump-probe (PP) and pump-repump-probe (PReP) measurements also provide detailed new information about the excited-state dynamics. The initially excited molecule must first cross a barrier on the excited-state potential energy surface before secondary excitation enhances the reaction. The PReP experiments demonstrate that the reaction path of a photochromic molecular switch can be selectively controlled through judicious use of time-delayed femtosecond laser pulses.

Photochemistry of fac-[Re(bpy)(CO)3Cl]

The photochemistry of fac-[Re(bpy)(CO)₃Cl] (1 a; bpy=2,2′-bipyridine) initiated by irradiation using <330 nm light has been investigated. Isomerization proceeded in THF to give the corresponding mer-isomer 1 b. However, in the presence of a small amount of MeCN, the main product was the CO-ligand-substituted complex (OC-6-24)-[Re(bpy)(CO)₂Cl(MeCN)] (2 c; bpy=2,2′-bipyridine). In MeCN, two isomers, 2 c and its (OC-6-34) form (2 a), were produced. Only 2 c thermally isomerized to produce the (OC-6-44) form 2 b. A detailed investigation led to the conclusion that both 1 b and 2 c are produced by a dissociative mechanism, whereas 2 a forms by an associative mechanism. A comparison of the ultrafast transient UV-visible absorption, emission, and IR spectra of 1 a acquired by excitation using higher-energy light (e.g., 270 nm) and lower-energy light (e.g., 400 nm) gave detailed information about the excited states, intermediates, and kinetics of the photochemical reactions and photophysical processes of 1 a. Irradiation of 1 a using the higher-energy light resulted in the generation of the higher singlet excited state with τ≤25 fs, from which intersystem crossing proceeded to give the higher triplet state (³HES(1)). In THF, ³HES(1) was competitively converted to both the triplet ligand field (³LF) and metal-to-ligand charge transfer (³mLCT) with lifetimes of 200 fs, in which the former is a reactive state that converts to [Re(bpy)(CO)₂Cl(thf)]⁺ (1 c) within 10 ps by means of a dissociative mechanism. Re-coordination of CO to 1 c gives both 1 a and 1 b. In MeCN, irradiation of 1 a by using high-energy light gives the coordinatively unsaturated complex, which rapidly converted to 2 c. A seven-coordinate complex is also produced within several hundred femtoseconds, which is converted to 2 a within several hundred picoseconds.

Modulation of a photoswitchable dual-color quantum dot containing a photochromic FRET acceptor and an internal standard

Photoswitchable semiconductor nanoparticles, quantum dots (QDs), couple the advantages of conventional QDs with the ability to reversibly modulate the QD emission, thereby improving signal detection by rejection of background signals. Using a simple coating methodology with polymers incorporating a diheteroarylethene photochromic FRET acceptor as well as a spectrally distinct organic fluorophore, photoswitchable QDs were prepared that are small, biocompatible, and feature ratiometric dual emission. With programmed irradiation, the fluorescence intensity ratio can be modified by up to ∼100%.

Gated photochromism in triarylborane-containing dithienylethenes: a new approach to a "lock-unlock" system

Photochromic inactive dithienylethene derivatives appended with 3- or 5-dimesitylboryl-2,2'-bithiophene have been synthesized. Upon fluoride ion-binding, the photochromic reactivity is "unlocked", displaying a novel gated photochromic property.