Towards Optical Information Recording: A Robust Visible-Light-Driven Molecular Photoswitch with the Ring-Closure Reaction Yield Exceeding 96.3

Diarylethene molecular photoswitches hold great fascination as optical information materials due to their unique bistability and exceptional reversible photoswitching properties. Conventional diarylethenes, however, rely on UV light for ring-closure reactions, typically with modest yields. For practical application, diarylethenes driven by visible lights are preferred but achieving high ring-closure reaction yield remains a significant challenge. Herein, we synthesized a novel all-visible-light-driven photoswitch, TPAP-DTE, by facilely endcapping the dithienylethene (DTE) core with triphenylamine phenyl (TPAP) groups. Owing to the electron-donating conjugation effect of TPAP, the open-form TPAP-DTE responds strongly to short-wavelength visible lights with considerable photocyclization quantum yields and molar absorption coefficient. Upon 405 nm visible-light irradiation, TPAP-DTE achieves a ring-closure reaction yield exceeding 96.3 % (confirmed by both nuclear magnetic resonance spectroscopy and high-performance liquid chromatography). Its ring-opening reaction yield is 100 % upon irradiation with long-wavelength visible light. TPAP-DTE could be regarded as a bidirectional "quasi"-quantitative conversion molecular switch. Furthermore, TPAP-DTE exhibits robust fatigue resistance over 100 full photoswitching cycles and great anti-aging property under 85 °C and 85 % humidity for at least 1000 h. Consequently, its rewritable QR-code, multilevel data storage, and anti-counterfeiting/encryption applications are successfully demonstrated exclusively using visible lights, positioning TPAP-DTE as a highly promising medium for information recording.

Non-Conjugated Bis-(Dithienylethene)-Naphthalenediimide as a Dynamic Anti-Counterfeiting Agent: Driving the Wheel of Photoswitching Enactment

Photochromic fluorescent molecules dramatically extend their fields of applications ranging from optical memories, bioimaging, photoswitches, photonic devices, anti-counterfeiting technology and many more. Here, we have logically designed and synthesized a triazole appended bis-(dithienylethene)-naphthalenediimide based photo-responsive material, 5, which demonstrated fluorescence enhancement property upon photocyclization (ΦF =0.42), with high photocyclization (44 s, ksolution =0.0355 s-1 , ksolid =0.0135 s-1 ) and photocycloreversion (160 s, ksolution =0.0181 s-1 , ksolid =0.0085 s-1 ) rate and decent photoreaction quantum yield (Φo→c =0.93 and Φc→o =0.11). The open isomer almost converted to the closed isomer at photo-stationary state (PSS) with distinct color change from colorless to blue with 92.85 % conversion yield. A reversible noninvasive modulation of fluorescence through efficient photoinduced electron transfer (PET) process was observed both in solution as well as in solid state. The fluorescence modulation through PET process was further corroborated with thermodynamic calculations using the Rehm-Weller equation and quantum chemical studies (DFT). The thermally stable compound 5 exhibits high fatigue resistance property (up to 50 cycles) both in solution and solid state. Furthermore, the compound 5 was successfully applied as erasable ink and in deciphering secret codes (Quick Response/bar code) portending potential promising application in anti-counterfeiting.

Spectroscopic, photochromic and kinetic behavior of photo conversion of Aberchrome 540™ in an ionic liquid media

The photo-bleaching reaction of the chemical actinometer, Aberchrome 540™, is reported for the first time in a series of ionic liquids (ILs). This fulgide in its C-form undergoes an opening reaction to yield its E-form, when it is irradiated with UV light at wavelengths >400 nm. The magnitude of bleaching was monitored spectrophotometrically and their kinetics evaluated, obtaining first-order rate constants (k_obs). The results obtained in different ILs suggest that changes in the rate constants (k_obs) of the opening reaction of Aberchrome 540™ are mainly governed by weaker interactions such as coulombic (π* parameter) and Van der Waals interactions (δ_H², parameter). In addition, the photochemical fatigue resistance was also studied in ILs media and compared with conventional solvents (benzene, toluene, etc.). In particular, we found that three different tendencies dependent on the ILs used exist. The first group of ILs where the reversibility of the fulgide is favored (for example, [Bmim][BF₄], [Bmim][PF₆] and [Bmim][OTf]), behaviour similar to conventional solvents. The second group corresponds to ILs where photo reversibility is partial; and finally, other group of ILs that prevented photo reversibility. It is proposed that depending on the ILs used, the stabilization of different forms of the fulgide can be controlled, thus resulting important in terms of choosing the appropriate ILs for a specific photochemical reaction.

An accessible visible-light actinometer for the determination of photon flux and optical pathlength in flow photo microreactors

Coupling photochemistry with flow microreactors enables novel synthesis strategies with higher efficiencies compared to batch systems. Improving the reproducibility and understanding of the photochemical reaction mechanisms requires quantitative tools such as chemical actinometry. However, the choice of actinometric systems which can be applied in microreactors is limited, due to their short optical pathlength in combination with a large received photon flux. Furthermore, actinometers for the characterization of reactions driven by visible light between 500 and 600 nm (e.g. photosensitized oxidations) are largely missing. In this paper, we propose a new visible-light actinometer which can be applied in flow microreactors between 480 and 620 nm. This actinometric system is based on the photoisomerization reaction of a diarylethene derivative from its closed to the open form. The experimental protocol for actinometric measurements is facile and characterized by excellent reproducibility and we also present an analytical estimation to calculate the photon flux. Furthermore, we propose an experimental methodology to determine the average pathlength in microreactors using actinometric measurements. In the context of a growing research interest on using flow microreactors for photochemical reactions, the proposed visible-light actinometer facilitates the determination of the received photon flux and average pathlength in confined geometries.

Improving the fatigue resistance of diarylethene switches

When applying photochromic switches as functional units in light-responsive materials or devices, an often disregarded yet crucial property is their resistance to fatigue during photoisomerization. In the large family of diarylethene photoswitches, formation of an annulated isomer as a byproduct of the photochromic reaction turns out to prevent the desired high reversibility for many different derivatives. To overcome this general problem, we have synthesized and thoroughly investigated the fatigue behavior of a series of diarylethenes, varying the nature of the hetaryl moieties, the bridging units, and the substituents. By analysis of photokinetic data, a quantification of the tendency for byproduct formation in terms of quantum yields could be achieved, and a strong dependency on the electronic properties of the substituents was observed. In particular, substitution with 3,5-bis(trifluoromethyl)phenyl or 3,5-bis(pentafluorosulfanyl)phenyl groups strongly suppresses the byproduct formation and opens up a general strategy to construct highly fatigue-resistant diarylethene photochromic systems with a large structural flexibility.

Photoirradiation wavelength dependence of cycloreversion quantum yields of diarylethenes

The cycloreversion quantum yields of compounds 1b and 2b increased upon irradiation with light at shorter wavelengths. The irradiation wavelength dependence is ascribed to the energy barrier on the 2A potential energy surface in the excited electronic state.

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.

Efficient photochromic transformation of a new fluorenyl diarylethene: one- and two-photon absorption spectroscopy

Efficient reversible phototransformation of a new diarylethene-fluorene derivative, 1,2-bis(5-(9,9-didecyl-7-nitro-9H-fluoren-2-yl)-2-methylthiophen-3-yl)cyclopent-1-ene (1), was demonstrated in organic media under low-intensity laser excitation. Linear photophysical characterization of 1 was performed at room temperature in solvents of different polarity and viscosity. Significantly, close to unity quantum yield for the cyclization reaction of 1 was shown in nonpolar solutions. The lifetimes of the excited states of the open (OF) and closed (CF) forms of 1 were measured by a femtosecond transient absorption technique, and corresponding values of ∼0.7 and ∼0.9 ps were shown in dichloromethane (DCM), respectively. Degenerate two-photon absorption (2PA) spectra of the OF and CF of 1 were obtained over a broad spectral range by the open aperture Z-scan method under 1 kHz femtosecond excitation. The values of 2PA cross sections of the OF in DCM (∼50-70 GM) were found to increase up to 1 order of magnitude (∼600 GM) after cyclization to the CF. The nature of cyclization and cylcoreversion processes were investigated by quantum chemistry with employment of DFT-based methods implemented in the Gaussian'09 program. The potential of 1 for application in optical data storage was shown using poly(methyl methacrylate)-doped films and two-photon fluorescence microscopy readout.