Photoisomerization-coupled electron transfer

Photochromic molecular structures constitute a unique platform for constructing molecular switches, sensors, and memory devices. One of their most promising applications is as light-switchable electron acceptor or donor units. Here, we investigate a previously unexplored process that we postulate may occur in such systems: an ultrafast electron transfer triggered by a simultaneous photoisomerization of the donor or the acceptor moiety. We propose a theoretical model for this phenomenon and, with the aid of density functional theory calculations, apply it to the case of a dihydropyrene-type photochromic molecular donor. By considering the wavepacket dynamics and the photoisomerization yield, we show that the two processes involved, electron transfer and photoisomerization, are in general inseparable and need to be treated in a unified manner. We finish by discussing how the efficiency of photoisomerization-coupled electron transfer can be controlled experimentally.

Deciphering Erasing/Writing/Reading of Near-Infrared Fluorophore for Nonvolatile Optical Memory

A near-infrared fluorescence-switchable molecule, dithienylethene-terrylenediimide (TDI-4DTE) exhibits high near-infrared fluorescence and on/off ratio, decent reversibility, and fatigue resistance upon alternating UV/vis (305/621 nm) irradiation. Photoinduced electron transfer mainly contributes to the fluorescence quenching of TDI-4DTE. As an information storage unit, single molecular TDI-4DTE in the polymer film can be written by red light (621 nm) and erased by UV light (305 nm), while nondestructive fluorescence readout (750 nm) of a single molecular memory has been obtained upon excitation with near-infrared light (720 nm). The fluorescence patterning of TDI-4DTE in the polymer film demonstrates that the erasing/writing/reading wavelengths are deciphered to minimize the signal crosstalk in nonvolatile fluorescent molecular memories.

Photoactivatable Fluorophores

Photoactivatable fluorophores switch from a nonemissive to an emissive state upon illumination at an activating wavelength and then emit after irradiation at an exciting wavelength. The interplay of such activation and excitation events can be exploited to switch fluorescence on in a defined region of space at a given interval of time. In turn, the spatiotemporal control of fluorescence translates into the opportunity to implement imaging and spectroscopic schemes that are not possible with conventional fluorophores. Specifically, photoactivatable fluorophores permit the monitoring of dynamic processes in real time as well as the reconstruction of images with subdiffraction resolution. These promising applications can have a significant impact on the characterization of the structures and functions of biomolecular systems. As a result, strategies to implement mechanisms for fluorescence photoactivation with synthetic fluorophores are particularly valuable. In fact, a number of versatile operating principles have already been identified to activate the fluorescence of numerous members of the main families of synthetic dyes. These methods are based on either the irreversible cleavage of covalent bonds or the reversible opening and closing of rings. This paper overviews the fundamental mechanisms that govern the behavior of these photoresponsive systems, illustrates structural designs for fluorescence photoactivation, and provides representative examples of photoactivatable fluorophores in actions.

Syntheses and fully diastereospecific photochromic reactions of thiophenophan-1-enes with chiral bridges

Thiophenophan-1-enes with chiral polyether bridges were prepared and their diastereospecific photochromic reactions were studied. The coupling reaction of substituted dithienylethenes and various chiral synthons afforded thiophenophan-1-enes, namely, bridged dithienylethenes, as single enantiomers without optical resolution, thus indicating that these reactions occurred diastereoselectively. Upon UV irradiation, each optically active thiophenophan-1-ene isomerized to the corresponding enantiomer of the closed form and returned to the initial enantiomer of the open form upon visible irradiation. Because thiophenophan-1-enes never isomerized to other diastereomers even at a high temperature, they underwent diastereospecific photochromic reactions. Large changes were observed in the measurement of the optical rotations of the solutions of thiophenophan-1-enes at 588 nm according to their photochromic reactions. As there was no absorption at this wavelength for both isomers of each thiophenophan-1-enes, the nondestructive readout of the photochromic reaction could be carried out by using these chiral thiophenophan-1-enes.

In situ preparation of highly fluorescent dyes upon photoirradiation

Photoswitchable or photoactivatable fluorescent dyes are potentially applicable to ultrahigh density optical memory media as well as super-resolution fluorescence imaging when the dyes are highly fluorescent and have large absorption coefficients. Here, we report on highly fluorescent photochromic dyes, which are initially nonluminous in solution under irradiation with visible light but activated to emit green or red fluorescence upon irradiation with ultraviolet (UV) light. The dyes 5a-9a are sulfone derivatives of 1,2-bis(2-ethyl-6-phenyl(or thienyl)-1-benzothiophen-3-yl)perfluorocyclopentene. It was found that substitution of phenyl or thiophene rings at 6 and 6' positions of the benzothiophene-1,1-dioxide groups is effective to increase the fluorescence quantum yields of the closed-ring isomers over 0.7 and absorption coefficients over 4 × 10(4) M(-1) cm(-1). The phenyl-substituted derivatives 5a-7a undergo photocyclization reactions to produce yellow closed-ring isomers 5b-7b, which emit brilliant green fluorescence at around 550 nm (Φ(F) = 0.87-0.88) under irradiation with 488 nm light. Any absorption intensity change of the closed-ring isomers was not observed even after 100 h storage in the dark at 80 °C. The closed-ring isomers slowly returned to the initial open-ring isomers upon irradiation with visible (λ > 480 nm) light. The ring-opening quantum yields (Φ(C→O)) were measured to be (1.6-4.0) × 10(-4). When the phenyl substituents are replaced with thiophene rings, such as compounds 8a and 9a, the absorption bands of the closed-ring isomers shift to longer than 500 nm. The closed-ring isomers exhibit brilliant red fluorescences at around 620 nm (Φ(F) = 0.61-0.78) under irradiation with 532 nm light. The ring-opening reactions are very slow (Φ(C→O) < 1 × 10(-5)). The fluorescence lifetimes of these sulfone derivatives were measured to be around 2-3 ns, which is much longer than the value of the closed-ring isomer of 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene (τ(F) = 4 and 22 ps). The closed-ring isomer 8b in 1,4-dioxane exhibits excellent fatigue resistant property under irradiation with visible light (λ > 440 nm) superior to the stability of Rhodamine 101 in ethanol.