The electrophilic cyclization of substituted propargylic aryl ethers by I2, ICl, and PhSeBr produces 3,4-disubstituted 2H-benzopyrans in excellent yields. This methodology results in vinylic halides or selenides under mild reaction conditions and tolerates a variety of functional groups, including methoxy, alcohol, aldehyde, and nitro groups.

Synthesis of 3,4-Disubstituted 2H-Benzopyrans Through C-C Bond Formation via Electrophilic Cyclization

Photolysis of regioisomeric helical chromenes 1 and 2 leads to colored reactive intermediates. While the latter generally decay quite rapidly, they are found to be longer lived in 1 and highly persistent in 2. The remarkable stability of the otherwise fleeting transient in 2 allowed isolation and structural characterization by X-ray crystallography. The structural analyses revealed that steric force inherent to the helical scaffold is the origin of stability as well as differentiation in the persistence of the intermediates of 1 and 2 (1Q and 2Q). The structure further shows that diphenylvinyl moiety in the TT isomer of 2Q gets splayed over the helical scaffold such that it is fraught with a huge steric strain to undergo required bond rotations to regenerate the precursor chromene. Otherwise, reversion of 2Q was found to occur at higher temperatures. Aazahelical chromenes 3 and 4 with varying magnitudes of helicity were designed in pursuit of o-quinonoid intermediates with graded activation barriers. Thei...

Helicity as a steric force: stabilization and helicity-dependent reversion of colored o-quinonoid intermediates of helical chromenes.

In this paper, the photodynamics of three chromenes (2,2-spiro-adamantyl-7,8-benzo(2H)chromene, 2,2-diphenyl(2H)chromene and 2,2-diphenyl-5,6-benzo(2H)chromene) has been investigated by nano- and femtosecond time resolved techniques in hydrocarbon solutions at room temperature. Using pump–probe techniques, ultrafast steps of molecular dynamics characterizing the photoresponse of chromenes have been investigated: the breakage of the C–O bond was found to occur within a few hundred femtoseconds producing a short-lived transient that converts to the cisoid-cis open form in less than one picosecond and then to the metastable transoid-cis structure within a few picoseconds. The effect of different excitation wavelengths on the formation rate of the first shortest-lived transient of 5,6DPBC is in agreement with the model previously proposed to explain the wavelength dependence of the reaction and emission quantum yields in photostationary experiments. The results obtained in the nanosecond time domain show that the triplet marginally participates in the photoreaction for only one of the studied compounds (2,2-spiro-adamantyl-7,8-benzo(2H)chromene).

Dynamics of the excited states of chromenes studied by fast and ultrafast spectroscopies

Diarylbenzo- and naphthopyrans are one of the important and well-known classses of organic photochromic systems. Their photolysis leads to highly colored o -quinonoid intermediates. The mechanistic details and spectrokinectic properties of the photogenerated colored intermediates have been extensively studied. Accordingly, stereoelectronic effects have been shown to play a crucial role in controlling properties such as spectrokinetics, fatigue resistance and colorability. Much research continues unabated to develop photochromic materials with properties that surpass the existing ones for application in ophthalmic lenses as well as optical data storage devices. In the realm of materials chemistry, the diarylpyran unit is presently exploited as an essential design element to produce functional materials. Given the facile synthetic access to molecular systems that respond to light as an external stimulus leading to widely distinct physicochemical properties, we believe that more exciting research based on diarylpyran photochemistry will germinate in materials chemistry and biology in the near future. This review with a coverage of the phenomenon since its inception to the present state of application for functional materials–through synthesis and mechanistic details–should serve as an invaluable resource to an uninhibited chemist to exploit the photochromism of diarylpyrans for diverse functions/applications.

Phenomenon to functions: Photochromism of diarylpyrans, spectrokinetic properties and functional materials

The photocolouration of 2,2-diphenyl-5,6-benzo(2H)chromene (Chr), upon irradiation of the ring-closed form (Np), and the subsequent relaxation kinetics of one merocyanine isomer into Np were studied in several solvents at different temperatures. Deactivation of the excited Np form leads via the singlet manifold predominantly to the ring-opened cis–trans (ct) isomer which converts thermally into Np. The activation energy, E ct→ Np ≈77 kJ mol −1 , and the pre-exponential factor, A≈1012 s−1, are essentially independent of the solvent polarity. The trans–trans (tt) isomer, which is suggested to be formed photochemically in a competing step, is thermally more stable. The triplet pathway of photocolouration can be induced by energy transfer using several sensitizers with a triplet energy of >200 kJ mol−1. The thermal relaxation time after either direct or naphthalene-sensitized colouration is the same. The mechanisms of ring opening and ring closure are discussed.

Photoprocesses in 2,2-diphenyl-5,6-benzo(2H)chromene

Picosecond to submillisecond photochromic reactions of 2,4-diphenyl-2H-benzopyran and 2,2,4-triphenyl-2H-benzopyran have been investigated by time-resolved absorption spectroscopy. The C−O bond cleavage of the benzopyrans (closed forms) occurs via the first excited singlet state within 2 ps to produce vibrationally excited open forms in the ground electronic state. In the subnanosecond to submillisecond time domain, several decay components with almost the same spectral profiles are observed. These components are assigned to respective stereoisomers with respect to two double bonds and one single bond of the open enone forms. From the pump-laser power dependencies of the yields of the open forms, it is suggested that the photocleavage gives at first only the open forms revertible to the closed form by a single-bond rotation, and that the photoexcitation of the first generated open forms gives rise to other open forms which need a double-bond rotation for reversion to the closed form. The photochromic reac...

Yoichi Kodama

Papers

Time-Resolved Absorption Studies on the Photochromic Process of 2H-Benzopyrans in the Picosecond to Submillisecond Time Domain