Photochromism of 2H-Naphtho[1,2-b]pyrans:  A Spectroscopic Investigation

Abstract

The photophysical and photochromic properties of 2,2-diphenyl-2H-naphtho [1,2-b]pyran and substituted derivatives were investigated by steady state and time-resolved optical absorption and emission spectroscopy in solution at room temperature and in a frozen matrix at 77 K. Fluorescence quantum yields, fluorescence lifetimes, and singlet energies depend strongly on the substitution patterns. Photoexcitation of the naphthopyrans (A) leads to efficient ring opening to produce the merocyanines (B) and (C). The optical absorption of the merocyanines can be tuned by the substituents and can cover most of the visible spectrum (400-700 nm). The decoloration kinetics of the open forms (merocyanines) B and C to produce naphthopyrans (A) depends strongly on the substituents. The ring closure rate constants (k BfA ) range from 0.0009 to 0.04 s -1 . IntroductionIn the past few years, photochromic dyes, which show reversible photoisomerization, have attracted considerable attention because of their implications in a great number of practical applications including data storage, optical filters, displays, sensor protection, waveguides, and ophthalmic plastic lenses. 1-3 Chromenes, a class of photochromic compounds, have been studied in their photochromic properties in the pioneering work by Becker et al. more than 30 years ago. [4][5][6][7][8][9][10][11] Naphthopyran is a chromene derivative (benzochromene) and can be divided into three isomers, 3H-naphtho[2,1-b]pyrans, 2H-naphtho[1,2-b]pyrans, and 2H-naphtho[2,3-b]pyrans (Scheme 1). The photochemistry and photophysics of 3H-naphtho[2,1-b]pyrans were extensively studied. 9,10,[12][13][14][15][16][17][18] Although, 2H-naphtho[1,2-b]pyrans are commercially used in plastic ophthalmic lenses, 19 systematic studies of the photophysics and photochemistry are surprisingly rare. 8,20,21 Most studies concentrated on synthetic aspects. 15,17,22,23 Therefore, we investigated in detail the photophysics and photochemistry of 2,2-diphenyl-2H-naphtho[1,2-b]pyran, 1(A), and four of its derivatives, 2(A)-5(A), (Scheme 1). The derivatives 2(A)-5(A) were chosen to study influences of substituents on the properties.Ottavi et al. showed that irradiation with UV light of 3,3-diphenyl-3H-naphtho[2,1-b]pyran produces two colored forms of ring opening products (cis-trans and trans-trans). 12 Applying this reaction mechanism to 1(A) leads to Scheme 2 in which A is the (uncolored) closed form (pyran form) and B (cis-trans) and C (trans-trans) are open (colored) forms (merocyanine forms). For this investigation, the photophysical properties of the closed form (A) and the open forms (B and C) were determined separately, and afterward, the ring closure kinetics (B and C to A) were investigated. Experimental SectionThe naphthopyrans 1(A)-5(A) (PPG Industries, Inc.) were recrystallized from ethanol. Acetonitrile, ethanol, and methylcyclohexane (Aldrich, spectroscopic grade) were used as received.The UV-vis spectra were recorded on a HP 8452A diode array spectrophotometer using quartz cells with path lengths...

The photochemical ring opening reaction of chromene as seen by transient absorption and fluorescence spectroscopy

“…2, dashed); the thermal reaction from OF (tc) -OF (tt) → CF was found to occur on the timescale of ∼10 s as shown earlier. 12,13,15,22 No cw-fluorescence could be detected in agreement with earlier studies on the system.…”

“…Centred at 425 nm a new peak arises which is assigned to a mixture of OF (tc) -OF (tt) isomers; all results are in accordance with published results. 12,13,15,22 Before ring-opening, the absorption is given by a superposition of two benzene moieties (∼240 nm, not shown) and a napthopyrane group. After the ring-opening reaction a larger system of delocalized π-electrons extending over both benzene rings and the napthopyrane moiety causes additional absorption in the visible region.…”

“…12,13 A large number of experimental and theoretical studies are currently available 13,14,16,[18][19][20][21] concentrating on the molecule DPBC (Scheme 1) or related compounds. These compounds were investigated in different solvents ( polar vs. non-polar) 13 and for different excitation conditions, (S 1 vs. S N excitation) 19 using continuous-wave spectroscopy 12,13,15,22,23 or timeresolved techniques, e.g., transient absorption, TA. 14,[18][19][20]24,25 Based on the data from transient absorption measurements and the spectral dynamics observed after photoexcitation, at first a sequential model with fast excited state decay was suggested for the ring-opening reaction (Fig.…”

A photochromic naphthopyran dye activated by the intramolecular hydrogen bond and its photodynamics in the ormosil matrix coating

“…The J values are given in Hz. 1 The single crystal of the NPase for the X-ray crystal analysis is obtained by slow evaporation of its ethyl acetate solution and the following filtration. The X-ray diffraction data are collected at 293 K on a CAD4 DIFFACTIS 586 diffractometer (EN-RAF-NONIUS) with the graphite monochromated Mo-Ka radiation (k = 0.71073 Å ), l = 0.184 mm -1 .…”

“…In the recent decades, naphthopyran molecules have been demonstrated to be one of the best families in organic photochromic dyes, which possess the low solvachromism, high color density, good fatigue resistance and variable color gamut in comparison with the other families [1][2][3]. Molecular design and synthesis of novel naphthopyran systems with large optical densities and fast photodynamics at ambient temperature are, therefore, essential for their industrial applications.…”