Photochromic copolymers containing 3-indolylfulgides/indolylfulgimides: Synthesis and photochemical properties in toluene and as films

Abstract

Photochromic indolylfulgimides covalently attached to polymers have beneficial properties for optical switching. A 3-indolylfulgide and two 3-indolylfulgimides with one or two polymerizable styrene groups attached on the nitrogen atom(s) were synthesized. Copolymerization with methyl methacrylate (MMA) provided linear copolymers (one styrene group) or a cross-linked copolymer (two styrene groups). The properties of the monomers and copolymers in toluene or as thin films were characterized. The new copolymers were photochromic (reversible Z-to-C isomerization), absorbed visible light, and revealed good thermal and photochemical stability. At room temperature, all copolymer films showed no loss of absorbance after 5 weeks. At 80 °C in either toluene or as films, the Z-forms copolymers were less stable than the C-form copolymers, which showed little or no degradation after 400 h. The degradation rate due to repeated ring-closing – ring opening cycles was less than 3% per 100 cycles. The cross-linked copolymer showed photochemical stability comparable to monomeric fulgides in toluene, <1% per 100 cycles. In general, the properties of the linear and cross-linked copolymers were similar to the corresponding monomers in toluene. In films, the conformations of the Z-form were restricted due to the matrix indicating that the preparation of films from the C-form is advantageous.

Synthesis and Characterization of Photochromic Copolymers Containing 3-Indolylfulgides/Indolylfulgimides

“…18 Indolylfulgides and indolylfulgimides are regarded as promising candidates to be used in optical data storage devices because they usually display great thermal stability in solvents and polymer films. 18,27,28,34 A previous study found that the fluorinated indolylfulgides had outstanding resistance to thermal stress in the closed form, however, the open form was less stable and degraded rapidly at 80°C.…”

“…46 Indolylfulgides and indolylfulgimides can endure hundreds to thousands of photochemical cycles (back and forth conversion between the two key forms) before degrading by 20%. 2,[27][28][29]41 A mechanism of photochemical degradation for a methyl indolylfulgide was proposed by Yokoyama (Scheme 20). 28 The major degradation pathway involves a 1,5-hydrogen migration from a methyl group to the carbonyl oxygen on the anhydride ring.…”

“…27 The fulgimide was then copolymerized with the methyl methacrylate (MMA) to obtain a photochromic linear copolymer. The resulting copolymer retained the photochromic properties as the monomeric fulgimide and exhibited good thermal and photochemical stabilities in toluene.…”

“…Among photochromic compounds, 3-indolylfulgimides are promising because of their enhanced photochromic properties, such as enhanced thermal stability, increased photochemical stability (fatigue resistance), highly efficient photoreactions, large molar absorption coefficients and hydrolytic stability. 22,27,35,39,81 The photochromism of methyl and trifluoromethyl-3-indolylfulgimides was shown in Scheme 32, where the fulgimides can be converted between the open forms and the closed form reversibly by light.…”

“…My previous study indicated that the CF 3 fulgimide 3 has better fatigue resistance than 5. 27 For the copolymers in aqueous solutions, the cycle numbers were lower and the cycling times were longer than their monomers in toluene, probably because the fulgimides have lower quantum yields in more polar solution. 31 Copolymer 5-co-PAA displayed better photochemical stability than the hydrolyzed 3-co-PAA.…”