Patterned Fluorescence Images with Indigo Precursors in Polymer Film

Abstract

Herein, we report a new approach to the generation of patterned fluorescence images in polymer film using indigo precursors as latent imaging materials. Fabrication of fluorescencepatterns is of great technological importance in a variety of fields including sensor, display and electronic industries.1-5 Among the various techniques such as photoirradiation, inkjet/screen printing,vapor deposition,and microcontact printing methods developed to date,the photoirradiation method represents the most popular and practical approach for the fabrication of patterned fluorescence images on a solid support.In a typical photoirradiation method, functional molecules having photochemically removable protecting groups are immobilized or coated on the solid substrate. Photoinduced removal of the protectinggroups allows formation of reactive species, such as amines, carboxylic acids, or alcohols in the photoirradiated areas.Incubation of the photo-treated substrate in a fluorescent dye solution results in the attachment of the fluorophore molecules to the reactive functional groups by covalent, ionic or hydrogen bonding interactions, allowing generation of patterned fluorescence images.Owing to the intrinsic limitations (employment of wetdeveloping process and difficulty of sufficient dye penetration into the matrix layer) associated with the conventional photochemical method, a new method called “precursor approach” has been developed recently.The “precursor approach” enables rapid and cost-effective generation of patterned images without the need for additional wet developing processes. Conceptually, the “precursor approach” is based on the use of a substrate containing a dye molecule that has different excited state electronic properties before and after photochemical treatment.In the course of the development of indigo derived polymeric materials studied in our group, we came up with an intriguing idea of generating patterned fluorescence images with indigo precursors. It has been well known that hydrolysis of acetoxyindole 1 produces unstable hydroxyindole 2 which spontaneously undergoes an oxidative couplingreaction to yield stable indigo 3 (Scheme 1(a)). If the acetyl group of 1 could be removed photochemically, generation of the same indigo dye 3 should be possible.Literature survey indicates that photochemical cleavage of the benzoyl ester group of the phenylbenzoxazole derivative 4 was achieved in polymer film (Scheme 1(b)).The photoinduced chemical transformation was applied to the fluorescence patterningsince the protected molecule 4 was nonfluorescent while the hydroxyphenylbenzoxazole 5 was strongly fluorescent.Thus, selective irradiation of a polymer film containing the protected benzoxazole derivative 4 allowed patterned fluorescence images.