Patterned Fluorescence Images Using a Photocleavable NVOC-Protected Quinizarin

Abstract

Photolabile protecting groups have widely been used in the fundamental and applied research areas. Among various photocleavable protecting groups reported, 6-nitroveratroyloxycarbonyl (NVOC) group, an o-nitrobenzyl derivative, has gained much attention due to efficient removal upon UV irradiation. Accordingly, the NVOC protecting strategy has been employed in a variety of solution and solid-phase chemistry. Generation of patterned color images in the polymer film using a photolabile a-methylnitroveratryloxycarbonyl (α-MNVOC) protecting group was reported. In this note, we report the first application of the NVOC protecting group to the direct generation of patterned fluorescent images. Previously, we and other groups described ‘precursor approach’ for the patterned functional images in the polymer film without employing wet-developing processes. The concept of the ‘precursor approach’ is to use different electronic properties between the protected and unprotected forms (Figure 1). Thus, a dye molecule is nonfluorescent when the key functional group of the dye molecule is protected with a protecting group (PG). If the protecting group is removed under photoinduced chemical transformation, the fluorescence can be regenerated, allowing patterned fluorescent images in the polymer film by selective removal of the protecting group in the exposed areas. The protecting group investigated in our previous research was tert-butyloxycarbonyl (t-Boc) group which required a photoacid generator to cleave the protecting group in the exposed areas. The NVOC group, however, does not require acidic conditions for deprotection. Thus, the NVOCprotecting strategy should be a useful alternative for patterned images in polymer film where acidic conditions could not be employed. In order to test the feasibility of generation of patterned images with a photolabile protecting group-containg precursor molecule, the NVOC-protected compound 2 was readily prepared from quinizarin (1) and 4,5-dimethoxy-5nitrobenzyl chloroformate as shown in Scheme 1. The NVOC-protected quinizarin 2 was found to have a maximum absorption wavelength in the UV region (350 nm) and was observed to show no fluorescence, similar to the t-Bocprotected quinizarin. In order to investigate the feasibility of generating quinizarin moieties by direct photolysis from NVOC-protected precursor 2, a thin PMMA film containing the precursor 2 (20 wt%) on a quartz substrate was irradiated with 360 nm UV light.