Tunable Photochromism of Spirooxazine in the Solid State: A New Design Strategy Based on the Hypochromic Effect.

As an important organic photo-functional material, spirooxazine usually does not exhibit photochromism in the solid state since the intermolecular π-π stacking impedes photoisomerization. Developing photochromic spirooxazine in the solid state is crucial for practical applications but is still full of challenges. Here, we design and synthesize a series of spirooxazine derivatives (SO1-SO4) with bulky aromatic substituents at the 4- and 7-positions of the skeleton, providing a large volume for them to undergo solid-state photochromism under mild conditions. All compounds SO1-SO4 exhibit tunable solid photochromism without ground colors, excellent fatigue resistance, and high thermal stability. Notably, it takes only 15 s for SO4 to reach the saturation of absorption intensity, representing the fastest solid-state photoresponse of spirooxazines to the best of our knowledge. X-ray crystal structures of the intermediate compound SO0 and the products SO1-SO2 as well as the computational studies suggest that the bulky aromatic groups can lead to a hypochromic effect, allowing for the photochromism of SO in the solid state. The ideal photochromic properties of these spirooxazines open a new avenue for their applications in UV printing, Quick Response (QR) code, and related fields. This article is protected by copyright. All rights reserved.