1. [See for example]:
2. Weigand U, Zinner H. Photochromic fluorenopyrans with defined dibenzo[b,d]pyrano fused attachment. International Patent No. WO009020 (A1), 2014.
3. Takenada J, Momoda J, Teranishi K, Takahashi T, Sando M, Izumi S. Chromene compound. United States Patent No. US0054520 (A1), 2014.
4. Xiao W, Van Gemert B. Indeno-fused naphthopyrans having ethylenically unsaturated groups. International Patent No. WO090220 (A1), 2013.
5. Shimizu Y, Izumi S, Momoda J. Chromene compound and curable composition. International Patent No. WO042800 (A1), 2013.