Fei Deng, Dongsheng Sun, Shixu Yanga,Wei Huanga,Chunfang Huanga, Zhaochao Xu , Limin Liu .Comparison of rhodamine 6G, rhodamine B and rhodamine 101 spirolactam based fluorescent probes. A case of pH detection.  Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

https://www.sciencedirect.com/science/article/abs/pii/S1386142521012397