The electronic spectra of the 1:1 rhodamine B base (RBB: leuco dye) with ethyl gallate (EG: developer) have been studied in solution and in the solid state (i.e., in spin-coated films and in single crystals) on the basis of the crystal structure analysis. There are two crystalline phases in the 1:1 "RBB/EG" colorant at low (93 K) and room temperatures. In solution, the maximum color intensity occurs with the 1:1 molar ratio of RBB with HCl, giving an absorption band at about 556 nm. In the solid state of spin-coated RBB/EG layers, an absorption band appears around 577 nm due to the ring opening caused by the hydrogen bond formation between RBB and EG. However, the color intensity is found to be limited to about 80% of the maximum available value. This has been attributed to a residual fraction of RBB molecules whose lactone ring is still closed due to steric hindrance. In addition, the polarized reflection spectra measured on single crystals of RBB/EG exhibit a drastically different spectrum (i.e., absorption maximum about 480 nm) from that of spin-coated films of the amorphous state. This result suggests that strong excitonic interactions of the H-aggregate type are operative in single crystals of RBB/EG that significantly displace the absorption band toward shorter wavelengths.
K. Sato, H. Shima, J. Mizuguchi, "Electronic Spectra of the 1:1 Rhodamine B Base with Ethyl Gallate in Solution and in the Solid State" in Journal of Imaging Science and Technology, 2009, pp 50303-1 - 50303-7, https://doi.org/10.2352/J.ImagingSci.Technol.2009.53.5.050303