Abstract
The mol-ecular structure of the title compound {systematic name: 3',6'-bis(di-ethyl-amino)-2-[(2-hy-droxy-benzyl-idene)amino]-spiro-[isoindoline-1,9'-xan-then]-3-one}, C35H36N4O3 or RbSa, can be seen as being composed of two parts sharing a central quaternary carbon atom. Both the xanthene and iso-indole moieties are nearly planar: 14 atoms in the former moiety show an r.m.s. deviation of 0.0411 Å and eleven atoms in the latter moiety show an r.m.s. deviation of 0.0545 Å. These two planes are almost perpendicular to each other, the angle between the mean planes being 87.71 (2)°. The title compound appears to be in its enol form. The corresponding H atom was located and freely refined at a distance of 1.02 (3) Å from the O atom and 1.72 (2) Å from the N atom. The strong intra-molecular hydrogen bond O-H⋯N bridging the hydroxyl group and its neighboring nitro-gen atom forms an S(6) graph-set motif. Apart from the intra-molecular O-H⋯N hydrogen bond, C-H⋯O inter-actions are observed between two neighbouring RbSa mol-ecules related by an inversion center. The C-O donor-acceptor distance is 3.474 (2) Å. Moreover, C-H⋯π inter-actions are observed between the C-H bond of one of the ethyl groups and the centroid of the benzene ring of the iso-indole moiety. The C⋯centroid distance is 3.8191 (15) Å. No π-π inter-actions are observed in the crystal structure as the shortest distance between ring centroids is more than 4 Å. A Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are from H⋯H, C⋯H/H⋯C, O⋯H/H⋯O and N⋯H/H⋯N inter-actions. DFT calculations at the CAM-B3LYP/6-31 G(d) level were carried out to gain a better understanding of the relative energies and the tautomerization process between two possible conformers (keto and enol), as well as the transition state of the title compound.
Highlights
B derivative is usually used as a chemosensor because of its good photophysical properties, sensitivity, the low cost of chemical reagents and the ease of modifying its structure
The title compound crystallizes in the monoclinic space group P21/c, and the asymmetric unit contains a single molecule in a general position
The major tautomer of the rhodamine B – salicylaldehyde Schiff-base derivative is usually the enol form for compounds having hydroxy and azamethylene groups attached to the benzene ring in the ortho positions (Veranitisagul et al, 2012; Wattanathana et al, 2012, 2016)
Summary
Rhodamine B derivatives are employed extensively as molecular probes in the study of complex biological systems because of their high absorption coefficients, high fluorescence quantum yields and long-wavelength absorptions and emissions (Bao et al, 2013; Biswal & Bag, 2013). On the basis of the spirolactam/ring-opened amide equilibrium of rhodamine, several fluorescence-based sensing systems for metal ions have been developed. Most of the reported sensors based on rhodamine B derivatives are fluorescent chemosensors for metal ion detection (Quang & Kim, 2010; Kim et al, 2008; Kwon et al, 2005; Liu et al, 2013; Ni et al, 2013). Rhodamine B derivatives have naked-eye detection and show off–on fluorescent property when reacting with metal ions
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