Highly emissive, naked-eye solvatochromic probe based on styryl tetrahydrodibenzo[a,i]phenanthridine for acidochromic applications†
A new series of 5-styryl tetrahydrodibenzo[a,i]phenanthridines was readily synthesized from β-tetralone, ammonium acetate and cinnamaldehydes. These dyes emitted light with large Stokes shift values of 3894–7265 cm−1, depending on the nature of the substituent. The dyes displayed positive solvatochromism in their emission spectra, suggestive of a more polar excited state. Dye 3c exhibited aggregation-induced emission (AIE) property in 10% THF–water mixture besides a ratiometric response to pH from 10.75 to 2.16 with blue shift of 84 nm in emission. The emission ratio (I526 : I442) further displayed a good linearity with pH in the range of 4.1 to 6.8 and the corresponding pKa value was found to be 4.55 ± 0.03. The addition of TFA led to a prominent bathochromic shift in spectra along with clear colour changes owing to protonation at the phenanthridine segment. Further, a second protonation of the phenanthrinium segment could take place based on the pKa strength of the acid added. The pH dependent spectral changes were successfully applied to quantitatively detect pH in biological fluids and acid impurities in solvents.