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 pK_a 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 pK_a 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.