A bright red-emitting flavonoid derivative was synthesized, which exhibited a large Stokes shift (Δλ > 150 nm) and high fluorescence quantum yields (φfl = 0.10-0.35). The probe could form a stable complex with Al3+ in 1 : 1 binding stoichiometry, generating a large bathochromic shift in both absorption and fluorescence (Δλ ≈ 70 nm) to enable ratiometric determination of cellular Al3+.