Off-line multidimensional high performance thin-layer chromatography for fractionation of Japanese knotweed rhizome bark extract and isolation of flavan-3-ols, proanthocyanidins and anthraquinones

•Methodology for isolation of bioactive compounds from Japanese knotweed bark.•HPTLC, HPTLC-densitometry, HPTLC-MSn, 1H NMR and 2D NMR analyses were performed.•Flavan-3-ols, proanthocyanidins and anthraquinones were isolated and characterized.•1st isolation: procyanidins B1 and B2 (pB1, pB2) and emodin-8-O-malonyl-glucoside.•1st detection: pB1, pB2, derivatives of resveratrol, taxifolin and emodin bianthrone.