Thienyl-and bithienyl-1,3-benzothiazoles 1 and 2 were synthesised by reacting various formyl thienyl and bithienyl derivatives with o-aminobenzenethiol in moderate to excellent yields. Evaluation of the solvatochromic and fluorescence properties of these compounds was carried out. Due to their strong fluorescence and also the strong push-pull character, benzothiazole derivatives 1 and 2 can be used as potential NLO materials or as fluorescent markers. Introduction The benzothiazole nucleus appears in many fluorescent compounds that have useful applications as a result of the ease of synthesis of this heterocycle and the high fluorescence quantum yields obtained when this small, rigid moiety is present [1]. The practical interest upon benzothiazole derivatives (2-aryl-benzothiazole, 2-(2-phenylethenyl)-benzothiazole, 2-indolyl-benzothiazole, 2-carbazolyl-benzothiazole and thiazole push-pull alkenes, etc.) is based on their widespread application [2]. In addition to intense fluorescence, some compounds of this type have interesting biological properties. They have been investigated for their clinical efficacy as cytotoxic, antimicrobial, antifungal, anti-tumour and anti-HIV activity. Such compounds can also have application as polidentate ligands, in laser technology as laser dyes, organic luminophores, thermo-and LTV-stabilizers for polymer materials, diazotype materials, for organic electroluminescent devices and materials with non-linear optical (NLO) properties [3-5]. As part of our continuing interest in heterocycles with non-linear optical and fluorescent properties [6-13] we describe here the synthesis, the UV-visible, the solvatochromic and fluorescence studies of a series of heterocyclic fluorophores of the benzothiazole type. Results and discussion Synthesis. A series of benzothiazoles 1 and 2 were synthesized with either fluoro-, alkoxy-or N,N-dialkylamino-donors or with a nitro acceptor on the thienylaryl or on the bithienyl moiety across a conjugated -bridge containing a thienylaryl or a bithienyl moiety (Scheme 1). Formyl precursors were synthesized by Suzuki cross-coupling reaction of thienyl-or aryl boronic acid with bromoaryl or bromothienyl compounds, catalyzed by Pd(PPh 3) 4 . These compounds were synthesized with different -bridges and several donor and acceptor groups in order to evaluate the effect of the conjugating bridge and the donating and accepting strength of these groups on the solvatochromic and fluorescent properties of benzothiazoles 1 and 2. The benzothiazole moiety was obtained by the simple reaction of o-aminobenzenethiol with formyl precursors, in DMSO at 120 °C for 30-60 min.