1 N-Alkyl-N-[5-(propylamino)-9H-benzo[a]phenoxazin-9-ylidene]alkan-1- aminium chlorides: synthesis and photophysical studies Sarala Naik, Paulo J. G. Coutinho a and M. Sameiro T. Gonçalves* Centro de Química, Universidade do Minho, Gualtar, 4710-057 Braga, Portugal a Centro de Física, Universidade do Minho, Gualtar, 4710-057 Braga, Portugal e-mail: msameiro@quimica.uminho.pt Abstract: Fluorescent benzo[a]phenoxazinium chlorides with dipropyl-, dioctyl- didecyl- and didodecyl-chains as the amino substituents of the 9-positions of the tetracyclic system were synthesised in moderate to good yields. All the compounds absorbed in the wavelength range of 638-641 nm and emitted at 673 or 676 nm with quantum yields of 0.17-0.19 in ethanol. Keywords: Benzo[a]phenoxazinium dyes; Near-infrared fluorescence probes; Cationic dyes; Nile Blue derivatives. 1. Introduction Fluorescent chromophores have gained pivotal importance in life sciences, particularly in detection, labelling, diagnosis and analysis. 1-5 The fluorophores with absorption and emission at longer wavelengths (600-1000) nm are suitable for bio-applications, taking into account their minimum interference from absorption scattering and the natural auto-fluorescence of biological molecules. 6,7 Oxazine derivatives, such as phenoxazines and benzo[a]phenoxazines have been reported for various spectroscopic research studies in the near-infrared region. 8 Taking cues from previous results of benzo[a]phenoxazinium dyes with long aliphatic chains at the 5-position of the aromatic system, 9 together with our current research interest in the synthesis and characterisation of fluorescent probes, 10 we decided to synthesise new near- infrared labels based on benzo[a]phenoxazinium chlorides with double long alkyl side-chains at the 9-position of the aromatic systems, which would function as anchors in biological structures such as lipids, membranes, and proteins. 2. Results and Discussion Benzo[a]phenoxazinium chlorides 1a-d were synthesised by condensation of 5- (dialkylamino)-2-nitrosophenol hydrochlorides 2a-d with N-propylnaphthalen-1-amine 3 in