Synthesis of new benzo[a]phenoxazinium probes possessing carboxylic ester, hydroxyl and amino functional groups: Photophysical studies in dry ethanol and conjugation with CdTe quantum dots

• A new set of (bi)functionalised benzo[a]phenoxazinium chlorides was synthesized.
• The photophysics in dry ethanol was distinct from solvent grade absolute ethanol.
• The proton dissociation equilibrium constants in dry ethanol were determined.
• The conjugation of dye with CdTe QDs evidenced energy transfer to the attached dye.
• These dyes can function as fluorescent probes for biotechnological applications.

A new series of benzo[a]phenoxazinium chlorides possessing hydroxyl, ethyl ester and amino functional groups as terminal substituents at the 9- and 5-positions of the tetracyclic aromatic system in different combinations, was synthesized. A photophysical study was carried out in anhydrous ethanol and water, as a function of solution pH. Acid dissociation constants were estimated and found to depend on the nature of the terminal groups. Experimental evidence for an additional molecular form is presented and is compatible with J-aggregates of the deprotonated form of some of the earlier studied benzo[a]phenoxazinium chlorides. The benzo[a]phenoxazinium chloride possessing the amino terminal group was reacted with the carboxylic acid of CdTe quantum dots (QDs) to obtain a conjugate of the dye and QD. Initial photophysical characterization indicates both photoinduced electron and energy transfer between the QD and the attached benzo[a]phenoxazinium chloride.

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