SYNTHESIS, SPECTRAL PROPERTIES AND PHOTOSTABILITIES OF SYMMETRICAL AND UNSYMMETRICAL SQUARAINES - A NEW CLASS OF FLUOROPHORES WITH LONG-WAVELENGTH EXCITATION AND EMISSION

The absorption and fluorescence properties of squaraine derivatives in different organic solvents, and in water in presence of bovine serum albumin were investigated. The objective is to identify long-wavelength probes with reasonable quantum yields, reasonably long lifetimes and good photostabilities for use in fluorescence-based assays and/or imaging. Both symmetrical and unsymmetrical squaraines were studied and a correlation is made between the spectral properties and the electronic symmetry of the molecules. Like normal cyanines, squaraines show negative solvatochromy and band width broadening with increasing asymmetry. Based on this investigation the most suitable probes for use in a biological application were found to be the symmetrical indolenine derivatives of the squaraines, which display the highest photostability. Importantly, their quantum yields and lifetimes increase significantly upon binding to bovine serum albumin, suggesting that a conjugatable derivative of these indolenine squaraines will be suitable for use in labeling proteins. The squaraine absorbance maxima between 630 and 650-nm allows the use of the new commercially available 635- and 650-nm diode lasers.