Aggregation of bis(2,4,6-trihydroxyphenyl) squaraine in different solutions

The theoretical and experimental aggregation behavior of bis(2,4,6-trihydroxyphenyl) squaraine (OHSQ) in ethanol and tetrahydrofuran (THF) is compared. In ethanol, two broad absorption bands at 508 and 540 nm undergo bathochromic shifts to 521 and 580 nm with an increase in concentration. In THF, the single narrow absorption band at 568 nm undergoes a bathochromic shift to 579 nm and a hypsochromic shift to 508 nm when the concentration increases. Quantum calculations suggest that in ethanol the dimer aggregate forms in a face-toface manner with a separation distance of approximatly 0.4¯0.5 nm. In THF, it is suggested that two OHSQ molecules are hydrogen bonded in a head-to-tail manner with angles between them of approximately 100.7 and 143.6°. Similar calculations relate to the trimer in both solvents. Changes of absorption bands with pH explain the H-bonding in aggregates.