Aggregated States of Chalcogenorhodamine Dyes on Nanocrystalline Titania Revealed by Doubly Resonant Sum Frequency Spectroscopy

Broadband doubly resonant sum frequency generation (DR-SFG) spectroscopy was used to probe selectively the vibrational signatures of a series of chalcogenorhodamine dyes tethered to TiO₂ either as monomers or as H-aggregates. This selectivity was achieved by virtue of the vibronic coupling associated with the unique electronic transitions for the corresponding states. It was found that clear spectral differences and polarization dependences were observed contingent on whether the outgoing SFG wavelength was in resonance with either the monomer or H-aggregate electronic excited states. Our results also indicate that the orientation and proximity of the xanthylium core to the semiconducting surface play an important role in this vibrational-electronic coupling and may affect the character of the charge-transfer complex. Our DR-SFG results also provide important clues into the anchoring motifs for carboxylic and phosphonic acid groups. (Figure Presented).