Spectroscopic properties of persistence-type dye aggregate in wide temperature range

Spectroscopic properties of the monolayer assemblies, S9(J) + S11(J), which incorporate the mixed J-aggregate (JA) of oxacyanine dye; N,N′-dioctadecyl-oxacyanine perchlorate (S9) and thiacyanine dye; N,N′-dioctadecyl-thiacyanine perchlorate (S11), have been studied in the wide temperature range of T = 6.0–293 K. The mole fraction X of S11, X = [S11]/([S9] + [S11]), in the S9(J) + S11(J) assemblies has been varied from 0 to 1. At room temperature, absorption and fluorescence spectra of the monolayer assemblies incorporating the individual dye aggregate, S9 J-aggregate (S9(J)) or S11 J-aggregate (S11(J)), are characterized by a distinct J-band and resonance fluorescence. The S9(J) + S11(J) assemblies with X = 0.1–0.9 display two absorption bands; shorter-wavelength (SW) band and longer-wavelength (LW) band and a single fluorescence band (LW-fluorescence). With lowering temperature, the blue shift and narrowing of the absorption bands are observed. The low-energy tails of the J-band of the S9(J) (X = 0) and the S11(J) (X = 1) assemblies, as well as the LW-band of the S9(J) + S11(J) assemblies obey the Urbach–Martienssen (U–M) rule. The steepness parameter σ versus T curves show characteristic changes caused by mutual mixing of S9 and S11, which would supply us with a supporting evidence for the formation of the homogeneous aggregate of persistence-type (HP-aggregate) in the S9(J) + S11(J) assemblies.