Size and molecular configuration of dye aggregates in mixed Langmuir–Blodgett films based on merocyanine dye

Abstract

We have studied the blue-shifted and the red-shifted bands formed in mixed Langmuir–Blodgett films of the merocyanine dye (MS)-arachidic acid (C20)-n-octadecane (AL18) ternary system with the molar mixing ratio [MS]:[C20]:[AL18]=1:2:x(0.5≦x≦5.0). The formation of the blue-shifted and the red-shifted bands depends on the AL18 content, and shows that the aggregation state can be modulated by changing the AL18 content. The observed overlapping spectra of the blue-shifted and the red-shifted bands are deconvoluted into two original bands. The extended dipole model has been applied to examine the aggregation state of MS referring to the deconvoluted spectra. Thus the estimated minimum aggregation number Nmin and the slip angle α between the long axis of the aggregate and the transition dipole moment are Nmin=40 and α=30° and Nmin=40 and α=50° for fully-developed J- and H-aggregates, respectively, seen for x≦1.5, and Nmin tends to decrease with increasing x.

Modification of optical characteristics in langmuir-blodgett films of merocyanine dye by reduced pressure treatment

“…The blue-shifted band is assigned to H-band accompanied by MS chromophore alignments of side-by-side type, based on the energy shift, the sharpness and the in-plane anisotropy of the band [6]. In Figure 2(c), the blue-shifted and the red-shifted bands are located at 525 nm and 590 nm, respectively.…”

“…In Figure 2(c), the blue-shifted and the red-shifted bands are located at 525 nm and 590 nm, respectively. The overlapped spectra can be deconvoluted into the blue-shifted band at 520 nm and the red-shifted J-band at 590 nm, with their bands showing R > 1 as well [6]. (data not shown).…”

“…The MS concentration was of the order of 10 À4 M. Solutions were spread on the air-water interface to form mixed monolayers after solvent evaporation. Aqueous subphase containing Cd 2 þ and deposition conditions were the same as given in the previous papers [5][6][7]. A glass substrate precoated with five monolayers of cadmium arachidate (CdC 20 ) was used.…”

“…. The red-shifted band with R > 1 at 590 nm is ascribed to J-aggregation elongated in shape, which is associated with MS chromophore alignments of head-to-tail type in the aggregates [5][6][7][8][9][10][11].…”

“…1) as a prototype, since various aggregation states including red-shifted J-and blue-shifted H-aggregates can be easily tuned in the ultra-thin films based on MS [5][6][7]. In the course of the study, we have so far explored a key for the blue-shifted aggregation by characterizing the structures of the red-shifted and the blue-shifted aggregates [5][6][7].…”