Controlled Formation of the Two-Dimensional TTBC J-Aggregates in an Aqueous Solution

Abstract

Strong experimental and theoretical evidence was provided on the controlled formation of the two-dimensional J-aggregates that were assembled in the herringbone morphology. The exciton-band structure formation of 1,1′,3,3′-tetraethyl-5,5′,6,6′-tetrachlorobenzimidazolocarbocyanine (TTBC) J-aggregates was investigated in ionic (NaOH) aqueous solution at room temperature. The control was achieved by changing the [TTBC] at a given [NaOH], or vice versa, and was monitored through the changes in the absorption, fluorescence excitation, and emission spectra. Specific attention was paid to expose the excited-state structure and dynamics through simulations of the excitonic properties, which included diagonal energetic disorder and phononassisted exciton relaxation. Aggregates were characterized by an asymmetrically split Davydov pair, an H-band (∼500 nm, 1300 cm -1 wide, Lorentzian-like) and a J-band (∼590 nm, 235 cm -1 wide, with a band shape typical of a one-dimensional J-aggregate), whose relative intensities showed a strong dependence on the [TTBC]/ [NaOH]. The H-band is favored by high [TTBC] or high [NaOH]. An explanation of the control on the aggregate formation was given by correlating the changes in the absorption with the structural modifications and the subsequent changes in the dynamics, which were induced by variations in the dye and NaOH concentrations. The J-band shape/width was attributed to disorder and disorder-induced intraband phononassisted exciton relaxation. The intraband processes in both bands were estimated to occur in the same time scale (about a picosecond). It has been suggested that the wide energetic gap between the Davydov split bands (3000 cm -1 ) could get bridged by the excitonic states of the loosely coupled chains, in addition to the monomeric species at low [TTBC]. Phonon-assisted interband relaxation, through the band gap states and/or directly from the H-to the J-band, are suggested for accounting the difference between the bandwidths and shapes of the two bands. Energy transfer between the H-band and the monomeric species is suggested as crucial for tuning the relative strengths of the two bands.

Polarized absorption spectra of highly oriented two-dimensional aggregates of tetrachlorobenzimidazolocarbocyanine in thin films

“…It is well known that the band positions depend to a large extent on the internal molecular arrangement, whereas the band shapes/widths are determined basically by the excited state dynamics (see e.g., Ref. [5]). The immediate conclusion following this comparison is that TTBC aggregates assume very similar internal molecular packing and dynamics of excited states in ionic aqueous solution and in thin films.…”

“…We have already carried out a comprehensive analysis of the exciton-band structure and the dynamics of the excited states in solution [5]. This analysis was based on the simulations of the A iso in the Frenkel exciton formalism [5][6][7][8][9] and on the discussion of the fluorescence excitation and emission data.…”

“…This analysis was based on the simulations of the A iso in the Frenkel exciton formalism [5][6][7][8][9] and on the discussion of the fluorescence excitation and emission data. A typical simulation is shown and is detailed in Fig.…”

“…To this end, we investigated the structure-spectroscopy -function relationship of the tetrachlorobenzimidazolocarbocyanine (TTBC) aggregates in (poly-vinyl alcohol) PVA thin films prepared by the VSCM employing polarized absorption spectroscopy, and discussed the experimental results within a model that we have recently proposed for the internal molecular packing and the structure and dynamics of excited states of TTBC aggregates [5].…”

Hierarchial Coassembly of a Cyanine Dye in Poly(vinyl alcohol) Fibrous Films by Electrospinning

“…Aggregation is driven mainly by the presence of a strong transition dipole moment formed by conjugated π-electrons of the polymethine backbone of cyanine dyes. 4 The construction of sophisticated molecular dye assemblies and highly ordered dye aggregates plays a significant role in designing smart materials because their optical properties show alteration depending on the structural organization of the dyes. 5 The understanding of internal organization and controlling the mesoscopic morphology of cyanine aggregates has provided a powerful way of developing techniques for fabrication of functional structures designed for specific purposes.…”