Optical Response Sensitive to the Assembly in a Molecular Material: Ultrathin Film with a Vanishing Electronic Absorption

Abstract

Optical properties of single-component molecular materials can often be described in terms of modest perturbations on the responses of the molecular building blocks; in specific systems, the modifications may be more pronounced. In all cases, however, the effects are broadly independent of the form of assembly, such as thin films, nanostructures and crystals, even though variations may exist at the quantitative level. We present a very different situation encountered with an amphiphilic molecule possessing a strongly polar head group that shows similar electronic absorption spectra in the solution as well as the bulk solid state, but a dramatically different one in ultrathin Langmuir-Blodgett (LB) films. The lowest energy absorption is switched off completely in the latter. It is demonstrated further that a metastable form of the LB film exhibiting the normal molecular spectrum can be generated, although its lowest energy absorption decays and vanishes over a period of time. The Langmuir films formed at the air/water interface and the corresponding LB films are examined through pressure-area isotherms, microscopy and spectroscopy. The experimental observations coupled with models constructed through computational investigations allowed us to interpret the unusual optical responses, which are a result of the specific supramolecular 2D assembly in the LB film and the impact of the neighboring molecular dipoles on the intramolecular charge transfer transition. The present study illustrates that selective molecular design and directed assembly can be exploited to elicit dramatic changes in physical responses of molecular materials and realize novel effects in molecular nanostructures.

Mechanical Control of Molecular Aggregation and Fluorescence Switching/Enhancement in an Ultrathin Film

“…Electronic absorption spectra of BODEPDQ LB films fabricated at different surface pressures are collected in Figure a. The characteristic UV absorption peak of DADQs at ∼240 nm is seen in all cases. The lowest energy absorption due to the intramolecular charge transfer in the zwitterionic chromophore occurs at ∼440 nm in the films deposited at ≤25 mN/m; the small blue shift relative to the absorption of BODEPDQ in chloroform solution may be attributed to the medium effects on the strongly dipolar chromophore.…”

“…Such a model that takes into account the most significant effect of the local electric field due to the neighboring, strongly dipolar zwitterionic molecules has been used to explain the vanishing electronic absorption in LB films of a related molecule. 34 The lowest energy excitations with significant oscillator strengths computed in the two cases (Figure 7c and 7d) are 443.0 and 373.4 nm, respectively. 28 The excitation energies computed at both levels are clearly consistent with the lowest-energy electronic absorptions manifested by the LB The final significant observation in this study is the evolution of the fluorescence emission from the LB films as a function of their deposition pressure.…”

“…Variations of the absorbance at the two λ max are shown in Figure 5b. It may be noted that the LB film of a related amphiphile, 7,7bis(octadecylamino)-8,8-dicyanoquinodimethane fabricated at 45 mN/m showed no trace of a low energy charge transfer absorption; 34 this was explained on the basis of the specific organization of the zwitterionic chromophores in the 2D assembly. The control case of OEOEP + Br − films deposited at similar surface pressures showed a steady increase in the absorbance (Figure 5, panels c and d); the slight broadening of the spectra of films formed at higher pressures may be attributed to the emergence of an absorption due to aggregates.…”

Extensively Reversible Thermal Transformations of a Bistable, Fluorescence‐Switchable Molecular Solid: Entry into Functional Molecular Phase‐Change Materials

“…Electronic absorption spectra of BEADQ in solution, microcrystalline solid and the Aand Cforms are collected in Figure 3a;t he inset shows the photographs of the latter. Broadening of the absorption spectra of the solids arises owing to the intermolecular interactions;the relatively higher broadening in Apoints to the disordered environment of the chromophores.T ou nderstand the influence of molecular interactions on the optical spectra of the crystals and thin films, [25,30] we have computed the electronic excitation energies of BEADQ molecule in different environments,using ab initio TD-DFT calculations at the B3LYP/6-31G* level. [26] Table 1s hows the good agreement between the l max computed for the lowest energy excitation having appreciable oscillator strength, and the relevant experimental observations;i nt he case of the solids,t he experimental value corresponds to the approximate midpoint of the broad peak.…”

Extensively Reversible Thermal Transformations of a Bistable, Fluorescence‐Switchable Molecular Solid: Entry into Functional Molecular Phase‐Change Materials

“…Broadening of the absorption spectra of the solids arises owing to the intermolecular interactions; the relatively higher broadening in A points to the disordered environment of the chromophores. To understand the influence of molecular interactions on the optical spectra of the crystals and thin films,25, 30 we have computed the electronic excitation energies of BEADQ molecule in different environments, using ab initio TD‐DFT calculations at the B3LYP/6‐31G* level 26. Table 1 shows the good agreement between the λ max computed for the lowest energy excitation having appreciable oscillator strength, and the relevant experimental observations; in the case of the solids, the experimental value corresponds to the approximate midpoint of the broad peak.…”