Chiroptical Properties in Thin Films of π-Conjugated Systems

Abstract

Chiral π-conjugated molecules provide new materials with outstanding features for current and perspective applications, especially in the field of optoelectronic devices. In thin films, processes such as charge conduction, light absorption, and emission are governed not only by the structure of the individual molecules but also by their supramolecular structures and intermolecular interactions to a large extent. Electronic circular dichroism, ECD, and its emission counterpart, circularly polarized luminescence, CPL, provide tools for studying aggregated states and the key properties to be sought for designing innovative devices. In this review, we shall present a comprehensive coverage of chiroptical properties measured on thin films of organic π-conjugated molecules. In the first part, we shall discuss some general concepts of ECD, CPL, and other chiroptical spectroscopies, with a focus on their applications to thin film samples. In the following, we will overview the existing literature on chiral π-conjugated systems whose thin films have been characterized by ECD and/or CPL, as well other chiroptical spectroscopies. Special emphasis will be put on systems with large dissymmetry factors (g abs and g lum) and on the application of ECD and CPL to derive structural information on aggregated states.

On the Optical Activity of Poly(l-lactic acid) (PLLA) Oligomers and Polymer: Detection of Multiple Cotton Effect on Thin PLLA Solid Film Loaded with Two Dyes

“…When several molecules are in close contact in a thin chiral film, exciton coupling occurs, which produces very a strong optical rotation signal, as the gigantic [α] λ values we are observing in the ORD spectrum of Figure 8. The exciton coupling is easily recognized in chiroptical spectra because it gives rise to two bands of opposite sign and similar amplitude [31]. For instance the exciton couplet at 353 and 405 nm in Figure 8 is an example, followed by another one at 480 and 600 nm.…”

“…The most intriguing result of this paper is certainly the ORD measurement made on the green thin PLLA film from used water bottles reported in Figure 8 showing a multiple Cotton effect with a certain degree of complexity, while the ORD curve of a transparent PLLA thin film is just a plain negative curve, as shown previously in Figure 4. The multiple Cotton effect in the green PLLA film is due to an extrinsic Cotton effect derived from the interaction of the helical structure of PLLA in the solid state and the two dyes SY93 and PB16 [53], as well as the mutual interaction of the two dyes in a chiral environment represented by the PLLA matrix in the solid state [31]. Similar extrinsic Cotton effect, but with considerably smaller ORD amplitude and complexity, has been detected also in our earlier works dealing with PLLA-iodine and PLLA-C 60 complexes [26,27].…”

“…Thus, a new Cotton effect in the ozonized PLLA film occurs below 300 nm and is due to the ozonolysis products of the dyes SY93 and PB16, which remain trapped in the bleached PLLA film. For the interpretation of the complex ORD spectrum of PLLA green film with two dyes, we will refer to the excellent review work on chiroptical properties of thin films of π-conjugated systems made by Albano, Pescitelli, and di Bari [31].…”

“…The form of the bisignate exciton couplets in Figure 8 may suggest a preferential stacking of the dye molecules following left-handed helices, the same helical conformation observed on PLLA in the solid state, i.e., the chiral medium embedding the dyes. For the interpretation of the complex ORD spectrum of PLLA green film with two dyes, we will refer to the excellent review work on chiroptical properties of thin films of π-conjugated systems made by Albano, Pescitelli, and di Bari [31].…”

“…The thermal stability of PLAO was subjected to a thorough investigation [14]. Then, the PLLA optical activity was studied in thin solid films derived from recycled PLLA water bottles, showing not only the anomalous and multiple Cotton effect due to a pigment embedded in the PLLA matrix but also gigantic solid state ORD properties which are easily accessible and may be exploited in new chirooptical devices involving display and wearable technologies, as well as the promise of efficient, large-area solar cells [31].…”