Photoswitchable Molecular Units with Tunable Nonlinear Optical Activity: A Theoretical Investigation

Abstract

The first-, second-, and third-order molecular nonlinear optical properties, including two-photon absorption of a series of derivatives, involving two dithienylethene (DTE) groups connected by several molecular linkers (bis(ethylene-1,2-dithiolato)Ni- (NiBDT), naphthalene, quasilinear oligothiophene chains), are investigated by employing density functional theory (DFT). These properties can be efficiently controlled by DTE switches, in connection with light of appropriate frequency. NiBDT, as a linker, is associated with a greater contrast, in comparison to naphthalene, between the first and second hyperpolarizabilities of the “open–open” and the “closed–closed” isomers. This is explained by invoking the low-lying excited states of NiBDT. It is shown that the second hyperpolarizability can be used as an index, which follows the structural changes induced by photochromism. Assuming a Förster type transfer mechanism, the intramolecular excited-state energy transfer (EET) mechanism is studied. Two important parameters related to this are computed: the electronic coupling (VDA) between the donor and acceptor fragments as well as the overlap between the absorption and emission spectra of the donor and acceptor groups. NiBDT as a linker is associated with a low electronic coupling, VDA, value. We found that VDA is affected by molecular geometry. Our results predict that the linker strongly influences the communication between the open–closed DTE groups. The sensitivity of the molecular nonlinear optical properties could assist with identification of molecular isomers.

HOMA Index Establishes Similarity to a Reference Molecule

“…Use of the HOMA Formula to Study Acyclic Molecules. The HOMA index revealed both an increase in delocalization in polyene-like structures and an increase in insulation in unsaturated hydrocarbons, 12,13,16 and a correlation with the boiling point in n-alkanes. 17 However, juxtaposing the HOMA indices taken against ethane and ethene or acetylene references in analogy to that presented in Section 3.3.1 could uncover interesting new molecular features.…”

“…The above data indicate that the HOMA index is more than just a geometrical aromaticity index. However, why it may be so is known . First, the HOMA index is connected to a distance in the abstract n -dimensional molecular space and, second, to a statistical value such as a variance .…”

“… 13 , 15 This can suggest the increase of π-electron delocalization with an increase in the number of units. 13 , 16 The HOMA indices were also used to study linear and branched alkanes, and for n -alkanes, a good correlation with the boiling points was found. On the other hand, for n -alkane constitutional isomers, the HOMA index tends to increase with the boiling points from the most branched to the most extended isomer.…”

“…However, why it may be so is known. 16 First, the HOMA index is connected to a distance in the abstract n -dimensional molecular space and, second, to a statistical value such as a variance. 17 Third, the association of HOMA with aromaticity is only through R opt , taken from the aromaticity standard: benzene.…”