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

Authors: Aggelos Avramopoulos, Heribert Reis, Demeter Tzeli, Robert Zaleśny and Manthos G. Papadopoulos

DOI: 10.3390/ molecules28155646

Abstract: The first-, second-, and third-order molecular nonlinear optical properties, including twophoton
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.