Optical Properties of DMA-π-DCV Derivatives: A Theoretical Inspection under the DFT Microscope

Joaquín Calbo

doi:10.1155/2016/3520698

Abstract

The optical properties of a series of donor-acceptor N,N-dimethylaniline-π-dicyanovinylene (DMA-π-DCV) chromophores have been investigated under the density functional theory framework. Focus has been made on the low-lying charge-transfer (CT) electronic transitions for which experimental data is available. The effect of theπ-conjugated bridge length and type was analysed between the families of oligoene and oligoyne derivatives of increasing size. Theoretical calculations demonstrate that the ethylene bridge is a betterπ-communicator and allows for more delocalized frontier molecular orbitals compared to the acetylene spacer. TheΛdiagnostic test allowed rationalization of the orbital spatial overlap in the main CT excitations. The performance of different density functional rungs was assessed in the prediction of the lowest-lying CT electronic transition. Surprisingly, most modern long-range corrected functionals demonstrated to provide among the largest errors, whereas hybrid functionals showed the best performance. Solvatochromism was confirmed in both oligoene and oligoyne compounds. A donor-acceptor-donor triad based on tetrathiafulvalene was utilised as a test system for the prediction of its two CT bands of different nature, energy, and intensity. The hybrid PBE0 (or a similar hybrid analogue) consolidates as the best choice for the prediction of CT excitations in the DMA-π-DCV push-pull family.

Highlights

Organic chromophores have attracted increasing attention in a variety of fields such as molecular electronics and optoelectronics due to their high degree of π-conjugation, giving them unique electronic and optical properties [1,2,3,4]
The highestoccupied molecular orbital (HOMO) tends to localize in the donor DMA moiety, whereas the lowest-unoccupied molecular orbital (LUMO) is mainly located on the acceptor DCV fragment
We report on the optical features of a list of pushpull chromophores based on the donor N,N-dimethylaniline and acceptor dicyanovinylene moieties through calculations using density functional theory

Summary

Introduction

Organic chromophores have attracted increasing attention in a variety of fields such as molecular electronics and optoelectronics due to their high degree of π-conjugation, giving them unique electronic and optical properties [1,2,3,4]. Donoracceptor (D-A) conjugated organic molecules constitute one of the most promising derivatives in this family, so-called push-pull systems, with particular interest in their secondand third-order nonlinear optical properties [5]. The charge transfer occurring in these systems is highly dependent on the nature (strength) of the donor and acceptor moieties, as well as on the π-conjugation efficiency and length of the bridge connecting both parts. In push-pull systems, the highestoccupied molecular orbital (HOMO) is usually placed in the donor moiety, whereas the lowest-unoccupied molecular orbital (LUMO) is found in the acceptor fragment. The photoexcitation of D-A entities produces a lowlying charge-transfer electronic excitation described by the one-electron promotion from the HOMO to the LUMO, where energy absorption highly depends on the linear D-A conjugation pathways [6, 7]. Stronger D-A coupling across shorter π-conjugated spacers leads to higher-energy absorptions, whereas weaker D-A coupling across longer spacers results in lower HOMO-LUMO gaps [8,9,10]

Methods

Results

Conclusion