Tuning of the Emission
Efficiency and HOMO–LUMO
Band Gap for Ester-Functionalized {Al(salophen)(H<sub>2</sub>O)<sub>2</sub>}<sup>+</sup> Blue Luminophors
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Abstract
A series of [Al<b>L</b>(H<sub>2</sub>O)<sub>2</sub>(NO<sub>3</sub>)] complexes, with <b>L</b> standing for an
ester substituted
salophen-type ligand, has been synthesized, and the luminescence properties
have been investigated. These derivatives differ by the nature of
the ester-R group introduced at the C5 position of their salicylidene
rings (i.e., phenyl, <b>7a,a</b>′; naphthyl, <b>7b,b</b>′; pentafluorophenyl, <b>7c,c</b>′; and <i>p</i>-nitrophenyl, <b>7d</b>) and by the bis-imino bridge
(i.e., 1,2- phenylene, <b>7a</b>–<b>d</b>; and
1,2-naphthalene, <b>7a</b>′<b>–c</b>′).
All the complexes are characterized by luminescence in the blue range,
the chemical diversity having no effect on the emission wavelength
(480–485 nm). However, the emission efficiency was found to
be strongly dependent on the Schiff-base ligand with quantum yields
ranging from ϕ = 22% to 44%, the highest values being for the
salophen derivatives with the electron-withdrawing ester-R groups
(<b>7a</b>, 34%; <b>7a</b>′, 23%; <b>7b</b>, 31%; <b>7b</b>′, 22%; <b>7c</b>, 40%; <b>7c</b>′, 29%, and <b>7d</b>, 44%). Both the electrochemical
data and DFT calculations show that the HOMO–LUMO band gap
is modified as a function of the ester R group (from 2.92 to 3.16
eV, based on the redox potentials). The crystal structures for the <i>N,N</i>′-bis(5-(phenoxycarbonyl)salicylidene)-1,2-phenylenediamine
and the <i>N,N</i>′-bis(5-(<i>p</i>-nitrophenoxycarbonyl)salicylidene)-1,2-phenylenediamine
aluminum complexes (<b>7a</b> and <b>7d</b>) are reported