posted on 2022-11-16, 20:33authored byMorris
E. Olumba, Ryan M. O’Donnell, Thomas N. Rohrabaugh, Thomas S. Teets
In this work, we introduce a series of cyclometalated iridium complexes
and evaluate the suitability of this class of compounds in nonlinear
optical (NLO) applications, with an emphasis on long-lived, panchromatic
reverse-saturable absorption (RSA). The investigated complexes are
represented by the general formula [Ir(C^N)2(CNArdmp)2]+, (C^N = cyclometalating ligand, CNArdmp = 2,6-dimethylphenyl isocyanide). Seven such complexes
were synthesized and characterized, including in-depth analysis of
their photophysical properties (UV–vis absorption, photoluminescence,
and transient absorption). This series of compounds contains seven
different cyclometalating ligands (2-phenylbenzothiazole (bt) (Ir1), 5-nitro-2-phenylpyridine (ppyNO2) (Ir2), 5-nitro-2-(9-phenanthryl)pyridine (phenNO2) (Ir3), 2-(benzo[b]thiophen-2-yl)quinoline
(btq) (Ir4), 6-(benzo[b]thiophen-2′-yl)phenanthridine
(btph) (Ir5), 2,4-diphenylquinoline (dpq) (Ir6), and 6-nitro-2,4-diphenylquinoline (NO2dpq) (Ir7)), which have profound effects on their ground-state and excited-state
absorption spectra. To evaluate the effects of the isocyanide ancillary
ligands, some heteroleptic bis-cyclometalated iridium(III) acetylacetonate
(acac) analogue complexes are included as points of comparison. In
the ground state, the bis-isocyanide complexes display UV–vis
absorption with the characteristic 1LC (π →
π*) band at λ < 350 nm and 1MLCT bands at
350–500 nm. Five of the complexes (Ir1, Ir2, Ir4, Ir5, and Ir6) display broad, intense triplet excited-state absorption with no
ground-state bleach (GSB) over the spectral window of 400–900
nm, with excited-state lifetimes spanning three orders of magnitude
from ∼32 ns to 12 μs. The photophysical data suggests
that the isocyanide ancillary ligand blue-shifts the GSB transient
into the UV, which is normally found in the middle of the visible
region for cyclometalated iridium complexes. This study demonstrates
the applicability of cationic cyclometalated iridium(III) bis-isocyanide
complexes as candidate RSA materials.