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Influence of Structural Isomerism on the Photophysical Properties of a Series of Donor–Acceptor 1‑Naphthalenecarbonitrile Derivatives Possessing Amine Substituents
journal contribution
posted on 2020-03-04, 21:29 authored by Alexis
T. Phillips, Zhenning Yu, David J. Stewart, Thomas M. Cooper, Joy E. Haley, Loon-Seng Tan, Tod A. GrusenmeyerIn
an effort to probe the influence of structural isomerism on
the excited-state properties of a naphthalene-based donor–acceptor
(D–A) system, four 1-naphthalenecarbonitrile compounds with
amine substituents in the 2-, 3-, and 4-positions were synthesized
and their photophysical properties were examined. Specifically, the
molecules 2-dimethylamino-1-naphthalenecarbonitrile (2DA), 2-(1-piperidinyl)-1-naphthalenecarbonitrile (2P),
3-dimethylamino-1-naphthalenecarbonitrile (3DA), and
4-(1-piperidinyl)-1-naphthalenecarbonitrile (4P) were
studied. The substitution position of the amine donor has a significant
impact on both the ground-state absorption and excited-state properties
of the complexes in toluene
solution. The energy, band shape, and extinction coefficient of the
ground-state absorption spectra are highly dependent on the substitution
position of the amine donor. All of the derivatives exhibit fluorescence
at room temperature. The fluorescence observed from 2DA, 2P, and 3DA
demonstrates a vibronic structure with all three molecules possessing
Stokes shifts on the order of 40 nm, whereas the fluorescence observed
from 4P is broad and has a Stokes shift 2 times greater than the other
derivatives. The fluorescence lifetimes, fluorescence quantum yields,
and intersystem crossing quantum yields vary greatly with the substitution
position of the amine donor. 2DA and 2P display intermediate fluorescence
lifetimes (2.7 ns) and fluorescence quantum yields (0.20) while possessing
the greatest intersystem quantum yield (0.80). 3DA has a much greater
fluorescence lifetime (16.9 ns) and fluorescence quantum yield (0.82)
at the expense of the intersystem crossing quantum yield (0.12). 4P
has the shortest lifetime (0.53 ns), with the lowest fluorescence
and intersystem crossing quantum yields (<0.05). The singlet–triplet
energy gaps are nearly identical for 2DA, 2P, and 3DA with values
on the order of 0.70 eV. This singlet–triplet gap is larger
in 4P, with a calculated value of 0.94 eV. The triplet–triplet
absorption spectra of 2DA, 2P, and 3DA are similar. Broad peaks in
the UV and visible regions with maxima around 330 and 500 nm characterize
all three spectra. The triplet excited-state extinction coefficient
values for 3DA were found to be 1.5 times larger than those in 2DA
and 2P. The triplet–triplet absorption spectrum of 4P is markedly
different from the triplet–triplet absorption spectra of the
other derivatives. The spectrum is broad, with the four local maxima
observed at 374, 445, 624, and 774 nm. All four molecules display
delayed fluorescence and laser-power-dependent triplet excited-state
decay kinetics, indicating the involvement of triplet–triplet
annihilation in the deactivation of the triplet excited states. Both
the intrinsic triplet lifetimes and triplet–triplet annihilation
rate constants were determined. These values are similar for all of
the derivatives with triplet lifetimes on the order of 100 μs
and diffusion-controlled rates of triplet–triplet annihilation.
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triplet lifetimes3 DA1- naphthalenecarbonitrile compounds4 P2 PUV2 DAtriplet excited-state extinction coefficient values2 P displayexcited-state propertiesmolecules 2- dimethylamino -1-naphthalenecarbonitrileground-state absorption spectrafluorescence quantum yieldssubstitution positionlaser-power-dependent triplet excited-state decay kineticsStokes shift 2 timesamine donorderivatives exhibit fluorescence
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