120
S. Singto et al. / Journal of Molecular Structure 1154 (2018) 114e130
isoquinoline under nitrogen at ambient room temperature. After
the solution was continuously stirred for 8 h, it was heated under
reflux and maintained at 220 ꢀC for 12 h. The solution was allowed
to cool to ambient room temperature, followed by slowly adding it
to vigorously stirred methanol. The resulting precipitated polymer
was collected by vacuum filtration, washed with cold methanol,
and dried under reduced pressure at 200 ꢀC for 24 h. The polymers
were obtained in 89e95% yields.
Eq. (1) defines Ff
!
ꢀ
ꢁ
ꢀ
ꢁ
Astd
A
n2
D
Dstd
Ff
¼
Ff ;std
(1)
n2std
where A is the absorbance at the excitation wavelength, n is the
refractive index of the solvent, and D is the integrated area under
the corrected fluorescence spectrum. Solutions of both the stan-
dard and sample were prepared initially with the absorbance at
lmax approximately equal to 0.5, followed by an accurate tenfold
dilution. Fluorescence spectra used to determine Ff were corrected
with sets of correction factors that were obtained by measuring the
spectra of compounds with known emission spectra [32].
Fluorescence lifetime (tf) is equal to the time after which the
fluorescence intensity of a fluorophore has dropped to 1/e of its
initial value after a pulsed excitation (where “e” is the Euler base).
In other words, it is a measure of the time a fluorophore spends in
the excited singlet state before emitting a photon and returning
back to the ground singlet state [17]. Eqs. (2) and (3) define tf
All polyimide compounds were structurally characterized by
1H NMR and ATR-IR spectroscopy. The Supplementary Information
provides the graphical 1H NMR and ATR-IR spectra. From the IR
characterization data, the absorption band of the amine functional
group was not observed, indicating the absence of precursor
amine monomer compounds in the polyimide compounds. In
addition, no absorption bands of both the amide and carboxylic
acid functional groups of the poly(amic acid) intermediate
appeared in the spectra, indicating that the poly(amic acid) was
completely converted to the corresponding polyimides during the
refluxing step. Complete conversion to the polyimides was
confirmed by the presence of the absorption bands at 1784 cmꢁ1
(asymmetric stretching C]O), 1728 cmꢁ1 (symmetric stretching
C]O), 1365 cmꢁ1 (CeN stretching), and 721 cmꢁ1 (C]O bending),
which are the characteristic absorption bands of the polyimide.
Polyimides in series I and II containing the free base cis- and trans-
DATPP have the internal NeH group in the porphyrin structure.
However, the characteristic NeH absorption band was not
observed in the IR spectra and this is potentially attributed to the
rigid structure of the polymer backbone chain, making it difficult
for NeH vibration to occur. 1H NMR spectral data of all polyimides
!
t
tf
IðtÞ ¼ I0ðtÞexp
ꢁ
(2)
(3)
1
tf
¼
kf þ knr
where I(t) and I0(t) are the relative fluorescence intensities at time,
t, and at time, 0; tf is the fluorescence lifetime, kf is the radiative
decay rate constant, and knr is the nonradiative decay rate constant
(knr ¼ kIC þ kISC).
shows a signal at
d 8.90 ppm, attributed to the hydrogen atoms
located at the positions on the pyrrole rings of the porphyrin
b
moiety. Polyimides in series I and II containing the free base cis-
and trans-DATPP showed the signal of the internal NeH proton at
Fluorescence lifetimes were measured with a Photon Technol-
ogy International® TM-3 time resolved spectrofluorometer with
pulsed nitrogen/dye laser excitation. All solutions were degassed
with N2 to prevent excited state quenching by molecular oxygen.
Felix32 analysis module was the computer software program used
for generating the time dependent fluorescence decay spectra. Data
analysis involved using a curve fitting procedure and best fit curves
were chosen on how well the statistically fitted curve overlaid with
d
ꢁ2.80 ppm, which was not present in the 1H NMR spectra for the
polyimides in series III and IV containing metalated cis- and trans-
ZnDATPP, confirming the occurrence of zinc metalation within the
porphyrin macrocycle. Furthermore, 1H NMR confirmed the
incorporation of the porphyrin units in the polymer structure.
the sample decay curve. The chi-squared (c
2) value had to lie within
2.4. Photophysical and fluorescence quenching measurements
the range of 0.9e1.2 for accurate measurements. In addition, the
fluorescence decay of an Instrument Response Function (IRF) was
measured to be at about the same maximum intensity as that of the
sample decay. The IRF used was an aqueous non-dairy creamer
solution.
Photophysical properties involved first measuring the UVeVi-
sible absorption spectra and fluorescence spectra of the target
compounds in dichloromethane and N,N-dimethylacetamide to
obtain the spectral maxima locations. Fluorescence quantum yield
(
Ff) is defined as the fraction of chromophore molecules under-
Fluorescence quenching was measured and observed by the
Stern-Volmer relationship, using 9,10-anthraquinone as the
quencher molecule. The Stern-Volmer relationship is expressed by
Eq. (4)
going fluorescence from the excited singlet state to the ground
singlet state, indicating a measure of fluorescence efficiency. Fluo-
rescence quantum yields were determined by comparing the cor-
rected integrated fluorescence spectrum of the sample with that of
a known standard using a set of correction factors for the in-
strument's photomultiplier tube (PMT) detector according to [32].
For the free base porphyrin compounds TPP, cis-DATPP, trans-
DATPP, and their corresponding polyimides, the following stan-
F0
Ft
¼ 1 þ KQ ½Qꢃ
(4)
where F0 and Ft are the relative fluorescence intensities in the
absence and presence of quencher (Q), [Q] is the molar concen-
tration of the quencher, and KQ is the Stern-Volmer constant
dards were used: TPP in toluene (Ff ¼ 0.1
0.001) [33], TPP in
dichloromethane (Ff ¼ 0.116, measured value), and TPP in N,N-
dimethylacetamide (Ff ¼ 0.15, measured value). For the Zn meta-
lated porphyrinic compounds ZnTPP, cis-ZnDATPP, trans-ZnDATPP,
and their corresponding polyimides, the following standards were
used: ZnTPP in toluene (Ff ¼ 0.033) [34], ZnTPP in dichloromethane
(KQ
¼
tfkq, where tf is the fluorescence lifetime in the absence of
quencher and kq is the bimolecular quenching rate constant).
The value of kq is approximately equal to the diffusion rate
constant (kd), which generally shows the rate which reactant
components A and B diffuse together in solution and collide,
defined by Eq. (5)
(
(
Ff ¼ 0.02, measured value), and ZnTPP in N,N-dimethylacetamide
Ff ¼ 0.024, measured value).