M. Canlıca
Journal of Photochemistry & Photobiology A: Chemistry 384 (2019) 112043
symmetry [58,59].
The IR spectrum of 3 exhibited a vibrational peak at 2232 cm−1,
indicating the presence CN. A diagnostic feature of 4–7 formation from
was the disappearance of this sharp CN vibrational peak. Otherwise,
The presence of two sharp peaks at 670 nm and 701 nm in the
spectrum of 5 suggests partial demetallation. Complex 5 also demon-
strated a blue-shift of the Q band relative to complex 4. Complex 4
exhibited only one Q band (at 684 nm), while both complexes showed
two vibrational bands at around 623 and 642 nm and a shoulder at
around 718 nm. Complex 7 also exhibited two vibration bands, at
611 nm and 631 nm. Complex 5 exhibited broad peaks at 606 nm and at
672 nm. Typical of ball-type phthalocyanines, the intensities of the B
bands were high relative to the Q band for all the complexes, which
may be due to intramolecular interactions between the Pc rings. The
aromatic groups and the distance between the two Pc units of ball-type
molecules substantially influence the degree of interaction between the
rings. It can be seen from the electron absorption spectrum of com-
plexes 4–7 that the Q band is broad. In addition, the Q band maximum
wavelength positions of all the complexes were shifted roughly 2 nm
longer at low concentrations. Thus, broadening of the Q band can be
attributed to intramolecular interactions.
3
the IR spectra of complexes 4–7 resembled that of compound 3.
Complexes 4–7 showed Ar-O-Ar peaks at 1246, 1254, 1271, 1261, and
−1
1
276 cm , respectively.
The H NMR spectra of complexes 3–5 and 7 recorded in DMSO
1
were also similar, although 4, 5, and 7 demonstrated more intricate
spectral patterns as they are mixed isomers. The H NMR spectrum of
complex 6 (CoPc) could not be obtained due to the presence of a
paramagnetic metal. The aromatic protons of 3 appeared at 8.31, 8.04,
1
7
1
.71, 7.46, and 7.35 ppm and integration yielded the expected total of
0 protons. The H NMR spectra of complexes 4, 5, and 7 exhibited
1
aromatic protons between 7.80 and 7.10 ppm, and integration yielded
totals of 44, 40, and 40 for protons, respectively (Please see Fig. S1 in
supplementary data).
Elemental analysis results were also consistent with the proposed
structures for compounds 3–7, yielding percentage carbon, hydrogen,
and nitrogene values within acceptable range for Pc complexes.
These purified Pc complexes 4–7 were further characterized using
mass spectroscopy. For their expected mass values, in the positive ion
and negative ion MALDI-MS spectra, the protonated, deprotonated,
3.2. Fluorescence spectra, lifetime (τF) and quantum yields (Φ
F
)
The fluorescence emission from MPcs is usually short lived, on the
−8
molecular ion peak, and fragment ions as adducted Li, Na, K, H
2
O were
order of 10
s. MPc fluorescence properties such as intensity and
observed [50]. This indicates that leaving groups are available for these
complexes under MALDI matrix conditions and under laser energy
quantum yield are influenced by multiple factors including aggregation,
solvent properties, concentration (due to quenching), the nature of the
central metal atom, substituent type (particularly halogenation), and
photo-induced energy transfer [10]. Fluorescence is reduced sub-
stantially in the presence of paramagnetic metals and metals of high
atomic number (due to the heavy atom effect). These types of com-
pounds encourage intersystem crossing (ISC), a spin-forbidden process
that occurs as a consequence of spin-orbit coupling. As a result of the
lower energy of emitted photons, Pc fluorescence emission spectra are
red-shifted relative to the absorption spectra (Fig. 3). This difference in
spectral position is known as the Stokes shift. Minimal re-arrangement
of the atomic coordinates during photoexcitation results in smaller
Stokes shifts [60]. In general, the shapes of excitation spectra are si-
milar to the corresponding absorption spectra; however, conforma-
tional re-organization during excitation may alter the shape of emission
spectra.
(
Please see Fig. S2 in supplementary data). The MALDI spectra con-
firmed further that the complexes were synthesized successfully and
separated efficiently.
The absorption spectra of monomeric metal phthalocyanines (MPcs)
are characterized by intense electron absorption between 600–750 nm
and the less intense broad band at ˜350 nm is the B band [51]. For
metal-free Pcs, Q band splitting is indicative of D2h symmetry while the
symmetry of MPcs is generally D4h. The absorption spectra of Pcs are
sensitive to the central metal, solvent, substitution pattern, and ag-
gregation [52–55]. Altered absorption features can also indicate the
presence of additional electronic levels in aggregates. The specific
molecular arrangement of Pcs in aggregates can result in broadening
and splitting of the main absorption Q band, with loss vibrational
component resolution and both a hypsochromic or bathochromic shift
[
56,57].
Complexes 4–7 exhibited Q band absorptions at 684, 670/701, 672,
The absorption, fluorescence excitation, and emission spectra of
complex 7 in DMSO are shown in Fig. 3a-b, and the band features of all
the complexes are summarized in Table 1 together with fluorescence
lifetime data (Please see Fig. S3 in supplementary data for the other
complexes). Complex 4 exhibited peak fluorescence emission at
698 nm, complex 5 at 694 nm, and complex 7 at 692 nm. Emission
maxima were blue-shifted owing to the atom size. All the complexes
showed similar fluorescence behavior, including the same Q band
maxima in absorption and excitation spectra. However, absorption and
emission spectra were broadened compared to emission spectra due to
molecular aggregation. The spectra of complexes 4 and 5 were much
broader than that of 7, suggesting more extensive aggregation. The
proximity of the Q band absorption peak to the Q band excitation
maximum for all the complexes suggests that the nuclear configurations
of the ground and excited states are similar and not altered during
excitation. In contrast to monomeric metal-free Pcs [30,32], the emis-
sion spectrum of complex 4 was not split and was broader. This may
explain which metal-free Pcs are known to fluoresce with only a main
peak assigned as the 0–0 transition of the fluorescence, similar to
complex 5 [26]. The emission peak of complex 4 was narrower since
aggregates are not known to fluoresce. The Stoke’s shifts of complexes
and 681 nm in DMSO, respectively (Fig. 2, Table 1). The Uv–Vis spectra
of these ball-type Pcs resemble those of other Pcs but with several in-
teresting differences. All showed unique spectral behavior in DMSO,
with a very broad band extending from 550 nm to 720 nm. The broad
vibrational band around 611–642 nm observed in the spectra of 4–7 has
been reported to indicate aggregation of Pc complexes, including ball-
type complexes. In addition, complex 5 showed low symmetry. The
unexpected differences in Q band shape among complexes, particularly
the split in complex 5 not observed in the others, suggest differences in
4
, 5, and 7 were 14, 24, 11 nm, respectively, typical for MPc complexes
30–33].
The fluorescence quantum yield Φ
complexes (in DMSO, 0.033 for 4, 0.084 for 5, and 0.058 for 7). The Φ
[
F
values were also typical of MPc
F
values are typically lower for ball-type Pc derivatives than for other
derivatives, as the ball-type structure may encourage ISC to the triplet
Fig. 2. Uv–Vis absorption spectra for compounds 4–7 in DMSO.
3