A negligible CT character of the lowest excited state of a novel complex of zinc tetraphenylporphyrin with axially bonded 2-(4-methoxy-trans-styryl)quinoline-1-oxide ligand: Experimental studies and TD DFT/CAM B3LYP [6-31G(d,p)] calculations

Article abstract of DOI:10.1016/j.poly.2014.12.025

The crystal structure and photophysical behavior of the novel (1:1) ZnTPP-MSQNO complex composed of ZnTPP unit with axially bonded 2-(4-methoxy-trans-styryl)quinoline-1-oxide ligand (MSQNO) has been extensively investigated both in solution and in the sol

Full text of DOI:10.1016/j.poly.2014.12.025

Polyhedron 88 (2015) 190–198
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
A negligible CT character of the lowest excited state of a novel complex
of zinc tetraphenylporphyrin with axially bonded 2-(4-methoxy-trans-
styryl)quinoline-1-oxide ligand: Experimental studies and TD DFT/CAM
B3LYP [6-31G(d,p)] calculations
a
b
c
Anna Szemik-Hojniak a, , Krzysztof Oberda , Irena Deperasinska , Yakov P. Nizhnik ,
⇑
´
Lucjan Jerzykiewicz a
a Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14 St, 50-383 Wroclaw, Poland
b Institute of Physics, Pol. Acad. of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland
c Chemistry Department, Bioo Scientific Corp., 7050 Burleson Rd., Austin, TX 78744, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The crystal structure and photophysical behavior of the novel (1:1) ZnTPP–MSQNO complex composed of
ZnTPP unit with axially bonded 2-(4-methoxy-trans-styryl)quinoline-1-oxide ligand (MSQNO) has been
extensively investigated both in solution and in the solid state. The single-crystal X-ray measurements,
stationary absorption and emission spectra and the time-resolved fluorescence spectroscopy combined
with TD DFT/CAM B3LYP [6-31G(d,p)] calculations were employed. The crystal structure is characterized
Received 13 October 2014
Accepted 19 December 2014
Available online 7 January 2015
Keywords:
Crystal structure
ꢀ
by the centrosymmetric triclinic unit cell, space group P1. Zinc atom in the studied complex is pulled out
0.365 Å from the porphyrin macrocycle and the MSQNO ligand occurs in the anti-rotameric form of the
trans-isomer. The complex formation in the solution is confirmed by the appropriate red shift of the Qx
and Qy band in the absorption spectrum in toluene, which is in a good agreement with the results
obtained by theoretical calculations. We have found that effective mixing of electronic configurations
of the locally excited states with those of the charge transfer type leads to a negligible CT nature of
the S1 state of ZnTPP–MSQNO complex. We also hypothesized, that due to an overlap of the absorption
spectrum of MSQNO and ZnTPP in the Soret band region, the electronic coupling becomes possible and
the nonradiative energy transfer process as an additional deactivation channel may take place. An unusu-
ally short lifetime (0.6 ns) was revealed for the solid complex, while in toluene (1.5 ns) and in n-propanol
(2.7 ns), they shown to be several times longer.
Zinc tetraphenylporphyrin
2-(4-Methoxy-trans-styryl)quinoline-1-
oxide
Lifetimes
CAM B3LYP calculations
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
the most extensively studied among metalloporphyrins [8–12],
often leads on irradiation to photoinduced electron transfer and
Porphyrins and metalloporphyrins are specific building blocks
for synthesis of donor–acceptor systems, in which photoinduced
electron transfer process and the charge transport mechanisms
can be studied [1]. The latter is important in understanding
reaction mechanisms both in natural and artificial systems and
is crucial in development of technologies connected to mole-
cule-based devices [2–6]. One way of obtaining such type of con-
structs is the ligand coordination to metalloporphyrin moiety. It
is known, in fact, that interaction of organic ligands with Lewis
acids may strongly affect the energy and electron transfer
processes in the systems of interest [7]. Hence, coordination of
ligand, for example, to zinc tetraphenylporphyrin (ZnTPP), as
appearance of excited states of the charge transfer (CT) nature
[13].
Also investigations of covalently linked porphyrin-fulleren
dyads have shown that ligation provides a practical tool to control
the charge separation (CS) and the charge recombination (CR)
reactions [14].
In these systems, the quantitative analysis of both intramolecu-
lar exciplex formation and electron transfer reactions have been
investigated by means of time-resolved transient absorption and
emission decay measurements in a range of solvents with different
polarities [15,16].
In turn, systems consisting of multiple porphyrin units may be
explored as models for practical charge-separation devices, as for
example, in organic photovoltaics solar cells [17–20] or photo
sensors [21].
⇑
Corresponding author. Tel.: +48 (71) 375 7366.
E-mail address: anna.szemik@chem.uni.wroc.pl (A. Szemik-Hojniak).
http://dx.doi.org/10.1016/j.poly.2014.12.025
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

A. Szemik-Hojniak et al. / Polyhedron 88 (2015) 190–198
191
An interesting sources of the results on electron and energy
2. Materials and methods
transfer are the molecular complexes of ZnTPP with axially
coordinated ligands. In recent years a variety of such systems have
been designed [8–11]. Their photophysical properties and
specifically, the excited electronic states, have been the subject of
considerable interest and study. Many of them involve
heteroaromatic N-oxides [22–24], which show a push–pull behav-
ior and are the components of great amount of natural products
with specific biological properties [25]. Hence, their constructs
with ZnTPP moiety forms a new sub-group of D-A systems
of unique photophysical properties and promising future
applications.
Recently, we also used the iso-quinoline N-oxide (IQNO) and
1,4-dioxane as ligands to obtain ZnTPP–IQNO [26] and ZnTPP–
(dioxane)2 [27] complexes, respectively, where they are axially
coordinated to the ZnTPP unit. It should be clarified in this place,
however, that the ZnTPP in the excited state may become an elec-
tron donor, despite that in the ground state it is a Lewis acid. This is
the case for the ZnTPP–IQNO complex [26]. From TD DFT CAM-
B3LYP/6-31G(d,p) calculations and specifically from analysis of
electronic configurations, we found that upon photoexcitation,
the internal transfer of charge occurs between the ZnTPP moiety
acting here as an electron donor, and the iso-quinoline N-oxide
as electron acceptor. In consequence, the S1 excited state shows
the CT nature.
On the other hand, in a unique structure of ZnTPP–(dioxane)2
complex [27], where two dioxane molecules are connected to
one ZnTPP unit in equatorial and in axial position, the ZnTPP dem-
onstrates a state rarely observed in the excited state the role of
electron acceptor. Analysis of TD DFT B3LYP/6-31G(d,p) electronic
wave functions of this complex, also identifies new CT bands,
absent in its components. These bands correspond to the excited
state transitions from the HOMO (Highest Occupied Molecular
Orbital) localized on dioxane molecule into direction of two LUMO
(Lowest Unoccupied Molecular Orbitals) orbitals placed on the
ZnTPP unit.
2.1. Materials
Zn-tetraphenylporphyrin, 2-methylquinoline-1-oxide, 4-metho-
xybenzaldehyde, and CH3OK as well as methanol, ethanol and
acetone for synthesis and recrystallizations were of analytical
grade (POCH, Sigma Aldrich). Solvents for absorption, steady state
and time-resolved fluorescence were of spectroscopic grade and
used fresh as purchased (Merck, Uvasol).
2.1.1. Synthesis of 2-(4-methoxy-trans-styryl)quinoline-1-oxide
2-(4-Methoxy-trans-styryl)quinoline-1-oxide [MSQNO] was
synthesized using the method described in literature [38] as fol-
lows: 1.6 g (0.01 mol) of 2-methylquinoline-1-oxide and 1.9 g
(1.7 mL, 0.014 mol) of 4-methoxybenzaldehyde were placed into
5 mL of 10% CH3OK in methanol. The mixture was refluxed 3 h
under nitrogen and then the solvent was evaporated in vacuo.
The substance was twice recrystallized from ethanol. Yield:
1.94 g (70%), Yellow powder; M.p. 142.5–143 °C (140 °C [38]).
2.1.2. Synthesis of ZnTPP–MSQNO complex
To 34 mg (0.05 mM) of Zn-TPP (obtained as described in [26])
dissolved in 15 mL of acetone, 1 mL of acetonic solution of 2-(4-
methoxy-trans-styryl)quinoline-1-oxide (13.9 mg, 0.05 mM) was
added. The red-violet crystals appeared thereafter and were
washed with acetone (1 mL, 2 times) and dried in air. Yield: 70%
(33.5 mg). Anal. Calc. for C62H43N5O2Zn: C, 77.94; H, 4.54; N,
7.33. Found: C, 77.96; H, 4.62; N, 7.24%. The substance was slowly
recrystallized from acetone before picking up the crystal for X-ray
analysis.
2.2. Methods
2.2.1. X-ray structure determination and refinement
Intensity data collection for X-ray structure determination was
In the present work a continuation of the two previously
published papers, the ZnTPP moiety is covalently linked to 2-(4-
methoxy-trans-styryl)quinoline-1-oxide (MSQNO) molecule to
form the ZnTPP–MSQNO complex.
The choice of this ligand, however, was not accidental. Recently,
we studied the excited state processes in the N-oxide of another
styryl derivative, i.e. trans-(dimethylamino-styryl) pyridine N-
oxide (DPO) [28] where in a wide range of various solvents, an adi-
abatic, ultra-fast photoinduced electron-transfer and an ultra-fast
solvent relaxation leading to the S1 emissive state of the CT charac-
ter have been confirmed. Our results on DPO are in line with
previously extensively discussed in the literature data on the
trans-4-(dimethylamino)-40-cyanostilbene (DCS) [29].
Another reason for undertaking this study is the biological
activity of heterocyclic N-oxides related to their ligation to metal-
loporphyrins [30]. Our experimental results indicate that the most
important process leading to apoptosis of the leukemic cell of the
K562 type is coordination of these N-oxides to the cytochrome b,
c1, and P450 [31,32].
carried out on a KUMA KM4 j-axis diffractometer equipped with a
CCD camera and an Oxford Cryo-system. All data were corrected
for Lorentz and polarization effects. Data reduction and analysis
were carried out with the KUMA diffraction programs [39]. The
structure was solved by the direct methods and refined by the
full-matrix least squares method on F2 data using the SHELXTL (ver-
sion 5.1) program [40]. The X-ray crystal structure of ZnTPP–
MSQNO complex is presented in Fig. 1 whereas the corresponding
experimental details are summarized in Table 1. The data of crystal
structure of ZnTPP–MSQNO complex [(
phenyl)-trans-ethen-1-yl}quinolone-1-oxide]-(
j
O1-2-{2-4-methoxy-
,b, ,d,-tetra-
a
c
phenylporphinato) zinc (II)] have been deposited at the Cambridge
Crystallographic Data Centre. The data have been assigned the fol-
lowing deposition numbers: CCDC (szem11) 983906.
2.2.2. Electronic absorption spectra
Electronic UV–Vis absorption spectra of ZnTPP–MSQNO com-
plex in toluene (spectral grade) was recorded on Perkin Elmer
Lambda 25 Spectrophotometer with concentration of 4 ꢀ 10ꢁ5M.
Finally, all the above mentioned reasons and important roles of
styryl quinolines both in pharmacology [33,34] and in material sci-
ences [35–37], have focused our interest in studying the optical
properties of the complex where MSQNO ligand, as styryl deriva-
tive, is linked to the ZnTPP unit. Our experimental results involve
the X-ray crystal structure of this complex as well as the stationary
absorption and emission spectra along with the time-resolved
emission. The whole experimental data are complemented by the
results of theoretical calculations on DFT and TD DFT/CAM level
of theory.
2.2.3. Steady state emission spectra
Emission spectra both in toluene and in the solid state were
recorded on FSL920 combined fluorescence lifetime and steady
state spectrofluorimeter (Edinburgh Instruments Ltd.) using as
excitation source Xe900, 450 W steady state xenon lamp (ozone
free) with computer controlled excitation shutter and with spectral
bandwidth of 65 nm for both excitation and emission spectra.
Luminescence was detected using a red sensitive (185–850 nm)
single photon counting photomultiplier tube (R928-Hamamatsu)

192
A. Szemik-Hojniak et al. / Polyhedron 88 (2015) 190–198
CAM-B3LYP/6-31G(d,p) level with the GAUSSIAN 09 [41] package of
programs. The geometric structures optimized by DFT B3LYP/6-
31G(d,p) method and experimentally obtained from the X-ray
studies have been taken into consideration.
3. Results and discussion
3.1. Crystal structure of ZnTPP–MSQNO complex
3.1.1. Monomer of ZnTPP–MSQNO complex
Mixing of acetone solutions of Zn-TPP and 2-(4-methoxy-trans-
styryl)quinoline-1-oxide produced the red-violet crystals of 1:1
ZnTPP–MSQNO complex. Its X-ray structure is shown in Fig. 1.
Single-crystal X-ray studies revealed that the ZnTPP–MSQNO
crystal is characterized by a centrosymmetric triclinic unit cell
ꢀ
containing two molecules of the complex, space group P1 (see
Fig. 1. X-ray, crystal structure of ZnTPP–MSQNO complex. The hydrogen atoms are
omitted for clarity.
Table 1 for other details).
The X-ray study of the complex showed, that both the bond
lengths and the valence angles in ZnTPP moiety resemble those
found in other complexes of ZnTPP unit. The same situation can
be observed in the case of the ligand molecule. Some geometric
parameters are described in Table 2.
Table 1
Crystallographic parameters of ZnTPP–MSQNO complex, and details of the structure
refinements.
In the case of trans-2-styrylquinolines, two possible conforma-
tional stereoisomers of the syn- and anti-type may be distinguished
[42–44]. These isomers can be described by relations between the
two axes (A and B) occurring between particular bonds or atoms, as
illustrated in Fig. 2. The A axis, passing through the N–O bond
(Fig. 2, right) and the B axis, passing through the carbon atoms of
the ethene moiety, may form the angle of about 60° (Fig. 2, left).
Our DFT B3LYP/6-31G(d,p) energy calculations for the syn- and
anti-isomers of the targeted 2-(4-methoxy-trans-styryl)quinoline-
Compound
[j
O1-2-{2-(4-methoxyphenyl)-trans-ethen-1-
yl}quinoline-1-oxide]-(
a,b,c,d-tetraphenylporphinato)
zinc (II)]
C62H43N5O2Zn
955.40
0.71073
100(1)
Formula
Formula weight
k (Å)
T (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
ꢀ
P1
9.796 (3)
13.587 (2)
19.223 (5)
72.72 (3)
89.96 (2
70.81 (3)
2293.9 (10)
2
1-oxide ligand, showed a quite small energy difference [Esyn
anti = ꢁ0.20 kcal/mol] between them. This means that both of
them can exist in equilibrium.
–
E
a
(°)
b (°)
c
(°)
The crystal structure analysis of the ZnTPP–MSQNO complex
reveals that the MSQNO ligand is represented by an anti-rotamer
with dihedral angle between the quinoline part and the benzene
rings of about 20°. This angle, results probably from a steric inter-
action between the styryl residue and the atoms of the porphyrin
macrocycle.
It is known that in the 1:1 complexes of Zn-TPP: ligand, the zinc
atom is usually slightly pulled out of the porphyrine plane towards
the ligand [45–47]. The degree of such a shift depends on the Zn–L
distance. Examples of the corresponding distances in the N- and O-
centered ligands are given in Table 1S of Supplementary Data (SD).
Using them, the plot in Fig. 3, presenting the Zn–L distances versus
porphyrine plane-Zn distance for both types of ligands, was con-
structed. It shows that the N- and O-ligands form two separate
groups with a larger shift of the zinc atom for the group of the
V (Å3)
Z
Dcalc (g cmꢁ3
)
1.383
0.590
l
(mmꢁ1
n measured
)
44916
19463
13026
n independent
n observations
(I P 2
R(I P 2 (I))
Rw(I P 2 (I))
r(I))
r
0.0404
0.0895
0.914
r
Goodness-of-fit
(GOF)
in a Peltier cooled housing. Spectra were corrected for detector
response and excitation source. The concentration of each solution
was about 10ꢁ5 M.
Table 2
Selected experimental values of bond lengths (Å) and angles (°) of ZnTPP–MSQNO
complex.
2.2.4. Time-resolved emission
Fluorescence decay curves were recorded using nanosecond
Time-Correlated Single Photon Counting (TCSPC) option of
FSL920 setup (Edinburgh Instruments Ltd.) with LED as the excita-
tion source. The excitation was done at 375 nm. Data acquisition
ensured Plug-in PC Card Model TCC900 with Maximum Count Rate
3 MHz, Time Channels per Curve up to 4096, Minimum Time per
Channel 610 fs. To detect the emission, the MiniRed (185–
850 nm) single photon counting photomultiplier was applied.
Distance Bond length
Type of angle
Valence or dihedral angle
(°)
0
(AÅ)
Zn–O1
Zn–N1
Zn–N2
Zn–N3
Zn–N4
2.136 (2)
2.054 (2)
2.073 (2)
2.055 (2)
2.094 (2)
Zn1–O1–N5
O1–Zn1–N1
O1–Zn1–N2
O1–Zn1–N3
O1–Zn1-N4
C59–O2–C62
N5–C45–C54–
C55
122.42 (8)
97.52 (5)
94.54 (5)
98.48 (5)
106.96 (5)
117.28 (12)
ꢁ156.08 (13)
N5––O1 1.335 (2)
O2–C59
1.372 (2)
2.2.5. Theoretical calculations
Theoretical calculations of electronic spectra for ZnTPP–
MSIQNO complex and its components were performed at TD
O2–C62
1.430 (2)
C46–C45–C54–
C55
21.6 (2)

A. Szemik-Hojniak et al. / Polyhedron 88 (2015) 190–198
193
N-ligands. For a relatively small value (2.136 Å) of the Zn–L dis-
tance in ZnTPP–MSQNO complex, the ‘‘outlet’’ value is 0.365 Å
(Fig. 3, red crossed square point). It seems, that such pushing of
the Zn atom out of the porphyrin plane may be caused by the
van der Waals repulsion induced between the porphyrin ring and
the ligand.
In the ZnTPP–MSQNO complex, the oxygen atom of the N–O
group is sp3-hybridized since the dihedral angle between the least
square planes drawn on the quinoline system and Zn–O–N atoms
is 81.00°. Hence, because the dihedral angle formed between the
porphyrin and the quinoline ring is 33.32°, thus it is too big for
We have employed the CAM-B3LYP functional because it is known
that the TD DFT B3LYP method has a tendency to underestimate
the energy of charge transfer excited states and to increase the
dipole moment values [53]. Our decision was further supported
by the literature data showing that this method was employed
with reliable results both for ZnTPP molecule and for its supramo-
lecular arrays [54,55]. Furthermore, we have also used this
functional for previously studied complex of ZnTPP with
any
hydrogen atoms of adjacent phenyl group of ZnTPP unit can form
a quite strong C–Hꢀ ꢀ ꢀ interactions.
p–p interactions between them to occur. Nonetheless, the
p
3.1.2. Supramolecular interactions in ZnTPP–MSQNO complex
The ZnTPP–MSQNO system forms a supramolecular assembly
where particular molecules of the complex interact with each
other mainly via C–Hꢀ ꢀ ꢀ
p interactions. The two molecules of the
complex form a supramolecular Van der Waals dimer (Fig. 4), in
which the hydrogen atoms (H23A) of the phenyl rings give quite
strong C–Hꢀ ꢀ ꢀ
p interactions with the phenyl group (C39–C44) of
adjacent complex molecule.
The dimers interact with each other resulting in formation of
the infinite ribbons (thickness 7.740 Å, width 19.639 Å) propagat-
Fig. 3. The zinc atom shift versus the Zn–L(ligand) distance. The squares indicate
the Zn-TPP complexes with the O-type ligands and the rhombs, for those with the
N-ligands. The crossed square (2.136; 0.365) is the point for the ZnTPP–MSQNO
complex.
ing along the b-axis and bound to each other by the C–Hꢀ ꢀ ꢀ
p inter-
actions. The four phenyls of the four adjacent molecules of the
complex form a curious Van der Waals cycle while the ribbons
lie parallel to each other, forming a quasi two dimensional layer
on account of the multiple short contacts (C62–H12 2.881 Å; O2–
H12 2.657 Å; C58–H13 2.809 Å; O1–H36 2.537 Å; C14–H37
2.824 Å), (Fig. 1S of Supplementary Data). In turn, the layers lie
parallel to each other with 3.666 Å displacement perpendicular
to the b-axis due to which 3D crystal lattice is being formed
(Fig. 2S of Supplementary Data). The examples of the short contacts
are the following: c40–c41 3.282 Å; c34–c60 3.339 Å; C8–C47
3.368 Å; C8–H47 2.829 Å; N1–H26 2.733 Å; C4–H26 2.646 Å).
3.2. Absorption spectra and theoretical calculations
Generally, complex formation between an axially coordinated
ligand and the ZnTPP unit is confirmed by a substantial red shift
of both vibronic Q sub-bands of parental ZnTPP molecule [48–
52]. As seen in Fig. 5, an experimental red shift confirming exis-
tence of ZnTPP–MSQNO complex in toluene solution is distinctly
noticeable and for the Qx and Qy band, is 12.5 and 14.0 nm,
respectively.
Fig. 4. Supramolecular dimer of ZnTPP–MSQNO complex. The ligand moieties are
omitted for clarity. The H23A, the phenyl ring centroid distance is 2.465 Å; C39–
H23A, 2.799 Å; C41–H23A, 2.899 Å; C42–H23A, 2.876 Å; C43–H23A, 2.761 Å; C44–
H23A, 2.711 Å.
Such spectral behavior was also predicted on the basis of TD
DFT/CAM B3LYP [6-31G(d,p)] calculations, as presented below.
Fig. 2. Two rotamers of trans-2-(4-MSQNO) ligand: anti-rotamer (left) and syn-rotamer (right); B3LYP/6-31G(d,p) calculations.

194
A. Szemik-Hojniak et al. / Polyhedron 88 (2015) 190–198
iso-quinoline N-oxide ligand (ZnTPP–IQNO) for which we have
obtained quite realistic energy estimates of CT states [26].
In the present case, the calculations were performed both for
the complex in its experimentally determined crystal structure,
as described in the Section 3.1 (Table 2), as well as for its isolated
components in the gas phase. Now, we attempt to analyze the orbi-
tal energy diagram and frontiers of molecular HOMO and LUMO
orbitals to learn how they contribute to electronic configurations
forming the lowest excited states of the complex. Their nature is
demonstrated in Fig. 3S (Supplementary Data). All the orbitals of
ZnTPP–MSQNO, contrary to those observed in the recently studied
ZnTPP–(dioxane)2 complex [27], are virtually localized exclusively
on one component, i.e., either on MSQNO ligand or on ZnTPP moi-
ety. As can be seen, during the HOMO ? LUMO transition in the
complex, only partial charge redistribution from the ZnTPP-elec-
tron donor to MSQNO fragment takes place. For this reason, a
resulting electronic S1 state exhibits only a weak CT character.
The TD DFT/CAM-B3LYP/6-31G(d,p) calculated excitation ener-
gies and oscillator strengths of electronic transitions in the crystal
of ZnTPP–MSQNO complex and its components in the gas phase
are gathered in Table 3. In this table, the contributions of Hi, Lj elec-
tronic configurations (Hi means i-th HOMO orbital and Lj-means j-
th LUMO orbital) to the particular transitions (with Hi and Lj
showed in Fig. 3S of Supplementary Data) are also presented.
It should also be noted that due to interactions in the complex,
the orbital degeneracy in the isolated, symmetric ZnTPP moiety is
removed [26,27].
In order to obtain a more transparent and understandable
description of electronic configurations, they have been divided
into three groups configurations localized exclusively on ZnTPP
moiety, those of the LE and CT character in the crystal structure
of the complex, as well as the orbitals distributed on MSQNO
ligand in the gas phase. Thus, in Table 3, one can find, for example,
that the first singlet state (S1) of the complex (571.7 nm), which is
red shifted in comparison to the S1 state of isolated ZnTPP unit
(553.5 nm), has only about 9% admixture of the CT character.
This is somewhat surprising that in the lowest electronic state
only a small fraction of electron is transferred from the ZnTPP
donor to the remainder of the molecule while a significantly large
contribution appears only in the higher excited states. These CT
states are present in the spectral region between the Soret band
and the Q band. In the S3 (491.0 nm) state, for example, which
results from the HOMO ? LUMO transition, and in the S4
(437.5 nm) state formed due to the HOMOꢁ1 ? LUMO transition,
the CT nature reaches 81% and 90%, respectively. Also the two high
energy bands lying above the Soret band of ZnTPP unit are charac-
teristic of a relatively strong CT character (346 nm, 62% and
342 nm, 34%).
To explain the loss of CT character in the S1 state of ZnTPP–
MSQNO complex and analyzing its absorption spectrum, one may
note that the band belonging to MSQNO ligand overlays the Soret
band region of ZnTPP unit. Hence, in consequence, electronic cou-
pling between the ligand electronic states and the higher lying
states of ZnTPP in the complex seems to be possible. We may
hypothesize that excitation of the ZnTPP–MSQNO complex band
at wavelengths shorter than the Soret band may not lead to elec-
tron transfer followed by the CT (S1) emissive state, but rather to
the nonradiative S1 energy transfer, as it was observed in the
donor–bridge–acceptor (D–B–A) systems containing zinc tetra-
phenylporphyrin [56]. To investigate this process in more detail,
the studies of excited-state dynamics of ZnTPP–MSQNO complex
by means of transient absorption are required.
The TD DFT/CAM-B3LYP/6-31G(d,p) simulated absorption spec-
tra of the ZnTPP–MSQNO complex and its components in their
structure as they reside in the crystal are shown in Fig. 6 while
those of the complex and its isolated components (ZnTPP and
MSQNO) in the gas phase optimized geometries are presented in
Fig. 4S of Supplementary Data. In both figures the overlap of higher
energetic bands of ZnTPP with those of MSQNO ligand is readily
visible.
Our further aim of simulating the absorption spectra of this
complex was to demonstrate a red shift of the Q-band of ZnTPP
unit as evidence of its coordination to the ligand.
The results of calculations using the GaussSum program [57],
with a fwhm of 3000 cmꢁ1, suggest that the complex formation
should lead to a 18 nm red shift of the Q-band with respect to that
of the ZnTTP moiety; these calculations agree with experimentally
determined 14 nm red shift. The same similarities occur between
experimental (Fig. 5) and calculated vertical excitation energies
and oscillator strengths (Fig. 6, Table 3), which supports our choice
of using here the CAM B3LYP functional.
In Fig. 6, apart from the redshift of the Q-band, a general visu-
alization of changes anticipated also in the Soret band region and
resulting from the mixing of all three types of electronic configura-
tions is captured. Due to its abovementioned overlap with the cor-
responding band of the MSQNO ligand, the analysis of the Soret
band in solution is difficult because to ensure stability of the com-
plex a relatively large excess of the ligand is used. Nonetheless,
examination of the Soret band in this type of complexes may be
an important source of information about the structure and envi-
ronment of the complex [58,59].
Results of TD DFT/CAM-B3LYP/6-31G(d,p) calculations show
the ZnTPP–MSQNO complex as a strongly polar system with a large
ground state dipole moment (7.37 D). Its direction is nearly parallel
to the short inertial axis of the molecule and perpendicular to the
porphyrin plane.
Further proof of ZnTPP–MSQNO complex formation comes out
from the experimental emission spectra presented in Fig. 7. Its
fluorescence spectral profile in toluene shows the Qx and Qy bands
that are centered at ꢂ610 and 653 nm, respectively. Relative to
parental fluorescence bands in ZnTPP moiety, red shift of 15 and
18 nm, respectively, are achieved.
At low temperatures, the solvent viscosity gradually increases
leading to fluorophore emission prior to solvent relaxation. Then,
the fluorescence may origin from the unrelaxed Franck–Condon
(F–C) state and in this case the fluorescence maximum shifts blue.
In the lowest temperatures, the excited molecule may fluoresce
from a relaxed state and then maximum red shift is observed [60].
The Qx and Qy fluorescence bands of studied ZnTPP–MSQNO
complex in ethanol in the temperature range 240–80 K are pre-
sented in Fig. 8.
Fig. 5. Changes in the electronic absorption spectrum of Zn-TPP in toluene upon
addition of MSQNO ligand (2–120 excess). Concentration of Zn-TPP = 4 ꢀ 10ꢁ5 M.
Stability constant = 689 31 Lꢀmolꢁ1
. Spectrum of Zn-TPP: 549.5 nm; 588 nm.
Spectrum of the complex: 562 nm; 602 nm.
Dk(Qx) = 12.5 nm, Dk(Qy) = 14 nm.

A. Szemik-Hojniak et al. / Polyhedron 88 (2015) 190–198
195
Table 3
The wavelengths and oscillator strengths corresponding to the calculated vertical transition energies for ZnTPP–MSQNO complex along with description of their different
electronic configurations (see text and Fig. 3S). In the last entries, the corresponding data for the isolated components of the complex; TD DFT/CAM-B3LYP/6-31G(d,p) method.
ZnTPP–MSQNO
E [nm]
ZnTPP
MSQNO
D
f
Description of transition
LE (ZnTPP) [%]
D
E [nm]
f
D
E [nm]
f
CT [%]
LE (MSQNO) [%]
–
571.7
569.2
491.0
0.031
0.014
0.005
H(a) ? L(b)+1
Hꢁ1 ? L+2
H ? L+2
Hꢁ1 ? L+1
H ? L+2
H ? L+1
–
ꢁ49%
29%
H ? L
ꢁ9%
ꢁ4%
553.5
549.9
–
0.016
0.009
–
–
–
–
–
–
–
–
–
ꢁ46%
H ? L
–
–
32%
Hꢁ1 ? L
ꢁ10%
ꢁ7%
H ? L
ꢁ81%
ꢁ90%
437.5
384.7
0.029
0.522
Hꢁ1 ? L
–
–
–
Hꢁ1 ? L+1
H ? L+2
Hꢁ1 ? L+2
H ? L+1
–
51%
ꢁ31%
60%
ꢁ31%
Hꢁ2 ? L
10%
366.3
1.387
–
–
–
–
377.0
1.024
–
365.0
1.456
346.5
342.4
0.312
0.823
H ? L+3
H ? L+3
ꢁ62%
ꢁ34%
Hꢁ2 ? L
Hꢁ2 ? L
28%
48%
–
–
–
–
Hꢁ1 ? L+1
4%
349.3
0.499
The bold state shows a strong CT nature.
(a)
H – means HOMO orbital (Highest Occupied Molecular Orbital).
L – means LUMO orbital (Lowest Unoccupied Molecular Orbital).
(b)
With decreasing temperature (lines 1–7), the position of the Qx
situation was observed in previously studied solid ZnTPP–IQNO
complex [26]. The fluorescence spectra of both complexes are pre-
sented in Fig. 9.
and Qy bands shifts blue with respect to the room temperature
conditions (599, 655 nm) and at 120 K they peak at 595 and
652 nm, respectively. Only at 100 K, both peaks move slightly red
(line 8), reaching at 80 K (line 9) the appropriate maxima at 602
and 658 nm. Only at 100 K, they move slightly red (line 8), reaching
at 80 K (line 9) the appropriate maxima at 602 and 658 nm. This
means that in this case, reduction of temperature leads in ethanol
solution to a better ground state stabilization, while the relaxed
excited state is stabilized only at the lowest temperature condi-
tions (100–80 K) [61]. A multiple increase in the fluorescence
intensity in the given temperature range, compared to the room
temperature, implies that in this rigid medium the non-radiative
processes are distinctly weakened.
3.3. Time-resolved emission
Fluorescence decays of excited ZnTPP–MSQNO complex were
monitored by the nanosecond Time-Correlated Single Photon
Counting (TCSPC) method. Analysis of the decay curves was per-
formed by non-linear least square iterative deconvolution proce-
dure exploring the Lavenberg–Marquardt algorithm [62]. The
lifetimes were estimated accordingly to Eq. (1):
X
IðtÞ ¼
Ai expðꢁt=si
Þ
ð1Þ
In the solid state, the fluorescence spectrum of the complex
shows only the Q band while the Soret band is totally quenched.
A red shift, which compared to the Qy band maximum in the solid
ZnTPP molecule (654 nm) is more than 20 nm (675 nm). It gives an
additional support confirming the complex formation. A similar
i
where: Ai is the abundance of the ith component contributing to the
P
fluorescence lifetime
s
i, where Ai = 1. Credibility of the measure-
ments was expressed by the reduced chi-square (v2) value.
Fig. 6. TD DFT/CAM-B3LYP/6-31G(d,p) simulated absorption spectra of the ZnTPP–MSQNO complex and its individual components (ZnTPP and MSQNO) in their experimental
crystal structure. The vertical lines indicate the location and the oscillator strength values (the scale on the right side of the figure) of electronic transitions (all data are given
in Table 3). The inset demonstrates the Q band of ZnTPP moiety and that of the complex.

196
A. Szemik-Hojniak et al. / Polyhedron 88 (2015) 190–198
Fig. 7. Fluorescence spectra of ZnTPP (1) and ZnTPP–MSQNO complex (2) in toluene
Fig. 9. Emission spectra of solid complexes: (1) ZnTPP–IQNO [26], (2) ZnTPP–
(exc. 550 nm).
MSQNO (exc. 500 nm).
In toluene solution, the fluorescence decay (Fig. 5S of Supple-
mentary Data), exhibits a single exponential behavior with the life-
time of 1.5 ns, which, as indicate theoretical calculations (Table 3),
may be ascribed to the excited pp⁄ state with a small admixture
(ꢂ9%) of CT configuration. This state is shorter than the lifetime
of the ZnTPP unit (ꢂ2 ns) which additionally confirms the complex
formation between the MSQNO ligand and ZnTPP unit [63,64].
In n-propanol, however, the excited state of ZnTPP–MSQNO
decays bi-exponentially and due to a slower solvent relaxation, a
shorter component representing the lifetime of the monomer is
elongated up to 2.7 ns. A longer component (23.4 ns), may be
assigned to the hydrogen bonded associates formed between
excited fluorophore and the propanol molecules.
The fluorescence decay of the solid complex while probing it
both in the fluorescence maximum (676 nm) and on the blue
(640 nm) side of the emission spectrum, fits to a single exponential
function with a very short (0.6 ns) lifetime. On the other hand the
monitoring at the longer wavelength (700 nm) gives a bi-exponen-
tial decay, with prevailing abundance of the same short (0.62 ns,
86%) component and with a longer lifetime (5 ns, 14%) that may
be assigned to dimers, ribbons and other supramolecular struc-
tures of the complex, as confirmed by the X-ray experiment. One
may suppose that so much shortened lifetime (ꢂ0.6 ns) of
ZnTPP–MSQNO complex, which is significantly smaller than that
of the previously studied solid ZnTPP–IQNO complex (1.36 ns)
[26] may be in line with our abovementioned hypothesis on the
nonradiative energy transfer process taking place in the S1 elec-
tronic excited state.
4. Conclusions
We have investigated the crystal structure and electronic prop-
erties of ZnTPP–MSQNO complex, where MSQNO ligand is axially
coordinated to ZnTPP moiety. The single-crystal X-ray studies
revealed that the crystal of the complex is characterized by a cen-
trosymmetric triclinic unit cell, space group P-1 and that the zinc
atom in the complex is 0.365 Å pulled out of the porphyrin plane.
In the solid state, besides the monomer of the targeted complex,
different supramolecular assemblies such as dimers, ribbons and
layers are also present.
Using DFT calculations, it was proven that the MSQNO ligand
occurs as the trans isomer in the anti-rotameric form. The TD
DFT/CAM B3LYP [6-31G(d,p)] calculations allowed us to obtain
Fig. 8. Temperature effect on the Qx (580 nm) and Qy (635 nm) emission bands of ZnTPP–MSQNO complex in EtOH (exc. 350 nm) in 240–80 K temperature range; the
temperature was changed with the step of 20 K and decreases upwards from band 1 to 9 (80 K). See text for further explanation.

A. Szemik-Hojniak et al. / Polyhedron 88 (2015) 190–198
197
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