Photoinduced electron transfer in pentacoordinated complex of zinc tetraphenylporphyrin and isoquinoline N-oxide. Crystal structure, spectroscopy and DFT studies

Article abstract of DOI:10.1016/j.poly.2011.06.009

A novel, pentacoordinated complex of (1:1) zinc tetraphenylporphyrin and isoquinoline N-oxide (ZnTPP-IQNO) was synthesized and its crystal structure along with photophysical properties by experimental methods (absorption, steady state and time-resolved emission) in conjunction with DFT and TD DFT calculations were investigated. In ZnTPP-IQNO complex, the isoquinoline N-oxide ligand (IQNO) is directly coordinated to the central zinc atom of the ZnTPP unit through the oxygen atom of the NO group and crystallizes in centrosymmetric triclinic unit, in the space group P1. Particular contacts between the two monomeric units (hydrogen bonds, O...H-C interactions, ...etc.) lead to a supramolecular dimer which forms the layers propagating both along the a and the b-axis. The electronic locally excited and the charge-transfer states of the complex were calculated by TDDFT CAM-B3LYP/6-31G(d,p) method. A surprising presence of the charge transfer states between the Soret and the Q bands leads to excitation energy dissipation processes involving the opening of the radiationless channels of excited ZnTPP-IQNO complex. Emission from the S ₁ state (Q band) in ethyl acetate decays accordingly to monoexponential function (1.92 ns) while a bi-exponential decay is found in n-propanol [(2.5 ns (87%); 14.4 ns (13%)] and in the solid state [1.36 ns (67.5%), 7.31 ns (32.5%)].

Full text of DOI:10.1016/j.poly.2011.06.009

Polyhedron 30 (2011) 2391–2399
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Photoinduced electron transfer in pentacoordinated complex of zinc
tetraphenylporphyrin and isoquinoline N-oxide. Crystal structure, spectroscopy
and DFT studies
a
b
c
a
a,
⇑
K. Oberda , I. Deperasi n´ ska , Y. Nizhnik , L. Jerzykiewicz , A. Szemik-Hojniak
a
Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14 St., 50-383 Wrocław, Poland
Institute of Physics, Pol. Acad. of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland
Department of Molecular Biology, Biological and Organic Chemistry, Petrozavodsk State University, pr. Lenina 33, 185910 Republic Karelia, Russia
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel, pentacoordinated complex of (1:1) zinc tetraphenylporphyrin and isoquinoline N-oxide (ZnTPP–
IQNO) was synthesized and its crystal structure along with photophysical properties by experimental
methods (absorption, steady state and time-resolved emission) in conjunction with DFT and TD DFT cal-
culations were investigated. In ZnTPP–IQNO complex, the isoquinoline N-oxide ligand (IQNO) is directly
coordinated to the central zinc atom of the ZnTPP unit through the oxygen atom of the NO group and
Received 18 April 2011
Accepted 14 June 2011
Available online 6 July 2011
Keywords:
ꢀ
crystallizes in centrosymmetric triclinic unit, in the space group P1. Particular contacts between the
Crystal structure
Zinc tetraphenylporphyrin
Isoquinoline N-oxide
Excited states
Electron transfer
Lifetimes
two monomeric units (hydrogen bonds, OÁ Á ÁH–C interactions, . . .etc.) lead to a supramolecular dimer
which forms the layers propagating both along the a and the b-axis. The electronic locally excited and
the charge-transfer states of the complex were calculated by TDDFT CAM-B3LYP/6-31G(d,p) method. A
surprising presence of the charge transfer states between the Soret and the Q bands leads to excitation
energy dissipation processes involving the opening of the radiationless channels of excited ZnTPP–IQNO
1
complex. Emission from the S state (Q band) in ethyl acetate decays accordingly to monoexponential
function (1.92 ns) while a bi-exponential decay is found in n-propanol [(2.5 ns (87%); 14.4 ns (13%)]
and in the solid state [1.36 ns (67.5%), 7.31 ns (32.5%)].
Ó 2011 Elsevier Ltd. All rights reserved.
1
. Introduction
Much work has been done in the recent years on the structure
porphyrins and metalloporphyrins became a ‘‘hot’’ subject of re-
cent photophysical studies devoted to these macrocycles.
Despite that metalloporphyrins are relatively simple and photo-
stable on excitation in the near-UV of the very allowed Soret band
region (oscillator strength close to unity), nevertheless identifica-
tion of their electronic states, as for example in derivatives of
ZnTPP is still an open question. The literature data concerned with
and properties of porphyrins, metalloporphyrins and their com-
plexes [1]. Metalloporphyrins, for example, are very useful and
play an important role in the active site of many proteins. They also
seem to be potential candidates both for artificial solar energy con-
version systems and for electronic storage devices [2]. Besides that,
metalloporphyrins find application in photodynamic therapy [3],
in optoelectronics – as appropriate materials for molecular logic
circuits [4], and as switchers in which by using different energies
of light, the electron transfer between particular components
may be controlled [5]. Many articles have been devoted, to a model
and symmetric (D4h) zinc tetraphenyl porphyrin (ZnTPP) [6–10]
and to complementary investigations of photophysical properties
of its unsymmetrical supramolecular arrays in the form of
donor–acceptor dyads, triads, and other multicomponent molecu-
lar systems which are the subject of a deep ongoing interest [5].
The relaxation dynamics and assignment of excited states of
photophysical parameters like the quantum yield and the decay
⁄
constants of the lowest two
p–p
excited states (S
state, for example,
state is in the
1 2
and S ) of the
latter, considerably differ. The lifetime of the S
is of the order of about 2 ns while that for the S
broad ps–fs range [11–15].
1
2
On the other hand, the literature, in a fact, contains only few
structural as well as spectral data describing the interaction of
metalloporphyrins with Lewis bases. The latter may be for exam-
ple, heteroaromatic N-oxides [16–18] that represent a prominent
biological activity [19–21]. Their combination with metallopor-
phyrins may result in a new type of materials with unique photo-
physical and electronic properties.
Zinc(II) tetraphenylporphyrin is particularly well suited for
studying such combined molecular systems. It does not undergo
neither changes in oxidation nor in the spin state, usually accepts
⇑
Corresponding author. Tel.: +48 71 375 7366; fax: +48 71 328 2348.
E-mail address: anias@wchuwr.pl (A. Szemik-Hojniak).
0
277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.06.009

2
392
K. Oberda et al. / Polyhedron 30 (2011) 2391–2399
only one axial ligand, does not have empty d-orbitals that might
participate in bonding, providing in this way a very convenient
model for tuning the effects of axial ligation upon its electronic
structure.
chloroform–hexane 2:1) for Zn-TPP purity determination. Yield
0.43 g (77%).
2.1.4. Synthesis of ZnTPP–IQNO complex
The main goal of this work, is to get a deeper insight into elec-
tronic structure of the target (jO-isoquinoline-N-oxide)-(a,b,c,d-
To 34 mg (0.05 mM) of Zn-TPP dissolved in 15 mL of acetone,
1 mL of an acetone solution of isoquinoline N-oxide (7.3 mg,
0.05 mM) was added. The red-violet crystals appeared in 30 min,
which were washed with acetone (1 mL, 2 times) and air dried.
Yield 70% (29 mg).
tetraphenylporphinato) zinc(II), [ZnTPP–IQNO] complex, where
the isoquinoline N-oxide ligand is axially coordinated to ZnTPP
unit. Specifically, by means of absorption and emission spectros-
copy as well as TD DFT calculations, we identify the nature of its
electronic states and in particular we check whether photoinduced
electron transfer (PET) takes place on excitation.
Anal. Calc. for C53H35N OZn (%): C, 77.32; H, 4.29; N, 8.51. Found
5
(%): C, 77.29; H, 4.44; N, 8.33%. The substance was slowly recrystal-
lized from acetone before picking up the crystal for X-ray analysis.
The X-ray crystal structure and DFT optimized molecular struc-
ture along with absorption and preliminary emissive properties of
ZnTPP–IQNO complex are also presented.
2.2. Methods
2.2.1. Electronic absorption spectra
2
. Materials and methods
Electronic absorption spectra in chloroform, ethyl acetate and
n-propanol solutions were measured using UV–Vis Cary 50, Varian
2
2
.1. Materials
spectrophotometer. All solvents were of spectral grade.
.1.1. Preparation of isoquinoline N-oxide
2.2.2. Emission spectra
Isoquinoline N-oxide was synthesized by the method of Ochiai
from isoquinoline by peroxide oxidation in acetic acid.
Luminescence spectra were recorded on FSL920 combined fluo-
rescence lifetime and steady state spectrometer (Edinburgh Instru-
ments Ltd.) using as excitation source Xe900, 450 W steady state
xenon lamp (ozone free) with computer controlled excitation shut-
ter and with spectral bandwidth of 65 nm for both excitation and
emission spectra.
2
.1.2. Synthesis of tetraphenylporphine
Tetraphenylporphine (TPP) was prepared from pyrrole and
benzaldehyde in boiling propionic acid [22]. Pyrrole was obtained
by the known method [23] using the thermal decomposition of
diammonium salt of mucic acid which was prepared from mucic
2.2.3. X-ray structure determination and refinement
acid and NH
tion with HNO
4
OH. Mucic acid was synthesized by galactose oxida-
upon heating [24].
A crystal suitable for X-ray diffraction determinations was
grown from acetone as mentioned above. Intensity data collection
3
was carried out on a KUMA KM4
j-axis diffractometer equipped
2
.1.3. Synthesis of Zn-tetraphenylporphine
Zn-tetraphenylporphine (Zn-TPP) was synthesized according to
known procedure [25] with some changes. Tetraphenylporphine
TPP) in CHCl was kept over PbO during 2 days to remove admix-
with a CCD camera and an Oxford Cryo-system. All data were cor-
rected for Lorentz and polarization effects. Data reduction and
analysis were carried out with the KUMA Diffraction programs
[27]. The structure was solved by the direct methods and refined
(
3
2
2
ture of tetraphenylchlorine (2–10% of which can be formed at TPP
synthesis) and a radical of unknown structure [26]. A mixture of
by the full-matrix least squares method on F data using the SHELXTL
(version 5.1) program [28]. The X-ray crystal structure of complex
is presented in Fig. 1 and corresponding experimental details are
summarized in Table 1.
0
(
.5 g (0.813 mmol) TPP, 0.25 g (1.14 mmol, 1.4-fold excess) of
CH COO) O, 50 mL of chloroform and 250 mL of glacial
ZnÁ2H
acetic acid was boiled for 1 h (using of 7-fold excess of
CH COO) Zn leads to the final substance almost without the initial
3
2
2
(
3
2
2.2.4. Time-resolved emission
TPP). The resultant dark blue crystals of Zn-TPP were washed with
acetic acid. Then the substance was chromatographed on an
alumina column with chloroform. TLC was used (Silufol,
Luminescence decay curves were recorded using nanosecond
Time-Correlated Single Photon Counting (TCSPC) option of
FSL920 setup (Edinburgh Instruments Ltd.). Excitation was
Fig. 1. X-ray crystal structure of ZnTPP-IQNO complex.

K. Oberda et al. / Polyhedron 30 (2011) 2391–2399
2393
Table 1
Table 2
Crystallographic parameters and details of the structure refinements of
ZnTPP-IQNO complex.
Selected experimental values of bond length (Å) and angles (°) of ZnTPP–IQNO
complex.
Compound
(
jO-isoquinoline-N-oxide)-(
a
,b,c
,d-
Type of
Bond length (Å)
Type of angle
Value of the valence
tetraphenylporphinato)zinc(II)
distance
or dihedral angle (°)
Formula
FW
Radiation (Å)
C
53
H
35
N
5
OZn
N5–O5
1.325(2)
2.097(2)
2.066(2)
2.066(2)
2.080(2)
2.089(2)
Zn1–O5–N5
115.67(10)
À77.96(16)
61.47
823.23
0.71073
Zn1–O5
Zn1–N1
Zn1–N2
Zn1–N3
Zn1–N4
C51–N5–O5–Zn5
C9–C10–C27–C28
C16–C15–C33–C34
N1–Zn1–O5–N5
N2–Zn1–N1–C1
(graph. monochromated)
À69.16
T (K)
100(1)
triclinic
P1ꢀ
131.33(11)
171.83(13)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
10.192(4)
13.569(5)
16.345(6)
67.06(3)
82.11(3)
70.51(3)
1962.3(13)
2
1.393
0.68
2418
8960
The Zn–O (2.097 Å) distance is quite small in comparison to
complexes of Zn-TPP with other N- or O-donors (Table 1-S, Sup-
porting materials) where the average value ranges between 2.1
and 2.4 Å. One may assume that steric availability of the N-oxide
oxygen might be responsible for that phenomenon.
On the other hand, according to literature data (see Table 1-S, in
Supporting materials), an average distance between the zinc atom
and the porphyrin plane in complexes with N- and O-donors is al-
most the same (0.32 Å) as in the target complex (0.337 Å).
The oxygen atom is sp -hybridized in the ZnTPP–IQNO complex
and the dihedral angle between the least square planes drawn on
the isoquinoline system and the Zn, O, and N atoms is 77.33°. Obvi-
ously, that in the case of sp -oxygen hybridization, the angle would
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
l
calc (g cm-3)
(mm-1
)
n measd
n ind
n obs (I P 2
R (I P 2 (I))
(I P 2 (I))
GOF
r
(I))
7529
0.036
0.096
1.04
3
r
R
w
r
2
likely to be around 0° (see Fig. 3, Structure B).
The ZnTPP–IQNO complex resembles the structure of Zn-TPP
complexes with anilines; indeed the nitrogen in anilines is sp -
provided by nF900 nitrogen filled nanosecond flashlamp under
computer control, with typical pulses width 1 ns, Pulse Repetition
Rate typical 40 kHz and possibility of measuring decays from
2
hybridized (Fig. 3, Structure D) but in complexes with acceptors
1
00 ps to 50 ls. Data acquisition ensured Plug-in PC Card Model
3
it is in sp -hybridization state (Fig. 3, Structure C). For example,
TCC900 with Maximum Count Rate 3 MHz, Time Channels per
Curve up to 4096, and Minimum Time per Channel 610 fs. A Ham-
amatsu (R928-Hamamatsu) in Peltier Cooled Housing was used as
detector.
in the complex of 3-nitroaniline with Zn-TPP, the angle between
porphyrin plane and the Zn–N–C(phenyl)–Cphenyl plane is 86.96°
(
CSD refcode: HAMLAI-Table 1-S, Supporting materials) clearly
indicating the Structure C.
The dihedral angle formed with the porphyrin and isoquinoline
rings is 24.45°, and it is too big for any p–p-interactions between
porphyrin ring and isoquinoline system, but the hydrogen atom
of adjacent phenyl group of Zn-TPP can form a quite strong
2
.2.5. Theoretical calculations
The calculations of electronic spectra of complex ZnTPP–IQNO
and its components were performed at the TDDFT B3LYP and
CAM-B3LYP/6-31G(d,p) level with GAUSSIAN09 [29] package of
programs. Different geometric structures optimised by DFT
B3LYP/6-31G(d,p) method and experimentally obtained the X-ray
structure have been taken into consideration.
C–HÁ Á Á
p interaction [30]. The corresponding distance between
H34A and the isoquinoline is 2.607 Å.
3
.2. Supramolecular interactions in ZnTPP–IQNO complex
3
. Results and discussion
Two molecules of adduct form a supramolecular dimer (Fig. 4)
in which the oxygen atoms (from the >NO group) form the weak
OÁ Á ÁH–C hydrogen bonds with the hydrogen of the phenyl ring of
the neighboring molecule (2.675 Å, O5–H40A) and the short
3
.1. Crystal structure of ZnTPP–IQNO complex
Mixing of acetone solutions of Zn-TPP and isoquinoline N-oxide
produced the red-violet crystals of 1:1 ZnTPP–IQNO complex, de-
picted in Fig. 1. A single-crystal X-ray studies revealed that they
are characterized by a centrosymmetric triclinic unit cell contain-
ꢀ
ing two molecules of ZnTPP–IQNO, space group P1 (see Table 1
for details).
The X-ray study of the complex showed that the bond lengths
and angles in Zn-TPP moiety are essentially the same as in other
complexes of Zn-TPP. The same situation can be viewed in the case
of isoquinoline fragment; some geometric parameters are de-
scribed in Table 2. The X-ray structure of isoquinoline N-oxide
has not yet been determined, but according to the literature data
for isoquinoline analogs [30] one can assume that the structure
of the isoquinoline N-oxide fragment is essentially the same as
for the free base.
-
+
+
N
N
-
O
O
Elongation of the N–O distance in comparison to free N-oxide is
2
3
caused by the oxygen coordination to the zinc atom and its sp –sp
rehybridization leading to contribution increase of the canonical
31] form I, presented in Fig. 2.
I
II
[
Fig. 2. Equilibrium of two canonical forms of isoquinoline N-oxide molecule.

2
394
K. Oberda et al. / Polyhedron 30 (2011) 2391–2399
the above mentioned types of interaction, as for example:
O5Á Á ÁH40A = 2.676 Å; Zn1AÁ Á ÁC56 = 2.273 Å or C35Á Á ÁH30A =
.679 Å. (Other appropriate interatomic distances are displayed
.
.
2
.
.
.
.
.
in Table 2-S of Supporting materials).
N O
N O
.
3.3. Electronic absorption and emission spectra
.
.
3.3.1. Experimental absorption spectra
Absorption spectrum of ZnTPP consists of a double set of clearly
A
B
separated Q
Soret band located around 420 nm. The former ones, origin from
the transition to S state while the latter corresponds to S ? S
x y
, and Q bands (540–600 nm) and the higher energetic
.
1
0
2
.
transition. The longest wavelength Q band is broad and structured
while the Soret band shows a strong intensity although is relatively
narrow. In the absorption spectrum, the Q band is significantly
weaker as compared to a totally allowed the Soret band.
Upon adding the excess of iso-quinoline N-oxide ligand to the
chloroform solution of ZnTPP (see Fig. 5), a bathochromic shift of
R
R
N
N
R
R
x y
the Soret, Q and Q bands was found in the UV–Vis spectrum
(
D
k, nm: 9.6, 12.5, 14.5, respectively) what is common for axial
C
D
coordination of metalloporphyrines in solutions [32–35].
A more detailed absorption spectrum of ZnTPP–IQNO in a larger
spectral range (250–650 nm) is presented in Fig. 6.
Fig. 3. Electronic structures of N-oxide and aniline in different hybrid conditions of
donor centers.
In this figure a third absorption band of ZnTPP–IQNO complex is
shown in 250–400 nm region. It is higher in energy scale than both,
the Q and the Soret band and its intensity is comparable to that of
the Q band although is slightly larger than absorption of parent
ZnTPP in the same spectral region.
contacts with two carbon atoms of the phenyl ring (3.130 Å
O5–C40; 3.215 Å O5–C41). Also the same hydrogens from the phe-
nyl rings, are involved in the C–HÁ Á Á
.821 Å porphyrine plane–H40A) with the porphyrin system.
The interplanar distance in the dimer (4.373 Å) is much larger
p contact (2.831 Å C4–H40A,
2
than the sum of van der Waals radii and, consequently, shows
the impossibility of a strong direct interaction of the two porphyrin
moieties. Such dimers form the infinite chains propagating along
the a-axis.
Different types of van der Waals interactions may occur in these
dimers: C–HÁ Á Á
line ring and the
well as possible the
p
interactions formed by the hydrogen of isoquino-
-system of the adjacent porphyrin molecule as
interactions (distances between the least
p
p–p
square plane drawn through the porphyrin moiety and atoms
H55A, H56A, C55, C66 are: 2.786, 2.572, 3.066, 2.932 Å conse-
quently). On the other hand, the dimers of ZnTPP–IQNO may prop-
agate along the b-axis (Fig. 1-S-Supporting materials) forming
between each other the infinite quasi two-dimensional layers with
C–HÁ Á Á
p contacts. It is interesting to view the projections of such
layers (Figs. 1-S and 2-S in supporting materials). These layers in
Fig. 5. Changes in the electronic absorption spectrum of Zn-TPP in chloroform upon
addition of isoquinoline N-oxide; Q-band intensity shown in the 12.5-fold
enlargement compared to the intensity of the Soret band.
turn, forming a 3D crystal structure are tightly connected through
Fig. 4. A supramolecular dimer formed by two molecules of ZnTPP-IQNO complex. The ligand moieties are drawn as a ball and stick models.

K. Oberda et al. / Polyhedron 30 (2011) 2391–2399
2395
the complex under investigation. However, taking into consider-
ations the fact that the accuracy of TD DFT B3LYP method with re-
spect to the energy gaps between the LE and the CT states should
be treated with caution [41–44], we carried out additional calcula-
tions by using the CAM-B3LYP functional, which provides the best
description of the overall excitation energy.
The results obtained from the calculations showed that the geo-
metric structure of the ZnTPP–IQNO complex, optimized by B3LYP/
6
-31G(d,p) method, is in good agreement with the above described
crystal structure (Table 2).
It is worthwhile to note, that accordingly to calculation results,
the N–O bond length in the iso-quinoline N-oxide molecule, opti-
mized as a free, unbounded species, is 1.28 Å, while that found in
the calculations for the complex (1.309 Å) is only a little bit smaller
than the experimental (1.325 Å) value. Hence, the N–O bond elon-
gated about 0.03 Å in the ZnTPP–IQNO complex supports the iso-
quinoline N-oxide binding to the parent ZnTPP moiety.
The calculations show also that a significant fraction of charge
(dq = 0.146) separated in the electronic ground state between the
two components of the studied complex results in a relatively large
value of the dipole moment [7.1D – as calculated by B3LYP/
Fig. 6. Absorption spectrum of ZnTPP-IQNO in ethyl acetate. Over the fragments of
the spectrum shown in the magnification, the value of the expanding factor for their
intensity is presented.
6
-31G(d,p), and 7.5D – obtained by HF/6-31G(d,p) method]. Its
More detailed analysis of absorption spectrum in this region re-
quires relevant quantum chemical calculations performed in the
next chapter.
direction is presented in Fig. 7.
The calculated vertical energies and oscillator strength values of
electronic transitions for ZnTPP–IQNO complex both with the use
of TDDFT B3LYP/6-31G(d,p) and TDDFT CAM-B3LYP/6-31G(d,p)
methods are presented in Table 3.
3.3.2. The calculated electronic transitions
Since the literature data confirm applicability of the DFT B3LYP
and TD DFT B3LYP method for ZnTPP and its supramolecular sys-
tems [33,36–40], hence the same methods have been applied for
The latter calculations are performed for the same geometry of
the complex that was used for the B3LYP/6-31G(d,p) optimization.
The character of electronic transitions is revealed in terms of
Fig. 7. The DFT B3LYP/6-31G(d,p) calculated direction of the ground state dipole moment in ZnTPP-IQNO complex.
Table 3
Comparison of electronic transition energies, oscillator strengths, and the character of transition for ZnTPP-IQNO complex calculated by TDDFT B3LYP/6-31G(d,p)//B3LYP/6-
1G(d,p) and TDDFT CAM-B3LYP/6-31G(d,p)//B3LYP/6-31G(d,p). (For electronic configurations see Fig. 8). In the last three entries are the results of TDDFT CAM-B3LYP/6-31G(d,p)
3
calculations for the crystal structure.
TDDFT B3LYP/6-31G(d,p)//DFT B3LYP/6-31G(d,p)
TDDFT CAM-B3LYP/6-31G(d,p)//DFT B3LYP/6-
1G(d,p)
TDDFT CAM-B3LYP/6-31G(d,p)//crystal structure
3
Transition energy Oscillator
Character of
transition
Transition energy Oscillator
Character of
transition
Transition energy Oscillator
Character of
transition
cm-1
]
strength
[cm
-1
]
strength
[cm
-1
]
strength
[
1
1
1
1
2
2
2
2
5492
0.0099
0.0004
0.0234
0.0222
0.0037
0.0067
0.8613
0.8408
CT1
CT2
LE1
LE2
CT3
CT4
LE3
LE4
7579
8064
8109
1180
3354
5480
5669
17843
17893
22625
24625
26862
27034
0.0129
0.0132
0.0111
0.0029
1.1955
1.3247
LE1
LE2
CT1
CT2
LE3
LE4
17679
17738
22489
24675
26694
26879
0.0208
0.0121
0.0038
0.0136
1.1304
1.1980
LE1
LE2
CT1
CT2
LE3
LE4

2
396
K. Oberda et al. / Polyhedron 30 (2011) 2391–2399
Fig. 8. The frontiers HOMO and the LUMO orbitals of ZnTPP-IQNO complex presented in two projections to facilitate their proper localization. It is seen that HOMO-a, HOMO-
b, LUMO-a and LUMO-b are localized on ZnTPP component while LUMO-c and LUMO-d are localized on IQNO fragment. The electronic configurations of HOMO-b ? LUMO-c
and HOMO-a ? LUMO-c contribute to the electronic states CT1 and CT2, respectively (see Tab.3). The CT3 and CT4 states are described by the configurations HOMO-b ?
LUMO-d and HOMO-a ? LUMO-d. The locally excited states LE1 and LE3 are described by symmetric and anti-symmetric combinations of HOMO-a ? LUMO-a and HOMO-b
?
LUMO-b configurations while LE2 and LE4 by the combinations of HOMO-a ? LUMO-b and HOMO-b? LUMO-a, respectively.
electronic configurations described by the frontier molecular orbi-
tals, shaped as shown in Fig. 8.
Fig. 9 shows also two spectra calculated for the isolated ZnTPP
unit. One of them corresponds to ZnTPP in the same geometry,
which it has in the ZnTPP–IQNO complex, and the second one re-
fers to ZnTPP geometry of CI symmetry which it adopted in its
own crystal [45]. A similar procedure was applied also to IQNO
molecule.
As you can see, there are two HOMO orbitals localized on ZnTPP
and four LUMO orbitals, two of which are located on the ZnTPP and
two on NOIQ moiety. Due to the fact that the energy ordering of
LUMO orbitals calculated in the two methods are not identical, in
presentation in Fig. 8, literal marks were used to characterize them
appropriately (HOMO – a, b and LUMO – a, b, c, d). It is worthwhile
to note that in TDDFT B3LYP method, the LUMO – c orbital is the
lowest of LUMO orbitals whereas in TDDFT CAM-B3LYP method,
the lowest is the LUMO – a.
As seen in both cases, adoption by the isolated molecule of such
geometry which it keeps in the complex, is associated with the
band redshift, the effect that is strengthened by the complex inter-
action. For example, the calculated maximum of the Soret band for
À1
the isolated ZnTPP in its own geometry is 28 300 cm , for isolated
A reflection of differences in the orbital energy ordering is also
manifested in different arrangement of the excited states calcu-
lated in the two ways. As shown in Table 3, in the description of
the lowest excited state of the complex calculated by TDFFT
B3LYP method, the CT1 configuration dominates, whereas in the
CAM-B3LYP method, the dominating is the LE1 configuration. The
excited state which has a dominant CT1 configuration lies above.
Hence, this result is yet another example of a more confirmatory
data and arguments given in reference [44].
Table 3, moreover, shows that the above-mentioned similarity
between the experimental crystal structure and B3LYP optimized
molecular structure is reflected in the similarity of their corre-
sponding electronic spectra.
ZnTPP but with geometry which it has in the complex is
À1
À1
27 600 cm and finally in the complex it is 26 800 cm . Indeed,
such a red shift is observed in reality, what illustrate the spectra
shown in Fig. 5.
Comparing the two spectra of isolated ZnTPP one can also see
the effect of lowering the symmetry of ZnTPP unit in combined
ZnTPP–IQNO complex.
The fact, that coordination of the axial ligand to ZnTPP unit
leads to a loss of its symmetry in the complex has been established
by the X-ray experiment and manifested by differentiation of geo-
metrical parameters in the complex relative to those in parent
ZnTPP unit. As can be seen from Fig. 9, the symmetry reduction
of ZnTPP in its complex is linked to appearance in its absorption
spectrum, the bands that are absent in the spectrum of symmetric
In Fig. 9, the simulated absorption spectrum of ZnTPP–IQNO
complex and its components, as based on the data of Table 2S of
Supplementary data, are presented.Comparison of theoretical
À1
ZnTPP moiety (e.g. ꢀ32 070 and ꢀ32 770 cm ). A detailed spectral
analysis shows that in spectrum of the complex, there are also the
CT bands characteristic only of the complex as a whole, not present
in the spectra of its components separately.
(Table 3 and Fig. 9) and experimental (Figs. 5 and 6) results allows
us to note a fairly good agreement between them, although all the
calculated vertical energies for the complex are slightly larger
Apparently, you can see it among the calculated transitions for
-
1
-1
À1
(
ꢀ800 cm in the case of the Q band and ꢀ3000 cm in the case
the ZnTPP–IQNO complex: ꢀ22 500 and ꢀ24 700 cm , located in
of the Soret band) with respect to experiment. The ratio of the Q
band relative to the Soret band, resulting from the calculations, is
smaller than the measured one (0.016 versus 0.045).
the region between the Q and the Soret band.
Unfortunately, these are weak bands with a small oscillator
strength, which in experiment are additionally covered by the

K. Oberda et al. / Polyhedron 30 (2011) 2391–2399
2397
300
350
400
450
500 [nm]
IQNO
1
0
2
*
12
ZnTPP
*
12
*50
1
0
ZnTPP-IQNO
2
*
12
*50
1
0
3
7000
32000
27000
wavenumber [cm
22000
17000
-1
]
Fig. 9. Simulated absorption spectrum of ZnTPP-IQNO complex and its components, based on the data of Table 3 (in the full form in Table 3S-of Supplementary Data). Each of
the calculated line was convoluted with a Gaussian distribution of 250 cm-1 width [such operation allowed the experimental width of the Soret band (see Fig. 6) in the
simulated spectrum to be reproduced]. Spectra of each component are calculated for two geometries: the same, which a component has in the complex and their own
geometries: For ZnTPP-in its crystal and for IQNO-in DFT B3LYP;6-31G(d,p) optimized geometry. All the calculations were performed by TD CAM-B3LYP/6-31G(d,p) method.
In the main scale the Soret band is shown. Intensity of the remaining bands are presented in enlargement described by the appropriate numbers (⁄50 in the case of the long
wavelength bands and ⁄12 in the case of the short wavelength bands. See text for discussion).
components of
40–44,46].
Hence, at the present time, from all the above mentioned rea-
sons we are unable to demonstrate a direct experimental confirma-
tion of predicted CT states. In a fact, for this kind of experiment, a
low temperature spectroscopy with a higher spectral resolution is
required. Nevertheless, such CT transitions were found in transient
absorption experiments of other porphyrin complexes [33–35,37,
a
rich vibrational structure of the band
Q
although the excitation at the Q band [S
(and S ) state of the complex] leads, accordingly to Fig. 9, to the
LE fluorescence from the S state of the complex. On the other
hand, excitation at the Soret band of the complex [S state of ZnTPP
but S (and S ) state of the complex], leads to a dual fluorescence
1 3
state of ZnTPP, but S
[
4
3
2
7
8
from both LE states of the complex.
Besides that, the excitation fluorescence spectra observed for
both fluorescence maxima match agreeably with the absorption
spectrum. Hence, it allows to conclude that there are no other
pathways of excitation energy dissipation than just a return to
4
6–48] where they were used for explanation and modeling of
the results related to electron transfer process. Difficulties with
interpretation and assignment of the calculated transitions to
those experimentally observed in ZnTPP–IQNO complex, are also
connected with the higher (in the energetic scale) energy region
than the Soret band. In this region, as Fig. 9 shows, many electronic
transitions of different type [LEZnTPP, LEIQNO and CTcomplex) contrib-
ute to particular absorption bands of the complex. It is evident that
only the measurements with a high spectral resolution can help to
resolve this problem.
0
the S state. Of course, a question arises regarding a quantitative
explanation of this finding. In a fact, many other questions are
not cleared out yet, as for example, whether the presence of the
two CT bands between the Soret and Q band, does affect radiation-
less rate constants of deactivation processes? In other words, a per-
tinent question appears whether additional radiationless channels
in the complex are or can be opened? Such channels, like for exam-
5 6 3 4 3 4 1 2
ple the S (or S ) ? S (or S ), and S (or S ) ? S (or S ), are in real-
ity, the radiationless charge separated (CS) transitions and charge
recombination (CR) processes, respectively.
3.3.3. Preliminary fluorescence spectra
The fluorescence spectra of ZnTPP–IQNO performed at different
excitation wavelength are presented in Fig. 10.
With excitation in the higher energy region (280 nm), an addi-
tional band exhibiting maximum at 380 nm in the fluorescence
spectrum of ZnTPP–IQNO (Fig. 10) is observed. Excitation spectrum
observed for this band, however, does not overlap with absorption
The findings related to fluorescence in ꢀ400–700 nm region of
the complex are similar to those for unbounded ZnTPP [6,11,12]

2
398
K. Oberda et al. / Polyhedron 30 (2011) 2391–2399
the emission decay of the solid complex, follows a bi-exponential
function. The lifetime in the S state relative to free ZnTPP
11–15] is considerably reduced [1.36 ns (67.5%)], indicating a
quenching due to a dynamic process like, for example, photoin-
duced electron transfer that may occur on the nanosecond time
scale. The longer lived component [7.31 ns (32.5%)] of the
crystalline complex may be attributed to dimeric structures of
ZnTPP–IQNO that were found in the X-ray experiment.
1
exc.300 nm
exc.400 nm
exc.555 nm
[
4
. Conclusions
On the basis of the results of TD DFT CAM-B3LYP/6-31G(d,p)
calculations and analysis described in terms of both electronic con-
figurations and frontiers of molecular orbitals, has been revealed
that excited photoinduced electron transfer (ET) can takes place
in ZnTPP–IQNO complex. The transfer of charge occurs from the
HOMO orbitals of ZnTPP moiety, to the LUMO orbitals localized
exclusively on axially coordinated isoquinoline N-oxide ligand.
Presence of the CT states between the Q-band and the Soret
band can run additional deactivation channels in the form of radi-
ationless charge separation and charge recombination transitions.
Excitation above the Soret band in the complex gives also the
third emission band exhibiting the maximum around 380 nm.
We suppose that the latter may origin from a photoproduct. The
fluorescence of the complex decays bi-exponentially in alcohols
and in the solid state and the longer time constant in the former
may be referred to the hydrogen bonded associates (14.4 ns in
n-propanol) and to dimeric structures in the latter (7.31 ns), what,
in a fact, has been found in the X-ray experiment. On the other
hand, the shortening of the second time constant in the solid com-
plex [1.36 ns (67.5%)] may be due to photoinduced electron trans-
fer as a dynamic, quenching process occurring on the nanosecond
time scale.
3
50
400
400
450
500
550
350
450
500
550
600
650
wavelength [nm]
Fig. 10. Fluorescence spectra of ZnTPP-IONO complex in ethyl acetate, excited at
00, 400 and 555 nm. All the spectra were normalized at 655 nm.
3
spectrum of the complex indicating a different source of this fluo-
rescence, as for example, a photoproduct.
The emission decay curve of ZnTPP–IQNO complex in ethyl
acetate solution, excited around the Soret band (385 nm) and
observed at the Q band (656 nm), is monoexponential and the esti-
mated lifetime (1.92 ns) agrees with the value of about 2 ns found
in the literature for the parent ZnTPP system [11–15]. However, in
the protic medium of n-propanol, a bi-exponential decay was ob-
served. In this case, the shorter component connected with the life-
time of the S
unbounded ZnTPP in ethyl acetate, while the longer one [14.4 ns
13%)] seems to origin from the hydrogen bonded associates
1
state is a little bit elongated [2.5 ns (87%)] relative to
Electron transfer properties of the above investigated donor-
acceptor photo-molecular system seem to be promising regarding
studies related to solar energy harvesting processes.
(
formed between the complex and the alcohol molecule. Somewhat
peculiar seems to look the emission spectrum of the solid ZnTPP–
IQNO complex, presented in Fig. 11.
Acknowledgements
The sample excited both at 420 and 553 nm, does not show the
Soret band at all and the Q band is represented only in the form of a
single band peaking at 670 nm (0-0 transition). As the result of
ZnTPP coordination to isoquinoline N-oxide ligand, the Q band is
shifted to the red by 20 nm with respect to fluorescence maximum
in the solid (654 nm), free ZnTPP unit [49]. Likewise to n-propanol,
The authors wish to thank to Interdisciplinary Centre for Math-
ematical and Computational Modeling in Warsaw (grant G32-10)
and partly, to the Wroclaw Center for Networking and Supercom-
putomg (Grant nr 61) for the use of their computational facilities.
The authors thank the reviewer of this work for his stimulating
guidance.
Appendix A. Supplementary data
CCDC <szem09n> contains the supplementary crystallographic
data for (jO-isoquinoline-N-oxide)-(a,b,c,d-tetraphenylporphina-
to)zinc(II). These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2011.06.009.
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