Influence of protonation of peripheral substituents on photophysical and photochemical properties of tetrapyrazinoporphyrazines

Article abstract of DOI:10.1142/S1088424610002458

Octasubstituted zinc tetrapyrazinoporphyrazines with four N,N-dimethylaminophenyls and four phenyl or pyridin-3-yl substituents were synthesized and fully characterized. Their fluorescence quantum yields in DMF or pyridine were very low, almost undetectable, as a consequence of ultrafast intramolecular charge transfer. Titration of their DMF solutions with sulfuric acid led to increase of the fluorescence quantum yields by two orders of magnitude when the full protonation of peripheral substituents was achieved. Intramolecular charge transfer is no longer a favorable way of excited-state relaxation at full protonation of N,N-dimethylaminophenyl substituents because of loss of donor centers (free electron pair on its nitrogen). Similarly, singlet oxygen quantum yields also increased by two orders of magnitude when sulfuric acid was added to tetrapyrazinoporphyrazine solutions in DMF. Protonation at azomethine nitrogens of tetrapyrazinoporphyrazine macrocycle was observed at higher acid concentrations and it led to considerable decrease of fluorescence quantum yields. Octaphenyl zinc tetrapyrazinoporphyrazine and octa(pyridin-3-yl) zinc tetrapyrazinoporphyrazine were used as controls without intramolecular charge transfer. Their fluorescence and singlet oxygen quantum yields were high in DMF and decreased at higher concentrations of sulfuric acid due to protonation of azomethine nitrogens. The results suggest that the photophysical and photochemical properties of studied compounds may be controlled by changes of pH of medium.

Full text of DOI:10.1142/S1088424610002458

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2010; 14: 582–591
DOI: 10.1142/S1088424610002458
Published at http://www.worldscinet.com/jpp/
Influence of protonation of peripheral substituents
on photophysical and photochemical properties of
tetrapyrazinoporphyrazines
Veronika Novakovaa, Eva H. Mørkvedb, Miroslav Miletina and Petr Zimcik*a
a Department of Pharmaceutical Chemistry and Drug Control, Faculty of Pharmacy in Hradec Kralove,
Charles University in Prague, Heyrovskeho 1203, 50005 Hradec Kralove, Czech Republic
b Norwegian University of Science and Technology, Department of Chemistry, N-7491 Trondheim, Norway
Received 31 March 2010
Accepted 15 May 2010
ABSTRACT: Octasubstituted zinc tetrapyrazinoporphyrazines with four N,N-dimethylaminophenyls
and four phenyl or pyridin-3-yl substituents were synthesized and fully characterized. Their fluorescence
quantum yields in DMF or pyridine were very low, almost undetectable, as a consequence of ultrafast
intramolecular charge transfer. Titration of their DMF solutions with sulfuric acid led to increase of
the fluorescence quantum yields by two orders of magnitude when the full protonation of peripheral
substituents was achieved. Intramolecular charge transfer is no longer a favorable way of excited-state
relaxation at full protonation of N,N-dimethylaminophenyl substituents because of loss of donor centers
(free electron pair on its nitrogen). Similarly, singlet oxygen quantum yields also increased by two
orders of magnitude when sulfuric acid was added to tetrapyrazinoporphyrazine solutions in DMF.
Protonation at azomethine nitrogens of tetrapyrazinoporphyrazine macrocycle was observed at higher
acid concentrations and it led to considerable decrease of fluorescence quantum yields. Octaphenyl zinc
tetrapyrazinoporphyrazine and octa(pyridin-3-yl) zinc tetrapyrazinoporphyrazine were used as controls
without intramolecular charge transfer. Their fluorescence and singlet oxygen quantum yields were
high in DMF and decreased at higher concentrations of sulfuric acid due to protonation of azomethine
nitrogens. The results suggest that the photophysical and photochemical properties of studied compounds
may be controlled by changes of pH of medium.
KEYWORDS: azaphthalocyanine, fluorescence, intramolecular charge transfer, negative Stokes shift,
singlet oxygen.
pH of the environment may play a very important role in
INTRODUCTION
changing the electronic absorption, photophysical or pho-
Aza-analogs of phthalocyanines (Pc) are well-known
tochemical properties. As shown on several examples of
organic dyes, with some of the carbons in Pc ring replaced
TPyPz or parent Pc, basic azomethine nitrogens undergo
with nitrogens. Of them, tetrapyrazinoporphyrazines
protonation easily, which is accompanied by change of
(TPyPz) are the most widely investigated group and they
absorption spectra [10–16].Also, other parts of Pc or related
showed important fluorescent [1, 2], nonlinear optical [3],
compounds may be protonated in acidic media, leading to
redox [4, 5], catalytic [6] and photodynamic properties
significant spectral changes [17, 18]. The influence of pro-
[7–9]. In a number of the abovementioned applications, the
tonation of azomethine nitrogens on photostability [19] and
photophysical and photochemical properties [20, 21] have
SPP full member in good standing
been also reported, as well as molecular calculations of the
protonated species [22, 23].
SPP student member
Recently, we showed that excited states of octakis(N,N-
diethylamino)zincTPyPz (1, Fig. 1) are preferentially
*Correspondence to: Petr Zimcik, email: petr.zimcik@faf.cuni.
cz, tel: +420 495067257, fax: +420 495067167
Copyright © 2010 World Scientific Publishing Company

InfluenCe Of PrOtOnatIOn Of PerIPheral SubStItuentS
583
Southend-on-Sea, Essex, Great Britain). Infrared spec-
tra were measured on Nicolet Impact 400 IR spectrom-
N
N
1
eter (KBr) or Nicolet 6700 (in ATR mode). H and 13C
N
N
N
N
N
N
NMR spectra were recorded on Varian Mercury Vx BB
300 (299.95 MHz – 1H and 75.43 MHz – 13C). Chemical
shifts reported are relative to Si(CH3)4. The elemental
analysis was carried out on Automatic Microanalyser
EA1110CE (Fisons Instruments S.p.A., Milano, Italy).
MALDI-TOF mass spectra were recorded in negative
reflectron mode on a Voyager-DE STR mass spectrome-
ter (Applied Biosystems, Framingham, MA, USA). The
instrument was calibrated externally with a five-point
calibration using Peptide Calibration Mix1 (LaserBio
Labs, Sophia-Antipolis, France). For each sample, 0.5
µl of the mixture was spotted onto the target plate, air-
N
Zn
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
1
Fig. 1. Structure of compound 1
deactivated by ultrafast intramolecular charge transfer
(ICT) leading to very low, almost undetectable fluo-
rescence emission and singlet oxygen production [24].
Peripheral N,N-diethylamino substituents (donors) in 1
play a crucial role in ICT and their protonation led to an
increase of fluorescence as a consequence of free electron
pair blockade. However, basicity of N,N-diethylamino
substituents in 1 is decreased by very strong deactivat-
ing effect of pyrazine and the increase of fluorescence
was therefore limited. The aim of the presented work is
the investigation of influence of protonation on photo-
physical as well as photochemical properties of TPyPz
substituted with more basic substituents. The idea is that
increased basicity of the amino groups may enable full
protonation of a donor center responsible for ICT and as
a consequence fluorescence or singlet oxygen produc-
tion may be restored. Because the donor center must be
conjugated with macrocyclic system in order to allow
efficient ICT, N,N-dimethylaniline was chosen as the
suitable substituent.
dried and covered with 0.5 µl of matrix solution con-
sisting of 10 mg of α-cyano-4-hydroxycinnamic acid in
100 µl of 50% ACN.
Synthesis and characterization of studied compounds
Preparation of 1-(4-(N,N-dimethylamino)phenyl)-
2-hydroxy-2-phenylethanone (2). This compound was
prepared similarly as published before [25] by conden-
sation of equimolar amounts of benzaldehyde (425 mg,
4 mmol) and 4-(N,N-dimethylamino)benzaldehyde (597
mg, 4 mmol) with 10% aqueous solution of potassium
cyanide (0.5 mL) in ethanol (3 mL). The solution was
heated at reflux for 48 h, left to cool to rt and yellow
crystals were filtered out. Water (50 mL) was added to
the filtrate and was subsequently extracted with ethylac-
etate, organic layer was dried over anhydrous Na2SO4,
evaporated and added to crystals. The crude product
was purified on silica with ethylacetate/hexane 1:2 and
recrystallized from EtOH/water. Yield 300 mg (29%) of
white solid, mp 157.0–159.8 °C (lit. [25] 162 °C (EtOH)).
Anal. calcd. for C16H17NO2: C, 75.27; H, 6.71; N, 5.49%.
Found: C, 74.70; H, 6.76; N, 5.47%. 1H NMR (300 MHz,
(CD3)2SO, Me4Si): δ, ppm 2.96 (6H, s, CH3), 5.69 (1H, d,
J = 6Hz, OH), 5.95 (1H, d, J = 6Hz, CHOH), 6.61–6.65
(2H, m,ArH(aniline)), 7.18–7.31 (3H, mArH(Ph)), 7.39–
7.41 (2H, m, ArH(Ph)), 7.83–7.88 (2H, m, ArH(aniline)).
13C NMR (75 MHz, (CD3)2SO, Me4Si): δ, ppm 74.87,
110.76, 121.65, 127.26, 127.56, 128.50, 131.17, 141.15,
153.47 and 196.41, signal of CH3 is overlapped by signal
of solvent. IR (ATR): ν, cm-1 3413, 2939, 1650, 1603,
1547, 1489, 1453, 1410, 1385, 1343, 1318, 1305, 1264,
1215, 1193, 1169, 1129, 1091, 1066, 1030, 1003, 976,
947 and 856.
EXPERIMENTAL
General
All organic solvents used were of analytical grade.
1,3-diphenylisobenzofuran (DPBF) for singlet oxy-
gen studies was purchased from Aldrich, quinoline
from Acros Organics, other organics from Lachema
(Czech Republic). Sulfuric acid used for titrations was
obtained from Penta (Czech Republic) and contained
96% sulfuric acid and 4% water. Zinc phthalocyanine
(ZnPc) was purchased from Eastman Organic Chemi-
cals (New York, USA). All chemicals were used as
received without further purification except for quino-
line which was freshly distilled prior to use. TLC was
performed on Merck aluminum sheets with silica gel 60
F254. Merck Kieselgel 60 (0.040–0.063 mm) was used
for column chromatography. Melting points were mea-
sured on Electrothermal IA9200 Series Digital Melt-
ing Point apparatus (Electrothermal Engineeering Ltd.,
Preparation of 1-(4-(N,N-dimethylamino)phenyl)-2-
hydroxy-2-(pyridin-3-yl)ethanone (3). This compound
was prepared similarly as published before [26] by con-
densation of equimolar amounts of pyridine-3-carboxal-
dehyde (428 mg, 4 mmol) and 4-(N,N-dimethylamino)
benzaldehyde (597 mg, 4 mmol) with 10% aqueous solu-
tion of potassium cyanide (0.5 mL) in ethanol (3 mL).
The solution was heated at reflux for 48 h, left to cool
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 583–591

584
V. nOVakOVa et al.
and yellow crystals were filtered out. Water (50 mL)
was added to the filtrate and was subsequently extracted
with ethylacetate, organic layer was dried over anhy-
drous Na2SO4 and added to crystals. The crude product
was purified on silica with ethylacetate/hexane 1:1 and
recrystallized from EtOH/water. Yield 320 mg (31%) of
yellow solid, mp 134.6–135.0 °C (lit. [26] 156–157 °C
(EtOH/water)) and 50 mg of its oxidized form (compound
5). Anal. calcd. for C15H16N2O2: C, 70.29; H, 6.29; N,
10.93%. Found: C, 70.03; H, 6.33; N, 11.04%. 1H NMR
(300 MHz, (CD3)2SO, Me4Si): δ, ppm 2.98 (6H, s, CH3),
5.93 (1H, d, J = 6Hz, OH), 6.06 (1H, d, J = 6Hz, CHOH),
6.64–6.68 (2H, m,ArH(aniline)), 7.32 (1H, ddd, J1 = 8 Hz,
J2 = 5 Hz, J3 = 1 Hz, ArH(Pyr)), 7.74 (1H, dt, J1 = 8 Hz,
J2 = 2 Hz, ArH(Pyr)), 7.86–7.90 (2H, m, ArH(aniline)),
8.43 (1H, dd, J1 = 5 Hz, J2 = 2 Hz, ArH(Pyr)), 8.64 (1H,
d, J = 2 Hz, ArH(Pyr)). 13C NMR (75 MHz, (CD3)2SO,
Me4Si): δ, ppm 72.58, 110.86, 121.46, 123.69, 131.19,
134.76, 136.70, 148.78, 148.80, 153.62 and 195.78, sig-
nal of CH3 is overlapped by signal of solvent. IR (ATR):
ν, cm-1 3415, 2921, 1653, 1609, 1547, 1478, 1426, 1389,
1350, 1318, 1280, 1231, 1197, 1170, 1131, 1087, 1064,
1023, 1005, 975, 944 and 858.
Preparation of 1-(4-(N,N-dimethylamino)phenyl)-
2-phenylethane-1,2-dione (4). Amonium nitrate and
copper(II)acetate were used to oxidize 2 as described
for similar compounds [27] to obtain yellow solid in a
yield of 84%, mp 112.1–113.5 °C (lit. [25] 114–116 °C
(EtOH)). 1H NMR (300 MHz, (CD3)2CO, Me4Si): δ, ppm
3.11 (6H, s, CH3), 6.77–6.82 (2H, m,ArH(aniline)), 7.55–
7.60 (2H, m, ArH(Ph)), 7.69–7.78 (3H, m, ArH(Ph)),
7.91–7.94 (2H, m, ArH(aniline)).
Preparation of 1-(4-(N,N-dimethylamino)phenyl)-
2-(pyridin-3-yl)ethane-1,2-dione (5). Amonium nitrate
and copper(II)acetate were used to oxidize 3 as described
in literature [26, 27] to obtain yellow solid in a yield
of 81%, mp 130.9–131.2 °C (lit. [26, 27] 132–133 °C).
Anal. calcd. for C15H14N2O2: C, 70.85; H, 5.55; N,
11.02%. Found: C, 70.96; H, 5.87; N 10.94%. 1H NMR
(300 MHz, (CD3)2CO, Me4Si): δ, ppm 3.12 (6H, s,
CH3), 6.81 (2H, m, ArH(aniline)), 7.60 (1H, ddd, J1 =
8 Hz, J2 = 5 Hz, J3 = 1 Hz, ArH(Pyr)), 7.78–7.83 (2H,
m, ArH(aniline)), 8.28 (1H, dt, J1 = 8 Hz, J2 = 2 Hz,
ArH(Pyr)), 8.85 (1H, dd, J1 = 5 Hz, J2 = 2 Hz, ArH(Pyr)),
9.06 (1H, dd, J1 = 2 Hz, J2 = 1 Hz). 13C NMR (75 MHz,
(CD3)2CO, Me4Si): δ, ppm, 40.05, 111.99, 120.80,
124.90, 130.11, 132.83, 137.34, 151.56, 155.42, 155.81,
191.59 and 195.26. IR (ATR): ν, cm-1 2924, 2854, 1673,
1653, 1633, 1596, 1578, 1541, 1487, 1472, 1457, 1442,
1418, 1382, 1343, 1322, 1229, 1176, 1117, 1067, 1020,
996, 944 and 871.
acetone. The undissolved residue was filtered off and the
crude extract was purified on silica with hexane/ethyl ace-
tate 3:1 as an eluent. Yield 90 mg (35%) of orange solid,
mp 147–156 °C. Anal. calcd. for C20H15N5: C, 73.83; H,
4.65; N, 21.52%. Found: C, 73.30; H, 4.87; N 20.00%. 1H
NMR (300 MHz, (CD3)2CO, Me4Si): δ, ppm 3.02 (6H, s,
CH3), 6.62–6.78 (2H, m, ArH(aniline)), 7.41–7.53 (5H,
m, ArH(Ph)), 7.60–7.64 (2H, m, ArH(aniline)). 13C NMR
(75 MHz, (CD3)2CO, Me4Si): δ, ppm 39.96, 112.09,
114.94, 115.13, 128.05, 129.54, 129.89, 130.05, 130.25,
131.12, 132.47, 138.28, 153.20, 154.88 and 155.92. IR
(KBr): ν, cm-1 3447, 2232, 1606, 1533, 1504, 1447,
1396, 1375, 1315, 1235, 1195, 1173, 1124, 1066, 1028,
946, 829, 807 and 778.
Preparation of 5-(4-(N,N-dimethylamino)phenyl)-6-
(pyridin-3-yl)pyrazine-2,3-dicarbonitrile (7). Diami-
nomaleonitrile (701.5, 6.5 mmol) was added to a solution
of 5 (550 mg, 2.2 mmol) in glacial acetic acid (15 mL)
and the mixture was heated under reflux for 3 hours. The
solvent was evaporated and the product extracted with
acetone. The undissolved residue was filtered off and
the crude extract was purified on silica with chloroform/
acetone 20:1 as an eluent. Yield 598 mg (85%) of red
solid, mp 136.3–137.7 °C. Anal. calcd. for C19H14N6: C,
69.93; H, 4.32; N, 25.75%. Found: C, 69.16; H, 4.38; N
26.04%. 1H NMR (300 MHz, (CD3)2CO, Me4Si): δ, ppm
3.04 (6H, s, CH3), 6.66–6.72 (2H, m, ArH(aniline)),
7.44–7.50 (3H, m, ArH(aniline) and ArH(pyridyl)), 8.02
(1H, ddd, J1 = 9 Hz, J2 = 2 Hz, J3 = 2 Hz, ArH(pyridyl)),
8.66 (1H, dd, J1 = 5 Hz, J2 = 2 Hz, ArH(pyridyl)), 8.77
(1H, d, J = 2 Hz, ArH(pyridyl)). 13C NMR (75 MHz,
(CD3)2CO, Me4Si): δ, ppm 41.00, 113.32, 115.84,
116.01, 122.92, 125.31, 129.30, 132.38, 133.61, 135.35,
138.35, 151.60, 152.75, 153.56, 154.32 and 157.39. IR
(ATR): ν, cm-1 3014, 2985, 2243, 1753, 1737, 1726,
1604, 1586, 1535, 1503, 1478, 1415, 1396, 1375, 1309,
1287, 1229, 1217, 1194, 1124, 1065, 1024, 944, 933,
834, 818 and 797.
Preparation of {[2,9,16,23-tetrakis[(4-(N,N-di-
methylamino)phenyl]-3,10,17,24-tetraphenyl-1,
4,8,11,15,18,22,25-(octazaphthalocyaninato)]}
zinc(II) (8). Compound 6 (180 mg, 0.5 mmol) and
zinc(quinoline)2Cl2 (217 mg, 0.5 mmol, prepared
according to the published procedure [28]) were ground
in a mortar, transferred into a flask and flushed with
nitrogen. Freshly distilled quinoline (0.5 mL) was added
and the mixture was heated at 170 °C for 20 min. Water
(20 mL) was added, a precipitate was collected and
washed thoroughly with water/methanol 1:1 and metha-
nol. The crude product was dissolved in 10% HCl and
filtered. The solution was neutralized with 5% NaOH.
The precipitate was collected, adsorbed on silica and
washed thoroughly with methanol, acetone and chloro-
form/toluene/pyridine 5:10:1. Subsequently, the prod-
uct was extracted from silica with pyridine, the solvent
was removed under reduced pressure and the product
was washed thoroughly with methanol. Yield 128 mg
Preparation of 5-(4-(N,N-dimethylamino)phenyl)-
6-phenylpyrazine-2,3-dicarbonitrile (6). Diaminoma-
leonitrile (256 mg, 2.4 mmol) was added to a solution
of 4 (200 mg, 0.8 mmol) in glacial acetic acid (15 mL)
and the mixture was heated under reflux for 3 hours. The
solvent was evaporated and the product extracted with
Copyright © 2010 World Scientific Publishing Company
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InfluenCe Of PrOtOnatIOn Of PerIPheral SubStItuentS
585
(68%) of green solid. Anal. calcd. for C80H60N20Zn +
Singlet oxygen measurements
4H2O: C, 66.78; H, 4.76; N, 19.47%. Found: C, 66.58;
H, 4.71; N 19.11%. 1H NMR (300 MHz, C5D5N,
Me4Si): δ, ppm 2.73 (12H, s, CH3), 2.78 (12H, s, CH3),
6.33–6.65brs (8H, ArH(aniline)), 7.27–7.49brs (12H,
ArH(Ph)), 7.61–7.85brs (8H, ArH(Ph)), 7.89–8.11brs
(8H, ArH(aniline)). 13C NMR (75 MHz, C5D5N, Me4Si):
δ, ppm 39.78, 39.81, 111.79, 126.71, 128.67, 131.05,
132.53, 141.35, 147.71, 151.02, 152.90, 154.09, 154.93.
All the samples for both singlet oxygen and fluores-
cence quantum yield determination were further purified
by TLC in order to ensure absolute purity of the samples.
The samples were eluted on TLC plate (Silufol Kavalier
Czechoslovakia without fluorescence indicator, eluents:
toluene/pyridine 1:1 for 8 and 10, toluene/pyridine 1:5
for 9, pyridine/MeOH 5:2 for 11), corresponding parts
were scrapped and TPyPz were eluted from silica using
pyridine/MeOH 10:1. Quantum yields of singlet oxygen
(ΦΔ) were determined according to a previously pub-
lished procedure using the decomposition of 1,3-diphe-
nylisobenzofuran [29]. Three solvents were considered:
pyridine, DMF and DMF with 10% H2SO4 (v/v). Zinc
phthalocyanine (ZnPc) was used as a reference (ΦΔ = 0.61
inpyridine[30],ΦΔ =0.56inDMF[31,32]).Dataobtained
for ZnPc in DMF were used also as the reference data for
the calculations in a medium composed of DMF and 10%
of H2SO4 because of significant protonation of ZnPc at
this concentration of sulfuric acid (Fig. S2). Absorption
of the dyes in the Q band area was set approximately to
0.5. All experiments were performed three times and data
presented in the paper represent a mean of these three
experiments. Estimated error ± 10%.
1
Signals in H NMR were broad due to aggregation. IR
(KBr): ν, cm-1 3417, 3057, 2861, 2799, 1636, 1607,
1538, 1484, 1445, 1405, 1347, 1245, 1195, 1171, 1108,
1064, 1051, 942, 830, 814 and 751. UV-vis (DMF): λ,
nm (ε, M-1.cm-1) 674 (163 000), 618 (42 500), 402 (105
200). MS (MALDI-TOF): m/z 1365 (calcd. for [M + H]+
1365), 1387 [M + Na]+, 1403 [M + K]+, 1426 [M + Na +
K]+, 1442 [M + 2K]+.
Preparation of {[2,9,16,23-tetrakis[(4-(N,N-di-
methylamino)phenyl]-3,10,17,24-tetrakis(pyridin-3-
yl)-1,4,8,11,15,18,22,25-(octazaphthalocyaninato)]}
zinc(II) (9). Compound 7 (120 mg, 0.4 mmol) and
zinc(quinoline)2Cl2 (156 mg, 0.4 mmol, prepared accord-
ing to the published procedure [28]) were ground in a
mortar, transferred into a flask and flushed with nitro-
gen. Freshly distilled quinoline (0.5 mL) was added and
the mixture was heated at 170 °C for 60 min. Water (20
mL) was added, the formed precipitate was collected and
washed thoroughly with water/methanol 1:1, methanol
and chloroform. The crude product was dissolved in 10%
HCl and filtered. The solution was neutralized with 5%
NaOH. The formed precipitate was collected, dissolved
in pyridine (0.5 mL) and precipitated again by dropping
slowly into methanol (30 mL). The precipitate was col-
lected and washed thoroughly with methanol. The prod-
uct was adsorbed on silica (5 g) and washed thoroughly
with methanol. Subsequently, the product was extracted
from silica with pyridine/methanol 8:1, the solvent was
removed under reduced pressure and the product was
washed thoroughly with methanol. Yield 61 mg (48%)
of green solid. Anal. calcd. for C76H56N24Zn + 4H2O: C,
63.26; H, 4.47; N, 23.30%. Found: C, 62.81; H, 4.42;
N 23.33%. 1H NMR (300 MHz, C5D5N, Me4Si): δ, ppm
2.60 (12H, s, CH3), 2.74 (12H, s, CH3), 5.93–6.57brs
(8H, m, ArH(aniline)), 7.23–7.51brs (16H, m, ArH),
8.59–8.92brs (4H, m, ArH), 9.08–9.48brs (4H, m, ArH).
13C NMR (75 MHz, C5D5N, Me4Si): δ, ppm 39.61, 39.73,
Fluorescence quantum yields measurements
Fluorescence quantum yields (ΦF) were determined by
the comparative method using ZnPc as a reference (ΦF =
0.17 in DMF). Stock solution of the studied compound
(8–11) in DMF (2.2 mL, 2.9 μM) was transferred into a
quartz optical cell (10 × 10 mm), defined amount of concen-
trated sulfuric acid was added into the sample, and absorp-
tion and emission spectra were collected. The presented
absorption spectra (e.g. Fig. 2) were corrected for dilution.
Both reference and sample were excited at 382 nm. ΦF was
calculated using the following equation (Equation 1):







2
S
−AR
S


F
1−10
1−10
n
ΦSF = ΦR
(1)




F 


−AS
FR
nR



where F is the integrated area under the emission spec-
trum, A is absorbance at excitation wavelength (382 nm),
n is the refractive index of the solvent. Superscripts R
and S correspond to the reference and sample, respec-
tively. ZnPc as a reference was measured only in DMF
without addition of sulfuric acid. The refractive index of
DMF solutions with increasing amount of sulfuric acid
was determined for a series of concentrations (from 0%
to 35% (v/v)) and calculated from nonlinear regression of
experimentally measured dependence between refractive
index and amount of sulfuric acid in DMF solution (see
Supporting information section). Excitation spectra were
collected with emission wavelength fixed at 700 nm.
All experiments were performed three times and data
presented in the paper represent a mean of these three
experiments. Estimated error ±15%.
1
111.49, 132.55, 136.91, 138.42, 154.90. Signals in H
NMR were broad due to aggregation; some signals in
13C NMR were not even detected for the same reason.
IR (ATR): ν, cm-1 3424, 2960, 2918, 2860, 1608, 1535,
1522, 1478, 1496, 1362, 1349, 1251, 1195, 1143, 1110,
1094, 974, 946, 886 and 783. UV-vis (DMF): λ, nm (ε,
M-1.cm-1) 676 (165 000), 616 (42 000), 404 (95 300). MS
(MALDI-TOF): m/z 1369 (calcd. for [M + H]+ 1369),
1391 [M + Na]+, 1407 [M + K]+, 1414 [M + 2Na]+, 1446
[M + 2K]+, 1430 [M + Na + K]+.
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586
V. nOVakOVa et al.
Fig. 2. Changes in absorption spectra and ΦF (▲) of compounds 8 (A), 9 (B), 10 (C) and 11 (D) in DMF in dependence on added
sulfuric acid. A1 (0–6% of sulfuric acid (v/v)), A2 (6–37%), B1 (0–12%), B2 (12–37%), C1 (0–12%), C2 (12–42%), D1 (0–33%),
D2 (33–47%). For the last column, absorption changes were monitored at: A3: 655 nm (○) and 682 nm (■), B3: 659 nm (○) and
682 nm (■), C3: 654 nm (○) and 684 nm (■), D3: 660 nm (○) and 684 nm (■). The letter “a” indicates the spectrum in pure DMF
for α-hydroxyketones (acyloins) preparation [33]. That
is why it was chosen as the most suitable for the synthe-
sis of acyloins 2 and 3. Generally, two symmetrical and
two unsymmetrical acyloins (isomers) can be expected
in the case of mixed acyloin synthesis. The amount of
the unsymmetrical one depends considerably on the sub-
stitution of aromatic aldehydes used for condensation.
The increased electron density on aldehyde group in
aldehydes with negative half-wave potential makes the
reaction with cyanide anion and formation of cyanohy-
drine anion difficult. However, such intermediate under-
goes easily the subsequent nucleophilic addition of other
aromatic aldehyde, which is the kinetic determining step
RESULTS AND DISCUSSION
Synthesis
The substituted pyrazine-2,3-dicarbonitriles (6, 7)
were prepared by condensation of corresponding dike-
tones (4, 5) with diaminomaleonitrile in glacial acetic
acid under reflux (Scheme 1). Appropriate ethan-1,2-di-
ones 4 and 5 arose from oxidization of acyloins 2 and 3,
respectively, by ammonium nitrate and copper(II)acetate
in a good yield. The benzoin condensation, which is a
reaction family leading to carbon-carbon bond forma-
tion, has been thoroughly investigated as the best method
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InfluenCe Of PrOtOnatIOn Of PerIPheral SubStItuentS
587
O
X
O
N
O
(ii)
N
OH
(i)
O
X
O
X
N
4, 5
2, 3
N
N
N
N
N
O
NH2
NH2
N
(iii)
N
N
X
O
4, 5
6, 7
X
N
X
X
X
N
N
N
N
N
X
X
X
X
X
N
Zn
N
N
N
N
N
N
N
N
N
N
N
(iv)
N
N
N
X = CH
X = N
2, 4, 6, 8, 10
3, 5, 7, 9, 11
6, 7
N
N
Zn
N
N
N
N
N
N
N
X
N
N
N
N
N
X
X
X
10, 11
8, 9
N
Scheme 1. Synthesis of studied compounds. (i) EtOH, H2O, KCN, reflux, 48 h; (ii) NH4NO3, Cu(CH3COO)2, AcOH, reflux; 90 min;
(iii) AcOH, reflux, 3 h; (iv) Zn(quinoline)2Cl2, quinoline, 170 °C, 20–60 min
for benzoin condensation [26, 34]. The reaction is fin-
ished by proton transfer and elimination of cyanide anion
that leads to the unsymmetrical acyloin. If the electron
density is particularly high (as in 4-(N,N-dimethylamino)
benzaldehyde), the reaction with cyanide anion is not
possible and corresponding symmetrical acyloin is not
formed [34]. However, such aldehyde may react with the
other aldehydes to form unsymmetrical acyloin.
The acyloin 2 was prepared similarly as published for
this compound before [25]. The synthesis of compound
3 has been reported to yield 92% [26]. Nonetheless, sig-
nificant amounts of symmetrical acyloin 2-hydroxy-1,2-
diphenylethanone were detected during the synthesis. That
is why isolated yields of 3 were substantially lower (31%)
than those published. From the reaction mechanism of
benzoin condensation it is obvious that carbonyl group in
acyloin arises from the formyl group of aromatic aldehyde
that started the reaction as cyanohydrine anion. 2D NMR
analysis (see Supporting Information) of 2 and 3 revealed
that only isomers with carbonyl group adjacent to phenyl
bearing dimethylamino group were prepared. This is also in
agreement with the similar acyloins published before [35].
The method of cyclotetramerization using high-boiling
solvent and an appropriate salt of metal was used to
prepare final TPyPz. Thus, compounds 8 and 9 were
obtained by heating 6 and 7, respectively, in quinoline
with zinc(quinoline)2Cl2 in reasonable yields (68% and
48%, respectively). Both compounds were most likely
prepared as a mixture of four isomers due to unsymmetri-
cal substitution of starting material but TLC did not indi-
cate any separation of isomers. However, the compounds
8 and 9 showed two different signals of enantiotopic
methyl groups of N,N-dimethylaniline substituent both
in 1H NMR (2.73; 2.78 ppm and 2.60; 2.74 ppm for 8 and
9, respectively ) and 13C NMR spectra (39.78; 39.81 ppm
and 39.61; 39.73 ppm for 8 and 9, respectively). It can be
caused either by restriction in free rotation of N,N-dime-
thylaniline substituents or by unsymmetrical composition
of TPyPz (isomers). As the precursors (6 and 7) did not
indicate any restrictions in free rotation in NMR, the lat-
ter explanation seems to be more likely. The compounds
10 and 11 were synthesized according to the procedures
published before [36, 37].
UV-vis absorption
Compounds 10 and 11 were chosen for compara-
tive purposes as controls without ICT. Their absorption
spectra in DMF and pyridine showed shape typical for
TPyPz or Pc with Q band maxima at 654 nm and 660 nm,
respectively. Stepwise titration of DMF solution of com-
pound 10 with H2SO4 led to slow decrease of absorption
in a Q band maximum up to approx. 12% (v/v) H2SO4
in solution. From this point, the decrease became steep
Copyright © 2010 World Scientific Publishing Company
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588
V. nOVakOVa et al.
with concomitant increase of absorption of at 684 nm
(Fig. 2, C1–3). Compared with published results [15,
21], the new maximum can be easily attributed to TPyPz
monoprotonated at azomethine-bridging nitrogen. The
whole macrocycle looses D4h symmetry and the Q band
of monoprotonated form is split due to decreased sym-
metry (C2v). Similar changes were observed also during
titration of 11 (Fig. 2, D1–3). However, the intensity
of absorption at the Q band was increasing in the begin-
ning. At the same time, a small shoulder at 750 nm was
slowly vanishing and finally disappeared at higher con-
centrations of H2SO4. Similar changes in absorption
were observed by Donzello et al. [38, 39] for similar
octa(pyridin-2-yl)TPyPz complexes but without addition
of any acid. They attributed these changes to slow disso-
lution of formed aggregates within a couple of hours. In
our case, addition of H2SO4 into solution caused imme-
diate protonation of the peripheral pyridyl rings and
subsequent monomerization of 11 in solution due to elec-
trostatic repulsion forces between peripherally charged
macrocycles. The Q band of 11 started decreasing from
approx. 33% (v/v) of H2SO4 in solution with increase of
azomethine protonated form absorption at 684 nm. The
azomethine protonation of 11 started at higher concentra-
tion than for 10 due to competitive protonation of more
basic pyridyl nitrogens.
system (Table 1). The spectra noticeably changed the
shape after the first addition of a small amount of H2SO4
(0.1% v/v) (see e.g. Fig. 2, A1, B1). This change is most
likely caused by rapid change of conditions in medium
from weak basic (pure DMF) to weak acidic after the first
addition of H2SO4. Similarly, a small shift of the Q band
maximum was observed after the first addition also in
the case of 10 and 11. After further addition of H2SO4
into DMF solutions of 8 and 9, the Q band shifted hypso-
and hyperchromically. This change can be attributed to
sequential addition of protons on N,N-dimethylamino
substituents hence the free electron pair is not further
available for conjugation with TPyPz macrocycle. The
protonation of these basic sites seems to be finished at
approx. 6% and 12% (v/v) of H2SO4 for 8 and 9, respec-
tively, as deduced from the changes of absorption (Fig. 2,
A1–A3 and B1–B3). The full protonation is, however,
most likely reached at somewhat higher concentration
of acid (10% and 20%, respectively, see below) but the
changes are not well observable in absorption spectra.
The equilibrium reached at higher amounts of H2SO4
in solution of 9 than 8 is due to a simultaneous proto-
nation of the peripheral pyridyl rings that have similar
pKa to N,N-dimethylaniline (pKa(pyridine) = 5.27 [40],
pKa(N,N-dimethylaniline) = 5.15 [41]). The real pKa
of these groups in compounds 8 and 9 are lower due to
deactivating effect of AzaPc macrocycle but the effect is
the same for both substituents. It is noteworthy, that the
observed blue shift of the Q band during titration stopped
at 655 nm and 659 nm for 8 and 9 (Fig. 3), respectively,
the values which are well comparable with 654 nm and
660 nm of corresponding 10 and 11 in DMF, respectively.
A different behavior was observed during the titra-
tions of 8 and 9 carrying peripheral N,N-dimethylamino
substituents. Their absorption spectra in DMF and
pyridine were considerably red-shifted to 674 nm and
676 nm, respectively, due to conjugation of free electron
pair of N,N-dimethylamino substituent with macrocyclic
Table 1. Photophysical and photochemical data of studied compounds
Compound
ΦΔ
ΦΔ
ΦΔ increase
(times)
ΦΔ
ΦF
ΦF
ΦF increase
(times)
ΦF
(pyridine)
(DMF) (DMF + 10% H2SO4)
(pyridine) (DMF) (DMF + 10% H2SO4)
8
9
0.0024
0.0016
—
0.23
0.14 (0.16a)
—
96
88 (100a)
—
0.0030
0.0048
0.49c
0.00095
0.0014
0.12
0.13
0.14b
0.12
0.12
137
100
1.0
1.1
n.d.e
n.d.e
0.24c
0.19d
10
11
0.47
0.77
1.6
0.51d
0.11
a Estimated value at 20% of H2SO4 in DMF solution. b Value at 20% of H2SO4 in DMF solution. c Data from reference 37. d Data
from reference 36. e Fluorescence signal not detected.
Fig. 3. Absorption maxima (○) and maxima of fluorescence emission (▲) of compounds 8 (A), 9 (B) and 10 (C) in DMF in depen-
dence on added sulfuric acid
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InfluenCe Of PrOtOnatIOn Of PerIPheral SubStItuentS
589
It indicates that fully protonated N,N-dimethylamino
compared to data in DMF alone and reached the val-
ues comparable to 10 and 11 (Table 1). Similarly to the
observations described for 10 and 11, further addition of
acid caused significant decrease of ΦF values as a conse-
quence of azomethine nitrogen protonation.
A peculiar behavior was detected when emission and
absorption maxima of all compounds were observed in
dependence on added amount of H2SO4 (Fig. 3). The λmax
of absorption was detected at longer wavelengths than the
substituents of 8 and 9 do not contribute to red-shift of
the Q band which is then found almost at the same wave-
lengths as for 10 and 11, respectively. Further addition
of H2SO4 over 6% and 12% for 8 and 9, respectively, led
to similar changes in absorption spectra as in the case of
10 and 11 as a result of azomethine nitrogen protonation.
The protonation on azomethine nitrogens of 8 and 9 was
manifested by appearance of a new maximum at 684 nm
in both cases.
λ
max of emission in the case of 8 and 9 in DMF alone or at
low concentrations of H2SO4 in solution (Fig. 4). It would
indicate a negative Stokes shift which is not possible and
the observed phenomenon must have different explana-
tion. All four N,N-dimethylamino substituents of com-
pounds 8 or 9 are most likely present in DMF solution (or
after addition of H2SO4) in equilibrium between proto-
nated and non-protonated form. As a consequence, non-,
mono-, di-, tri- or tetraprotonated forms will be always
present although some of them at very low concentra-
tions. The low amounts are valid particularly for the tri-
and tetra protonated forms at low H2SO4 concentration
or DMF alone. As we have shown in a recent report [24],
the ICT is very strong effect if two or more donors are
present and it will quench almost completely the excited
states of TPyPz. It may lead to suggestion that only tri-
and tetraprotonated forms of 8 and 9 will be fluorescent.
As has been shown above on the changes of absorption
spectra, the absorption λmax of the non-protonated 8 and
9 in DMF or pyridine is significantly red-shifted due to
conjugation of N,N-dimethylamino groups with TPyPz
macrocycle, whereas the non-conjugated tetraprotonated
form absorbs at lower wavelengths. This may explain
the controversy of the observed negative Stokes shift. At
lower H2SO4 amounts in DMF solution, the non-, mono-
and di-protonated forms of 8 and 9 prevail and the shape
of the absorption spectra is given mostly by them. The
fluorescence, however, is emitted only by tri- and tet-
raprotonated forms that contribute only insignificantly to
the shape of absorption spectrum at these concentrations
of acid. This fact was further confirmed by collecting the
excitation spectra at different acid concentrations.
Fluorescence
Fluorescence emission spectra of 10 and 11 in DMF
or pyridine showed a shape typical for TPyPz with small
Stokes shifts. Their fluorescence quantum yields (ΦF) in
pyridine reached higher values than in DMF (Table 1),
most likely as a consequence of small amount of aggre-
gates formed in DMF. Titration of DMF solution of 10
and 11 with H2SO4 led to the changes in ΦF values which
corresponded well with the changes in absorption spectra
monitored at Q band maximum at 654 nm and 660 nm,
respectively (Fig. 2, C3, D3). For 10, the slow decrease in
ΦF became steep when protonation on azomethine nitro-
gen started. For 11, moderate increase in ΦF values due to
dissolution of aggregates was followed by large decrease
from 30% (v/v) H2SO4 in the solution. From these results,
it is deduced that protonation on azomethine nitrogens
lead to lowering of the fluorescence quantum yields. The
significant decrease in ΦF values of azomethine proto-
nated forms of Pc were observed also by other authors
[20, 21] or described in our recent work for TPyPz [24].
In contrast to 10 and 11, ΦF values of 8 and 9 in pure
DMFwereverylow,almostundetectable.Nofluorescence
signal was even detected in pyridine (Table 1). Since the
absorption spectra had the shape typical for monomers
in both solvents, the aggregation can be excluded as the
main reason of no or very low fluorescence. Recently, we
confirmed that such loss of fluorescent properties may be
attributed to ICT due to the presence of peripheral ter-
tiary amino group with free electron pair conjugated with
TPyPz macrocycle [24]. Removal of the donor center may
restore the fluorescence properties as the ICT is no longer
favorable way of a S1 excited state relaxation. Therefore,
a stepwise addition of H2SO4 into DMF solution of 8 and
9 caused increase in their ΦF values up to approx. 10%
and 20% (v/v) H2SO4 in the solution as a consequence of
protonation of N,N-dimethylamino substituent, a donor
center for ICT. The progress of the changes resembled
the changes in absorption spectra monitored at maximum
of 8 and 9 with fully protonated N,N-dimethylamino sub-
stituents, i.e. at 655 nm and 659 nm, respectively (Fig. 2,
A3, B3). The only difference is that the fluorescence
quantum yield peaked at higher acid concentrations (see
below for explanation). It is worth noting that the ΦF val-
ues of 8 and 9 fully protonated on N,N-dimethylamino
substituents increased by two orders of magnitude when
Fig. 4. Normalized absorption (dashed line) and fluorescence
emission (solid line) spectra of compound 8 in DMF with 0.1%
of sulfuric acid (v/v). Excitation wavelength = 382 nm
Copyright © 2010 World Scientific Publishing Company
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590
V. nOVakOVa et al.
specific chemical trap of singlet oxygen. No changes in
DPBF concentration were observed in any of the tested
solvents (with or without sulfuric acid) in the dark or
after irradiation without the tested dyes. ZnPc was cho-
sen as the reference for determination of ΦΔ. However,
the azomethine nitrogens of ZnPc appeared to be more
basic than for the studied compounds. Absorption spec-
trum of ZnPc indicated presence of a significant degree
of azomethine protonated form in DMF with 10% H2SO4.
For this reason, the ZnPc data from pure DMF were taken
as the reference. That is why the data collected in the
acidic medium cannot be considered as absolute and
serves only as orientation.
The results are summarized in Table 1. Data collected
for 10 and 11 in pyridine or DMF (only 11) showed high
production of singlet oxygen by both compounds. On the
other hand, similarly to what was observed during the flu-
orescence measurements, the ΦΔ values of 8 and 9 in DMF
and pyridine were very low as a consequence of quench-
ing of TPyPz excited states by ICT. After the addition of
H2SO4, the efficient blockade of ICT by protonation of
N,N-dimethylamino substituents increased the ΦΔ values
of 8 and 9 by two orders of magnitude. Since the full pro-
tonation of peripheral N,N-dimethylamino substituents of
9 is not achieved at 10% of H2SO4 in the solution, its ΦΔ
may reach even higher values. Based on the fluorescence
measurements (Fig. 2, B3), we estimate the ΦΔ value of 9
at 20% of H2SO4 in solution to be approx. 0.16. ΦΔ values
of 11 also considerably increased after addition of 10%
H2SO4 but not as much as of 8 and 9. As discussed above,
in the case of this compound it may be simply attributed
to dissolution of the aggregates formed in DMF.
Fig. 5. Normalized absorption and fluorescence excitation
spectra of compound 9 in DMF with 1% of sulfuric acid (v/v)
(absorption: dashed-dotted line, excitation: solid line) and 20%
of sulfuric acid (v/v) (absorption: dotted line, excitation: dashed
line). For excitation spectra, emission wavelength was fixed at
700 nm
The shape of the excitation spectra of 8 remained
unchanged from 0.5% to 10% H2SO4 in solution and super-
imposed the absorption spectra taken at 10% of H2SO4 in
DMF solution. The same was valid also for 9 but from 1%
to 20% H2SO4 (Fig. 5). The excitation spectra could not
be taken in DMF alone due to very weak fluorescence sig-
nal. The constant shape of excitation spectra indicates that
only one form may be responsible for the emission of fluo-
rescence. On the other hand, the emission maxima shifted
bathochromically with increasing amount of H2SO4 (Fig.
3A,B), suggesting equilibrium between at least two forms
(tri- and tetraprotonated) with the same excitation spectra
but different emission spectra. If we take into consideration
that the substantial changes in absorption spectra of 8 and
9 appeared only up to 6% and 12% and the increase of flu-
orescence still continued up to 10% and 20%, respectively
(Fig. 2, A3 and B3), we may conclude that both tri- and
tetraprotonated forms have the same absorption spectra.
The fluorescence signal was therefore emitted by tri- and
tetraprotonated forms with the same absorption but differ-
ent emission spectra. For comparison, the shape of emis-
sion and excitation spectra and the position of emission
maximum did not change for 10 or 11 in a range 0–18% of
H2SO4 in DMF solution (Fig. 3C).
CONCLUSION
In conclusion, two new TPyPz bearing peripheral four
N,N-dimethylanilino substituents responsible for quench-
ing of the singlet excited state by ICT were synthesized
and characterized. Several experiments with added sulfuric
acid documented that fluorescence as well as singlet oxy-
gen quantum yields are strongly influenced by protonation
of these peripheral donor centers. Values of both quantum
yields increased by two orders of magnitude when the N,N-
dimethylamino substituents were fully protonated. This
finding implies that the photochemical and photophysical
properties of TPyPz with substituents allowing ICT may
be controlled by changes in pH of the environment. The
differences in properties between non-protonated and fully
protonated species are sufficiently large and may lead to
the development of new pH-sensitive photosensitizers for
photodynamic therapy or pH-sensitive fluorescent probes.
Work on such derivatives is in progress.
Singlet oxygen
Singlet oxygen quantum yields (ΦΔ) of studied TPyPz
were determined in DMF, pyridine and DMF with 10%
H2SO4. The concentration of H2SO4 was chosen as the
concentration when the N,N-dimethylamino substitu-
ents of 8 and 9 should be almost fully protonated and,
at the same time, no significant azomethine protonation
occurs for any of the tested compounds. Compound 10
could not be properly measured in DMF and DMF with
10% H2SO4 because of considerable aggregation at the
concentrations used for the measurements.
Acknowledgements
The singlet oxygen determination was based on
decomposition of 1,3-diphenylisobenzofuran (DPBF), a
This work has been supported by research project
MSM 0021620822. Authors would like to thank to Jiří
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 590–591

InfluenCe Of PrOtOnatIOn Of PerIPheral SubStItuentS
591
Kuneš for measuring NMR and to Vojtěch Tambor for
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Supporting information
2D NMR spectra (gHMQC technique), additional
absorption, emission and excitation spectra are given in
the supplementary material. This material is available
free of charge via the Internet at http://www.worldscinet.
com/jpp/jpp.shtml.
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