Structural factors influencing the intramolecular charge transfer and photoinduced electron transfer in tetrapyrazinoporphyrazines

Article abstract of DOI:10.1039/c3cp54731k

A series of unsymmetrical tetrapyrazinoporphyrazines (TPyzPzs) from the group of azaphthalocyanines with one peripherally attached amino substituent (donor) were synthesized, and their photophysical properties (fluorescence quantum yield and singlet oxygen quantum yield) were determined. The synthesized TPyzPzs were expected to undergo intramolecular charge transfer (ICT) as the main pathway for deactivating their excited states. Several structural factors were found to play a critical role in ICT efficiency. The substituent in the ortho position to the donor center significantly influences the ICT, with tert-butylsulfanyl and butoxy substituents inducing the strongest ICTs, whereas chloro, methyl, phenyl, and hydrogen substituents in this position reduce the efficiency. The strength of the donor positively influences the ICT efficiency and correlates well with the oxidation potential of the amines used as the substituents on the TPyzPz as follows: n-butylamine < N,N-diethylamine < aniline < phenothiazine. The ICT (with conjugated donors and acceptors) in the TPyzPz also proved to be much stronger than a photoinduced electron transfer in which the donor and the acceptor are connected through an aliphatic linker.

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PAPER
Structural factors influencing the intramolecular
charge transfer and photoinduced electron
transfer in tetrapyrazinoporphyrazines†
Cite this: Phys.Chem.Chem.Phys.,
2014, 16, 5440
Veronika Novakova,*a Petr Hladık,b Tereza Filandrova,b Ivana Zajıcova,b
´
´
´
´
Veronika Krepsova,b Miroslav Miletin,b Juraj Lencocd and Petr Zimcik*b
´
ˇ
A series of unsymmetrical tetrapyrazinoporphyrazines (TPyzPzs) from the group of azaphthalocyanines with
one peripherally attached amino substituent (donor) were synthesized, and their photophysical properties
(fluorescence quantum yield and singlet oxygen quantum yield) were determined. The synthesized TPyzPzs
were expected to undergo intramolecular charge transfer (ICT) as the main pathway for deactivating their
excited states. Several structural factors were found to play a critical role in ICT efficiency. The substituent
in the ortho position to the donor center significantly influences the ICT, with tert-butylsulfanyl and butoxy
substituents inducing the strongest ICTs, whereas chloro, methyl, phenyl, and hydrogen substituents in this
position reduce the efficiency. The strength of the donor positively influences the ICT efficiency and
correlates well with the oxidation potential of the amines used as the substituents on the TPyzPz as
follows: n-butylamine o N,N-diethylamine o aniline o phenothiazine. The ICT (with conjugated donors
and acceptors) in the TPyzPz also proved to be much stronger than a photoinduced electron transfer in
which the donor and the acceptor are connected through an aliphatic linker.
Received 8th November 2013,
Accepted 27th January 2014
DOI: 10.1039/c3cp54731k
www.rsc.org/pccp
(or internal) charge transfer (ICT)5 where charge transfer states
between fully conjugated donors and acceptors are formed after
Introduction
Photoinduced electron transfer (PET) is one of the numerous excitation.6 In this article, we use the acronym PET to indicate
pathways for deactivating the excited states of molecules and non-conjugated linkers and the acronym ICT to refer to the
has been widely used to develop diverse sensors based on the fully conjugated connection.
ON–OFF switching of this highly efficient quenching mechanism.
Tetrapyrazinoporphyrazines (TPyzPzs) are the aza-analogs of
A PET requires the presence of donor and acceptor molecules/ phthalocyanines (Pcs) in which the benzene rings are replaced
moieties that can be fully separated as individual species in with pyrazines. Interest in their synthesis and photophysical
solution1,2 or connected through an aliphatic (non-conjugated) properties has increased during the last decade because of their
linker.3,4 A radical cation on the donor and a radical anion on strong absorption and emission in the red region of the visible
the acceptor are formed after excitation. Another deactivation spectrum, which is desirable for biological applications.7–9
process that is used for a design of the sensors is intramolecular Moreover, the presence of additional nitrogen atoms in the
macrocyclic core of TPyzPzs imparts it with remarkable
electron-deficient properties,10,11 which cause the macrocycle
a Department of Biophysics and Physical Chemistry, Faculty of Pharmacy in Hradec
to behave as an electron acceptor. In addition, PET12 or ICT13
Kralove, Charles University in Prague, Heyrovskeho 1203, 50005, Hradec Kralove,
Czech Republic. E-mail: veronika.novakova@faf.cuni.cz; Tel: +420 495067380
b Department of Pharmaceutical Chemistry and Drug Control, Faculty of Pharmacy
in Hradec Kralove, Charles University in Prague, Heyrovskeho 1203, 50005,
Hradec Kralove, Czech Republic. E-mail: petr.zimcik@faf.cuni.cz; Web: http://
portal.faf.cuni.cz/Groups/Azaphthalocyanine-group/; Tel: +420 495067257
c Department of Biochemical Sciences, Faculty of Pharmacy in Hradec Kralove,
Charles University in Prague, Heyrovskeho 1203, 50005, Hradec Kralove,
Czech Republic
may occur after combination with a donor. The ICT process, in
particular, is efficient in TPyzPzs and has recently been used to
develop red-emitting pH or metal–cation sensors.14,15 However,
since the first observation of this excited-state quenching in
TPyzPzs, no systematic study has focused on determining the
structural factors that influence the efficiency of these two
deactivation processes.12 The only study of the structural factors
was recently published and suggested that the ICT is influenced
by the electron-deficient properties of the macrocycle core.16
Therefore, in this study, we aim to scrutinize the impact of
structural modifications in close proximity to the donor on the
d Institute of Molecular Pathology, Faculty of Military Health Sciences, University of
´
´
Defence, Trebesska 1575, 500 01 Hradec Kralove, Czech Republic
† Electronic supplementary information (ESI) available: Synthesis of studied
compounds, correlations between Hammett substituent constants and photo-
physical parameters, cyclic voltammograms. See DOI: 10.1039/c3cp54731k
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efficiency of an ICT and to compare ICT and PET in TPyzPzs to Electrochemical measurements
allow rational design of sensors based on the TPyzPz macrocycle.
The electrochemical measurements were performed at room
temperature using an Autolab PGSTAT101 potentiostat (Metrohm,
Czech Republic). The three-electrode experimental setup consisted
of a Pt working electrode, a Pt counter electrode, and an Ag/AgCl
reference electrode separated from the bulk solution by an
integrated salt bridge. Samples (typically 1 ꢂ 10ꢀ3 M) were
dissolved in 0.1 M tetrabutylammonium hexafluorophosphate
in anhydrous acetonitrile (Sigma-Aldrich) as a supporting electrolyte.
The dissolved oxygen was removed via vigorous purging with N2
gas for 5 min. All values in Table 2 are referenced to an SCE
with ferrocene as the internal standard (half-wave potential
E1/2(Fc/Fc+) = +0.38 V vs. SCE20).
Experimental
Synthesis
Details regarding the synthesis of both precursors (substituted
pyrazine-2,3-dicarbonitriles) and TPyzPzs and their full characteriza-
tion are described in the ESI† and are discussed in the Results
section. All samples were re-purified using preparative TLC for
the photophysical measurements (both FF and FD) to ensure
that they were highly pure.
Fluorescence
The fluorescence spectra were obtained using an AMINCO
Bowman Series 2 luminescence spectrometer. Emission spectra
Results
were corrected for instrument response. The fluorescence Synthesis
quantum yields (FF) were determined in THF via the comparative
The studied TPyzPzs were synthesized from corresponding
method using unsubstituted zinc phthalocyanine (ZnPc, Sigma-
Aldrich) as a reference (FF = 0.32 in THF17). Both the reference
and sample were excited at 595 nm. The absorbance at the
excitation wavelength was held below 0.015, and the absorbance
at the Q-band maximum was held below 0.05 to limit the inner filter
effect. The value of FF was calculated using eqn (1):
precursors, i.e., substituted pyrazine-2,3-dicarbonitriles (1–8).
The precursors 1–8 were prepared via the nucleophilic substitution
of chlorine atoms in 5-chloro-6-phenylpyrazine-2,3-dicarbonitrile,
5-chloro-6-methylpyrazine-2,3-dicarbonitrile, or 5,6-dichloropyrazine-
2,3-dicarbonitrile. With two exceptions, the reactions proceeded with
a high yield (Scheme 1) after the starting material was mixed
with an excess of the nucleophile (amine or thiolate). First,
the extremely low nucleophilicity of nitrogen in phenothiazine
prohibited the direct substitution and synthesis of 4, but the
reaction required the addition of CsF to form the highly reactive
phenothiazin-10-ide. Similar attempts to synthesize a precursor
substituted with diphenylamine were unsuccessful. The reaction
indicated no trace of a product (5-diphenylamino-6-phenyl-
pyrazine-2,3-dicarbonitrile) despite a number of bases and con-
ditions being attempted (e.g., NaOH, NaH, K2CO3, CsF, DBU in
DMF or THF). Second, the isolation of compound 6, which is an
intermediate in the synthesis of 7, proved to be difficult due to
the similar Rf values of 6 in all mobile phases tested with both
the starting material and the side product 5,6-dibutoxypyrazine-
!
ꢀAR
FS 1 ꢀ 10
FSF ¼ FRF
(1)
S
FR
1 ꢀ 10ꢀA
where F is the integrated area under the emission spectrum and
A is the absorbance at the excitation wavelength. The super-
scripts R and S correspond to the reference and the sample,
respectively. All experiments were performed in triplicate with
the data representing the mean (estimated error ꢁ15%). The
excitation spectra were collected by observing the fluorescence
signal at lem E 690–720 nm.
Determination of the singlet oxygen production
The quantum yields of the singlet oxygen (FD) were determined 2,3-dicarbonitrile. Consequently, only a small quantity was isolated
in THF according to a previously published procedure18 using for analytical purposes. The precursor 7 (with different retention
the decomposition of a chemical trap 1,3-diphenylisobenzofuran properties on silica) was synthesized in one pot with a satisfactory
(DPBF) with ZnPc as a reference (FD = 0.53 in THF19). The yield of 30% without isolation of the intermediate 6.
detailed procedure was as follows: 2.5 mL of a DPBF stock
Some of the studied TPyzPzs were prepared via published
solution in THF (5 ꢂ 10ꢀ5 M) was transferred into a 10 ꢂ procedures (see ESI†). The rest were synthesized (Fig. 1 and
10 mm quartz optical cell and was saturated with oxygen for Scheme 1) via a mixed cyclotetramerization of 3 eq. of 5,6-bis-
1 min. A stock solution of the tested compound in THF (typically (tert-butylsulfanyl)pyrazine-2,3-dicarbonitrile (precursor A) with
30 mL) was then added to achieve an absorbance of the final 1 eq. of one of the aforementioned precursors (1–8, precursor
solution in the Q-band maximum of approximately 0.1. The B). The reaction initiated by magnesium butoxide yielded a
solution was stirred and irradiated using a xenon lamp (100 W, statistical mixture of magnesium complexes with six congeners
ozone-free XE DC short-arc lamp, Newport). The incident light (AAAA, AAAB, ABAB, AABB, ABBB, and BBBB). Treatment of the
was filtered through a water filter (6 cm) and an OG530 cut-off mixture with p-toluenesulfonic acid converted the magnesium
filter (Newport) to remove the heat and the light below 523 nm, complexes to metal-free derivatives that typically allow an easier
respectively. Decrease of DPBF in solution with irradiation time isolation of the required AAAB-type congener.13–15 Zinc and
was monitored at 414 nm. All experiments were performed in magnesium were complexed to the center of the metal-free
triplicate, and the data presented herein represent the mean of TPyzPz after the successful isolation of the AAAB-type congener
the three experiments (estimated error: ꢁ10%).
using either zinc acetate or magnesium acetate, respectively.
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Fig. 1 Structures of studied TPyzPzs. Substituent R1 contains always a
donor group.
Scheme 1 Synthetic procedures: (i) n-butylamine, THF, rt, 2 h. (ii) Aniline,
THF, reflux, 8 h. (iii) NH4OH, THF, rt, 4 h. (iv) Phenothiazine, CsF, anhydr.
DMF, rt, 5 h. (v) 2-Diethylaminoethanethiol hydrochloride, aq. NaOH, THF,
rt, 15 min. (vi) n-Butanol aq. NaOH, ꢀ10 1C, 5 min. (vii) Anhydr. n-butanol,
Mg, reflux, B6 h. (vii) p-Toluenesulfonic acid, THF, rt, 2 h, isolation of
congeners. (ix) Anhydr. Zn(CH3COO)2, pyridine, reflux, 2 h. (x) Anhydr.
Mg(CH3COO)2, pyridine, reflux, 2 h.
Table 1 Photophysical data of studied TPyzPzs in THF. Absorption max-
imum in the Q-band (lmax), fluorescence emission maximum (Fmax),
fluorescence quantum yield (FF), singlet oxygen quantum yield (FD)
a
a
M
lmax
Fmax
FF
FD
FF + FD
9b
Mg
Zn
Mg
Zn
Mg
Zn
Mg
Zn
Mg
Zn
Mg
Zn
Mg
Zn
Mg
Zn
Zn
Mg
Zn
Mg
Zn
Mg
Zn
Mg
Zn
Mg
Zn
Mg
Zn
654
653
652
651
648
646
662
660
655
653
658
656
657
655
654
652
655
652
650
653
651
655
654
648
646
650
649
651
649
664
663
667
667
655
662
674
673
669
668
668
668
664
664
661
660
665
659
666
672
666
669
666
653
652
655
654
659
657
0.508
0.245
0.428
0.175
0.308
0.174
0.447
0.202
0.326
0.123
0.497
0.210
0.268
0.080
0.038
0.014
0.005
0.245
0.165
0.092
0.018
0.007
0.002
0.178
0.130
0.011
0.002
0.468
0.299
0.303
0.551
0.356
0.647
0.138
0.306
0.316
0.623
0.430
0.622
0.348
0.582
0.215
0.255
0.036
0.043
0.021
0.126
0.286
0.128
0.190
0.043
0.107
0.143
0.344
0.056
0.094
0.30
0.81
0.80
0.78
0.82
0.45
0.48
0.76
0.83
0.76
0.75
0.85
0.79
0.48
0.34
0.07
0.06
0.03
0.37
0.45
0.22
0.21
0.05
0.11
0.32
0.47
0.07
0.10
0.77
0.85
10ab
10f
A cyclotetramerization reaction with compound 3 proceeded as
well, but the resulting mixture of congeners was inseparable due
to strong tailing on the TLC and overlapping of the fractions. For
that reason, the corresponding TPyzPz could not be isolated.
11b
12ab
12b
12c
12d
Absorption spectra
Absorption spectra of 9–16 (both Mg and Zn complexes) in THF
exhibited a shape typical of TPyzPzs with a high-energy B-band
(B360 nm) and a low energy Q-band (B660 nm) (see ESI,† Fig. S2).
The position of the Q-band varied slightly with the substitution
pattern (Table 1); however, the bands of the magnesium complexes
were systematically red-shifted by approximately 1–2 nm when
compared with the bands of the zinc derivatives. The sharp
Q-band with well-resolved vibrational bands indicated that no
aggregation occurred at the tested concentrations (0.05–5 mM).
Aggregation of the planar TPyzPz macrocycles can result in a
misinterpretation of the photophysical results because it typically
decreases both FF and FD independently of the presence or the
absence of ICT or PET.
12e
12f
13b
14ab
14f
15
16c
0.55
Fluorescence and singlet oxygen
a
Fluorescence data were corrected for instrument response. In the case
of published data, the values were recalculated using original spectra.
Data from ref. 14. Data from ref. 17.
The studied TPyzPzs all emitted red fluorescence after excitation.
The signal intensity, however, varied strongly depending on the
b
c
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one ligand yielded comparable values, indicating that similar
competitive processes (PET or ICT) are responsible for the
deactivation of the first excited states in both complexes. The
only difference between the zinc and magnesium derivatives of
the ligand was in the ratio between the intersystem crossing
and photon emission, which is driven by the heavy-atom effect.
Differences in the sum of FD + FF caused by the peripheral
substituents are examined in detail in the Discussion section.
Electrochemical behavior
The electrochemistry of the studied amines and the corresponding
pyrazine-2,3-dicarbonitriles was assessed via cyclic voltammetry in
acetonitrile at room temperature with ferrocene as an internal
standard. The experiments focused on the oxidation process to
evaluate the electron-donating properties of the engaged nitrogen
atoms. n-Butylamine, N,N-diethylamine, aniline, and pheno-
thiazine underwent the first oxidation best expressed by the
half-wave potentials E1/2 = 1.70, 1.14, 0.98, and 0.60 vs. SCE,
respectively. A similar trend was observed in the corresponding
series of pyrazine-2,3-dicarbonitriles (see Table 2). The process can
be attributed solely to the oxidation of the attached nitrogens
because 5-chloro-6-phenylpyrazine-2,3-dicarbonitrile—a similar
compound without any donor amines—underwent no oxidation
up to 2.8 V vs. SCE (see Fig. S6 in the ESI†). Chemically irreversible
oxidations were observed in all cases except for phenothiazine and
its corresponding pyrazine-2,3-dicarbonitrile 4, in which reversible
oxidations occurred (Fig. 3 and Fig. S4 and S5 in the ESI†). Notably,
the final TPyzPzs showed evidence of corresponding oxidation
processes, although the appropriate half-wave potentials could
not be properly determined. Nevertheless, the relationship
observed in the amine series and the pyrazine-2,3-dicarbonitriles
suggests that the oxidation feasibility of the peripheral donor
nitrogens in TPyzPzs increases in the order 12b o 12a o 12c o 12d.
Fig. 2 Normalized absorption (black, full line), fluorescence emission
(blue, dotted line) and excitation (red, dashed line) spectra of compound
14fMg in THF. The spectra are typical for all studied TPyzPzs. Emission
spectra were corrected for instrument response.
peripheral substitution (see below). The shapes of the emission
spectra were typical for this type of compounds (Fig. 2), with only
a small Stokes shift observed in the range of B6–13 nm. The
excitation spectra were also recorded and matched the absorption
spectra perfectly (Fig. 2), thereby demonstrating that only monomers
were present. This result confirmed that bulky tert-butylsulfanyl
substituents on 9–14 and 16 (or N,N-diethylaminoethylsulfanyl
substituents in 15) fully inhibited the aggregation. The subsequently
discussed changes in the quantum yields can therefore not be
attributed to an aggregation phenomenon. The fluorescence
quantum yields (FF) were determined in THF using a compara-
tive method with ZnPc as the reference compound. The FF
values (Table 1) of the zinc complexes varied from 0.30 (for
compounds without a donor or those in which the ICT or PET
was inefficient) to 0.002 (in compounds with highly efficient ICT
or PET). In magnesium complexes, FF varied from 0.508 to 0.007.
The magnesium complexes were always characterized by FF
values that were greater than those of the corresponding zinc
complexes, which is consistent with a number of previous
observations of TPyzPzs and is explained by the heavy-atom
Discussion
effect.17,21 The heavy-atom effect supposes that substitution with A systematic observation of the low FD and FF values in TPyzPzs
heavier atoms increases the probability of intersystem crossing with alkylamino peripheral substituents led to a thorough
and thus decreases the photon emission from the first excited investigation of this problem. Using a combination of experiments,
state.22
including changes in the solvent polarity and acidity, cyclic
Singlet oxygen production was determined via the chemical voltammetry measurements, steady-state and time-resolved
trapping of singlet oxygen by DPBF with ZnPc as the reference emission spectra, and transient absorption studies, we subse-
compound. Consistent with the heavy-atom effect, the FD quently confirmed that ICT is the major deactivating pathway for
values were greater for zinc complexes (0.647–0.021) than for the excited states in such derivatives (e.g. in compound 13 that
the corresponding magnesium derivatives (0.356–0.036). The was previously studied in detail).13 ICT was found to be compe-
sum of FD + FF for both the zinc and magnesium complexes of titive to both fluorescence and intersystem crossing. Quenching
Table 2 Oxidation potentials expressed as E1/2 (in V vs. SCE) of amines and corresponding 5-substituted-6-phenylpyrazine-2,3-dicarbonitriles in
acetonitrile
Amine
1st
2nd
Pyrazine-2,3-dicarbonitrile
1st
2nd
3rd
4th
n-Butylamine
N,N-Diethylamine
Aniline
1.70
1.14
0.98
0.60
—
—
—
0.91
Compound 1
5-Diethylamino-6-phenylpyrazine-2,3-dicarbonitrile
Compound 2
Compound 4
1.75
1.63
1.50
1.05
1.98
1.80
—
—
1.96
—
—
—
—
1.90
Phenothiazine
1.51
1.68
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acceptor (i.e., a comparison between ICT and PET). TPyzPzs
9–15 were synthesized, and their photophysical properties were
determined and compared in a selected series with compound 16,
which does not contain any donor center. Both the magnesium and
zinc complexes with differing FD-to-FF ratios were investigated. As
documented in Table 1 and Fig. 4, the trends in ICT efficiency
(expressed as the sum of FD and FF) were similar for both
complexes.
The effect of a substituent in the ortho position has been
recently described and used to develop new cation sensors
without any serious discussion.14 As indicated in Fig. 4a, the
presence of Cl (9Mg, 9Zn), CH3 (10aMg, 10aZn) or hydrogen
(11Mg, 11Zn) ortho to the donor (the N,N-diethylamino sub-
stituent) eliminates any ICT, and the sum of FD and FF is fully
comparable with compounds without any donor center (16Mg,
16Zn). The ICT is slightly less deactivated with phenyl (12aMg,
12aZn) as the ortho substituent. The strongest ICT is observed
with alkylsulfanyl (13Mg, 13Zn) or alkoxy (14aMg, 14aZn) sub-
stituents in the ortho position. These effects were correlated
with the steric or electronic effects (Hammett substituent
constants sp and sm, see ESI†); however, these correlations
were consistently tenuous and provided insufficient answers.
The rationale behind this strong effect of the ortho substituent
remains unexplained and, for future use, must only be taken
empirically. However, the effect is clearly connected with the
Fig. 3 Cyclic voltammogram (acetonitrile, 100 mV sꢀ1, 0.1 M tetrabutyl-
ammonium hexafluorophosphate, 25 1C) of phenothiazine (a) and corres-
ponding pyrazin-2,3-dicarbonitrile 4 (b, only first oxidation presented).
of the TPyzPz (or Pc) triplet states by oxygen in organic solvents close proximity of the substituent to the donor. A strong ICT
(and in saturated oxygen) typically approaches 100%; therefore, the efficiency was achieved when a linker (benzene ring, 12eZn) was
FD values can serve as indicators of intersystem crossing.23 The inserted between the donor and the acceptor (compare FD + FF
sum of FD and FF can thus quickly determine the ICT efficiency (12eZn) = 0.03 with FD + FF (12aZn) = 0.75).
and can be used to compare a series of similar compounds.
For further study, a substituent was fixed in the ortho
Little attention has been previously paid to the structural position, and the substituents on the donor were altered
factors that can influence ICT. Substituents on or nearby the by increasing their electron-donating properties (12a-dMg,
donor center were considered to have only a marginal effect, and 12a-dZn). Phenyl, as an ortho substituent, was selected due
research efforts were primarily aimed at determining the recognition to its moderate deactivating effect on the ICT (see previous
properties of the donor moiety in sensing applications.15 discussion), which allows an evaluation of the potentially
If differences were observed, limited details were provided, with positive effects of the electron-donating substituents on the
no further explanation.14 Hence, this study is focused on deeper donor. The electron-donating properties were estimated based
investigation and discussion on some structural modifications in on the electrochemical measurements of the ease of oxidation.
close proximity to the donor center. The study can be divided into The electron-donating properties of the amines alone or when
three particular goals: (a) substituents in a position ortho to the attached as substituents on the pyrazine-2,3-dicarbonitrile increase
donor center, (b) substituents on the donor center, (c) the influence in the order n-butylamine o N,N-diethylamine o aniline o
of a conjugated ꢂ non-conjugated linker between the donor and the phenothiazine, as deduced from the half-wave oxidation potentials
Fig. 4 Sum of singlet oxygen quantum yield (FD, striped parts of the columns) and fluorescence quantum yield (FF, full parts of the columns) of studied
TPyzPzs grouped according to the studied phenomenon: (a) effect of the ortho substituent, (b) effect of donor strength, (c) comparison of PET and ICT.
Horizontal dotted line represents a mean (from Mg and Zn complex) value of the sum FD + FF for control complex 16 that has no donor center.
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in Table 2. This series correlates well with the ICT efficiency for assess the influence of substitution on the efficiency of the
compounds 12a-dMg and 12a-dZn (Fig. 4b). Therefore, the deactivation processes. Several factors were recognized. A substituent
substituents on the donor clearly play an important role, and in the position ortho to the donor center plays an important role in
sufficiently strong donors (e.g., phenothiazine in 12dZn and ICT for reasons that have heretofore not been fully revealed. Chloro,
12dMg) can suppress the effect of a substituent in the ortho methyl, hydrogen, or phenyl substituents were found to inhibit
position (see previous discussion).
efficient ICT processes, whereas alkylsulfanyl or alkoxy substituents
After the excitation, the charge transfer state is formed in ICT supported the ICT. The effect of an ortho substituent can be
when the donor and the acceptor are in conjugation; however, the eliminated through the introduction of a linker between the TPyzPz
radical anion and the radical cation are formed in PET (no macrocycle and the donor. Strong electron donors can also suppress
conjugation between the donor and the acceptor). A comparison the ‘‘ortho effect.’’ The ICT efficiency depends significantly on the
of the efficiencies of these two processes in TPyzPzs is interesting electron-donating properties of the donor, which may easily be
because both principles have been recognized in other classes of characterized using its oxidation potential. Phenothiazine-10-yl was
molecules for the development of a number of sensors or activable the strongest donor in the studied series. Conjugation of the donor
compounds.5,24,25 In this study, PET was demonstrated in with an acceptor (i.e., ICT) leads to a more efficient deactivation of
compounds with one N,N-diethylaminoethylsulfanyl substituent the excited states in TPyzPzs than in the case of an aliphatic linker
(10fMg, 10fZn, 12fMg, 12fZn, 14fMg, or 14fZn) or with eight such between the donor and the acceptor (i.e., PET). These factors should
substituents (15Mg, 15Zn). The first important observation is that a be further considered for the design of new fluorescence sensors
substituent in the ortho position has no effect on the PET efficiency. based on the ON–OFF switching in TPyzPzs.
All compounds from the 10f, 12f, 14f series with one donor center
were characterized by a similar sum of FD and FF (approximately
half the value of a compound without any donor—16Mg, 16Zn)
Acknowledgements
irrespective of the adjacent substituent (Fig. 4c). Apparently, the
The work was supported by the Czech Science Foundation
longer distance eliminates the ‘‘ortho’’ effect. However, the overall
ˇ´
ˇ
(P207-11-1200). The authors would like to thank Jirı Kunes
PET efficiency is significantly below that of ICT, as demonstrated by
a comparison with compound 12eZn, in which the donor (in
conjugation) is approximately at the same distance from the
acceptor and the ‘‘ortho effect’’ is limited (Fig. 4c). The sum of
FD and FF for the compound 12eZn is approximately one order of
magnitude less than those for compounds 10fMg, 10fZn, 12fMg,
12fZn, 14fMg, and 14fZn with PET. An intensification of the PET12
or ICT13 efficiency with an increasing number of donor centers has
been demonstrated in the literature. Thus, the decrease in the FD
and FF values in compounds 15Mg and 15Zn substituted with eight
donor centers for PET was unsurprising. However, the interesting
fact is that only the presence of eight donor centers in these
compounds brought the PET efficiency closer to that of the ICT
in 12eZn with one donor center.
Another experiment was performed to demonstrate the key
role played by the lone pair on the donor nitrogen in the
deactivation process of the PET in TPyzPzs. The fluorescence
quantum yield of compound 14fZn in THF is 0.13. The addition of
a small quantity of sulfuric acid (0.1% v/v) into this solution
increased FF to 0.23, a value comparable to that of compound
16Zn with no donor center. An increase of the sulfuric acid
concentration to 1% (v/v) or 2% (v/v) did not change the fluores-
cence intensity further, which indicates that the donor center was
fully protonated after the first addition. A similar recovery of
photophysical properties was noted for ICT in TPyzPzs15,26 or for
PET in Pc after protonation of the donor.27–29
ˇ
ˇ
and Zdenek Kucera for NMR measurements.
Notes and references
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