The long and winding road to new porphycenes

Article abstract of DOI:10.1142/S1088424612500733

We describe attempts - not always successful - made over the years to improve the efficiency of porphycene synthesis and to produce novel compounds, custom-designed for specific purposes. New porphycenes are reported, some of them obtained rather unexpectedly as by-products of the planned reactions. Structure and energy computations of possible tautomeric forms in porphycenes substituted by one, two, three, and four tert-butyl groups lead to predictions regarding the kinetics and mechanisms of intramolecular double hydrogen transfer. The occurrence of tautomerization in single molecules of tert-butylsubstituted porphycenes is demonstrated by using fluorescence polarization techniques.

Full text of DOI:10.1142/S1088424612500733

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2012; 16: 589–602
DOI: 10.1142/S1088424612500733
Published at http://www.worldscinet.com/jpp/
The long and winding road to new porphycenes
Igor Czerskia, Arkadiusz Listkowskia,b, Jan Nawrockia, Natalia Urban´skaa, Hubert
Piwon´skia, Adam Sokołowskia, Oksana Pietraszkiewicza, Marek Pietraszkiewicz*a
and Jacek Waluk*a,b◊
a Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warsaw 01-224, Poland
b Faculty of Mathematics and Natural Sciences, College of Science, Cardinal Stefan Wyszyński University,
Dewajtis 5, Warsaw 01-815, Poland
Dedicated to Professor Emanuel Vogel in memoriam
Received 9 October 2011
Accepted 10 February 2012
ABSTRACT: We describe attempts — not always successful — made over the years to improve the
efficiency of porphycene synthesis and to produce novel compounds, custom-designed for specific
purposes. New porphycenes are reported, some of them obtained rather unexpectedly as by-products of
the planned reactions. Structure and energy computations of possible tautomeric forms in porphycenes
substituted by one, two, three, and four tert-butyl groups lead to predictions regarding the kinetics and
mechanisms of intramolecular double hydrogen transfer. The occurrence of tautomerization in single
molecules of tert-butyl-substituted porphycenes is demonstrated by using fluorescence polarization
techniques.
KEYWORDS: porphycene, synthesis, tautomerism, photodynamic therapy, porphyrin isomers,
McMurry, single molecule spectroscopy.
it was dubbed “inverted” [17] or “N-confused” [18]
INTRODUCTION
porphyrin (6).
The synthesis of porphycene by Emanuel Vogel and
Porphyrins have been suitably described as “pigments
coworkers [1] has created a new branch of chemical
of life” [19]; it was therefore to be expected that
research, devoted to the studies of constitutional isomers
their isomers can prove useful in those areas where
of porphyrin [2–9]. In a theoretical paper [10], inspired
the interaction of light and matter is crucial. Indeed,
by the work of Vogel, all other possible “nitrogen-in”
porphycene derivatives turned out to be extremely
isomers have been discussed (Scheme 1), all except
promising in photodynamic therapy (PDT) [20]. For
porphyrin (1) and porphycene (2) nonexistent at that
time. Since then, three more have been synthesized (as
alkyl or aryl derivatives): hemiporphycene (3) [11,12]
corrphycene (4) [13,14], and isoporphycene (5) [15,16],
but three (7–9) still await experimental realization.
Another isomer has also been obtained, characterized
example, derivatives carrying methoxyethyl side chains
were found to be between 17 and 220 times more active
against SSK2 murine fibrosarcoma cells than Photofrin,
one of the few approved and clinically used sensitizers
[21]. Interestingly, this result cannot be solely due to a
better absorption of porphycenes in the red part of the
by one nitrogen atom located outside the inner cavity;
spectrum, because the absorption coefficients differ
only by a factor of about ten. Moreover, the yields of
◊SPP full member in good standing
single oxygen generation are lower in porphycenes [22].
Evidently, structural differences may lead to different
kinetics of cell membrane penetration, and to different
localization inside the cell. Studies of structure-properties
*Correspondence to: Marek Pietraszkiewicz, email: pietrasz@
ichf.edu.pl, fax: +48 22-343-3333 and Jacek Waluk, email:
waluk@ichf.edu.pl, fax: +48 22-343-3333
Copyright © 2012 World Scientific Publishing Company

590
I. CZERSKI ET AL.
Scheme 1.
correlations require testing of differently substituted
porphycenes.
In addition, tautomerization in porphycenes can also be
monitored on a single molecule level [36, 51]. On the
application side, using tautomerization in porphycenes for
construction of ultrafast switches has been proposed [58].A
related possibility, described in several patent applications,
is to employ porphycenes for information storage.
Another area where the research on porphycenes has
been very intense concerns tautomerism and hydrogen
bond studies [9,23–60]. Two internal hydrogens can
migrate between the four inner nitrogen atoms. Formally,
this process is the same as in free-base porphyrins, but
both the kinetics and the mechanism are totally different.
In porphyrin, the ground state tautomerization proceeds in
microseconds as a stepwise process [61–63], whereas in
porphycenes two hydrogen move in a concerted fashion,
and the reaction rates range between 1010 and 1013 s-1 [49,
56, 59]. This huge acceleration is caused by very strong
intramolecular hydrogen bonds in porphycene. Variations
of the rate constants over four orders of magnitude are
suggestive of tunneling. In fact, studies of porphycenes
isolated in supersonic jets [29, 37] and helium nanodroplets
[53] revealed tunneling splittings. Most interesting was the
finding that the splittings are vibrational-mode-specific:
excitation of certain modes lowers the tautomerization
barrier, for other modes the barrier is not influenced,
and finally, it becomes higher for some vibrations. All
these results make porphycenes role models for detailed
studies of multidimensional double hydrogen tunneling.
The above examples show the importance of
obtaining new porphycenes, custom-designed for specific
purposes. Unfortunately, syntheses of porphycenes are
rather difficult. They are usually based on the McMurry
reductive cyclization of two 2,2′-bipyrrole moieties
bearing carbonyl substituents in 5,5′-positions. The yields
are small: 2–3% was reported for the parent porphycene
[1]. The amount of the product can be enhanced by
substituents in the pyrrole rings, the more bulky ones
leading to higher yields. Thus, 10% has been reported
for 2,7,12,17-tetrapropylporphycene [64], whereas the
formation of 2,7,12,17-tetra-tert-butylporphycene (10)
occurs with 20% yield [65]. Noting this higher yield,
we produced parent porphycene by first obtaining
2,7,12,17-tetra-tert-butylporphycene and then removing
the tert-butyl groups [66]. A similar approach has been
employed earlier for the preparation of parent porphyrin
[67, 68] and for the unsubstitued “inverted” isomer [69].
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 590–602

THE LONG AND WINDING ROAD TO NEW PORPHYCENES
591
In this work, we describe the attempts to (i) optimize
the yields in the syntheses of porphycenes and
their building blocks; (ii) produce new compounds,
relevant for the studies of tautomerization, hydrogen
bond properties, spectroscopy, photophysics, and
photodynamic therapy. Several new porphycenes
are described; interestingly, some of them have been
obtained somewhat by chance, as unexpected reaction
by-products. We then focus on those molecules which
exhibit lower symmetry, and thus different potential
governing the intramolecular double hydrogen transfer.
The results of calculations of optimized ground state
geometry are presented for trans and cis tautomeric
forms of tert-butyl-substituted porphycenes. On the
basis of the analysis of relative energies and structural
parameters, conclusions are presented regarding the
tautomerization kinetics and mechanism: concerted vs.
stepwise. Finally, using single molecule spectroscopy
with polarized light, we show that tautomerization can be
observed not only for porphycenes exhibiting symmetric
double minimum potential, but also for asymmetrically
substituted derivatives.
N
N
N
N
N
N
N
N
O
R
O
R
base
R = P(O)(OEt)2, PPh3
Scheme 3.
lack of protection makes the compounds so unstable that
they decompose during the reaction, or during attempts at
purification using traditional chromatographic methods.
From the viewpoint of syntheses of porphycenes, nitrogen
in the pyrrole ring cannot be associated with substituents
larger than hydrogen: for steric reasons, this would make
impossible the process of cyclization reaction shown in
Scheme 3.
Attempts to use methathesis
In the next stage of research we decided to use a
metathesis reaction leading to the transformation of
porphycene, according to the synthetic pathways shown
in Scheme 4.
RESULTS AND DISCUSSION
In order to identify potential applications of this
transformation we conducted a series of model reactions
(Scheme 5). As catalysts, we used commercially
available ruthenium complexes (Fig. 1). It should be
noted that this is the first case of the use of metathesis
reactions using unprotected vinyl pyrrole derivatives.
The reactions were carried out in both homo-and
cross-metathesis.
The highest yield (25%) was obtained for (E)-3-(1H-
Pyrrol-2-yl)-acrylic acid ethyl ester (14a) [71] using an
indenylidene catalyst, wherein the ratio of E to Z was
1/3. As a result of cross-metathesis reaction leading to
compounds 17a and 17b, only E,E and E,Z products were
obtained, not suitable for our purposes. Notwithstanding
the low yield of metathesis, we attempted a synthesis
of porphycene from “vinyl derivatives of bipyrrole”
(Scheme 6). However, despite repeated attempts, no
expected macrocycle was obtained. Getting porphycene
requires two Z double bonds in the precursors. The
Attempts to use Wittig reaction and its modifications
Based on the literature synthesis of 2,2′-(Z)-ethene-
1,2-diylbis(1H-pyrrole) (13, Scheme 2), [70] we decided
to use the Wittig reaction and its modifications for the
synthesis of porphycenes.
We performed a series of control reactions on model
compounds. In the study, we obtained derivatives
of 2,2′-(Z)-ethene-1,2-diylbis(1H-pyrrole) 13, but it
turned out that pyrrole phosphorus ylide derivative (or
phosphonate) must be protected on the nitrogen atom. The
base
N
+
PO(OEt)2
N
N
Z:E 2:1
N
O
11
12
13
Scheme 2.
N
H
H
N
N
H
H
N
N
H
N
N
H
N
ruthenium cat.
metathesis
[O2]
N
H
N
H
2
Scheme 4.
Copyright © 2012 World Scientific Publishing Company
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592
I. CZERSKI ET AL.
N
H
H
N
metathesis
R2
catalysts used in metathesis
are shown in Figure 1
N
H
E, Z isomers
R1
14
15
R3
R3
R3
R3
N
H
H
N
N
H
H
N
metathesis
+
N
H
N
N
H
R2
H
R1
R4
R4
14
16
E, Z isomers
17
14a: R1 = H, R2 = CO2Me
14b: R1 = H, R2 = CN
16a: R3 = H, R4 = CO2Me
16b: R3 = tBu, R4 = H
17a: R3 = H
16c: R3 = tBu, R4 = CO2Me
14c: R1 = CO2Me, R2 = CN
17b: R3 = tBu
Scheme 5.
Mes
Cl
N
N Mes
Mes
N
N Mes
Mes
Cl
N
N Mes
Ph
PCy3
Ru
Ru
O
Cl
Cl
Cl
Ru
Cl
Cl
Ru
PCy3
Grubbs cat. 2nd gen.
Cl
Ph
Ph
PCy3
indenylidene cat.
PCy3
Grubbs cat. 1st gen.
Hoveyda - Grubbs cat.
Fig. 1. The catalysts used in the thesis
R1
Synthesis of singly, doubly, and triply tert-butyl-
R1
substituted porphycenes
R1
R1
N
N
H
metathesis
In the reaction of 2,7,12,17-tetra-tert-butylporphycene
(10)withhotsulfuricacidweobtainedremarkableamount
of the unsubstituted porphycene [66] and a mixture of
derivatives of not fully removed alkyl groups: 2-tert-
H
N
N
H
N
H
N
R2
R2
R1
R1
butylporphycene (18), di-tert-butylporphycene
—
16a: R1 = H, R2 = CO2Et
16b: R1 = tBu, R2 = H
16c: R1 = tBu, R2 = CO2Et
2: R1 = H
10: R1 = tBu
three regioisomers (19, 20, 21) and 2,7,12-tri-tert-
butylporphycene (22) (Scheme 7). The amount of mono-
di- and trisubstituted derivatives could be modified
by changing the reaction conditions, e.g., reducing the
temperature or shortening the reaction time.
Scheme 6.
It was feasible to isolate mono-substituted 18 and tri-
tert-butylporphycene 22. The mixture of disubstituted
porphycenes has also been isolated, but the separation of
individual regioisomers was not possible. However, we
probability of obtaining such a system is very low due
to the preferential formation of E double bond. Such
precursors lead to the formation of polymers.
R2
R1
2: R1, R2, R3, R4 = H
18: R1 = tBu; R2, R3, R4 = H
N
N
N
H
N
H
19: R1, R2 = tBu; R3, R4 = H
H2SO4
20: R1, R3 = tBu; R2, R4 = H
H
N
H
∆
N
N
N
21: R1, R4 = tBu; R2, R3 = H
22: R1, R2, R3 = tBu; R4 = H
R3
R4
10
Scheme 7.
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 592–602

THE LONG AND WINDING ROAD TO NEW PORPHYCENES
593
R1
R1
N
H
N
H
O
O
O
O
N
N
H
23
24
TiCl4, CuCl, Zn
H
N
H
N
H
N
N
R2
R2
2: R1, R2 = H
19: R1 = H; R2 = tBu
10: R1, R2 = tBu
Scheme 8.
Fig. 3. The electronic absorption spectra in the Soret region,
recorded in acetonitrile at 293 K. From bottom to top:
porphycene (2), 2-tert-butylporphycene (18), 2,7-di-tert-
butylporphycene (19), 2,7,12-tri-tert-butylporphycene (22),
2,7,12,17-tetra-tert-butylporphycene (10)
for the cis forms the same species correspond to the location
of protons at N1 and N2 or N3 and N4 (for numbering,
see Scheme 1). Another pair of equivalent cis tautomers is
obtained by placing the protons either at N1 and N4 or N2
and N3. The latter structures lie at much higher energy than
the other two pairs and are therefore not populated in the
ground state, although they may play a crucial role in the
radiationless deactivation of the excited state [52].
Fig. 2. The electronic absorption spectra in the Q region,
recorded in acetonitrile at 293 K. From bottom to top:
porphycene (2), 2-tert-butylporphycene (18), 2,7-di-tert-
butylporphycene (19), 2,7,12-tri-tert-butylporphycene (22),
2,7,12,17-tetra-tert-butylporphycene (10)
The pairwise degeneracy is broken for some tert-butyl-
substituted porphycenes. In the monosubstituted derivative
18 both trans and cis tautomeric forms should have
different energies. The same situation is encountered for 22,
the compound bearing three tert-butyl groups. In doubly
substituted derivatives, lifting of degeneracy is expected for
the trans forms of 21 and for the cis forms of 19, whereas
for 20 the pairwise symmetry should be preserved. In order
to estimate the magnitude of these effects we optimized the
geometry for the trans and cis structures of 2, 10, 18–22.
The results of calculations of relative energies (Table 1),
inner cavity parameters (Table 2), and dipole moment values
(Table 3), confirm the above considerations. For all the
derivatives, the cis-trans energy difference is quite similar
to that in the parent porphycene 2; the largest differences,
about 0.2 kcal/mol, are predicted for 21 and 10. On the
other hand, the energy splittings caused by lower symmetry
are quite substantial, above 0.7 kcal/mol for the trans forms
of 21 and about 0.4 kcal/mol for 18 and 22. These numbers
indicate that the asymmetry effects caused by a tert-butyl
group are additive: in the doubly substituted porphycene 21
the stabilization of one trans tautomer vs. the other is about
twice as large as in monosubstituted derivatives 18 and 22.
Interestingly, the computed splittings are much lower for
the inequivalent cis forms.
synthesized 2,7-di-tert-butylporphycene 19 according to
the reaction shown in Scheme 8.
Substituting porphycene by tert-butyl groups in β
positions(2,7,12,17)hasnosignificantimpactontheshape
and location of spectral bands (Fig. 2). In the electronic
absorption spectra of unsubstituted porphycene (2), 2-tert-
butylporphycene (18), 2,7-di-tert-butylporphycene (19),
2,7,12-tri-tert-butylporphycene (22) and 2,7,12,17-tetra-
tert-butylporphycene (10) small shifts in the position of
Q-bands towards lower frequencies are observed. The
shapes of the bands remain almost identical.
With the increasing number of alkyl groups, evident
shift of the Soret bands is observed toward longer
wavelengths, exceeding 10 nm between porphycene (2)
and 2,7,12,17-tetra-tert-butylporphycene (10) (Fig. 3).
Structure and tautomerism of tert-butyl-substituted
porphycenes
In parent porphycene, symmetry dictates that two trans
tautomers, with the inner protons located either at N1 and
N3 or at N2 and N4 are chemically equivalent. Similarly,
While the energy difference of less than 1 kcal/
mol may at first seem not very significant, we recall
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594
I. CZERSKI ET AL.
Table 1. Calculated energy differences (in kcal/mol) between
different tautomeric forms of parent and tert-butyl-substituted
porphycenes. In trans1, the inner protons are located at N1 and
N3, in trans2, at N2 and N4 (cf. Scheme 1). In cis1 the protons
reside on N1 and N2, in cis2, on N3 and N4
of tautomerization may have dramatic consequences
for both the kinetics and the mechanism of the reaction.
This conjecture is strengthened by the analysis of the
geometry of the inner cavity (Table 2). The values of
NH–N distances and NHN angles, crucial for the rate
of tautomerization, change upon substitution in a way
which may either accelerate or slow down the process.
For instance, the N1N4/N2N3 distances in the trans
forms of 18 are longer than in parent 2, whereas they
become shorter for 22. We have demonstrated [49] that
very small variations of the inner cavity parameters
lead to large changes in the tautomerization rates: upon
passing from 2 to 10, the rate of the ground state process
increases from 5.8 × 1011 s-1 to 13.7 × 1011 s-1, and the
rate in S1 from 7.4 × 1010 s-1 to 2.0 × 1011 s-1. Comparison
of the calculated geometry parameters of 2 with those
of 10, 18 and 22 leads to a prediction of the variability
of tautomerization rates in singly and triply substituted
porphycenes being greater than in the tetra-tert-butyl
derivative. Moreover, the mechanism of the reaction need
not be the same in all compounds. Inspection of Table 2
reveals for the trans form of 20 a large disparity between
the N1N4 and N2N3 distances (267.0 and 264.1 pm,
respectively). For such a case, the exchange of the two
inner hydrogens may no longer be concerted, and the
DE(cis1-trans1)
DE(trans2-trans1)
DE (cis2-cis1)
2
2.24 (1.59)a
2.37 (1.77)
2.15 (1.49)
2.45 (1.89)
2.10 (1.47)
2.23 (1.60)
2.03 (1.37)
0
0
18
19
21
20
22
10
0.38 (0.41)
-0.02 (0.02)
0
-0.01 (0.07)
0.72 (0.79)
0
0
0
0.37 (0.35)
0
0.01 (0.07)
0
aIn parentheses, the values obtained after zero-point-energy
correction.
that the experimentally determined activation energy
for tautomerization in parent porphycene is about
0.5 kcal/mol [42]. This finding led to postulating
that double hydrogen transfer occurs via vibrational
gating of concerted tunneling. Therefore, lifting of
symmetric double minimum character for the potential
Table 2. Calculated geometries of the inner cavity for different tautomeric forms of parent and tert-butyl-substituted
porphycenes
N1N4a
N2N3
N1N2
N3N4
NH1/NH2
N1HN4b
N2HN3
2-trans
2-cis
265.5
261.0
266.3
266.3
261.9
262.1
265.7
261.3
261.7
265.7
265.5
261.4
267.0
263.0
264.9
264.8
260.7
260.9
265.8
261.9
265.5
261.0
264.9
264.7
260.5
260.7
265.7
261.3
261.7
265.7
265.5
261.6
264.1
260.1
266.3
266.1
262.1
262.3
265.8
261.9
283.8
289.3
286.0
286.4
291.6
286.1
288.5
293.7
288.3
285.5
286.1
285.8
285.7
291.1
288.3
288.5
293.5
288.1
287.9
293.0
283.8
284.0
283.4
283.5
283.6
289.0
283.1
283.3
288.6
285.5
286.1
291.1
286.0
285.8
285.4
285.8
285.6
291.0
287.9
287.6
104.9
152.6
154.2
154.6
152.0
156.1
153.4
152.2
155.8
153.9
154.6
152.1
153.3
154.0
155.3
154.9
152.3
156.3
153.8
154.2
155.6
152.6
154.2
152.8
152.8
154.0
154.6
154.6
155.8
153.9
154.6
152.1
156.4
152.9
154.4
154.2
154.1
155.3
155.9
154.2
155.6
106.7
18-trans1
18-trans2
18-cis1
18-cis2
19-trans
19-cis1
19-cis2
21-trans1
21-trans2
21-cis
105.1/105.0c
105.0/104.8
106.7/106.8
106.7/106.4
105.1/104.9
106.8
106.5
105.1
104.9
106.7/106.6
105.0/105.1
106.5/106.9
105.3/105.1
105.1/105.0
106.9/106.7
106.6/106.7
105.1
20-trans
20-cis
22-trans1
22-trans2
22-cis1
22-cis2
10-trans
10-cis
106.7
aDistances in picometers; bangles in degrees; cin trans2 forms, NH1/NH2 correspond to N2H/N4H; in cis2, to N3H/N4H.
Copyright © 2012 World Scientific Publishing Company
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THE LONG AND WINDING ROAD TO NEW PORPHYCENES
595
Table 3. Dipole moment values (in
Debyes) calculated for different
tautomeric forms of parent and tert-
butyl-substituted porphycenes
two separate bands of similar IR intensity, corresponding
to the N1H and N3H stretches in the trans form (N1H
and N2H in the cis species). In other words, the motions
of the inner protons become decoupled after lowering
of the overall molecular symmetry. It is crucial to note
that the tautomerization potential in 20 still retains the
symmetric double minimum character, the two trans and
two cis forms being pairwise similar.
Loss of symmetry in porphycene derivatives should
drastically influence the tunneling splitting due to the
delocalization of two inner protons between the nitrogen
atoms [29, 37, 53]. For this reason, tert-butyl-substituted
porphycenes provide interesting objects for research
on proton tunneling in supersonic jets. However, the
consequences of symmetry changes may also be detected in
condensed phase. Such effects may also be solvent-polarity-
specific, since the calculations indicate different values
of dipole moments in molecules for which the symmetry
reduction destroys the pairwise degeneracy (Table 3).
2-trans
0.0
2-cis
1.15
0.80
0.62
1.78
0.85
0.93
2.17
0.34
0.0
18-trans1
18-trans2
18-cis1
18-cis2
19-trans
19-cis1
19-cis2
21-trans1
21-trans2
21-cis
0.0
1.27
1.00
1.59
0.55
0.73
1.78
1.02
0.0
20-trans
20-cis
Experimental demonstration of tautomerism in single
molecules of tert-butyl-substituted porphycenes
22-trans1
22-trans2
22-cis1
22-cis2
10-trans
10-cis
In previous studies of 2 [36] and 10 [51], we have shown
that tautomerism can be monitored on a single molecule
level by observing the spatial pattern of the fluorescence
from a single chromophore embedded in a thin polymer
matrix. Due to the symmetry lowering expected for some
tert-butyl derivatives, it seemed important to probe whether
the reaction still proceeds in such molecules. Moreover,
for chromophores located in a rigid polymer additional
asymmetryduetotheenvironemntcanalsocontributetothe
decrease in the tautomerization rate. This may be the origin
of the finding of Meixner and coworkers, who observed
unusually long tautomerization times for single molecules
of symmetric 2,3,7,8,12,13,17,18-octaethylporphyrin in
polymer matrices [72].
1.36
reaction can proceed according to a stepwise mechanism.
Such a possibility is also suggested by the form of the
NH stretching vibrations. In porphycenes possessing
the centre of inversion and equal values of N1N4 and
N2N3 separation, the calculations yield an in-phase and
an out-of-phase combinations of the NH stretches. The
former should be observed by Raman, the latter, by IR
spectroscopy. The mutual exclusion principle is no longer
valid for 20. The calculations for this molecule predict
Figure 4 presents the spatial patterns of the emission
spectra of single molecules of 19 and 22 recorded in
a thin (ca. 30 nm) poly(methyl methacrylate) film. To
Fig. 4. (a) Image of room temperature fluorescence from single molecules of 2,7,12-tri-tert-butylporphycene 22 embedded in
PMMA sheet, recorded from an area of 15 × 15 mm2 with the azimuthal polarization of the exciting laser beam; (b) expanded view
of the emission in the upper left corner of (a); (c) fluorescence spectrum from the molecule shown in (b); (d) fluorescence from
single molecules of 2,7-di-tert-butylporphycene 19
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 595–602

596
I. CZERSKI ET AL.
ensure that the emitting objects do really correspond
to porphycenes, not only the total emission intensity
was recorded, but also its spectral distribution, yielding
fluorescence typical for porphycenes (Fig. 4c). The most
important finding is the detection of ring shape patterns
for both porphycenes. Such a shape is characteristic for a
double–dipole emitter, i.e., a molecule for which the S1–
S0 transition cannot be represented by a single transition
dipole. Double hydrogen transfer in porphycenes
is usually accompanied by rotation of transition
Syntheses of phenyl-substituted porphycenes
In 2008, Srinivasan and coworkers published the
synthesis of porphycene using a novel, three-step
procedure [73] based on acid-catalysed oxidative
coupling of 2,2′-(1,2-diphenylethane-1,2-diyl)bis-1H-
pyrrole. This method, however, seems to be
limited only to a narrow group of meso-substituted
porphycenes, which does not fully cover our
expectations and, unfortunately, it failed in our hands.
Therefore we decided to prepare this compound on
a more classic route involving McMurry coupling
of the proper diketone. One of the advantages of the
meso-substituted porphycenes is that they can be
obtained on a relatively short pathway, limited to,
as in the method mentioned before, just three steps
(Scheme 9). Thus, oxidative coupling of pyrrole
mediated by phenyliodine(III) bis(trifluoroacetate)
in the presence of bromotrimethylsilane afforded
2,2′-bipyrrole [74] which was immediately converted
to the proper diphenyl ketone under Villsmeier–Haack
conditions [75]. Further reaction under conditions
elaborated by Vogel and coworkers [1] led to the
expected 9,10,19,20-tetraphenyl porphycene (25) in
4% yield. Similarly, 9,20-diphenylporphycene (26)
can be prepared by reaction of 5,5′-dibenzoyl-2,2′-
bipyrrole (27) and 5,5′-diformyl-2,2′-bipyrrole (23), as
it was confirmed by the MS analysis of the porphycene
fraction isolated by column chromatography. However,
separation of such formed product in a pure form from
porphycene (1) and 9,10,19,20-tetraphenylporphycene
(26), also obtained in this reaction, has not been so far
completed, due to the very small difference in polarity
of all of these compounds.
moments. A value of 72
3o has been determined
for the angle formed by the S1–S0 transition moment
directions in the two trans tautomeric forms of the
parent porphycene [36]. Similar results have been also
observed for 10 [51]. The present findings demonstrate
that tautomerization occurs at room temperature even
for asymmetric porphycene derivatives. Moreover,
the observation that the reaction in asymmetric 22
is accompanied by transition moment rotation is
by no means trivial: in a recent study by Nonell and
coworkers on another asymmetric derivative, 9-amino-
2,7,12,17-tetraphenylporphycene, it was postulated
that the transition dipoles do not change direction upon
tautomerization [60].
For some molecules, not a ring, but a double-
lobe emission pattern was observed. This can be
explained by either (i) lack of tautomerization in
certain molecules; (ii) different spatial orientation of
the emitting chromophore, with its plane perpendicular
to the plane of the sample. Our studies for 10 proved
that the latter situation occurs. However, in the case of
asymmetric porphycene the former explanation cannot
be a priori excluded. Further studies are planned in this
regard.
Ph
Ph
Ph
Ph
N
H
N
H
N
N
Zn, CuCl, TiCl4
25
PhCONMe2
POCl3
PIFA
TMSBr
Ph
Ph
N
H
N
H
N
H
CH2Cl2
N
H
CHCl3
N
H
O
O
27
23 (1 equiv.)
Zn, CuCl, TiCl4
N
H
N
2
25
+
+
H
N
N
Ph
Ph
26
Scheme 9.
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 596–602

THE LONG AND WINDING ROAD TO NEW PORPHYCENES
597
I
Br
N
N
N
N
H
N
N
NIS (95% pure)
H
N
H
N
+
H
N
H
N
H
N
N
I
I
10
29
28
Scheme 10.
Synthesis and structure of halogenated porphycenes
The presence of a heavy atom in an organic
molecule can increase the speed of all singlet–triplet
conversion processes, both radiative and non-radiative.
In order to enhance and explore the phenomenon of
phosphorescence in porphycenes, we attempted to obtain
halogen derivatives of porphycene. The first choice
was 2,7,12,17-tetra-tert-butylporphycene (10), as the
cheapest and most available substrate. The reaction was
carried out using the N-iodosuccinimide (NIS) (95%
pure) as a iodinating reagent. Because the iodination
reaction is not selective, we received a very large number
of products, but one of them deserves special attention.
After a long and arduous process of separating a mixture
of porphycene derivatives, it turned out that one of the
purified reaction products had a mass corresponding to
the product of substitution of two iodine atoms. NMR
analysis demonstrated that the obtained compound
is 3,13-diiodo-2,7,12,17-tetra-tert-butylporphycene (28)
However, mass spectrometry analysis also revealed the
existence of a compound with the mass corresponding
to the product of substitution of one iodine atom and one
... bromine atom! It turned out that the reagent was not
completely pure and contained the N-bromosuccinimide
(NBS). It is known that the bromonium cation is more
reactive than the iodonium cation, and thus, NBS
reacted more rapidly than NIS. Not only mass spectra,
but also careful NMR analysis confirmed that the
obtained compound is 3-bromo-13-iodo-2,7,12,17-
tetra-tert-butylporphycene (29) (Scheme 10). The
crystallographic data could not be fully refined: in
particular, it was difficult to distinguish bromine and
iodine, most probably due to orientational disorder. In
addition, the positions of the inner protons could not be
determined. The latter effect, which can be due to both
disorder and delocalization of the inner protons, has been
known since the very first synthesis of parent porphycene
[1]. Even though the X-ray data were not complete, they
provided the unequivocal evidence for the 3,13-halogen
positions on the porphycene ring.
Fig. 5. The electronic absorption spectrum of 3,13-diiodo-
2,7,12,17-tetra-tert-butylporphycene (28) (red) and 3-bromo-
13-iodo-2,7,12,17-tetra-tert-butylporphycene (29) (black),
recorded in acetone at 293 K
13C spectrum exhibits 26 signals, suggesting a strong
perturbation of symmetry by the presence of bromine and
iodine in the molecule. Proton NMR spectra revealed a
presence of two identical and two nonidentical tert-butyl
groups. The latter are due to the presence of iodine or
bromine atoms in the vicinity. This finding excluded the
presence of two identical halogen atoms in the molecule,
confirming the results from mass spectroscopy and X-ray
studies.
More advanced 2D NMR investigations allowed to
determine that the proton doublet splittings are due to the
coupling of the C6 and C16 protons with the protons of
the inner cavity. An analogous effect has been described
by Vogel and coworkers for 2,3-dihydroporphycene [76].
In the electronic absorption spectra of halogenated
derivatives (Fig. 5), a significant shift of the Q-bands
towards the red (above 30 nm with respect to 10) and
a specific two-band shape in the Soret region were
observed.
An interesting effect has been observed during NMR
studies of 29. Based on the X-ray data, we expected to
observe two singlet peaks due to inequivalent protons
bonded to C6 and C16. Instead, two doublets were
observed, each with the coupling constant of 2 Hz. The
Structure and tautomerism of 3,13-dihalogenated-
2,7,12,17-tetra-tert-butylporphycenes
B3LYP/6-311G(d,p) calculations performed for
trans and cis tautomers of 28 and 29 reveal significant
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 597–602

598
I. CZERSKI ET AL.
Table 4. Calculated energy differences (in kcal/mol) between
different tautomeric forms of 28 and 29. See Table 1 for
labeling conventions
halogen substituents win over the energy gain obtained
by forming stronger hydrogen bonds.
DE(cis1-trans1)
DE(trans2-
trans1)
DE (cis2-cis1)
EXPERIMENTAL AND COMPUTATIO-
NAL DETAILS
28
29
0.97 (0.58)
0.87 (0.72)
-3.97 (-3.95)
-3.89 (-3.46)
0
Electronic absorption spectra were recorded on a
Shimadzu UV 3100 spectrophotometer. Single molecule
measurements were performed using a home-built
confocal scanning microscope equipped with a piezo
nanopositioning stage (PI-517.3CD, Physik Instrumente).
Higher order azimuthally polarized doughnut mode
of a HeNe laser (l = 594.1 nm) was generated by a
polarization converter as described previously [78], and
served as the excitation light source. The collimated beam
was reflected by a dichroic beamsplitter (Z594RDC,
Chroma) and focused by an oil immersion objective
(CP-Achromat 100x/1.25 NA, Zeiss) onto the sample
surface. Fluorescence from the sample was collected with
the same objective, transmitted first through the dichroic
beamsplitter mentioned above, and then through an
emission filter (ET605LP, Chroma) for separation from
scattered laser light and detected by a photon-counting
avalanche photodiode (SPCM AQRH-14, Perkin Elmer).
For spectral characterization the collected fluorescence
was redirected to an EMCCD camera (Newton, Andor)
equipped with an Andor Shamrock 303i spectrograph.
The calculations were carried out using Gaussian
09 program package (Revision B.01). Geometry
optimizations have been performed using density
functional theory, with the B3LYP functional and
6–31G(d,p) basis set. Vibrational structure has been
calculated for each optimized stucture to ensure that no
negative values of vibrational frequencies are present.
For the bromine and iodine derivatives, the 6–311G(d,p)
basis set was used [79].
1.00 (0.86)
differences in energies and structure with regard to 10,
the non-halogenated derivative (Tables 4 and 5). First
of all, the energies of two trans forms are no longer the
same, the tautomeric species with protons located at N2
and N4 (trans2) being stabilized by nearly 4 kcal/mol
with respect to the trans1 form. The geometry of the inner
cavity also changes dramatically. The distances between
hydrogen-bonded nitrogen atoms (N1N4 and N2N3)
increase by more than 10 pm with respect to 10, while the
N1N2 and N3N4 separations become shorter by 5–7 pm.
Thus, upon halogen substitution, the cavity becomes
square-like, resembling that of porphyrin, but with a
side length smaller by about 10 pm. We have recently
demonstrated that tautomerization rate in porphyrin
can be strongly accelerated by introducing substituents
which make the inner cavity rectangular, similar in size
to that of porphycenes [77]. The behavior caused by
3,13-dihalogen substitution is the opposite, showing a
possibility of making porphyrin-like porphycenes by
using appropriate substituents.
The computational results suggest a much weaker
intramolecular hydrogen bonding for the halogenated
derivatives 28 and 29 than in the corresponding non-
halogenated compound. A consequence of this would
be a much slower tautomerization rate. Actually, due
to the considerable energy difference between the two
trans forms, the inner protons may become localized
in the trans2 configuration. Finally, we note that in
the more energetically stable trans2 form all the inner
geometry parameters (NHN and NH distances and NHN
angles) suggest weaker hydrogen bonds than in trans1
(Table 5). Thus, overall electronic effects caused by
2,2′-(E,Z)-ethene-1,2-diylbis(1H-pyrrole) (15)
Schlenk flask was charged under argon with the (E)-
3-(1H-pyrrol-2-yl)-acrylic acid methyl ester (14a)
(155 mg, 1 mmol) and 10 mL of freshly distilled benzene,
Table 5. Calculated geometries of the inner cavity for different tautomeric forms 28 and 29. See Table 1 for labeling
conventions
N1N4
N2N3
N1N2
N3N4
NH1/NH2
N1HN4
N2HN3
28-trans1
28-trans2
28-cis
276.5
278.2
270.3
275.3
276.3
268.7
270.6
276.5
278.2
271.1
276.1
277.0
270.2
270.3
281.7
281.6
287.8
282.7
282.7
288.9
283.2
281.7
281.6
283.2
281.5
282.0
283.3
288.2
104.1
152.2
149.4
154.7
152.8
149.8
155.2
151.4
152.2
149.4
150.9
151.9
150.1
151.7
154.7
103.2
105.6/104.6
104.2/104.1
103.4/103.4
105.8/104.7
105.6/104.7
29-trans1
29-trans2
29-cis1
29-cis2
Copyright © 2012 World Scientific Publishing Company
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THE LONG AND WINDING ROAD TO NEW PORPHYCENES
599
then indenylidene catalyst (48 mg, 0.05 mmol) was
added. The reaction mixture was magnetically stirred at
room temperature for 24 h under argon atmosphere. After
completion of the reaction, the volatiles were removed
under vacuum. The crude residue was purified by flash
column chromatography (hexane/ethyl acetate 4:1)
to give 15 (40 mg, 0.25 mmol, E/Z ratio 3/1). Spectral
characteristics were in agreement with the data published
previously [80].
over anhydrous Na2SO4, crude product was isolated by
column chromatography (hexane:ethyl acetate 4:1) to
1
give 19 (8 mg, 0.02 mmol) as a blue solid. H NMR
3
(200 MHz, CDCl3): d, ppm 10.15 (d, 2H, H-9, 20, J =
3
11.3 Hz), 9.80 (d, 2H, H-10, 19, J = 11.3 Hz), 9.52 (d,
2H, H-13, 16, 3J = 4.4 Hz), 9.44 (d, 2H, H-3, 6, 4J = 0.9
Hz), 9.21 (d, 2H, H-12, 17, 3J = 4.4 Hz), 3.38 (br s, 2H,
NH), 2.25 (s, 18H, 2*tBu). UV-vis (acetonitrile): lmax, nm
627.5, 596.5, 558, 375, 362.
Partial removal of tert-butyl groups from 2,7,12,17-
tetra-tert-butylporphycene (10)
9,10,19,20-tetraphenylporphycene (25)
To the suspension of Zn (7.26 g, 111 mmol) and CuCl
(1.13 g, 11.1 mmol) in anhydrous THF (250 mL) TiCl4
(6.30 mL, 55.4 mmol) was added slowly at 0 °C. The
reaction mixture was heated to the reflux for 1 h and
5,5′-dibenzoyl-2,2′-bipyrrole (27, 0.942 g, 2.77 mmol)
was added quickly. Heating was prolonged for additional
2 h, then reaction mixture was cooled down to room
temperature and ammonia solution (ca. 10%, 200 mL)
was added. Such resulted mixture was stirred on air
overnight, then it was filtered through Celite pad, which
was additionally washed with THF (3 × 70 mL). Organic
solvents were removed from the filtrate under reduced
pressure and the residue was extracted with CH2Cl2 (3
× 70 mL). The organic layer was dried over anhydrous
Na2SO4, filtered through silica pad (hexane: CH2Cl2; 7:3)
and finally the expected product was isolated by column
chromatography (hexane:CH2Cl2 7:3) to give 25 (34 mg,
0.053 mmol) as a blue solid. The spectral characteristics
were in agreement with the data published previously
[73].
2,7,12,17-tetra-tert-butylporphycene (10, 100 mg,
0.187 mmol) was dissolved in concentrated H2SO4 (14
mL), then water (6 mL) was slowly added under argon.
The mixture was heated at 150 °C for 10 min. 1-butanol
(40 mL) and chloroform (200 mL) were added to the cold
mixture, which was then neutralized with dilute NaOH
and water. The solvent was removed under reduced
pressure. The residue was extracted with CH2Cl2 The
solution was evaporated. Porphycenes present in the
mixture were separated by HPLC (hexane:THF:MeOtBu
100:1:1).
1
2-tert-butylporphycene (18). H NMR (200 MHz,
CDCl3): d, ppm 10.17 (d, 1H, H-20, 3J = 11.3 Hz), 9.83
3
(d, 1H, H-19, J = 11.3 Hz), 9.80 (s, 2H, H-9, 10), 9.65
(m, 3H, H-6, 13, 16), 9.46 (d, 2H, H-3, 4J = 1.4 Hz), 9.22
(m, 3H, H-7, 12, 17), 3.41 (br s, 1H, NH), 3.07 (br s, 1H,
t
NH), 2.27 (s, 9H, Bu). UV-vis (acetonitrile): lmax, nm
627.5, 594.5, 555.5, 372.5, 358.
1
2,7,12-tri-tert-butylporphycene (22). H NMR (200
MHz, CDCl3): d, ppm 10.16 (s, 2H, H-9, 10), 10.11 (d,
3
3
1H, H-20, J = 11.3 Hz), 9.77 (d, 1H, H-19, J = 11.3
Iodination of 2,7,12,17-tetra-tert-butylporphycene (10)
3
Hz)), 9.62 (d, 1H, H-16, J = 4.1 Hz), 9.44; 9.42; 9.40
2,7,12,17-tetra-tert-butylporphycene (10, 30mg,
0.056mmol) was dissolved in 5 mL of THF, then
NIS (18.96 mg, 0.084 mmol) was added slowly to the
intensively stirred solution. After addition stirring was
continued for 1 h. The reaction mixture was evaporated
and chromatographed on silica gel (eluent hexane/
CH2Cl2 3:1), followed by another chromatography
(eluent hexane) to give 28 (5.1 mg 0.0069 mmol) and 29
(6.7 mg 0.0085 mmol).
3
(3H, H-3, 6, 13), 9.18 (d, 1H, H-17, J = 4.2 Hz), 3.58
(br s, 1H, NH), 3.28 (br s, 1H, NH), 2.27; 2.26; 2.25
(s, 3*9H, 3*tBu). UV-vis (acetonitrile): lmax, nm 629.5,
597, 558.5, 378.5, 364.
2,7-di-tert-butylporphycene (19)
To the suspension of Zn (2.615 g, 40 mmol) and
CuCl (0.396 g, 4 mmol) in anhydrous THF (250 mL)
TiCl4*2THF (6.48 g, 20 mmol) was added slowly at
0 °C. The reaction mixture was heated to the reflux for
3 h and 1H,1′H-[2,2′]bipyrrolyl-5,5′-dicarbaldehyde (23,
0.186 g, 1 mmol) and 4,4′-di-tert-butyl-1H,1′H-[2,2′]
bipyrrolyl-5,5′-dicarbaldehyde (24, 0.300 g, 1 mmol)
were added quickly. Heating was prolonged for
additional 2 h, then the reaction mixture was cooled
down to room temperature and ammonia solution (ca.
10%, 200 mL) was added. Such resulted mixture was
stirred in air overnight, then it was filtered through Celite
pad, which was additionally washed with THF (3 × 70
mL). Organic solvents were removed from the filtrate
under reduced pressure and the residue was extracted
with CH2Cl2 (3 × 70 mL). The organic layer was dried
3,13-diiodo-2,7,12,17-tetra-tert-butylporphycene
1
(28). H NMR (500 MHz, CDCl3): d, ppm 10.85 (d,
4
3
2H, H-6,16, J = 2.2 Hz), 10.33 (d, 2H, H-10,20, J =
3
12.0 Hz), 9.96 (d, 2H, H-9,10, J = 12.0 Hz), 2.42 (s,
18H, 2xtBu, C-2, 12), 2.23 (s, 18H, 2xtBu, C-7, 17).
13C NMR (500 MHz, CDCl3): d, ppm 155.29, 150.55,
149.53, 138.93, 135.30, 129.65, 123.21, 115.38,
109.77, 82.31, 36.62, 35.51, 34.44, 33.74. MS (ESI):
m/z 787 [M + H+]. UV-vis (acetone): lmax, nm 378, 391,
580, 626, 668.
3-bromo-13-iodo-2,7,12,17-tetra-tert-butylpor-
phycene (29). 1H NMR (500 MHz, CDCl3): d, ppm 10.86
4
4
(d, 1H, H-6, J = 2.2 Hz), 10.49 (d, 1H, H-16, J = 2.2
3
Hz), 10.36 (d, 1H, H-20, J = 12.0 Hz), 10.28 (d, 1H,
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 599–602

600
I. CZERSKI ET AL.
3
3
H-10, J = 11.9 Hz), 10.00 (d, 1H, H-9, J = 12.4 Hz),
9.97 (d, 1H, H-19, 3J = 12.8 Hz), 2.43 (s, 9H, tBu, C-2),
2.37 (s, 9H, C-12), 2.24 (s, 18H, C-7,17), 1.39 (br s, 1H,
NH), 1.32 (br s, 1H, NH). 13C NMR (500 MHz, CDCl3):
d, ppm 155.20, 150.96, 150.60, 150.29, 149.36, 148.65,
138.78, 136.49, 135.39, 135.25, 129.68, 128.86, 123.23,
122.38, 115.63, 115.42, 114.08, 110.10, 109.83, 82.34,
36.64, 36.53, 35.51, 34.66, 34.35, 33.62. MS (ESI): m/z
741.2 [M + H+]. UV-vis (acetone): lmax, nm 376, 390,
578, 624, 665.
doubly, and triply tert-butyl-substituted derivatives
18–22, of which the structure has been discussed above.
Investigations of these molecules by high-resolution
spectroscopy techniques should contribute to the
understanding of such phenomena as the role of symmetry
in tunneling splitting, coupling of low and high frequency
modes involved in hydrogen bond, cooperativity in double
hydrogen transfer, and the influence of conformational
isomerism on the tunneling characteristics. Compounds
18–22 bridge a gap between the parent porphycene and
2,7,12,17-tetra-tert-butylporphycene 10. Supersonic jet
studies of the latter [35] revealed a very complicated
pattern of vibronic transitions, making it very difficult
to distinguish features due to tunneling from the
contributions of different conformers. Such distinction
may become possible by carrying out systematic studies
of singly, doubly and triply-substitued compounds.
All the newly obtained porphycenes will be screened
against their photooxidizing properties. It has been
demonstrated many times that minor structural variations
can lead to huge changes in phototherapeutic activity
[20]. A systematic study of such variations will be
possible for the series of 10, 18–22. These group of
compounds will also enable a particularly challenging,
but possibly rewarding task: getting insight into
differences in spectral and photophysical behavior along
the series on a single molecule level. For this, we plan to
combine fluorescence, Raman, and atomic and tunneling
microscopy techniques.
SUMMARY AND PERSPECTIVES
The new porphycenes will be used as objects in studies
encompassing at least five different research areas. The
first two are spectroscopy and photophysics. Regarding
the former, it should be recalled that, due to the “hard-
chromophore” nature of porphycene chromophore
(DHOMO << DLUMO), substitution should not strongly
affect the intensity of the electronic transitions [9, 24].
On the other hand, the transition energy can be shifted
by suitable substitution. For instance, in meso-alkyl- or
aryl-substituted porphycenes, the origin of the electronic
absorption is red-shifted with respect to the unsubstitued
chromophore by more than 30 nm. Interestingly, the
second transition seems to be shifted more than the first
one, which leads to the coalescence of two transitions,
resulting in the broadening of the lowest absorption band.
Initial studies for the doubly meso-substituted derivatives
show that the red shift is roughly about half that observed
for tetrasubstituted porphycenes. In contrast to the latter,
the S1–S2 separation seems to be preserved.
Regarding photophysics, comparison of the excited
state depopulation patterns in meso doubly- and
tetrasubstituted derivatives will be a crucial test for
a recently suggested mechanism of excited state
depopulation in porphycenes [52]. According to this
model, the deactivation channel involves both twisting of
two pyrrole units and a single hydrogen transfer from a
transtoacistautomer,thelatterwithtwoprotonsonseparate
bipyrrole moieties. A consequence of this mechanism is a
large viscosity dependence of fluorescence quantum yields
and lifetimes in meso-tetraalkylsubstituted derivatives
[56]. The model predicts a much weaker dependence for
the 9,10,19,20-tetraphenylporphycene 25 and for doubly-
substituted derivatives. Preliminary measurements reveal
much stronger fluorescence in all these compounds.
In summary, it can be stated without any doubt that
the pioneering work on porphycene isomers initiated by
Emanuel Vogel and his coworkers a quarter of century
ago has been providing inspiration, fascination, and fun
for many researchers from diverse areas. The field is
growing, and one can expect exciting news in the years
to come.
Acknowledgements
The authors thank Dr. Roman Luboradzki for the
X-ray structures of halogenated porphycenes. The
research was partially supported by the European Union
within European Regional Development Fund, through
grant Innovative Economy (POIG.01.01.02-00-008/08),
and by the Ministry of Science and Higher Education
grants, PBZ/MNiSW07/2006/44-PolPostdoc III Project
and N N204 355040. We acknowledge the computing
grant (No. G17–14) from the Interdisciplinary Centre
for Mathematical and Computational Modeling of the
Warsaw University. We are grateful to Prof. A. Meixner
for providing us with blueprints of a confocal microscope.
Porphycenes are particularly suitable for studies of
intramolecular hydrogen bonding and tautomerism,
because the strength of their hydrogen bonds may be
strongly altered by substituents, due to changes in size
of the internal cavity. Extremely instructive are the
measurements performed for ultracold molecules isolated
in supersonic jets [29, 37] or helium nanodroplets [53].
Such studies allow to pinpoint mode-dependent tunneling
splittings due to coherent delocalization of two internal
protons. Very promising in this regard are the singly,
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