Phototautomerization in Pyrrolylphenylpyridine Terphenyl Systems

Article abstract of DOI:10.1021/acs.joc.5b00275

[4-(2-Pyrrolyl)phenyl]pyridines 2-4 were synthesized and their photophysical properties and reactivity in phototautomerization reactions investigated by fluorescence spectroscopy and laser flash photolysis (LFP). The pK_a for the protonation of the pyridine nitrogen in 2-4 was determined by UV-vis and fluorescence titration (pK_a = 5.5 for 4). On excitation in polar protic solvents, 2-4 populate charge-transfer states leading to an enhanced basicity of the pyridine (pK_a^? ≈ 12) and enhanced acidity of pyrrole (pK_a^? ≈ 8-9) enabling excited-state proton transfer (ESPT). ESPT gives rise to phototautomers and significantly quenches the fluorescence of 2-4. Phototautomers 2-T and 4-T were detected by LFP with strong transient absorption maxima at 390 nm. Phototautomers 2-T and 4-T decayed by competing uni- and bimolecular reactions. However, at pH 11 the decay of 4-T followed exponential kinetics with a rate constant of 4.2 × 10⁶ s^-1. The pyridinium salt 4H⁺ forms a stable complex with cucurbit[7]uril (CB[7]) with 1:1 stoichiometry (β₁₁ = (1.0 ± 0.2) × 10⁵ M^-1, [Na⁺] = 39 mM). Complexation to CB[7] increased the pK_a for 4H⁺ (pK_a = 6.9) and changed its photochemical reactivity. Homolytic cleavage of the pyrrole NH leads to the formation of an N-radical because of the decreased acidity of the pyrrole in the inclusion complex. (Figure Presented).

Full text of DOI:10.1021/acs.joc.5b00275

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Article
Phototautomerization in pyrrolylphenylpyridine terphenyl systems
Nikola Basaric, Suma S Thomas, Vesna Blazek Bregovic, Nikola Cindro, and Cornelia Bohne
J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 30 Mar 2015
Downloaded from http://pubs.acs.org on March 31, 2015
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Phototautomerization in pyrrolylphenylpyridine terphenyl systems
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Nikola Basarić,†* Suma S. Thomas,‡ Vesna Blažek Bregović,† Nikola Cindro,† and Cornelia
Bohne‡*
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† Department of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Bijenička cesta
54, 10 000 Zagreb, Croatia. Fax: + 385 1 4680 195; Tel: +385 1 4561 141
‡ Department of Chemistry, University of Victoria, Box 3065 STN CSC, Victoria BC, V8W
3V6, Canada. Fax:+ 250 721 7147; Tel:+ 250 721 7151
Corresponding authors' Eꢀmail addresses: NB nbasaric@irb.hr; CB cornelia.bohne@gmail.com
Graphical abstract
N
H
N
H
N
N
2
4
3
-Cl+HN
N
N
H
N
H
4H+
ν
h
N
HN
N
N
H2O
H
4
4-T
Abstract: [4ꢀ(2ꢀPyrrolyl)phenyl]pyridines 2ꢀ4 were synthesized and their photophysical
properties and reactivity in phototautomerization reactions investigated by fluorescence
spectroscopy and laser flash photolysis (LFP). The pKa for the protonation of the pyridine
nitrogen in 2ꢀ4 was determined by UVꢀvis and fluorescence titration (pKa = 5.5 for 4). On
excitation in polar protic solvents 2ꢀ4 populate charge transfer (CT) states leading to an enhanced
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basicity of the pyridine (pKa*≈ 12) and enhanced acidity of pyrrole (pKa*≈ 8ꢀ9) enabling excited
state proton transfer (ESPT). ESPT gives rise to phototautomers and significantly quenches the
fluorescence of 2ꢀ4. Phototautomers 2-T and 4-T were detected by LFP with strong transient
absorption maxima at 390 nm. Phototautomers 2-T and 4-T decayed by competing uniꢀ and
bimolecular reactions. However, at pH 11 the decay of 4-T followed exponential kinetics with a
rate constant of 4.2 ×106 sꢀ1. The pyridinium salt 4H+ forms a stable complex with cucurbit[7]uril
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(CB[7]) with 1:1 stoichiometry (β11 = (1.0 ± 0.2) × 105 Mꢀ1 [Na+] = 39 mM). Complexation to
,
CB[7] increased the pKa for 4H+ (pKa =6.9) and changed its photochemical reactivity. Homolytic
cleavage of the pyrrole NH leads to the formation of an Nꢀradical because of the decreased
acidity of the pyrrole in the inclusion complex.
Key words: excited state proton transfer (ESPT), pyridine, pyrrole, laser flash photolysis,
inclusion complexes, cucurbit[7]uril
Introduction
Proton transfer is a fundamental reaction in chemistry and biology1 that has received much
attention due to fundamental aspects, as well as numerous applications.2 Upon electronic
excitation, some organic functional groups exhibit enhanced acidity or basicity.3,4 If these sites
are in close proximity, excitation can lead to excitedꢀstate intramolecular proton transfer
(ESIPT).5,6,7,8 However, if the basic and acidic sites are not at a short distance, proton transfer
can be feasible either via double proton transfer or via a relay mechanism over bridges of protic
molecules.9 The latter process is particularly interesting in biological systems and involves
chains of polar amino acids and H2O molecules,10,11,12 or coupled electron transfer and proton
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transfer.13,14 Longꢀrange proton transfer also takes place in respiratory complexes,15 whereas
absorption of light in bacteriorhodopsin enables the function of a proton pump that moves
protons through a membrane against a gradient.16,17 The mechanism of ESPT that involves
solventꢀrelay shuffling of a proton has been documented for 7ꢀazaindoles18 and 7ꢀ
hydroxyquinoline,19,20,21,22,23 and has been used for the probing of structural dynamics in
proteins.24 Furthermore, proton transfer taking place via relay of H2Oꢀmolecules has been used to
study dynamics of membranes and micelles.25,26,27
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ESIPT in the pyrrolylpyridine systems has been well documented.28 In these examples the
pyrrole or indole NH is the acidic site, whereas the pyridine nitrogen is the basic site.29,30,31,32,33
For example, photoexcitation of pyrrolylpyridine 1 in nonpolar solvents leads to ESIPT and to
the population of phototautomer 1-T (Eq. 1) which was detected by fluorescence spectroscopy.31
In the corresponding meta and para pyridine derivatives wherein the acidic and basic sites are
distant, solventꢀassisted double proton transfer occurs giving rise to phototautomers, or Hꢀ
bonding complexes with the solvent are formed, leading to a deꢀexcitation via an internal
conversion channel.34,35,36
ν
h
(1)
ESIPT
N
N
N
NH
1-T
H
1
Herein we report on the investigation of solventꢀassisted phototautomerization (formal proton
transfer) in a series of pyrrolylphenylpyridine terphenyl derivatives 2ꢀ4. The photophysical
properties of molecules 2ꢀ4 were investigated by fluorescence spectroscopy, whereas formation
of the phototautomers was probed by laser flash photolysis (LFP). Furthermore, the
complexation of 4H+ with cucurbit[7]uril (CB[7]) was investigated, because inclusion complexes
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with CB[n]s were shown to alter photochemical reactivity37 and these complexes have been the
focus of intensive research,38 particularly owing to the potential applicability as drug delivery
vehicles39,40,41 or photoswitches.42,43 Herein we demonstrate that complexation of 4H+ with
CB[7] changes its photochemical reactivity and prevents phototautomerization.
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N
H
N
H
N
N
2
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3
N
-Cl+HN
N
H
N
H
4H+
Results
Synthesis
Pyrrole derivatives 2ꢀ4 were prepared from the corresponding (4ꢀbromophenyl)pyridines
(Schemes 1ꢀ3). For the ortho derivative, 4ꢀbromophenyl boronic acid (5) was prepared first, that
in the Suzuki coupling with 2ꢀbromopyridine afforded 2ꢀ(4ꢀbromophenyl)pyridine (6)44,45 in the
yield of 29%. The subsequent Suzuki reaction with the pyrrole boronic acid,46 according to the
optimized conditions for the arylation with pyrrole47 and the Bocꢀdeprotection in basic
conditions (Scheme 1) gave the target compound 2.48
Scheme 1.
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1) Mg
Br
Br
Br
B(OH)2
2) B(OMe)3
3) HCl
5
7
8
9
Pd(PPh3)4
K2CO3
N
Br
Br
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1)
N
B(OH)2
Boc
N
Pd(PPh3)4
N
H
N
CsCO3
6
2
2) NaOMe / MeOH
Synthesis of meta derivative 3 started from the commercially available 3ꢀpyridine boronic acid
that was arylated in a Suzuki coupling to afford bromide 7,49,50 and subsequently in another
Suzuki coupling with pyrrole boronic acid gave 3-Boc derivative. Bocꢀdeprotection in basic
conditions gave the target compound in the overall yield of ≈20% (Scheme 2).
Scheme 2.
Br
B(OH)2
Pd(PPh3)4
Br
+
K2CO3
N
N
7
Br
Pd(PPh3)4
CsCO3
B(OH)2
N
Boc
NaOMe
MeOH
N
N
N
N
H
Boc
3
3-Boc
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The synthetic strategy for paraꢀderivative 4 was different than for 2 and 3 since bromide 8 was
not prepared in a metalꢀcatalyzed crossꢀcoupling. Instead, according to a modification of the
published procedure,51 pyridine was activated by transforming it to a triflate salt, which enabled
nucleophilic addition of pꢀbromophenyllithium generated in situ. The reaction gave the 1,2ꢀ
(minor) and 1,4ꢀadducts (major) which were isolated as a mixture and without characterization
treated with a base to afford 848 as the major product, isolated in the overall yield of 25%.
Subsequent arylation with pyrrole boronic acid, as for the ortho and meta derivative, and Bocꢀ
deprotection gave the target compound (Scheme 3).
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Scheme 3.
1) Tf2O
N
Br
2) n-BuLi
Br
N
8
Br
Pd(PPh3)4
CsCO3
B(OH)2
3) NaOH
N
Boc
NaOMe
MeOH
N
N
N
N
H
Boc
4
4-Boc
Fluorescence measurements
Absorption spectra of 2ꢀ4 taken in CH3CN (Fig. 1 top) exhibit an absorption band with a
maximum at around 327 nm corresponding to the HOMOꢀLUMO transition and population of
S1. The geometries of the two conformers of 4 and orbitals involved in the electronic transition
are presented in the supporting information (Fig. S1 and S2, Tables S1, S3 and S4 in the SI). The
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excitation has a significant charge transfer (CT) character, leading to an electron density
enhancement on the pyridine and a decrease on the pyrrole.
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Fluorescence spectra of 2ꢀ4 were measured in cyclohexane, CH3CN and CH3CNꢀH2O (1:1, Fig.
1 bottom and Fig. S5ꢀS12 in the SI). In cyclohexane, the fluorescence spectra of 2ꢀ4 are
structured with a vibronic progression of 1400 cmꢀ1. The increase in solvent polarity shifts the
maximum of the emission to longer wavelengths (Fig. 1 bottom, and Fig. S7, S9, S11 in the SI),
and leads to a disruption of the vibronic structure. These findings suggest an increase of the
dipole moment for the excited state, in agreement with the calculation for 4 (see Table S3 in the
SI).
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35000
30000
2
3
4
25000
4H+
20000
15000
10000
5000
0
200
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Wavelength / nm
cyclohexane
CH3CN
CH3CN-H2O (1:1)
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Fig 1. Absorption spectra of 2ꢀ4, and 4H+ in CH3CN (top), and normalized fluorescence spectra
of 4 in different solvents (bottom).
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Quantum yields of fluorescence for 2ꢀ4 were measured by use of quinine sulfate / 0.05 M H2SO4
as a reference (see equation S1 in the SI), whereas lifetimes were measured by timeꢀcorrelated
single photon counting (SPC). Similar quantum yields were measured for 2ꢀ4 in cyclohexane and
CH3CN (Table 1). The decays from S1 for 2ꢀ4 were faster in cyclohexane than in CH3CN, and in
cyclohexane the kinetics were fit to a sum of two exponentials, while the kinetics was fit to a
monoexponential function in CH3CN (Table 1). Although straightforward assignment of decay
components in cyclohexane is not possible at this point, the observation could be interpreted as
due to locally excited (LE) and charge transfer (CT) states, or aggregation of molecules in that
solvent. However, fine vibronic structure observed in the steady state spectra in cyclohexane
does not suggest aggregation. Nevertheless, in CH3CN only one S1 state is populated with a
significant CT character wherein the pyrrole moiety becomes relatively positively charged (and
therefore, more acidic than in S0) and the pyridine relatively negatively charged (more basic than
in S0).
Table 1. Photophysical properties of 2ꢀ4 and 4H+
Φ a
τ b
Φ a
τ b
Φ a
τ b
(cyclo.)
(cyclo.)/ns
(CH3CN)
0.95 ± 0.05
(CH3CN)/ns
2.14 ± 0.01
(CH3CNꢀH2O)
0.046 ± 0.003
(CH3CNꢀH2O)/ns
(90ꢀ120)×10ꢀ3
0.40 ± 0.05
< 30×10ꢀ3
0.90±0.03 (60ꢀ120)×10ꢀ3
1.25±0.01
2
3
0.90±0.02 (70ꢀ100)×10ꢀ3
1.25±0.01
0.83 ± 0.03
2.28 ± 0.01
(2.7 ± 0.2)×10ꢀ3
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0.95±0.02 (60ꢀ100)×10ꢀ3
1.41±0.01
0.91 ± 0.03
2.29 ± 0.01
0.049 ± 0.003
(90ꢀ150)×10ꢀ3
4
0.3 ± 0.1
ꢀ
4H+
ꢀ
ꢀ
0.012±0.002 0.11 ± 0.01
ꢀ
c
0.68 ± 0.02
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a
Fluorescence quantum yields measured by use of quinine sulfate in 0.05 M H2SO4 as a
reference (Φf = 0.53).52 Errors correspond to averaged data measured at three different
wavelengths.
b
Measured by SPC. Errors correspond to those obtained by global fitting of three decays at
different emission wavelengths. The preꢀexponential factors for the fastest lifetime of the nonꢀ
exponential decays were: 0.03ꢀ0.05 in cyclohexane, 0.6ꢀ0.95 in CH3CNꢀH2O with a significant
dependence on the emission wavelengths, and 0.4ꢀ0.5 for 4H+ in CH3CN.
c
Estimated fluorescence quantum yield measured for the emission band at 570 nm by use of
acridine yellow in CH3OH as a reference (Φf = 0.57).53 Errors correspond to averaged data
measured at three different wavelengths.
Protonation of the pyridine nitrogen in 4H+ significantly shifts the position of the absorption
maximum bathochromically to 393 nm (Fig. 1, top) owing to a larger stabilization of S1 than S0
by protonation of 4. However, dilution of the solution (from 5 ×10ꢀ5 M to 5 ×10ꢀ6 M) changes the
appearance of the spectrum with the shift of the maximum to 327 nm (Fig. S13 in the SI).
Although these spectral changes could be related to deaggregation of the molecules by dilution,
the spectral changes are consistent with the deprotonation of 4H+ because the changes parallel
those observed in the pH titration. Therefore, 4H+ in CH3CN behaves as a weak acid suggesting
that both 4 and 4H+ are present in solution.
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Fluorescence spectra of 4H+ in CH3CN are also strongly dependent on concentration (Fig. S14 in
the SI) with two emission maxima at 430 and 570 nm, corresponding to 4 and 4H+, respectively.
Moreover, the fluorescence decay of 4H+ in CH3CN is biꢀexponential, due the presence of 4 and
4H+. The fluorescence quantum yield of 4H+ in CH3CN (c = 5 ×10ꢀ6 M) measured for the
emission between 430 and 750 nm was estimated by exciting the sample at ca. 410 nm where
only 4H+ absorbs, giving a value about 75 times lower than for 4. Increase of temperature also
led to the deprotonation of 4H+, as indicated by the change of the relative intensities of the bands
at 430 nm and 570 nm (Fig. S15 in the SI). These fluorescence results are consistent with the
changes observed in the absorption spectra when the concentration of 4H+ was altered.
Addition of a protic solvent (H2O) to the CH3CN solution strongly quenches the fluorescence of
2ꢀ4. This finding indicates that a protic solvent opens an efficient deactivation channel from S1.
As discussed above, 2ꢀ4 populate CT states in a polar solvent wherein the pyridine nitrogen
becomes more basic and the pyrrole more acidic. Therefore, quenching of fluorescence in the
presence of protic solvent can be rationalized by ESPT leading to the protonation of the pyridine
nitrogen and/or deprotonation of the pyrrole NH (Scheme 4). Furthermore, a new shoulder
appears (λ > 550 nm) in the fluorescence spectrum of 4 taken in CH3CNꢀH2O (Fig. S5, S6, S11
and S12 in the SI) that is associated with the fluorescence of 4H+ formed by ESPT (see below).
Decays of fluorescence of 2ꢀ4 in CH3CNꢀH2O were multiꢀexponential, but due to the presence of
a very fast component, analysis of the decay components was not possible with the SPC
equipment used.
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Scheme 4. Phototautomerization of 4 and formation of its radical in aqueous solution depending
5
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on pH.
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9
S1
*
ν
h
h
pH > 12
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N
N
N
HN
N
N
N
N
4-
4-T
4-T
pKa* ca 8
pKa ca 17.5
S1
*
ν
pH = 8-12
H2O
N
HN
N
H
N
H
4
pKa = 5.5
pKa* = 12.7
S1
*
ν
h
pH = 5-8
+HN
+HN
+HN
N
N
H
N
H
9'
4H+
-H+
N
N
10
Acidꢀbase properties were investigated for 2ꢀ4 by UVꢀvis and fluorescence titrations. For para
derivative 4, the pH titrations were performed in H2O in the absence of a buffer, and in the
presence of phosphate or citrate buffers at two different concentrations of 4. The variation of pH
in the range 3ꢀ7 induced UVꢀvis and fluorescence spectral changes (Fig. 2). These spectra were
processed by multivariate nonlinear regression analysis using the SPECFIT software54,55,56 to
reveal the pKa of 5.5 ± 0.1 determined using different methods (Fig. S26ꢀS37 and Table S9 in the
SI). This value matches with the pKa for the protonation of the pyridine nitrogen (pKa = 5.2).57
Similar spectral changes were observed in the UVꢀvis pH titrations of 2 and 3 (Figs S16, S17,
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S22 and S23) which were processed by SPECFIT software to reveal the pKa values of ≈ 4.8 (see
Tables S7 and S8 in the SI).
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9
The pKa* value for the protonation of the pyridine nitrogen in 4 in S1 was estimated from the
fluorescence titration by use of the Förster cycle.3 Nonlinear regression analysis of the
fluorescence titration data revealed the position of the emission maxima in aqueous solution for 4
and 4H+ at 484 and 580 nm, respectively (Table S9 in the SI). These values correspond to the
increase of basicity of the pyridine nitrogen on excitation to S1 of ∆pK = 7.2 ± 0.2; that is, the
estimated value of pKa* is 12.7 ± 0.2. For 2 and 3 Förster cycle analysis could not be applied to
determine pKa* for the protonation of pyridine since the corresponding protonated form 2H+ is
not fluorescent, and due to generally very weak fluorescence of both 3 and 3H+ (see Figs S18ꢀ
S21, S24 and S25 in the SI). In contrast to pyridine, pyrrole behaves as a weak acid. The reported
pKa value for deprotonation of pyrrole is 17.5.57 Therefore, we could not determine the pKa for
the deprotonation of pyrrole for compound 4 in the aqueous solution.
10
11
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59
60
1.2
1.0
H+
0.8
H+
A
0.6
0.4
0.2
0.0
300
350
400
450
Wavelength / nm
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H +
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58
59
60
H +
450
500
550
600
650
Wavelength / nm
Fig. 2. UVꢀvis (top, [4] =5.3 × 10ꢀ5 M) and fluorescence spectra (bottom, λex=350 nm, [4] =5.3 ×
10ꢀ6 M) at different pH values (from 3.0 to 7.5) in the presence of citrate buffer (0.05 M).
Inclusion complex with cucurbit[7]uril (CB[7])
Positively charged 4H+ is a good candidate to form a hostꢀguest complex with CB[7]. This
macrocyclic host is known to bind well guests with positive charges and hydrophobic moieties,42
where the hydrophobic moiety fits within the cavity of the CB[n] and the positive charge is
stabilized by interaction with the carbonyl groups. The association equilibrium constant was
determined by UVꢀvis titration. To assure that the solution contained only 4H+, the titration was
performed at pH 3.5 in the presence of citrate buffer. Addition of CB[7] induced a hypochromic
and very weak bathochromic shift indicative of complex formation (Fig. 3). An overall binding
model with the formation of a 1:1 complex between 4H+ and CB[7] was employed (see the SI,
page S29) where the binding of sodium cations (39 mM) to CB[7] was not accounted for in
separate equilibria. The overall binding constant58 β11 of (1.0 ± 0.2) × 105 Mꢀ1 was determined
from two independent experiments for the absorption change at 360, 380 and 390 nm (Fig. S38ꢀ
S39 and Table S11 in the SI).
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Fig 3. Absorption spectra for 4H+ (5 ꢁM) in citrate buffer (47 mM, pH = 3.5, [Na+] 39 mM) in
the absence and presence of different concentrations of CB[7].
Complexation with CB[n] changes the pKa of bound guest molecules,59,60 and therefore
influences ESIPT reactivity of complexed guests.61 This variation of pKa upon complexation was
used for logic gates62 or drug delivery systems.63,64,65 Therefore, we investigated the use of CB[n]
to modulate the pKa of 4 by performing titrations with CB[7] in nonꢀbuffered solution wherein
the formation of the inclusion complex is anticipated to lead to the pyridine protonation. In the
titration experiment, an aqueous solution of CB[7] containing 100 mM NaCl was added to
solutions of 4 in CH3CNꢀH2O (1:9 or 1:99, both containing 100 mM NaCl). The band at 330 nm
disappeared with concomitant formation of a new band at 390 nm (Fig. 4 and Fig. S40 in the SI).
The titration data resemble the ones seen in the pH titration (Fig. 2 top), where the absorption
with maximum at 390 nm corresponds to 4H+. This finding indicates that CB[7] enhances the
basicity of 4, leading to a stabilization of 4H+ in the inclusion complex.
To determine the pKa of 4H+ in the CB[7] complex we performed a pH titration of 4 in the
presence of a large excess of CB[7] (0.5 mM) to ensure that all 4 is bound to the complex, since
it is known that the neutral form of guests have a lower stability constants than the cationic
forms.38,42,43 The resulting UVꢀvis spectra (Fig. S41 in the SI) were processed by multivariate
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nonlinear regression analysis using the SPECFIT program54,55,56 to reveal the pKa value of 6.97 ±
0.05. The higher pKa value (ꢂpKa ≈1.5) is in accordance with the precedent literature.59,60
3
4
5
6
7
8
9
1.4
1.2
1.0
10
11
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57
58
59
60
0.8
A
0.6
0.4
0.2
0.0
250
300
350
400
450
500
Wavelength / nm
Fig. 4. Absorption spectra for the titration of 4 (5 × 10ꢀ5 M in CH3CNꢀH2O 1:10, 100 mM NaCl)
with CB[7] (1 mM in H2O, 100 mM NaCl, CB[7] = 0ꢀ6×10ꢀ5 M) in the absence of buffer. The
curves were corrected for dilution.
Laser Flash Photolysis (LFP)
LFP measurements for solutions in CH3CN and CH3CNꢀH2O were performed for isomers 2ꢀ4, as
well as for the salt of 4H+ to probe for the formation of phototautomers by ESPT. No
phototautomers are expected to be formed in CH3CN. Transient absorption spectra for isomers 2ꢀ
4 in CH3CN showed an absorption band with a maximum at 410 nm, and a weaker band at 680
nm (Fig. 5). Addition of H2O to the CH3CN solution significantly changed the appearance of the
transient absorption (Fig. 5 and Fig S42 in the SI). The results in CH3CN (Fig. 5, black lines)
will be described first where for 2, 3 and 4 the same transients were observed (Fig. S43ꢀS49 in
the SI). These were assigned to radicalꢀcations (9), which absorb at 680 nm, and pyrrolyl Nꢀ
centered (10) radicals, which absorb at 410 nm (Scheme 5) according to the comparison with
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published transient spectra for phenylpyrroles66 and indoles.67,68 The decay of the transient
absorption measured in CH3CN was fit to a sum of two exponentials (k ≈ 9×105 sꢀ1 and 1×105 sꢀ1
for 2, 1×106 sꢀ1 and 1×105 sꢀ1 for 3, and 3×105 sꢀ1 and ≈104 sꢀ1 for 4). The lifetimes were not
affected by O2, in agreement with a previous report that O2 does not quench Nꢀcentered
radicals69 and radicalꢀcations.66,67,68,69 Shortꢀlived 9 absorbing in the visible part of the spectrum,
and longꢀlived 10 absorbing at shorter wavelengths (380ꢀ420 nm), are probably formed in
parallel processes. However, the observation of a growth kinetics with small amplitude in the
transient absorption at ≈ 420 nm (Fig. S48 in the SI) suggested that sequential formation of 9 and
decay of this transient leading to the longer lived 10 may also take place. Since the absorption of
solvated electron was not detected, an electron acceptor in the formation of 9 could have been H·
radicals which are produced in the parallel process, traces of O2 or H2O molecules, or 2ꢀ4 in S0
(less likely since radicalꢀanions of 2ꢀ4 were not detected). Furthermore, in the homolytic
cleavage of the pyrrole NꢀH bond and formation of 10, Hꢀacceptors could have been 2ꢀ4 in S0,
CH3CN or O2ꢀmolecules.
3
4
5
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7
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0.20
0.15
0.10
0.05
0.00
0.20
A
CH3CN
ꢀ
CH3CN-H2O (1:1)
0.15
0.10
0.05
0.00
A
Tim1e0 /
ꢁs
0
5
15
20
400
500
600
700
Wavelength / nm
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0.15
0.10
0.05
0.00
0.15
CH3CN
ꢀ
A
CH3CN-H2O (1:1)
0.10
0.05
0.00
A
10
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58
59
60
0
5
10
15
20
Tim e /
ꢁs
400
500
600
700
Wavelength / nm
Fig. 5. Transient absorption spectra in O2 purged and optically matched (A355 ≈ 0.35) solutions
for 2 (top) in CH3CN (delay = 500 ns) and in CH3CNꢀH2O (delay = 200 ns); the inset shows the
decays at 380 nm). Transient absorption spectra in O2 purged and optically matched (A355 ≈ 0.32)
solutions for 3 (bottom) in CH3CN (delay = 600 ns) and CH3CNꢀH2O (delay = 200 ns); the inset
shows the decays at 380 nm.
Scheme 5. Photochemistry of 2ꢀ4 in aprotic solvent.
ν
h
N
CH3CN
N
N
H
N
H
2-4
9
ν
h
-H+
CH3CN
N
N
10
In CH3CNꢀH2O solution for 2ꢀ4 the transient absorption band in the visible region was not
detected (Fig. 5, Fig. S43ꢀS47 and S49 in the SI), in accordance with the assignment of the
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9
transient observed in CH3CN to radicalꢀcations, which in aqueous solution rapidly decay by
proton transfer giving pyrrolyl radicals. Therefore, transients 9 are not observed when water is
present but the absorption for transient 10 is present (vide infra for discussion on lifetimes). In
the case of derivatives 2 and 4, but not 3, a new transient with absorption between 380 and 420
nm (Fig. 5, Fig. S42ꢀS47 and S49 in the SI) was observed that is assigned to phototautomers 2-T
(vide infra, scheme 7) and 4-T (Scheme 4) formed by ESPT. These species gave rise to a higher
intensity of the transient absorbance at 380ꢀ420 nm, compared to the spectra in CH3CN solutions
when the absorbance values at the excitation wavelengths were matched (Fig. 5 top). In the case
of 3, analysis of the kinetics (vide infra) showed that 3-T was not observed on the nanosecond
time scale (Fig. S42, S45 and S46 in the SI). Therefore, the spectrum in CH3CNꢀH2O (Fig. 5
bottom) corresponds only to intermediate 10.
10
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54
55
56
57
58
59
60
The relative contribution of the species in the photochemistry of 2ꢀ4 (Scheme 4) is expected to
be pH dependent for reactions in CH3CNꢀH2O because the formation of phototautomers is pH
dependent. At pH 11 (Fig. 6), the decay of the transient absorption was fit to a singleꢀexponential
function (4.1 × 106 sꢀ1, τ = 240 ns) and was assigned to phototautomer 4-T that at this pH decays
through a unimolecular process. A small offꢀset was observed at longer times which suggests
that a longꢀlived transient was present. This transient was more prominent at lower pHs and
corresponds to 10 (vide infra).
Decrease of the pH from 11 to 9ꢀ10 led to slower kinetics and the decay could not be fit to a
monoꢀexponential function. As will be shown below the kinetics correspond to a combination of
bimolecular and unimolecular processes, where the bimolecular process is assigned to the
reaction between two molecules of 4-T which competes with the unimolecular decay of 4-T. The
bimolecular component only becomes apparent when the unimolecular decay of 4-T becomes
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slower at these lower pH values. The contribution of the bimolecular process was minimized by
decreasing the energy of the laser pulse and the decay was fit by starting the fits at successively
longer delay times after the laser pulse until the kinetics fit to a monoexponential function (eq.
S3ꢀS4 and Fig. S50 in the SI). The lifetime for the unimolecular decays of 4-T were ≈ 2 ꢁs at pH
10, and 4 ꢁs at pH 9. A lengthening of the lifetime of 4-T is expected since 4-T undergoes base
catalysis to recover 4. In addition to the decay of 4-T, another transient was observed that
decayed over a much longer time window. This longer decay was fit to a monoexponential
function and had a lifetime of 120 µs (k = 8.3 × 103 sꢀ1). This transient is assigned to 10 based on
precedent in the literature.66,67,68,69
10
11
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54
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58
59
60
On decrease of the pH below 8ꢀ9, the characteristic strong signal assigned to 4-T disappeared.
Thus, in neutral and acidic solution only the longerꢀlived transient species was detected assigned
to radical 10, decaying through a unimolecular process (k = 8.3 × 103 sꢀ1, τ = 120 ꢁs). However,
between pH values of 3 and 8 a new transient is observed (Fig. S51 in the SI). The lifetime for
this transient was estimated to be shorter than 100 ns, where this decay occurs over a short time
window and levels off before the longꢀlived decay occurs. Since the shortꢀlived transient was not
detected in the visible part of the spectrum (at 680 nm), it cannot correspond to radicalꢀcation 9'.
According to the observation of the transient only at pH 3ꢀ8, its short lifetime and the absorption
at 390 nm, it may be related to the equilibrium between 4 and 4H+ in the ground state (Scheme
4). Upon excitation, singlet excited state 4 is protonated leading to 4H+ because of the higher
basicity in S1. Therefore this transient, was tentatively assigned to an excess of 4H+ in S0 which
then decays to reꢀestablish the groundꢀstate equilibrium between 4 and 4H+.
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5
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7
8
0.09
0.06
0.03
0.00
-0.03
-0.06
-0.09
60 ns
150 ns
320 ns
720 ns
9
A
10
11
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60
350
400
450
500
Wavelength / nm
0.20
0.10
0.08
0.06
0.04
0.02
0.00
ꢀ
A
0.15
0.10
0.05
0.00
A
2
4
8
10
12
6 pH
0.0
0.5
1.0
Time /
1.5
2.0
ꢁ
s
Fig. 6. Transient absorption spectra of 4 (top) in CH3CNꢀH2O (5:95), and decay at 380 nm at pH
= 11. The inset shows the pH dependence of the transient absorbance intensity at 390 nm, right
after laser excitation).
Formation of 4-T by ESPT can only be facilitated in the pH range between the pKa* values for
the pyridine protonation and the pyrrole deprotonation. These pKa* values were estimated in LFP
experiments from the dependence of the initial transient absorption intensity right after the laser
pulse collected at different pH values (Fig. 6 bottom, inset). Although this value is related to the
formation of all transients, significantly stronger signal intensities were observed in the pH
region between 9 and 12 where the characteristic transient assigned to 4-T was detected. Thus,
the estimated pKa* from the LFP experiment for the pyrrole deprotonation in S1 is in the range of
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8ꢀ9, and for the protonation of the pyridine this range is between 11 and 12 (Scheme 4). The
value for the pyridine pKa* in S1 for 4H+ obtained by LFP (11ꢀ12) is somewhat lower than the
value obtained by Förster cycle analysis (12.7). However, it should be noted that the
determination of pKa* by Förster cycle is usually inacurrate.3 Irrespective of the accuracy for the
pKa* values, LFP measurements clearly showed that the formation and decay kinetics of
phototautomer 4-T in aqueous solution is pH dependent.
10
11
12
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58
59
60
In some ESPT systems double proton transfer takes place through a bimolecular reaction
involving two phototautomers, leading to nonꢀexponential decays for the phototautomers.9
Therefore, the origin of the nonꢀexponential decays for 2-T and 4-T was investigated using LFP
by changing the energy of the laser pulses. For competitive uniꢀ and bimolecular reactions, the
decrease of the laser pulse energy leads to a decreased contribution of the bimolecular reaction to
the observed kinetics, because the bimolecular reaction depends on the concentration of
phototautomer, whereas unimolecular reactions are independent of the concentrations of
reactants. The contribution of the bimolecular reaction appears as an initial nonꢀlinear decay in
the semiꢀlog plot of the kinetics (Fig. 7). Fourfold decrease of the laser pulse energy decreased
the amplitude of the bimolecular contribution of the decay. This finding indicates that only one
species is detected decaying by competing uniꢀ and bimolecular reactions. Similar findings have
already been reported in the photochemistry of pyridylphenols.70
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8
0.1
0.01
1E-3
9
A
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59
60
0
5
10
15
20
Time / ꢁs
Fig. 7. Decay of the log of the transient absorbance at 400 nm for the solution of 4 in CH3CNꢀ
H2O as a function of the relative laser pulse energy, estimated from the intensity of the
benzophenone triplet at 525 nm (black line ꢂA525 = 0.08; red line ꢂA525 = 0.31) for optically
matched solutions at the excitation wavelength (A355 = 0.31).
Proton transfer processes are usually characterized by a large primary isotope effect.71 To verify
the assignment of the transient absorption to 2-T and 4-T we conducted LFP measurements for 2
and 4 in optically matched CH3CNꢀH2O and CH3CNꢀD2O solutions (Fig. 8 and Fig. S52 in the
SI). Change of H2O to D2O resulted in weaker signal intensities and longer decay times for both
compounds. From the ratio of the transient absorbance intensity immediately after the laser pulse
the estimate for the isotope effect for the formation of tautomers is in the range 1.3.ꢀ2.5.
However, the precise values of the isotope effects for the decay of transients was not warranted
due to complex decay kinetics imposed by competing uniꢀ and bimolecular reactions. The
observed changes in the decay kinetics and the intensity of the transient absorption are due to the
primary deuterium isotope effect for the formation and the decay of the transient species. These
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results strongly indicate that the observed transient absorption corresponds to species formed by
ESPT.
7
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0.08
CH3CN-H2O
CH3CN-D2O
0.06
A
0.04
0.02
0.00
0
2
4
6
8
10
Time / ꢁs
0.15
0.12
0.09
0.06
0.03
0.00
CH3CN-H2O
CH3CN-D2O
A
0
5
10
Time /
15
20
ꢁs
Fig. 8. Decay of transient absorbance at 420 nm in CH3CNꢀH2O (1:1) and CH3CNꢀD2O (1:1) for
2 (top) and 4 (bottom).
LFP measurements were also conducted for the salt of 4H+ in CH3CN and CH3CNꢀH2O (Fig.
S53ꢀS56 and S57 left). In CH3CN relatively weak transient absorption was observed with a
maximum at 650 nm. The decay was fit to a sum of two exponentials with rate constants of 5 ×
106 and 2 × 104 sꢀ1. Due to the similarity of the transient absorption with radicalꢀcation 9 the fast
component was tentatively assigned to tautomeric radicalꢀcation 9' formed by homolytic NꢀH
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cleavage of 4H+, whereas the slow component was assigned to radical 10 (Scheme 6). The
measurement in CH3CNꢀH2O performed at neutral or slightly basic conditions gave rise to the
same transients as observed by LFP of 4 since 4H+ at pH > 5.5 dissociates. Measurements in the
acidic solutions (H2SO4, pH 2) gave rise to a transient absorption with a maximum at 370 nm and
weaker signals at 390 and 420ꢀ500 nm decaying with a rate constant of 4 × 104 sꢀ1 that in analogy
was assigned to radical 10 (Fig S56 in the SI).
3
4
5
6
7
8
9
10
11
12
13
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59
60
Scheme 6. Homolytic cleavage of 4H+ observed in aprotic solvent.
ν
h
-H
+HN
Cl-
-Cl+HN
N
CH3CN
N
H
4H+
9'
-HCl
-HCl
N
N
N
H
N
Cl-
10
9
LFP experiments were conducted with the 4H+·CB[7] complex to probe if the
phototautomerization can take place within the cavity of CB[7]. Interestingly, the complexation
with CB[7] completely changed the photochemistry of 4H+. The signals corresponding to the
phototautomer 4-T were not detected. Instead, the transient absorption showed a maximum at
370 nm and a weaker broad band at 500ꢀ700 nm, similar to the transients observed in neat
CH3CN after decay of the radicalꢀcation (Fig. 9 and Fig. S58 in the SI). Therefore we assigned
the observed transient absorption in the presence of CB[7] to the pyrrolyl radical 10.
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CH3CN
CB[7]
0.015
0.010
0.005
0.000
-0.005
-0.010
9
10
11
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A
300
400
500
600
700
Wavelength / nm
Fig. 9. Transient absorption spectra of 4H+ (4.8 ×10ꢀ5 M) in CH3CN and in aqueous NaCl in the
presence of CB[7] (2 × 10ꢀ4 M) taken 700 ns after the laser pulse. The solutions were optically
matched (A355 = 0.35).
Discussion
Irradiation of 2ꢀ4 does not yield any stable photoproduct. However, quenching of fluorescence in
aqueous solutions and LFP measurements indicate that ESPT takes place in H2O leading to the
formation of phototautomers. Since the acidic site (pyrrole) and the basic site (pyridine) are not
in proximity, a polar protic solvent is essential to stabilize the CTꢀcharacter of the S1 state, and
even more important, to act as a proton donor (acid) and acceptor (base).
Formation of phototautomer 2-T from the ortho derivative in aqueous solution was assigned to
the transient absorption with maximum at 390 nm. In nearꢀneutral solution the decay for 2-T was
not exponential due to competing uniꢀ and bimolecular reactions, as indicated by the dependence
of the kinetics on the laser pulse energy. Only an estimate was possible for the decay time of 2-
T, τ ≈ 1.5 ꢁs. The assignment of the transient to 2-T was corroborated by a primary isotope
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effect in D2O solutions for its formation and decay (Fig 8). Moreover, 2-T can in principle exist
in two isomeric forms (2-Ta and 2-Tb, Scheme 7). The observed nonꢀexponential decay of 2-T
could in principle originate from the presence of these two isomers that undergo photochemical
EꢀZ isomerization, and are characterized by different decay rate constants. However, the
photochemical EꢀZꢀisomerization would require a second photon. Furthermore, we observe
similar nonꢀexponential decay kinetics for 4-T which cannot have two stereoisomers. Since the
fast decay depends largely on the laser pulse energy, this kinetics is more likely due to the
bimolecular reaction of 2-T involving two proton transfers.
10
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Scheme 7. Phototautomerization of 2.
S1
*
N
N
ν
H
h
solvent-assisted
ESPT
2-Ta
N
H
N
CH3CN-H2O
+
N
N
H
N
(H2O)m
(H2O)n
2
N
H
2*
2-Tb
Meta derivative 3 also bears the basic pyridine nitrogen and the acidic pyrrole NH. Strong
quenching of fluorescence in aqueous solution strongly indicates deactivation from S1 by ESPT.
Contrary to the transient spectra of 2, the LFP for the meta isomer 3 did not give rise to a
characteristic transient absorption that could be assigned to the phototautomer (Fig. S42 in the
SI). Instead, in aqueous solution the transient absorption spectrum was narrower (370ꢀ390 nm),
and the kinetics was slower than for 2-T, that is the fast decay was missing. The spectrum for the
photolysis of 3 was assigned to pyrrolyl radical 10 (Fig. 5 bottom). We do not have data for the
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acidity/basicity of 3 in S1 or spectroscopic evidence for the formation of the corresponding
phototautomer 3-T. Nevertheless, 3-T is a zwitterion that cannot be represented by a Kekulé
structure. Therefore, if this transient was formed in aqueous solution it would probably be very
shortꢀlived (probably in the picosecond timeꢀscale) and could not be detected by nanosecond
LFP. However, structurally related more stable zwitterions formed in photodehydration of
phenols have been reported and characterized by LFP.72 The other plausible explanation for the
observed quenching of fluorescence of 3 in aquoeus solution does not involve ESPT and
formation of phototautomer 3-T. Quenching may occur due to Hꢀbonding with the solvent,
leading to a deꢀexcitation via an internal conversion channel.34,35,36
10
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N
N+
-
H
3-T
Excitation of 4 to S1 leads to a significant enhancement of the pyridine basicity (pKa* ≈ 12), as
indicated by fluorescence measurements. In addition, concomitant enhancement of the pyrrole
acidity in aqueous solution leads to ESPT and formation of phototautomer 4-T, providing that
the solution pH is in the range between the pKa* of pyrrole (pH≈8ꢀ9), and the pKa* of pyridine
(pH≈11ꢀ12). A water molecule protonates the pyridine nitrogen and the pyrrole is deprotonated
to another water molecule from the solvent leading to the formation of phototautomer 4-T.
Therefore, in the appropriate pH range phototautomer 4-T was detected by LFP by its
characteristic strong transient absorption with a maximum at 390 nm. Phototautomer 4-T in nearꢀ
neutral solution (pH 8ꢀ10) decays through uniꢀ and bimolecular reactions, as indicated by the
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dependence of decay on the laser pulse energy. As anticipated the formation and decay of 4-T is
pH dependent and susceptible to deuterium isotope effect. The decay kinetics for 4-T is probably
governed by general and specific acid and base catalysis. Such acid/base catalysis was
demonstrated for example in the hydration reactions of quinone methides,73 or ketoꢀenol
tautomerisations.74,75
10
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The driving force for the formation of phototautomer 4-T was investigated by molecular
modeling. The energy for the vertical excitation of 4 to S1 calculated at the B3LYP/6ꢀ311G level
of theory in the gas phase is 366 kJmolꢀ1 (Table S1 in the SI), which perfectly matches the
experimentally observed absorption of 4 in CH3CN (λmax = 327 nm, 365.8 kJmolꢀ1). The S1 state
of 4 has a significant CT character, as evidenced by the fluoroꢀsolvatochromic properties and the
calculated dipole moment (Table S3 in th SI). The CT character is the driving force for ESPT to
occur. The calculated energy of the S1 state of 4-T in the gas phase is higher, 390 kJmolꢀ1 (Fig.
S3 and S4, and Tables S2, and S5 in the SI). However, we have no evidence that 4-T is formed
in the excited state. Its formation may involve double ESPT by solvent molecules in an adiabatic
exergonic reaction, or passing through a conical intersection, leading to 4-T in S0.
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Scheme 8. Energy diagram for 4 and 4-T.
5
6
7
S1
390 kJmol-1
366 kJmol-1
S1
8
9
10
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ν
h
145 kJ mol-1
S0
ν
h
HN
N
S0
N
N
H
Kinetics for 4-T formation could not be studied since the tautomerization takes place within the
laser pulse (10 ns). Protonation and deprotonation may take place simultaneously, or
sequentially. Furthermore, we have no experimental evidence if the tautomerization involves a
H2Oꢀrelay mechanism as was suggested for 7ꢀazaindoles18 and 7ꢀhydroxyquinoline.19,20,21,22,23
Phototautomer 4-T is 145 kJmolꢀ1 higher in energy than 4, which sets the stage for the
tautomerization in the ground state back to the starting molecule, taking place between 200 ns to
4 ꢁs, depending on the pH of the solution.
In an aprotic solvent, excitation of 4 to S1 leads to parallel reactions, where homolytic cleavage
of the pyrrole NꢀH forms radical 10, and photoionization of 4 forms radicalꢀcation 9. The
radicalꢀcation is acidic and deprotonates to pyrrolyl radical 10. Similarly, excitation of 4H+ in an
aprotic solvent ultimately leads to the formation of 10, involving homolytic cleavage of the
pyrrole NꢀH bond to form radicalꢀcation 9' followed by deprotonation to 10. These processes
probably take place also in aqueous solution, but with a quantum efficiency significantly lower
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