Excited state intramolecular proton transfer in π-expanded phenazine-derived phenols

Article abstract of DOI:10.1021/jp411395c

Two previously inaccessible analogs of 10-hydroxybenzo[h]quinoline were prepared via a straightforward strategy comprising the formation of π-expanded phenazines skeleton followed by C-H acetoxylation at position 10. Two bis-phenols possessing C₂ and D₂ symmetry were obtained in yields of 52% and 15%, respectively. The occurrence of excited state intramolecular proton transfer (ESIPT) was detected in all cases because steady state emission was observed only from the excited keto-tautomer. Additionally, a short-lived, ～0.1 ps, emission decay was resolved by the femtosecond up-conversion technique at the blue side of the keto-tautomer emission band, 610 nm, and was attributed to the ESIPT, i.e., conversion from enol to keto tautomer. In comparison with the corresponding 10-hydroxybenzo[h]quinoline emissions, the emission spectrum of the π-expanded phenazine analogues were weaker but displayed a characteristic bathochromically shift into NIR region. These phenazine analogues constitute one of largest heterocycles for which ESIPT was unambiguously detected.

Full text of DOI:10.1021/jp411395c

Article
pubs.acs.org/JPCA
Excited State Intramolecular Proton Transfer in π‑Expanded
Phenazine-Derived Phenols
Joanna Piechowska,†,‡ Kirsi Virkki,§ Bartłomiej Sadowski,† Helge Lemmetyinen,§ Nikolai V. Tkachenko,*,§
and Daniel T. Gryko*,†
†Institute of Organic Chemistry of the Polish Academy of Sciences, 01-224 Warsaw, Poland
‡Institute of Physical Chemistry of the Polish Academy of Sciences, 01-224 Warsaw, Poland
§Department of Chemistry and Bioengineering, Tampere University of Technology, 33720 Tampere, Finland
ABSTRACT: Two previously inaccessible analogs of 10-hydroxybenzo[h]-
quinoline were prepared via a straightforward strategy comprising the
formation of π-expanded phenazines skeleton followed by C−H acetoxylation
at position 10. Two bis-phenols possessing C2 and D2 symmetry were obtained
in yields of 52% and 15%, respectively. The occurrence of excited state
intramolecular proton transfer (ESIPT) was detected in all cases because
steady state emission was observed only from the excited keto-tautomer.
Additionally, a short-lived, ∼0.1 ps, emission decay was resolved by the femtosecond up-conversion technique at the blue side of
the keto-tautomer emission band, 610 nm, and was attributed to the ESIPT, i.e., conversion from enol to keto tautomer. In
comparison with the corresponding 10-hydroxybenzo[h]quinoline emissions, the emission spectrum of the π-expanded
phenazine analogues were weaker but displayed a characteristic bathochromically shift into NIR region. These phenazine
analogues constitute one of largest heterocycles for which ESIPT was unambiguously detected.
π-expanded pyrazine analogues with these new synthetic
methodologies could provide easy access to novel structural
analogues of 10-hydroxybenzo[h]quinoline. The aim of this
study was to apply the new synthetic strategy to obtain a range
of unique analogues of HBQ and to investigate their
fundamental optical properties. This approach addresses the
tunability of chromophore absorption as well as proton transfer
emission, key features involved in the ESIPT system
INTRODUCTION
■
Excited state inter- and intramolecular proton transfer
(ESIPT)1−11 has emerged as an interesting phenomenon that
can be utilized in the design of fluorescent sensors.12−15
Compounds displaying ESIPT include benzoxazoles,16−21
flavones,22−24 imidazoles,25−34 imidazo[1,2-a]pyridines,35−42
and anthraquinones.43−47 The photophysical characteristics of
ESIPT-capable compounds make them excellent candidates for
applications in such diverse areas as laser dyes,48,49 fluorescence
recording,50,51 ultraviolet stabilizers,52 probes for solvation
dynamics,53 probes for biological environments,54 and recently,
organic light emitting devices.55,56 Although 10-hydroxybenzo-
[h]quinoline (HBQ) represents one of the fundamental
heterocyclic systems, in which ESIPT occurs, it was only in
1992 when the fundamental studies of Martinez, Chou, and
colleagues identified ESIPT as the process responsible for the
strongly bathochromically shifted fluorescence of HBQ.57,58
Detailed photophysical and theoretical studies of HBQ showed
very fast and solvent-independent ESIPT,59−62 and recently
more elaborated derivatives were also investigated.63−68
Although the photophysical properties of dibenzo[a,c]-
phenazine have been extensively studied,69−76 more compre-
hensive studies involving its more complex derivatives have yet
to be made. This situation can be partly attributed to its
problematic derivatization.77−82 The recent discovery of
coordination-assisted acetoxylation of derivatives and analogues
of 2-phenylpyridine, by Sanford and co-workers, opened up
new possibilities in heterocyclic chemistry.83−91 Acetate
derivatives of 10-hydroxybenzo[h]quinoline prepared by this
method can be easily hydrolyzed to the corresponding phenols.
We envisioned that the combination of the rich photophysics of
EXPERIMENTAL SECTION
■
All chemicals were used as received unless otherwise noted.
Reagent grade solvents (CH2Cl2, hexanes) were distilled prior
1
to use. All reported H NMR and 13C NMR spectra were
collected using 600, 500, 400, or 200 MHz spectrometers.
Chemical shifts (δ ppm) were determined with TMS as the
internal reference; J values are given in hertz. The UV/vis
absorption spectra were recorded in CH2Cl2, toluene, TFA, and
MeCN. The absorption wavelengths are reported in nanome-
ters with the extinction coefficient in M−1 cm−1 in parentheses.
The melting points of compounds were determined using a
capillary type apparatus. Chromatography was performed on
silica (230−400 mesh) or neutral alumina. Dry column vacuum
chromatography (DCVC)92 was performed on preparative
thin-layer chromatography alumina. The mass spectra were
obtained via field desorption MS (FD-MS), electrospray
ionization (ESI-MS), and electron impact MS (EI-MS).
Compound 193 was prepared according to established
Received: November 20, 2013
Revised: December 18, 2013
Published: December 18, 2013
© 2013 American Chemical Society
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protocols. A spectrophotometer and a spectrofluorometer were
used to acquire the absorption and emission spectra.
Spectrophotometric grade solvents were used without further
purification.
measured at three wavelengths when possible: at the expected
emission maximum of the enol-form and close to the maximum
and at the red sides of the keto-form emission band. The
longest delay time is limited by 1.2 ns for the up-conversion
instrument. Therefore, the samples with emission lifetime
longer than few hundredths of picoseconds were also measured
using TCSPC instrumentation described elsewhere.94 In brief,
the samples were excited with a pulsed diode laser at 405 nm,
the emission detection range was 450−840 nm, and the time
resolution was 60−70 ps.
1,8-Dihydroxydibenzo[a,c]phenazine (2). A sealed tube
was charged with dibenzo[a,c]phenazine (1) (70 mg, 0.25
mmol), PhI(OAc)2 (330 mg, 1 mmol), Pd(OAc)2 (4.6 mg, 0,02
mmol), and CHCl3 (2.1 mL). The reaction was stirred at 120
°C for 24 h. The mixture was transferred to a round-bottom
flask and evaporated to dryness. The precipitate was dissolved
in MeOH, and NaOH (5 equiv) was added to hydrolyze the
formed bis-acetate. After stirring at room temperature for a few
hours, the mixture was acidified with 10% HCl until neutral pH
was achieved. The solvent was removed under reduced pressure
and extracted between CH2Cl2 and water. A thin layer of
diatomaceous earth was used to remove the insoluble black
solid from the formed emulsion. The organic layer was dried
over Na2SO4 and removed under vacuum. The solid was
chromatographed (DCVC, Al2O3, hexanes → hexanes/CH2Cl2
4:1, 3:1, 3:2). The pure product was obtained after
RESULTS AND DISCUSSION
■
Design and Synthesis. No pyrazine derivatives undergoing
ESIPT were known prior to this study. We designed two π-
expanded pyrazines with appropriate architecture namely
dibenzo[a,c]phenazine (1)93 and dibenzo[a,h]phenazine (5)
as precursors of the corresponding bis-phenols. Phenazine 4 has
been synthesized by air-controlled oxidation of 1-amino-
naphthalene following procedure by Itoho and Kosugi.95
Initially, compound 1 was subjected to optimized conditions
(i.e., MeCN, 150 °C, 14 h) with 2 equiv of oxidant
(PhI(OAc)2). Although a high conversion did occur, the
reaction led to the formation of numerous products and
therefore required optimization. On the basis of this result, we
tested other solvents and determined that using CHCl3 at 120
°C in conjunction with higher concentration of Pd(OAc)2 (3
times) resulted in full conversion (Scheme 1). After purification
1
crystallization from hot acetone (40.6 mg, 52%): H NMR
(400 MHz, CDCl3, δ) 3.23−5.10 (br s, 1H, NH), 7.28 (d, 1H, J
= 0.7), 7.68 (t, 2H, J = 8.0), 7.82−7.98 (m, 2H), 8.06 (d, 2H, J
= 7.7), 8.15−8.34 (m, 2H), 14.81 (s, 1H, OH); 13C NMR (125
MHz, CDCl3, δ) 112.9, 114.6, 116.7, 127.3, 130.5, 132.7, 133.7,
137.2, 145.5, 161.6; ESI-LR found 313.1 [M + H+], calcd 313.1
(C20H13N2O2); λabs (CH2Cl2, ε × 10−3) 428 (17.9), 263 (64.8)
nm.
1,8-Dihydroxydibenzo[a,h]phenazine (5). A sealed tube
was charged with dibenzo[a,h]phenazine (4) (300 mg, 1.07
mmol), PhI(OAc)2 (1.38 g, 4.28 mmol), Pd(OAc)2 (24 mg,
0.107 mmol), and CHCl3 (10 mL). The reaction was stirred at
120 °C for 24 h. The mixture was transferred into a round-
bottom flask and evaporated to dryness. The precipitate was
dissolved in MeOH/THF, and NaOH (5 equiv) was added to
hydrolyze the formed bis-acetate. After stirring at room
temperature for a few hours, the mixture was acidified with
10% HCl until neutral pH was achieved. The solvent was
removed under reduced pressure and extracted with CHCl3.
The organic layer was dried over Na2SO4 and removed under
vacuum. The solid was chromatographed (DCVC, Al2O3,
hexanes → hexanes/CH2Cl2 4:1, 3:1, 3:2 → CHCl3). The pure
product was obtained after crystallization using THF and
filtrated hot mixture through crepe filter paper (52 mg, 15%);
1H NMR (600 MHz, DMSO, δ) 7.55 (d, 2H, J = 7.9), 7.76 (d,
2H, J = 7.6), 7.89 (t, 2H, J = 7.8), 8.17 (d, 2H, J = 9.1), 8.40 (d,
2H, J = 9.3), 11.43 (s, 2H); 13C NMR (150 MHz, DMSO, δ)
110.1, 116.6, 119.4, 122.4, 133.7, 134.0, 135.0, 136.9, 138.7,
156.1; EI-HR found 312.0905 [M·+], calcd 312.0899
(C20H12N2O2); λabs (CH2Cl2, ε × 10−3) 484 (8.7), 266
(26.2) nm.
Spectroscopy Measurements. Steady state absorption
spectra were measured using a Shimadzu UV-3600 spectropho-
tometer. Corrected emission spectra were acquired using a
Fluorolog 3 fluorometer (SPEX Inc.), with excitation at 328−
409 nm, depending on the compound. Emission quantum
yields of 2 and 5 were determined using Rhodamine 6G as a
standard (ethanol, quantum yield 0.95). Emission decays were
measured using two methods, femtosecond up-conversion and
picosecond time correlated single photon counting (TCSPC).
The up-conversion instrument has been described elsewhere.94
Excitation wavelengths were 380−400 nm with a time
resolution of approximately 150 fs. Emission decays were
Scheme 1. Preparation of Hydroxyphenazine 2
using chromatography, the newly optimized procedure
produced the desired product 2 in 52% yield. The same
conditions were applied for phenazine 4 (Scheme 2) to afford
compound 5 in 15% yield (after hydrolysis).
Optical Properties. Previously reported studies show that
pyrazine derivatives do not display a strong fluorescence.
Indeed, their photostability results from the loss of excited state
energy through vibrational relaxation.96,97 Absorption and
emission spectra of 2 and 5 are presented in Figure 1. Close
examination of the spectra revealed a virtual solvent
independence with the exception of a slightly higher emission
intensity of 2 in toluene in comparison to the cases of other
studied solvents. For both compounds large Stokes shifts can
be noted at 9800 and 7500 cm−1 for dyes 2 and 5, respectively
(Table 1). The optical data collected emphasize that, in analogy
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probing100 compounds, because coordination of various cations
disables proton transfer and in turn usually leads to a significant
increase in fluorescence quantum yields.66,67,101,102 Given
relatively high molar-absorption coefficients, one can also
envision applications as photostabilizers.
Scheme 2. Preparation of Hydroxyphenazine 5
To experimentally determine the fluorescence lifetime
emission, we employed both femtosecond up-conversion and
picosecond time-resolved emission spectroscopy (Figure 2). As
expected, emission decay measurements of phenol 2 confirmed
shorter lifetimes composed of two dominating emission decay
component having lifetimes of 0.16 and 8.4 ps in THF. The
shorter 0.16 ps lifetime is clearly observed at the blue side of
the emission band and is the only decay component at 610 nm.
These findings can be attributed to the emission of the enol-
tautomer, or the singlet excited state with conformation
corresponding the ground state. The longer lifetime of 8.4 ps
occurred at similar wavelengths observed in the steady state
emission spectrum of 2 and therefore can be ascribed to the
lifetime of the emission band. The lifetime of 8.4 ps is still
rather short and indicates that nonradiative decay is dominating
the relaxation process, which is in agreement with the low
emission quantum yield of the compound. Similar measure-
ments were also carried out in MeCN and toluene, the results
were essentially the same showing a fast (∼0.1 ps) emission
decay at 610 nm and decays with 11 and 14 ps time constants
at longer wavelengths (710 nm) in MeCN and toluene,
respectively. The somewhat longer emission lifetime in toluene
corroborates with the slightly high emission quantum yield in
this solvent as compared to the cases of two other solvents.
Unfortunately, similar measurements were impossible for 5
because the limitation imposed by the up-conversion instru-
ment, namely a low signal-to-noise ratio near the fundamental
wavelength of the femtosecond laser, 800 nm, as well as a weak
absorption of the compound 5 at the excitation wavelength,
400 nm. However, considering that the emission quantum yield
of 5 is roughly 3 times larger than that of 2, one can expect that
the nonradiative decay of 5 is slower and its time constant is
expected to be close to 25 ps.
to HBQ,57−59 ESIPT for phenols 2 and 5 is free from solvent
perturbation. Large Stokes shifts (characteristic for ESIPT),
along with the lack of emission at the red end of the absorption
band indicated that the proton transfer was a fast and efficient
process, similar to the parent HBQ compound. Additionally,
the fluorescence emission generated from the proton trans-
ferred state was relatively weak. Emission quantum yields were
estimated to be roughly 0.01% and 0.03% for 2 and 5,
respectively, which indicated a relatively short lifetime of the
proton transferred state.
The fundamental study by Park and co-workers showed that
ESIPT-derived fluorescence from the excited keto state is
usually low if the six-membered nitrogen containing ring is
involved.98 10-Hydroxybenzo[h]quinoline and all its derivatives
and analogues possessing various functional groups have rather
low fluorescence quantum yields.63−68 For this reason the
parent compound has been used as an optical filter agent in
photographic emulsions, for many years.99 ESIPT-based
functional dyes characterized by low fluorescence quantum
yields can find applications in research aiming at switch-on
Three main solvents used in this study were MeCN, THF,
and toluene, which represent polar, moderately polar, and
nonpolar media, and all these solvents are nonprotic ones.
Unfortunately, good protic solvents, such as methanol or
ethanol, could not be used with these compounds because of
solubility issues. Therefore, test measurements were carried out
in mixtures of dichoromethane and methanol at 1:1 volume
ratio. Virtually no change in position or intensity of absorption
and emission bands was observed, and only a slight (<20%)
Figure 1. Normalized absorption (solid lines) and emission (dashed lines) spectra of 2 (a) and 5 (b) in different solvents.
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Table 1. Summary of the Absorption and Emission Properties of Compounds 2 and 5 in MeCN, THF, Toluene, and CH2Cl2−
MeOH mixture
compound
solvent
MeCN
λabs (nm)
10−3
ε
λem (nm)
Δv (cm−1
)
Φ (%)
2
423
432
429
431
477
485
488
484
14.2
15.4
15.5
17.0
16.7
19.8
22.0
22.1
724
722
722
708
743
756
754
740
9830
9300
9500
9100
7510
7390
7230
7150
0.01
0.01
0.02
0.03
0.03
0.03
0.03
0.03
THF
toluene
CH2Cl2/MeOH 1:1
MeCN
5
THF
toluene
CH2Cl2/MeOH 1:1
Figure 2. Time-resolved emission decays of 2 in THF at 610, 710, and 750 nm. The colored symbols are experimental data, and the solid lines are
exponential fits of the decays. (a) and (b) represents the same data at different time scales.
increase in emission quantum yield was noticed. This agrees
with previous reports indicating almost no solvent dependence
for similar compounds.59−68
of extension of the π-conjugated system on the optical
properties of ESIPT dyes? A limited number of papers
published on this aspect103−107 revealed that the π-expansion
of basic ESIPT-capable chromophores can, in some cases, lead
to quenching of excited state intramolecular proton transfer.104
Our earlier studies of this aspect led to the conclusion that in
the case of π-expanded benzo[h]quinoline analogues, the
energetics of ESIPT depends on the actual position of π-
expansion.65−68 Park and co-workers33 proved that the
observed behavior concerning ESIPT of π-expanded derivatives
of imidazoles and related molecules can be explained by
considering the nodal pattern of the wave function along with
the delocalization of the electrons in the excited state (so-called
nodal-plane model developed by Nagaoka).108,109 In the case of
dyes 2 and 5 the weak keto-tautomer emission is parallel to
many other ESIPT molecules and can be generally interpreted
in terms of either rapid intersystem crossing or other fast
nonradiative pathways. As was proved by Chou and co-workers
for HBQ, the excited state resonance charge transfer triggers
the proton transfer reaction, which simultaneously enhances
the degree of resonance charge transfer effect. The optical
properties of compounds 2 and 5 (Table 1, Figures 1 and 2)
made it clear that π-expanding of the chromophore of 10-
hydroxybenzo[h]quinoline in the presented way does not
interfere with ESIPT. The presence of neither additional
benzene rings nor additional nitrogen atoms interferes with the
favorable relative energy levels of E* and K*. This is in strong
contrast with the case of 12-hydroxy-1-azaperylene, when π-
expansion of HBQ leads to a drastic change of energy levels,
hence disabling ESIPT.65,68 In conjunction with our previous
studies on π-expanded analogs of HBQ, the current pyrazine-
Comparison of the emission and absorption spectra of two
compounds shows a greater difference between the steady-state
absorption bands than that of the emission bands. The
absorption band energies for 2 and 5 are 23 640 (2.93 eV)
and 20 960 cm−1 (2.60 eV), which correspond to a difference of
2680 cm−1 (0.36 eV), whereas the energies of the emission
bands are 13 800 (1.71 eV) and 13 500 cm−1 (1.67 eV) for 2
and 5, respectively, which result in a difference of only 300
cm−1 (0.04 eV) (nearly an order or magnitude smaller than the
absorption energy difference). This can be rationalized by
considering the conjugation systems of the enol- and keto-
tautomers. In the ground state, the enol-tautomer is dominant,
which results in a conjugation system that spans the entire
molecular backbone of both compounds. Because each
compound has a distinct arrangement of aromatic systems,
the electronic configurations of 2 and 5 are quite different,
which leads to the difference in absorbance spectra. Conversely,
for the keto-tautomers, ESIPT occurs and the formation of each
keto group displaces the aromaticity of the connecting ring,
effectively removing it from the extended π-system. The
decreased degree of conjugation diminishes the extended π-
systems of both bis-phenols to a common tricyclic
chromophore. Thus the energy differences of the emissions
bands between compounds 2 and 5 are smaller than the steady-
state absorbance energies because they are generated by a
similar excited state electronic configuration.
The vast majority of ESIPT-related studies performed within
last the 60 years focused on the small heterocyclic systems. This
leaves an intriguing question unanswered: what is the influence
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Steady-state fluorescence emission maxima measurements
showed emission by the keto-tautomer in the NIR spectral
range. The two compounds have different symmetries, and the
positions of their absorption maxima differ by 2680 cm−1.
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AUTHOR INFORMATION
Corresponding Authors
*N. V. Tkachenko: e-mail, nikolai.tkachenko@tut.fl.
*D. T. Gryko: e-mail, dtgryko@icho.edu.pl.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was funded by the Foundation for Polish Science
(TEAM-2009-4/3). We thank Jillian Larsen (UC Riverside) for
amending manuscript.
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