Proton transfer triggered proton transfer: A self-assisted twin excited state intramolecular proton transfer

Article abstract of DOI:10.1039/c8cp03835j

The double excited state intramolecular proton transfer (ESIPT) of 3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazole (bis-HPTA), a molecule possessing two intramolecular hydrogen bonded donor-acceptor pairs, has been investigated. The molecule undergoes not only

Full text of DOI:10.1039/c8cp03835j

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Proton Transfer Triggered Proton Transfer: A Selfꢀ
Assisted Twin Excited State Intramolecular Proton
Transfer§
Saugata Sahu, Minati Das, Aditya Kumar Bharti, G. Krishnamoorthy*
Department of chemistry, Indian Institute of Technology Guwahati, Guwahati, Assam 781039,
India.
Corresponding Author,
*Email: gkrishna@iitg.ernet.in
§Dedicated to Professor Jack Saltiel on his 81st birthday.
1

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ABSTRACT. Double excited state intramolecular proton transfer (ESIPT) of 3,5ꢀbis(2ꢀ
hydroxyphenyl)ꢀ1Hꢀ1,2,4ꢀtriazole (bisꢀHPTA), a molecule possessing two intramolecular
hydrogen bonded donorꢀacceptor pairs, has been investigated. The molecule undergoes not only
a single ESIPT, but also the rare twin ESIPT. The most interesting fact is that initially, only one
acidꢀbase pair is ESIPT active and the other pair is ESIPT inactive. The first proton transfer
triggers the proton transfer in the second acidꢀbase pair by creating an appropriate condition.
This new class of sequential proton transfer is labeled as ‘proton transfer triggered proton
transfer’ (PTTPT). The PTTPT has been demonstrated with steady state and time resolved
fluorescence techniques. It was further substantiated by the theoretical data and using a
chemically modified system (3ꢀ(2ꢀhydroxyphenyl)ꢀ5ꢀ(2ꢀmethoxyphenyl)ꢀ1Hꢀ1,2,4ꢀtriazole.
KEYWORDS: ESIPT, Double Proton Transfer, Cooperative Proton Transfer, Triazole, Annular
Tautomer.
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1. Introduction
Photoinduced proton transfer plays a fundamental role in several different chemical and
biological processes1ꢀ7 such as genetic mutation in the DNA base pair,1 synthesis of ATP,2
catalytic reduction of methionine sulfoxide to methionine.3 The proton transfer may be an
intermolecular or intramolecular process. Excited state intermolecular proton transfer has found
9
many uses8ꢀ12 for example in ratiometric sensing,8, pH indicating,10 microꢀenvironment
probing,11 photolithography.12 The intramolecular proton transfer process has also drawn much
attention in scientific research due to the potential applications of these molecules in laser dye,13
16
solar concentrator,14 photostabilizer,15,
sensing,19ꢀ22 probing23.
molecular data storage,17 light emitting diode,18
Different types of photoinduced proton transfer have been reported. For example, 1ꢀnaphthol,24
2ꢀhydroxynaphthaleneꢀ6ꢀsulfonate,25 7ꢀhydroxycoumarin,26 luciferin27 undergo solvent assisted
excited state intermolecular proton transfer and produce anion in the excited state. The dimer of
7ꢀazaindole exhibits a concerted intermolecular double proton transfer.28 Most recently, a triple
proton transfer mechanism was reported with a cyclic trimer of azaindole.29 A solvent assisted
relay proton transfer was observed in 7ꢀhydroxyquinoline.30 Weller first reported the excited
state intramolecular proton transfer (ESIPT) which follows a four level cyclic process.31 It occurs
via the intramolecular hydrogen bond which is a prerequisite for ESIPT. Different proton donorꢀ
acceptor coupled ESIPT molecules lead to different kinds of tautomerism such as enolꢀketo,
imineꢀamine, enolꢀamine, imineꢀketo. Most of these molecules follow a single proton transfer
process in the excited state. Recently, the development of double/multiple intramolecular proton
transfer systems has gained interest.32ꢀ35 However, the chemical modification of a system with a
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single ESIPT unit to a system with multiple ESIPT units often does not lead to a multiple proton
transfer system.34, 36ꢀ38 Upon photoexcitation the modified systems underwent single proton
transfer rather than twin or multiple proton transfer. Chou and his coꢀworkers reported
intramolecular relay type double proton transfer with 7ꢀhydroxyquinolineꢀ8ꢀcarboxylic acid and
1,8ꢀdihydroxyꢀ2ꢀnaphthaldehyde.32, 33 The interesting aspect of these systems is that the initial
proton transfer is carried forward to another unit(s) in the excited state. The dual ESIPT systems
have to undergo a twin proton transfer to generate a dual tautomerized state. The phenomenon is
unusual and such systems are rarely reported.32, 33, 35, 39, 40 A cooperative twin proton transfer
upon photoexcitation were reported earlier in 2,5ꢀbis(benzoxazolꢀ2ꢀyl)thiopheneꢀ3,4ꢀdiol,40, 41
42ꢀ44
2,2'ꢀbipyridineꢀ3,3'ꢀdiol and 2,2'ꢀbipyridineꢀ3,3'ꢀdiamine.39,
The double proton transfer
mechanism of 2,2'ꢀbipyridineꢀ3,3'ꢀdiol and 2,2'ꢀbipyridineꢀ3,3'ꢀdiamine are well established.42, 43,
45ꢀ49 They comprised of two identical donorꢀacceptor units and show a sequential proton transfer
which can commence from any of the donorꢀacceptor unit. Due to the fast proton transfer from
the monoketo to the diketo, the steady state tautomer emission appears only from the diketo
species of 2,2'ꢀbipyridineꢀ3,3'ꢀdiol and 2,2'ꢀbipyridineꢀ3,3'ꢀdiamine.
In the present work, we report a new class of proton transfer, proton transfer triggered proton
transfer (PTTPT) in 3,5ꢀbis(2ꢀhydroxyphenyl)ꢀ1Hꢀ1,2,4ꢀtriazole (bisꢀHPTA, Chart 1A). bisꢀ
HPTA possesses two phenolic units (proton donor) attached with a 1,2,4ꢀtriazole unit (proton
acceptor). The donorꢀacceptor systems in bisꢀHPTA (α and β, Chart 1) are not electronically
identical and they should yield different keto forms upon photoꢀexcitation. The first proton
transfer (ESIPTꢀI) produces emission from mono keto tautomer. The second proton transfer
(ESIPTꢀII) produces a different emission from a double tautomerized state. We observed that the
ESIPTꢀI triggers the ESIPTꢀII. Otherwise ESIPTꢀII is not feasible in the solution. In system such
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as 2,2'ꢀbipyridineꢀ3,3'ꢀdiol or diamine, ESIPT in both the donorꢀacceptor systems are feasible
and equally probable. Thus, PTTPT of bisꢀHPTA produces a twin ESIPT emission by a rare twin
ESIPT mechanism.
Chart 1. Molecular structures of (A) 3,5ꢀbis(2ꢀhydroxyphenyl)ꢀ1Hꢀ1,2,4ꢀtriazole (bisꢀHPTA),
(B) 3ꢀ(2ꢀhydroxyphenyl)ꢀ5ꢀ(2ꢀmethoxyphenyl)ꢀ1Hꢀ1,2,4ꢀtriazole (HPMPTA).
α
O
(A)
H
4
N
N
NH
1
O
2
H
β
(B)
O
H
N
N
N
O
H
2. Materials and methods
bisꢀHPTA was prepared in two steps (Scheme S1A).50 In the first step, 2ꢀ(2’ꢀhydroxyphenyl)ꢀ
4Hꢀ1,3ꢀbenzoxazinꢀ4ꢀone was synthesized by the reaction of equimolar amount of salicylic acid
and salicylamide in the presence of thionyl chloride and a catalytic amount of pyridine in xylene.
The purified product (by column chromatography) was reacted with an equivalent amount of
hydrazine monohydrate in diethyl ether to yield the final product.
1
BisꢀHPTA: white solid; H NMR (600 MHz, DMSOꢀd6) δ 11.46 (s, 2H), 8.02 (d, J = 7.8 Hz,
2H), 7.35 (m, J = 7.7 Hz, 2H), 7.03 (d, J = 8.1 Hz, 2H), 7.00 (m, 2H) (Figure S1, ESI). 13C NMR
(600 MHz, DMSO) δ 155.90, 131.36, 127.52, 119.52, 116.61, 113.42. (two carbons did not
appear, Figure S2, ESI). HRMS (M+1) 254.093 (254.093 cal.).
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The monoꢀmethoxy derivative of bisꢀHPTA, 3ꢀ(2ꢀhydroxyphenyl)ꢀ5ꢀ(2ꢀmethoxyphenyl)ꢀ1Hꢀ
1,2,4ꢀtriazole (HPMPTA) was synthesized in a two step process (Scheme S1B). First, 2ꢀhydroxyꢀ
Nꢀ(2ꢀmethoxybenzoyl)benzamide (HMBB) was synthesized by the reaction of an equimolar
amount of salicylamide and 2ꢀmethoxybenzoicacid in the presence of thionyl chloride and a
catalytic amount of pyridine in xylene. It was purified with column chromatography. Benzamide
1 mmole, hydrazine hydrate 1 mmole were added to 3 mL 30% acetic acid and the solution was
refluxed at 118° C for 24 h. A clear solution was observed at the end of the reaction. It was
partially neutralized with 2% sodium bicarbonate solution. The product was extracted with
chloroform and purified by column chromatography. HPMPTA was characterized with 1H NMR,
13C NMR and HRMS.
HMBB: pale yellow solid; 1H NMR (600 MHz, DMSOꢀd6) δ 7.89 (d, J = 7.9 Hz, 1H), 7.83 (d, J
= 7.7 Hz, 1H), 7.58 (m, 1H), 7.44 (m, 1H), 7.21 (d, J = 8.4 Hz, 1H), 7.10 (m, 1H), 7.03 (d, J =
8.2 Hz, 1H), 6.96 (m, 1H), (Figure S3, ESI). HRMS (M+1) 272.093 (272.093 cal.).
1
HPMPTA: white solid; H NMR (600 MHz, CDCl3): δ 12.21 (s, 1H), 11.28 (s, 1H), 8.28 (d, J =
7.7 Hz, 1H), 8.10 (d, J = 7.7 Hz, 1H), 7.46 (m, 1H), 7.30 (m, 1H), 7.17 (m, 1H), 7.04 (d, J = 8.2
13
Hz, 1H), 6.99 (d, J = 8.3 Hz, 1H), 6.92 (m, 1H), 3.94 (s, 3H) (Figure S4, ESI). C NMR (600
MHz, CDCl3) δ 160.80, 157.28, 157.02, 152.08, 132.35, 131.15, 129.78, 126.87, 121.92, 119.52,
117.38, 114.47, 111.47, 56.12, (Figure S5, ESI). HRMS (M+1) 268.108 (268.109 cal.).
All the required chemicals for the synthesis were purchased from Sigma Aldrich, USA. All
solvents were purchased from Spectrochem, India.
The absorption spectra were recorded with a Perkin Elmer Lambda 750 UVꢀVisible
spectrometer. The steady state emission and excitation spectra were acquired using a Horiba
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Fluoromaxꢀ4 spectrometer. HPLC grade solvents were used for spectral studies. The
concentration of dyes was 5 ꢁM for all photophysical experiments. The fluorescence quantum
yields were measured using quinine sulphate solution in 1 N sulphuric acid (φf = 0.55) as
reference.51 Edinburgh instrument LifeSpecꢀII was used to record the fluorescence lifetime with
a Hamamatsu MCP detector with response time of ~ 300 ps for the rising time and ~ 500 ps for
the decay time. A 308 nm LED light sources with full width at half maximum 635 picosecond
was used for the lifetime measurement. The pulse repetition rate was adjusted to 5 MHz. The
timeꢀresolved emission spectra were constructed using the method reported by Koti et al.52
Theoretical calculations were performed using Gaussian 09W program to obtain different
molecular parameters.53 GaussView 5.0 was used to draw the molecular structures, to obtain the
coordinates and to feed the inputs. The density functional theory (DFT)54, 55 and the time
dependent DFT (TDDFT) were used to optimize the ground and the excited state geometries,
respectively.56 A 6ꢀ31+G(d,p) basis set with Becke’s threeꢀparameter exchange functional along
with LeeꢀYangꢀParr nonlocal correlation functional (B3LYP) was used for the geometry
optimization.57, 58 The excitation and the emission energies were calculated from the optimized
ground state and excited state geometries, respectively using TDDFT/B3LYP/6ꢀ31+G(d,p)
calculations.59 The solvent stabilization of the molecule was included by using integral equation
formalismꢀpolarizable continuum model (IEFꢀPCM) and choosing tetrahydro furan (THF) as
solvent. 60
3. Results and discussion
3.1. Intrinsic photophysics of bisꢀHPTA and twin ketoꢀemission
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1.2
0.9
0.6
0.3
0
Appears in polar
protic solvents
280
300
320
340
360
380
Wavelength (nm)
Figure 1. Normalized absorption spectra of bisꢀHPTA in different solvents.
1.2
1
8
0.9
0.6
0.3
0.0
320
370
420
470
520
Wavelength (nm)
Figure 2. Normalized emission spectra of bisꢀHPTA in different solvents 1) ethyl acetate, 2)
tetrahydrofuran, 3) dioxane, 4) chloroform, 5) 1ꢀbutanol, 6) 1ꢀpropanol, 7) 2ꢀpropanol, 8)
methanol. λex = 308 nm.
Figure 1 shows the absorption spectra of bisꢀHPTA in different solvents. The absorption
maximum of bisꢀHPTA appears at ~ 308 nm and it shows a weak dependency on solvent
polarity. The molecule possesses a high molar extinction coꢀefficient (2 x 104 Mꢀ1cmꢀ1, in
methanol). The vibrational structure of the absorption spectrum slowly blurs with increasing
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solvent polarity. In polar protic solvents, along with the main absorption band a small band also
appears around 340 nm (Figure 1). Upon excitation, depending on the solvent, a less Stokes
shifted UV emission and a highly Stokes shifted visible emission are observed (Figure 2 and
Table 1). In low polarity media such as cyclohexane, chloroform, tetrahydrofuran, dioxane, only
the visible emission appears. The Stokes shift of the visible emission is as large as 11,000 cmꢀ1.
The emission quantum yield is 0.04ꢀ0.08 in those solvents. With increasing solvent polarity, the
visible emission gradually shifts toward the shorter wavelength. The ratio of the shorter
wavelength emission band to the longer wavelength emission band gradually increases with
hydrogen bond forming ability of solvent. In a solvent such as ethyl acetate, the ratio is 0.01.
But, in a highly polar protic solvent such as methanol, the intensity ratio is 0.8. The Stokes shift
of the UV emission in methanol is 3100 cmꢀ1. The less Stokes shifted UV emission suggests that
it is the normal emission from the enol tautomer. The highly Stokes shifted visible emission
exhibits a characteristic negative solvatochromic effect of the tautomer emission,61, 62 and is
therefore attributed to the emission from the keto species.
The proton transfer occurs from the phenolic unit to the triazole nitrogen of bisꢀHPTA
molecule via the intramolecular hydrogen bond. In protic solvents, the intramolecular hydrogen
bond is interrupted by the formation of intermolecular hydrogen bonds with solvent. The
breaking of the intramolecular hydrogen bond results in the formation of solvated open
conformer (see later) where the ESIPT is not feasible. Thus, results in the normal emission (340
nm emission). The excitation spectra also suggest the presence of two different conformers (see
later). The fluorescence lifetime of bisꢀHPTA was recorded at normal and tautomer emission
maxima in different solvents (Table 1, Figure 3). The excited enol and keto tautomer of
imidazoleꢀbased ESIPT systems have fluorescence lifetimes of ~ 1.5 ns and ~ 3.5 ns,
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respectively.63ꢀ65 Apart from these two fluorescence lifetimes, another emission lifetime is
observed when the tautomer emission of bisꢀHPTA was monitored (Table 1). It indicates the
existence of more than one keto tautomer in the excited state.
Table 1. Absorption maxima (λꢀꢁꢃꢁꢄꢂ, nm) emission maxima (λꢅꢀꢀꢁꢂ, nm) and fluorescence lifetime
(τ, ns)a of bisꢀHPTA in different solvents, λex = 308 nm.
τ b
τ c
ꢅꢀ
ꢁꢃꢄ
Solvent
λ
λ
ꢀꢁꢂ
ꢀꢁꢂ
Ethyl acetate
307
308
308
307
308
308
460
460
459
0.7 (72)
3.7 (28)
0.8 (61)
3.9 (39)
0.8 (62)
4.0 (38)
0.7 (48)
3.0 (52)
1.8 (70)
4.1 (30)
2.4 (68)
4.0 (32)
Dioxane
Tetrahydrofuran
Methanol
340, 438
340, 442
340, 444
1.4
(100)
1ꢀPropanol
1ꢀButanol
1.6
(100)
1.6
(100)
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aRelative amplitude (in %) of different emitting species are given in parenthesis.
b
c
ꢉꢆ
Emission monitored at shorter λꢉꢆꢆꢇꢈ. Emission monitored at longer λ
.
ꢆꢇꢈ
1.0E+4
1.0E+3
1.0E+2
a
b
c
1.0E+1
1.0E+0
5
2.0 7.0 12.0 17.0 22.0 27.0
Time (ns)
0
ꢀ5
Figure 3. Fluorescence decays of (a) tautomer emission in dioxane (recorded at 460 nm) and (b)
normal emission in methanol (recorded at 350 nm) and (c) the instrument response function, λex
= 308 nm. The corresponding residual plots are shown below the decay profile.
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0.16
0.12
0.08
0.04
0.00
0 ns
10 ns
0 ns
10 ns
420
470
Wavelength (nm)
520
Figure 4. Time resolved area normalized emission spectra of bisꢀHPTA in dioxane obtained at
different times 0 ns, 0.8 ns, 1.4 ns, 1.8 ns, 2.2 ns, 2.8 ns, 3.5 ns and 10 ns. λex = 308 nm.
To obtain a better insight on the spectralꢀnature of the dual keto emission, time resolved area
normalized emission spectra (TRANES) were constructed in dioxane where the interference
from the enol emission is negligible. TRANES reveal the existence of two different emission
bands with an isoemissive point. The emission maxima appear at 455 nm and 465 nm (Figure 4).
The 455 nm emission is associated to 3.9 ns lifetime species and the 465 nm emission is
associated with a 0.8 ns lifetime species. The traces of decays are shown in Figure S6. All decays
were fitted well with two components emission decay (Table S1).
3.2 Role of ground state conformers
Scheme 1. Different possible conformers of bisꢀHPTA.
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H
α
O
O
H
O
O
O
H
H
H
N
N
N
N
N
N
N
N
N
bisꢀHPTAꢀIVb
H
O
H
bisꢀHPTAꢀIV
H
H
H
β
O
O
bisꢀHPTAꢀI
H
O
H
N
N
N
N
N
H
N
N
N
N
bisꢀHPTAꢀIVa
O
O
O
H
H
H
H
H
bisꢀHPTAꢀIII
bisꢀHPTAꢀII
Table 2. Relative ground state energy of different bisꢀHPTA conformers with respect to bisꢀ
HPTAꢀI in THF.
Conformer
Relative Energy (eV)
bisꢀHPTAꢀI
0.000
0.113
0.083
0.119
0.301
0.294
bisꢀHPTAꢀII
bisꢀHPTAꢀIII
bisꢀHPTAꢀIV
bisꢀHPTAꢀIVa
bisꢀHPTAꢀIVb
bisꢀHPTA has two phenolic units. The ꢀOH groups of the phenolic units form intramolecular
hydrogen bonds with the triazole nitrogens. The two six membered cyclic structures formed
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through intramolecular hydrogen bonds differ from each other by the nitrogen that is involved in
the hydrogen bond. We labeled them as α (colored in green) and β (colored in red) (Scheme 1).
α involves the nitrogen that is attached to two carbon atoms. β involves the nitrogen that is
attached to a carbon atom and a nitrogen atom. The torsional rotation results in different
conformers (Scheme 1). However, due to the presence of strong intramolecular hydrogen bonds,
the bisꢀHPTAꢀI would be the most stable conformer. The ground state geometries of different
conformers of bisꢀHPTA were optimized (Table S2ꢀS7, ESI). Theoretical calculations also
support that the bisꢀHPTAꢀI conformer is the most stable conformer (Table 2). The Boltzmann
populations contribution of other conformers at ambient temperature are less than 2% compared
to bisꢀHPTAꢀI. The calculated lowest excitation energy of bisꢀHPTAꢀI in tetrahydrofuran is 305
nm and it agrees well with the experimental value. The ‘OꢀH’ bond lengths in α and β are 0.99 Å
and 0.98 Å, respectively. The distances between the acidic proton and the receptor sp2 nitrogen
are 1.75 Å and 1.80 Å in α and β, respectively. Thus, it appears that the intramolecular hydrogen
bond in the α ring is stronger than in the β ring.
Scheme 2. Conformer in the protic solvents.
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Excitation spectra were recorded at different wavelengths of the visible emission (Figure
5). The complete overlapping of the excitation spectra suggests that both visible emissions
originate from the same ground state precursor. It further rules out the formation of two different
keto through the ESIPT of two different conformers. We recorded the excitation spectra in polar
protic solvent also, to understand the origin of the enol emission (Figure S7, ESI). The excitation
spectra recorded at 350 nm and 460 nm of bisꢀHPTAꢀI emission in methanol are not identical.
This is consistent with the presence two different conformers, bisꢀHPTAꢀI and openꢀenol
(Scheme 2). The formation of strong intermolecular hydrogen bond in protic solvents also
facilitates the formation of anion through the intermolecular proton transfer in the ground state.61
The weak absorbance/excitation band at ~ 340 nm observed in polar protic solvents may be due
to the formation of anion (Figure 1, Figure S7, ESI). To verify our prediction, bisꢀHPTA was
deprotonated further to form anion by adding sodium hydroxide to the methanol solution. The
emission maximum of the anion appears at 400 nm and matches with the maximum of the
emission band obtained upon excitation at 340 nm in methanol and other polar protic solvents
(Figure S8, ESI). The absorption spectrum of the alkaline solution also has band at ~ 340 nm
(ESI, Figure S8).
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1.2
0.9
0.6
0.3
0.0
λ (nm)
430
440
450
460
470
480
490
260
285
310
335
360
Wavelength (nm)
Figure 5. The normalized excitation spectra of bisꢀHPTA in dioxane, recorded at different
emission wavelengths (λ).
3.3. The dual ESIPT and the pathway for the diketo tautomer.
Figure 6. HOMO and LUMO of bisꢀHPTAꢀI, calculated in tetrahydrofuran.
Scheme 3. Plausible ways of single and dual ESIPT in bisꢀHPTA in the excited state.
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*
O
H
N
α
*
O
*
H
N
N
NH
H
O
O
H
K1*
N
N
NH
*
O
O
N
O
H
N
N
H
H
H
β
DK*
E*
N
NH
O
H
K2*
Table 3. Theoretically calculated energiesa (eV) and emission energies (eV) of different keto
forms in THF.
Species
Excited state
Corresponding
Franck Condon
ground state
0.858
Emission energyb
Oscillator strength (f)
K1
K2
DK
3.725
3.411
4.180
2.867 (2.723)
2.050
2.330 (2.667)
0.33
0.07
0.23
1.361
1.850
aThe energies are relative energies with respected enol (bisꢀHPTAꢀI). The values in parenthesis
b
are experimental values.
Since bisꢀHPTAꢀI is the most stable conformer with strong intramolecular hydrogen bonds, the
ESIPT should occur from bisꢀHPTAꢀI. The HOMOꢀLUMO corresponding to the longest
wavelength electronic transition of bisꢀHPTAꢀI is given in Figure 6. Compared to the HOMO,
the electron density on the acceptor triazole unit increases in the LUMO. Among the proton
receptors, the N4 atom in the α ring becomes more electron rich than N2 in the β ring. The
ESIPT processes in α and β rings are labeled as ESIPTꢀI and ESIPTꢀII, respectively (Scheme 3).
The ESIPTꢀI, the ESIPT in α ring, will produce K1 tautomer. Similarly, ESIPTꢀII, the ESIPT in
β ring will produce K2 tautomer. In K1 tautomer conjugation is within one of the donorꢀaccepor
unit, but in K2 the conjugation is more and it is extended over the entire molecule (Chart S1).
TRANES reveal that both the keto emission maxima differ by small extent 455 nm and 465 nm
(Figure 4). This indicates that the second keto emission is not from K2. The proton transfer in
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the β ring (ESIPTꢀII) of the K1 tautomer yields a double tautomerized diketo (DK) tautomer
(Scheme 3). In DK though the conjugation is present in both the of the donorꢀaccepor units, but
unlike in K2 they are separated. Therefore, the emission maximum of DK is expected to differ
less that of K1 than the emission maximum of K2. To confirm further the keto tautomers, K1, K2
and DK of bisꢀHPTA were fully optimized in the excited state using THF as solvent (Table S8ꢀ
S11). The theoretically calculated emission energy of K2 is less than that of DK which is less
than that of K1 (Table 3). The theoretical transition energies of K1 and DK are compared with
the experimental energies (Table 3) and there is a good agreement between theoretical and
experimental emission energies. Therefore, the twoꢀketo emissions in the visible region
correspond to the emissions from K1 and DK. This is also consistent with the existence of the
isoemissive point in the TRANES. It is a rare consecutive twin ESIPT process that produced the
DK tautomer. To further substantiate the twin ESIPT process, the mono methylated derivative of
bisꢀHPTA, HPMPTA (Chart 1) was designed and synthesized. HPMPTA produces a tautomer
emission at 450 nm (Figure S9). Unlike those of bisꢀHPTA, the tautomer emission of HPMPTA
exhibits a monoꢀexponential decay with a fluorescence lifetime of 4.5 ns in dioxane. This
substantiates the conclusion. The probable excited paths of the excited bisꢀHPTAꢀI is shown
Scheme 4. Upon excitation, enol is excited to the FrankꢀCondon state (a higher vibrational level)
of the excited electronic state. The ESIPT process is an ultrafast process.33, 66 The absence of
enol emission in nonpolar solvents suggest that ESIPTꢀI occurs faster than the vibrational
relaxation to produce K1. It appears that ESIPTꢀII competes with vibrational relaxation of K1.
Therefore, the emissions observed from both K1* and DK*. The proton transfers and vibrational
relaxations are seemed to be faster than our instrument’s response time therefore no rising
components are observed for K1* and DK*. Since the ESIPT processes occur from higher
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vibrational state and the emission occurs from the relaxed vibration state (v = 0 state of the first
electronic excited state), the fluorescence lifetimes obtained are the corresponding lifetime of the
emitting states. In ESIPT molecules, torsional rotation of the phenyl ring of the keto tautomer in
the excited state is reported as a possible path of deꢀexcitation.64, 65, 67, 68 The K1* has one such
possibility, but DK* has two such possibilities. This may be the cause of the shorter lifetime of
DK* as compared to that of K1*.
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Scheme 4. The plausible excited state processes of excited enol of bisꢀHPTA in THF along with
the calculated, (relative) energies, transition energies and the oscillator strengths (f). The
transition energies and lifetime in parenthesis are experimental values. Corresponding Franck
Condon states are denoted as FC state
3.4. Annular tautomerism and proton transfer triggered proton transfer
The absence of K2 emission in solution indicates that the ESIPT process through β unit (red in
color, Scheme 2) is prevented by some factor in the solution. This anomalous behavior of the
molecule can be explained with the annular tautomerism that takes place between N1ꢀH and N2
of the triazole system (Scheme 3).69ꢀ73 Due to the presence of electron withdrawing sp2 nitrogen
(N2 and N4), the N1ꢀH proton in the azole ring is fairly acidic and the proton is being shared
between N1 and N2.74, 75 The mobility of the NꢀH proton in 1,2,4ꢀtriazole also produces extra
stabilization through aromatization.76 The intramolecular hydrogen bond in the β unit is strongly
perturbed due to this shuttling of the acidic proton. The shifting of the proton from N1 to N2
provides a less stable tautomer than bisꢀHPTAꢀI, which is basically the bisꢀHPTAꢀIII conformer
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(Scheme 4 and Scheme 1). Eventually, this annular tautomer would switch back to the more
stable bisꢀHPTAꢀI conformer by the torsional rotation of the phenyl rings (Scheme 4). Therefore,
in solution, β ring is weakened and fails to commence the ESIPTꢀII process. However, after the
first proton transfer (when the proton is transferred to N4), the acidity of N1ꢀH decreases and the
efficiency of the annular tautomerism also diminishes. It facilitates the ESIPTꢀII in β. Thereby,
the ESIPTꢀI triggers the ESIPTꢀII. We named this new class of proton transfer as ‘proton transfer
triggered proton transfer’ (PTTPT). In PTTPT a proton transfer, which is otherwise not viable, is
made feasible by the other proton transfer. In bisꢀHPTA, the ESIPTꢀII is blocked because of
annular tautomerism. The ESIPTꢀI weakened the annular tautomerism, thereby strengthened the
intramolecular hydrogen bond in the β ring. This facilitates and triggers the ESIPTꢀII. Therefore,
in bisꢀHPTA the PTTPT is a selfꢀassisted twin ESIPT processes. Unlike in relay proton transfer
where the proton acceptor which donates the proton in subsequent steps, in this PTTPT the
proton acceptor in the first step is not donating the proton in the second step. The first proton
transfer aided the (otherwise not feasible) second proton transfer. All these make this PTTPT a
unique class of proton transfer.
Scheme 4. Annular tautomerism and conformational equilibrium of bisꢀHPTA in solution.
O
O
O
H
H
H
N4
4N
4 N
180o rotation
along vertical axis
Annular Tautomerism
N
N
N
N
N
1
N
O
1
O
2
2
O
2
1 H
H
H
H
H
H
bisꢀHPTAꢀI
bisꢀHPTAꢀIII
3.5. Tautomer emissions in the powder sample
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1.2
0.9
0.6
0.3
0.0
1.3
1.0
460 nm
430 nm
0.7
520 nm
0.4
a
b
0.1
390
490
590
Wavelength (nm)
c
d
385
435
485
535
585
Wavelength (nm)
Figure 7. Emission spectra of bisꢀHPTA powder at different excitation wavelengths (a) 340 nm,
(b) 350 nm, (c) 360 nm, (d) 370 nm. The spectrum ‘a’ was deconvoluted intro three gaussian
spectra (inset).
In solution medium, the ESIPT cannot be initiated in β ring due to annular tautomerism
and it takes place only after the occurrence of the ESIPTꢀI. Such a fast exchange of proton by
annular tautomerism may reduce in the solid. Therefore, we studied the ESIPT process of bisꢀ
HPTA with powder sample. In powder sample, a longer wavelength emission appears at ~520
nm along with other emissions (Figure 7). The emission maximum recorded for the powder
sample is excitation wavelength dependent. The wavelength dependency of the emission spectra
indicates the existence of multiple ground state species. However, our interest mainly lies on the
appearance of the longest wavelength emitting species. A triexponential fluorescence decay with
lifetime 4.0 ns, 0.3 ns and 1.1 ns is obtained for the emission monitored at 430 nm, 460 nm and
520 nm (Table 4). The relative population of 1.1 ns lifetime species increases when monitored at
longer wavelength. The longest wavelength emitting species with fluorescence lifetime 1.1 ns
does not appear in solution. This longest wavelength emission can appear only if the ESIPT
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process is initiated in β ring not in the other ring. In solid state, due to the structural rigidity the
torsional rotation and the annular tautomerism may have been reduced. Subsequently, in solid
state, the proton may be more localized on N1 nitrogen, and ESIPTꢀII also commenced in β ring
directly from the enol form. Therefore, not as in the solution, K2 species is also present along
with K1 and DK in the powder sample. The shortening of lifetime to 0.3 ns may be due to
aggregation induced quenching and the molecular aggregation is reported to enhance the
intersystem crossing in some molecules.77ꢀ80
Table 4. Emission lifetime of bisꢀHPTA powder recorded at different emission wavelengths (λ).
λex = 336 nm. Relative population is provided in the parenthesis.
χ2
0.95
1.00
0.94
λ (nm)
τ1 (ns)
0.3 (34)
0.3 (49)
0.3 (37)
τ2 (ns)
1.1 (35)
1.1 (27)
1.1 (43)
τ3 (ns)
4.0 (31)
4.0 (24)
4.0 (20)
430
460
520
4. Conclusion.
We have reported a new kind of proton transfer ‘proton transfer triggered proton transfer’. It is a
twin ESIPT in which two consecutive ESIPT produce a dual keto tautomer, DK. In bisꢀHPTA
both the protons are transferred from different ring to the acceptors in the same ring i.e. the
triazole. In solution the process is initiated first in the α ring. The ESIPT process cannot
commence from the other phenolic unit due to the annular tautomerism. After the first proton
transfer, ESIPTꢀI, the acidity of the N1ꢀH proton decreases and the second ESIPT, ESIPTꢀII
takes place. However, in solid sample, due to the structural rigidity annular tautomerism is not
efficient enough, and both the phenol unit undergoes ESIPT. Due to the structural resemblance,
the emission maxima of the K1 (monoketo) and the DK species are close to each other. The
experimental findings are supported well by the theoretical predictions.
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Electronic Supplementary Information (ESI). NMR spectra of bisꢀHPTA, HMBB, HPMPTA.
Excitation spectra of bisꢀHPTA in methanol. Emission and fluorescence decay of HPMPTA in
dioxane. Anionic emission of bisꢀHPTA. XYZ coꢀordinates of different ground and excited
tautomer.
ACKNOWLEDGMENT
The authors acknowledge Central Instruments Facility, IIT Guwahati. Saugata Sahu thanks the
Council of Scientific and Industrial Research, India for the fellowship. The authors thank
Professor Jack Saltiel, Florida State University for his valuable comments and suggestions.
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