Frustrated excited state intramolecular proton transfer (ESIPT) in 10-hydroxy-11H-benzo[b]fluoren-11-one: Synthesis and photophysics

Article abstract of DOI:10.1016/j.dyepig.2019.02.031

A new hydroxybenzofluorenone has been designed and synthesised in order to investigate the origin of excited state intramolecular proton transfer reactions in this familiy of compounds. 10-Hydroxy-11H-benzo [b]fluoren-11-one (10-HHBF) does not show dual f

Full text of DOI:10.1016/j.dyepig.2019.02.031

Accepted Manuscript
Frustrated excited state intramolecular proton transfer (ESIPT) in 10-hydroxy-11H-
benzo[b]fluoren-11-one: Synthesis and photophysics
Joanna Piechowska, Gonzalo Angulo
PII:
S0143-7208(18)32063-1
https://doi.org/10.1016/j.dyepig.2019.02.031
DYPI 7365
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 18 September 2018
Revised Date: 18 February 2019
Accepted Date: 19 February 2019
Please cite this article as: Piechowska J, Angulo G, Frustrated excited state intramolecular proton
transfer (ESIPT) in 10-hydroxy-11H-benzo[b]fluoren-11-one: Synthesis and photophysics, Dyes and
Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.02.031.
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ACCEPTED MANUSCRIPT
Frustrated Excited State Intramolecular Proton Transfer (ESIPT) in 10-
Hydroxy-11H-benzo[b]fluoren-11-one: synthesis and photophysics
a
a
Joanna Piechowska* and Gonzalo Angulo
a
Institute of Physical Chemistry Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw,
Poland;
*
corresponding author.
e-mail address: jpiechowska@ichf.edu.pl.
Abstract
A new hydroxybenzofluorenone has been designed and synthesized in order to investigate the origin
of excited state intramolecular proton transfer reactions in this familiy of compounds. 10-Hydroxy-
1
1H-benzo[b]fluoren-11-one (10-HHBF) does not show dual fluorescence in contrast to its well
known analogue 1-hydroxy-11H-benzo[b]fluoren-11-one (1-HHBF). It is suggested that in 10-HHBF
ESIPT is not occuring. The origin of the difference between these isomers is likely to be found in the
increased donor-acceptor distance in 10-HHBF and in the lower stability of its excited state tautomer.
This streses the large impact in the photophysics of this class of materials of subtle structural changes
which must be taken into account for the desingn of single-molecule white light emitters.
Keywords: excited state intramolecular proton transfer, white light fluorescence,
hydroxybenzofluorenone, Fries rearrangement, regioselectivity, spontaneous cyclization

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1
. Introduction
The search of molecules emitting white light has been attracting much attention for
already several decades. They are intended to substitute combinations of multiple emitters in
applications like white organic light emitting diodes (WOLEDs) [1]. There are however not
many examples of single white light emitters reported in the literature [2], moreover,
molecules with high emission quantum yields are still lacking, except for some
heteroaromatics [3].
In single white light emitters two emissive states of relatively close energies are
connected through an excited state intramolecular reaction, usually electron or proton
transfer. Whether the final emission will arise from only one or both of the states is difficult
to predict as the phenomenon depends not only on the states energies, but also on the
kinetic rates of their intrinsic decays with respect to those of the reaction connecting them
[
4]. Even more difficult is to make predictions based on the compounds chemical structures.
Therefore, more examples are required to help rationalizing the occurrence of white light
emission.
Of the several candidates showing excited-state intramolecular proton transfer (ESIPT), 1-
hydroxy-11H-benzo[b]fluoren-11-one (1-HHBF, Chart 1) got our attention: its simple structure
with a relatively straightforward synthesis, consists of a benzofluorenone core and a hydroxyl
group attached to the benzene ring and exhibits broadband emission (CIE 1931 chromaticity
coordinates: (0.30 0.27)) [2c]. This emission consists of two bands originating from two
energetically close excited states (normal and tautomeric forms) after the excitation of one
dominant ground state tautomer.
The work on molecules structurally related to 1-HHBF has to be tracked back to the
findings of ESIPT in 7-hydroxy-1-indanone (
I, Chart 1) [5]. By expanding the ring to 1-hydroxy-
9
H-fluoren-9-one (II, Chart 1) the ESIPT seems to be hindered, though contradicting reports
can be found in the literature about the occurrence of the tautomeric emission [2c,6]. Finally,
in 2011 the group of Chou extended the conjugated system of hydroxyfluorenone (II) from
the benzene ring side, leading to white light emission in 1-HHBF [2c].

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Chart 1 10-HHBF and molecules structurally related to it.
Recently, the study of this family has been expanded to tert-butyl substituted 1-HHBF
molecules [7, 8, 9]. In ref. 7 it was experimentally demonstrated the high sensitivity of the ESIPT to
the inductive–resonance effects of the substituents. The yield of the ESIPT reaction was correlated
with the distance between the oxygen atoms of the donor and acceptor groups in the enol form, in
line with the theoretical predictions by Siskos et al [10]. This adds interest to the study of the
structural modifications of these molecules as their sensitivity to them is relatively large.
There are two different explanations given to the occurrence of ESIPT in this family of
compounds in the literature: the first one reasons that the changes in conjugation and aromaticity
upon ESIPT tune the energy of the tautomeric form [2c, 11], while the second one relies on the
amount of charge that the keto oxygen acquires after excitation of the dominant enolic form in the
ground state [12, 13].
We have synthesised a structural isomer of 1-HHBF, placing the additional benzene ring fused
to that containing the OH group in hydroxyfluorenone: 10-HHBF (Chart 1). The two above
explanations for the occurrence of ESIPT lead to contradicting predictions for this new molecule:
according to the first, it is possible that 10-HHBF does not show ESIPT, while in agreement with the
second the behaviour should be very similar to that of 1-HHBF. A recent theoretical paper has not
found major differences between these two isomers [14].
In this work we present a new synthesis method for 1- and 10-HHBF, their photophysics in a
1
collection of organic solvents and quantum mechanical calculations.
1
The quantum-mechanical calculations are done following the methods tested and recommended for this class
of compounds by previous authors [15, 12, 2c] and are used as a guidance although they are not the major
target of the present work.

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2
. Experimental
.1 Materials and methods
2
The starting materials, 2-naphthoyl chloride, phenol, benzoyl chloride, 1-naphthol and
AlCl , as well as hydrogen chloride solution (2.0 M) in diethyl ether were purchased from
3
Aldrich. The reactions were carried out under Argon (< 1 ppm H O). TLC analyses were carried
2
out using aluminium plates coated with Silica gel 60 F254 from Merck. Column
chromatography was carried out on silica gel 70–230 mesh. Tetrahydrofuran (THF) was dried
by distillation from LiAlH and stored with molecular sieves A3. NMR spectra were recorded
4
on a Varian 400 MHz spectrometer. Chemical shifts (δ ppm) were determined with TMS as
the internal reference, J values are given in Hz.
The solvents used for spectroscopic measurements, cyclohexane (CHEX), amyl ether (AE),
ethyl ether (EE), toluene (TOL), butyl acetate (BA), tetrahydrofuran (THF), acetonitrile (ACN),
methanol (MeOH), were of spectroscopic quality and further dried with molecular sieves
when needed.
Absorption spectra were recorded with a Shimadzu UV 3100 spectrophotometer.
Extinction coefficients were measured for four independently prepared samples by precise
weighting. The measurements of emission spectra were carried out using a Fluorolog-3
Horiba spectrofluorimeter. The luminescence spectra were subsequently corrected by
subtraction of the background due to the solvent (e.g., Raman lines, and second order of the
Rayleigh scattering) and for the sensitivity of the detection. Luminescence quantum yields
were determined using quinine sulfate in 0.1 N H SO as a standard (ΦF=0.54) and corrected
2
4
for the refractive index of the solvent [16]. For quantum yield determinations, optical density
of both the sample and the standard were around 0.05 at the excitation wavelengths. The
quantum yields were determined exciting at wavelengths between 385 and 396 nm, and at
shorter wavelengths to check the independency of the result on the position of the
excitation. Lifetime measurements were performed with a home-made setup equipped with
a Time Harp 100 TCSPC card and a 379 nm pulsed excitation laser from PicoQuant.
Fluorescence decay curves were measured at narrow emission wavelengths after passing
through a monochromator from Spectral Product CM110 by a PMT from Becker&Hickl. The
FWHM for IRF was 0.39 ns. The crystal structure was solved by direct methods SHELXS-2014
[
17] and refined with full-matrix least-squares calculations on F2 using SHELX-20144. All non-
hydrogen atoms were refined anisotropically. The hydrogen atom positions were
geometrically idealized and allowed to ride on their parent atoms. The infrared (IR)
spectroscopy measurements by ATR (Attenuated total reflectance) technique on ZnSe crystals

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were performed on a FTIR SHIMADZU IRTracer-100 spectrometer. Electrospray ionization
high-resolution mass spectra (ESI-HR) were recorded with MALDISynapt G2-S HDMS (Waters
Inc.) mass spectrometer equipped with an electrospray ion source and q-TOF type mass
analyser. ESI-MS spectra were recorded in the negative ion mode.
2
.2 Synthesis and characterization
2
.2.1 2-(2-Hydroxybenzoyl)-naphthalene (2) [18]
A Schlenk tube was charged with phenyl 2-naphthoate (
1
, 100 mg, 0.40 mmol) and a 10 times
the quantity of equimolar mixture of NaCl and AlCl in argon atmosphere. The sealed tube
3
was placed in oil bath preheated to 215⁰C and melted together while stirring. After 4 hours
water was added to hot reaction mixture and the product was extracted with AcOEt (3x).
Combined organic layers were washed with diluted NH Cl and saturated NaCl, dried over
4
Na SO , then filtered. The filtrate was concentrated and purified by flash chromatography,
2
4
1
hexane : dichloromethane (9:1) to afford the product as yellowish powder, 66 mg (66%).
H
NMR (400 MHz, CDCl ) δ 12.03 (s, 1H), 8.17 (s, 1H), 7.88 - 8.00 (m, 3H), 7.77 (dd, J = 8.5, 1.6
3
Hz, 1H), 7.67 (dd, J = 8.0, 1.5 Hz, 1H), 7.49 - 7.64 (m, 3H), 7.10 (dd, J = 8.4, 0.8 Hz, 1H), 6.92 –
1
.85 (m, 1H). C NMR (101 MHz, CDCl ) δ 201.4, 163.2, 136.3, 135.1, 134.9, 133.6, 132.2,
3
6
1
3
30.4, 129.2, 128.3, 128.2, 127.8, 127.0, 125.3, 119.4, 118.7, 118.4.
2
.2.2 2-(4-Hydroxybenzoyl)-naphthalene (3) [19]
A Schlenk tube was charged with phenyl 2-naphthoate (
1
, 104 mg, 0.42 mmol) and a 10 times
the quantity of equimolar mixture of NaCl and AlCl in air atmosphere. The open tube was
3
placed in oil bath preheated to 215⁰C and melted together with stirring. After 4 hours water
was added to the hot reaction mixture and products were extracted with AcOEt (3x).
Combined organic layers were washed with diluted NH Cl and saturated NaCl, dried over
4
Na SO , then filtered. The filtrate was concentrated and purified by flash chromatography on
2
4
silica gel 70-230 Mesh, hexane : ethyl acetate (95:5 (
whitish powder, 34 mg (36%) and product
2
) and 8:2 (
3
)). Product
3
was obtained as
1
as yellowish powder 38 mg (40%). H NMR (400
2
MHz, Acetone-d6) δ 9.25 (s, 1H), 8.27 (s, 1H), 7.97 - 8.09 (m, 3H), 7.86 (dd, J = 8.5, 1.7 Hz, 1H),
1
3
7
.82 (d, J = 8.7 Hz, 2H), 7.54 - 7.69 (m, 2H), 7.01 (d, J = 8.7 Hz, 2H). C NMR (101 MHz,
Acetone-d6) δ 195.3, 162.6, 136.8, 135.9, 133.5, 133.4, 131.4, 130.4, 130.2, 129.0, 128.9,
28.7, 127.7, 126.6, 116.1.
1
2
.2.3 1-Hydroxy-11H-benzo[b]fluoren-11-one (1-HHBF) [2c]

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Pd(II)-catalyzed dehydrogenative cyclization. Under air atmosphere, Pd(TFA) (0.0075 mmol,
2
2
.48 mg, 5 mol%), Ag O (0.22 mmol, 51.8 mg, 1.5 eq), K CO (0.37 mmol, 51.5 mg, 2.5 eq) and
2 2 3
the 2-(2-hydroxybenzoyl)-naphthalene (4) (0.15 mmol, 37 mg) were added into a dry Schlenk
tube. The tube was stopped and vacuumed and filled with Argon for three times. Then
trifluoroacetic acid (TFA), (0.4 mL) was added by syringe. The mixture was stirred under Argon
atmosphere at 140
℃ for 48 h. Then the mixture was cooled down to room temperature and
evaporated in vacuum and further purified by flash chromatography on silica gel with mixture
hexane : dichloromethane (95:5) contaminated with 2.0 M hydrogen chloride solution in
diethyl ether (approximately 5 drop in 100 ml of eluent) to give the product 1-HHBF (6.6 mg,
1
8%) as a yellow solid. H NMR (400 MHz, CDCl ) δ 8.67 (s, 1H), 8.12 (s, 1H), 7.87 (d, J = 8.1 Hz,
1
1
7
1
1
3
H), 7.83 (s, 1H), 7.81 (d, J = 8.1 Hz, 1H), 7.55 (ddd, J = 8.2, 7.0, 1.3 Hz, 1H), 7.47 (ddd, J = 8.1,
.0, 1.3 Hz, 1H), 7.42 (dd, J = 8.4, 7.2 Hz, 1H), 7.19 (dd, J = 7.3, 0.5 Hz, 1H), 6.78 (d, J = 8.3 Hz,
1
3
H). C NMR (101 MHz, CDCl ) δ 195.5, 157.7, 144.3, 138.1, 137.8, 136.8, 133.5, 132.8, 130.8,
3
29.1, 128.9, 127.1, 125.6, 120.22, 120.0, 117.4, 113.0, 77.3, 77.0, 76.7; HRMS (ESI-TOF):
+
+
-1
calcd m/z C H O ([M + H] ) 247.0759 found 247.0755; IR (neet): νmax /cm = 3347, 3049,
1
7
11
2
1
676, 802.
.2.4 10-Hydroxy-11H-benzo[b]fluoren-11-one (10-HHBF)
The Schlenk tube was charged with 1-naphthyl benzoate (
2
8
, 107 mg, 0.43 mmol) and a 10
times the quantity of equimolar mixture of NaCl and AlCl in argon atmosphere. The open
3
o
tube was placed into preheated to 215 C oil bath and melted together with steering. After 4
hours water was added to hot reaction mixture and products were extracted with AcOEt (3x).
Combined organic layers were washed with diluted NH Cl and saturated NaCl, dried over
4
Na SO , then filtered. The filtrate was concentrated and purified by flash chromatography on
2
4
silica gel with mixture hexane: dichloromethane (95:5) contaminated with HCl in Et O to give
2
1
the product 10-HHBF (56 mg, 56%) as a yellow solid. H NMR (400 MHz, CDCl ) δ 9.48 (s, 1H),
3
8
7
.22 (d, J = 8.2 Hz, 1H), 7.70 (t, J = 7.3 Hz, 2H), 7.65 (d, J = 7.6 Hz, 1H), 7.48 – 7.57 (m, 2H),
1
.44 (ddd, J = 8.2, 7.0, 1.2 Hz, 1H), 7.36 (s, 1H), 7.32 (td, J = 7.5, 0.9 Hz, 1H); C NMR (101
3
MHz, CDCl ) δ 195.4, 155.9, 144.4, 138.8, 136.9, 136.3, 134.6, 130.1, 129.0, 128.9, 126.4,
3
–
-
1
2
26.1, 124.4, 124.0, 121.5, 112.7, 112.5. HRMS (ESI-TOF): calcd m/z C H O ([M - H] )
17 9 2
-
1
45.0603 found 245.0603; IR (neet): νmax /cm = 3321, 3053, 1676, 978.
2
.3 Computational Methodology

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The calculations were performed with the Gaussian 09 program [20]. Geometry
optimization for both states, the ground state and the first singlet excited state, of molecules
(
for both tautomers) was carried out using density functional theory (DFT) and time-
dependent density functional theory (TDDFT) respectively, with CAM-B3LYP hybrid function
and the 6-311+G(2d,2p) basis sets. The solvent, cyclohexane, was modelled with IEFPCM
within the SCRF method. No constrains for symmetry, bonds, angles, or dihedral angles were
employed in the geometry optimization calculations. The oscillator strengths are taken from
the calculation of the relaxed excited states.
3
. Results and discussion
Synthesis
3
.1
Scheme 1 Synthesis of 1-HHBF and 10-HHBF.
We planned to access 1-HHBF and 10-HHBF in one pot reaction, through a sequence
of Fries rearrangement of appropriate esters followed by oxidative cyclization. To the best of
our knowledge, such a straightforward and atom-economical strategy was reported only once
for the synthesis of 1-hydroxybenzanthrone directly from 2-naphtyl benzoate, employing 10-
fold excess of the mixture of NaCl and AlCl (1:1) in presence of oxygen [21, 22]. A molten
3
mixture of AlCl and NaCl (2.2 : 1) was also used in cyclization of aryl vinyl ketones to
3
indanones [23]. Moreover, one example of sequential Fries rearrangement and cyclization of
quinol dicrotonate yielding 4,7-dihydroxy-3-methylindan-1-one was shown to proceed under

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similar conditions [23]. Conversely, phenyl benzoate underwent only Fries rearrangement,
without any sign of nascent cyclization [23]. Furthermore, two examples of AlCl -mediated
3
oxidative coupling of diaryl ketones have been reported so far: (3-hydroxy-2-naphthyl)
phenylmethanone to 6-hydroxy-7H-benzo[c]fluoren-7-one [24], and 2-duroylresorcinol to
1
,2,3,4-tetramethyl-8-hydroxyfluorenone [25]. Notably, in both cases starting ketones were
substituted by free OH group.
The synthesis of 1-HHBF (Scheme 1) starts by obtaining phenyl 2-naphthoate (1) according to
the reported procedure [26]. Then, ester 1 was subjected to conditions reported by Kranzlein et al.
[21], however the reaction required a higher temperature (215⁰C) and argon atmosphere to provide
complete selectivity towards ketone 2. (Table 1, Entry 1). Moreover, only ketone 2 resulting from
Fries rearrangement was formed, without even a sign of nascent oxidative cyclization. Under air
conditions at 215⁰C (Table 1, Entry 2) or at a lower temperature (180⁰C) (Table 1, Entry 3) formation
of the mixture of isomers (2 and 3) was observed.
Table 1 Fries Rearrangement of phenyl 2-naphthoate (1) – evaluation of the reaction conditions.
[
a]
Entry NaCl/AlCl
3
Atmosphere
θ [⁰C]
t [h]
Yield [%]
(
1:1) (eq)
(Product)
66 (2)
1
2
3
10
10
10
Argon
Air
215
215
180
4
4
4
40 (2), 36 (3)
30 (2), 36 (3)
Argon
[
a]
isolated yields
To complete the synthesis of the desired 1-HHBF, we envisioned oxidative cyclization
of ketone based on the palladium-catalyzed protocol developed by Shi for synthesis of
fluorenones from benzophenones [20]. Although we observed a high conversion of ketone
2
2
under these conditions, the desired 1-HHBF was isolated in low 10% yield (Table 2, Entry 1).
Extension of the reaction time to 48h only slightly improved the yield to 13% (Table 2, Entry
2
). We have tested various combinations of Pd- catalysts and bases (Table 2, Entries 1-6) of
which Pd(OAc) /Cs CO or Pd(TFA) /K CO performed the best, enabling the isolation of 1-
2
2
3
2
2
3
HHBF in 18% yield (Table 2, Entries 4-5).
Interestingly, decreasing the reaction temperature by only 10⁰C almost completely shuts
down the reactivity, resulting in negligible conversions and yields (Table 2, Entries 7-8). The relatively
low yields of the desired product (accompanied by high conversion of 2) could be attributed to its
decomposition under the conditions of oxidative coupling. It is worth mentioning that severe drop in
yield was also observed by Shi for cyclization leading to hydroxy-substitued fluorenone [27].

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[
a]
Table 2 Pd-catalyzed dehydrogenative cyclization – evaluation of the reaction conditions.
[
b]
Entry Catalyst
Base
θ[⁰C] t [h]
Yield [%]
Conv. [%]
[
c]
1
2
3
4
5
6
7
8
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
K
K
2
CO
CO
3
140
140
140
140
140
140
130
130
24
48
24
44
48
48
21
21
10
71
2
2
3
13
77
2
2
Cs
Cs
2
CO
CO
3
7
33
2
3
18
75
Pd(TFA)
Pd(TFA)
Pd(TFA)
Pd(TFA)
2
K
2
CO
3
18
75
2
2
Cs CO
3
12
73
2
2
K
2
CO
3
Trace
1.5
small
small
2
Cs CO
3
[
a]
reactions were conducted with 1, catalyst (5 mol %), Ag O (1.5 eq), base (2.5 eq), TFA (0.5 ml), Argon
2
[
b]
[c]
atmosphere; isolated yields; original conditions proposed by Shi [27].
As for the synthesis of 10-HHBF (Scheme 1), 1-Naphthyl benzoate
4 was obtained in
analogous way as phenyl naphtoate
leading to ketone , ester underwent the expected sequence of rearrangements followed
by spontaneous cyclization providing exclusively 10-HHBF in good yield (55%) (Table 3, Entry
1. Fortunately, in contrast to Fries rearrangement of 1
2
4
1
3
). The regioselectivity is independent of the atmosphere and temperature (Table 3, Entry 2-
). Interestingly when ester was treated with pure AlCl in chlorobenzene corresponding
4
3
ketone resulting from Fires rearrangement was formed as the sole product [28].
Table 3 One-pot reaction from 1-naphthyl benzoate (4) to 10-HHBF – evaluation of reaction conditions.
[
a]
Entry NaCl/AlCl
1:1) (eq)
3
Atmosphere
θ [⁰C]
t [h]
Yield [%]
(
1
2
3
10
10
10
Argon
Air
215
215
180
4
4
4
55
53
42
Argon
[
a]
isolated yields
3
.2 Spectroscopic Properties
The molecular geometry of 1-HHBF and of 10-HHBF enables the occurrence of
intramolecular hydrogen bonding between the hydroxyl and keto groups. It is to be expected
that in both cases the proton may be transferred from the donor to the acceptor only in the
excited state.
In the case of 1-HHBF, the ESIPT reaction happens in a six-membered ring, and a broadband
dual emission is observed because of the quickly established equilibrium between the excited states
of normal and tautomer forms. The latter form emits with a large Stokes shift. So far the emission of
1
-HHBF has been measured in cyclohexane (Φ = 0.020 + 0.006) and in the solid state (Φ ≈ 0.1) [2c].
F
F
We have further characterized the photophysics of 1-HHBF in other solvents.

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The position of absorption spectra for 1-HHBF is independent of solvent polarity (Fig. 1, c),
although the structure of the band changes. In some solvents an additional band between 450 and
5
00 nm appears, most likely due to water impurities as its intensity depends on the dryness of the
solvent [ESI section 7]. In other words from the absorption spectra it seems that only one tautomeric
form is present in the ground state. In cyclohexane the extinction coefficients at the absorption
-
1
-1
-
maxima amounts to 2635± 170 M cm . This value is at odds with the reported in the past (1300 M
1
-1
cm ) though it fits better to the series published in there [7].
a) b)
c)
d)
Fig. 1 Absorption spectra of 10-HHBF (a), and of 1-HHBF (c) , fluorescence emission spectra of 10-HHBF (b)
and of 1-HHBF (d) in eight solvents: CHEX (cyclohexane, black lines), AE (amyl ether, dark blue lines), EE (ethyl
ether, light blue lines), TOL (toluene, red lines), BA (butyl acetate, light green lines), THF (tetrahydrofuran, dark
green lines), ACN (acetonitrile, grey lines), MeOH (methanol, pink lines). None of the solvents in these
measurements were further dried. The absorption spectra are scaled arbitrarily for a clearer presentation,
while the fluorescence is normalized to the maximum of the blue band. See ESI for a representation in
wavenumbers scale Fig. A.8.
In aprotic solvents dual fluorescence in 1-HHBF is observed independently of the
polarity and mostly the ratio between the bands is affected, with the contribution of the
tautomeric form emission larger the lower the solvent polarity (Fig.1, d). In more polar
solvents, like acetonitrile, this second band is barely visible, and a bathochromic shift of the

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normal form emission is noticeable, indicative of a larger dipole moment in the excited state
of the normal form. In protic methanol just one emission band occurs with a maximum
strongly red-shifted with respect to that of the normal form in acetonitrile.
The total quantum yield of fluorescence for 1-HHBF is in the range of 3% in all solvents
except MeOH (6%) (Table 4) and it slightly increases with polarity. The fluorescence time
decay is about 0.5 ns in cyclohexane and also grows together with the polarity of the solvent
to 2.44 ns in acetonitrile, and 3.1 ns in MeOH. These decay time constants are independent of
the wavelength of emission. We do not observe shorter components due to the limited time
resolution of our set-up (0.39 ns) indicating that the ESIPT rate constants are in the range of
times shorter than that, as measured in the past [2c].
Table 4 Fluorescence experimental data for 1-HHBF and 10-HHBF: Φ - total fluorescence quantum yield,   
F
F
time decay constant of fluorescence, apparent radiative, k , and non-radiative, k , rate constants.
R
NR
[
F
a]
[b]
-1
/ ns
-1
Solvent
Φ
τ
F
/ ns (λem / nm)
k
R
kNR / ns
1
-HHBF
CHEX
AE
0.023
0.025
0.024
0.029
0.029
0.033
0.043
0.063
0.54(447), 0.53(447)
0.75(458), 0.67(578)
0.84(461), 0.81(576)
0.95(470), 0.92(587)
1.14(470), 1.13(582)
1.21(470), 1.17(582)
2.46(493), 2.42(585)
3.10(545)
0.042
0.035
0.029
0.031
0.025
0.027
0.017
0.017
1.809
1.373
1.190
1.044
0.851
0.812
0.392
0.252
EE
TOL
BA
THF
ACN
MeOH
1
0-HHBF
CHEX
AE
0.062
0.063
0.073
0.094
0.104
0.104
0.095
0.162
1.22(445)
1.50(453)
1.96(462)
1.91(464)
1.94(461)
1.93(464)
2.95(475)
0.050
0.042
0.037
0.049
0.053
0.053
0.032
0.080
0.768
0.624
0.473
0.474
0.461
0.464
0.306
0.414
EE
TOL
BA
THF
ACN
MeOH
[
c]
4.46(505)
[
a]
[b]
[c]
Φ measured with λ 385 - 396 nm depending on the solvent; detection wavelength; weighted lifetime
F
ex
average from bi-exponential decay (2.02 ns (0.4) and 6.09 ns (0.6) ).
For the excitation spectra of 1-HHBF we find a good agreement with the absorption
spectra, independently of the emission wavelength in most of the solvents [ESI Fig. A.7a-h].
For THF an additional band appears with an intensity dependent on the emission wavelength
(
ESI Fig. A.7f). This change becomes stronger when setting the emission at wavelengths above
4
80 nm, that is, for the range corresponding to the second emission band.
The absorption spectra of 10-HHBF are very similar in position to those of 1-HHBF. The major
-
difference is that the extinction coefficient of 10-HHBF is close to three times larger (6470 ± 52 M
1
-1
-1
-1
cm ) than that of 1-HHBF (2635± 170 M cm ) (Fig. 1, a vs c). An additional band is observed in
several solvents, except cyclohexane and toluene, if water traces are present (ESI section 7).

ACCEPTED MANUSCRIPT
Dual fluorescence for 10-HHBF is not observed at any solvent polarity (b, Fig.1).
Moreover, the fluorescence emission appears to be from the normal form what is not
characteristic for most of the ESIPT molecules in which a large Stokes shift is expected
(
emission from tautomeric form) [1c]. Such lack of ESIPT was found only in few examples
[
29].
The quantum yield of emission for 10-HHBF (around 7% in low polar solvents) is larger
than that of 1-HHBF (3%), and the time decay constants are longer by a factor of two (Table
4
). Both the quantum yield and the lifetime increase with the polarity of the solvent.
In the fluorescence excitation spectra [ESI Fig. A.6a-h] we observed an additional band,
associated with a second species, in all solvents, even for CHEX when placing the emission
wavelength at 555 nm (the red edge of the emission band). When recorded at emission
wavelengths closer to the blue edge of the fluorescence band, we did not observe additional
bands in the fluorescence excitation spectra.
3
.3 Computational Results
The calculated energies for the excited states of both tautomers of 1-HHBF in
cyclohexane are not the same as in reference 2c, due to the different DFT method used in
here which is a long-range-corrected version of B3LYP, CAM-B3LYP (Tables 5 and 6). When
using B3LYP as in ref. 2c we have obtained very similar results (ESI Table A.2). Once this
checking has been performed we proceeded to calculate also 10-HHBF. The results in
cyclohexane are collected in Table 5. The main result of interest for the occurrence of ESIPT is
the difference in energy between both tautomers in the excited state. According to this
calculation in none of the cases the reaction is thermodynamically favoured. Moreover, the
ESIPT reaction should be easier for 10-HHBF than for 1-HHBF. A slightly different picture
arises from the B3LYP calculation without correction, as performed in ref. 2c: both energy
differences between N* and T* are more similar (3.37 and 4.46 kcal/mol for 10- and 1-HHBF
respectively, see ESI Table A.2).
Both transition energies and oscillator strengths are similar also for 1- and 10-HHBF. In
both cases the oscillator strength for the N form transition is smaller than for that of the T
form.
Therefore, according to these calculations, if just the energy difference between the excited
states is to be corrected, in the manner it was performed in ref. 2c, 10-HHBF should

ACCEPTED MANUSCRIPT
experience a tautomerization reaction in the excited state leading to a strong emission from
the T* state.
Table 5 Calculated energy in relaxed state (E) and oscillator strength (f) for 1-HHBF and 10-HHBF with CAM-
B3LYP in cyclohexane.
1
-HHBF
10-HHBF
-
1
-1
-1
-1
E/kcal mol
∆E/kcal mol
λ/nm (f)
E/kcal mol
∆E/kcal mol
λ/nm (f)
4
3
5
4
60 (0.053)
63 (0.178)
20 (0.314)
47 (0.117)
448 (0.088)
379 (0.190)
517 (0.175)
446 (0.324)
N*
T*
72.92
72.96
74.00
4
.79
1.04
77.70
N
T
1.06
0
16.02
8.95
Additional information can be obtained from the calculated structures. In Table 6 the
distance between the oxygen atoms of the donor and acceptor groups for all the states
relevant to the ESIPT reaction are collected. The experimental distance in the solid state,
obtained from X-ray dispersion, is also provided in the table. Interestingly, the discrepancy
between the experimental and the calculated value is the largest for 10-HHBF. The calculated
distances are quite similar in both excited states, being the largest difference for the excited
tautomers. Again, according to the calculations, if the kinetics of the ESIPT reaction are
controlled by the distance between these groups in the N* state, both molecules should be
reactive.
Table 6 Calculated distance between donor and acceptor oxygen atoms D-A [Å] for optimized geometric
structures in the ground state for normal (N) and tautomer (T) forms and first singlet excited state for normal
(N*) and tautomer ( T*) forms.
1
-HHBF
10-HHBF
2.84 (3.00)
2.80
(
a)
[b]
N
2.86 (2.88)
2.78
N*
T*
T
2.76
2.63
2.69
2.73
[
a]
(b)
reference [30] X-Ray measurements
The frontier orbitals for both molecules are represented in the (ESI Fig. A.15). Taking
into account the different position of the OH group, state by state the structures of 1- and 10-
HHBF are also very similar: in both cases a large increase of the charge density is expected to
happen upon excitation of the N form.

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3
.4 Discussion
The major question rising from the photophysical properties of 1-HHBF and 10-HHBF is
why there is a dual fluorescence in the former and not in the latter. Two possible
explanations, not mutually exclusive, are that either the T* form in 10-HHBF has a very small
radiative rate constant, or that the forward ESIPT rate constant to form it is much smaller
than the decay rate constant of the N* state. Comparing the collected properties for both
molecules might help clarifying this thrilling mystery.
The X-ray data provide with an important clue regarding the strength of the intramolecular
H-bond: in 10-HHBF the distance between the enol O and the keto O is larger than in 1-HHBF
-3.00 vs 2.88 Å (Table 6). This leaves 10-HHBF out of range (2.713-2.919 Å) in the Table 2 of
reference [7] for the compounds of the same family showing ESIPT. However, there is a
fundamental difference in these structures: in 10-HHBF a trimer linked by intermolecular H-
bonds is deduced (ESI Fig. A.14a,b).
1
The H-NMR data are not congruent with the trends observed for the tert-butyl derivatives
[
7] of the chemical shift of the enol H with the strength of the intramolecular H-bond: for 10-
1
HHBF this corresponds to 9.48 ppm (broad signal) (ESI Fig. A.1a,b H NMR), while for 1-HHBF
1
we found 8.67 ppm (narrow signal) (ESI Fig. A.2a,b H NMR). The same is true for the position
in the infrared (IR) spectrum of the OH stretching transition: it is placed at similar
wavenumbers for both molecules within the range typically attributed to the H bonded OH
(
ESI Fig. A.3-4). IR experiments, performed in the solid state and with a large packing of the
molecules, probe the H atom and suggest that in both molecules it is involved in a H-bond,
though don’t allow to discern between intra- and intermolecular classes.
Regarding the structure of the electronic absorption spectra, in apolar solvents it is much
more pronounced for 10-HHBF than for 1-HHBF suggesting a weaker H-bonding for the
former. In polar solvents the structure becomes less pronounced in both molecules. Adding
water to THF has the same effect (ESI Section7).
The extinction coefficient of the least energetic absorption transition is larger by a factor
2
.5 for 10-HHBF than for 1-HHBF in CHEX. On the contrary, the next transition has similar
extinction coefficient in both molecules. This points to a quite important effect of the H-
bonding also in this quantity and being this bond weaker in 10-HHBF than in 1-HHBF.
The sensitivity to water impurities, as shown in the ESI (Fig. A.9-10), in an experiment in
which water was added in a controlled manner to dry THF, is much larger in the case of 10-
HHBF than in that of 1-HHBF. This also points towards a more accessible H-bonding site for

ACCEPTED MANUSCRIPT
the solvent in the former molecule, and therefore to a weaker intramolecular H-bond. This
1
fact could explain the observations in IR and H-NMR if we admit small traces of water in
1
these samples, despite being the H-NMR probe for 10-HHBF much drier than for 1-HHBF (see
1
the region around 1.5 ppm in the H-NMR spectra collected in the ESI Fig. A.1a and Fig. A.2a).
The emission spectra of the normal form shifts to the red in both cases with the solvent
polarity. On the contrary, in 1-HHBF the emission band of the tautomer doesn’t shift and
simply gets weaker with the same change of medium. The former observed shift indicates
that the excited state of the normal form has a larger dipole moment than its ground Franck-
Condon state, while the tautomer dipole moments are similar in both states. The absolute
values of the N* and T* dipole moments cannot be accessed from these measurements,
although a smaller dipole moment for T* in 1-HHBF would be congruent with the decrease of
its emission intensity. This is because its relative energy would then increase in polar media
with respect to the more stabilized N* due to its larger polarity. However, this speculation is
purely based on the relative energies of the states and do not take into account the polarity
effects on the energy barrier for the ESIPT, which could also be important.
In both molecules the quantum yields of fluorescence are quite modest, as it is usually the
case for keto containing chromophores subject to large intersystem crossing to the triplet
state (ISC), in agreement with the El-Sayed rules [31,5]. The quantum yield of 10-HHBF is
larger by a factor of 3 than that of 1-HHBF in all solvents of polarity below that of ACN. The
lifetimes are larger by a factor of 2. From the extinction coefficient in CHEX, the radiative rate
constant of 10-HHB is expected to be larger in its normal form than that of 1-HHBF. Although
it is difficult to access this quantity for 1-HHBF because of the presence of excited state
reaction, we can make a comparison using a solvent in which most of the emission appears to
be from the N* state: ACN. Even in this solvent the ratio φF/τF is about 2 times larger in 10-
HHBF, in line with the mentioned difference in extinction coefficients (Table 4). Then, the
drop in the lifetime and quantum yield of 10-HHBF with reducing the polarity of the medium
can be attributed to the accessibility to the triplet nπ* state and a larger ISC. This would
explain this variation without needing to invoke ESIPT. In summary, the observed behaviour
of the fluorescence of 10-HHBF can be explained assuming a complete lack of ESIPT, rather
than a small radiative rate constant of the tautomer.
The intermolecular H-bonding deserves a discussion apart. We have already
mentioned the effects on the absorption spectra, quite simple to interpret in terms of
formation of a complex, or even total abstraction of the proton of the enol group; the
difference between both molecules, according to the experiment in THF with added water,
(
ESI section 7) relies on the better availability of 10-HHBF to intermolecular H-bonding. The

ACCEPTED MANUSCRIPT
comparison of the emission spectra in THF and MeOH leads us to think that MeOH binds
preferentially to the keto group of 1-HHBF or that the H-bonding to this group affects more
strongly the photophysics of the molecule. This argument is based on the similarity of the
Kamlett-Taft parameters for THF and MeOH, which differ significantly only in the H-bond
donating ability of the solvent (parameter alfa) [32]. On the other hand, the emission bands in
MeOH could be attributed to deprotonated forms of the HHBF molecules (see ESI section 7
for further discussion). The similarities in 1-HHBF and 10-HHBF spectra in this solvent would
be indicative of similar basicities of their keto groups. Therefore, the differences in ESIPT
cannot be attributed to the increased charging of this atom upon excitation of the normal
forms, as suggested in [12,13,15]. It is clear in any case that such an increase in charge density
is rather a necessary condition for the ESIPT to happen. Finally, a representation of the
spectra in the wavenumber scale shows a congruent and continuous shift of the emission
spectra explainable by dipolar solvation and the discussed specific interaction in MeOH (ESI
Fig. A.8).
In summary most of the data point towards a lack of ESIPT in 10-HHBF rather than to a
very low quantum yield of the tautomeric form. Apparently, the O-O distance is larger than in
the 1-HHBF ground state, hindering kinetically the ESIPT reaction. Additionally, it is quite
likely that the reason is a higher energetic cost of the tautomer formation in this molecule
from the normal precursor. This may be explained considering that the structure of 10-HHBF
plays against the ESIPT: such a reaction would lead to an important decrease of the
aromaticity in both the benzene and the naphthol rings (Scheme 2). [2c, 11]. Unfortunately,
the performed TDDFT calculations do not help in this case as they fail to reproduce the
correct energies of the tautomers. This deficiency has been observed in the past with many
different methods [2c,12, 15]. Nevertheless, if the error in the energy of the tautomeric forms
is systematic, and thus correctable by constant factor, the trend obtained from the TDDFT
calculations is an increase of the ESIPT reaction on going from 1- to 10-HHBF. If this is the
case, the explanation for the lack of reaction in the latter molecule should be found in a larger
kinetic barrier than in the former. This would be in agreement with the larger distance
between donor and acceptor groups found in the X-ray measurement, although not in the
calculation.

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Scheme 2 Representative resonant tautomer structures of 10-HHBF and 1-HHBF. 1-HHBF retains the
aromaticity of the naphthalene moiety, 10-HHBF doesn’t. The opposite is true for the benzene moiety,
although in 1-HHBF it is compensated by the extended pi-conjugation with the keto group and the 5-
carbon ring.
4
. Conclusions
We have presented a comparative study of two structural isomers: the known 1-HHBF
and the newly designed 10-HHBF, fluorophores with appropriate structures for ESIPT.
We have found a straightforward and selective method for the synthesis of 10-HHBF, based
on the sequence of Fries rearrangement of 1-naphthyl benzoate followed by spontaneous
cyclization. Conversely, under the same conditions, phenyl 2-naphthoate doesn't provide the
expected 1-HHBF, but undergoes only Fries rearrangement, still however with complete
ortho-regioselectivity.
Both compounds are highly sensitive to water and possibly to other proton donating
agents, as we have observed in absorption and, even more pronouncedly, in emission and
excitation spectra.
1
0-HHBF does not show dual emission, in contrast to 1-HHBF. There are two possible
explanations for this: either there is no ESIPT or the tautomer has a very small fluorescence
quantum yield. From the comparison between the spectroscopic data collected for both
compounds we conclude that the ESIPT reaction in 10-HHBF is not occurring.
The possible reasons for this change of behaviour with the position of the hydroxyl
group would be a larger separation between the donor and the acceptor groups in 10-HHBF
and a less stable excited state tautomer due to a drastic decrease of its aromaticity. Very
small molecular details may control the appearance of secondary emission bands from

ACCEPTED MANUSCRIPT
excited tautomers and much care must be taken when designing new molecules intended as
single emitters for WOLED applications.
Acknowledgements
This work has been supported by the Ministry of Science and Higher Education [IP2014
0
35773]. We thank dr Wojciech Chaładaj for his help with QM calculations.
Appendix A: Supplementary data
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Appendix A
Supporting Information for
Frustrated Excited State Intramolecular Proton Transfer
in 10-Hydroxy-11H-benzo[b]fluoren-11-one: synthesis and photophysics
a
a
Joanna Piechowska* and Gonzalo Angulo
a
Institute of Physical Chemistry Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland; jpiechowska@ichf.edu.pl.
Table of contents
1
2
3
4
5
6
7
. Synthesis
. NMR spectra
. IR spectra
. Crystallographic data
. Absorption spectra vs excitation spectra in different solvents
. Absorption and emission spectra in another representation
. Additional data and discussion about the influence of water in the spectroscopic
measurements
8
9
. Computational Data
. References

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1. Synthesis
O-acylation of phenol in diluted TfOH according to the known procedure [1].
Phenyl 2-naphthoate (1) [2]
O
O
Phenol (263 mg, 2.8 mmol) and 2-naphthoyl chloride (1.07 g, 5.6 mmol, 2 eq.) were dissolved
in 1% TfOH/CH CN (10 ml). The reaction mixture was stirred at room temperature for an
3
hour, then poured into cold water and extracted with ethyl acetate. The organic layer was
washed with 1 M HCl, saturated NaHCO , and saturated NaCl, and dried over Na SO , then
filtered. The filtrate was concentrated and chromatographed SiO 70-230 Mesh, hexane :
3
2
4
2
1
ethyl acetate (95:9) to afford O-acylated products (1) as a white powder, 686 mg (99%). H
NMR (400 MHz, CDCl ) δ 8.79 (s, 1H), 8.20 (dd, J = 8.6, 1.7 Hz, 1H), 8.00 (dd, J = 8.0, 0.6 Hz,
3
1
1
1
H), 7.93 (dd, J = 8.5, 11.4 Hz, 2H), 7.53 - 7.67 (m, 2H), 7.41 - 7.49 (m, 2H), 7.25 - 7.32 (m, 3H);
3C NMR (101 MHz, CDCl3) δ 165.3, 151.1, 135.8, 132.5, 131.9, 129.5, 129.5, 128.6, 128.4,
27.8, 126.8, 125.9, 125.5, 121.8.
1
-naphthyl benzoate (4) [1]
O
O
1-Naphthol (500 mg, 3.47 mmol) and benzoyl chloride (1.2 ml, 10.4 mmol, 3 eq.) were
dissolved in 1% TfOH/CH CN (10 ml). The reaction mixture was stirred at room temperature
3
for an hour, then poured into cold water and extracted with ethyl acetate. The organic layer
was washed with 1 M HCl, saturated NaHCO , and saturated NaCl, and dried over Na SO ,
3
2
4
then filtered. The filtrate was concentrated and purified by flash chromatography on silica gel
0-230 Mesh with hexane: ethyl acetate (95:5) to afford O-acylated product as a colorless
oil 766 mg (89 %). H NMR (400 MHz, CDCl ) δ 8.29 - 8.41 (m, 1H), 7.92 - 7.98 (m, 1H), 7.89
7
4
1
3
(
(
dd, J = 6.9, 2.5 Hz, 1H), 7.78 (d, J = 8.3 Hz, 1H), 7.64 - 7.71 (m, 1H), 7.45 - 7.58 (m, 3H), 7.38
dd, J = 7.5, 1.0 Hz, 1H).

ACCEPTED MANUSCRIPT
1.04
9.48
8
8
7
7.70
7.68
7.66
7.64
7.56
7.56
.23
.21
.72
1
.00
2.00
1.07
2.13
1.07
1.04
1.10
7.54
7.54
7.54
7.53
7.52
7.52
7.51
7.50
7.50
7.46
7.46
7.44
7.44
7.44
7.42
7.42
7.36
7.34
7.33
7.32
7.32
7.30
7.30
7.25
0.00

ACCEPTED MANUSCRIPT
195.41
8
8
.23
.21
1.00
155.93
138.75
136.84
136.26
134.56
130.08
129.03
128.91
126.38
126.08
124.39
123.97
1
21.48
1
12.68
112.49
7
7
7
7
.72
.70
.68
.66
2
.00
.07
1
7.64
77.32
77.20
77.00
76.68
7.54
7.54
7.54
7.53
2
.13
.07
7.52
7.52
7.51
7.50
7.46
7.46
7.44
7.44
1
1
.04
.10
7.36
7.34
7.33
7.32
7.32
1
7
.30
7
.25

ACCEPTED MANUSCRIPT
195.41
1
1
1
0
1
1
1
1
1
.01
.01
.04
.99
.03
.05
.01
.01
.02
8.67
8.12
7.88
7.86
7.83
7.82
7.80
7.57
7.56
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
7
6
6
.55
.55
.54
.53
.53
.49
.49
.47
.47
.47
.45
.45
.44
.42
.42
.40
.26
.20
.19
.18
.18
.79
.77
1
55.93
144.42
138.75
136.84
136.26
134.56
130.08
129.03
128.91
-
0.00
126.38
126.08
124.39
123.97
121.48
1
1
12.68
12.49

ACCEPTED MANUSCRIPT
1
95.54
1
.01
8.67
1
57.74
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
44.30
38.06
37.83
36.81
33.48
32.76
30.81
29.15
28.87
27.10
25.57
20.22
20.03
17.44
13.01
1.01
8.12
7.88
7.86
7.83
7.82
7.80
1.04
0.99
1.03
7
7
7
7
7
7.49
7.49
7.47
.55
.55
.54
.53
.53
7
7
7
7.32
7.00
6.68
1.05
1
1
.01
.01
7
7.47
.47
7
7
7
.44
.42
.42
7
.40
7
.26
7
7
7.18
.20
.19
1
.02
7
.18
6
6
.79
.77

ACCEPTED MANUSCRIPT
1
57.74
1
44.30
1
1
1
38.06
37.83
36.81
1
1
33.48
32.76
1
30.81
1
1
29.15
28.87
1
1
27.10
25.57
1
1
20.22
20.03
1
17.44
1
13.01