Spectral manifestation of protonation of photochromic naphthopyrans

Article abstract of DOI:10.1016/j.dyepig.2020.108833

The formation of proton complexes of the photoinduced colored open form of naphthopyrans in solutions was first discovered and investigated by spectral-kinetic method. The complexes exhibit a new absorption band in the visible region, which is bathochromically shifted relative to the absorption band of the photoinduced colored open form. With an increase in the concentration of perchloric or hydrochloric acid, this long-wavelength absorption band appears immediately after the addition of acids to solutions of photochromic compounds. The protonation efficiency depends on the strength of the acids.

Full text of DOI:10.1016/j.dyepig.2020.108833

Dyes and Pigments 184 (2021) 108833
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Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Spectral manifestation of protonation of photochromic naphthopyrans
Alexander М. Gorelika, Оlga V. Venidiktovaa, Оlga I. Kobelevaa, Тatyana М. Valovaa,
,b,*
Valery А. Barachevskya
a Photochemistry Center FSRC “Crystallography and Photonics” of the Russian Academy of Sciences, Novatorov Str.,7a, Bl.1, Moscow, 119421, Russia
b Interdepartmental Center of Analytical Research of the Russian Academy of Sciences, Moscow, Profsouyznaya Str., 65, Bl.6, Moscow, 117997, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords:
The formation of proton complexes of the photoinduced colored open form of naphthopyrans in solutions was
first discovered and investigated by spectral-kinetic method. The complexes exhibit a new absorption band in the
visible region, which is bathochromically shifted relative to the absorption band of the photoinduced colored
open form. With an increase in the concentration of perchloric or hydrochloric acid, this long-wavelength ab-
sorption band appears immediately after the addition of acids to solutions of photochromic compounds. The
protonation efficiency depends on the strength of the acids.
Photochromism
Naphthopyran
Proton complex
Spectroscopy
Rate constant
Acid
1. Introduction
aryl-substituted naphthopyrans, nitrogen-containing chromenes, when
react with acids, become colored and give rise to a new long-wavelength
The protonation processes of spiropyrans have been most widely
studied to date [1–5]. Studying the protonation of nitro-substituted
indoline spiropyrans has shown that adding acids to solutions of
photochromic compounds results in proton complexes with molecules of
the merocyanine form of spiropyrans. The complexes absorb in a
shorter-wavelength spectral region (400–430 nm) than the merocyanine
form of these compounds (500–650 nm).
absorption band without UV irradiation [21]. N-Protonated pyr-
anoquinolines lose the photochromic properties [22].
This paper describes a spectral-kinetic study of the protonation of
naphthopyrans containing no functional substituents.
2. Experimental
Unlike the protonation of spiropyrans, the protonation of photo-
chromic naphthopyrans (chromenes) has not been adequately studied.
These compounds are of particular scientific and practical interest
because of their use for the manufacture of photochromic ophthalmic
lenses [6–9] and the prospects for their application as chemosensors for
metal ions [10–12], neuromorphic engineering [13], photoswitches,
photocontrolled molecular electronic devices, etc. [7].
3,3-Diphenyl-3H-naphtho[2,1-b]pyran
(I),
3,3-di(4-methox-
yphenyl)-3H-naphtho[2,1-b]pyran (II), and 3,3-di(4-methoxyphenyl)-
3H-anthro[2,1-b]pyran (III) were used in the study as photochromic
compounds (Scheme 2).
Naphthopyrans I and II were obtained using 2-naphthol and 1,1-
diphenyl-2-propyn-1-ol or 1,1-(4,4′-dimethoxyphenyl)-2-propyn-1-ol,
respectively, by heating 2-naphthol with a propynol in toluene at 50–60
oС in the presence of toluene sulfonic acid as a catalyst. Naphthopyran
III was synthesized according to a similar procedure, but using 2-anthrol
as a substrate. Propargyl alcohols were obtained from benzophenone
and 4,4′-dimethoxybenzophenone by procedures commonly used for
reactions of this type. Thus, 1,1-(4,4′-dimethoxyphenyl)-2-propyn-1-ol
was isolated from the reaction of 4,4′-dimethoxybenzophenone with the
ethylenediamine complex of lithium acetylide in anhydrous dimethyl
sulfoxide. 1,1-Diphenyl-2-propyn-1-ol was a commercial chemical.
For preparing solutions, acetonitrile (Aldrich) was used as a solvent.
Photochromic transformations of chromenes are based on reversible
photodissociation of the —C—O— bond in the pyran ring of the initial
colorless cyclic compound A and the subsequent cis-trans isomerization
forming the colored open form B. The back reaction is spontaneous and
is accelerated by heating or by the action of visible light (Scheme 1).
The key studies concerned with the formation of proton complexes of
the photoinduced colored open form B of chromenes addressed the ef-
fect of intramolecular hydrogen bonds on the thermal stability of form B
of functionalized chromenes [14–20]. It was found that, in contrast to
* Corresponding author. Photochemistry Center FSRC "Crystallography and Photonics" of the Russian Academy of Sciences, Novatorov Str.,7a, bl.1, Moscow,
119421, Russia.
E-mail address: barva@photonics.ru (V.А. Barachevsky).
https://doi.org/10.1016/j.dyepig.2020.108833
Received 7 May 2020; Received in revised form 4 August 2020; Accepted 3 September 2020
Available online 7 September 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

A.М. Gorelik et al.
Dyes and Pigments 184 (2021) 108833
Table 1
Spectral-kinetic characteristics of photochromic transformations of naph-
thopyrans I-III in acetonitrile.
Compound
λA max, nm
εА, cmꢀ 1.Мꢀ 1
λB max, nm
ΔDpBhot
kdb, sꢀ 1
0.158
I
302
315
345
358
303
316
347
357
324
344
362
385 sh
402
424
7600
9000
6000
5800
5900
6825
4575
4400
4350
3950
4075
3475
5400
4600
425
0.49
Scheme 1. Photochromic transformations of chromenes.
II
475
485
0.01
0.66
0.869
0.092
The initial concentration of photochromic compounds in the solution
was C = 4 × 10ꢀ 4 M. Protonation processes were studied using citric,
hydrochloric (HCl), and perchloric (HClO4) acids, with naphthopyran to
acid molar ratios being 1:10 and 1:100. When studying the effect of
acids, solutions of photochromic compounds and acids were mixed in a
ratio of 1:1 (by volume). In these mixed solutions, the concentration of
photochromic compounds was constant (C = 2 × 10ꢀ 4 M), while the
concentration of acids varied from C = 2 × 10ꢀ 2 М to C = 2 × 10ꢀ 4 M.
The spectral-kinetic measurements were performed on the Cary 60
UV–Vis spectrophotometer (Varian) in a quartz cell with an optical path
length of 0.2 cm.
III
Note: λmA ax, λmВ ax are the wavelengths of the absorption maxima for initial and
photoinduced forms, respectively;
ε is the molar extinction coefficient at the
absorption maximum of form A; ΔDpBhot is the photoinduced change of absor-
bance at the absorption maximum of the photoinduced form at the photo-
equilibrium; kdb is the rate constant of dark relaxation of absorbance at the
photoinduced long-wavelength absorption maximum.
The UV irradiation of the solutions was carried out with the light of a
LC-4 L8253 xenon lamp (Hamamatsu) through a UFS-1 glass light filter
that transmits UV light.
3. Results and discussion
The spectral-kinetic characteristics of the photochromic trans-
formations of naphthopyrans in acetonitrile are presented in Table 1.
Table 1 shows that the introduction of methoxy groups into the
phenyl moieties of naphthopyran I virtually does not affect the positions
of the absorption bands of the initial closed form A, but leads to a
bathochromic shift of the photoinduced absorption maximum of form B
for compound II by 50 nm. In this case, the rate of dark bleaching of
photoinduced form B increases, which leads to a decrease in the
photoinduced change in optical density at the absorption maximum of
this form located in the visible region.
In the case of compound III, annulation of the benzene ring causes a
bathochromic shift of not only the absorption band of the photoinduced
open form B, but also the absorption bands of the closed isomer A. In
contrast to compounds I and II, this compound is colored in the initial
state, since it absorbs in the 400–450 nm range. It discolors most slowly
in the dark and, therefore, it is characterized by the most pronounced
change in the optical density under UV light.
Fig. 1. Absorption spectra of naphthopyran I in acetonitrile in the absence (1,
3) and in the presence (2, 4) of hydrochloric acid HCl (1:1) before (1, 2) and
after (3, 4) UV irradiation.
Fig. 1 and Table 2 show that, in contrast to the absorption spectra of
the initial colorless closed form A (Fig. 1, curve 1) and photoinduced
colored open form B (Fig. 1, curve 3) of naphthopyran I, in this case, the
addition of hydrochloric acid to the solution gives rise to a new long-
wavelength absorption band with a maximum at 600 nm (Fig. 1,
curve 4). This band is absent in the absorption spectrum of a photo-
chromic solution containing an acid before irradiation (Fig. 1, curve 2).
The intensity of the photoinduced absorption band with a maximum at
420 nm decreases upon addition of the acid compared to that for acid-
free solution (Fig. 1, curve 3). This can be attributed to the formation
of the protonated colored open form, which absorbs in the long-
wavelength spectral range.
Similar spectral changes are also observed in the presence of
perchloric acid (Table 2). With increasing acid concentration in solution,
the magnitude of the photoinduced absorbance at the absorption
maximum of the photoinduced protonated form increases simulta-
neously with an increase in the rate of its dark relaxation to the initial
Scheme 2. Structures of chromenes.
2

A.М. Gorelik et al.
Dyes and Pigments 184 (2021) 108833
Table 2
Table 3
Spectral-kinetic characteristics of naphthopyran I and its protonated complexes
in acetonitrile.
Spectral-kinetic characteristics of naphthopyran II and its protonated complexes
in acetonitrile.
Acid (chromene: acid molar ratio)
λB max, nm
ΔDpBhot
kdb, sꢀ 1
Acid (chromene:acid molar ratio)
λmB ax, nm
ΔDpBhot
kdb, sꢀ 1
–
425
420
600
420
595
595
0.49
0.41
0.02
0.28
0.18
0.44
0.158
–
475
475
620
478
620
480
620
480
620
0.01
0.14
0.30
0.17
0.50
0,62
1.70
0.70
1.80
0.869
0.522
HCl (1:100)
–
HCl (1:10)
HClO4 (1:10)
HClO4 (1:100)
0.173
0.177
0.257
HCl (1:100)
0.520
0.076
0.075
HClO4 (1:10)
HClO4 (1:100)
closed form (Table 1).
Similar photoinduced spectral changes were also observed in solu-
tions of naphthopyran II, which differs from compound I by the presence
of methoxy groups in the p-positions of the benzene rings (Fig. 2,
Table 3). The absorption bands of the colored open form B are bath-
ochromically shifted by 50 nm, and the absorption band of the photo-
induced protonated colored open form B undergoes a bathochromic shift
of 25 nm relative to the corresponding bands of naphthopyran I. The
introduction of the methoxy groups into the molecule increases the in-
tensity of the long-wavelength absorption band of the protonated form.
It can be seen from Fig. 2 that at a 1:10 concentration ratio of
photochromic chromene II and hydrochloric acid, the long-wavelength
absorption band appears immediately after the addition of the acid, i.
e., before UV irradiation of solutions. Its intensity increases with
increasing acid concentration in the solution (Fig. 3, curves 5 and 7).
This is the dark protonated form.
Dark and photoinduced spectral changes of the same type are also
observed for naphthopyran II in acetonitrile in the presence of perchloric
acid HClO4 (Fig. 3, Table 2).
In contrast to naphthopyran I, the rate constant of dark relaxation of
the photoinduced protonated form of naphthopyran II decreases with
increasing acid strength in comparison with the relaxation rate constant
of the photoinduced colored open form B in the absence of an acid in
solution. The constant is not significantly dependent on the acid con-
centration, but decreases sharply with increasing acid strength.
Compound II in acetonitrile, in the presence of HCl and HClO4, shows
reversibility of changes in the intensity of the long-wavelength bands of
the photoinduced protonated complex during alternate UV irradiation
and dark relaxation (Fig. 4). However, visible light irradiation of the
photoinduced colored solution does not change the intensity of these
absorption bands.
Fig. 3. Absorption spectra of naphthopyran II in acetonitrile in the presence of
HClO4 in 1:100 M ratio before (1) and after UV irradiation (2) and during dark
relaxation (3–5).
The absorption band observed in the visible spectral region and
appearing immediately when an acid solution is added to a solution of a
photochromic compound (a dark protonated complex) gradually but
slowly disappears during storage of the solution in the dark (Fig. 5). As
Fig. 4. Kinetics of UV coloration (1) and dark bleaching (2) for naphthopyran II
in acetonitrile in the presence of HClO4 (1:100) measured at a wavelength of
620 nm.
in the case of photoinduced coloration, visible light does not affect the
spontaneous bleaching of the dark protonated complex in the solution.
The results obtained for naphthopyran III are similar to those
observed for compounds I and II (Fig. 6, Table 4). The difference is only
in the shift of the absorption maxima observed in the visible region. The
short-wavelength photoinduced absorption band is shifted bath-
ochromically by 10 nm and the long-wavelength band is shifted by 75
Fig. 2. Absorption spectra of naphthopyran II in acetonitrile in the absence (1,
2) and in the presence of hydrochloric acid for molar ratios of 1 : 1 (3, 4), 1 : 10
(5, 6); and 1 : 100 (7, 8) before (1, 3, 5, 7) and after (2, 4, 6, 8) UV irradiation.
3

A.М. Gorelik et al.
Dyes and Pigments 184 (2021) 108833
Fig. 5. Kinetics of dark coloration and subsequent bleaching of naphthopyran II
in acetonitrile after addition of HClO4 (1 : 100) to the photochromic compound
solution measured at a wavelength of 620 nm.
Fig. 7. Absorption spectra of naphthopyran III in acetonitrile without (1,2) and
with citric acid (3,4) at 1:100 M ratio before (1,3) and after UV irradia-
tion (2,4).
Thus, unlike the proton complexes of spiropyrans, which are char-
acterized by a hypsochromic shift of the absorption band of the mer-
ocyanine form to the UV spectral range, photoinduced protonation of
naphthopyrans is manifested, on the contrary, in the appearance of an
absorption band in the long-wavelength spectral region with
a
maximum more than 120 nm away from the maximum of the photoin-
duced absorption band of chromene observed in the absence of acid in
solution. The intensity of this band increases with increasing acid con-
centration. Photoinduced colored proton complexes experience revers-
ible dark relaxation, but are insensitive to visible light. No absorption
band of the protonated complexes appears when weak citric acid (pKa =
3.1) is added to the solution. The band is observed in the presence of
strong hydrochloric (pKa = ꢀ 9) or perchloric (pKa = ꢀ 10) acid.
As the acid concentration in the solution increases, a similar ab-
sorption band, along with the absorption band of the colored open form
B of chromenes, appears in the absorption spectra of chromene solutions
before irradiation. This is a result of equilibrium shift from the initial
closed form A towards the formation of protonated complexes, which do
not bleach under visible light, but slowly disappear in the dark.
Experimental data indicate that proton complexes are formed in
acetonitrile solutions of chromenes in the presence of strong acids. These
complexes arise as a result of interaction between negatively charged
phenoxide oxygen of the open form and a proton. The photoinduced
proton complexes are thermally unstable, insensitive to visible light, and
are transformed into their original closed form A in the dark. The
colored proton complexes formed in the dark in the presence of high
concentrations of strong acids and possessing spectral properties of the
photoinduced complexes do not exhibit photochromic properties and
are extremely slowly spontaneously bleached. Perhaps, their appearance
is attributable to the formation of associates of proton complexes with a
closed and/or photoinduced open form.
Fig. 6. Absorption spectra of naphthopyran III in acetonitrile in the presence of
HClO4 (1:100) before (1) and after UV irradiation (2) and during dark relaxa-
tion (3–5).
Table 4
Spectral-kinetic characteristics of naphthopyran III and its protonated com-
plexes in acetonitrile.
Acid (chromene: acid molar ratio)
λmB ax, nm
ΔDpBhot
kdb, sꢀ 1
–
485
487
488
480
695
470
695
470
695
470
695
0.66
0.55
0.52
0.63
0.19
0.74
0.95
1.00
1.62
1.10
1.93
0.092
Citric acid (1:10)
Citric acid (1:100)
HCl (1:10)
–
–
0.068
HCl (1:100)
0.066
0.016
0.014
HClO4 (1:10)
HClO4 (1:100)
nm on going from naphthopyran II to compound III in the presence of
perchloric acid HClO4 (1: 100).
When weak citric acid is introduced into the solution of naph-
thopyran III in acetonitrile, the long-wavelength absorption band is not
observed either before or after UV irradiation (Fig. 7, Table 4). Conse-
quently, the efficiency of naphtopyran protonation depends on the acid
strength [20].
4

A.М. Gorelik et al.
Dyes and Pigments 184 (2021) 108833
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CRediT authorship contribution statement
Alexander М. Gorelik: Methodology, Investigation, Synthetic
Investigation. Оlga V. Venidiktova: Methodology, Investigation,
Spectral – Kinetic Investigation, Data curation. Оlga I. Kobeleva:
Methodology, Investigation, Spectral – Kinetic Investigation, Data
curation. Тatyana М. Valova: Investigation. Valery А. Barachevsky:
Project administration, Conceptualization, Investigation, Writing -
original draft, Writing - review & editing, Funding acquisition.
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ligands: insights into mutual relations between complexing and photochromic
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Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Acknowledgments
This work was supported by the Ministry of Science and Higher
Education within State Assignments of the Interdepartment Centre of
Analytical Study, RAS (in the part of studying the processes of proton-
ation of chromenes) and FSRC “Crystallography and Photonics”, RAS (in
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compounds).
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108833.
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