The photochromic, electrochemical and third-order nonlinear optical properties of the novel naphthopyran derivatives

Article abstract of DOI:10.1016/j.dyepig.2020.108689

A series of novel naphthopyran derivatives featuring nonplanar and spiro-conjugation structure have been successfully synthesized and characterized. In this paper, we investigated the effects of the electron donating ability and volume of substituents on the photochromic and third-order nonlinear properties of novel naphthopyran derivatives. The UV–Vis absorption spectroscopy was used to analyse the photophysical and photochromic properties. Under the dual role of electron-donating group and bulky substituent group, maximum absorption peaks (λ_max) of the closed form and the open-form of the naphthopyran derivatives were both red shifted, and all molecules showed relatively good photochromic properties and fatigue resistance. The electrochemical properties have been investigated by the cyclic voltammetry (CV) measurement, the band gap of NP-C, NP-O and NP-N narrowed in sequence, and the level of HOMO decreased in turn. The third-order nonlinear optical (NLO) properties of the compounds were studied by the Z-scan techniques at 532 nm, which indicated that all molecules showed the reverse saturable absorption (RSA) behavior and the optical susceptibility χ⁽³⁾ reached 10^?14. These interesting findings provide a new insight on the practical application of naphthopyran molecules and the design of the NLO materials.

Full text of DOI:10.1016/j.dyepig.2020.108689

Journal Pre-proof
The Photochromic, Electrochemical and Third-order Nonlinear Optical Properties of
the Novel Naphthopyran Derivatives
Pengzhen Zhao, Hong Gao, Dong Wang, Zhou Yang, Hui Cao, Wanli He
PII:
S0143-7208(20)31386-3
DOI:
https://doi.org/10.1016/j.dyepig.2020.108689
DYPI 108689
Reference:
To appear in: Dyes and Pigments
Received Date: 15 April 2020
Revised Date: 19 June 2020
Accepted Date: 3 July 2020
Please cite this article as: Zhao P, Gao H, Wang D, Yang Z, Cao H, He W, The Photochromic,
Electrochemical and Third-order Nonlinear Optical Properties of the Novel Naphthopyran Derivatives,
Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.108689.
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Author statement
Manuscript title: The Photochromic, Electrochemical and
Third-order Nonlinear Optical Properties of the Novel
Naphthopyran Derivatives
I have made substantial contributions to the conception or design of the
work; or the acquisition, analysis, or interpretation of data for the work; AND
I have drafted the work or revised it critically for important intellectual
content; AND I have approved the final version to be published; AND
I agree to be accountable for all aspects of the work in ensuring that
questions related to the accuracy or integrity of any part of the work are
appropriately investigated and resolved.
All persons who have made substantial contributions to the work reported
in the manuscript, including those who provided editing and writing assistance
but who are not authors, are named in the Acknowledgments section of the
manuscript and have given their written permission to be named. If the
manuscript does not include Acknowledgments, it is because the authors have
not received substantial contributions from nonauthors.
Pengzhen Zhao
Corresponding author:
Name: Dong Wang
E-mail: wangdong@ustb.edu.cn
11th May, 2020

The Photochromic, Electrochemical and Third-order Nonlinear Optical
Properties of the Novel Naphthopyran Derivatives
Pengzhen Zhao a, Hong Gao*b, Dong Wang*a, Zhou Yanga, Hui Caoa and Wanli Hea
a School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
b Division of Material Engineering, China Academy of Space Technology, Beijing, 100094, PR China
A series of novel highly efficient photochromic naphthopyran dyes were synthesized and the photochromic
behavior, electrochemical and third-order nonlinear optical properties of naphthopyran derivatives were fully
investigated.

The Photochromic, Electrochemical and Third-order Nonlinear Optical
Properties of the Novel Naphthopyran Derivatives
Pengzhen zhaoa, Dong Wang*a, Hong Gao*b, Zhou Yanga, Hui Caoa and Wanli Hea
a School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
b Division of Material Engineering, China Academy of Space Technology, Beijing, 100094, PR China
Abstract
A series of novel naphthopyran derivatives featuring nonplanar and spiro-conjugation structure have been successfully synthesized and characterized. In this
paper, we investigated the effects of the electron donating ability and volume of substituents on the photochromic and third-order nonlinear properties of novel
naphthopyran derivatives. The UV-Vis absorption spectroscopy was used to analyse the photophysical and photochromic properties. Under the dual role of
electron-donating group and bulky substituent group, maximum absorption peaks (λmax) of the closed form and the open-form of the naphthopyran derivatives
were both red shifted, and all molecules showed relatively good photochromic properties and fatigue resistance. The electrochemical properties have been
investigated by the cyclic voltammetry (CV) measurement, the band gap of NP-C, NP-O and NP-N narrowed in sequence, and the level of HOMO decreased
in turn. The third-order nonlinear optical (NLO) properties of the compounds were studied by the Z-scan techniques at 532 nm, which indicated that all
reached 10-14. These interesting findings provide a new
ꢀ ꢂ
ꢁ
molecules showed the reverse saturable absorption (RSA) behavior and the optical susceptibility χ
insight on the practical application of naphthopyran molecules and the design of the NLO materials.
Key words: Naphthopyran, Photochromic properties, Third-order NLO properties, Electron donating ability
formation of the TT isomer is to prevent the C=C isomerization
of the short-lived TC isomer to the more stable TT isomer.
Sousa reported that the formation of the TT form can be
suppressed by incorporating an alkyl bridge between the pyran
ring and the naphthalenic ring[20, 22, 23]. Yuki Inagaki found
that the amount of the undesirable long-lived colored TT form
could be reduced by introducing an alkoxy group at the
1-position of 8H-pyranoquinazolines. The alkoxy group
effectively reduced the formation of the TT form due to
C-H···O intramolecular hydrogen bonding in the TC form[6].
1. Introduction
Photochromism of chromenes was first reported by Becker and
Michl in 1966[1]. Then the photochromic materials have
attracted widespread research interest because of the physical
and chemical properties could be changed by light irradiation,
Among the classes of organic photochromic materials,
azobenzenes and diarylethenes, spiropyrans and naphthopyran
have received the most attention because of their excellent
performance and broad utility. The photochromic materials
have been used in many fields, such as optical switches[2-5,
35], photochromic lenses[6, 7], photochromic fibers and
dyes[8-10], optical storage[11-13, 33] and molecule
imaging[14-16], optical actuators[33]. Naphthopyrans as an
important class of photochromic molecules which exhibit a
reversible color change when exposed to UV light and are well
known for the commercial application in the ophthalmic lens
industry[17].
Céu M. Sousa synthesized
a polycyclic photochromic
fused-naphthopyran which could produce a single colored
photoisomer under UV irradiation, and in the dark, the single
colored photoisomer returned to the uncolored much faster than
the parent unfused naphthopyran.[31] Stuart Aiken reported a
series of bi-naphthopyrans, recorded the absorption spectra at
lower temperature, bi-naphthopyrans showed an appreciable
shoulder on the main absorption band accompanied by a change
in colour. [32]
The exposure of the closed form (CF) of naphthopyrans to
UV light or sunlight caused the opening of the C(sp3)-O bond
of the pyran ring, that generates two isomers, the transoid-cis
In this paper, we synthesized a series of novel highly
efficient photochromic naphthopyran derivatives with different
substituents, in order to study the effects of the volume of
substituents and the electron donating ability of substituents on
the photochromism, electrochemistry and third-order nonlinear
optical properties of naphthopyran derivatives. Especially the
(TC) and transoid-trans (TT) forms. As shown in Scheme 1
The TC form is converted to the TT form by the rotation of the
C bonds[18]. The TC form thermally returns to CF in a few
,
C=
seconds/minutes, whereas the thermal reversion of TT to CF is third-order nonlinear optical properties of naphthopyrans,
much slower (minutes/hour) because TT is thermostable and because in our previous work, we studied the third-order
the relatively large activation energy isomerization barrier to nonlinear properties of spiropyrans. Hong Zhao designed and
TC must be overcome[6, 19]. Thus, although the coloration is synthesize two new photochromic cyclometalated (N^C^N)
very fast, the decolorization process is generally slower with an
initial rapid discoloration and then a slower decolorization step,
resulting in a residual color that lasts for a few minutes/hours
.
The naphthopyrans with large optical density and fast return
to the uncoloured closed form is a key requirement for their
industrial application[20]. The slow thermal reversion of TT to
CF and the residual colour of the TT form are considered to be
one of the key issues that photochromic lenses and optical
switches applications need to solve[6, 7, 18, 21]. One way to
overcome the problem of the residual colour and avoid the
Scheme 1 Photochromic reactions of naphthopyran

platinum(II) complexes, and the efficient photochromism
allowed a significant NLO photomodulation, both in solution
and in thin films[34]. we hope to explore this non-planar
naphthopyrans with spiro conjugated whether the molecule has
good third-order nonlinear performance. The volume of
substituents and the electron donating ability of substituents on
photochromism, electrochemistry and third-order nonlinear
optical properties of naphthalpyran derivatives were fully
studied in this paper.
Scheme 2
.
Synthesis
of
3,3-Diphenyl-8-[2-(trimethylsilyl)ethynyl]-3H-naphtho
[2,1-b]pyran (2)
2
was prepared according to literature procedures[24]. 1 (1652
mg, 4.00 mmol) and ethynyltrimethylsilane (784 mg, 8.00
mmol) were added in the solvent of triethylamine (70 mL).
After the mixture degassed under Ar. for 40 min, catalytic
agents Pd(PPh3)2Cl2 (563 mg, 0.80 mmol) and CuI (8.2 mg,
0.042 mmol, 3 mol%) were then added. The reaction mixture
was stirred for 24h at 70 °C under Ar. atmosphere. After the
evaporation of the solvent, the crude mixture was purified by
2. Experimental
silica gel column chromatography to give
2 as a white solid
All the reagents were purchased as reagent grade from
1
(1.38 g, 80%). Rf (dichloromethane: petroleum ether = 1:2):
commercial sources and used without further purification. H
1
0.45. H NMR (500 MHz, Chloroform-d): δ 7.91-7.85 (m, 2H),
NMR spectra were measured on a BRAKER AVANCE III HD
NMR spectrometer (500 MHz) at 20 °C. Chemical shifts were
reported in ppm downfield from SiMe4, using the solvent's
residual signal as an internal reference. FT-IR spectroscopy was
recorded on a Perkin Elmer LR-64912C Fourier transform
infrared spectrometer. All MALDI-TOF-MS spectra were
measured on a Shimadzu AXIMA-CFR mass spectrometer.
Elemental analyses were performed at the Institute of
Chemistry Chinese Academy of Sciences, with a Flash EA
1112 instrument. The UV/Vis spectra were recorded in a quartz
7.61 (d, J = 9.0 Hz, 1H), 7.55-7.48 (m, 5H), 7.38-7.32 (m, 4H),
7.30-7.26 (m, 3H), 7.24-7.21 (m, 1H), 6.29 (d, J = 10.0 Hz,
1H), 0.32 (s, 9H) ppm. The reaction phenomenon and chemical
shifts of the resulting product are consistent with previous
literature.
procedures[25].
1 was also prepared according to literature
Synthesis of 8-Ethynyl-3,3-diphenyl-3H-naphtho[2,1-b]
pyran (3)
cuvette on
a
JASCOV-570 spectrophotometer. Cyclic
3
was synthesized according to the previous literature[24]. To a
voltammetric measurements were carried out in a conventional
three-electrode cell using glassy carbon working electrodes of 2
mm diameter, a platinum wire counter electrode, and an
Ag/AgCl reference electrode on a computer-controlled CHI
660C instrument in dichloromethane/Bu4NPF6 at room
temperature. All potentials were referenced to the
ferrocenium/ferrocene (Fc/Fc+) couple used as an internal
standard. The NLO properties response measured by means of
the Z-scan technique, employing 15 ps laser pulses at 532 nm
delivered by a mode-locked Nd:YAG laser, the intensity of the
laser was 1 uj. The linearly polarized laser beam was focused
with a 300 mm focal length lens and the sample was moved
stirred solution of 2 (1385 mg, 3.22 mmol) in CH2Cl2 (25 mL),
A solution of KOH (540 mg, 9.66 mmol) in MeOH (17 mL)
was added dropwise. And then stirred for 3 h at room
temperature in the dark. The solvent was evaporated and the
crude mixture was purified by silica gel column
chromatography to give 3 as a white solid (1029mg, 89%), Rf
1
(dichloromethane: petroleum ether = 2:3): 0.50. H NMR (500
MHz, Chloroform-
1H), 7.51 (dd,
7.38-7.32 (m, 4H), 7.28 (d,
1H), 7.22 (d, = 9.0 Hz, 1H), 6.29 (d,
d
): δ 7.91-7.85 (m, 2H), 7.61 (d,
= 8.5, 1.5 Hz, 1H), 7.49-7.45 (m, 4H),
= 3.0 Hz, 2H), 7.25 (d, = 1.0 Hz,
= 10.0 Hz, 1H), 3.10 (s,
J = 9.0 Hz,
J
J
J
J
J
1H)ppm. The appearance and chemical shifts of the resulting
product were consistent with previous literature[24].
across the focus by means of
a computer controlled
micrometric translation stage. All the samples were studied
from a 10-4 mol/L solution in dichloromethane, the thickness of
the colorimetric dish was 1 mm. In addition, the NLO property
of pure dichloromethane was also measured at the same
conditions to clarify the solvent contribtion.
Synthesis
of
4-((3,3-diphenyl-3H-benzo[f]chromen-8-yl)ethynyl)-N,
N-dihexadecylaniline (NP-N)
3
(790 mg, 2.21 mmol) and N,N-Dihexadecyl-4-iodoaniline
The synthetic routes and molecular structures are shown in
(1472 mg, 2.21 mmol) were added in Et3N (20 mL). After the
Scheme 2 Chemical structures and synthetic routes of NP-N, NP-O, NP-C.

mixture degassed under Ar for 40 min, catalytic agents
Pd(PPh3)2Cl2 (110 mg, 0.16 mmol) and CuI (31 mg, 0.16
mmol) were added successively in and stirred overnight under
Ar at 90 °C. After cooling to room temperature, and the Et3N
was evaporated, the residue was purified by silica gel column
chromatography to give NP-N as a light-yellow solid (1.506 g,
76%). Rf (PE:DCM = 3:2): 0.75; 1H NMR (500 MHz,
Chloroform-d): δ 7.87-7.81 (m, 2H), 7.57 (d, J = 8.5 Hz, 1H),
7.52 (dd, J = 8.9, 1.5 Hz, 1H), 7.49-7.44 (m, 4H), 7.38 (d, J =
9.0 Hz, 2H), 7.30 (t, J = 7.5 Hz, 4H), 7.24-7.22 (m, 2H), 7.22-
7.19 (m, 1H), 7.17 (d, J = 9.0 Hz, 1H), 6.56 (d, J = 9.0 Hz, 2H),
6.24 (d, J = 10.0 Hz, 1H), 3.30-3.19 (m, 4H), 1.56 (s, 4H), 1.28
(d, J = 21.5 Hz, 52H), 0.87 (t, J = 7.0 Hz, 6H)ppm; FT-IR (KBr,
Fig.
in THF before UV irradiation.
cm-1) :
v = 3060, 2923, 2852, 2200, 1604, 1519, 1465, 1371,
1.7 Hz, 1H), 7.50 (d,
J
= 7.5 Hz, 5H), 7.40 (d,
= 7.5 Hz, 2H), 6.81 (d,
= 10.0 Hz, 1H)ppm; FT-IR (KBr, cm-1):
= 3386, 3060, 2955, 2925, 2853, 2204, 1893, 1633, 1605,
J = 8.5 Hz, 2H),
1266, 1245, 1219, 1184, 1089, 1007, 812, 755, 730, 698;
MALDI-TOF-MS (dithranol): m/z: calcd for C65H87NO: 897.7
g mol-1, found: 896.5 g mol-1 [MH]+; elemental analysis calcd
(%) for C65H87NO (897.7): C 86.90, H 9.76, N 1.56, O 1.78;
found: C 86.92, H 9.74, N 1.56, O 1.78.
7.36 (t,
J
= 7.5 Hz, 5H), 7.27 (t,
J
J =
8.5 Hz, 2H), 6.64 (d,
v
J
1582, 1512, 1465, 1447, 1379, 1265, 1245, 1220, 1168, 1091,
1054, 1006, 944, 891, 833, 761, 733, 699, 639, 582, 534;
MALDI-TOF-MS (dithranol): m/z: calcd for C33H22O2: 450.2
g·mol-1, found: 450.5 g·mol-1 [MH]+; Elemental analysis calcd
(%) for C65H87NO (450.2): C 87.98, H 4.92, O 7.10; found: C
87.97, H 4.92, O 7.11.
Synthesis
of
4-((3,3-diphenyl-3H-benzo[f]chromen-8-yl)ethynyl)
phenol (NP-O)
3
(345 mg, 0.96 mmol) and 4-iodophenol (212 mg, 0.96 mmol)
were added in the mixed solvent of triethylamine (16 mL).
After the mixture degassed under Ar for 40 min, catalytic
agents Pd(PPh3)2Cl2 (50 mg, 0. 07 mmol) and CuI (14 mg, 0.07
mmol) were added successively in and stirred overnight under
Ar at 90 °C. After cooling to r. t. and the solvent was
evaporated, the residue was purified by silica gel column
chromatography to give NP-O as a white powder with a little
purple (317 mg, 73%). Rf (PE:EAC = 3:1): 0.40; 1H NMR (500
Synthesis
of
3,3-diphenyl-8-(p-tolylethynyl)-3H-benzo[f] chromene
(NP-C)
3
(358 mg, 1.00 mmol) and 4-iodotoluene (218 mg, 1.00 mmol)
were added in triethylamine (Et3N/THF=1:1, 12 mL), 20 mL).
After the mixture degassed under Ar for 40 min, catalytic
agents Pd(PPh3)2Cl2 (50 mg, 0.07 mmol) and CuI (14 mg, 0.07
mmol) were added successively in and stirred overnight under
Ar at 90 °C. After cooling to r. t. and the solvent was
MHz, DMSO-
d
6): δ 9.93 (s, 1H), 8.12 (d,
J
= 9.0 Hz, 1H),
= 8.5,
8.05-7.99 (m, 1H), 7.83 (d,
J
= 9.0 Hz, 1H), 7.55 (dd,
J
Fig. 2 The UV-Vis absorption spectra and the images of the solution of (a) NP-N, (b) NP-O and (c) NP-N in THF (5x10-5 M) before UV irradiation, under UV
irradiation (365nm, 100mW/cm2) and after visible light irradiation. (d) The UV-Vis absorption spectra of the three molecules in THF (5x10-5 M) under UV
irradiation.

evaporated, the residue was purified by silica gel column
chromatography to give NP-C as a white solid (320 mg, 71%).
1
Rf (PE:DCM = 2:1): 0.60; H NMR (500 MHz, Chloroform-
d
):
J
=
δ 7.91 (d,
= 8.5, 1.8 Hz, 1H), 7.53-7.47 (m, 5H), 7.46 (s, 1H), 7.34 (t,
7.5 Hz, 4H), 7.31-7.26 (m, 3H), 7.23 (d,
(d, = 7.5 Hz, 2H), 6.29 (d, = 10.0 Hz, 1H), 2.39 (s, 3H)ppm;
FT-IR (KBr, cm-1) :
= 3059, 3028, 2920, 2851, 1900, 1629,
J = 8.5 Hz, 2H), 7.63 (d, J = 9.0 Hz, 1H), 7.57 (dd,
J
J
= 8.5 Hz, 1H), 7.18
J
J
v
1580, 1511, 1492, 1465, 1447, 1381, 1353, 1266, 1246, 1217,
1183, 1158, 1084, 1054, 1006, 954, 889, 812, 757, 735, 699,
638, 523; MALDI-TOF-MS (dithranol): m/z: calcd for
C34H24O: 448.2 g·mol-1, found: 447.5 g·mol-1 [MH]+; elemental
analysis calcd (%) for C34H24O (448.2): C 91.04, H 5.39, O
3.57; found: C 91.05, H 5.38, O 3.57.
Fig. 3 The images of the films of PMMA doped with NP-N, NP-O and NP-C (3 wt%)
before UV irradiation, under UV irradiation (365nm, 100mW/cm2, 5min) and after
visible light irradiation
3. Results and discussion
3.1 Photochromic behavior
NP-N showed a more obvious blue shift. With the enhancement of
the electron donating ability of substituents, the absorbance of NP-C,
NP-O and NP-N decreased in turn, the decrease in absorbance of
NP-N was more obvious, the stronger the electron donating ability
of substituents, the more difficult it is to transform from closed-form
to open-form, because the steric repulsion of the bulky substitutes, it
was more difficult for NP-N to switch from closed-form to
open-form.
Furthermore, the photochromic behavior of the
polymer-doped films were investigated. The films of PMMA
doped with the three novel molecules (3 wt%) were prepared by
mold method, as shown in Fig. 3, the films exhibitd the same
color change as in the solvent, the three novel molecules also
behaved obvious photochromism, these molecules maybe be
used in photochromic lenses, dyes and nail equipment.
Fig. 1 showed the absorption spectra and the images of the
solution of molecules in THF. By comparing the absorption
spectra of NP-N
maximum absorption peaks (λmax
,
NP-O and NP-C, it was found that the
of the closed-loop
)
naphthopyran derivative shifted from NP-C (300 nm) to NP-O
(304 nm) and NP-N (348 nm). Compared with NP-C, the
absorption tails of the closed form of the NP-N showed a
bathochromic shift of about 30 nm, because of the enhancement
of the electron donating ability of the substituents which could
conduce to the compounds becoming a more powerful and
complicated intramolecular charge-transfer (ICT). As shown in
the UV−Vis absorption spectra of the closed form of NP-N, the
introduction of an amine group on the 3-position induced an
intense absorption band in the region of 250-400 nm.
3.2 Reversibility and fatigue resistance
After stable ultraviolet irradiation (365 nm, 100 mW/cm2), the
solutions of NP-N, NP-O and NP-C in THF appeared
photochromism, the naphthopyran derivatives changed from the
closed-form to the open-form. As shown in Fig. 2(a), (b) and (c), the
color of the THF solution of NP-N in THF changed from colorless
to red, and that of the NP-O and NP-C changed from colorless to
orange. By comparing the UV-Vis absorption spectra of the
solutions before UV irradiation, under UV irradiation and after
visible light irradiation, it was found that all three molecules could
return to their original state after visible light irradiation, The THF
solutions of these compounds actually showed bright and deep color
changes before and after UV irradiation (Movie S1).
The open-form of the novel naphthopyran derivatives showed
an intense absorption band in the region of 400-600 nm, the
maximum absorption peaks (λmax) of NP-C, NP-O and NP-N
are 466 nm, 472 nm and 502 nm, respectively. All molecules
behaved relatively good photochromic properties. As shown in
Fig. 2 (d), by comparing the UV-Vis absorption spectra of the
open-form of the 3H-naphthopyran derivatives, it was found that the
maximum absorption peaks (λmax) of the open-form of molecules
bathochromically shifted from NP-C (466 nm) to NP-O (472 nm),
and NP-N (502 nm), which could be reflected by the photography of
products in Fig. 2 (a) , (d) and (c), the color of the solutions had
hanged from orange to red, because the enhancement of the electron
donating ability of substituents which could conduce to the
compounds becoming a more powerful and complicated ICT
systems. The electron donating ability of alkylamine group was
obviously stronger than that of hydroxyl group and methyl group, so
Reversibility and stability are important factors for the application of
naphthopyran. As portrayed in Fig. 4 (a), (b) and (c), The
molecules were irradiated by ultraviolet light and visible light. After
ten cycles of irradiation, the decreases of absorbance of the three
molecules were almost negligible, so the NP-N, NP-O and NP-C
performed relatively good fatigue resistance. The thermal
fading curves after ceasing the UV irradiation were showed in
Fig. 4 (d), the thermal fading curves after ceasing the light
irradiation consisted of an initial fast decay attributable to the
thermal isomerization of the TC form to the CF and a following
slow decay of the long-lived TT form. On the thermal fading
curves, the absorbance of NP-C
turn. As shown in Fig. 4, in the initial rapid decay phase, it
could be inferred that the decay rates of NP-O NP-N and
, NP-O and NP-N decreased in
,
Fig. 4 Reversibility of (a) NP-N, (b) NP-O and (c) NP-C in THF solution upon
UV-Vis cycles. (d) Time variation of the change in absorbance at λmax of NP-N, NP-O
and NP-C after continuous UV irradiation (365 nm, 100 mW/cm2, 20 s) at 298 K.

NP-C decreased in sequentially, the reason for the phenomenon repulsion of the bulky substitutes.
was strongly related to the half-life of the colored isomers. The
half-lives (τ1/2) of the TC form were calculated from the fitting 3.3 Electrochemical properties
curves of the time variation of the absorbance using the double
In order to investigate the electrochemical properties of the
exponential function below[6, 8]:
closed-form of the novel molecules, the cyclic voltammetry
(CV) were performed in dichloromethane solution containing
0.1 M Tetrabuty-lammonium hexafluorophosphate (Bu4NPF6)
as the supporting. Electrolyte (a standard ferrocene/ferrocenium
(Fc/Fc+) redox system as the internal standard) at room
temperature. The HOMO and the LUMO energy levels of the
molecules could be calculated by the following equations.
ꢃ ꢄ = Aꢅꢆꢇꢈ ꢊ + Aꢋꢆꢇꢈ ꢊ
ꢀ1ꢂ
ꢉ
ꢌ
ꢀ ꢂ
Where ꢍꢅ is the rate constant for the thermal back reaction
of the TC form and ꢍꢋ is that of the TT form, Aꢅ and Aꢋ are
the initial absorbances of the two species, the formation ratios
of the TT form are defined as A2/(A1+A2) by assuming the
molar extinction coefficients of the TC and TT forms are same.
ꢑꢖ
ꢐ
ꢚ
HOMO = −e Eꢑꢒꢓꢔꢕ − ꢗꢘꢙ + 4.8V
ꢀ2ꢂ
ꢀ3ꢂ
Table
1 The λmax for the colored form at the photostationary state, Half-Lives
ꢜꢔꢝ
(τ1/2=ln(2)/K1) of the TC forms, and formation ratios of the TT form in THF (5 × 10−5
LUMO = −eꢛE
− ꢗꢘꢙ + 4.8Vꢞ
ꢑꢒꢓꢔꢕ
M) at 298 K.
Table 2 The electrochemical properties of the closed form of all products.
τ
1/2(TC)/ TT form
ꢑꢖ ꢐꢟꢚ
ꢑꢒꢓꢔꢕ
ꢜꢔꢝ ꢐꢟꢚ
ꢑꢒꢓꢔꢕ
ꢗ
ꢗ
HOMO LUMO
ꢗꢠꢐꢡꢚ
compound λmax/nm A1
A2
k1
k2
Sample
s
/%
[eV]
[eV]
[V]
[V]
NP-N
NP-O
NP-C
502 0.257 0.015 0.166 0.002 4.17
472 0.731 0.068 0.177 0.003 3.91
466 0.766 0.085 0.137 0.004 5.06
5
9
10
NP-N
NP-O
NP-C
0.68
1.10
1.19
-0.20 -5.30 -4.42 0.88
0.19
0.12
-5.72 -4.81 0.91
-5.81 -4.74 1.07
These results were reported in Table 1. The ꢎꢏ of NP-O
NP-N and NP-C decreased in sequentially, so in the initial
rapid decay phase, the decay rates of NP-O NP-N and NP-C
decreased in turn. Formation ratio of the TT form of NP-C
NP-O and NP-N were 10%, 9% and 5%, respectively. The
formation ratio NP-C is slightly higher than that of NP-O, but
significantly higher than that of NP-N, because the volume of
alkylamine group was larger than that of hydroxyl group and
had a stronger electron donating ability electron donating
ability, as shown on the thermal fading curves, the absorbance
of NP-C, NP-O and NP-N reduced in turn. The half-life (τ1/2) of
,
[a]Onset oxidation and reduction potentials determined from cyclic voltammograms in
CH2Cl2/Bu4NPF6. [b]Band gaps estimated from the onset potentials.
,
Where ꢗꢘꢙ is the measured oxidation-reduction potential
of Fc (vs Ag/AgCl) is 0.18 V. The experimental results were
depicted in Table 2 and Fig. 5. Table 2 showed that the onset
oxidation potential of the novel 3H-naphthopyran derivatives
decreased with the enhancement of electron donating ability of
substituents.
Fig. 5(b) visually reflect the same phenomenon. By
comparing the band gaps and the HOMO levels of the closed
,
form of NP-C, NP-O and NP-N in the diagrammatic sketch
the TC form of NP-C, NP-N and NP-O were shortened in turn,
(Fig. 5(a)), it was clearly found that the HOMO energy level of
the molecules increased and the band gap narrowed with the
electron-donating ability of the substituent increased, because
the electron-donating ability of substituents promoted the
intramolecular electron transfer.
NP-O had a longer half-life than NP-C, because of the electron
donating ability of the hydroxyl group was greater than that of
the methyl group. However, the half-life (τ1/2) of the TC form
of NP-N was longer than that of NP-O, because of the double
effect of the strong electron-donating ability and the steric
3.4 Third-order nonlinear optical properties
The Z-scan technique was performed to study the third-order
nonlinear optical properties of the molecules, the nonlinear
absorption coefficient (
ꢢ ) was measured by means of
open-aperture Z-scan techniques, the nonlinear index (n2) was
measured by the closed-aperture Z-scan techniques in
dichloromethane. However, the third-order nonlinear refractive
properties of these novel molecules were so poor that can’t be
discerned. Therefore, the third-order nonlinear optical
ꢀ ꢂ
ꢁ
susceptibility
coefficient (
Z-scan traces of the molecules were shown in Fig. 6 (a)
χ
only involved the nonlinear absorption
) in this paper. The normalized open aperture
(b)
ꢢ
,
and (c), a trough of transmittance could be seen near the focus,
as the sample moved toward the focus, the transmittance
decreased and reached a minimum at the focus. All molecules
exhibited the reverse saturable absorption (RSA) behavior,
which could be used for optical limiting of laser protection and
indicating that the nonlinear absorption coefficient is
positive[26, 27].
The parameters of third-order nonlinear optical (NLO)
property were summarized in Table 3, the third-order nonlinear
Fig. 5 (a) The diagrammatic sketch of HOMO and LUMO energy levels. (b) The
cyclic voltammogram of the closed form of NP-N, NP-O and NP-C in
dichloromethane.

of the electron donating ability of the substituent which could
be visually observed in Fig. 6 (d). Usually, the strong electron
donating and the conjugated structures are the key factors for
enhancing NLO properties. The abilities of π-electron
delocalization and intramolecular charge transfer are the
foundation for enhancing NLO properties. Comparing to NP-C
NP-O had a strong electron donating group, so NP-O showed
better third-order nonlinear optical properties than NP-C
Comparing to NP-C and NP-O NP-N exhibited much better
,
.
,
third-order nonlinear optical properties, because the substituent
of NP-N had the stronger electron-donating ability than that of
NP-C and NP-O, which could promote the intramolecular
electron transfer and enhance third-order NLO properties.
Fig. 6 The red lines are the theoretical fitting curves and the blue scatter dots are
the Z-scan experimental data: the normalized open aperture Z-scan traces
measured for (a) NP-N, (b) NP-O, (c) NP-C. (d) The comparison of the
normalized open aperture Z-scan traces.
4. Conclusion
In summary, a series of novel highly efficient photochromic
naphthopyran derivatives with different electron donating
ability substituents were synthesized, and the effects of
substituents on photochromic behavior, electrochemical and
third-order nonlinear optical properties of naphthopyran
derivatives have been studied in this paper. A very interesting
phenomenon was found, with the enhancement of electron
donating ability of substituents, the maximum absorption peaks
(λmax) and the absorption tails of the closed form and the
open-form of the molecules both showed strong bathochromic
shift, the absorbance at λmax of the open-form decreased, the
half-lives (τ1/2) of the compounds’ TC form shorten and the
formation ratios of the TT form increased, the HOMO energy
level of the molecules decreased and the band gaps narrowed,
absorption coefficient (
equation[28, 29]:
ꢢ) was calculated by the following
ꢀ
ꢂ
ꢦ ꢦꢐꢏ − ꢧ ꢨ = ꢩ ꢚ
√
ꢀ
⁄ ꢂ
ꢣ ꢤ ꢥ =
ꢀꢮꢂ
ꢪꢩꢫꢬꢭꢭ
Where,
ꢧ is the normalized transmittance at the focus (Z=0),
ꢪꢩ is the intensity of the laser beam at the focal point. The
Com-
β
Imχ(3)
χ(3)
pounds
(×10-13 m/W) (×10-14 esu) (×10-14 esu)
NP-N
NP-O
NP-C
11.80
4.50
4.00
2.55
0.97
0.86
2.55
0.97
0.86
the third-order nonlinear absorption coefficient (
ꢢ
) and optical
effective thickness of the sample can be estimated as ꢫꢬꢭꢭ
=
ꢀ ꢂ
ꢁ
susceptibility
χ
both increased. These findings provide a
ꢐ
ꢀ
ꢂꢚ
ꢏ − ꢯꢰꢱ −ꢲꢫ /ꢲ
,
ꢫ
is the thickness of the sample and α is
guidance for the design of naphthopyran molecules and are of
great significance for the practical application of naphthopyran
molecules.
the linear absorption coefficient of the sample at the laser
excitation wavelength.
The value of the third-order nonlinear optical susceptibility
only includes the real parts in this paper, which can be
calculated from the third-order nonlinear absorption coefficient
(ꢢ) by the following equation[27]:
Conflicts of interest
There are no conflicts to declare
ꢸꢩꢦꢹꢦꢣ
ꢀꢵꢂꢀ
ꢳꢯꢴ ꢯꢶꢷ =
ꢂ
ꢀꢼꢂ
ꢦꢮꢩꢺꢦꢻ
Where, ꢽꢾ is the linear refractive index of solution,
c
is the
ꢋꢿ
speed of light and ω =
is the fundamental frequency in
ꣀ
Acknowledgments
cycles s-1. The value of the third-order nonlinear optical
ꢀ ꢂ
ꢁ
This work was supported by the National Key Basic Research
Program of China (2014CB931804), the National Natrual
Science Foundation of China (Grant No. 51673023, 51773017,
61370048, 51333001), the State Key Laboratory for Advanced
Metals and Materials (No. 2018Z-06), the Fundamental
Research Funds for the Central Universities (Grant No.
FRF-GF-17-B8).
susceptibility
equation[28]:
χ
could be calculated through the
ꢀ ꢂ
ꢵ
ꢀꢵꢂ ꢦ
ꣁ
|
|
ꢴ
=
ꢳꢯꢴ
ꢀꣂꢂ
Table 3 Third-order nonlinear optical parameters of compounds.
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As shown in Table 3, the third-order absorption coefficient
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ꢢ
Fig. 6 The third-order nonlinear absorption coefficient (
ꢢ
) and
ꢀ ꢂ
ꢁ
optical susceptibility
χ
both increased with the enhancement

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Highlights:
1. A series of novel naphthopyran derivatives with different substituents was
synthesized.
2. The novel naphthopyran derivatives showed excellent photochromic
behavior.
3. The photochromic behavior, electrochemical and third-order nonlinear
optical properties of naphthopyran derivatives have been fully studied.

Declaration of interests
☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: