Systemic study on fluorescent switching systems composed of naphthopyran and benzimidazole in solution and film forms

Article abstract of DOI:10.1016/j.molstruc.2016.03.031

The fluorescent photo-switching systems were prepared based on fluorescent benzimidazole and photochromic naphthopyran. Naphthopyran in this systems displayed excellent photochromic performance in tetrahydrofuran solutions and in PMMA films. The fluoresce

Full text of DOI:10.1016/j.molstruc.2016.03.031

Accepted Manuscript
Systemic study on fluorescent switching systems composed of naphthopyran and
benzimidazole in solution and film forms
Yi He, Guang Wang, Mingxin Wang
PII:
S0022-2860(16)30226-5
10.1016/j.molstruc.2016.03.031
MOLSTR 22342
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 20 January 2016
Revised Date: 10 March 2016
Accepted Date: 10 March 2016
Please cite this article as: Y. He, G. Wang, M. Wang, Systemic study on fluorescent switching systems
composed of naphthopyran and benzimidazole in solution and film forms, Journal of Molecular Structure
(2016), doi: 10.1016/j.molstruc.2016.03.031.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Systemic study on fluorescent switching systems composed of
naphthopyran and benzimidazole in solution and film forms
Yi Hea*, Guang Wangb, Mingxin Wangb,
aCollege of Chemistry, Jilin University, Changchun 130025, P. R. China
bFaculty of Chemistry, Northeast Normal University, Changchun 130024, P. R. China
Corresponding author: Yi He
Tel: 86+431+85095144;
Fax: 86+431+85095144.
E-mail address:
heyi@jlu.edu.cn (Y. He)

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Abstract
The fluorescent photo-switching systems were prepared based on fluorescent benzimidazole
and photochromic naphthopyran. Naphthopyran in this systems displayed excellent photochromic
performance in tetrahydrofuran solutions and in PMMA films. The fluorescent emission of
benzimidazole was modulated between “on” and “off” via the photoisomerization of naphthopyran
in high-contrast due to the photoinduced energy transfer from benzimidazole to the open-form
naphthopyran. Both the fluorescent photoswitching and the photochromism of
benzimidazole-naphthopyran dyads in solutions and films displayed excellent fatigue resistance.
The spaces between benzimidazole and naphthopyran affect the absorbance and fluorescence
spectra of benzimidazole-naphthopyran dyads. The non-destructive readout ability of synthesized
dyads in doped PMMA film was achieved.
Keywords: Fluorescence switching; Photochromism; Naphthopyran; Benzimidazole;
Photoinduced energy transfer.
1. Introduction:
Fluorescence switchings have attracted increasing interest due to their potential applications in
molecular switches, optical data storage, fluorescent biological markers and other molecular
electronic and photonic devices [1-5]. The photochromic fluorescence switching is one important
kind of fluorescence switching and received more and more research interesting. In photochromic
fluorescence switching systems, the fluorescent emission of fluorophore can be modulated
between “on” and “off” via the photochromic reaction of photochromic moieties under irradiation
of different wavelength lights or thermal effect. In most cases, the fluorescence quenching is due
to the intramolecular energy transfer and/or electron transfer [6-7]. Most of photochromic
fluorescence switchings are usual prepared in the following three formations: the fluorescent units
and photochromic groups are combined to be organic dyad or in copolymer or doped in polymeric
matrices. Diarylethene, spiropyran, spirooxazine, azobenzene derivates and other photochromic
unit are often used as photochromic units to prepare the photochromic fluorescence switchings
[8-11]. Naphthopyran (NP) derivatives are an important class of photochromic dyes due to their
breadth of color generated, absence of background color, good thermal reversibility, fine control
over fading kinetics and good resistance to photochemical fatigue [12-13]. Under UV light

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irradiation, the uncolored naphthopyran undergoes an electrocyclic pyran-ring opening with
cleavage of the C (sp3)-O bond and a subsequent structural reorganization to be more planar
structures (the so called ‘open form’, OF) with greater conjugation, which is responsible for the
increased absorption in the visible part of the spectrum, their molecular structures are shown in
Scheme 1 [14-16]. The OF is constituted by a set of colored stereoisomers, with similar absorption
characteristics but with diverse thermal stabilities. Under continuous UV irradiation, a
photostationary equilibrium is attained between the uncolored ‘closed form’ (CF) and the colored
OF form. This isomerization process is accompanied by color change of the system. Up to now,
only our group has reported on naphthopyran as photochromic unit to be used in fluorescence
switching [17-20].
O
O
+
O
eat
or h
hv'
TT
TC
CF
Scheme 1. The photochromic reaction of naphthopyran molecule.
In the paper [18], we fabricated the fluorescence photoswitching using naphthopyran as
photochromic unit and benzimidazole as fluorophore. In this switching molecule, the
naphthopyran connected with benzimidazole via ester bond on the nitrogen atom of imidazole ring
and benzimidazole connected with a pyridine ring, but its shortcoming is the large degradation of
fluorescent intensity after first fluorescent switch. Compared to the above report, the connect
manner of naphthopyran with benzimidazole is not same in this paper. In this system, we designed
a
systematic naphthopran-benzimidazole fluorescence switching, the spaces between
naphthopyran and benzimidazole were ester bond, two-carbon chain and six-carbon chain,
respectively. In addition, the linked atom on the benzimidazole is oxygen atom of hydroxy on
benzene ring but not nitrogen of imidazole ring. Their molecular structures and synthetic routes
are shown in scheme 2. The performance of their fluorescent switching in solutions and films were
investigated and compared, the effect of spaces between naphthopyran and benzimidazole on the
photophysical properties were also studied.

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N
HO
NH2
NH2
HN
NaHSO3
OH
N
O
HO
CHO
HO
O
HO
O
Br
COOH
CH3CH2OH
DMF
CH3CH2OH
HN
Br
SO3Na
O
PBI
O
4
O
3
5
O
Br
O
DMF 80 o
K2CO3 KI
C
DMF 80 o
K2CO3 KI
C
OH
EDC, 25 o
C
DMAP
3
O
1
PPh3
O
O
O
CBr4
+
O
CH2Cl2, rt
O
O
O
O
Br
C
O
O
O
OH
O
HN
N
4
HN
N
O
2
N
HN
NP-PBI
C6NP-PBI
C2NP-PBI
Scheme 2. The synthetic route of the systematic photochromic fluorescent switching.
2. Experimental
2.1. Materials and characterization
4-dimethylaminopyridine (DMAP), 1-ethyl-3-(3’-dimethylaminopropyl)carbodiimide (EDC),
6-hydroxy-2-naphthoic acid and 2,6-dihydroxynaphthalene were purchased from Aldrich. The
other materials were commercial products and used without further purification. Solvents were
purified by normal procedures and handled under moisture free atmosphere. All chromatography
examinations were performed using silica gel, and TLC was performed on silica plates (made in
China).
1H and 13C NMR spectra were recorded on a Varian Unity Inova Spectrometer at room
temperature using d-chloroform and dimethylsulfoxide-d6 as solvent. Mass spectra were recorded
on BRUKER AutoflexIII Smartbeam MS-spectrometer. Fluorescence and UV-Vis absorbance
spectra were measured on Cary Eclipse Fluorescence Spectrophotometer and Varian Cary 500
spectrophotometer, respectively. Photochromic performances of the films were examined using a
Varian Cary 500 UV-vis-NIR spectrophotometer. The ultraviolet source for irradiation was
CHF-XM35 parallel light system (Beijing Changming Technology Co. Ltd) with a 500 W xenon
lamp and monochromatic filter (360 nm). The irradiating light reached to the samples was parallel
and its intensity was about 5 mW/cm2.
2.2. Synthesis
2.2.1 Compound 1, 2 and 5 was synthesized according to our previous reports [18, 21].
2.2.2 Synthesis of PBI：
After the mixture of p-hydroxybenzaldehyde (6.1 g, 50 mmol), sodium bisulfite (5.2 g, 50
mmol) and anhydrous ethanol (30 mL) was stirred at room temperature for 4 h, the solution of

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o-diaminobenzene (5.4 g, 50 mmol) in DMF (30 mL) was added and the reaction mixture was
then stirred at 120 for 3 h. After cooling down to room temperature, the reaction mixture was
poured into 300 mL water and the precipitate was obtained by filtering. The pure product was
1
obtained by recrystallization in ethanol, faint yellow solid, 9.42 g，yield 89.7%. H NMR (400
MHz, DMSO-d6) δppm: 12.63 (s, 1H,), 9.99 (s, 1H), 8.02-8.00 (t, 2H, J = 7.0), 7.55-7.53 (dd, 2H,
J = 3.0, 5.6 Hz), 7.18-7.14 (m, 2H), 6.94-6.91 (t, 2H, J = 6.5).
2.2.3 Synthesis of compound 3 and 4
Compound 3 and 4 were synthesized by the following procedure.
For compound 3: Compound 1 (2.0 g, 5 mmol), carbon tetrabromide (2.7 g, 8 mmol) and
triphenylphosphine (2.1 g, 8 mmol) were dissolved in purified CH2Cl2 (30 mL) and stirred for 24
h at room temperature. The reaction mixture was poured into 50 mL water and the product was
extracted with CH2Cl2 (30 mL×3). The organic phase was dried with anhydrous sodium sulfate,
the purified compound 3 was obtained by column chromatography on silica gel (eluent: petroleum
1
ether: CH2Cl2 = 3:1) to give white powder, 1.91 g, yield 83.6%. H NMR (CDCl3, 500MHz) δ
ppm: 7.88-7.86 (d, 1H, J=11.5 Hz), 7.54-7.52 (d, 1H, J=9.0 Hz), 7.48-7.46 (d, 4H, J=9.0 Hz),
7.33-7.15 (m, 9H), 7.04-7.03 (d, 1H, J=3.0 Hz), 6.29-6.26 (d, 1H, J=10.0 Hz), 4.41-4.32 (t, 2H,
J=7.5 Hz), 3.68-3.63 (m, 2H, J=7.5 Hz).
For compound 4: White powder, yield 87.2%. 1H NMR (CDCl3, 500MHz) δ ppm: 7.86-7.84 (d,
1H, J=9.0 Hz), 7.54-7.51 (d, 1H, J=9.0 Hz), 7.49-7.47 (d, 4H, J=9.0 Hz), 7.32-7.12 (m, 9H),
7.01-7.03 (d, 1H, J=2.0 Hz), 6.28-6.25 (t, 1H, J=10.0 Hz), 4.0-4.01 (t, 2H, J=7.0 Hz), 3.45-3.41 (t,
2H, J=7.0 Hz), 1.91-1.82 (m, 4H), 1.56-1.52 (m, 4H).
2.2.4 Synthesis of C2NP-PBI and C6NP-PBI
Compound C2NP-PBI and C6NP-PBI were synthesized by the following procedure.
A mixture of PBI (0.63 g, 3 mmol), anhydrous K2CO3 (0.6 g, 4.5 mmol) and a pinch of KI in
DMF (50 mL) was stirred at room temperature for 1 h, then compound 3 (1.4 g, 3 mmol) was
added into the above reaction mixture. The reaction mixture was heated to 80 ℃ and kept for 24 h.
After cooling down to room temperature, the reaction mixture was poured into 200 mL water. The
precipitate was obtained by filtering and dissolved with ethyl acetate. The organic phase was dried
with anhydrous sodium sulfate, the purified C2NP- PBI was obtained by column chromatography
on silica gel (eluent: petroleum ether: ethyl acetate = 9:2) to give white powder, 1.31 g, yield

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1
74.7%. H NMR (400 MHz, DMSO) δppm: 8.14-8.11 (d, 2H, J = 8.8 Hz), 8.06-8.03 (d, 1H, J =
9.2 Hz), 7.72-7.69 (d, 2H, J = 8.8 Hz), 7.61-7.58 (m, 2H), 7.52-7.45 (m, 5H), 7.33-7.15 (m, 12H),
6.62-6.59 (d, 1H, J = 9.6 Hz ), 4.48-4.40 (m, 4H). MS (ESI-TOF): m/z calcd for C40H30N2O3:
587.2351; found: 587.2329 [M ]+.
For C6NP-PBI: White powder, yield 78.2%。1H NMR (400 MHz, DMSO) δppm: 7.96-7.93
(d, 2H, J = 8.8 Hz), 7.86-7.83 (d, 1H, J = 9.2 Hz), 7.65 (s, 1H), 7.53-7.46 (m, 6H), 7.33-7.21 (m,
10H), 7.17-7.11 (m, 2H), 7.03-6.97 (m, 3H), 6.25-6.27 (d, 1H, J = 9.6 Hz ), 4.06-4.01(m, 4),
1.86-1.85(m, 4H), 1.59-1.57 (m, 4H). MS (ESI-TOF): m/z calcd for C44H38N2O3: 643.2990; found:
643.2955 [M ]+.
2.2.5 Synthesis of NP-PBI
PBI (0.7 g, 3 mmol), 3,3-Diphenyl-[3H]naphtho[2,1-b]pyran-8-carboxylic Acid (5) (1.1 g, 3
mmol) and DMAP (0.072 g, 0.6 mmol) were dissolved in purified CH2Cl2 (30 mL) and stirred for
1 h at room temperature. After addition of EDC (0.7 g, 3.6 mmol), the reaction mixture was
continuously stirred 24 h. The organic phase was washed with water for three times and dried with
anhydrous sodium sulfate. The purified NP-PBI was obtained by column chromatography on
silica gel (eluent: petroleum ether: ethyl acetate= 3:1), white powder 1.2 g, yield 70.2%. 1H NMR
(400 MHz, CDCl3) δ 8.64 (d, 1H, J = 1.4 Hz), 8.18 – 8.11 (m, 2H), 8.04 (d, 1H, J = 8.9 Hz), 7.81
(d, 1H, J = 8.8 Hz), 7.67 (s, 2H), 7.49 (d, 3H, J = 7.2 Hz), 7.41 (d, 2H, J = 8.5 Hz), 7.37 – 7.25 (m,
13H), 6.32 (d, 1H, J = 9.9 Hz). 13C NMR (101 MHz, DMSO, δ): 128.77, 128.18, 126.76, 123.13,
40.60, 40.39, 40.18, 39.97, 39.76, 39.55. MS (ESI-TOF): m/z calcd for C39H26N2O3: 571.2006;
found: 571.2016 [M ]+.
2.3 Preparation of doped PMMA films
The three compounds doped-PMMA films were prepared by the following procedure. After the
mixture of 1.25 g polymethylmethacrylate (PMMA, Mw=350000 g mol-1) and 30 mL
tetrahydrofuran (THF) was stirred very well, 5 mg of compound NP-PBI was added and the whole
mixture was further stirred to be homogenous. A 1.5 mL of the above solution was dropped on the
quartz plate to prepare the sample film by spin-coating at 300 rpm for 40 s using a spin coater
SIYOUYEN (KW-4A). The film was dried in air and then in a vacuum at room temperature and
kept in darkness.

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3. Results and discussion
3.1 The photochromic performance of synthesized compounds in solutions and films
Naphthopyran in three synthesized compounds displayed excellent photochromic performance
in tetrahydrofuran (THF) solutions (1×10-5 M) and displayed very fast photochromic rates. Three
compounds presented similar absorbance spectra profiles, Fig.1a is the absorbance spectra of
C2NP-PBI before and after UV irradiation and represents the photochromic patterns of NP-PBI
and C6NP-PBI. The absorbance peaks in the left wavelength range of 335 nm are ascribed to PBI
unit and a part of NP moiety, and the other two absorbance peaks among the 335 nm ~ 395 nm are
attributed to NP moiety. In the longer wavelength range than 400 nm, there is no any absorbance
peaks. For the absorbance spectra of three synthesized compounds in THF solutions, after UV
irradiation, the new absorption bands at 390 nm～580 nm with the peak at about 470 nm appeared,
their intensities increased gradually with the irradiation time and reached the maximum values
within 3 minutes. The new absorbance bands are attributed to the open-form NP and this coloring
process was accompanied by the color change of solution from colorless to orange. When the UV
irradiation was ceased and the solutions were put in the dark, the absorption bands of open-form
NP decreased with the time and disappeared finally without any residual within 2.5 min. At the
same time, the color of the solution faded completely (Fig.1a). The mathematical biexponential
equation, A(t) = A1e-k1t+ A2e-k2t+ Ath, was suitable to evaluate the photochromic decoloration rates.
Here, A(t) is the optical density at the λmax, A1 and A2 are contributions to the initial optical density
A0, k1 and k2 are the fading rates of the fast and slow components, respectively, and Ath is the
residual coloration [22]. The decoloration curve was fitted using the above biexponential equation
with Origin software, which was found to agree best with the experimental data (Fig.1b). All the
fading kinetics data are shown in Table 1, the fading rate of NP-PBI is smaller than that of
C2NP-PBI and C6NP-PBI. PBI unit is close to NP in NP-PBI molecule and provided larger steric
hindrance for NP moiety to isomerization, which reduced the fading rate of open-form NP. The
fatigue resistance is one important parameter for photochromic materials in their application.
Fig.1c is the typical illustration of fatigue resistance of C2NP-PBI, which represents the similar
behaviors of NP-PBI and C6NP-PBI. In the measurement of fatigue resistance of C2NP-PBI in
THF solution (1×10-5 M), the times of alternative UV irradiation and put in the dark were

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controlled to be 2 min and 3 min, respectively. The fluorescent intensity of C2NP-PBI did not
have distinct degradation and a little residual color were observed after 5 cycles [Fig.1c]. All three
compounds in THF solutions displayed fast photochromic rate and excellent fatigue resistance, the
remarkable reproducibility can be seen after each coloration/fading sequence monitored at
absorption maximum.
0.40
3.5
UV
C2NP-PBI
UV-2 min
In the dark-2 min
0.6
0.5
0.4
0.3
0.2
0.1
0.0
UV
UV
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
-0.05
UV
Experimental data
Fit of experimental data
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
UV
c
b
a
Put in the dark
0
1
2
3
4
5
6
0
200
400
600
800
300
400
500
600
Cycle Number
Time (s)
Wavelength (nm)
Fig. 1 Photochromic performances and the fatigate resistances of C2NP-PBI in THF solution (1×10-5 M). (a) The
changes of absorbance spectra. (b) The fading curve. (c) The photochromic fatigue resistance monitored at 475 nm
under alternative UV irradiation 2 min and put in the dark 3 min.
Table 1 Photokinetics of the bleaching of compounds in solutions,
and absorption and fluorescence spectral characteristics in solutions and films
Ath
A1
k1 (S-1)
0.22408
0.62344
0.96965
A2
k2 (S-1)
0.01042
0.03698
0.04539
NP-PBI
0.00198
0.06278
0.05287
0.01607
0.21071
0.39344
0.06027
0.24957
0.13708
C2NP-PBI
C6NP-PBI
Absorption peak of open-form NP (λmax, nm)
Fluorescence peaks ( λ, nm)
In solution
444
In film
455
In solution
435
In film
NP-PBI
390
C2NP-PBI
C6NP-PBI
468
440
350, 389
350, 391
347, 402
350, 407
474
450
The photochromic properties of photochromic material in film are more important for their
application. Three synthesized compounds in PMMA films also displayed normal photochromic
behaviors and have similar photochromic performances. Fig 2 illustrates the representative
photochromic performance of the synthesized compounds in PMMA films. Fig2.a is the
absorbance spectra of NP-PBI, there are four main absorption peaks at 250 nm, 310nm, 360 nm
and 378 nm belonging to naphthopyran and benzimidazole. After UV irradiation, the absorption
bands of open-form naphthopyran at 390 nm~550 nm centered at about 450 nm emerged and

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increased with the irradiation time, Fig.2b shows the change of absorbance spectra of C2NP-PBI
in PMMA film after UV irradiation for 5 min. For all three compounds, the complete color process
needed about 7 min under UV irradiation, which is a little slower than in THF solutions. However,
their fading rates in films are significant slower than those in THF solutions. If the irradiated film
was put in the dark, the absorbance band of open-form NP only decreased to half of its maximum
after 7 h and could not drop to its original after 36 h. The slow photochromic rate should be
attributed to the molecular compact aggregation in solid state and the free-volume for
intramolecular bond rotation of NP molecules to transform from open-form to closed form was
reduced. Due to the longer fading time, the fatigue resistance in film was also investigated under
alternative UV irradiation and put in the dark in short time. For C6NP-PBI, the times of alternative
UV irradiation and put in the dark were only 1 min and 5 min, respectively. Fig.2c shows the fine
fatigue resistance of C6NP-PBI and the absorbance intensity did not show obvious degradation
after 5 cycles. NP-PBI and C2NP-PBI also displayed similar fatigue resistance as C6NP-PBI.
The spaces between NP and PBI have smaller effects on the absorbance spectra but obvious
effects on their photochromic kinetics of NP unit in three synthesized compounds. The excellent
photochromic performance of three synthesized compounds in solutions endows them potential
application as photoswitching materials.
0.08
0.06
0.04
0.02
0.044
0.040
0.036
0.032
0.028
0.6
0.4
0.2
0.0
C6NP-PBI
C2NP-PBI
UV irradiation
b
NP-PBI
c
UV-5 min
In the dark-5 min
Put in the dark
C2NP-PBI
300
400
500
600
700
400
500
600
700
800
1
2
3
4
5
6
Wavelength / nm
)
Wavelength (nm)
Cycle Number
Fig. 2 Photochromic performances and the fatigate resistances of the synthesized compounds in PMMA films. (a)
The absorbance spectra of NP-PBI. (b) The changes of absorbance spectra of C2NP-PBI under UV irradiation and
put in the dark. (c) The photochromic fatigue resistance of C6NP-PBI monitored at 450 nm.
3.2 Comparison analysis of the absorbance and fluorescence spectra of three synthesized
compounds in THF solutions and PMMA films
All three synthesized compounds showed intensive fluorescent emission and different spectra
pattern in THF solutions and films (Fig. 3). In THF solutions, NP-PBI presented a single emission
peak at 435 nm under the excitation of 370 nm. But C2NP-PBI and C6NP-PBI showed obvious

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different fluorescence spectra and presented two main peaks at 349 nm and 390 nm under the
excitation of 336 nm. The quantum yield values of three compounds were measured using quinine
sulphate in 0.05 M H2SO4 as a standard (Фf = 0.55) to be 0.12, 0.27 and 0.23 for NP-PBI,
C2NP-PBI and C6NP-PBI, respectively. The fluorescence emission of three synthesized
compounds originates from the PBI moiety because that naphthopyran moiety just shows very
weak fluorescence emission. Although three compounds are composed of NP and PBI, their
fluorescent spectra presented different patterns upon the molecular structures. The spaces between
NP and PBI provided dominative effects on the fluorescence spectra. The naphthopyran moiety
provides large effect on the fluorescence spectra of NP-PBI dyad due to the ester space, but very
small effect on that of C2NP-PBI and C6NP-PBI due to carbon chain. Although three compounds
have different spectra patterns, for each compound, the fluorescent spectra in PMMA film had
similar spectra with that in solutions, respectively. The emission spectrum of NP-PBI under the
excitation of 330 nm in film did not present difference but showed distinct hypochromatic shift
(44 nm) compared with that in THF solution (Fig.3a). On the contrary, the emission bands of
C2NP-PBI and C6NP-PBI under the excitation of 290 nm in films have no obvious shift compared
with that in THF solutions, but the peak intensity at 390 nm in film relatively reduced compared
with that in solution (Fig.3b,c). Based on the photoinduced energy transfer mechanism, in this
system of photochromic fluorescence switching, the larger overlapped extent of the emission band
of PBI with the absorbance band of photochromic open-form NP is in favor of the energy transfer
1.0
0.8
0.6
0.4
0.2
1.0
0.8
0.6
0.4
0.2
NP-PBI in solution
1.0
0.8
0.6
0.4
0.2
C6NP-PBI in the solution
C2NP-PBI in solution
Emission
a
Emission
Emission
c
After UV irradiation
b
After UV irraditation
Before UV irraditation
Before UV irradiation
After UV irradiation
Before UV irradiation
0.0
1.0
0.0
1.0
0.0
1.0
C6NP-PBI in the film
Emission
NP-PBI in the film
Emisssion
C2NP-PBI in the film
Emission
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
After UV irradiation
Before UV irradiation
Before UV irradiation
After UV irradiation
Before UV irradiation
After UV irradiation
300 400 500 600 700 800
Wavelength (nm)
300 400 500 600 700 800
Wavelength (nm)
300
400
500
600
700
Wavelength (nm)
Fig.3 The absorbance and fluorescence spectra of compounds, NP-PBI, C2NP-PBI and C6NP-PBI in
solutions and films.

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from excited state of PBI to open-form NP, in other words, it benefit the fluorescent quenching of
PBI. From Fig.3, it is easy to see that the overlapped extents of the absorption band of open-form
NP with the emission band of PBI for three compounds have large difference. Among them, the
overlapped extent of NP-PBI is the largest than that of C2NP-PBI and C6NP-PBI whether in
solutions or in films. The emission band of NP-PBI in solution completely overlapped with the
absorption band of open-form NP, but the overlapped extent in films is smaller than that in
solution. On the contrary, For the C2NP-PBI and C6NP-PBI, the overlapped extents both in
solutions and films are smaller than that of NP-PBI, in respect their own, the overlapped extents in
solutions are a little bigger than that in films, respectively. From the above results, the efficiency
of photoinduced energy transfer in NP-PBI is the biggest than those in C2NP-PBI and C6NP-PBI.
In addition, the spaces between NP and PBI are dominating factor on photophysical features and
efficiencies of the photoinduced energy transfer in three synthesized compounds.
3.3 Fluorescence photoswitching
Based on the above spectral analysis, the absorption bands of open-form naphthopyran in three
compounds overlapped partly or completely with the emission spectra of PBI in solutions and
films. Therefore, the photoinduced energy transfer (PET) from excited PBI to open-form NP can
be used to modulate the fluorescence emission of PBI. Fig.4a, b and c illustrate the changes of
fluorescence spectra of NP-PBI, C2NP-PBI and C6NP-PBI in THF solution (1×10-5 M) under UV
irradiation and put in the dark, respectively. The fluorescent intensities decreased with the UV
irradiation time and reached to the lowest values within 40 min, at this time the fluorescence was
at “off” state. The emission intensities of NP-PBI at 435 nm and C2NP-PBI at 390 nm and
C6NP-PBI at 390 nm decreased to 94.4％, 49.5% and 40.8% of their original value, respectively.
The fluorescence quenching efficiency in NP-PBI is the biggest, which is due to the PET
efficiency is the highest in NP-PBI. The higher modulation efficiencies, especially of NP-PBI are
satisfied for their application in fluorescence photoswitching. This result is coincident with the
above spectra analysis and also proved that the photoinduced energy transfer is responsible for
fluorescence quenching of PBI. On the contrary, these irradiated solutions were put in the dark, the
fluorescence intensities of solution recovered gradually to the “on” state within 10 min. When the
samples were alternately irradiated by UV light and put in the dark, the fluorescent emission can
be modulated between “off” and “on”. Fig.4 d, e and f illustrate the sequential switching cycles of

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fluorescence of three compounds in THF solutions. The fluorescent intensities of three compounds
in solutions degraded in small extent with the number of switching cycles, which indicated that the
three compounds possessed excellent fluorescent fatigue resistance. This higher modulation
efficiency of fluorescence and excellent fatigue resistance indicate that the three
naphthopyran-benzimidazole dyads, especially for NP-PBI, are better candidates in potential
application of fluorescent photoswitching in solution.
4000
3000
2000
1000
0
4000
3000
2000
1000
0
NP-PBI
0 min
5 min
C6NP-PBI
In the dark 2 min
C2NP-PBI
b
c
10 min
15 min
20 min
25 min
30 min
40 min
3000
2000
1000
0
UV irradiation
In the dark 2 min
UV-3 min
a
UV-2 min
400
500
600
300 350 400 450 500 550 600 650
Wavelength (nm)
300
400
500
600
Wavelength (nm)
Wavelength (nm)
4000
C6NP-PBI
NP-PBI
C2NP-PBI
2400
Put in the dark
Put in the dark
3500
3000
2500
2000
1500
Put in the dark
3000
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1000
2000
e
d
f
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UV on
UV on
UV on
1
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3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
Cycle Number
Cycle Number
Cycle Number
Fig. 4 The changes of fluorescence spectra of three compounds and their fluorescent fatigue in THF
solutions (1×10-5 M). a) the changes of the fluorescence of NP-PBI under UV irradiation; b and c) the
changes of the fluorescence of C2NP-PBI and C6NP-PBI under UV irradiation and put in the dark,
respectively; d, e and f) the fluorescent fatigue resistance of three compounds monitored at 435 nm,
390 nm, 390 nm), respectively. Test conditions for fatigue resistance: for NP-PBI, UV irradiation 5 min,
put in the dark 8 min; for C2NP-PBI, UV irradiation 3 min, put in the dark 5 min; for C6NP-PBI, UV
irradiation 2 min, put in the dark 5 min.
Indeed, the higher modulation efficiency of fluorescence and excellent fatigue resistance in
solid state are more essential for photoswitching molecules to be used in fluorescent
photoswitching than in solution. All three synthesized compounds displayed intensive
fluorescence in PMMA films. The fluorescent spectra of NP-PBI in film narrowed down and had
obvious hypochromatic shift (about 44 nm) than that in THF solution (Fig.3a), the overlapped part
of emission band of PBI with the open-form NP significantly reduced. However, the fluorescent
spectra of C2NP-PBI and C6NP-PBI did not shift relative to those in solutions but showed
obvious peak tailing. Upon UV irradiation, the fluorescent intensities of three doped PMMA films
decreased continuously and reached the lowest value after 20 min. Fig.5a and b shows the changes

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of fluorescent spectra of NP-PBI and C2NP-PBI upon UV irradiation. After UV irradiation for 5
min, the quenched efficiencies of fluorescent intensities at maximum wavelengths of three doped
films are 28.6% (NP-PBI), 24.7% (C2NP-PBI) and 23.1% (C6NP-PBI), respectively. The
fluorescence switching is safely ascribed to the photochromic reaction of the naphthopyran. When
the irradiated films were put in the dark, the open-form NP spontaneously transformed to the
closed-form and the fluorescence intensities of PBI increased constantly. Compared with that in
solutions, the fluorescent intensities of three doped films increased slowly due to the slow
isomerization rate of naphthopyran. Their fluorescent intensities needed at least 40 h to recover to
their initial states, which indicated that the fluorescence of PBI in films restored very slowly.
Because of the long fading time, the fluorescent fatigue resistances in films were investigated
under alternative UV irradiation and putting in the dark in short time. Fig.5c is the representative
switching reproduction of C2NP-PBI monitored at 346 nm in film measured under alternative UV
irradiation 5 min and put in the dark 8 min for 5 cycles. The fluorescent intensity decayed a little
bigger than that in solution, which may be attributed to the incomplete isomerization of NP from
open-form to closed-form. NP-PBI and C6NP-PBI in films have similar performance and showed
satisfied fatigue resistances. The above results indicate that the synthesized compounds are
potential candidates of fluorescence photoswitching.
1500
NP-PBI
C2NP-PBI
Put in the dark
2400
2000
1600
C6NP-PBI
c
6000
4000
2000
0
b
1200
900
600
300
0
UV irradiation 30 s
UV irradiation 30 s
a
UV irradiation 3 min
UV irradiation 3 min
UV on
1
2
3
4
5
6
350
400
450
500
300 350 400 450 500 550
Cycle number
Wavelength (nm)
Wavelength (nm)
Fig 5 The changes of fluorescence spectra of NP-PBI and C6NP-PBI in PMMA films under UV
irradiation and the fluorescent fatigue resistance of C2NP-PBI monitored at 346 nm in PMMA film
under alternative UV irradiation for 5 min and put in the dark for 8 min.
3.4 Rewritable optical recording and non-destructive readout ability
Fluorescent photochromic materials attract more interest for their possible application in
optical memory, which display reversible change in fluorescent intensity along with the
photochromic reaction and are also used in non-destructive optical readout systems [23-24]. Three
synthesized compounds showed high-density fluorescence contrast in PMMA film before and

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after UV irradiation. Here, NP-PBI was used as an example to account for the potential application
in non-destructive readout. The fluorescence is at two discrete states, “on” before UV irradiation
and “off” after UV irradiation, respectively. The photochromic fading of NP-PBI in PMMA film is
very slow and the quenched fluorescence needs more 40 h to restore. The above results indicate
that NP-PBI possessed the feasibility to be used in non-destructive readout.
Scheme 3 presents the optical switching function of NP-PBI in PMMA film. The open-form
and closed-form naphthopyrans represent two binary digital codes, “0” and “1” states. It exhibits
fine photo-regulating luminescence of NP-PBI with very high-contrast in PMMA film. The strong
fluorescence emission with high-contrast switching in the solid film could increase the density of
the stored information. NP-PBI has high-contrast fluorescence switching characteristics and the
photochromic performance of naphthopyran, so it can be used as a promising candidate for
erasable optical data storage. Here, it should be pointed out that if the more stable open-form
naphthopyran in solid state could be obtained, this system will be more suitable for the rewritable
optical recording and non-destructive fluorescence readout.
380 nm
380 nm
Readout
Energy
transfer
Readout
365 nm
(write)
Energy
transfer
NH
N
O
450 nm
(Erase)
NH
N
O
O
C
O
O
C
O
Closed-form NP
No Fluorescence
Open-form NP
Fluorescence
Scheme 3 The fluorescence switching behavior of NP-PBI in PMMA film
Non-destructive fluorescence readout ability is another important property for an optical device,
so the feasibility of non-destructive readout for NP-PBI-doped PMMA film was investigated.
From Fig.3a, both the closed-from and open-form naphthopyrans in PMMA film have very small
absorbance at 380 nm, which means the irradiation of 380 nm light does not induce the
photoisomerization of naphthopyran in the NP-PBI doped PMMA film and fluorescence change.
Our experimental results proved this assumption, no obvious color change appeared after the
NP-PBI doped PMMA film was irradiated with the 380 nm light for 5 min. Conversely, if the film
was irradiated by the light of 365 nm or 300 nm, remarkable color changes were observed. These
results indicated that there is no generating of photochromic reactions of naphthopyran if the 380

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nm light is used as the fluorescence excitation wavelength during the fluorescence readout process,
so the non-destructive readout for NP-PBI doped PMMA film was realized.
4. Conclusion:
The fluorescent photo-switching dyads, composed of fluorophore “benzimidazole” and
photochromic naphthopyran with ester bond and alkyl as spaces, were designed and prepared.
Naphthopyran in the synthesized dyads displayed excellent photochromic performance. The
fluorescent emission of PBI could be modulated between “off” and “on” via the
photoisomerization of naphthopyran under alternate irradiation of UV light and put in the dark.
The fluorescent switchings have excellent fatigue resistance in solutions and PMMA films. The
spaces between PBI and NP affect fluorescence spectra of NP-PBI dyads and the efficiencies of
photoinduced energy transfer in three synthesized compounds. The high-contrast fluorescence
photoswitching characteristics and capacity of non-destructive readout of information indicates
that the synthesized benzimidazole-naphthopyran dyads are promising candidates in erasable
optical information technology.
Acknowledgement
This work was supported by the Research Fund for the Doctoral Program of Higher
Education of China (20120043110007).
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Figure captions
Scheme 1. The photochromic reaction of naphthopyran molecule.
Scheme 2. The synthetic route of the systemic photochromic fluorescent switching.
Scheme 3. The fluorescence switching behavior of NP-PBI in PMMA film
Figure 1. Photochromic performances and the fatigate resistances of C2NP-PBI in THF solution
(1×10-5 M). (a) The changes of absorbance spectra. (b) The fading curve. (c) The
photochromic fatigue resistance monitored at 475 nm under alternative UV irradiation
2 min and put in the dark 3 min.
Figure 2. Fig. 2 Photochromic performances and the fatigate resistances of the synthesized
compounds in PMMA film. (a) The absorbance spectra of NP-PBI. (b) The changes of
absorbance spectra of C2NP-PBI under UV irradiation and put in the dark. (c) The
photochromic fatigue resistance of C6NP-PBI monitored at 450 nm.
Figure 3. The absorbance and fluorescence spectra of compounds, NP-PBI, C2NP-PBI and
C6NP-PBI in solutions and films.
Figure 4. The changes of fluorescence spectra of three compounds and their fluorescent fatigue in
THF solutions (1×10-5 M). a) the changes of the fluorescence of NP-PBI under UV
irradiation; b and c) the changes of the fluorescence of C2NP-PBI and C6NP-PBI
under UV irradiation and put in the dark, respectively; d, e and f) the fluorescent
fatigue resistance of three compounds monitored at 435 nm, 390 nm, 390 nm),
respectively. Test conditions: for NP-PBI, UV irradiation 5 min, put in the dark 8 min;
for C2NP-PBI, UV irradiation 3 min, put in the dark 5 min; for C6NP-PBI, UV
irradiation 2 min, put in the dark 5 min.
Figure 5. The changes of fluorescence spectra of NP-PBI and C6NP-PBI in PMMA films under
UV irradiation and the fluorescent fatigue resistance of C2NP-PBI monitored at 346 nm
in PMMA film under alternative UV irradiation for 5 min and put in the dark for 8 min.
Table 1 Photokinetics of the bleaching of compounds in solutions and absorption and fluorescence
spectral characteristics in solutions and films

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Highlights
► A system of fluorescent photoswitchings based on benzimidazole-naphthopyran
dyads was designed and prepared.
► The fluorescence emission of benzimidazole can be efficiently modulated by
photochromism of naphthopyran.
► Photoinduced energy transfer is responsible for the modulation of fluorescence
emission.
► The non-destructive readout ability of synthesized dyads in doped PMMA film was
achieved.