Development of molecular photoswitch with very fast photoresponse based on asymmetrical bis-azospiropyran

Article abstract of DOI:10.1016/j.saa.2015.07.110

To study the effects of an extended bis-azo conjugated bridge with two different photochemical functions on a molecule in photochromic responses, a novel asymmetrical bifunctional bis-azo spiropyran photochromic dye was designed and synthesized. The obtai

Full text of DOI:10.1016/j.saa.2015.07.110

Development of Molecular Photoswitch with Very Fast Photoresponse based
on Asymmetrical Bis-Azospiropyran
Farahnaz Nourmohammadian, Ali Ashtiani Abdi
PII:
DOI:
Reference:
S1386-1425(15)30157-8
doi: 10.1016/j.saa.2015.07.110
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21 April 2015
29 July 2015
30 July 2015
Please cite this article as: Farahnaz Nourmohammadian, Ali Ashtiani Abdi, Development
of Molecular Photoswitch with Very Fast Photoresponse based on Asymmetrical Bis-
Azospiropyran, (2015), doi: 10.1016/j.saa.2015.07.110
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ACCEPTED MANUSCRIPT
Development of Molecular Photoswitch with Very Fast Photoresponse based on
Asymmetrical Bis-Azospiropyran
Farahnaz Nourmohammadian* a,b and Ali Ashtiani Abdia
a Department of Organic Colorants, Institute for Color Science and Technology P.O. Box 16765-654,
Tehran, Iran; b Centre of Excellence for Color Science and Technology, Tehran, Iran;
Corresponding authors: Tel. +98 21 22058314; fax: +98 21 22947537, E-mail address:
nour@icrc.ac.ir (F. Nourmohammadian)

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ABSTRACT
To study the effects of an extended bis-azo conjugated bridge with two different photochemical
functions on a molecule in photochromic responses, a novel asymmetrical bifunctional bis-azo
spiropyran photochromic dye was designed and synthesized. The obtained photoresponses were
compared with symmetrical bifunctional bis-azo spiropyran analogues, and relative mono-azo and
simple spiropyrans. Colorimetric behaviour, luminescence, and switching kinetics of all the dyes were
studied. The largest molar absorption coefficient in merocyanine form, quickest response to light, and
highest fluorescence quantum yield of the spiropyran form with a superior ratio of emission intensities
of spiropyran to merocyanine form were achieved for the asymmetric bis-azospiropyran,.
Solvatochromic effect was studied to observe the solvent effects on non-irradiation coloration of the
photochromic dyes. Furthermore, The molecular energy levels for optimized geometries of the
synthesized bis-azospiropyrans and their probable photochemical products were obtained at the
B3LYP/6-31G(d) level of theory.
Keywords: Photochromic, Solvatochromic, Density functional theory, Azospiropyrans, Bi-dose
response.
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1. Introduction
The design and synthesis of novel photochromic molecules with high sensitivity and signal
transduction capability are points of scientific interest and major activity in the field of color
chemistry. A large variety of applications for photoresponsive molecules has been investigated in high-
level technologies such as controlling the conformation and activity of biomolecules[1-3], sensors [4-
10], data recording, optical storage, optical switching [11-14], displays, non-linear optics [15-18], and
other photochemical and photophysical reactions [19, 20].
Molecular switches that exhibit reversible ring opening-closing behaviour have drawn much attention
recently, owing to the well-separated absorption bands of the two isomers [21-26]. In these molecules,
spiropyrans which have unique molecular binding abilities, and signal transduction functions are
known as important classes [12, 13, 27]. Because, the stability and photosensitivity of spiropyrans are
strongly dependent on the substituent, the design of novel structures of these photoswitches and the
investigation of their optical behaviour are interesting [14, 15, 28].
In the investigation of spiropyrans, few bis-spiropyrans have been reported [15, 18-20]. The
development of such spiropyrans has led to the development of bifunctional chromophores in one
molecule as highly sensitive molecular switches. The two provided zwitterionic merocyanine units,
which had a more sensible system, developed higher sensitivity and molar capacity than the mono-
spiropyran [21]. Few studies have been conducted on bis-spiropyran systems and their unique
properties [12, 15, 21, 29-31], and to the best of our knowledge, bis-azospiropyran systems have been
introduced by authors only recently [32].
Therefore, in keeping with our earlier research on the synthesis and properties of symmetrical bis-
azospiropyran dyes [32, 33], we developed in the current study an asymmetric azo spiropyran
derivative on an extended conjugation push-pull system, in which two different spiropyrans are linked
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by a bis-azo extended aromatic system to develop a very sensitive photochromic system with high-
level color strength in merocyanine form. The colorimetric behaviour, luminescence, and switching
kinetics as well as solvatochromic effects of all the synthesized photochemic compounds were also
investigated. Moreover, in order to expose the particular photo properties of the target structure, six
different derivatives of spiropyranes with relative structures were synthesized and studied as well.
Besides, molecular level of energy for each photoreactive form of the synthesized bis-azospiropyrans
was calculated using density functional theory method (DFT).
2. Experimental
2.1. Chemical and apparatus
Chemicals were purchased from Merck and used without further purification. Melting points were
determined with a Büchi B-545 melting-point apparatus; uncorrected. UV/Vis Spectra were recorded
with a Multispec-1501-Shimadzu UV/Vis spectrophotometer. Infrared spectra were recorded with a
Perkin-Elmer-Spectrum-One-BX FT-IR spectrometer. Fluorescence spectra were obtained by a Perkin-
1
Elmer-LS-55 spectrometer. H-NMR Spectra were obtained with a Bruker-500-Avance Fourier-
transform (FT) NMR instrument, at 500 MHz, in solvents that are indicated in parenthesis before the
chemical shift values (δ relative to TMS and given in ppm). Mass spectrometry analyses were
performed at the Finnigan-Mat-8430 mass spectrometer. Elemental analyses for C, H, and N were
recorded with a Heraeus-CHN-O-Rapid analyzer. A 365 nm UV handheld lamp (8-watt cm-2) were
used as the excitation light sources for the photochromic ring-opening reactions.
2.2. Computation
Our theoretical results for the synthesized spiropyrans are based on first-principles density functional
theory (DFT) calculations. Molecular geometries of the synthesized bis-azospiropyrans (1, 6a and 6b)
and their probable photochemical products were optimized at the B3LYP/6-31G (d) level of theory
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without applying any restrictions on the molecular symmetry. DFT calculations carried out using the
GAMESS set [21]. Graphical representations of quantum chemical calculation outputs were prepared
by MacMolPlt [34] and CHEMCRAFT [35] software.
2.4. Synthesis
2.4.1. Synthesis of azo salicylaldehydes
2.4.1.1. Mono-azosalicylaldehydes 2
A solution of NaNO2 (0.8 g, 11.6 mmol) in 5 ml H2O was added to a solution of salicylaldehyde (1.3
ml, 10.7 mmol) in 20 ml H2O including NaOH (0.43 g) and the temperature was kept lower than 5 °C.
The resulting solution was added dropwise to a solution of aniline (1 g, 10.7 mmol) and 3 ml HCl
(36%) in 10 ml H2O at 0 °C. The mixture was stirred for 1 h at room temperature; then the precipitate
was filtered and washed out with distilled water. Compounds based on monoazo-salicylaldehyde 2
were crystallized in EtOH to afford pure compounds.
2-hydroxy-5-[(E)-phenyldiazenyl]benzaldehyde 2a. Yellow powder (75%). M.p: 128-129°C.
1
IR(KBr) υ= 3430(OH), 2975(CH=),1619 (C=O), 1342 cm-1 (C-N). H-NMR (CDCl3) δ =7.18 (d, 1H,
J=8.83Hz, CH), 7.53 (t, 1H, J =7.32Hz, CH), 7.57 (t, 2H, J=7.32Hz, 2CH ), 7.95 (d, 2H, J= 7.32Hz,
2CH), 8.23 (d, 1H, J = 8.83Hz, CH), 8.26 (s, 1H, CH), 10.08 (s, 1H, CHO), 11.37 (s, 1H, OH).
Anal.calc. for C13H10N2O2 (226.235): C 69.02, H 4.45, N 12.38; found: C 68.99, H 4.47, N 12.42.
2.4.1.2. Synthesis of symmetrical bis-azo salicylaldehydes 3
The diazotation of 1,4-diphenylenediamine performed according to the reported procedure in our
previous work [32] as below:
A solution of NaNO2 (1.6 g, 23.2 mmol) in H2O (5 mL) was added to salicylaldehyde derivatives
(21.8 mmol) in H2O (10 mL) including NaOH (1.8 g) at 5 °C. The resulting solution was added
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dropwise to a solution of 1,4-diaminobenzene (11.6 mmol) in HCl (10 mL, 1 %) at 0 °C. The mixture
was stirred for 1 h at room temperature; then the precipitate was filtered and washed out with distilled
water. The compounds 3 were crystallized in EtOH to afford pure compounds.
3,3'-[Benzene-1,4-diylidi(E)diazene-2,1-diyl]bis(6-hydroxybenzaldehyde) 3a. Green powder (96%).
1
Mp: 207-209 °C. IR (KBr) υ=3435 (OH), 2971 (CH=), 1621 (C=O), 1349 cm-1 (C-N). H-NMR
(DMSO-d6) δ=6.95 (d, 2H, J =8.1Hz, 2CH), 7.41 (d, 2H, J= 8.1Hz, 2CH), 7.52 (s, 4H, 4CH), 7.65 (s,
1H, 2CH); 9.01 (s, 1H, 2CHO), 13.12 (s, 1H, 2OH). Anal. calc. for C20H14N4O4 (374.35): C 64.17, H
3.77, N 14.97; found: C 64.21, H 3.74, N 14.99.
3,3’-[benzene-1,4-diylidi(E)diazene-2,1-diyl]bis(4,4’-diethylamino-6-hydroxybenzaldehyde)
3b.
Reddish Yellow powder (95%). Mp: 190-192 °C. IR (KBr) υ= 3422 (OH); 2936 (CH=); 1619 (C=O);
1329 cm-1 (C-N). 1H-NMR (CDCl3) = 1.25 (t, 6H, J=7.2Hz, 2CH3), 3.46 (q, 4H, J=7.2Hz, 2CH2), 7.11
(s, 2H, 2CH), 7.67 (s, 4H, 4CH), 8.01 (s, 2H, 2CH), 8.46 (s, 2H, 2CHO), 13.12 (s, 2H, 2OH). Anal.
Calc. for C28H32N6O4 (516.60): C 65.10, H 6.24, N 16.27. Found C 65.14, H 6.18; N 16.30.
2.4.1.3. Synthesis of asymmetrical bis-azo salicylaldehyde 9
First, a solution of NaNO2 (1.14 g, 16.6 mmol) in H2O (5 mL) was added to a solution of
salicylaldehyde (16.6 mmol) in H2O (10 mL) including NaOH (0.6 g, 15 mmol) at 5 °C. The resulting
solution was added dropwise to the 1,4-phenylenediamine (2.6 g, 16.6 mmol) in HCl (60 mL, 1 %) at 0
°C. The mixture was stirred for 1h at room temperature; then the precipitate was filtered and washed
out with distilled water, and dried at 50 °C in vacuum oven. At the second step, a solution of NaNO2
(0.057 g, 0.82 mol) in H2O (3 ml) was added dropwise to
a
solution of 4-
(diethylamino)salicylaldehyde (0.16 g, 0.82 mmol) in H2O (5 mL) including NaOH (0.06 g, 1.5 mmol)
at 0-5 °C. Then the resulting solution was added to a solution of the obtained precipitate from the first
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step (0.2 g, 0.82 mol) in HCl (11 mL, 1 %) at 0°C. The mixture was stirred for 1 h at room
temperature; then the precipitate was filtered and washed out with distilled water. Compound 9 was
crystallized in EtOH to afford pure substance.
4-(diethylamino)-5-((E)-(4-((E)-(3-formyl-4-hydroxyphenyl)diazenyl)phenyl)diazenyl)-2-
hydroxybenzaldehyde 9. Deep red powder (58%). Mp: 230 °C. IR (KBr) υ= 3421 (OH), 2974 (CH=),
1
1610 (C=O), 1333 cm-1 (C-N). H-NMR (CDCl3) δ = 1.26 (t,6H, J = 7.10Hz, 2CH3), 3.46 (q,4H, J
=7.10Hz, 2 CH2), 6.24-721 (m,9H, Ar H), 8.49 (s, 1H, CHO), 8.50 (s, 1H, CHO), 13.8 (s, 1H, OH),
13.9 (s, 1H, OH). Anal. calc. for C24H23N5O4 (445.479): C 64.71, H 5.20, N 15.72, Found: C 64.75, H
5.18, N 15.69.
2.4.2. Synthesis of Spiropyran compounds
The simple spiropyrans 4 were prepared by one step reaction of Fischer's base and salicyaldehyde
derivatives according to known procedure[36]. Procedure to the synthesis of the other spiropyrans was
as below:
2.4.2.1. Synthesis of mono-azo spiropyran 5 and symmetrical bis-azo spiropyran 6
2-Methylene-1,3,3-trimethylindoline (1 mmol for mono-spiropyran and 2 mmol for bis-spiropyran
compounds) in 5 ml CHCl3 was added dropwise to a refluxing solution of salicylaldehyde derivatives
2 or 3 (1 mmol) in 20 ml chloroform, in 20 minutes. The mixture was refluxed for 2 h, and then cooled
to room temperature. Then the precipitate was filtered and washed out with distilled water, and
crystallized in EtOH to afford the pure spiropyran compounds 5 or 6 accordingly.
(E)-1',3',3'-trimethyl-6-(phenyldiazenyl)spiro[chromene-2,2'-indoline] 5a. Pink powder (87%). Mp:
180-182 °C. IR (KBr) υ= 2963 (CH=), 1654 (C(3)=C(4)), 1020 cm-1 (C(2)-O). 1H NMR (DMSO-d6) δ
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= 1.25 (s, 3H, CH3), 1.28 (s, 3H, CH3), 2.80 (s, 3H, NCH3), 4.29 (d, 1H, J=10.19 Hz, H-C(3)), 6.60-
7.25 (m, 14H, 12ArH, and 2H-C(4)). Anal. calc. for C25H25N3O (383.494): C 78.30, H 6.57, N 10.96;
found: C 78.34, H 6.53, N 10.92.
3,3'-[Benzene-1,4-diylidi(E)diazene-2,1-diyl]bis[1',3',3'-
trimethylspiro(2H-1-benzopyran-2,2'-
indoline)-6-yl] 6a. White powder (89%). Mp: 203-205°C. IR (KBr) υ= 2966 (CH=), 1605
1
(C(3)=C(4)), 1311 (C(2)-N), 1021 cm-1 (C(2)-O). H-NMR (DMSO-d6) δ = 1.25 (s, 6H, 2CH3), 1.28
(s,6H, 2CH3), 2.80 (s, 6H, 2NCH ), 4.20 (d, 2H, J=10.2Hz, 2H-C(3)), 6.60-7.26 (m, 20H, 18ArH, and
2H-C(4)). MS (EI) m/z (%): 685 (M+1, 1), 655 (M−2 Me, 1), 493 (15), 404 (2), 318 (88), 274 (25),
212 (100), 183 (18). Anal. calc. for C44H40N6O2 (684.84): C 77.17, H 5.89, N 12.27; found: C 77.20, H
5.85, N 12.30.
3,3’-[benzene-1,4-diylidi(E)diazene-2,1-diyl]bis[1',3',3'-trimethyl-6-diethylamino
spiro(2H-1-
benzopyran-2,2'-indoline)-6-yl] 6b. Cream color powder (80%) Mp: 216-218 °C. IR (KBr)
1
υ=2966(CH=), 1605 (C(3)=C(4)), 1313 (C(2)-N), 1020 cm-1 (C(2)-O). H NMR (CDCl3) = 1.13 (t,
6H, J=6.97 Hz, 2 CH3) 1.38 (s,6H, 2 CH3), 1.43 (s,6H, 2 CH3), 2.92 (s, 6H, 2NCH3), 3.28 (q,4H,
J=6.97Hz, 2 CH2), 4.3 (d,2H, J =10.1 Hz,, 2H-C(3)); 6.37-7.28 (m, 16H, ArH and 2 H-C(4)). MS (EI)
m/z (%): 828 (M+1, 1), 797 (M-2 CH3, 1), 493 (16), 404 (2), 318 (79), 274 (28), 212 (100), 183 (20).
Anal. Calc. for C52H58N8O2 (827.09): C 75.51, H 7.07, N 13.55; Found C 75.59, H 7.11, N 13.53.
2.4.2.2. Synthesis of asymmetrical bis-azo Spiropyran compound 1
2-Methylene-1,3,3-trimethylindoline (0.35 ml, 2 mmol) in 5 ml CHCl3 was added dropwise to a
refluxing solution of salicylaldehyde derivatives 9 (0.44 g, 1 mmol) in 20 ml chloroform, in 20
minutes. The mixture was refluxed for 2 h, and then cooled to room temperature. Then the precipitate
was filtered and washed out with distilled water, and crystallized in EtOH to afford the pure spiropyran
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compound 1.
N,N-diethyl-1',3',3'-trimethyl-6-((E)(4-((E)-(1',3',3'-trimethylspiro[chromene-2,2'-indoline]-6-
yl)diazenyl)phenyl) diazenyl) spiro [chromene-2,2'-indolin]-7-amine 1. Cream-colored powder (44%).
1
Mp: 185-186 °C. IR (KBr) υ= 2967(CH=), 1612(C(3)=C(4)), 1310 cm-1 (C(2)-N),1021(C(2)-O). H-
NMR (CDCl3) = 1.00 (t,3H, J=6.9Hz, CH3), 1.23 (s,3H, CH3), 1.25 (s,3H, CH3), 1.55 (s,3H, CH3),
1.56 (s,3H, CH3), 2.79 (s,3H, NCH3), 3.00 (s, 3H, NCH3), 3.21 (q, 2H, J=6.9Hz, CH2), 4.10 (d,1H,
J=9.98Hz, H-C(3)), 4.23 (d,1H, J=10.18Hz, H-C(3)), 6.15-7.17 (m,19H, 17Ar H, and 2H-C(4)). MS
(EI) m/z (%): 756 (M+, 1), 727 (M-2CH3, 1), 493 (10), 348 (100), 19 (31), 158 (81). Anal. Calc. for
C48H49N7O2 (755.967): C 76.26, H 6.5307, N 12.97; Found C 76.22, H 6.56, N 13.01.
CHO
N
OH
HO
N
CHO
i
H2N
N
N
N
OH
H2N
NH2
+
+
CHO
OHC
N
OH
8
3a
CHO
HO
N
CHO
i
8
OH
N
N
+
OHC
N
OH
Et2N
9
Et2N
4C 3C
N
2C
3'C
O
N
3'C
ii
2C
9
2
(
)
N
N
+
3C 4C
N
N
O
N
Et2N
1
i) NaNO2 - HCl 0-5°C
ii) CHCl3 - Reflux 4h
Scheme 1. Synthesis of asymmetric bis-azospiropyran photochromic dye 1
3. Results and Discussion
Synthesis of asymmetrical substituted of bis-azospiropyran 1 as a target dyad molecule was designed
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in order to study the effect of a highly conjugated bis-azo bridge between two different spiropyrans
with and without an electron donor group (Et2N-) as a push-pull arrangement (Scheme 1).
To study the effects of electron-donating group (Et2N-) and a conjugated azo bridge, simple
spiropyrans 4, mono-azo spiropyrans 5, and symmetrical bis-azospiropyranes 6, all with and without
Et2N- substitution were synthesized (Scheme 2). Their optical properties were compared with
asymmetric bis-azospiropyran 1 as an electron conjugated push-pull system. Photoresponses, color
intensities in merocyanine (Mc) forms, emission intensities in spiropyran (Sp) forms and their
photokinetic conversion of Sp into Mc forms were studied.
In the synthesis procedure, spiropyrans 4 were prepared according to the references.26
Azospiropyrans 5 were obtained by the coupling reaction of salicylaldehyde derivatives 2 with
Fischer's base, i.e. 1,3,3-trimethyl-2-methyleneindoline in good yields (≥80%) (Scheme 2), and 6-
hydroxybenzaldehyde derivatives 2 were obtained from the diazotization of aniline with the
subsequent coupling with salicylaldehydes 7 in high yields (≥95%).
Accordingly, bis-azospiropyrans 6 were obtained in good yields (≥80%) by the coupling reaction of
bissalicylaldehyde derivatives 3 with Fischer's base (Scheme 2), and bis(6-hydroxybenzaldehyde)
derivatives 3 in high yields (≥95%) were obtained from the bis-diazotization of p-phenylenediamine
with subsequent coupling with salicylaldehydes 7.
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CHO
OHC
N
i
OH
NH2
+
HO
N
2, a: R=H;
b: R=Et2N-
7, a: R=H;
b: R=Et2N-
R
R
R
CHO
OH
OHC
HO
N
OH
CHO
i
H2N
NH2
N
N
N
+
R
3, a: R=H;
b: R=Et2N-
R
R
N
N
O
O
4, a: R=H;
b: R=Et2N-
ii
+
7 or 2 or 3
N
R
Fischers base
N
5, a: R=H;
b: R=Et2N-
N
i) NaNO / HCl - 0-5°C
2
R
ii) CHCl - Reflux 4h
3
N
O
N
N
N
N
6, a: R=H;
b: R=Et2N-
O
N
R
Scheme 2. Synthesis of spiropyran derivatives 3-5
Asymmetrical bis-azospiropyran 1 was synthesized by the reaction of asymmetrical bis-azo
precursor 9 with Fischer's base in 2:1 mole ratios; the precursor 9 was prepared in two steps. First, the
mono-azo product 8 was obtained from the product of the diazotization and coupling of 1,4-
phenylenediamine with salicylaldehyde 7a in 1:1 mole ratios as a major product (70%) and the doubly
coupled product 3a as the minor product (30%), according to TLC. Then, the precursor 9 was achieved
from the diazotization and coupling of mono-azo product 8 with N,N-diethyl salicylaldehyde 7b in 1:1
1
mole ratios. The structures of products were identified from their MS, FT-IR, and H-NMR
spectroscopic data and CHN analysis.
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The absorption frequencies in FT-IR spectra of 1 revealed that the peak at 2967 cm-1 was due to C-H
(aromatic); the peak at 1612 cm-1 was related to C(3)=C(4) stretch (Scheme 1); the stretching vibration
of the C(2)-N was observed at 1310 cm-1; and the C(2)-O stretching frequencies occurred at 1021 cm-1.
The 1H-NMR spectra for 1 clearly showed four singlet signals at 1.23 and 1.25 ppm (6H) and 1.55 and
1.56 ppm (6H) for four CH3 at 2 C(3)-(CH3)2 protons (Scheme 1). The close proximity of two CH3 of
indoline moieties (C(3)-(CH3)2) to the olefinic proton or an oxygen moiety of the pyranyl group made
them magnetically non-equivalent [8]. Two singlet signals appeared in 2.79 and 3.00 ppm (6H) for two
N-CH3 protons of 1, which are chemically and magnetically non-equivalent. The olefinic protons of 1
revealed four doubles, two of them at 4.10 and 4.23 ppm for two H-C(3) with large coupling constants
(J=9.98-10.18 Hz), and the other two for olefinic protons 2 H-C(4) in the aromatic regions. The former
doublet for olefinic protons with large coupling is very characteristic of a spiropyran system. Fig. 1
shows the absorption spectrum of 10-5 M dyad 1, 6b and 5b and 10-4 M 4b, 4a, 5a and 6a. It shows the
maximum absorptions of the photochromic compounds in dark (OFF) and in their photostationary
states in dichloromethane (DCM) after exposure to UV light. All the photochromic compounds were
bleached in DMF at 125-130 °C.
Fig. 2 shows the color strengths of 10-5 M photochromic compounds 1 and 4-6 in dichloromethane
(DCM) after 0, 10, and 30 minutes exposure to UV light at the ambient temperature. The asymmetric
photochemically bifunctional compound 1 revealed the fastest speed and best color strength photo
response. The addition of a diethyl amine group to the pyran moieties of spiropyrans 4 and 5 as an
electron-donating substitution increased the rapidness and strength of photoresponses of photochromic
molecules. Although 6b with a high electron density is colored (yellow) in Sp form, it revealed a
different shade in Mc form due to the combination of the colors yellow (λmax 420 nm, ε 4.7104 M-
1.cm-1) and red (λmax 555 nm, ε 2.7104 M-1.cm-1).
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Fig. 1. UV-Vis spectra of the photochromic molecules before and after exposure to 366 nm UV light
and the Mc forms are reversible to Sp forms at 125-130˚ in DMF (Concentrations of 1, 6b and 5b is
10-5 M and 4b, 6a, 5a and 4a is 10-4 M to adjust the maximum absorbance of color form.)
As shown in Fig. 1-3, absorption intensity and subsequently the color strength of photochromic
molecules were in the order of 1>>6>5>4. The color strength of merocyanine forms of mono
azospiropyrans 5 were more than that of the corresponding spiropyrans 4.
As mentioned, a large molar absorption coefficient of as much 9.7104 M−1·cm−1 formed in
merocyanine state of 1. That is considerably high, even for a commercial non photochromic dye. The
superior strength of the color produced in Mc form of the asymmetric bis-azospiropyran 1 was
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astonishingly higher than that of the symmetric bis-azospiropyrans 6 (2.7 and 3.8 × 104 M−1·cm−1 for
6b and 6a respectively). Moreover, their absorption coefficient was much more than the corresponding
known spiropyran 4 and mono-azospiropyran 5 chromophores (ε = 3.1 × 103 M−1·cm−1 and 1.3 × 104
M−1·cm−1, respectively) (Fig. 3). Such a high absorption coefficient indicates high sensitivity to the
irradiation and thus such a distinguished response. Practically, we have developed a novel photoswitch
with improved light sensitivity and resolution between Sp (OFF) and Mc (ON) forms.
Fig. 2. Time and strength of photoresponses of 10-5 M photochromic molecules 1 and 4-6 in DCM
after 0, 10, and 30 minutes exposure to UV light at ambient temperature.
Prompt response to light is another important parameter for photoresponsive dyes. Hence,
absorbance variations at wavelengths of maximum absorption for the dyes 4a, 5a, 6a at 550 nm and
for 1 at 555nm under irradiation at 365 nm were studied and their photokinetic curves are presented in
Fig. 4. The response rates are in this order: asymmetric bis-azospiropyran 1>>symmetric bis-
azospiropyran 6a> azospiropyran 5a > spiropyran 4a.
Photoreversibility as an inherent property of spiropyrans is revealed in Fig. 5 for the product 1 as
well. In this regard, 10-3 M of photochromic molecule 1 in DCM was exposed to 365 nm UV and then
Vis light in 5 minute intervals.
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This highly sensitive light-driven molecular switch is particularly valuable for designing of
instruments with high resolution light sensing, and it could be used in optical storages [37, 38].
Fig. 3. UV-Vis spectra (right) and Spectroscopic data (Left) of the photochromic compounds in
photostationary states Mc forms.
a
b
c
Fig. 4. Time-evolution of the maximum absorbance of 10-5 M photochromic materials (4a, 5a, 6a at
550nm and at 555nm) under irradiation at 365 nm (a), zoom out of the curve for 1 (b) in DCM, and
UV-Vis absorption spectra of 1 during 20 min UV irradiation (c).
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Fig. 5. Photoreversibility of 10-3 M photochromic molecule 1 at 555 nm under irradiation at 365 nm
UV and Vis light in DCM.
The other possibility for photochemical process is the well-known cis-trans isomerization of azo-
bonds that leads to decrease of color intensity [39-43]. As we reported earlier [44], cis-trans
isomerization of bis-azo precursor (3a) was leaded to reduction of color intensity. Therefore, such
isomerization of azobenzene moieties should elicit a negligible decreasing impact on the resulted
immensely increased color intensities due to mero forms spiropyrans.
In addition, this novel bisazospiropyran revealed a high fluorescence quantum yield value in Sp
form with the maximum difference ratio in emission intensity in Sp to Mc forms (FSP/FMC) rather than
on the other structures (Table 1). These results indicate they are potentially suitable for photoswitch
applications with high sensitivity and resolution using fluorescence emission.
3.1. Fluorescence properties of the synthesized photoresponsive products
All the synthesized photoresponsive dyes, 4a-b, 5a-b, 6a-b, and 1, revealed fluorescence emission in
their colorless forms; their emission intensities diminished when exposed to UV irradiation. The
Stokes shift[45] was studied for all the dyes and the values were calculated for open and closed forms,
which yielded between 1752-9960 cm-1 for Sp forms and 3329-11711 cm-1 for Mc forms (Table 1).
16

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The fluorescence quantum yields [45, 46] of spiropyrans were calculated using anthracene
(Øref=0.27) as a reference. For Sp forms, the fluorescence quantum yields were obtained between 0.33-
0.45, thus, the minimum fluorescence quantum yield was observed in 5a and the maximum belonged
to 1 (0.45).
Ratio of emission intensity of Sp to Mc form (FSP/FMC) is another interesting parameter, indicating
signal to noise ratio in this issue. Therefore, higher FSP/FMC ratio makes the molecules potentially more
suitable for photoswitch application using fluorescence emission. This parameter was obtained for all
the synthesized photochromic molecules and the asymmetric bis-azospiropyran 1 revealed the highest
ratio of FSP/FMC (Table 1). Bis-azospiropyrans 1 and 6 declared higher FSP/FMC ratios than mono-azo 5
and simple 4 spiropyrans.
The novel asymmetric bis-azospiropyran structure 1 revealed a high ratio of FSP/FMC fluorescent which
make it potentially applicable in many molecular actuators including optical memory devices,
holographic gratings, and drug delivery vesicles [38, 47].
3.2. Exposure–response modelling to study the bifunctional photochromic activity of 1
Dose–response relationships are well-known methods to study biological processes [48-51]. Herein,
we have applied that to study the bifunctional photochromic activity of 1 by describing “absorbance of
the UV irradiated molecules” as response and “time of exposure” as dose. To the best of our
knowledge, application of this model for photochromic molecules is for the first time. Subsequently, to
define whether the two spiropyrans moieties of the bifunctional dyes will open or not, bi-dose response
(Eq. 1) and dose-response (Eq. 2) fitted on absorbance (558 nm)-time curve of photochromic molecule
1 (10-5 M in DCM) by employing Levenberg-Marquardt as an iteration algorithm (Fig. 6).
(1)
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(2)
Wherein, A0 is the absorbance at time = 0, A1 is the absorbance at stationary state. P is proportion of
response spans known as proportion of SP-MC to MC-MC photochromic products of 1. Logarithms of
T, T1 and T2 are centres of related spans in time logarithmic axis and h, h1 and h2 are hill slopes at T,
T1 and T2 respectively.
Consequently, by successfully fitted the obtained experimental data to a bi-dose exposure–response
relationship model, we observed that both photochromic moieties in the dye 1 are photoactive.
Contrary to the dyes 6a and 6b which failed in bi-dose response fitting, and cannot find direction to
change parameters to reduce Chi-sqr.
This algorithm could help us to show the reason of higher color ability of molecule 1 than 6a and 6b
owing to produce Mc forms of both spiro moieties in photochromic bifunctional molecule 1 during UV
light exposure.
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Table 1 Excitation and fluorescence characteristics of the produced photochromic dyes in DCM at 293
K (C= 1×10−5 mol.l−1).
Spectroscopic properties
λex (nm)
1
6b
5b
323
383
60
4b
335
411
76
6a
345
348
3
5a
4a
395
459
64
343
524
181
318 330
381 380
λem (nm)
Δλ(nm)
SP
63
50
νA-νB (cm-1)
3530
1510
0.45
361
440
79
10070 4850 5519
250 5199 3987
845 755 518
0.38 0.33 0.36
Intensity
φ
1491
0.39
343
582
0.36
344
437
93
480
0.37
330
430
100
λA (nm)
λF (nm)
Δλ (nm)
345
432
87
345 344
438 427
513
170
93
83
MC
νA-νB (cm-1)
4974
359
0.19
9661
418
6186 4047 5842 6154 5651
Intensity
500
440
0.33 0.016 0.02 0.23
1.1 2.5 1.3 1.8
337
593 295
φ
0.26
0.02
a
FSP/FMC
4.2
3.6
1.2
a Fluorescence Intensity of colorless spiropyran form (FSP), and merocyanine form (FMC) in stationary
state.
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d)
Residual
Sum of
Regular
Residual
Pattern
LogT1,
LogT
Adj. R-
Square
Reduced
Chi-Sqr
Model
A0
A1
P
LogT2
h1
h2
Squares
0.00279
BiDoseResp
DoseResp
0.13
0.25
2.52
2.75
0.45
-
0.67
0.95
1.00
-
2.05 4.55
2.49
0.99942
0.99648
0.00034 Succeeded
0.00213 Failed
-
0.02346
Fig. 6. (a) Bi-dose response fitted to absorbance (558 nm)-logarithmic time data for photochromic
molecule 1 (10-5 M in DCM), (b,c) the related residuals of related fitted curved for both bi-dose and
dose responses of 1; and (d) the obtained fit statistics and parameters
3.3. Solvatochromic effects
Photoresponsive materials with light controlled polarizability has been of significant interest in many
applications such as microfluidic devices, photocontrol of liquid motion, and self-cleaning surfaces.
Hence, it seems essential to find an appropriate controllable medium by spectroscopic evaluation of
media related impacts on the photoresponsive materials [52-55] .
Herein, the spectroscopic results of spontaneous and photochemical colorations of the synthesized
dyes 1 and 6b were studied in different solvents at room temperature. The solvents used in this study
were categorized in three classes of polar-aprotic, polar-protic and polar-halogenated, regarding to
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their dipole moments and polarity parameters (ET) according to Fig. 7.
The symmetric bis-azo spiropyran 6b in polar-aprotic solvents not only was not revealed coloration
spontaneously in dark, but also photochemically even after one hour exposing to 365 nm UV light.
Polar-protic solvents may stabilize Mc form of the photochromic dye 6b and led the Sp-Mc
equilibrium to Mc formation, thus the coloration was observed in dark. Moreover, the dye 6b was
diluted five-fold in methanol for spectroscopic study due to more Mc formation and high color
strength. As expected, water could not dissolve the dyes. According to the results, polar-halogenate
solvents as chloroform and dichloromethane (DCM) could stabilize the Mc form of 6b, and coloration
was observed in dark.
Fig. 7. The solvents categorized regarding to their dipole moments versus polarity parameters (ET).
The same solvatochromic study was achieved for asymmetrical bifunctional bis-azo spiropyran 1
(Table 2). In this case, only polar-protic solvents led Mc-Sp equilibrium to Mc form and caused
coloration. Aprotic-polar and also polar-halogenated solvents could not drive the dye to Mc form in
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dark. That means the polarity of the push-pull molecule 1 in Sp form is more than the symmetry
derivatives; so, only polar and protic solvents could push it to mero form in dark.
Table 2 The spectroscopic results of spontaneous coloration (in dark) of the synthesized photochromic
dye 1 and 6b in different solvents at 293 K.
6b
1
a
C
λmax
Time
C
λmax
Time
(min)
b
c
A0
A1
A0 A1
Solvents
(M) (nm)
(min) (M) (nm)
10-4
10-4
10-4
10-4
10-4
10-4
10-4
-
-
-
-
-
-
0
0
0
0
0
0
0
0
0
0
0
0
60
60
60
60
60
60
60
70
60
60
60
10-5
10-5
10-5
10-5
10-5
10-5
-
-
-
-
-
-
0
0
0
0
0
0
0
0
0
0
0
0
60
60
60
60
60
60
Acetone
THF
EtOAc
Acetonitrile
DMF
Dioxane
1-Butanol
Methanol
Ethanol
DCM
552 0.28 1.5
10-5 550 0.03 0.38 60
10-5 550 0.54 1.31 10
10-5 548 0 0.53 30
5×10-5 548 0.56 1.38
10-4
10-4
10-4
548 0.14 0.65
558 0.01 1.05
10-5
10-5
-
-
0
0
0
0
60
60
562
0
1.08
Chloroform
λmax a: wavelength of maximum absorbance for coloration; b A0: Visible light absorbance at time = 0; c
A1: Visible light absorbance at stationary state.
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3.4. Density functional theory study of the synthesized bis-azospiropyrans
To obtain the molecular energy levels for each photoreactive form, molecular structure of the
synthesized bis-azospiropyrans (1, 6a and 6b) were studied by quantum molecular calculation with
density functional theory method (DFT) at the B3LYP/6-31G (d) level of theory without applying any
restrictions on the molecular symmetry. The results summarized in Fig. 8 and Table 3. To compare the
obtained energy levels, the related Sp-Sp forms as a more stable isomer were chosen. As shown in Fig.
8, the energies of Sp-Mc isomers are higher than Sp-Sp forms. In photoreactive molecule 1 with two
different spiro moieties, diethyl amino substituted Mc form is only 1 kJ.Mol-1 less stable than the
unsubstituted moiety, however they both are less stable than the same isomers in the symmetric
molecules 6a and 6b. The interesting point is that the difference energy between Sp-Mc (and Mc-Sp)
with Mc-Mc form in structure 1 is less than the symmetric structures (6a and 6b), and it shows that
going to Mc-Mc form in this structure is easier than that in 6a and 6b. These findings are in
accordance with experimental results that revealed more color ability (higher ) in photostationary
state of photochromic molecule 1 than 6a and 6b. They also support exposure–response modelling
results which revealed bifunctional activity of asymmetric bis-azospyropyran dye 1 rather than
monofunctional activity of the symmetric bis-azospyropyrans.
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Fig. 8. Molecular energy level for each photoreactive forms of the synthesized bis-azospiropyrans (1,
6a and 6b)
Table 3 The obtained molecular level of energy, and C-O bond lengths for each photoreactive form of
the synthesized bis-azospiropyrans (1, 6a and 6b).
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Bond
Length (A)
C25-O26
1.48
Bond
Length (A)
C28-O27
1.44
E
ΔE
Dye Form
(kJ/mol)
(kJ/mol)
Sp-Sp
-5722660
-5722632
-5722608
-6838954
-6838941
-6838922
-6280823
-6280788
-6280787
-6280777
0.0
27.5
51.8
0.0
Sp-Mc
Mc-Mc
Sp-Sp
-
1.44
-
6a
6b
-
1.44
1.48
1.44
-
Sp-Mc
Mc-Mc
Sp-Sp
-
13.1
31.8
0.0
-
1.44
-
1.48
1.44
-
Sp-Mc
Mc-Sp
Mc-Mc
35.8
36.9
46.6
1
1.44
-
-
The obtained molecular energy levels, which schematically declare in Fig. 8, are presented in Table 3.
Also, the reactive C-O bond lengths in Sp forms of 1, 6a and 6b revealed that one of the C-O bond
lengths are longer than the second one.
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Sp-Sp Forms
6a
1
6b
HOMO
LUMO
Fig. 9. Molecular orbitals for Sp-Sp forms of the synthesized bis-azospiropyrans (1, 6a and 6b)
Moreover, HOMO and LUMO of Sp-Sp forms for the optimized structures of bis-
azospiropyrans are revealed in Fig. 9. Consistent with the frontier orbital theory [56], in the reactant
molecules, the chemistry of conjugated π systems is mainly influenced by the HOMO and LUMO π
orbitals. Here, the extended conjugation in HOMO π orbitals of the electron rich structure (6b) in Sp-
Sp form could confirm the yellow color of this derivative than two other colorless Sp-Sp forms.
Further detailed studies on the exact quantitative analysis of the dynamic behaviour of such
asymmetric bis-azospiropyn dyes are going to deliberate mathematically and will be reported
afterward.
4. Conclusions
Large molar absorption coefficient in the Mc form as 9.7×104 M−1·cm−1 was obtained for asymmetric
bis-azospiropyran 1. Whilst molar absorption coefficient about 3.0×104 M−1·cm−1 was obtained for
symmetric bis-azospiropyrans, and 1.3×104 M−1·cm−1 and 3.1×103 M−1·cm−1 for the corresponding
mono-azospiropyran and spiropyran chromophores respectively, which indicate the strength of
26

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produced color in the Mc form of the developed molecular photoswitch. Also, the asymmetric bis-
azospiropyran exposed quickest response to light in the synthesized photochromic molecules. In
addition, this novel bis-azospiropyran revealed high fluorescence quantum yield (0.45) in the spiro
form and higher ratio of emission intensity of spiro to Mc form (FSP/FMC) than the other structures,
which make it potentially suitable for photoswitch applications with high sensitivity and resolution
using fluorescence emission. Practically, it implies that we have novel photoswitch with improved
light sensitivity and superior discrimination between Sp (OFF) and Mc (ON) forms.
Density functional theory (DFT) calculation revealed that the difference energy between Sp-Mc (and
Mc-Sp) with Mc-Mc form in structure 1 is less than other derivatives (6a and 6b), and it shows that
going to Mc-Mc form in this structure is easier than that in 6a and 6b. This is in accordance with
experimental results that revealed more color ability (higher ) in the steady state form of
photochromic molecule 1 than 6a and 6b, and also exposure–response modelling results which showed
the bifunctional activity of photochromic dye 1 rather than monofunctional activity of photochromic
dyes 6a and 6b.
Acknowledgements
Authors are grateful to the Iran National Science Foundation (INSF) for financial support #92003297.
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