Synthesis and photochromic behavior of mono-, and biphotochromic system linked by p-phenylene bridge

Article abstract of DOI:10.1016/j.cclet.2010.04.036

The synthesis of mono- and bis-1,3-diazabicyclo[3.1.0]hex-3-ene derivatives with indole ring and p-phenylene spacer, which behave as photochromic materials, is reported. The structure-photochromic behavior relationship (SPBR) of the synthesized compounds

Full text of DOI:10.1016/j.cclet.2010.04.036

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Chinese Chemical Letters 21 (2010) 1199–1202
www.elsevier.com/locate/cclet
Synthesis and photochromic behavior of mono-,
and biphotochromic system linked by p-phenylene bridge
*
N.O. Mahmoodi , K. Tabatabaeian, A. Ghavidast
Department of Chemistry, Faculty of Science, University of Guilan, P.O. Box 41335-1914, Rasht, Iran
Received 3 March 2010
Abstract
The synthesis of mono- and bis-1,3-diazabicyclo[3.1.0]hex-3-ene derivatives with indole ring and p-phenylene spacer, which
behave as photochromic materials, is reported. The structure–photochromic behavior relationship (SPBR) of the synthesized
compounds has been analyzed.
# 2010 N.O. Mahmoodi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Biphotochromic; p-Phenylene bridge; Bis-1,3-diazabicyclo[3.1.0]hex-3-ene
Photochromic compounds undergo a photo-induced reversible isomerization process between two isomers with
different absorption spectra upon irradiation with light of appropriate wavelength [1–3]. Although many types of
photochromic compounds have been synthesized so far, few molecules that show photochromic reactivity in the
crystalline state are reported [4–6]. Solid-state organic photochromic molecules have attracted much attention due to
their potential applications in various optoelectronic devices such as optical memory, optical switch, electronic
display, information storage, and so on. Among them, the 1,3-diazabicyclo[3.1.0]hex-3-ene derivatives are a class of
unique photochromic compounds, which generate bistable molecules and undergo photochromism in the crystalline
state. In continuation to our prior work in the field of synthesis of 1,3-diazabicyclo[3.1.0]hex-3-ene systems and their
applications in photochromism organic chemistry [7–13].
In this paper, a novel mono-1, 2 and bis-3, 4 photochromic compounds contain indole moiety and p-phenylene
spacer for first time is reported (Scheme 1).
The general preparation of the photochromic compounds 1–4 is depicted in Scheme 2. Reaction of indole with 1,4-
bis(chloromethyl)benzene in the presence of NaH in dry DMF under nitrogen atmosphere leads to the preparation of
bis-indole 8 in 72% yield. This recrystalized pure bis-indole was taken for Vilsmeier–Haack reagent (POCl3, DMF) at
room temperature, to produce a bis-aldehyde 7 in 89% yield [14]. Reaction of both one equivalent of indole 3-
carbaldehyde or half-equivalent of bis-aldehyde 7 together with one equivalent of ketoaziridines (5 or 6) and 10 hg
equivalents of NH4OAc under anhydrous conditions afforded the desired compounds 1–4 in good yields (70–80%).
Ketoaziridines 5, 6 was prepared according to literature [7,9,10]. DMF was dried over activated 0.4 nm molecular
* Corresponding author.
E-mail address: mahmoodi@guilan.ac.ir (N.O. Mahmoodi).
1001-8417/$ – see front matter # 2010 N.O. Mahmoodi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.04.036

[(Scheme_1)TD$FIG]
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N.O. Mahmoodi et al. / Chinese Chemical Letters 21 (2010) 1199–1202
Scheme 1. Structure of photochromic compounds 1–4.
[(Scheme_2)TD$FIG]
Scheme 2. Synthetic route for the preparation of compounds 1–4.
sieves and EtOH was distilled from Mg, the other reagents and solvents were commercial and used without further
purification.
The colorless solids 1 and 2, after remaining for ꢀ5–10 s under the sunlight, changed the color; this due to the
formation of zwitterionic double charged imine ylide B (open formed) [7,10]. Compound 1Awith NO2 group in para
position changed to deep green 1B, while compound 2Awith NO2 group in meta position changed to pink 2B (Fig. 1).
Compounds 1 and 2 with one photochromic center in solid state were bleached by heating them in the oven for 4–
[(Fig._1)TD$FIG]
Fig. 1. The photoinduced (UVand VIS) transformations of colorless photochromic compounds 1A and 2A, and conversion to 1B(green) and 2B (pink).

[(Scheme_3)TD$FIG]
N.O. Mahmoodi et al. / Chinese Chemical Letters 21 (2010) 1199–1202
1201
Scheme 3. Photochromic reactions of bis-1,3-diazabicyclo[3.1.0]hex-3-enes 3 and 4.
5 min. However, compounds 3 and 4 with two photochromic centers after 10–12 min heating in oven was converted to
the initial forms. The repeatable cycle number in solid phase for 1 and 2 is estimated to be >104 times. Upon
alternating irradiation with UV and visible light, the bis- and mono-bicyclic aziridine derivatives undergo reversible
cycloreversion/cyclization photochromic reactions between the cc, co and oo isomers.
Compounds 3 and 4 contain two photochromic centers that linked by premade p-phenylene spacer, upon exposure
to the light sources undergo reversible photocyclization concerning their close–close ring isomer (cc), close–open ring
isomer (co) and open–open ring isomer (oo) forms. Photoisomerization of these photochromic compounds with a p-
phenylene spacer unit are illustrated in Scheme 3. The nitrile ylide of photoisomers 3 (oo) and 1B with a para-NO2
group is more highly stabilized than photoisomers 4 (oo) and 2B with meta-NO2 substitution. The pale orange 3 (cc)
after 15 s exposure to sunlight converted to the deep orange 3 (oo), while yellow 4 (cc) changes to orange 4 (oo).
The photochromic behavior of 1–4 in ethanol induced by photoirradiation at room temperature was also studied
[15]. Fig. 2 shows the absorption spectral change of EtOH solution of compound 3 upon irradiation with 254 nm light
(c = 1.0 Â 10À4 mol/L). Upon irradiation with UV light, colorless solution of the close-ring isomer 3 (cc) changed to
the yellow color open-ring isomers 3 (co) and 3 (oo). The absorption spectrum returned to its initial state when it was
irradiated with visible light at longer wavelength than 450 nm.
The wavelengths maximum absorption of 1,3-diazabicyclo[3.1.0]hex-3-ene derivatives 1–4 before (lAmax, nm) and
after (lBmax, nm) irradiation in EtOH at 254 nm wavelength are shown in Table 1. Photochromic compounds 1 and 3
with NO2 group in para position, exhibit a bathochromic shift in comparison with compounds 2 and 4 with NO2 group
in meta position. This noticeable bathochromic shift (ꢀ38–44 nm) means that para-NO2 group participates in the p
conjugation of the central photochromic backbone more effective than meta-NO2 group.
Detailed procedures for synthesis of compounds 1–4 and 8 and analytical data of compounds 1–4 and 8 are
available in supporting information.
[(Fig._2)TD$FIG]
Fig. 2. UV–vis absorption spectra of 3 (left) and 4 (right) upon irradiation under 254 nm light in EtOH (c = 1.0 Â 10À4 mol/L) before and after
successive UV irradiation for 65 s.

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N.O. Mahmoodi et al. / Chinese Chemical Letters 21 (2010) 1199–1202
Table 1
UV–vis data of the 1,3-diazabicyclo[3.1.0]hex-3-enes in EtOH.
lAmax (nm)
lBmax (nm)
Compound
1
2
3
4
201, 266
219, 254, 284, 434
213, 260, 307, 390
202, 257, 292, 429
202, 296, 391
219, 243
202, 217, 247, 296
212, 246, 288
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/
j.cclet.2010.04.036.
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[14] The typical procedure for the synthesis of 1-((4-((3-formyl-1H-indol-1-yl)methyl)phenyl)methyl)-1H-indole-3-carbaldehyde 7 was described
as follows: POCl3 (1.8 mL, 19.8 mmol) was added slowly to dry DMF (9.1 mL) at 0 8C, after 30 min a solution of 8 (2.2 g, 6.64 mmol) in DMF
(46 mL) was added drop wise over 1 h at 0 8C. The mixture was stirred at r.t. for 6 h and poured into 5 mol/L sodium hydroxide (95.24 mL).
The mixture heated at 100 8C for 30 min and allowed to cool. The white precipitate was filtered off. Washed with water and purified by silica gel
column chromatography to give a pale yellow solid 2.3 g (89%), mp = 294 8C, IR (KBr): 3100, 3050, 2910, 2750, 2800, 1657, 1610, 1528,
1461, 1400 cmÀ1; 1H NMR (500 MHz, DMSO-d6): d 9.92 (s, 2H), 8.43 (s, 2H), 8.11–8.09 (m, 2H), 7.56–7.54 (m, 2H), 7.29 (s, 4H), 7.26–7.23
(m, 4H), 5.5 (S, 4H); 13C NMR (125 MHz, DMSO-d6): d 185.56, 141.79, 137.79, 137.27, 128.60, 125.65, 124.5, 123.43, 121.95, 118.29,
112.21, 50.25; HRMS m/z (EI) calcd. for C26H20N2O2 392.1525. Found 392.1529; Anal. calcd. for C26H20N2O2: C, 79.57; H, 5.14; N, 7.14;
Found: C, 79.56; H, 5.13; N, 7.16.
[15] The photoinduced (open) form B was formed upon UV irradiation (Hg lamp DRSh-260+ UV-transmitting glass filters), and visible light was
generated by a 500 W Xe lamp with a standard band-pass filter 550 AF10 (omega).