Thermolysis of fluorinated cycloalkylidene fulgides yields a new class of photochromic compounds.

Article abstract of DOI:10.1039/b300010a

A series of fluorinated cycloalkylidene indolylfulgides has been designed, synthesized and characterized; most of the thermolysis products of these fulgides maintain photochromicity and display outstanding thermal and photochemical stability.

Full text of DOI:10.1039/b300010a

Thermolysis of fluorinated cycloalkylidene fulgides yields a new class of
photochromic compounds†
Mason A. Wolak, Nathan B. Gillespie, Robert R. Birge and Watson J. Lees*
1-014 CST, Department of Chemistry and W. M. Keck Center for Molecular Electronics, Syracuse
University, Syracuse, New York, USA. E-mail: wjlees@syr.edu; Fax: 315-443-4070; Tel: 315-443-5803
Received (in Cambridge, UK) 6th January 2003, Accepted 25th February 2003
First published as an Advance Article on the web 27th March 2003
A series of fluorinated cycloalkylidene indolylfulgides has
been designed, synthesized and characterized; most of the
thermolysis products of these fulgides maintain photo-
chromicity and display outstanding thermal and photo-
chemical stability.
irradiated the molecule with 436 nm light and found evidence of
E/Z and Z/C isomerization via thin layer chromatography.
Based upon this indication of photochromic behavior, we then
sought to design new photochromic compounds through the
controlled thermal degradation of fluorinated indolylfulgides.
The new materials, indolylethenylanhydrides, would be ex-
pected to maintain greater thermal stability than the parent
fulgides while absorbing well into the visible region (2E was
found to have an absorption maxima of 475 nm). To ensure that
the hexatriene unit of the newly designed chromophore existed
in the proper conformation for photochemical ring closure, the
isopropylidene unit of 1Z was replaced with a cycloalkylidene
unit.7 The resulting cycloalkylidene indolylfulgides would then
be thermally degraded to yield the desired indolyletheny-
lanhydrides.
Fulgides 3–6 were synthesized using standard methods and
displayed similar spectral characteristics in toluene to the
previously studied fluorinated isopropylidene fulgides (Table 1,
Scheme 3).5,8–10 As hypothesized, cyclopentylidene fulgides 3
and 4 rearranged to the respective products 7 and 8 after 3 d in
refluxing toluene ( > 70% yield, Scheme 3). Rearrangement of
the cyclohexylidene fulgides required more vigorous condi-
tions. Indolylethenylanhydrides 9 and 10 were generated in
approximately 30% yield after 5 d in refluxing o-xylene.
Each of the thermally rearranged indolylethenylanhydrides
displayed considerable bathochromic shifts in the Z-form
relative to their parent fulgides (Table 2). Methoxy-substituted
cyclopentenyl derivative 8 displayed the longest wavelength
maxima at nearly 500 nm. For compounds 8–10, an accompany-
ing blue shift of the C-form results in an overlap of the spectral
features of the two forms (Fig. 1). The greatest spectral
separation ( ~ 71 nm) between wavelength absorption maxima
Photoactive organic compounds such as fulgides and diaryle-
thenes have generated great interest as the photoactive media in
high-density rewriteable optical memory devices.1 The photo-
reversible nature of these compounds allows for repeated
conversion between two key isomers with readily distinguish-
able absorption spectra.2,3 Although numerous compounds have
been prepared, few photochromic materials maintain the desired
combination of photochemical and thermal stability essential
for optical memory media. Recently discovered fluorinated
indolylfulgides and their derivatives have demonstrated greatly
enhanced resistance to photochemical fatigue relative to non-
fluorinated analogues (Scheme 1).4,5 Unfortunately, the tri-
fluoromethyl group has been shown to destabilize the bridging
alkene of the Z-form, resulting in loss of absorbance when
subjected to elevated temperatures.6 Herein, we describe the
characterization of new thermally and photochemically stable
photochromic materials derived from the thermal degradation
of fluorinated indolylfulgides.
Table 1 Quantum yields and extinction coefficients at lmax for fulgides 3–6
in toluene
Scheme 1 Photoreactions of indolylfulgides. For fluorinated indolylful-
gides, the cyclizable form is the Z-form (IUPAC nomenclature). For
virtually all other fulgides the E and Z labels are reversed.
lmax/nm (emax/(mol21 L cm21))
427 nm
559 nm PSS427nm
FZC FCZ FCZ
C:Z:Ea
Z-form
E-form
C-form
As previously reported, thermolysis of trifluoromethyl-
substituted indolylfulgide 1Z in refluxing toluene results in the
formation of rearranged product 2E (Scheme 2).6 Noting that a
central hexatriene unit is preserved in compound 2E, we
1b 427 (5300) 441 (5600) 571 (6400) 0.20 0.10 0.043 95:3:2
3
4
5
6
431 (5600) 444 (5400) 570 (5500) 0.17 0.052 0.032 88:3:9
438 (5100) 457 (6600) 615 (5200) 0.14 0.131 0.012 90:9:1
428 (4500) 443 (6600) 572 (5100) 0.08 0.055 0.087 91:7:2
433 (4700) 451 (7200) 617 (5200) 0.06 0.075 0.036 76:15:9
a Photostationary State (PSS). b Reference 5.
Scheme 2 Thermolysis of indolylfulgide 1Z at 110 °C in toluene.
† Electronic supplementary information (ESI) available: Synthetic proce-
dures for 3–10. Optical and stability characterization methods. See http:
//www.rsc.org/suppdata/cc/b3/b300010a/
Scheme 3 Synthesis of indolylethenylanhydrides.
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CHEM. COMMUN., 2003, 992–933
This journal is © The Royal Society of Chemistry 2003

Table 2 Quantum yields and extinction coefficients at lmax for in-
dolylethenylanhydrides 7–10 in toluene
Table 3 Thermal and photochemical stability of compounds 3–10
Thermal stability
Photochemical stability
# of
lmax/nm (emax
/
(mol21 L cm21))
488 nm
FZC
—
A/A0
A/A0
(C-form)a
%/
Cycle
605 nm PSS488nm
(Z-form)a
Cycles
A/A0
Z-form
C-form
FCZ
FCZ
C+Z
3
4
5
6
7
8
9
10
0.29b
0.28b
0.27b
0.27b
1.00d
1.00d
1.00d
1.00d
0.86
0.92
0.49
0.13
1200
700
900
900
0.81
0.79
0.81
0.94
0.018
0.030
0.023
0.006
a
7
8
9
485 (2000)
—
—
—
—
495 (2400) 545 (5800) 0.016
460 (1200) 516 (6300) 0.071
10 464 (1100) 535 (6300) 0.063
a No C-form was obtained upon irradiation with visible light.
0.044
0.043
0.028
0.044
0.037
0.018
20+80
28+72
38+62
c
c
c
c
–
–
–
–
0.98
0.96
0.94
600
500
600
1.00d
1.00d
1.00d
0.00
0.00
0.00
a Absorbance at lmax relative to initial absorbance after 168 h at 80 °C.
b Residual absorbance due to thermolysis products. c No C-form generated.
d No loss of absorbance within experimental error
The most notable characteristic of indolylethenylanhydrides
was their outstanding thermal and photochemical stability
(Table 3). While the Z-forms of cycloalkylidene fulgides 3–6
were fully decomposed within 4 h at 80 °C in PMMA,
(poly(methyl methacrylate)), the corresponding indolyletheny-
lanhydrides lost no absorbance after 3 weeks at 80 °C. The C-
forms of 8–10 maintain comparable thermal stability. In
addition, compounds 8–10 exhibited no loss of absorbance after
500 or more photochemical cycles in toluene (irradiation to
90% PSS with 436 nm light followed by quantitative bleaching
with > 630 nm light). In comparison, bis(indolyl) diarylethenes
degrade by more than 20% after 100 cycles in cyclohexane and
are thermally unstable at room temperature.11 Monoindolylmo-
nothiophene diarylethenes are more thermally stable, > 12 h at
80 °C.1 The thermal and photochemical stabilities of cycloalk-
ylidene fulgides 3–6 were similar to previously studied
fluorinated indolylfulgides.5
In summary, we have designed and synthesized a new class of
photochromic compounds based upon the thermolysis of the
fluorinated indolylfulgide precursors. The resulting indolyle-
thenylanhydrides possess outstanding thermal and photo-
chemical stability, making them candidates for a wide range of
practical applications. Optimization of the spectral character-
istics and examination of the wavelength dependency of the
PSS compositions is ongoing.
Fig. 1 Absorption spectra of indolylethenylanhydride 8 in toluene.
for the Z- and C-forms is evident for cyclohexenyl methoxy-
substituted derivative 10.
As suggested by our initial results with 2, the Z-forms of
indolylethenylanhydrides 8–10 were found to undergo photo-
chemical electrocyclic ring closure, generating C-forms
(Scheme 4). Photostationary states (PSS) containing apprecia-
ble amounts of both the C- and Z-forms of compounds 8–10
were obtained upon irradiation with 488 nm light (Fig. 2). In
addition, each of the C-form compounds underwent quantitative
cycloreversion to Z-form upon irradiation with > 630 nm light.
Ring-closing quantum yields were greater for cyclohexenyl
compounds 9 and 10, while cyclopentenyl compound 8
displayed the highest ring-opening quantum yield when irra-
diated with 605 nm light.
Financial support from the National Science Foundation
(CHE9975076 to W.L.), the Army Research Office (MURI
program, DAAD19-99-1-0198 to R.B.), and the National
Institutes of Health (GM-34548 to R.B.) is acknowledged.
Notes and references
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2 H. Durr and H. Bousas-Laurent; (Eds.) Photochromism, Molecules and
Systems; Elsevier: Amsterdam, 1990.
Scheme 4 Photochemical reactions of methoxy-substituted indolyletheny-
lanhydride 10.
3 J. C. Crano and R. J. Guglielmetti; (Eds.) Organic Photochromic and
Thermochromic Compounds; Plenum: New York, 1999; Vol. 1.
4 Y. Yokoyama and K. Takahashi, Chem. Lett., 1996, 1037–1038.
5 M. A. Wolak, N. B. Gillespie, C. J. Thomas, R. R. Birge and W. J. Lees,
J. Photochem. Photobiol., A, 2001, 144, 83–91.
6 M. A. Wolak, J. M. Sullivan, C. J. Thomas, R. C. Finn, R. R. Birge and
W. J. Lees, J. Org. Chem., 2001, 66, 4739–4741.
7 A. M. Asiri and S. S. Al-Juaid, Indian J. Chem., Sect. B: Org. Chem.
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8 M. A. Wolak, N. B. Gillespie, C. J. Thomas, R. R. Birge and W. J. Lees,
J. Photochem. Photobiol. A, 2002, 147, 39–44.
9 C. J. Thomas, M. A. Wolak, R. R. Birge and W. J. Lees, J. Org. Chem.,
2001, 66, 1914–1918.
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Fig. 2 Changing absorbance of a solution of methoxy-substituted in-
dolylethenylanhydride 10Z upon irradiation with 488 nm light; (a) 0 min,
(b) 1 min, (c) 3 min, (d) 7 min.
CHEM. COMMUN., 2003, 992–933
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