Multiple addressing in a hybrid biphotochromic system

Article abstract of DOI:10.1002/anie.200462575

Affairs of state: Investigation of the photochromic properties of a combined naphthopyran and diarylethene biphotochromic system (1) has revealed that four different states with entirely different absorption behaviors are formed. Opening of the naphthopyr

Full text of DOI:10.1002/anie.200462575

Communications
Photochemistry
DOI: 10.1002/anie.200462575
Multiple Addressing in a Hybrid Biphotochromic
System
Michel Frigoli and Georg H. Mehl*
Addressing the optical and electronic properties of organic
materials by light is currently a very active research area and
such systems are expected to be of great importance for
optical computing—as logic gates, as the optical equivalent of
field-effect transistors, as high density (3D) data storage
systems, as well as for light rectification and harvesting.[1–3]
The goal is a miniaturization of functional elements down to
the molecular level, or at least close to that, which could result
in a markedly increased performance as a consequence of the
higher volume density of the functional elements compared to
that of current devices.
Irradiation of photochromic systems at selected wave-
lengths leads to the reversible formation of colored forms as a
result of a change of the structure of conjugated p systems.
Bistable materials or systems that are dependent on contin-
uous irradiation to maintain the colored (high-energy) form
are suitable candidates for such applications.Nanostructured
crystalline systems, high-density photochromic polymers, and
a range of systems which form anisotropic fluids either as
mixtures or as molecular materials in the condensed state,
have been reported recently.[4,5]
In this context, biphotochromic systems are particularly
attractive, as the controlled sequential interaction of photons
in one molecular system promises synergies in the design and
electronic properties (for example, a sequential change of the
electronic properties would be a precondition for a fully
photonic molecular logic gate).[2a,g,3g] Investigations on bipho-
tochromes based on naphthacene–diarylethene, intercon-
nected diarylethenes, and diarylethene–dihydroazulene sys-
tems have been reported recently.[6–8] The challenge with
biphotochromic systems is both the selective and controlled
addressing of individual photochromic units and the con-
version of the combined system.In this context, the combi-
nation of naphthopyrans and photochemically bistable and
very fatigue resistant diarylethene groups (Scheme 1) are
particularly interesting, and as yet not investigated.[2b]
Naphthopyrans are currently the most important class of
photochromes commercially as a result of their high stability,
fatigue resistance, and addressable bleaching kinetics.[9]
Herein we report the synthesis and investigation of the
photochromic properties of a combined system consisting of a
[*] Dr. M. Frigoli, Dr. G. H. Mehl
University of Hull
Department of Chemistry
Cottingham Road
Hull, HU67RX (UK)
Fax : (+44)1482-466-411
E-mail: g.h.mehl@hull.ac.uk
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“one-pot reaction” of 5 and 2-naphthol in the
presence of a catalytic amount of para-toluene-
sulfonic acid (PTSA; yield 48%).[11] Lithiation of
the chromene 6 by bromine/lithium exchange
using nBuLi in THF at ꢀ788C, followed by
quenching of the reaction mixture with
7
afforded the hybrid biphotochrome OD-CN
(52% yield).[12a]
The mode of action of the multiaddressable
photochromic system OD-CN is illustrated in
Scheme 3.The path from OD-CN to CD-ON
requires the interaction of OD-CN with two
photons.The photochromic properties were
investigated by a series of steady-state experi-
ments in which light was either switched on or off
(input: 0,1) and spectra were recorded until a
photostationary state (PS) was obtained.This
sequential approach revealed the formation of
four different states with entirely different
absorption behaviors, and five addressing
modes were investigated.[12b]
Scheme 1. Photochemical interconversion in the [3H]-naphthopyran and dithienyl-
ethene series; TT=transoid,trans; TC=transoid,cis.
naphthopyran unit and a diarylethene group.An important
feature in the design of this system is the different absorption
behaviors of the naphthopyran (CN) and the diarylethene
(OD) groups which are connected through a sp3-hybrdized
carbon center.Only the conversion of the closed naphtho-
pyran (CN) into the open form (ON) leads to conjugation and
an electronic connection.Closure of the open diarylethene
system (OD to CD) extends the p-electron system over the
whole molecule.The different behavior of these two photo-
chromes—diarylethenes are essentially bistable and naph-
thopyrans open on irradiation to a number of isomers, most of
which revert back thermally to the closed forms in the dark—
results in a multiaddressable hybrid photochrome.
The synthesis of OD-CN (Scheme 2) starts with the
preparation of ketone 4, which was obtained (89% yield)
from the reaction of 2-methyl-3-bromothiophene with ben-
zoyl chloride in the presence of AlCl3.The subsequent
condensation of the lithium acetylide diamine complex with 4
resulted in the corresponding propargylic alcohol 5 (65%
Scheme 3. Photochemical reactions of the hybrid OD-CN showing all
four photochromic states.
yield).[10] The intermediate chromene 6 was obtained by a
The associated investigated reac-
tion pathways are: 1) OD-CN!OD-
ON, 2) OD-CN!CD-ON (via CD-
CN), 3) CD-ON!CD-CN, 4) CD-
CN!CD-ON, 5) CD-CN!OD-CN.
Our experimental set-up did not
allow for the investigation of the
potential direct path from OD-ON to
CD-ON.Two of the states, OD-CN
and CD-CN, are bistable, while OD-
ON and CD-ON have to be triggered
by irradiation.The absorption spectra
of all four forms are shown in Figure 1
and the colored solutions are shown in
Figure 2.
Scheme 2. Synthesis of the biphotochromic hybrid OD-CN.
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Figure 3. Thermal bleaching of OD-ON monitored at 456 nm.
Figure 1. UV/Vis absorption spectra (cylohexane solution,
3.33”10ꢀ5 m) of the four photochromic states of the system OD-CN,
before irradiation (OD-CN), with irradiation at 366 nm (OD-ON), with
irradiation at 313 nm (CD-ON), after irradiation at 313 nm then left in
the dark (CD-CN).
closing reaction of the diarylethene, as the electronic proper-
ties of the ground states of the two photochromic moieties
overlap between 300 and 350 nm, which leads initially to the
formation of OD-ON and CD-CN.After irradiation for one
minute the absorption is centered at 539 nm with a shoulder at
434 nm and also with absorption above 600 nm (Figure 4a).
This is, however, not an equilibrium, CD-CN is bistable while
OD-ON is not and reverts to OD-CN.Continuing the
irradiation leads to an enrichment of CD-CN in the mixture,
which is subsequently converted into CD-ON on further
irradiation (path 2).The initially colorless solution turns
brown, and the UV/Vis spectrum shows absorption up to
800 nm with three absorption maxima at 434, 539, and
618 nm.
When the irradiation is stopped, the open forms of the
naphthopyran revert back to the closed form CD-CN (see
Figure 4b) and the solution turns slowly from brown to pink
(the maxima at 434 and 618 nm decrease and a new maximum
at 516 nm is observed; path 3).Monitoring the kinetics of the
conversion of CD-ON into CD-CN at 680 nm, where only the
fully conjugated system absorbs, gives a bleaching rate
constant of 1.95 ” 10ꢀ3 sꢀ1 (monoexponential; Figure 4d).
This comparatively high value indicates that the extension
of the p conjugation in the CD-ON isomer stabilizes the open
naphthopyran moiety to a large extent.[15]
Two photopathways are possible from the stationary state
of CD-CN.Ring opening of the diarylethene to recover the
OD-CN starting material (path 4) can be effected quantita-
tively from CD-CN with visible light (> 420 nm), a behavior
typical of dithienylethenes.Irradiation of CD-CN (path 5) at
313 nm (OD-CN and CD-CN have a similar absorption at
313 nm; see Figure 1) induces the formation of CD-ON.The
photostationary state (CD-ON) is reached faster from CD-
CN than starting from OD-CN (Figure 4c).
Figure 2. Colors obtained for each stationary state: OD-CN (initial
state, colorless), OD-ON (after irradiation at 366 nm, yellow), CD-ON
(after irradiation of OD-CN or CD-CN at 313 nm, brown), and CD-CN
(from thermal bleaching of OD-ON in the dark, red/pink).
An interesting feature of the OD-CN system is the
electronic decoupling of the two photochromic groups con-
nected through a sp3-hybridized carbon atom.Irradiation
above 350 nm transforms the naphthopyran group selectively
from the closed (colorless) form of OD-CN to the open form
(orange, lmax = 456 nm) of OD-ON (path 1; Figure 1).The
thermal bleaching kinetics of the majority of the photo-
products (ca.81%) can be described with a monoexponential
function (kD = 0.055 sꢀ1; Figure 3), with a value for the closure
rate similar to that observed for other chromene systems.[13]
The residual absorption (ca.19%) in the visible region is
typical for 2H-chromenes and is attributed to the presence of
stable isomers; such isomers have been observed with other
naphthopyrans.[14] These forms can be converted fully into the
closed form by irradiation above 400 nm.In other words, this
system has a read-out window between 350 and 380 nm,
where the naphthopyran group can be switched selectively
without affecting the diarylethene moiety.
The absorption characteristics of CD-ON are the same,
regardless of whether the conversion is from OD-CN or CD-
CN, which indicates that the naphthopyran group can be
selectively opened and closed using UV light without affect-
ing the diarylethene moiety.
Irradiation of OD-CN at 313 nm triggers both the ring-
opening reaction of the naphthopyran and also the ring-
In summary, we have investigated a hybrid biphotochro-
mic system based on the thermally reversible naphthopyran
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Figure 4. a) Time-dependent absorption spectra of a solution of OD-CN in cyclohexane (3.33”10ꢀ5 m) upon irradiation at 313 nm; b) fading of
CD-ON after irradiation at 313 nm; c) time-dependent absorption spectra of CD-CN upon irradiation at 313 nm; d) thermal bleaching of CD-ON
monitored at 680 nm.
Primak, P.A. Lidell, L. Otero, L. Sereno, J.J. Silber, A.L.
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very different absorption properties and which can be
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Received: November 10, 2004
Revised: March 23, 2005
Published online: July 20, 2005
Keywords: conjugation · molecular devices · photochemistry ·
.
photochromism · UV/Vis spectroscopy
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[12] a) A solution of nBuLi in hexane (2.5m, 0.53 mL) was added
dropwise to a solution of 6 (0.52 g, 1.20 mmol) in dry THF
(20 mL) at ꢀ788C.After the mixture had been stirred for 1 h at
this temperature, a solution of 7 (0.36 g, 1.20 mmol) in dry THF
(5 mL) was added dropwise.The reaction mixture was allowed
to warm up to room temperature and then an aqueous solution
of NH4Cl was added.The resulting mixture was extracted twice
with diethyl ether, washed with water, dried over magnesium
sulfate, then concentrated under vacuum.Column chromatog-
raphy (silica, hexane/CH2Cl2, 70/30) gave OD-CN as a slightly
1
orange pure product (0.4 g, 52%). M.p. 67.5–68.58C; H NMR
(400 MHz, CDCl3): d = 1.38 (s, 3H), 1.84 (s, 3H), 2.33 (s, 3H),
6.17 (d, J = 9.8 Hz, 1H,), 6.60 (s, 1H), 6.85 (s, 1H), 7.16 (d, J =
9.0 Hz, 1H), 7.29–7.58 (m, 8H), 7.68 (d, J = 9.0 Hz, 1H), 7.73 (d,
J = 8.2 Hz, 1H), 7.96 ppm (d, J = 8.4 Hz, 1H). Elemental
analysis calcd (%) for C35H24F6OS2: C 65.82, H 3.79; found: C
65.98, H 3.67; b) the experimental set-up for the photochemical
measurements is described in ref.[4d,g].
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this is typical for 2H-chromenes and indicates, in principle, the
presence of four photoproducts (CTC, TTC, TTT, and CTC).
The isomers differ only in the configuration at the double bond
attached to the naphthalene ring and at the position of the aryl
group at the terminal double bond.The isomers CTC and TTC
are thermoreversible whereas the other two are thermally stable
and bleach on exposure to visible light.(TTT = trans,transoid,
trans, CTT= cis,transoid,trans).
[15] Note: The thermal stability of CD-ON increased about 28 times
relative to OD-ON.This result, together with monoexponential
bleaching kinetics, might indicate that TT isomers, whose
formation is selectively photoinduced by the extended p conju-
gation throughout the molecule, are formed preferentially.
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