Molecular photoswitches based on spiro-acridans

Article abstract of DOI:10.1039/c3cc41135d

Thermally reversible photochromic spiro-acridans have been synthesized for the first time. They exhibit high ring opening efficiencies. As the formed zwitterions do not possess a merocyanine structure their lifetime is in the range of milliseconds to seconds. An observed side reaction can be avoided by suitable substitution of hydrogen atoms.

Full text of DOI:10.1039/c3cc41135d

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Molecular photoswitches based on spiro-acridans†
Alina Raskosova, Reinhard Stoßer and Werner Abraham*
¨
Cite this: Chem. Commun., 2013,
49, 3964
Received 11th February 2013,
Accepted 22nd March 2013
DOI: 10.1039/c3cc41135d
www.rsc.org/chemcomm
Thermally reversible photochromic spiro-acridans have been synthe-
sized for the first time. They exhibit high ring opening efficiencies. As
the formed zwitterions do not possess a merocyanine structure their
lifetime is in the range of milliseconds to seconds. An observed side
reaction can be avoided by suitable substitution of hydrogen atoms.
Photochromic molecules can reversibly switch between two
states having different absorption spectra. A starting material
is converted to a product by means of UV or visible light. The
back reaction can occur thermally or photochemically.1
The reversible reactions performed by photoswitches are
accompanied by drastic changes in properties of the compound
such as the shape or the dipole moment. Electrocyclic reactions and
trans–cis isomerization induce such changes in the molecular
structure and, as a consequence, the physical properties of the
species are altered. To induce mechanical movement at the mole-
cular level is just as important.2 Spiropyrans have been one of the
most highly investigated classes of photochromic compounds. In
the closed form, the two halves of the molecules are located in two
Scheme 1 Synthesis of spiro-acridans.
perpendicular planes. The photoreaction produces a more planar (see Scheme 1). After acidification, the acridinium compounds
zwitterionic merocyanine.3 Rhodamine spiroamides have been used are the primary products.
for switching fluorescent nanoscopy.4
Compounds 3a–3d were not isolated, but were transformed
We have recently introduced the 9-alkoxy-9-arylacridan into spiro-acridans under basic conditions because these spiro-
(9-alkoxy-9-aryl-9,10-dihydroacridine)/9-aryl-acridinium-photoswitch
acridans are more easily purified using column chromatography
for controlling the ring movement of rotaxanes.5 The photoreaction (for details see ESI†). The corresponding acridinium salts were
of acridans results in the formation of an acridinium salt with an prepared from the spiro-acridans by reacting with an acid.
alkoxide as the counterion. The present study describes a new class
Alcohol 6 containing an amino substituent was introduced
of photoactive heterocyclic spiro-acridans 4 (Scheme 1) that undergo into acridinium iodide 5 according to the method developed by
ring-opening, which results not in a salt, but in a zwitterion.
The routes employed for the synthesis of spiro-acridans are
Chupakhin et al.6 (Scheme 2).
Upon irradiation of the spiro-acridans, the furan- or pyran-ring
outlined in Schemes 1 and 2. N-Methyl-acridone 1 is reacted was opened with high efficiency to form a zwitterionic acridinium
with organolithium precursors obtained from alcohols 2a–2e compound 3Z which does not possess a merocyanine structure
(Scheme 3). The quantum yield obtained from the ring opening in
¨
Humboldt-Universitat zu Berlin, Institute for Chemistry, Brook-Taylor-Str. 2,
MeCN–MeOH solution is relatively high (Table 1).
D-12489 Berlin, Germany. E-mail: abraham@chemie.hu-berlin.de
Because of the spiro-structure (see the crystal structure of 4a,
Fig. 1) the UV-spectra of compounds 4 were dominated by the
acridan-absorption and were independent of R1 (see Fig. 2 and
Scheme 3). As 3Z has a more planar structure, the UV-Vis
spectra of the zwitterions were very similar to those of the
† Electronic supplementary information (ESI) available: Full synthetic details
pertaining to the preparation of new compounds, crystallographic information
file (CIF) for compound 4a; and analysis of thermal back reactions (transient
decay curves and absorption spectra). CCDC 919797. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3cc41135d
c
3964 Chem. Commun., 2013, 49, 3964--3966
This journal is The Royal Society of Chemistry 2013

View Article Online
Communication
ChemComm
Table 1 Spectroscopic and kinetic data of 4 and 3Z
Absorption maximum
Quantum yield
Fr MeCN–MeOH Lifetime of 3Z/s (solvent) 3Z/nm
3/nm
4a
4b 0.5
—
0.007 (MeOH)
13 (MeOH)
420
423
423
423
23 (MeCN–EtOH 1 : 1)
9 (MeOH)
30 (EtOH)
4c 0.4
4d 0.3
4e 0.5
426
423
431
428
426
432
36 (MeOH)
54 (MeCN–MeOH 4 : 1)
0.003 (MeOH)
4f
—
502
503
— Not determined.
Scheme 2 Synthesis of an amino-substituted spiro-acridan.
Fig. 1 Single crystal X-ray structure of compound 4a (the hydrogen atoms have
been omitted for clarity).
Scheme 3 Ring opening of spiro-acridans 4.
acridinium salts 3 (Table 1). The absorption maxima of acridinium
ions are dependent on the donor property of R1.7
Fig. 2 UV-Vis spectra of compounds 4b (––) and 3b (-----) in acetonitrile solution
(6 Â 10À5 M).
Since the stabilization by a merocyanine structure was
absent, the zwitterions 3Z rapidly converted back to 4, as
acridinium alkoxides do.5 The lifetime depended both on the
ring size and on the solvent (see Table 1 and Fig. S1–S4, ESI†).
The presence of alcohol in the solution was necessary to
obtain the reaction cycle. Obviously, the ionic path is already
preformed by the solvent and the solvent plays an active role in
the early stage of the ring opening process.8 Accordingly, the
question arises as to whether the reaction of the alcohol with 3Z
is able to compete with the ring closure reaction.
In contrast to the 9-alkoxy-9-arylacridans studied previously,5
the spiro-acridans were not transformed to the corresponding
acridinium ions in alcoholic solution. This finding emphasized
the increased thermodynamic stability of the cyclic acridans.
By repeating the switching cycle, we observed that the
acridinium chromophore was recovered in diminishing
amounts. By irradiation of 4a,b in aerated methanol solution
on a preparative scale, the photo-inactive 9-H-acridans 7a,b
could be isolated (Scheme 4).
In general, homolysis may compete with the heterolytic
bond fission of the C–O-bond such as was found for xanthenol.9
However, a recent mechanistic study of the photolysis of
NMR-spectroscopic investigations after irradiation of 4 in
ethanol or methanol solution showed that the alkoxy group had
not been introduced into the acridan moiety (see Scheme 3).
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 3964--3966 3965

View Article Online
ChemComm
Communication
Such hydride transfer was also not found with a fluoren-9-yl
cation.13
To avoid an irreversible hydrogen abstraction, compounds
4d and 4e were synthesized. With these structures, the decom-
position of the dimethyl-substituted derivatives was stopped
and the switching cycle could be repeated at least ten-times in
solvents such as methanol and ethanol or their mixtures with
acetonitrile (see Fig. S6–S8, ESI†).
For example, the transient UV-Vis absorption spectra
recorded at different times after the irradiation of 4e displayed
isosbestic points which indicated the presence of only two
components (see Fig. 3). Compound 4e was synthesized in
order to offer the possibility of elongating the chain for use
as the molecular thread of rotaxanes.
Work is in progress to prepare rotaxanes which include the
spiro-acridan unit within the molecular axle.
In conclusion, we have synthesised compounds of a novel
class of photochromic molecules which show high ring
opening efficiencies under UV-irradiation and rapid thermal
ring closure back reactions. With a suitable substitution mode,
at least ten switching cycles can be performed. The two parts of
the photochromic system differ strongly not only in the absorp-
tion spectra, but also in the molecular shape.
Scheme 4 Proposed radical ring opening process resulting in 9-H-acridans 7.
Notes and references
1 (a) Y. Hirshberg and E. Fischer, J. Chem. Phys., 1955, 23, 1723;
(b) Photochromism, ed. G. H. Brown, Wiley Interscience, New York,
1971; (c) Organic Photochromic and Thermochromic Compounds, ed.
J. C. Crano and R. J. Guglielmetti, Plenum Press, New York, 1999;
(d) Photochromism Molecules and Systems, ed. H. Du¨rr and
H. Bouas-Laurent, Elsevier, Amsterdam, 1990; (e) M. Irie, Chem.
´
Rev., 2000, 100, 1685; ( f ) D. Gust, J. Andreasson, U. Pischel,
T. A. Moore and A. L. Moore, Chem. Commun., 2012, 48, 1947.
2 (a) Molecular Switches, ed. B. L. Feringa and W. R. Browne, Wiley
VCH, Weinheim, 2nd edn, 2011; (b) J. N. Moorthy, S. Mandal and
A. Kumar, New J. Chem., 2013, 37, 82.
3 F. M. Raymo and S. Giordani, Org. Lett., 2001, 3, 3475–3478.
4 (a) K.-H. Knauer and R. Gleiter, Angew. Chem., Int. Ed. Engl., 1977,
¨
¨
16, 113; (b) J. Folling, V. Belov, R. Kunetsky, R. Medda, A. Schonle,
A. Egner, C. Eggeling, M. Bossi and S. W. Hell, Angew. Chem., Int.
¨
Ed., 2007, 46, 6266–6270; (c) V. N. Belov, M. L. Bossi, J. Folling,
V. P. Boyarski and S. W. Hell, Chem.–Eur. J., 2009, 15, 10762–10776.
Fig. 3 Time evolution (20 s, 1 min, 3 min) of the transient UV-Vis absorption
spectra of a solution of 4e in MeCN–MeOH 4 : 1 after irradiation for 5 s (>300 nm).
¨
5 (a) W. Abraham, K. Buck, M. Orda-Zgadzaj, S. Schmidt-Schaffer and
U.-W. Grummt, Chem. Commun., 2007, 3094–3096; (b) W. Abraham,
A. Wlosnewski, K. Buck and S. Jacob, Org. Biomol. Chem., 2009, 7,
142–154.
9-hydroxy/9-methoxy-9-phenyl-acridan demonstrated that in
methanol solution only the heterolytic bond fission occurred
while in acetonitrile solution, an intersystem crossing process
dominated.10 In our system we think that homolytic and,
mainly, heterolytic ring opening occurs after photoexcitation
of 4a,b. The formed biradical then undergoes hydrogen abstrac-
tion to yield 7. Actually ESR spectra recorded after irradiation of
4a in methanol or ethanol solution at 77 K exhibit signals of
radicals formed from the solvent11 as well as the typical ESR
fine structure of a biradical. The signals disappear when the
temperature is increased (see Fig. S5, ESI†).
6 (a) O. N. Chupakhin, V. A. Trofimov and Z. V. Pushkareva, Dokl.
Akad. Nauk SSSR, 1969, 188, 376–378; (b) V. A. Trofimov,
O. N. Chupakhin and Z. Pushkareva, Khim. Geterotsikl. Soedin.,
1971, 5, 653–654.
7 S. A. Jonker, S. I. van Dijk, K. Goubitz, C. A. Reiss and J. Verhoeven,
Mol. Cryst. Liq. Cryst., 1990, 183, 273.
8 G. G. Gurzadyan and S. Steenken, Chem.–Eur. J., 2001, 7, 1808–1815.
9 R. E. Minto and P. K. Das, J. Am. Chem. Soc., 1989, 111, 8858–8866.
10 D. Zhou, R. Khatmullin, J. Walpita, N. A. Miller, H. L. Luk,
S. Vyas, C. M. Hadad and K. D. Glusac, J. Am. Chem. Soc., 2012,
134, 11301–11303.
11 (a) R. W. Fessender and R. H. Schuler, J. Chem. Phys., 1963, 39, 2147;
(b) E. L. Cochran, F. J. Adrian and V. A. Bowers, J. Chem. Phys., 1964,
40, 213.
12 B. Zhou, K. Kano and S. Hashimoto, Bull. Chem. Soc. Jpn., 1988, 61,
1633–1640.
13 G. Mladenova, L. Chen, C. F. Rodriquez, K. W. M. Stu, L. J. Johnston,
A. C. Hopkinson and E. Lee-Ruff, J. Org. Chem., 2001, 66, 1109–1114.
An alternative hydride transfer from the zwitterion in the
ground state12 is unlikely, because the corresponding acridinium
ions are stable in methanol solution without any decomposition.
c
3966 Chem. Commun., 2013, 49, 3964--3966
This journal is The Royal Society of Chemistry 2013