Simultaneous fluorescence and redox modulation in an irreversible photochrome based on a strained dibenzo-acridinium cation

Article abstract of DOI:10.1039/b611092d

A highly strained non-luminescent dibenzo-acridinium cationic compound was identified in acetonitrile light-induced ring closure to create a highly fluorescent, planar, eight-ringed cationic anti-aromatic dye. It was found that the non-emissive compound u

Full text of DOI:10.1039/b611092d

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www.rsc.org/obc | Organic & Biomolecular Chemistry
Simultaneous fluorescence and redox modulation in an irreversible
photochrome based on a strained dibenzo-acridinium cation†
Andrew C. Benniston* and Dorota B. Rewinska
Received 1st August 2006, Accepted 12th September 2006
First published as an Advance Article on the web 28th September 2006
DOI: 10.1039/b611092d
A
highly strained non-luminescent dibenzo-acridinium
of degradation, but solid samples left exposed to ambient light
gradually turned red. Dilthey et al. attributed this red product to
the ring-closed form in which the 14-phenyl is fused to one side
only of the benzo group (Scheme 1). We found, however, that the
red solid obtained is in fact the fully closed form, Ph-RCAcr+. This
cationic compound is identified that undergoes in acetonitrile
light-induced ring closure to create a highly fluorescent,
planar, eight-ringed cationic anti-aromatic dye.
1
identification was based on a 500 MHz H NMR spectrum that
Interest in photochromic (PC) molecules that alter their colour
upon exposure to light has increased in recent years as new
applications have arisen in technological areas such as molecular
logic gates,1 field effect transistors2 and bitwise volumetric data
storage.3 All these technologies rely on the fact that light activation
of the PC species, at a specific wavelength, creates a molecular
entity that reverts back to the original form using light of different
frequency.4 However, in general the reverse reaction is also possible
thermally. This can be problematic if data need to be stored
for long periods without degradation of information. Thermally
irreversible PC molecules upon light activation create a new entity
that not only displays dissimilar photochemical properties to the
starting material but cannot revert back.5 We have been interested
not only in identifying suitable new irreversible PC molecules,
but ones which also display very high fluorescent quantum
yields and/or alter their redox properties. Such molecules have
been identified as candidates for multilayer recording of data.3
During our investigation into highly strained acridinium-based
donor–acceptor dyads6 the cation, Ph-ROAcr+, was prepared.
Interestingly, this non-emissive compound upon illumination
with white light, ring closes to generate the highly fluorescent
cation, Ph-RCAcr+. The modification is also accompanied by a
considerable alteration in reduction potential.
supported a compound with only 18 protons and a high level of
symmetry. The electrospray mass spectrum also revealed a cluster
of peaks at m/z = 428.3 which matches that of the [M − PF6]+
ion.
The synthetic procedure† used to prepare Ph-ROAcr+ was
adapted from work previously reported by Dilthey et al.7 Sim-
ple refluxing of N-phenyl-2-naphthylamine, benzaldehye and 2-
naphthol in glacial acetic acid produced, after recrystallisation, the
condensation product 1 in an unoptimised 22% yield. Oxidation of
this derivative to the carbinol 2 with MnO2 had to be performed in
the dark as the product was light-sensitive. Yields of the carbinol
varied (20–68%) and is attributed to unwanted photodegradation.
Aromatisation to generate the desired cation was carried out by
HCl acidification of 2, followed by conversion of the chloride
salt to the hexafluorophosphate derivative. Recrystallisation of
the material from CH3CN–Et2O (1 : 4) afforded golden yellow
needles. The product when stored in the dark showed no signs
Scheme 1 Outline of the methods used in the preparation of the cation
Ph-ROAcr+ and the ring-closed form Ph-RCAcr+. NB: the incorrectly
identified ring-closed product is also shown.
To collect detailed structural information on ring closure,
300 MHz 1H NMR spectra were monitored following illumination
of a sample of Ph-ROAcr+ in air-equilibrated and N2-purged P2O5-
dried CD3CN at 25 ◦C. Illustrated in Fig. 1 are a selection of 1H
NMR spectra collected during the course of the reaction for the
N2-purged sample. Within the first hour of irradiation four new
clear doublets are visible downfield from the rest of the aromatic
signals. As well as these series of new signals, clear doublet (d =
6.74) and singlet (d = 6.85) signals are visible. During irradiation
these signals grow and three apparent triplets emerge at d 7.0,
7.18 and 7.32, respectively. The spectrum in the region d = 7.5–8.0
also simplifies as resonances for the starting material disappear.
Molecular Photonics Laboratory, School of Natural Sciences (Chemistry),
Bedson Building, Newcastle University, Newcastle upon Tyne, UK NE1
7RU. E-mail: a.c.benniston@ncl.ac.uk; Fax: (0191) 222 5706; Tel: (0191)
222 6929
† Electronic supplementary information (ESI) available: experimental
1
procedures, full compound characterisation, electrochemistry, H NMR
spectra following irradiation experiments and excitation/emission spectra
for final product. See DOI: 10.1039/b611092d
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Fig.
2
Alterations in absorption spectra for Ph-ROAcr+ in dry
air-equilibrated acetonitrile following irradiation with white light.
of Ph-RCAcr+. Close inspection of the overlayed spectra revealed
there are no clean isosbestic points, which suggests conversion of
Ph-ROAcr+ to Ph-RCAcr+ proceeds via at least one intermediate.
Rather surprisingly, the ring closure carried out in N2-purged
acetontrile proceeded just as well as the O2-containing solutions.
Moreover, the cyclisation reaction proceeded faster when carried
out in N2-purged acetonitrile solution containing a few drops of
distilled water. This finding seems to suggest that during ring
closure a H-atom abstraction process is required.
The basic absorption spectrum for Ph-ROAcr+ in acetonitrile is
dominated by three major intense bands located at 446 (log e =
4.39), 421 (log e = 4.24) and 310 nm (log e = 4.65), respectively.
In contrast, the absorption spectrum of Ph-RCAcr+ is rich and
contains a number of bands, the most prominant being at 505 (log
e = 4.29) and 328 nm (log e = 4.47). Extremely weak fluorescence
from Ph-ROAcr+ is observed in acetonitrile solution at 25 ◦C,
though dependence of the band shape on the excitation wavelength
strongly supports this is an artifact caused by minute traces of an
impurity. By contrast, fluorescence from Ph-RCAcr+ in N2-purged
acetonitrile solution was readily observed centred at 530 nm,
the band shape being insensitive to excitation wavelength. The
excitation and absorption spectra were in good agreement (ESI†),
and the small Stokes’ shift (SS) of 934 cm−1 indicates a modest
change in structure after relaxation from the initially produced
Franck–Condon state. Considering the rigidity of the cation this
is not too surprising since only the N-phenyl group is free to rotate
in fluid solution. The radiative lifetime (sr) calculated using the
Strickler–Berg8 expression is 9.8 ns, whereas the measured lifetime
(ss) is 7 ns. Hence, the corresponding quantum yield of fluorescene
(ss/sr) for Ph-RCAcr+ is a respectable 0.71.
The redox chemistry of acridinium-based compounds is gen-
erally well understood.9 Lacking any subsitituent in the 10-
position the reductive electrochemistry of, for example, the
9-methylacridinium cation is irreversible because of chemical
breakdown of the generated radical.10 In contrast, the reductive
electrochemistry of 10-phenyl-9-methylacridinium is well behaved
consisting of a reversible reduction wave at −0.55 vs. SCE.11 In
view of these previous findings, cyclic voltammograms for Ph-
ROAcr+ were obtained in dry CH3CN. Upon oxidative scanning
an irreversible wave is observed just around the solvent cut-
off at ca. +2.4 V vs. SCE. The reduction portion of the cyclic
voltammogram is dominated by a reversible one-electron wave at
Fig. 1 Selected 300 MHz 1H NMR spectra for Ph-ROAcr+ in N2-purged
CD3CN following irradiation at time differences t = 0 (A), t = 1 h (B) and
t = 13 h (C). Arrows in panel (B) mark new resonances. In panel (C), ꢀ
represent final product and ᭹ represent intermediate product.
After a substantial irradiation time (Fig. 1C), signals associated
with the final product are visible, and the other signals clearly
diminish. The final spectrum at the end of the irradiation is
identical to that recorded for a purified sample of Ph-RCAcr+.
Matching irradiation experiments carried out in air-equilibrated
CD3CN (see the ESI†) did not show identical build up and loss
of resonances marked with dark circles (Fig. 1C). Instead, as well
as resonances for the final product only very minor signals were
observed in the d 6.5–7.0 ppm region; these disappered after ca.
1 h.
Further elucidation of the ring closure processes within Ph-
ROAcr+ were obtained by UV-visible spectroscopy. A solution of
Ph-ROAcr+ in acetonitrile at 25 ◦C was illuminated with white light
(kmax > 300 nm) for fixed time periods and the resultant spectra
recorded. Samples were irradiated following substantial purging
with either nitrogen, oxygen or air-equilibration. Illustrated in
Fig. 2 is a typical set of spectra obtained during an irradiation
experiment in air-equilibrated acetonitrile. The absorption band
associated with Ph-ROAcr+ (445 nm) gradually diminished and
a new absorption band grew at 505 nm, which is characteristic
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E1 = −0.50 V (65 mV) vs. SCE, an irreversible one-electron wave
at E2 = −1.03 V vs. SCE and an irreversible two-electron wave
at E3 = −2.36 V vs. SCE. The similarity of E1 to that of the first
reduction potential for the 10-methyl-9-methylacridinium cation
supports one-electron addition to the central acridinium ring to
create the neutral radical. Further addition of an electron to create
the anion presumably breaks conjugation throughout the molecule
to produce two isolated naphthalene units that are simultaneously
reduced. Cyclic voltammograms collected following steady-state
visible light illumination of an air-saturated solution of Ph-
ROAcr+, followed by N2-purging, were very different and are as-
signed to Ph-RCAcr+. The cyclic voltammogram in the oxidatative
portion is more resolved and contains a main irreversible wave at
2.1 V vs. SCE that supports a slight shoulder at ca. 1.75 V vs.
SCE. Thus, ring closure makes it easier to remove an electron
from the conjugated p-system as expected because of lowering
of the HOMO orbital energy. In contrast, the reductive segment
of the cyclic voltammogram contains only two main waves. The
first is a quasi-reversible wave situated at −0.81 V (140 mV) vs.
SCE, with a second quasi-reversible wave at −1.5 V (80 mV)
vs. SCE. Identical electrochemical behaviour was observed for
a synthesised and purified sample of Ph-RCAcr+. In light of the
electrochemical investigations it can be concluded that Ph-ROAcr+
is a better one-electron acceptor than Ph-RCAcr+ by ca. 300 mV.
These preliminary results support the basic tenet that light-
induced ring closure in a highly-strained acridinium cation
effectively modulates its luminscent and redox properties. That
the ring opening reaction appears irreversible means that any
‘information’ written into the molecule is permanently stored,
and in addition can be read out by two different means, i.e.
electrochemically or photochemically. Furthermore, the work also
throws up some interesting basic questions regarding the actual
ring closure process and the subsequent mechanism. For example,
why is no apparent oxidant required to complete the ring closure
reaction?12 Follow-up experiments are currently underway to
obtain a comprehensible picture of the pericyclic reaction.
Acknowledgements
This work was supported by the University of Newcastle. We
also thank the EPSRC sponsored Mass Spectrometry Service at
Swansea for the electrospray mass spectra.
Notes and references
1 J. Andre´asson, Y. Terazono, B. Albinsson, T. A. Moore, A. L. Moore
and D. Gust, Angew. Chem., Int. Ed., 2005, 44, 7591; X. Guo, D. Zhang,
T. Wang and D. Zhu, Chem. Commun., 2003, 914; D. Gust, T. A. Moore
and A. L. Moore, Chem. Commun., 2006, 1169.
2 M. Frigoli and G. H. Mehl, Angew. Chem., Int. Ed., 2005, 44, 5048;
X. Guo, L. Huang, S. O’Brien, P. Kim and C. Nuckolls, J. Am. Chem.
Soc., 2005, 127, 15045.
3 T. D. Milster and Y. Zhang, MRS Bull., 2006, 31, 318.
4 G. Berkovic, V. Krongauz and V. Weiss, Chem. Rev., 2000, 100, 1741.
5 Y. Yokoyama, S. M. Shrestha and Y. Yokoyama, Mol. Cryst. Liq. Cryst.,
2005, 431, 133; M. Irie and M. Mohri, J. Org. Chem., 1988, 53, 903; S.
Nakamura and M. Irie, J. Org. Chem., 1988, 53, 6136; Y. Nakayama,
K. Hayashi and M. Irie, J. Org. Chem., 1990, 55, 2592; T. Yamaguchi,
Y. Fujita and M. Irie, Chem. Commun., 2004, 1010; M. Hanazawa,
R. Sumiya, Y. Horikawa and M. Irie, J. Chem. Soc., Chem. Commun.,
1992, 206.
6 A. C. Benniston, A. Harriman, P. Li, J. P. Rostron, H. J. van
Ramesdonk, M. M. Groeneveld, H. Zhang and J. W. Verhoeven, J. Am.
Chem. Soc., 2005, 127, 16054.
7 W. Dilthey, F. Quint and J. Heinen, J. Prakt. Chem., 1939, 152, 49.
8 S. J. Strickler and R. A. Berg, J. Chem. Phys., 1962, 37, 814.
9 P. Hapiot, J. Moiroux and J.-M. Save´ant, J. Am. Chem. Soc., 1990, 112,
1337.
10 S. Fukuzumi, T. Kitano and K. Mochido, J. Chem. Soc., Chem.
Commun., 1990, 1236.
11 N. W. Koper, S. A. Jonker, J. W. Verhoeven and C. van Dijk, Recl. Trav.
Chim. Pays-Bas, 1985, 104, 296; S. Fukuzumi, K. Ohkub, T. Suenobu,
K. Kato, M. Fujitsuka and O. Ito, J. Am. Chem. Soc., 2001, 123, 8459.
12 If the reaction proceeded in an analogous manner for the conversion of
stilbene to phenanthrene, molecular oxygen would be required in order
to remove the two hydrogen atoms of the intermediate.
3888 | Org. Biomol. Chem., 2006, 4, 3886–3888
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The Royal Society of Chemistry 2006
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