Charge shift and triplet state formation in the 9-mesityl-10- methylacridinium cation

Article abstract of DOI:10.1021/ja052967e

The target donor-acceptor compound forms an acridinium-like, locally excited (LE) singlet state on illumination with blue or near-UV light. This LE state undergoes rapid charge transfer from the acridinium ion to the orthogonally sited mesityl group in po

Full text of DOI:10.1021/ja052967e

Published on Web 11/01/2005
Charge Shift and Triplet State Formation in the
9-Mesityl-10-methylacridinium Cation
Andrew C. Benniston,† Anthony Harriman,*,† Peiyi Li,† James P. Rostron,†
Hendrik J. van Ramesdonk,¶ Michiel M. Groeneveld,¶ Hong Zhang,¶ and
Jan W. Verhoeven*,¶
Contribution from the Molecular Photonics Laboratory, School of Natural Science,
UniVersity of Newcastle, Newcastle upon Tyne, NE1 7RU, United Kingdom, and
Chemistry Department, HIMS, UniVersity of Amsterdam, Nieuwe Achtergracht 129,
1018 WS Amsterdam, The Netherlands
Received May 6, 2005; E-mail: anthony.harriman@ncl.ac.uk
Abstract: The target donor-acceptor compound forms an acridinium-like, locally excited (LE) singlet state
on illumination with blue or near-UV light. This LE state undergoes rapid charge transfer from the acridinium
ion to the orthogonally sited mesityl group in polar solution. The resultant charge-transfer (CT) state
fluoresces in modest yield and decays on the nanosecond time scale. The LE and CT states reside in
thermal equilibrium at ambient temperature; decay of both states is weakly activated in fluid solution, but
decay of the CT state is activationless in a glassy matrix. Analysis of the fluorescence spectrum allows
precise location of the relevant energy levels. Intersystem crossing competes with radiative and nonradiative
decay of the CT state such that an acridinium-like, locally excited triplet state is formed in both fluid solution
and a glassy matrix. Phosphorescence spectra position the triplet energy well below that of the CT state.
The triplet decays via first-order kinetics with a lifetime of ca. 30 µs at room temperature in the absence of
oxygen but survives for ca. 5 ms in an ethanol glass at 77 K. The quantum yield for formation of the LE
triplet state is 0.38 but increases by a factor of 2.3-fold in the presence of iodomethane. The triplet reacts
with molecular oxygen to produce singlet molecular oxygen in high quantum yield. In sharp contradiction
to a recent literature report, there is no spectroscopic evidence to indicate the presence of an unusually
long-lived CT state.
desired long-lived photoredox state.7 The most satisfactory way
around this problem has involved attaching additional electron
Introduction
The formation of a long-lived charge-separated state is a
prime requirement for artificial photosynthetic systems, opto-
electronic devices, and photochemically driven machines.1 An
additional design feature inherent to all such molecular-scale
materials is that, to offset fatigue, the electron-transfer processes
should avoid diffusional motion. These realizations have led to
the continued search for simple molecular systems in which
rapid charge recombination is circumvented by competing
secondary events.2 In principle, the Marcus inverted effect3 could
be invoked to restrict the rate of charge recombination, while
favoring fast charge separation, but complications from triplet
formation,4 quantum mechanical effects,5 and nuclear tunneling6
obscure the full impact of high reaction exoergonicity. As such,
simple molecular dyads cannot be relied upon to display the
donors and/or acceptors to the central dyad.8 Such multicom-
ponent supermolecules can initiate a cascade of electron-transfer
steps leading to a spatially resolved radical pair.9 Each electron-
transfer event loses part of the initial excitation energy but,
because of the incremental separation distance, stabilizes the
(4) (a) Dutton, P. L.; Leigh, J. S.; Seibert, M. Biochim. Biophys. Res. Comm.
1972, 46, 406. (b) Thurnauer, M. C.; Katz, J. J.; Norris, J. R. Proc. Natl.
Acad. Sci. U.S.A. 1975, 72, 3270. (c) Hasharoni, K.; Levanon, H.;
Greenfield, S. R.; Gosztola, D. J.; Svec, W. A.; Wasielewski, M. R. J. Am.
Chem. Soc. 1995, 117, 8055. (d) Wasielewski, M. R.; Minsek, D. W.;
Niemczyk, M. P.; Svec, W. A.; Yang, N. C. J. Am. Chem. Soc. 1990, 112,
2823. (e) Brun, A. M.; Harriman, A.; Tsuboi, Y.; Okada, T.; Mataga, N. J.
Chem. Soc., Faraday Trans. 1995, 91, 4047.
(5) (a) Bixon, M.; Jortner, J. AdV. Chem. Phys. 1999, 106, 35. (b) Kroon, J.;
Oevering, H.; Verhoeven, J. W.; Warman, J. M.; Oliver, A. M.; Paddon-
Row, M. N. J. Phys. Chem. 1993, 97, 5065. (c) Smit, K. J.; Warman, J.
M.; de Haas, M. P.; Paddon-Row, M. N.; Oliver, A. M. Chem. Phys. Lett.
1988, 152, 177. (d) Liang, N.; Miller, J. R.; Closs, G. L. J. Am. Chem.
Soc. 1990, 112, 5353. (e) Warman, J. M.; Smit, K. J.; Jonker, S. A.;
Verhoeven, J. W.; Oevering, H.; Kroon, J.; Paddon-Row, M. N.; Oliver,
A. M. Chem. Phys. 1993, 170, 359.
† University of Newcastle.
¶ University of Amsterdam.
(1) (a) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2002, 35, 57.
(b) Shipway, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421. (c) Wosnick,
J. H.; Swager, T. M. Curr. Opin. Chem. Biol. 2000, 4, 715.
(2) (a) Sanchez, L.; Perez, I.; Martin, N.; Guldi, D. M. Chem. Eur. J. 2003, 6,
2457. (b) Imahori, H. Org. Biomol. Chem. 2004, 2, 1425. (c) Park, Y. S.;
Lee, E. J.; Chun, Y. S.; Yoon, Y. D.; Yoon, K. B. J. Am. Chem. Soc.
2002, 124, 7123. (d) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem.
Res. 2001, 34, 40. (e) Du¨rr, H.; Bossman, S. Acc. Chem. Res. 2001, 34,
905.
(6) (a) Kroon, J.; Oevering, H.; Verhoeven, J. W.; Warman, J. M.; Oliver, A.
M.; Paddon-Row, M. N. J. Phys. Chem. 1993, 97, 5065. (b) Franzen, S.;
Martin, J. Annu. ReV. Phys. Chem. 1995, 46, 453. (c) Benniston, A. C.;
Chapman, G. M.; Harriman, A.; Mehrabi, M. J. Phys. Chem. A 2004, 108,
9026. (d) Ohkita, H.; Benten, H.; Anada, A.; Noguchi, H.; Kido, N.; Ito,
S.; Yamamoto, M. Phys. Chem. Chem. Phys. 2004, 6, 3977.
(7) (a) Verhoeven, J. W.; van Ramesdonk, H. J.; Groenveld, M. M.; Benniston,
A. C.; Harriman, A. Chem. Phys. Chem., in press. (b) Verhoeven, J. W.;
van Ramesdonk, H. J.; Zang, H.; Groeneveld, M. M.; Benniston, A. C.;
Harriman, A. Int. J. Photoenergy 2005, 7, 103.
(3) Marcus, R. A. Discuss. Faraday Soc. 1960, 29, 21.
9
16054
J. AM. CHEM. SOC. 2005, 127, 16054-16064
10.1021/ja052967e CCC: $30.25 © 2005 American Chemical Society

9-Mesityl-10-methylacridinium Cation
A R T I C L E S
radical pair against charge recombination. In this way, despite
the synthetic challenge, molecular triads, tetrads, pentads, etc.
have become an essential part of our strategy to engineer
intelligent molecular photonic devices.10
nitrile at ambient temperature, but this process becomes first
order at higher temperatures and in frozen benzonitrile.16 We
have questioned these latter results17 and suggested that the
observed EPR signal might arise from a sacrificial photoredox
process involving trace impurities in the solvent. Such behavior
might also explain other results where long-lived photoredox
products have been observed under unusual circumstances.18
The significance of the results reported by Fukuzumi et al.16
cannot be overstated. In particular, the ability to stabilize a
simple molecular dyad against charge recombination, nuclear
tunneling, quantum mechanical effects, and triplet formation
would open the way to construct all manner of fascinating
optoelectronic devices without the need for elaborate synthesis.
Already, this dyad has been proposed as an improved sensitizer
for dye-injection solar cells,19 as an olefin oxygenation photo-
catalyst,20 and as a light harvester for photovoltaic cells.21
Several questions have to be asked of the present system,
however, before markedly changing strategy. A major issue
concerns the lack of triplet formation since closely related 9-aryl-
10-methylacridinium ions are known to exhibit efficient inter-
system crossing to the triplet manifold.22 Such donor-acceptor
systems also display charge-transfer fluorescence22,23 that decays
on the nanosecond time scale. Of course, the mesityl group is
held orthogonal to the acridinium nucleus, and this geometry
might have profound implications for the extent of electronic
coupling between the redox partners. It seems prudent, therefore,
to conduct a detailed spectroscopic investigation of this system
before drawing too many conclusions. We now describe the
results of such an examination and find that although the
9-mesityl-10-methylacridinium cation displays some interesting
photophysical properties these are very different from those
reported by Fukuzumi et al.16 and, in particular, do not include
formation of a long-lived charge-transfer state.
Apart from thermodynamic considerations, there are other
possible ways to decrease the rate of charge recombination in
simple molecular dyads.7 For example, concepts based on spin
restriction rules,11 conformational gating,12 applied magnetic
fields,13 orientational effects,14 and orbital symmetry15 have been
considered as ways to avoid the time-consuming synthesis
needed to produce the multicomponent arrays. The effects of
these approaches tend to be limited and/or of restricted ap-
plicability.7 A notable exception to this generic behavior,
however, was reported recently. Thus, Fukuzumi et al.16
described a donor-acceptor compound, namely the 9-mesityl-
10-methylacridinium ion, that exhibits truly remarkable proper-
ties. This simple system is said to produce a photoredox state
by way of a charge-shift reaction, in 98% yield, that survives
for more than 2 h in benzonitrile at 203 K. The charge-transfer
state is believed to store 2.37 eV and, because the reorganization
energy for reforming the ground state is only 0.79 eV, classical
Marcus theory predicts that the activation energy for this process
is unusually high. Bimolecular charge shift is noted in aceto-
(8) (a) Gust, D.; Moore, T. A.; Moore, A. L.; Macpherson, A. N.; Lopez, A.;
DeGraziano, J. M.; Gouni, I.; Bittersman, E.; Seely, G. R.; Gao, F.; Nieman,
R. A.; Ma, X. C.; Demanche, L.; Luttrell, D. K.; Lee, S. J.; Perrigan, P. K.
J. Am. Chem. Soc. 1993, 115, 11141. (b) Imahori, H.; Guldi, D. M.; Tamaki,
K.; Yoshida, Y.; Luo, C.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc.
2001, 123, 6617. (c) Kodis, G.; Liddell, P. A.; de la Garza, L.; Lin, S.;
Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. A 2002, 106, 2036.
(d) Lukas, A. S.; Miller, S. E.; Wasielewski, M. R. J. Phys. Chem. B 2000,
104, 931. (e) Osuka, A.; Okada, T.; Taniguchi, S.; Nozaki, K.; Ohno, T.;
Mataga, N. Tetrahedron Lett. 1995, 36, 5781. (f) Bell, T. D. M.; Jolliffe,
K. A.; Ghiggino, K. P.; Oliver, A. M.; Shephard, M. J.; Langford, S. J.;
Paddon-Row, M. N. J. Am. Chem. Soc. 2000, 122, 10661.
(9) (a) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (b) Maruyama, K.;
Osuka, A.; Mataga, N. Pure Appl. Chem. 1994, 66, 867. (c) Harriman, A.;
Sauvage, J.-P. Chem. Soc. ReV. 1996, 41. (d) Sakata, Y.; Imahori, H.; Tsue,
H.; Higashida, S.; Akiyama, T.; Yoshizawa, E.; Aoki, M.; Yamada, K.;
Hagiwara, K.; Taniguchi, S.; Okada, T. Pure Appl. Chem. 1997, 69, 1951.
(e) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (f)
Paddon-Row, M. N. Aust. J. Chem. 2003, 56, 729.
(10) (a) Linke, M.; Chambron, J.-C.; Heitz, V.; Sauvage, J.-P.; Encinas, S.;
Barigelletti, F.; Flamigni, L. J. Am. Chem. Soc. 2000, 122, 11834. (b)
Haycock, R. A.; Hunter, C. A.; James, D. A.; Micheksen, U.; Sutton, L.
R. Org. Lett. 2000, 2, 2435. (c) Kim, Y. H.; Jeoung, D. H.; Kim, D.; Jeoung,
S. C.; Cho, H. S.; Kim, S. K.; Aratani, N.; Osuka, A. J. Am. Chem. Soc.
2001, 123, 76. (d) Loewe, R. S.; Lammi, R. K.; Diers, J. R.; Kirmaier, C.;
Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Mater. Chem. 2002, 12, 1530.
(11) (a) Smit, K. J.; Warman, J. M. J. Lumin. 1988, 42, 149. (b) Karpiuk, J.
Phys. Chem. Chem. Phys. 2003, 5, 1078. (c) Anglos, D.; Bindra, V.; Kuki,
A. J. Chem. Soc., Chem. Commun. 1994, 2, 213. (d) van Dijk, S. I.; Groen,
C. P.; Hartl, F.; Brouwer, A. M.; Verhoeven, J. W. J. Am. Chem. Soc.
1996, 118, 8425. (e) Hviid, L.; Brouwer, A. M.; Paddon-Row, M. N.;
Verhoeven, J. W. Chem. Phys. Chem. 2001, 232. (f) Hviid, L.; Bouwman,
W. G.; Paddon-Row, M. N.; Verhoeven, J. W.; van Ramesdonk, H. J.;
Brouwer, A. M. Photochem. Photobiol. Sci. 2003, 2, 995. (g) Hviid, L.;
Verhoeven, J. W.; Brouwer, A. M.; Paddon-Row, M. N.; Yang, J.; George,
M. W. Photochem. Photobiol. Sci. 2004, 3, 246.
(12) (a) Osyczka, A.; Moser, C. C.; Daldal, F.; Dutton, P. L. Nature 2004, 427,
607. (b) Andreasson, J.; Kyrychenko, A.; Martensson, J.; Albinsson, B.
Photochem. Photobiol. Sci. 2002, 1, 111. (c) Davis, W. B.; Ratner, M. A.;
Wasielweski, M. R. J. Am. Chem. Soc. 2001, 123, 7877.
(13) (a) Wolff, H.-J.; Burssner, D.; Steiner, U. E. Pure Appl. Chem. 1995, 67,
167. (b) Schaffner, E.; Fischer, H. J. Phys. Chem. 1996, 100, 1657. (c)
Sinks, L. E.; Weiss, E. A.; Giaimo, J. M.; Wasielewski, M. R. Chem. Phys.
Lett. 2005, 404, 244.
(14) (a) Tsukahara, K.; Ueda, R. Bull. Chem. Soc. Jpn. 2003, 76, 561. (b)
Korchowiec, J. Int. J. Quantum Chem. 2005, 101, 714. (c) Sakata, Y.;
Imahori, H.; Tsue, H.; Higashida, S.; Akiyama, T.; Yoshizawa, E.; Aoki,
M.; Yamada, K.; Hagiwara, K.; Taniguchi, S.; Okada, T. Pure Appl. Chem.
1997, 69, 1951. (d) Asano-Someda, M.; Jinmon, A.; Toyama, N.; Kaizu,
Y. Inorg. Chim. Acta 2001, 324, 347.
(15) (a) Zeng, Y.; Zimmt, M. B. J. Am. Chem. Soc. 1991, 113, 5107. (b) Zeng,
Y.; Zimmt, M. B. J. Phys. Chem. 1992, 96, 8395. (c) Oliver, A. M.; Paddon-
Row, M. N.; Kroon, J.; Verhoeven, J. W. Chem. Phys. Lett. 1992, 191,
371.
(16) Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko, N. V.;
Lemmetyinen, H. J. Am. Chem. Soc. 2004, 126, 1600.
Experimental Section
All chemicals were purchased commercially and, unless stated
otherwise, were used as received. Solvents for synthetic procedures
were dried by standard literature methods before being distilled and
stored under nitrogen over 4 Å molecular sieves. N,N′-Dimethyl-4,4′-
(17) Benniston, A. C.; Harriman, A.; Li, P.; Rostron, J. P.; Verhoeven, J. W.
Chem. Commun. 2005, 2701.
(18) Guldi, D. M.; Imahori, H.; Tamaki, K.; Kashiwagi, Y.; Yamada, H.; Sakata,
Y.; Fukuzumi, S. J. Phys. Chem. A 2004, 108, 541.
(19) Hasobe, T.; Hattori, S.; Kamat, P. V.; Wada, Y.; Fukuzumi, S. J. Mater.
Chem. 2005, 15, 372.
(20) (a) Kotani, H.; Ohkubo, K.; Fukuzumi, S. J. Am. Chem. Soc. 2004, 126,
15999. (b) Ohkubo, K.; Nanjo, T.; Fukuzumi, S. Org. Lett. 2005, 7, 4265.
(21) Hasobe, T.; Hattori, S.; Kotani, H.; Ohkubo, K.; Hosomizu, K.; Imahori,
H.; Kamat, P. V.; Fukuzumi, S. Org. Lett. 2004, 6, 3103.
(22) van Willigen, H.; Jones, G., II; Farahat, M. S. J. Phys. Chem. 1996, 100,
3312.
(23) Jones, G., II; Farahat, M. S.; Greenfield, S. R.; Gosztola, D. J.; Wasielewski,
M. R. Chem. Phys. Lett. 1994, 229, 40.
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16055

A R T I C L E S
Benniston et al.
bipyridinium hexafluorophosphate (methyl viologen) was precipitated
from an aqueous solution of the commercially available chloride salt
by addition of KPF6(aq). The resultant solid was filtered, washed
thoroughly with water and Et2O, and dried under vacuum. Recrystal-
lization of the solid from CH3CN/Et2O afforded a pure white solid that
was dried and kept over silica gel in a desiccator. 2,4,6-Triphenyl-
pyrylium tetrafluoroborate (Aldrich Chemicals) was recrystallized
several times from ethanol and dried under vacuum. Mesitylene was
redistilled under vacuum. Benzonitrile was distilled under reduced
pressure from P2O5, not CaH2, since this latter drying agent leads to
(s, 2H, Ph-H), 7.84 (d, J ) 5 Hz, 4H, Acr-H), 8.37 (m, 2H, Acr-
H), 8.60 (d, J ) 9.3 Hz, 2H, Acr-H). ES-MS (m/z): 312.2 (calcd Mr
) 312.2 for M - ClO4]+). Anal. Calcd for C23H22NClO4: C, 67.07;
H, 5.38; N, 3.40%. Found: C, 67.07; H, 5.42; N, 3.42%. Note that the
possible shock sensitiVity of organic perchlorates requires that these
are handled carefully and in small quantities only.
Photophysical studies were made in parallel in both Amsterdam and
Newcastle with fully consistent results. Absorption spectra were
recorded with a Hitachi U3310 spectrophotometer, and luminescence
spectra were recorded with a fully corrected Yvon-Jobin Fluorolog tau-3
spectrophotometer. Emission spectra were recorded for optically dilute
solutions after purging with N2. Spectral curve fitting was carried out
as described before.26 The spectrum was fully corrected, reduced,27 and
deconvoluted into Gaussian components28 using the commercially
available software PEAKFIT. Luminescence quantum yields were
determined relative to that of 9-phenyl-10-methylacridinium chloride
in acetonitrile.22 Temperature dependence studies were made with sealed
sample cells housed in an Oxford Instruments Optistat DN cryostat.
Emission lifetimes were measured at room temperature with the tau-3
spectrophotometer. For lower temperatures, the luminescence lifetimes
were measured by time-correlated, single-photon counting using a high
repetition (40 MHz) picosecond laser diode operating at 440 nm.
Emission was isolated from scattered laser light with a high-radiance
monochromator and detected with a microchannel plate photocell. After
deconvolution of the instrument response function, the time resolution
of this setup was ca. 15 ps. The spectral resolution, being limited by
the need to obtain a reasonable count rate, was ca. 4 nm. Improved
time resolution was achieved using fluorescence upconversion methods
as described previously.29
1
chemical decomposition. H (referenced to residual protiated solvent
in CD3CN, δ ) 1.93) and 13C NMR spectra were recorded with a JEOL
Lambda 500 spectrometer. Routine mass spectra and elemental analyses
were obtained using in-house facilities. Synthesis of N-(2-methoxy-
ethoxymethyl)-9-acridone has been described previously.24
Preparation of 9-Mesitylacridine. To a solution of N-(2-methoxy-
ethoxymethyl)-9-acridone24 (1.0 g, 3.53 mmol) in tetrahydrofuran (THF)
(100 mL) at 0 °C under nitrogen was added over 15 min a THF solution
of 2-mesitylmagnesium bromide (freshly prepared from 2-bromomesi-
tylene (1.2 mL, 7.76 mmol) in THF (30 mL) and magnesium turnings
(0.22 g, 9.17 mmol)). The mixture was stirred at RT for 24 h and then
heated at 50 °C for 12 h. A solution of concentrated HCl (50 mL) and
H2O (50 mL) was poured into the brown suspension. The resultant
clear yellow solution was stirred overnight at RT. The mixture was
treated with K2CO3(aq) to give pH ca. 9 and extracted with ethyl acetate.
The organic layer was washed with brine, separated, and dried over
MgSO4. Removal of the solvent afforded the crude product, which was
purified by silica gel chromatography (petrol/EtOAc 2:1) to give a pale-
1
yellow crystalline solid (0.59 g, 56% yield). H NMR (δ, 500 MHz,
Flash photolysis studies were made with a variety of instruments.
An Applied Photophysics Ltd. LKS60 was used for nanosecond studies.
Excitation was made with 4-ns pulses at 355 nm delivered with a
frequency-tripled, Q-switched Nd:YAG laser, while detection was made
at 90° using a pulsed, high-intensity Xe arc lamp. The signal was
detected with a fast response PMT after passage through a high-radiance
monochromator. Transient differential absorption spectra were recorded
point-by-point with 5 individual records being averaged at each
wavelength. Kinetic measurements were made after averaging 50
individual records using global analysis methods. The sample was
purged with N2 before use. For some studies, iodomethane (10% v/v)
was added before photolysis. The laser intensity was calibrated by
reference to the triplet state of benzophenone in deoxygenated aceto-
nitrile.30 Quantum yield measurements were made by reference to
benzophenone in acetonitrile over a wide range of laser intensities. For
these studies, the triplet quantum yield and T1 - Tn molar absorption
coefficient at 525 nm for benzophenone were taken as 1.0 and 6500
M-1 cm-1, respectively.31 The molar absorption coefficient for Mes-
Acr+ at 500 nm was measured by the complete conversion method to
be 5500 M-1 cm-1; this latter value is in excellent agreement with that
determined for the conjugate acid of acridine.32 For the complete
bleaching studies, the excitation source was a pulsed Xe flash lamp
(fwhm <1 µs) and the optical cell was a 10 cm path length glass
cylinder. The solution was deoxygenated by freeze-pump-thaw cycles,
and the flash lamp was filtered to remove UV (λ < 330 nm) light.
CD3CN): 1.65 (s, 6H, CH3-), 2.42 (s, 3H, CH3-), 7.15 (s, 2H, Ph-
H), 7.45 (m, 4H, Acr-H), 7.80 (m, 2H, Acr-H), 8.21(d, J ) 8.8 Hz,
2H, Acr-H).
Preparation of 9-Mesityl-10-methylacridinium Hexafluorophos-
phate (Mes-Acr+). To a solution of 9-mesitylacridine (0.24 g, 0.81
mmol) in CH3CN (20 mL) was added CH3I (0.35 mL, 5.56 mmol).
The mixture was refluxed overnight. The volume of the mixture was
reduced to ca. 2 mL, and 100 mL of ether was added. The resultant
brown-red solid (0.15 g) was collected by filtration and washed with
diethyl ether. The solid was dissolved in a mixture of CH3CN (20 mL)/
H2O (10 mL), and KPF6 (0.70 g, 3.80 mmol) in H2O (20 mL) added
to afford a light-yellow crystalline solid that was collected by filtration
and washed with H2O and diethyl ether. Bright yellow crystals were
obtained by recrystallization from CH3OH and diethyl ether. Yield:
1
0.15 g (41%). H NMR (δ, 500 MHz, CD3CN): 1.68 (s, 6H, CH3-),
2.46 (s, 3H, CH3-), 4.82 (s, 3H, CH3-N), 7.23 (s, 2H, Ph-H), 7.84
(d, J ) 5 Hz), 4H, Acr-H), 8.38 (m, 2H, Acr-H), 8.61 (d, J ) 9.3
Hz, 2H, Acr-H). ES-MS (m/z): 312.1 (calcd Mr ) 312.2 for [M -
PF6]+). Anal. Calcd for C23H22NPF6: C, 60.40; H, 4.85; N, 3.06%.
Found: C, 60.40; H, 4.65; N, 3.17%.
Preparation of 9-Mesityl-10-methylacridinium Perchlorate. 9-
Mesityl-10-methylacridinium iodide (60 mg, 0.137 mmol) was dissolved
in a mixture of CH3CN (2 mL) and H2O (20 mL). To this yellow
solution was added NaClO4 (0.10 g) in H2O (20 mL). The resulting
light orange-yellow precipitate was collected by filtration and washed
with H2O and ether. Orange crystals were obtained by recrystallization
from CH3OH/CH3CN and ether.25 The authenticity of this sample was
confirmed by a single-crystal X-ray diffraction study (see the Supporting
(26) (a) Bixon, M.; Jortner, J.; Verhoeven, J. W. J. Am. Chem. Soc. 1994, 116,
7349. (b) Gould, I. R.; Young, R. H.; Meuller, L. J.; Albrechts, A. C.;
Farid, S. J. Am. Chem. Soc. 1994, 116, 8188. (c) Bixon, M.; Jortner, J.;
Corte´s, J.; Heitele, H.; Michel-Beyerle, M. E. J. Phys. Chem. 1994, 98,
7289. (d) Oevering, H.; Verhoeven, J. W.; Paddon-Row, M. N.; Warman,
J. M. Tetrahedron 1989, 45, 4751. (e) Corte´s, J.; Heitele, H.; Jortner, J. J.
Phys. Chem. 1994, 98, 2527.
1
Information). Yield: 50 mg (89%). H NMR (δ, 500 MHz, CD3CN):
1.68 (s, 6H, CH3-), 2.46 (s, 3H, CH3-), 4.81 (s, 3H, CH3-N), 7.23
(27) Parker, C. A.; Rees, W. T. Analyst 1960, 85, 587.
(28) Harriman, A.; Hissler, M.; Ziessel, R. Phys. Chem. Chem. Phys. 1999, 1,
4203.
(24) Tanaka, T.; Tasaki, T.; Aoyama, Y. J. Am. Chem. Soc. 2002, 124, 12454.
1
(25) The H NMR spectrum recorded for the perchlorate salt differs from that
reported in the literature (ref 20a). This is not due to ion pairing since our
spectra for perchlorate and hexafluorophosphate samples are indistinguish-
able. Furthermore, we note that all the chemical shifts differ by ca. 0.5
ppm (ref 20) and this effect is most likely caused by incorrect referencing
to the residual protiated solvent peak. Closely comparable spectra were
found in CDCl3 and between the two laboratories.
(29) Harriman, A.; Hissler, M.; Trompette, O.; Ziessel, R. J. Am. Chem. Soc.
1999, 121, 2516.
(30) Carmichael, I.; Hug, G. L. J. Phys. Chem. Ref. Data 1986, 15, 1.
(31) Bensasson, R. V.; Gramain, J.-C. J. Chem. Soc., Faraday Trans. 1 1980,
76, 1801.
(32) Nishida, Y.; Kikuchi, K.; Kokubun, H. J. Photochem. 1980, 13, 75.
9
16056 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

9-Mesityl-10-methylacridinium Cation
A R T I C L E S
Improved time resolution was achieved with a mode-locked,
frequency-doubled Nd:YAG laser (fwhm ) 20 ps). The excitation pulse
was passed through a Raman shifter in order to isolate the required
wavelength. The monitoring pulse was either a white light continuum,
delayed with respect to the excitation pulse with a computer-controlled
optical delay line, or a pulsed Xe arc lamp. For the former studies, the
two pulses were directed almost collinearly through the sample cell.
The monitoring pulse was dispersed with a Princeton Instruments
spectrograph and detected with a dual-diode array spectrometer.
Approximately 150 individual laser shots were averaged at each delay
time. Kinetic parameters were derived by overlaying spectra collected
at different delay times. All measurements were made with dilute
solutions after purging with N2. For the latter studies, a fast response
digitizer and PMT were used as detector at a single wavelength. The
temporal resolution of this setup was about 2 ns.
After decay of the triplet state,34a the residual spectrum contains
contributions from the neutral TPP‚ radical34b and the mesityl π-radical
cation.34c,d Global analysis of the kinetic data and subtracting the former
spectrum (ꢀ550 ) 2790 M-1 cm-1) provides access to the absorption
spectrum of the mesityl π-radical cation. Confirmation of this latter
assignment was made by addition of low concentrations of 4,4′-
dimethoxydiphenylmethanol, which was oxidized by the mesityl
π-radical cation to form the known spectrum (λMAX ) 510 nm) of the
4,4′-dimethoxydiphenylmethyl cation.34e
Electrochemical measurements were made with an HCH electro-
chemical analyzer. The working electrode was a highly polished glassy
carbon disk, and the counter electrode was a Pt wire. A Ag/AgCl
reference electrode was used, with ferrocene as internal standard. The
solution (ca. 1 mM) contained tetra-N-butylammonium hexafluoro-
phosphate (0.2 M) as background electrolyte and was purged thoroughly
with N2 prior to electrolysis. Peak potentials were reproducible to within
15 mV. The differential absorption spectrum of the acridinyl radical
was recorded by spectroelectrochemical reduction using a fixed potential
of -0.7 V vs SCE. The solution was deoxygenated and subjected to
exhaustive reduction, with absorption spectra being recorded at frequent
intervals. A similar experimental protocol was used to generate the
differential absorption spectrum for the one-electron reduced species
from methyl viologen in deoxygenated acetonitrile.
Fast transient spectroscopy was made by pump-probe techniques
using femtosecond pulses delivered from a Ti:sapphire generator
amplified with a multipass amplifier pumped via the second harmonic
of a Q-switched Nd:YAG laser. The amplified pulse energies varied
from 0.3 to 0.5 mJ, and the repetition rate was kept at 10 Hz. Part of
the beam (ca. 20%) was focused onto a second-harmonic generator in
order to produce the excitation pulse. The residual output was directed
onto a 4-mm sapphire plate so as to create a white light continuum for
detection purposes. The continuum was collimated and split into two
equal beams. The first beam was used as reference, while the second
beam was combined with the excitation pulse and used as the diagnostic
beam. The two beams were directed to different parts of the entrance
slit of a cooled CCD detector and used to calculate differential
absorbance values. The CCD shutter was kept open for 1 s, and the
accumulated spectra were averaged. This procedure was repeated until
about 100 individual spectra had been averaged. Time-resolved spectra
were recorded with a delay line stepped in increments of 100 fs for
short time measurements. This step length was increased for longer
time measurements. The decay profiles were fitted globally as the sum
of exponentials and deconvoluted with a Gaussian excitation pulse.
The group velocity dispersion across one spectrum (ca. 220 nm) was
of the order of 1 ps, and the overall temporal resolution of this setup
was ca. 0.8 ps. The sample, possessing an absorbance of ca. 1 at 430
nm, was flowed through a quartz cuvette (optical path length ) 1 mm)
and maintained under an atmosphere of N2.
Results and Discussion
Spectroscopic Studies. The 10-methylacridinium ion (Acr+)
exhibits highly structured fluorescence in fluid solution that is
characterized by a Stokes shift of ca. 30 nm in acetonitrile. The
fluorescence quantum yield (ΦF) is approximately 1.0, while
the excited singlet state lifetime (τS) is 31 ( 2 ns in the absence
of oxygen. Weak phosphorescence can be detected around 620
nm in an ethanol glass at 77 K after addition of iodomethane
(10% v/v). The lifetime of the triplet state under these conditions
is 5.5 ( 0.5 ms. Several research groups have described7,22,23,35
the photophysical properties of 9-aryl-substituted 10-methyl-
acridinium ions and, in particular, Fukuzumi et al.16 have
reported spectroscopic data for 9-mesityl-10-methylacridinium
perchlorate at room temperature. We now describe the outcome
of an independent investigation of the photophysical properties
of this latter cation in order to properly establish a detailed
understanding of the excited-state hierarchy. We have used the
hexafluorophosphate salt for reasons of safety and improved
solubility, but parallel studies were made with the perchlorate
salt to ensure the absence of specific counterion effects.25 In
all cases, the results obtained for the two salts were indistin-
guishable. An X-ray crystal structural determination made for
the perchlorate salt proved the authenticity of the compound,
while the close comparability of the 500 MHz 1H NMR spectra
was taken as evidence that the hexafluorophosphate salt was of
the appropriate purity.
Singlet molecular oxygen was detected by time-resolved lumines-
cence after excitation of the sample in O2-saturated acetonitrile with a
10-ns laser pulse delivered at 355 nm. The detector was a cryogenically
cooled Ge photocell operating at 1270 nm. The laser intensity was
varied over a wide range using metal screen filters and, in each case,
20 separate laser shots were averaged to produce the final signal. The
yield of singlet molecular oxygen was obtained by extrapolation of
the first-order decay trace to zero time. An optically matched solution
of perinaphthenone in O2-saturated acetonitrile was used as reference.33
The transient differential absorption spectrum of the mesityl π-radical
cation was recorded by laser flash photolysis (λ ) 355 nm, fwhm ) 4
ns) of 2,4,6-triphenylpyrylium tetrafluoroborate (TPP+) (2.5 × 10-5
M) in air-equilibrated acetonitrile containing mesitylene (0-0.1 M).34
At the end of the laser pulse, the transient differential absorption
spectrum comprises ground-state bleaching, triplet TPP+ (λMAX ) 340
and 470 nm), TPP‚ (λMAX ) 550 nm), and the mesityl π-radical cation.
The triplet lifetime is unaffected by the presence of mesitylene, but
pronounced fluorescence quenching occurs. The lifetime of the triplet
state, being set by the concentration of dissolved O2, was ca. 1 µs.
The absorption spectrum recorded for Mes-Acr+ in aceto-
nitrile solution closely resembles that found for Acr+, indicating
the dominant chromophore is the acridinium ion.35 Fluorescence
is observed under these conditions, but the spectrum differs
markedly from that characteristic of Acr+ (Figure 1). In
deoxygenated acetonitrile, ΦF is only 0.035 ( 0.004 while τS
is 6.0 ( 0.1 ns. Decay of the excited state is monoexponential
at all monitoring wavelengths and, in particular, contains no
(33) Oliveros, E.; Bossman, S. H.; Nonell, S.; Marti, C.; Heit, G.; Troscher, G.;
Neuner, A.; Martinez, G.; Braun, A. M. New J. Chem. 1999, 23, 85.
(34) (a) Branchi, B.; Bietti, M.; Ercolani, G.; Izquierdo, M. A.; Mitanda, M.
A.; Stella, L. J. Org. Chem. 2004, 69, 8874. (b) Jayanthi, S. S.;
Ramamurthy, P. J. Phys. Chem. A 1997, 101, 2016. (c) Sehested, K.;
Holcman, J.; Hart, E. J. J. Phys. Chem. 1977, 81, 1363. (d) Hubig, S. M.;
Kochi, J. K. J. Am. Chem. Soc. 2000, 122, 8279. (e) Bartl, J.; Steenken,
S.; Mayr, H.; McCelland, R. A. J. Am. Chem. Soc. 1990, 112, 6918.
(35) (a) Jonker, S. A.; Ariese, F.; Verhoeven, J. W. Recl. TraV. Chim. Pays-
Bas 1989, 108, 109. (b) Jonker, S. A.; van Dijk, S. I.; Goubitz, K.; Reiss,
C. A.; Schuddeboom, W.; Verhoeven, J. W. Mol. Cryst. Liq. Cryst. 1990,
183, 273. (c) Acheson, R. M., Ed. Acridines, 2nd ed.; Interscience
Publishers: New York, 1973; Chapter 11.
9
J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16057

A R T I C L E S
Benniston et al.
Figure 1. Normalized fluorescence spectra recorded for Acr+ (solid curve)
and Mes-Acr+ (broken curve) in deoxygenated acetonitrile at room
temperature.
contribution from a long-lived component. The corrected
excitation spectrum is in excellent agreement with the absorption
spectrum between 230 and 470 nm. Close examination of the
fluorescence spectrum, however, reveals the existence of some
fine structure on the high-energy side of the profile that coincides
with the structured emission typical for the acridinium chro-
mophore, while the low-energy side shows broad and unstruc-
tured emission similar to that reported for certain 9-aryl-10-
methylacridinium ions with an electron-donating aryl group
(Figure 1).7,22,23 In these latter cases, fluorescence was attributed
to a charge-transfer (CT) state in which the aryl group acts as
donor and the acridinium ion as acceptor. The spectrum
observed for Mes-Acr+ could be interpreted, therefore, as a
mixture of fluorescence both from a locally excited (LE),
acridinium-like singlet state and a CT state. It should be stressed
that repeated purification of the sample had no effect on the
fluorescence profile or lifetime.36 Identical results were obtained
for the perchlorate salt.
Figure 2. Transient differential absorption spectra recorded after laser
excitation of Mes-Acr+ in acetonitrile at room temperature. (a) Excitation
at 430 nm with a 0.2-ps laser pulse. (b) Excitation at 440 nm with a 20-ps
laser pulse. Note the isosbestic points and the absorbance rise in the near-
IR in panel b. The delay times are indicated on each panel.
On exposure to subpicosecond laser pulses delivered at 430
nm the S1-Sn transient absorption spectrum, which has a
maximum at 470 nm, can be observed (Figure 2a). This
spectrum shows pronounced absorption in the blue region but
very little absorption at longer wavelengths. Over a period of 5
ps, the signal assigned to the LE singlet state evolves into a
new absorption profile characterized by an absorption maximum
around 500 nm and a pronounced shoulder at 550 nm (Figure
2b). This latter signal persists into the nanosecond time regime
and is assigned to the singlet CT state. The strong absorption
band around 500 nm can be attributed to the acridinyl radical,17
while the mesityl radical cation is known to absorb around 470
nm.34d Neither species is expected to display significant absorp-
tion at 550 nm; the same situation is observed for 9-(1-naphthyl)-
10-methylacridinium where there is an additional absorption
transition superimposed over bands expected for the naphthyl
radical cation and the acridinyl radical.7 Given the close
proximity and high redox power of the reactants, the 550 nm
absorption band might be considered characteristic of strong
electronic coupling within the CT state.
Figure 3. Kinetic traces recorded at 480 nm following laser excitation of
Mes-Acr+ in acetonitrile at room temperature. (a) Excitation at 430 nm
with a 0.2-ps laser pulse. (b) Excitation at 440 nm with a 20-ps laser pulse.
(c) Excitation at 440 nm with a 20-ps laser pulse and with detection provided
by a fast response digitizer.
1011 s-1. This latter value is in good agreement with the lifetime
of the LE state (τLE) of 4.4 ( 0.9 ps derived by fluorescence
upconversion spectroscopy. On this basis, we would expect to
see much less LE fluorescence than is apparent in the spectrum
recorded in acetonitrile.37 In fact, a crude estimate based on
deconvolution of the entire spectrum into LE and CT fluores-
Formation of the singlet CT state occurs by way of first-
order kinetics with a rise time of 5 ( 1 ps in acetonitrile at
room temperature (Figure 3a), in good agreement with the value
reported by Fukuzumi et al.16 Thus, the LE singlet state is
heavily quenched compared to Acr+, and the rate constant for
intramolecular charge transfer (kCT) can be taken as ca. 2 ×
(36) The compound was subjected to repeated column chromatography, TLC
and recrystallization in an effort to minimize the contribution made by the
LE fluorescence. Simultaneous analysis was made by time-resolved
fluorescence to ensure the absence of a long-lived component in the decay
records.
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16058 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

9-Mesityl-10-methylacridinium Cation
A R T I C L E S
Figure 5. Effect of heating from 20 to 130 °C on the fluorescence spectrum
of Mes-Acr+ recorded in deoxygenated benzonitrile. The inset shows a
plot to the Boltzmann expression.
Figure 4. Deconvolution of the fluorescence spectrum recorded for Mes-
Acr+ in acetonitrile at room temperature into a series of Gaussian-shaped
components. The three dashed curves are attributed to LE fluorescence,
while the solid curves are assigned to fluorescence from the CT state. Note
the fourth CT band cannot be properly resolved on this wavenumber range.
rate constants of LE and CT states, can be well described in
terms of the Boltzmann expression with an energy gap of 8 (
1 kJ mol-1. This value is comparable to that estimated for Mes-
Acr+ in acetonitrile solution by deconvolution of the fluores-
cence spectrum and provides clear evidence that the LE and
CT states reside in thermal equilibrium. The same situation
arises in other solvents at ambient temperature, and the total
fluorescence spectral profile can be split into contributions from
LE and CT states. In all solvents, the amount of LE fluorescence
exceeds that expected on the basis of its spontaneous lifetime.
The results are easily explained, however, in terms of thermal
equilibration between LE and CT singlet states. The energy gap
remains around 8-11 kJ mol-1 in all solvents. It should be
stressed that Mes-Acr+ fluorescence does not follow Lippert-
Mataga behavior,42 presumably because there is no significant
change in dipole moment upon excitation.
In propylene carbonate, the fluorescence properties remain
similar to those described for acetonitrile with ΦF ) 0.035 and
τS ) 5.1 ns at 20 °C. However, on detection at the high-energy
side, where the contribution of CT fluorescence is minimal, a
short component could be picked up with a decay time of 5 (
1 ps. This coincides very well with the rate of formation of the
CT state from the LE state as detected by ultrafast transient
absorption spectroscopy under identical conditions. On cooling
the solution, the contribution of this short component increases
while, at the same time, in the continuous emission spectrum
the contribution of LE fluorescence increases progressively. As
such, the total fluorescence intensity also increases with
decreasing temperature (Figure 6). The temperature dependence
of the decay time of the short component, which corresponds
to the rate constant (kLE) for conversion of LE to CT states,
follows Arrhenius-type behavior in liquid solution with an
cence (Figure 4) indicates that the LE fluorescence contributes
ca. 13% toward the total emission under these conditions.38
Lifetime measurements eliminate the possibility that this residual
LE fluorescence arises from an impurity. Even so, the derived
kCT value, taken together with the radiative rate constant
measured for Acr+ (kRAD ) 3 × 107 s-1), implies that the LE
state should contribute only ca. 0.5% to the overall fluorescence
profile under these conditions. The observed fluorescence
spectrum can be reconstituted as a mixture of LE and CT states
in which the population of the LE state is 2.5%. On this basis,
it is considered that at room temperature virtually all of the
observed LE fluorescence arises from thermal repopulation from
the lower-energy CT state.39 The relative population of LE and
CT states corresponds to an energy gap of ca. 9 kJ mol-1 such
that the rate constant for repopulation of the LE state would be
ca. 5 × 109 s-1. The two states should now decay with a
common lifetime of 6 ns,40 as is observed at ambient temper-
ature.
Further support was obtained for the concept of rapid thermal
equilibration between LE and CT singlet states at and above
room temperature via a temperature dependence study made in
highly purified benzonitrile. With increasing temperature over
the range of 20-130 °C the amount of structured fluorescence
increases progressively, while there is a corresponding decrease
in the unstructured fluorescence profile attributed to the CT state
(Figure 5).41 The relative ratio, after correction for the radiative
(37) The contribution made by LE fluorescence is somewhat solvent dependent
and is notably higher in ethanol solution and a KBr disc. This is not a
consequence of impurities since separate studies confirmed that our time-
resolved fluorescence setup can easily resolve components due to Mes-
Acr+ and Acr+ in mixtures, even when the latter is present at very low
concentration.
(38) The fluorescence spectrum of Acr+ was used as a reference system. This
spectral profile was shifted to match that of the high-energy component
observed for Mes-Acr+, and it was assumed, on the basis of Gaussian
analyses, that the CT fluorescence is negligible at 510 nm. The CT “pure”
fluorescence spectrum was compiled from the Gaussian bands and the two
resolved spectra combined to give the best least-squares match to the
observed spectrum.
(41) (a) It should be noted that the apparent fluorescence maximum differs in
acetonitrile (λ ≈ 590 nm), benzonitrile (λ ≈ 560 nm), and propylene
carbonate (λ ≈ 570 nm) as shown in Figures 1, 5, and 6. This is due to
very slight changes in the equilibrium distribution of LE and CT states in
the different solvents. Because of the vastly different radiative rates, a trivial
increase in the percentage of the LE state moves the emission maximum
towards the blue. Detailed spectral curve fitting shows that the solvent has
minimal effect on the actual energy levels of these two states. The
equilibrium distribution is also changed slightly on addition of electrolytes
such as tetra-N-butylammonium hexafluorophosphate. Such effects have
been reported earlier for related 9-aryl-10-methylacridinium salts, see: (b)
Horng, M. L.; Dahl, K.; Jones, G., II; Maroncelli, M. Chem. Phys. Lett.
1999, 315, 363.
(39) (a) Wasielewski, M. R.; Johnson, D. G.; Svec, W. A.; Kersey, K. M.;
Minsek, D. W. J. Am. Chem. Soc. 1988, 110, 7219. (b) Heitele, H.; Frinckh,
P.; Weeren, S.; Pollinger, F.; Michel-Beyerle, M. E. J. Phys. Chem. 1989,
93, 5173. (b) Harriman, A.; Heitz, V.; Ebersole, M.; van Willigen, H. J.
Phys. Chem. 1994, 98, 4982.
(40) (a) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1992, 96, 2917. (b)
Hissler, M.; Harriman, A.; Khatyr, A.; Ziessel, R. Chem. Eur. J. 1999, 5,
3366.
(42) (a) Lippert, E. Z. Naturforsch. 1955, 10a, 541. (b) Mataga, N.; Kaifu, Y.;
Koizumi, M. Bull. Chem. Soc. Jpn. 1955, 28, 690.
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J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005 16059

A R T I C L E S
Benniston et al.
Figure 6. Effect of cooling from 300 to 130 K on the fluorescence spectrum
recorded for Mes-Acr+ in propylene carbonate. The inset shows an
expansion of the ambient temperature region so as to better reveal the CT
fluorescence, with spectra run each 5 K. Low-temperature spectra refer to
130, 140, 150, and 160 K.
Figure 8. Microsecond triplet-triplet differential absorption spectra
recorded at different times after laser excitation of Mes-Acr+ in deoxy-
genated acetonitrile with a 4-ns laser pulse at 355 nm. The inset shows a
kinetic trace recorded at 480 nm at very low laser power. The individual
spectra were recorded at delay times of 11, 20, 27, 40, 60, and 100 µs and
show the progressive recovery of the ground state.
Detection at low energy (λ ) 600 nm), where the CT
fluorescence makes the dominant contribution, allows study of
the temperature dependence of the processes by which the CT
state decays (kCT). It should be stipulated that kCT is the sum of
at least three rate constants; namely, thermal repopulation of
LE (k-LE), internal conversion to the ground state, and inter-
system crossing to the triplet manifold. In fluid solution, the
temperature dependence of kCT follows Arrhenius behavior with
EA ) 2.8 kJ mol-1 and an apparent preexponential factor (ν)
of 6 × 108 s-1 (Figure 7). The decay rate constant (kCT ≈ 6 ×
107 s-1) becomes activationless well below the glass transition
temperature. It is important to note that the dynamics of the
CT state vary only over a rather restricted range and that its
lifetime remains in the nanosecond domain even at the lowest
temperature attained (τCT ) 18.6 ns at 130 K) in sharp contrast
to the strong temperature dependence and lifetime of several
hours at low temperature reported by Fukuzumi et al., admittedly
in a different solvent.16 We find no obvious solvent dependence
for this process and no effect of the counterion. In fact, the
very weak temperature dependence of kCT observed here is quite
typical of electron-transfer processes involving a large energy
gap (i.e., inverted region conditions) where nuclear tunneling6
and other quantum chemical effects5 allow the process to avoid
the high activation barrier expected on the basis of a fully
classical description.3
The transient absorption studies indicate that the CT state
decays with a lifetime of 6 ( 1 ns (Figure 3b), in excellent
agreement with the time-resolved fluorescence spectral data, to
leave a residual transient signal (Figure 3c). Investigation of
the long-lived species by excitation of Mes-Acr+ in deoxygen-
ated acetonitrile with a 4-ns laser pulse and observation into
the microsecond domain gives rise to a transient spectrum that
shows bleaching around 350-450 nm and absorption in the
range of 450-600 nm (Figure 8). This spectrum is similar, but
not identical, to that observed at the end of the picosecond laser
flash photolysis records (Figure 9a). The transient decays via
first-order kinetics over a wide variation in laser power, but
there are indications for a second-order component at very high
intensities. The globally averaged lifetime is 30 ( 5 µs under
Figure 7. Arrhenius-type plots constructed for the different processes
observed for Mes-Acr+. The data points refer to LE (]) and CT (O)
fluorescence in fluid and glassy propylene carbonate while the solid lines
are least-squares fits as described in the text. The remaining data set refers
to the triplet excited state (4) in butyronitrile, as measured by transient
absorption spectroscopy. The gap in the time-resolved fluorescence data
sets corresponds to the phase transition of the solvent, and the derived rate
constants do not follow a continuous curve over this region.
activation energy (EA) of 6.5 kJ mol-1 and a preexponential
factor (ν ) 2.8 × 1012 s-1) that approaches the vibrational limit.
In a glassy matrix, kLE remains activated and fits better to the
modified Arrhenius expression given as eq 1
∆E1
k ) k + k exp -
(1)
(
)
0
1
RT
(Figure 7). Here, the activationless rate constant (k0) has a value
of 3.6 × 109 s-1, while the activated parameters k1 and ∆E1
have values of 4 × 1011 s-1 and 12 kJ mol-1, respectively. It is
important to note that even at the lowest temperature attained
(T ) 130 K) conversion of LE to CT dominates over radiative
decay from the LE state. This is evidenced by the fact that the
prompt fluorescence lifetime (τLE ) 0.27 ns) is very much
reduced relative to that of Acr+ (τS ) 31 ns), while the total
fluorescence spectrum contains important contributions from
both CT and LE states. The energy gap between these two states
is reduced somewhat at low temperature.
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16060 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

9-Mesityl-10-methylacridinium Cation
A R T I C L E S
that phosphorescence from Mes-Acr+ has been observed in an
ethanol glass at 77 K.17,43
The transient absorption signal now attributed to the locally
excited triplet state of Mes-Acr+ was quenched by molecular
oxygen in acetonitrile with a bimolecular rate constant of 2.0
( 0.3 × 109 M-1 s-1. This value is close to the one-ninth
diffusion controlled limit usually observed for O2 quenching
of a local triplet state. At all concentrations of dissolved O2,
the signal decayed to the prepulse baseline without leaving a
longer-lived species assignable to the mesityl π-radical cation.36
Instead, quenching resulted in the formation of singlet molecular
oxygen with a quantum yield of 0.42 ( 0.05. The so-formed
singlet molecular oxygen decayed via first-order kinetics with
a lifetime of 60 ( 5 µs. The perchlorate salt behaved
identically.44
Fukuzumi et al.16 have reported that the long-lived transient,
identified here as being the local triplet state, decays via a
strongly activated process. Indeed, their activation energy as
recalculated from the quoted reorganization energy and driving
force is 7.0 kJ mol-1 in benzonitrile solution. According to the
energy-gap law,45 nonradiative decay of a local triplet state is
not expected to show a significant temperature dependence, but
it is not unusual for CT states to decay by way of activated
routes. It was observed that the lifetime of triplet Mes-Acr+ in
butyronitrile increased with decreasing temperature over the
range 300 < T < 140 K but became essentially temperature
independent at lower temperatures (Figure 7). At 77 K, both
transient absorption spectroscopy and time-resolved phospho-
rescence indicated that the triplet lifetime was 5 ( 2 ms. Overall,
Figure 9. (a) Comparison of differential transient absorption spectra
recorded for the CT (solid line) and triplet (b) states of Mes-Acr+ in
deoxygenated acetonitrile at 20 °C. (b) Comparison of the differential
absorption spectra recorded for the triplet state (red), the acridinyl radical
(blue), and the mesityl π-radical cation (black) in deoxygenated acetonitrile.
these conditions. This species is formed with a quantum yield
of 0.38 ( 0.06. The quantum yield is increased a factor of 2.3-
fold on addition of iodomethane (10% v/v), where the lifetime
decreases to 700 ( 50 ns. Given that this species evolves from
the singlet CT state, and taking account of the effect of
iodomethane, it can be identified as being either a triplet CT
state or a locally excited triplet state. It should be noted that
the transient signal decays cleanly to the prepulse baseline and
that the spectral changes involve several isosbestic points. These
findings are taken to suggest that only one species is involved.
The transient differential absorption spectrum observed on
the microsecond time scale is similar to that reported by
Fukuzumi et al.,16 although their assignment as the singlet CT
state cannot be correct since this latter species has a lifetime of
only 6 ns. In any case, the spectrum differs from that observed
on subnanosecond time scales and that is clearly assignable to
the singlet CT state. The longer-lived spectral profile is
remarkably similar to that assigned to the LE triplet state of
certain aryl-substituted 10-methylacridinium ions,22,35 and in
particular there is close agreement with the spectrum of the
triplet state observed for 9-phenyl-10-methylacridinium (see the
Supporting Information). While it bares resemblance to the
differential spectrum observed for the Mes-Acr+ acridinyl
radical produced by spectroelectrochemistry,17 there are impor-
tant discrepancies (Figure 9b). The most obvious difference is
that the sharp absorption peak seen at 370 nm for the radical is
absent from the spectrum recorded for the 30-µs transient, while
only the latter transient shows a strong absorption peak around
330 nm. Furthermore, the difference between these two spectra
does not match the known absorption spectrum of the mesityl
π-radical cation, which absorbs weakly around 475 nm (Figure
9b).34 The 30-µs transient absorption persists into the far-red
region (see Figure 2b), showing weak absorbance as far as 750
nm, where the acridinyl radical is transparent. Consequently,
the transient species is most likely the LE triplet state arising
from intersystem crossing. In this respect, it is important to note
the kinetic data give a reasonable fit to eq 1 with k0 ) 200 s-1
,
k1 ) 3.2 × 107 s-1, and ∆E1 ) 13.8 kJ mol-1. This analysis
shows that the triplet state decays by both activated and
activationless processes.46 The latter step is most likely associ-
ated with nonradiative decay through vibrational modes and will
be controlled by the energy-gap law. The activated route is more
difficult to rationalize but does not refer to thermal repopulation
of singlet or triplet CT states since the derived energy barrier
(∆E1 ) 13.8 kJ mol-1) is significantly less than the spectro-
scopic energy gap (∆E ) 60 kJ mol-1) between these states. It
is more likely that the activated process is associated with a
conformational change accessible to the triplet state. As such,
it is important to note that the 9-phenyl-10-methylacridinium
ion shows similar behavior although intramolecular charge
transfer does not take place in this molecule.47
Energy Levels. An important aspect of this investigation
relates to the accurate positioning of the various excited-state
(43) The phosphorescence signal, which decays with a lifetime of 5.3 ms, is
enhanced a factor of 2.5-fold upon addition of iodomethane (10% v/v).
Under these latter conditions, the lifetime decreases to 3.3 ms. The
phosphorescence spectrum is comparable to that observed for Acr+ under
identical conditions, except for a small red shift.
(44) Illumination of Mes-Acr+ in O2-saturated acetonitrile has been reported
previously but only in the presence of 9,10-dimethylanthracene (ref 20),
where electron transfer might compete with singlet oxygen formation. In
view of the high quantum yield for singlet oxygen production found here,
together with the reinterpretation of the long-lived intermediate species,
the mechanism given for the photooxygenation of anthracene derivatives
needs to be reconsidered.
(45) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145.
(46) The activated process observed at higher temperatures is not too dissimilar
from that reported by Fukuzumi et al. (ref 16), allowing for the different
solvents, although the derived parameters are different. The real disparity,
however, is that whereas our results indicate the onset of an activationless
process at low temperature, Fukuzumi et al. (ref 16) reported Arrhenius-
type behavior even in frozen media. We have shown previously (ref 17)
that the low-temperature EPR results are better explained in terms of a
sacrificial photoreaction involving amine impurities in the solvent.
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A R T I C L E S
Benniston et al.
energy levels. The energy of the LE singlet state can be
identified as the intersection between fluorescence and excitation
spectra recorded in a glassy matrix at 77 K. This provides a
precise energy for the LE state of 2.70 ( 0.02 eV. An estimate
of the same energy gap in acetonitrile at room temperature, as
obtained from an overlap of normalized absorption and fluo-
rescence spectra after deconvolution into Gaussian components,
is 2.67 ( 0.03 eV. Fukuzumi et al.16 calculated the energy of
the CT state from electrochemical potentials measured in
acetonitrile, but this approach is likely to underestimate the
energy gap because of the irreversible nature of the mesityl
oxidation peak.48 A more reliable procedure involves analysis
of the reduced charge-transfer fluorescence spectrum recorded
in acetonitrile at room temperature.26 For this purpose, the
reduced fluorescence spectrum was deconvoluted into the
minimum number of Gaussian components and interpreted as
a mixture of LE and CT fluorescence bands (Figure 4). The
latter bands are easily identified by their relative broadness. Four
peaks are needed to fully fit the entire CT fluorescence profile
recorded under these conditions. The averaged separation
between these peaks of 1450 cm-1 can be assigned to a
vibrational bending mode that is coupled to deactivation of the
excited state. The highest energy CT fluorescence peak (E00) is
centered at 18 400 cm-1. The full width at half-maximum of
each Gaussian component increases with increasing temperature
and corresponds to a reorganization energy (λCT) of 0.29 ( 0.02
eV. The latter term is somewhat solvent dependent, decreasing
to 0.23 eV in butyronitrile and decreasing further to 0.14 eV in
octanitrile. Moreover, the sum of E00 and λCT corresponds to
the energy of the CT state in acetonitrile solution;49 the derived
value is 2.57 ( 0.04 eV. As expected, this latter value is
significantly higher than that estimated by electrochemical
methods (ECT ) 2.37 eV).16 It is also important to note that the
reorganization energy determined from spectral curve fitting is
markedly lower than that reported (λCT ) 0.79 eV) by Fukuzumi
et al.16 on the basis of an Arrhenius-type plot. A crude estimate
of the reorganization energy (λLE) associated with population
of the LE state, taken as half the relevant Stokes shift, is 0.04
eV in acetonitrile.50 Comparable values were derived for Mes-
Acr+ in a range of solvents of varying polarity.
tion of Mes-Acr+ in acetonitrile in the presence of anthracene
(0.2-5 mM) with a 4-ns laser pulse at 430 nm leads to the
production of the anthracene radical cation and the correspond-
ing acridinyl radical. The bimolecular rate constant for this
process is ca. 2.0 × 109 M-1 s-1. Taking account that the CT
state has a lifetime of only 6 ns under these conditions,51 it
seems more likely that electron transfer occurs to the triplet
state of Mes-Acr+. Indeed, the reduction potential for the triplet
state, calculated from the reversible reduction potential (ERED
) -0.49 V vs SCE)17 and the triplet energy (ET ) 1.94 eV), is
+1.45 V vs SCE. As such, oxidation of anthracene (EOX
)
+1.09 V vs SCE)52 by the triplet state is thermodynamically
favorable (∆G0 ) -0.36 eV).53 Related studies has confirmed
that the triplet state of 9-(1-naphthyl)-10-methylacridinium also
oxidizes anthracene under these conditions with a bimolecular
rate constant of ca. 1.5 × 109 M-1 s-1 7
.
On the basis of the derived energy levels, the Mes-Acr+
triplet state is not expected to reduce electron acceptors even
as strong as methyl viologen (MV2+) in polar solution since
the thermodynamic driving force is highly unfavorable (∆G0
> +40 kJ mol-1). Surprisingly, it was observed that steady-
state illumination (λ > 350 nm) of Mes-Acr+ in deoxygenated
acetonitrile containing MV2+ (5 mM) resulted in the appearance
of the characteristic blue color associated with reduced viologen
(MV+•). Although formed in very low concentration, MV+• was
found to persist in solution for many minutes unless molecular
oxygen was present. In fact, MV+• accumulates quickly but once
a steady-state concentration is reached further illumination has
no effect. The blue coloration does not develop if a few drops
of concentrated acid are added prior to illumination.
Control studies showed that MV2+ did not quench fluores-
cence from the CT state (kQ < 4 × 108 M-1 s-1) or the triplet
lifetime (kQ < 5 × 105 M-1 s-1) in deoxygenated acetonitrile.54
Even so, laser excitation of Mes-Acr+ in deoxygenated aceto-
nitrile in the presence of MV2+ resulted in formation of MV+•
as evidenced from transient absorption spectroscopy. It was
observed that, whereas the rate of formation of MV+• increased
linearly with increasing MV2+ concentration, corresponding to
a bimolecular rate constant of 1.3 × 107 M-1 s-1, the yield of
MV+• reached a maximum value and did not increase with
further addition of MV2+. The optimized quantum yield for
formation of MV+•, calculated by comparison to the triplet state
at high MV2+ concentration, was only 0.05 ( 0.01. Extensive
purification of the solvent led to a progressive decrease in the
yield of reduced viologen. All indications point to the appear-
ance of MV+• as arising from a minor sacrificial side-reaction
perhaps associated with the high instability of the mesityl
π-radical cation. This species deprotonates, even in acidic
solution, and reacts rapidly with nucleophiles.34c
From low-temperature phosphorescence spectra, the triplet
energy (ET) of Mes-Acr+ is calculated to be 1.94 ( 0.03 eV.
A similar value was observed for Acr+ (ET ) 2.00 eV) and for
several related 9-aryl-10-methylacridinium ions (ET ≈ 1.9).22
It is likely, therefore, that each triplet state in these ions is
localized on the acridinium nucleus. This analysis places the
triplet state of Mes-Acr+ some 0.63 eV below the singlet CT
state; a similar situation was reported previously for the 9-(1-
naphthyl)-10-methylacridinium ion.7 Fukuzumi et al.16 have
reported that the CT state of Mes-Acr+ oxidizes anthracene in
deaerated acetonitrile solution. We have confirmed that irradia-
(51) At the highest concentration of anthracene used (5 mM) ca. 15% of the
CT singlet state would be quenched, but triplet quenching would be
quantitative.
(47) For 9-phenyl-10-methylacridinium hexafluorophosphate, which does not
show an intramolecular CT state, the local triplet state decays via first-
order kinetics with τT ) 10 µs in deoxygenated CH2Cl2 at room temperature
(ref 22). The phosphorescence lifetime recorded at 77 K in ethanol
containing 10% v/v iodomethane is 4 ms.
(52) Baird, A., Lund, H., Eds. Encyclopedia of the Electrochemistry of the
Elements. Organic Section; Marcel Dekker: New York, 1984; Vol. XI.
(53) Given the relative triplet energies of Mes-Acr+ (ET ) 1.94 eV) and
anthracene (ET ) 1.84 eV), it might be argued that triplet energy transfer
should compete with electron transfer from triplet Mes-Acr+. However,
the resultant triplet excited state of anthracene is a good reductant and will
reduce ground-state Mes-Acr+ (∆G0 ) -0.26 eV). The net effect, therefore,
is the production of the anthracene π-radical cation by both routes.
(54) It should be noted that the triplet lifetime was lowered in the presence of
reduced viologen that accumulates under exposure to repeated laser pulses.
With the use of a flow cell and low repetition rate that allows the solution
to be replenished for each laser shot it was shown conclusively that MV2+
does not quench the triplet state under these conditions.
(48) (a) The reduction potential for one-electron oxidation of mesitylene in
acetonitrile is reported as 1.88 V (ref 48b) vs SCE or 2.1 V (ref 48c) vs
SCE. In our hands, the oxidative peak is highly irreversible due to a
following chemical process. (b) Fukuzumi, S.; Ohkubo, K.; Suenobu, T.;
Kato, K.; Fujitsuka, M.; Ito, O. J. Am. Chem. Soc. 2001, 123, 8459. (c)
Gould, I. R.; Moser, J. E.; Armitage, B.; Farid, S. J. Am. Chem. Soc. 1989,
111, 1917.
(49) Kjaer, A. M.; Ulstrup, J. J. Am. Chem. Soc. 1988, 110, 3874.
(50) Englman, R.; Jortner, J. Mol. Phys. 1970, 18, 145.
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16062 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005

9-Mesityl-10-methylacridinium Cation
A R T I C L E S
analysis shows that the energy gap between LE and CT states
is only ca. 0.1 eV. This close positioning of the energy levels
accounts for the delayed fluorescence observed for the LE state
in fluid solution, and it should be noted that the energy gaps
obtained by spectral curve fitting and Boltzmann analyses of
the continuous spectra are identical within experimental error.
Solvent polarity has only a small effect on this energy gap,
presumably because the process under investigation is charge
shift, not charge separation/recombination.
Decay of the LE singlet state to the CT state is weakly
activated in solution but approaches an activationless rate (kLE
) 3.6 × 109 s-1) in a glassy matrix. This latter process might
refer to nuclear tunneling, which is known to be favored at low
temperature.6 The apparent activation energy (EA ) 6.5 kJ
mol-1) found in fluid propylene carbonate is close to that
estimated (EA ≈ 5.4 kJ mol-1) from classical Marcus theory,3
while the limiting rate constant (kLE ≈ 3 × 1012 s-1) approaches
the vibrational limit. In the event that charge transfer occurs
within the framework of Marcus theory,3 the electronic coupling
matrix element can be calculated as being ca. 75 cm-1 at room
temperature. This derived value might be considered rather high
for charge transfer across an orthogonal geometry, but as noted
above, the transient absorption spectrum of the CT state indicates
that considerable electronic interaction between the two moieties
occurs.
Figure 10. Energy level diagram proposed for Mes-Acr+. All the excited
states indicated have been identified spectroscopically except for the triplet
CT state (*CTT). State energies are only weakly medium dependent; values
indicated refer to acetonitrile solution for singlets and to ethanol at 77 K
for the LE triplet. Solid arrows represent the major nonradiative decay
pathways following excitation of the acridinium chromophore. In addition,
dashed arrows represent the three (minor) radiative decay routes. The
following terms apply to individual rate constants: kLE, charge transfer from
the LE singlet to form the CT state; k-LE, reverse charge transfer to reform
the LE singlet state; kT, nonradiative decay of the LE triplet to reform the
ground state.
We note that radiative decay from the CT state is inefficient,
despite the relatively long lifetime of this state, and there is
clear evidence for intersystem crossing to the triplet manifold.
Earlier work22 has considered two mechanisms for triplet
formation in 9-aryl-10-methylacridinium ions. Thus, in a polar
solvent at ambient temperature, a likely route to the locally
excited triplet involves intersystem crossing within the CT state
to form the corresponding triplet CT state. Reverse charge
transfer within this latter species would result in population of
the acridinium-like LE triplet state, provided this species lies
at lower energy. The effect of iodomethane is to promote
intersystem crossing between the CT states. Alternatively the
LE triplet could arise directly from the singlet CT state via
coupled electron transfer and spin inversion, a process enhanced
by the spin-orbit coupling resulting from the nearly perpen-
dicular orientation of the two units.55 Spin polarization studies
of systems closely related to Mes-Acr+ have indicated that the
latter mechanism dominates,22 but later studies on other dyads
involving (nearly) orthogonal donor and acceptor moieties
showed the operation of both mechanisms.56 At low tempera-
tures, where formation of the CT state is incomplete, intersystem
crossing from singlet LE to triplet LE could contribute, but
because the triplet yield in Acr+ is small this path is considered
to be relatively unimportant. It is interesting to note that
iodomethane enhances triplet formation by a factor of ca. 2.4-
fold at both room temperature and 77 K. At room temperature,
the LE triplet state is formed with a quantum yield of only 0.38.
This means that direct internal conversion to the ground state
must occur from the singlet CT state, and in view of the low
CT fluorescence quantum yield this must be a nonradiative
process.
In propylene carbonate, decay of the CT state is activationless
in the glass (kCT ) 6 × 107 s-1) and weakly activated (EA
)
2.5 kJ mol-1; kCT ) 6 × 108 s-1) in fluid solution.57 These
derived kCT values are considerably smaller than the vibrational
limit (k ≈ 1013 s-1). It should be stressed that these activation
parameters relate to a process that bifurcates from the CT state
into a spin-forbidden process to the local triplet state (ca. 38%)
and a spin-allowed process to the ground state (g60%) (Figure
10). As such, the derived activation energy cannot be associated
with a particular decay route.
Concluding Remarks
Rapid charge transfer occurs within the LE singlet state of
Mes-Acr+ in fluid solution to generate the corresponding CT
state. This latter species fluoresces, thereby allowing precise
determination of its lifetime and energy, but decays mainly by
way of nonradiative processes. The fluorescence differs mark-
edly in profile and lifetime from that characterized for the LE
singlet state... and this difference can be unambiguously assigned
to a contribution of CT fluorescence.58 Similar fluorescence
properties have been observed for related 9-aryl-substituted
acridinium ions in solution, but the energy levels of Mes-Acr+
are such that the LE and CT states reside in thermal equilibrium
at ambient temperature. The singlet CT state is sufficiently long-
lived for this latter equilibrium to be established on the relevant
time scale and, because of the small energy gap, the rate of
repopulation of the LE state is relatively fast. It seems
remarkable that these two charge-transfer processes occur on
the subnanosecond time scale despite the fact that the mesityl
On the basis of the above discussion, an energy level diagram
can now be constructed for Mes-Acr+ (Figure 10). Our spectral
(57) (a) The apparent temperature dependence in fluid solution can be traced to
small changes in the energy gap, connected with frequency changes in
solvent librations. This effect disappears in a glassy matrix. (b) Claude, J.
P.; Meyer, T. J. J. Phys. Chem. 1995, 99, 51.
(58) Fukuzumi et al. (ref 16) report that the absorption and fluorescence spectra
of Mes-Acr+ are superpositions of the spectra of each component, that is
mesitylene and Acr+. There is no mention of CT fluorescence, although
this is well documented for related compounds (refs 22, 23, 34).
(55) Okada, T.; Karaki, I.; Matsuzawa, E.; Mataga, N.; Sakata, Y.; Misumi, S.
J. Phys. Chem. 1981, 85, 3957.
(56) Wiederrecht, G. P.; Svec, W. A.; Wasielewski, M. R.; Galili, T.; Levanon,
H. J. Am. Chem. Soc. 2000, 122, 9715.
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A R T I C L E S
Benniston et al.
and acridinium subunits are held in an orthogonal geometry.
Electronic coupling under such conditions might be expected
to be at a minimum,59 although the reactants are in close
proximity and charge transfer can proceed along the connecting
σ-bond. In fact, our analysis suggests that the coupling element
Under the conditions where Fukuzumi et al.16 report that the
singlet CT state survives for some hours, namely frozen
benzonitrile at 203 K, it is now apparent that the LE triplet will
be the longest-lived species. This triplet has a lifetime of 5.3
ms at 77 K, in the absence of a sacrificial electron donor.17
Formation of the local acridinium-based triplet with a micro-
second lifetime was also confirmed by transient absorption
spectroscopic studies (see the Supporting Information). Persistent
discolorations reported to result from continuous irradiation at
low temperature must be the result of photodecomposition.17,61
is ca. 75 cm-1
.
Once formed, the singlet state is deactivated primarily by way
of nonradiative processes. Our quantitative studies indicate that
the CT state contributes 97.5% to the equilibrium mixture of
excited singlet states but that the LE triplet is formed with a
yield of only 38%. Indeed, formation of the longest-lived
transient species, regardless of its assignment, must be less than
43% to account for the observed effect of iodomethane.60 For
9-(1-naphthyl)-10-methylacridinium, addition of iodomethane
causes a 7-fold increase in the yield of the LE triplet state.7 It
is most likely that the singlet CT state partitions between charge
transfer to reestablish the ground state (60%) and intersystem
crossing (38%) (via a triplet CT state) to the lowest LE triplet,55
which resides on the acridinium unit. The observation of long-
lived phosphorescence from Mes-Acr+, Acr+, and 9-(1-naph-
thyl)-10-methylacridinium at 77 K provides clear evidence that
the LE triplet lies at lower energy than the singlet CT state.
Acknowledgment. We thank the EPSRC (EP/C007721/1),
the University of Amsterdam, and the University of Newcastle
for financial support. All EPR studies were made at the EPSRC-
supported EPR Centre at the University of Manchester. Singlet
molecular oxygen measurements were made at the Free Radical
Research Facility at the Daresbury Laboratory.
Supporting Information Available: Analytical data for the
perchlorate salt of Mes-Acr+, including ES-MS, 1H NMR,
C/H/N analysis and X-ray structure, comparison of triplet
absorption spectra for Mes-Acr+ and 9-phenyl-10-methylacri-
dinium, and the difference between transient differential absorp-
tion spectra recorded for the triplet and acridinyl radical,
transient differential absorption spectrum recorded at 77 K. This
material is available free of charge via the Internet at
http//pubs.acs.org.
(59) Lappe, J.; Cave, R. J.; Newton, M. D.; Rostov, I. V. J. Phys. Chem. B
2005, 109, 6610.
(60) Fukuzumi et al. (ref 16) report this quantum yield as being 0.98, on the
basis of a comparative method apparently using anthracene as quencher,
but this value is inconsistent with the noted effect of iodomethane.
(61) Ohkubo, K.; Kotani, H.; Fukuzumi, S. Chem. Commun. 2005, 4520.
JA052967E
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16064 J. AM. CHEM. SOC. VOL. 127, NO. 46, 2005