Designing multifunctional expanded pyridiniums: Properties of branched and fused head-to-tail bipyridiniums

Article abstract of DOI:10.1021/ja108668h

The multifaceted potentialities of expanded pyridiniums (EPs), based on one pyridinium core bearing a 4-pyridyl or 4-pyridylium as the N-pyridinio group, are established at both experimental and theoretical levels. Two classes of head-to-tail (htt) EPs were designed, and their first representative elements were synthesized and fully characterized. The branched (B) family is made up of 2,6-diphenyl-4-aryl-1,4′-bipyridin-1-ium (or 1,1′-diium) species, denoted 1B and 2B for monocationic EPs (with aryl = phenyl and biphenyl, respectively) and 1B_Me and 2B_Me for related quaternarized dicationic species. The series of fused (F) analogues comprises 9-aryl-benzo[c]benzo[1,2]quinolizino[3,4,5,6-ija][1,6]naphthyridin-15-ium species, denoted 1F and 2F, and their 2,15-diium derivatives referred to as 1F_Me and 2F_Me. Electrochemistry (in MeCN vs SCE) reveals that branched EPs undergo a single reversible bielectronic reduction at ca. -0.92 V for 1B/2B and -0.59 V for 1B_Me/2B_Me, whereas pericondensed species show two reversible monoelectronic reductions at ca. -0.83 and -1.59 V for 1F/2F and ca. -0.42 and -1.07 V for 1F_Me/2F _Me. Regarding electronic absorption features, all htt-EP chromophores show absorptivity in the range of ca. 1-4 × 10⁴ M^-1 cm^-1, with red-edge absorptions extending toward 450 and 500 nm (in MeCN) for 2B_Me and 2F_Me, respectively. These lowest-energy pi-pi* transitions are ascribed to intramolecular charge transfer between the electron-releasing biphenyl group and the htt-bipyridinium electron-withdrawing subsystems. EPs display room-temperature photoemission quantum yields ranging from 10% to 50%, with the exception of 1B, and branched luminophores are characterized by larger Stokes shifts (8000-10 000 cm ^-1) than fused ones. Lastly, a method to predict the efficiency of photobiscyclization of branched EPs into fused ones, based on the analysis of computed difference maps in total electron density for singlet excited states, is proposed.

Full text of DOI:10.1021/ja108668h

Published on Web 10/28/2010
Designing Multifunctional Expanded Pyridiniums: Properties
of Branched and Fused Head-to-Tail Bipyridiniums
Je´roˆme Fortage,†,‡ Cyril Peltier,§ Francesco Nastasi,| Fausto Puntoriero,|
Fabien Tuye`ras,† Sophie Griveau,⊥ Fethi Bedioui,⊥ Carlo Adamo,§ Ilaria Ciofini,*,§
Sebastiano Campagna,*,| and Philippe P. Laine´*,†
Laboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques (CNRS UMR-8601),
UniVersite´ Paris Descartes, 45 rue des Saints Pe`res, F-75270 Paris Cedex 06, France, Institut
Parisien de Chimie Mole´culaire (CNRS UMR-7201), Equipe Chimie Supramole´culaire, Case 42,
UniVersite´ Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France, LECIME,
´
´
Laboratoire d′Electrochimie, Chimie des Interfaces et Mode´lisation pour l′Energie (CNRS
´
UMR-7575), Ecole Nationale Supe´rieure de Chimie de Paris-Chimie ParisTech, 11 rue Pierre et
Marie Curie, F-75231 Paris Cedex 05, France, Dipartimento di Chimica Inorganica, Chimica
Analitica e Chimica Fisica and Centro InteruniVersitario per la ConVersione Chimica
dell’Energia Solare, UniVersita` di Messina, Via Sperone 31, I-98166 Messina, Italy, and
Laboratoire de Pharmacologie Chimique et Ge´ne´tique (CNRS UMR-8151 and INSERM U-1022),
´
UniVersite´ Paris Descartes, Ecole Nationale Supe´rieure de Chimie de Paris-Chimie ParisTech,
11 rue Pierre et Marie Curie, F-75231 Paris Cedex 05, France
Received September 25, 2010; E-mail: philippe.laine@parisdescartes.fr; campagna@unime.it;
ilaria-ciofini@chimie-paristech.fr
Abstract: The multifaceted potentialities of expanded pyridiniums (EPs), based on one pyridinium core
bearing a 4-pyridyl or 4-pyridylium as the N-pyridinio group, are established at both experimental and
theoretical levels. Two classes of head-to-tail (htt) EPs were designed, and their first representative elements
were synthesized and fully characterized. The branched (B) family is made up of 2,6-diphenyl-4-aryl-1,4′-
bipyridin-1-ium (or 1,1′-diium) species, denoted 1B and 2B for monocationic EPs (with aryl ) phenyl and
biphenyl, respectively) and 1BMe and 2BMe for related quaternarized dicationic species. The series of fused
(F) analogues comprises 9-aryl-benzo[c]benzo[1,2]quinolizino[3,4,5,6-ija][1,6]naphthyridin-15-ium species,
denoted 1F and 2F, and their 2,15-diium derivatives referred to as 1FMe and 2FMe. Electrochemistry (in
MeCN vs SCE) reveals that branched EPs undergo a single reversible bielectronic reduction at ca. -0.92
V for 1B/2B and -0.59 V for 1BMe/2BMe, whereas pericondensed species show two reversible monoelectronic
reductions at ca. -0.83 and -1.59 V for 1F/2F and ca. -0.42 and -1.07 V for 1FMe/2FMe. Regarding
electronic absorption features, all htt-EP chromophores show absorptivity in the range of ca. 1-4 × 104
M-1 cm-1, with red-edge absorptions extending toward 450 and 500 nm (in MeCN) for 2BMe and 2FMe
,
respectively. These lowest-energy pi-pi* transitions are ascribed to intramolecular charge transfer between
the electron-releasing biphenyl group and the htt-bipyridinium electron-withdrawing subsystems. EPs display
room-temperature photoemission quantum yields ranging from 10% to 50%, with the exception of 1B, and
branched luminophores are characterized by larger Stokes shifts (8000-10 000 cm-1) than fused ones.
Lastly, a method to predict the efficiency of photobiscyclization of branched EPs into fused ones, based on
the analysis of computed difference maps in total electron density for singlet excited states, is proposed.
1. Introduction
ible reduction processes also make them choice components in
supramolecular assemblies designed to behave as electron-
storage systems (e.g., polyviologen dendrimers)9,10 or molecular
machines11,12 whose organization and machinery are based on
charge-transfer interactions and redox inputs, respectively.3,13
Pyridinium and bipyridinium1 species are among the most
extensively used functional entities2,3 because of their appealing
electron-withdrawing (pull effect)4,5 and electron-accepting
(redox)6,7 properties. By virtue of their reduction properties they
are suited to artificial systems developed for solar energy
conversion, both in bimolecular schemes and in supramolecular
(multicomponent) arrangements in which they play the role of
electron acceptors (i.e., electrophores).8 Their mild and revers-
(2) When performing a bibliometric analysis through the ISI Web of
Knowledge, a striking point is the close correlation observed in the
evolution over the years of the number of publications (and related
citations) dealing with the following topics: “viologen” or “bipyri-
dinium” and “ruthenium bipyridine”. Two remarkable inflection points
are found around 1980 and 1990. They are ascribable to the launching
of two related topics: “water splitting” and “artificial photosynthesis”,
on the one hand, and “molecular devices” including “molecular
machines”, on the other; see also ref 3.
† LCBPT-Universite´ Paris Descartes.
‡ IPCM-Universite´ Pierre et Marie Curie.
§ LECIME-ENSCP.
| Universita` di Messina.
⊥ LPCG-ENSCP-Universite´ Paris Descartes.
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16700 J. AM. CHEM. SOC. 2010, 132, 16700–16713
10.1021/ja108668h  2010 American Chemical Society

Designing Multifunctional Expanded Pyridiniums
A R T I C L E S
Chart 1. General Representation of Tetra- (TP)23,24 and
Pyridinium species, already known for their redox properties
as electrophores, have also attracted attention for their electro-
optical and photophysical properties.14-19 Indeed, there is great
interest in designing new types of pyridiniums that behave not
Hexabranched (XP) EPs of the First Generation along with
Corresponding Fused (fTP) and Hemi-Fused (hfXP) EPs16
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F. V.; Prasad, P. N. J. Phys. Chem. 1996, 100, 4521–4525.
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Yakymyshyn, C. P. Chem. Mater. 1994, 6, 1137–1147. (b) Marder,
S. R.; Perry, J. W.; Schaefer, W. P. Science 1989, 245, 626–628. For
inorganic NLO-phores, see, for instance: (c) Coe, B. J. Acc. Chem.
Res. 2006, 39, 383–393. (d) Konstantaki, M.; Koudoumas, E.; Couris,
S.; Laine´, P.; Amouyal, E.; Leach, S. J. Phys. Chem. B 2001, 105,
10797–10804. (e) Marder, S. R.; Perry, J. W.; Tiemann, B. G.;
Schaefer, W. P. Organometallics 1991, 10, 1896–1901.
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(7) Bird, C. L.; Kuhn, A. T. Chem. Soc. ReV. 1981, 10, 49–82.
(8) There are literally hundreds of examples of papers where pyridinium
and/or bipyridinium species act as electron acceptors in photoinduced
electron transfer processes. For a few selected early and/or seminal
and/or influential contributions involving bipyridiniums, see the
following references. Topic of prototypical photochemistry of the
ruthenium(II) tris(2,2′-bipyridine) complex with the 1,1′-dimethyl-4,4′-
bipyridinium electron-accepting quencher: (a) Bock, C. R.; Meyer,
T. J.; Whitten, D. G. J. Am. Chem. Soc. 1974, 96, 4710–4712. (b)
Whitten, D. G. Acc. Chem. Res. 1980, 13, 83–90. (c) Juris, A.; Balzani,
V.; Barigelletti, F.; Campagna, S.; Belser, P.; Von Zelewsky, A. Coord.
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conversion: (d) Balzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F.;
Gleria, M. Science 1975, 189, 852–856. (e) Young, R. C.; Meyer,
T. J.; Whitten, D. G. J. Am. Chem. Soc. 1975, 97, 4781–4782. (f)
Gra¨tzel, M. Acc. Chem. Res. 1981, 14, 376–384. (g) Kalyanasundaram,
K. Coord. Chem. ReV. 1982, 46, 159–244. Topic of multimolecular
systems for hydrogen evolution: (h) Lehn, J.-M.; Sauvage, J.-P. NouV.
J. Chim. 1977, 1, 449–451. (i) Moradpour, A.; Amouyal, E.; Keller,
P.; Kagan, H. NouV. J. Chim. 1978, 2, 547–549. (j) Kalyanasundaram,
K.; Kiwi, J.; Gra¨tzel, M. HelV. Chim. Acta 1978, 61, 2720–2730. (k)
Harriman, A.; Porter, G. J. Chem. Soc., Faraday Trans. 2 1982, 78,
1937–1943. Topic of photochemical devices (PMDs) in the field of
supramolecular photochemistry: (l) Balzani, V. Tetrahedron 1992, 48,
10443–10514. (m) Balzani, V.; Scandola, F. Supramolecular Photo-
chemistry; Ellis Horwood: Chichester, U.K., 1991; Chapter 12. (n)
Balzani, V.; Moggi, L.; Scandola, F. In Supramolecular Photochem-
istry; Balzani, V., Ed.; D. Reidel Publishing Co.: Dordrecht, The
Netherlands, 1987; pp 1-28. Topic of integrated supramolecular
functional assemblies devoted to artificial photosynthesis: (o) Meyer,
T. J. Acc. Chem. Res. 1989, 22, 163–170. (p) Sauvage, J.-P.; Collin,
J.-P.; Chambron, J.-C.; Guillerez, S.; Coudret, C.; Balzani, V.;
Barigelleti, F.; De Cola, L.; Flamigni, L. Chem. ReV. 1994, 94, 993–
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only as good electrophores but also as luminophores and
chromophores, that is, as truly multifunctional and polyvalent
entities. Efforts in this direction may go through the expansion
of the molecular scaffold of pyridiniums20 and, in particular,
of their π system.21 Similarly to macromolecular unsaturated
hydrocarbon architectures in which connecting patterns22 are
based on (i) single-bond linkages and/or (ii) edge-fused linkages,
skeletal expansion of pyridiniums can take the form of
aryl-substituted23,24 and pericondensed16 species, hereafter
referred to as branched (B) and fused (F) expanded pyridiniums
(EPs), respectively (Chart 1).
The approach adopted here to design integrated multifunc-
tional EPs consists in close redox tuning,6,7 combined with a
strategy relying on the extension of the π-conjugated system
of pyridiniums for absorption and emission properties.
Indeed, our previous investigations on EPs of the first
generation (Chart 1)16,23,24 established that the electronic,
photophysical, and electrochemical properties of pyridiniums
are all significantly affected by scaffold expansion (whether by
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Ciofini, I. J. Phys. Chem. A 2010, 114, 8434–8443.
(18) Balzani, V.; Credi, A.; Langford, S. J.; Prodi, A.; Stoddart, J. F.;
Venturi, M. Supramol. Chem. 2001, 13, 303–311.
(9) Ronconi, C. M.; Stoddart, J. F.; Balzani, V.; Baroncini, M.; Ceroni,
P.; Giansante, C.; Venturi, M. Chem.sEur. J. 2008, 14, 8365–8373.
(10) (a) Juris, A. Annu. Rep. Prog. Chem., Sect. C 2003, 99, 177–241. (b)
Toba, R.; Quintela, J. M.; Peinador, C.; Roman, E.; Kaifer, A. E. Chem.
Commun. 2002, 1768–1769. (c) Baker, W. S.; Lemon, B. I., III;
Crooks, R. M. J. Phys. Chem. B 2001, 105, 8885–8894. (d) Ceroni,
P.; Venturi, M. In Electrochemistry of Functional Supramolecular
Systems; Ceroni, P., Credi, A., Venturi, M., Eds.; Wiley: Hoboken,
NJ, 2010; pp 145-184.
(19) Ballardini, R.; Credi, A.; Gandolfi, M. T.; Giansante, C.; Marconi,
G.; Silvi, S.; Venturi, M. Inorg. Chim. Acta 2007, 360, 1072–1082.
(20) Arai, S.; Hida, M. AdV. Heterocycl. Chem. 1992, 55, 261–358.
(21) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004, 104,
4891–4945, and references therein.
(22) (a) Mu¨ller, S.; Mu¨llen, K. Phil. Trans. R. Soc. A 2007, 365, 1453–
1472. (b) Watson, M. D.; Fechtenko¨tter, A.; Mu¨llen, K. Chem. ReV.
2001, 101, 1267–1300.
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Amouyal, E. J. Am. Chem. Soc. 2002, 124, 1364–1377. (b) Laine´, P.;
Bedioui, F.; Amouyal, E.; Albin, V.; Berruyer-Penaud, F. Chem.sEur.
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N.; Stoddart., J. F. J. Am. Chem. Soc. 1991, 113, 5131–5133. (b) Anelli,
P. L.; et al. J. Am. Chem. Soc. 1992, 114, 193–218. (c) Ballardini, R.;
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A R T I C L E S
Fortage et al.
ring branching16,23,24 or by ring fusion16). Yet, these properties
are impacted differently when passing from branched to
corresponding fused molecular backbones (Chart 1).16 On the
one hand, the two types of EPs exhibit distinct electronic and
photophysical characteristics, i.e., they are basically different
chromophores and luminophores. On the other hand, their redox
properties barely change when aryl branches are pericondensed,
especially for the potential of the first reduction process (Ered).
This finding was ascribed to the fact that beyond substituent
effects attached to aryl appending (for branched EPs) and ring
fusion (e.g., benzoannulation in fused EPs), the features of the
LUMO are essentially the same in both types of EPs. In addition,
they are closely akin to those of the bare N-methylpyridinum.17
In other words, in the case of monopyridinium derivatives, the
LUMO remains essentially localized on the pyridinium motif
regardless of the type of skeletal expansion, including the
extension of the π-conjugated system by pericondensation of
branched EPs.16,17
Chart 2. Most Efficient Inter-Pyridinium Connections Discussed in
the Text along with the Adopted Numbering Scheme for the
Pyridinium Entity: C-4/C-4 (b), C-4/N-1 (c), and N-1/N-1 (d)a
a Red circles are pictorial representations of atomic contributions to the
LUMO (absolute values) of the N-methylpyridinium.
viologens,27 namely, 1,1′-dialkyl-4,4′-bipyridiniums (“paraquats”;
Chart 2b),28 1,1′-dialkyl-2,2′-bipyridiniums (“diquats”), and
other dimeric combinations.29 From the analysis of the features
of the LUMO of reference N-methylpyridinium (see Figure S-55,
Supporting Information), one can derive the most relevant
connection scheme to optimize the mutual influences of two
directly linked pyridiniums. Constraints related to forthcoming
uses of the new EPs, that is, stability (chemical, thermal, and
photochemical), must also be taken into account as well as their
possible well-defined attaching to other functional subunits.
Sorted by decreasing order of magnitude, atomic contributions
to the LUMO are lying at positions C-4 > N-1 > C-2 and C-6
. C-3 and C-5 (Chart 2a and Figure S-55, Supporting
Information). This observation suggests that the most efficient
interpyridinium connections should involve the following pairs
of positions: C-4/C-4 (rank 1), C-4/N-1 (rank 2), and then N-1/
N-1 (rank 3), given that the larger the associated coefficients,
the greater the mutual influence. Chart 2 illustrates the interpy-
ridinium connections above mentioned.
Expanding the molecular scaffold of pyridiniums by extending
their π-conjugated system is an efficient means to modify their
electronic and photophysical features to obtain good pyridinium-
based chromophores and luminophores. Yet, it is an inoperative
approach to control their electrochemical properties (especially
in the reduction regime).16,17 Therefore, improving the electro-
phoric properties of pyridiniums, that is, obtaining the anodic
shift of their first reduction potential to make these EPs more
easily reducible, must somehow rely on pyridinium substitution
with electron-withdrawing groups. Unfortunately, in spite of the
great chemical versatility of EPs of the first generation (Rn
substituents in Chart 1)23 peripheral substitution failed in
rendering thus-modified monopyridinium species competitive
with other electron acceptors commonly exploited to efficiently
monitor photoinduced electron transfers (Ered g -0.6 V vs
SCE).24 With regard to branched EPs, this failure was observed
even in the favorable cases of aryl branches that are (i) para
substituted with strongly electron-withdrawing nitro groups and
(ii) appended at the most influential C-2, C-4, and C-6 positions
of the pyridinium core.25 Clearly, the tilt of intervening aryl
rings with respect to the plane of the pyridinium center (Chart
1) contributes to weakening the efficiency of the propagation
of mesomeric electronic effects. The direct substitution of
pyridinium centers6,7 therefore appears desirable as the only way
to strongly modify their electrochemical features and reach the
targeted range of reduction potentials.26
Figure 1. Key elements of the molecular design. Circles are pictorial
representations of atomic contributions to the LUMO (absolute values) of
the pyridinium building block.
2. Molecular Design
Pyridinium is actually in itself a choice influential group to
improvetheelectron-acceptingabilityofpyridiniumelectrophores,7,16
as demonstrated by the benchmark electrochemical behavior of
However, due to the very similar contributions of atomic
orbitals of the C-4 and N-1 atoms to the LUMO, the efficiencies
of the first two pairs are comparable. The C-4/N-1 configuration
is dissymmetric, which permits one to distinguish the functions
of the two pyridiniums (Figure 1). Here, the pyridinium
engaging the lower coefficient (i.e., N-1) can be formally
considered as the substituent of the other pyridinium at the C-4
position. The former is hereafter referred to as the core
(25) Typical nitro substitutions of tetrabranched pyridinium (TP in Chart
1) give the following values for the first reduction potential (vs SCE
in MeCN):-0.70 (with R1 ) H and R2 ) NO2) and-0.65 V (with
R1 ) NO2 and R1 ) H); see ref 24b.
(26) As an illustration of the effect of direct substitution, one can cite the
case of N-(2-pyrimidyl)pyridinium derivatives, which are reversibly
one-electron reduced at ca.-0.36 V vs SCE in MeCN, see: (a) Coe,
B. J.; Beyer, T.; Jeffery, J. C.; Coles, S. J.; Gelbrich, T.; Hursthouse,
M. B.; Light, M. E. J. Chem. Soc., Dalton Trans. 2000, 797–803. (b)
Coe, B. J.; Harris, J. A.; Jones, L. A.; Brunschwig, B. S.; Song, K.;
Clays, K.; Garin, J.; Orduna, J.; Coles, S. J.; Hursthouse, M. B. J. Am.
Chem. Soc. 2005, 127, 4845–4859. (c) Coe, B. J.; Harris, J. A.;
Brunschwig, B. S.; Asselberghs, I.; Clays, K.; Garin, J.; Orduna, J.
J. Am. Chem. Soc. 2005, 127, 13399–13410.
(27) Michaelis, L. Biochem. Z. 1932, 250, 564–567.
(28) The first reduction process related to pyridinium occurs at a potential
value that is typically shifted from ca.-1.30 to ca.-0.44 V (vs SCE
in MeCN) on going from the N-methylpyridinium monomer to the
N-methylviologen (MV) dimer (i.e., “paraquat”).
(29) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637–1755, and
references therein.
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Designing Multifunctional Expanded Pyridiniums
A R T I C L E S
Chart 3. Molecular Structures along with Related Labels of the Branched and Fused EPs Studied, Including Three Reference EPs (Rf (this
work), RfB,16 and RfF16) and Two Additional Pyridinium-Based References (i.e., methylviologen, MV, and 2,7-dimethyl-diazapyrenium,
DMDAP)a
a
-
-
Counteranions are BF4 for monocationic expanded pyridiniums (EP+) and PF6 for dicationic expanded htt-bipyridiniums (EP2+).
Table 1. Reduction Data of the Studied Compounds and Some
Reference Species (vs SCE) at Pt Electrode for Acetonitrile
Solutions (+0.1 M NBu4PF6) at Room Temperaturea
pendant and the core pyridiniums, aryl groups are appended at
either side of the core Npyridinio atom (thereby also protecting
the interpyridinium linkage)30 and para to it,31 respectively
(Figure 1). Branched EPs made up of two redox sites forming
the htt-bipyridinium subsystem are anticipated to efficiently
manage with two electrons, as is the case for viologens.
However, this will be done differently due to intramolecular
steric hindrance and/or the central position of the core Npyridinio
atom. The pyridinium-localized nature of the LUMO can be
turned to good account by embedding the htt-bipyridinium
redox-active subsystem within the planar molecular scaffold of
fused polycyclic EPs, which allows for locking the two
heteroaromatic fragments in a coplanar conformation, hence
optimizing their synergistic interaction. At the same time,
pericondensation is also likely to favorably impact the electronic
and photophysical properties. Lastly, it is worth noting that
substituents R2, primarily, and to a lesser extent R1 and R0, can
be replaced with functional subunits to build multicomponent
architectures.
Here we report the syntheses, redox behavior, electro-optical
features, and luminescence properties of eight expanded pyri-
diniums featuring a “head-to-tail” (htt) topology. This investiga-
tion was conducted both experimentally and theoretically. The
compounds studied are shown in Chart 3. In our nomenclature,
1 identifies species that have a single phenyl at the C-4 position
of the pyridinium core, whereas species of the 2 series bear a
C-4 biphenyl group; moreover, B stays for branched compounds,
while F indicates fused ones (see Chart 3); finally, the Me
subscript indicates N-methylated species. It can be noted that
dicationic compounds 1BMe, 2BMe, 1FMe, and 2FMe are derived
from 1B, 2B, 1F, and 2F, respectively,32 by the quaternarization
entry
E1/2, V (∆Ep, mV)
n
E1/2, V (∆Ep, mV)
n
Rf
-1.07 (90)
-1.00 (77)
-0.93 (115)
-0.91 (115)
-0.60 (102)
-0.58 (102)
-1.00 (117)
-0.84 (114)
-0.83c
-0.43 (88)
-0.42 (85)
-0.44 (84)
-0.44e
1
1
2
2
2
2
1
1
1
1
1
1
1
1
1
1
-1.53 (97)
-1.16 (96)
1
1
RfBb
1B
2B
1BMe
2BMe
RfFb
1F
-1.77 (102)
-1.61 (83)
-1.57c
-1.08 (198)
-1.07 (213)
-0.86 (86)
- 0.93 (irr.)e
- 0.99/-0.70
-0.82
1
1
1
1
1
1
1
1
1
2F
1FMe
2FMe
MVd
DMDAPe
f
NI/PI
-0.48/-0.43
-0.44
-0.47
g
C60
BQh
a Unless otherwise noted, E1/2 (vs SCE) is calculated as (Epa + Epc)/2,
where Epa and Epc are the anodic and cathodic peak potentials measured
by cyclic voltammetry at 0.1 V s-1, ∆Ep is the difference of potential
between Epc and Epa, n is the number of electrons involved in the redox
process determined by rotating disk electrode voltammetry by
comparison with reference components or with respect to an internal
monoelectronic oxidation of ferrocene (see text below about EP-based
dyads). For more details, see the Experimental Section. b From ref 16.
c From square-wave measurement. d MV, see Chart 3; data determined
in this work; see also ref 33. e DMDAP; see Chart 3; data from ref 34;
irr., irreversible process; these reduction potential values should be
compared with those of MV measured in the same conditions, i.e.,
-0.40 and -0.82 V, see ref 34. f NI is naphthalene-tetracarboxydiimide,
and PI is perylene-tetracarboxydiimide; measured in DMF; see ref 35.
g Measured in CH2Cl2 vs SCE, from ref 36. h BQ is benzoquinone; see
ref 37.
pyridinium, while the latter is identified as the pendant pyri-
dinium (Figure 1). This functional distinction is tightly bound
to the notion of gradient and is of utmost interest when dealing
with vectorial electron and/or energy transfers or transports.
Thus, the (C-4/N-1) “head-to-tail” (htt) manner to assemble
pyridiniums deserves to be assessed as a promising alternative
to the symmetric (C-4/C-4) “tail-to-tail” dimerization embodied
in popular viologens (Chart 2).
(30) (a) Katritzky, A. R.; Marson, C. Angew. Chem., Int. Ed. Engl. 1984,
23, 420–429. (b) Katritzky, A. R. Tetrahedron 1980, 36, 679–699.
(31) Volke, J.; Dunsch, L.; Volkeova, V.; Petr, A.; Urban, J. Electrochim.
Acta 1997, 42, 1771–1780.
(32) 1B, 2,4,6-triphenyl-1,4′-bipyridin-1-ium; 1BMe, 1′-methyl-2,4,6-tri-
phenyl-1,4′-bipyridine-1,1′-diium; 2B, 4-(biphenyl-4-yl)-2,6-diphenyl-
1,4′-bipyridin-1-ium; 2BMe, 4-(biphenyl-4-yl)-1′-methyl-2,6-diphenyl-
1,4′-bipyridine-1,1′-diium;1F,9-phenyl-benzo[c]benzo[1,2]quinolizino[3,4,5,6-
ija][1,6]naphthyridin-15-ium; 1FMe, 2-methyl-9-phenyl-benzo[c]benzo-
[1,2]quinolizino[3,4,5,6-ija][1,6]naphthyridin-2,15-diium; 2F, 9-(bi-
phenyl-4-yl)-benzo[c]benzo[1,2]quinolizino[3,4,5,6-ija][1,6]naphthyridin-
15-ium; 2FMe, 9-(biphenyl-4-yl)-2-methyl-benzo[c]benzo[1,2]quinoli-
zino[3,4,5,6-ija][1,6]naphthyridin-2,15-diium.
To ensure the chemical and electrochemical stability of
pyridiniums vis-a`-vis nucleophilic attacks (including reductive
dimerization or pimerization) at the C-4 position of both the
9
J. AM. CHEM. SOC. VOL. 132, NO. 46, 2010 16703

A R T I C L E S
Fortage et al.
of the nitrogen atom of their pendant pyridine moiety, whereas
1F, 2F, 1FMe, and 2FMe are the fused analogues of 1B, 2B,
1BMe, and 2BMe branched species, respectively.
3. Results and Discussion
3.1. Expanded Bipyridiniums as Electrophores: Reduction
Behavior. The investigated species undergo reversible reduction
processes at mild potentials. Table 1 shows the data of the
studied EPs (see also Supporting Information) together with
relevant data of reference species and other well-known electron
acceptors.
As expected, the first reduction potential moves to less
negative values (anodic shift) when passing from monocharged
pyridiniums (EP+) to quaternarized, dicharged, bipyridinium
species (EP2+) by ca. +370 mV on average (Table 1 and Figure
2). The impact of pericondensation is comparatively smaller in
absolute values: ca. +85 and +165 mV on average for
monocationic (1B/1F and 2B/2F) and dicationic (1BMe/1FMe and
2BMe/2FMe) species, respectively. Comparison with other com-
monly used and well-known systems as electron-acceptor
species (see Table 1) indicates that the studied compounds are
fairly good electron acceptors since the quaternarized species
2BMe, 1BMe, 2FMe, and 1FMe have redox properties similar to
those of the best electron-acceptor systems.
The bielectronic nature of the first reduction process of the
four branched species (1B/2B-1BMe/2BMe) is a very interesting
result (Table 1). This electrochemical feature is unambiguously
demonstrated by performing rotating disk electrode voltammetry
on purposely synthesized ferrocenyl-substituted EPs, namely,
2B-Fc and 2F-Fc dyads (Figure 3). The well-established one-
electron oxidation attached to the ferrocene (Fc) subunit serves
as internal reference to determine the number of electron(s)
involved in the reduction processes of the EP parts of these
dyads (Figure 3).
Roughly, bielectronic processes usually occur when (i) two
identical and noninteracting (at least from the electrostatic
viewpoint) redox sites are present in the same (super)molecule
and (ii) when adding/removing a second electron is easier than
adding/removing the first one; the latter phenomenon, known
as “potential inversion”, is usually associated to structural
changes38 or gains in “electronic stability” (e.g., aromaticity).21
In the present case, the bielectronic reduction is most likely
due to a “potential inversion” phenomenon attached to a huge
structural change upon reduction. Typically, a change in the
hybridization of the Npyridinio atom of the pyridinium core,
accompanied by its pyramidalization, is hypothesized. Such a
redox-triggered pyramidalization has been recently computed
in the two following cases: (1) the two-electron-reduced
hexaaryl-substituted pyridinium16 (XP in Chart 1) and (2) the
relaxed singlet excited charge-transfer state of pyridinium-N-
Figure 2. Illustration of the impact of quaternarization: EP+ (solid) and
EP2+ (dotted). Cyclic voltammograms: (a) 1B (c ) 6 × 10-4 M) and 1BMe
(c ) 5 × 10-4 M), (b) 2B (c ) 5.5 × 10-4 M) and 2BMe (c ) 6 × 10-4
M), and (c) 2F (c ) 3 × 10-4 M) and 2FMe (c ) 6 × 10-4 M) in MeCN
(a, b) or DMSO (c) + 0.1 M TBAPF6 (Pt electrode, V ) 100 mV s-1).
phenolate betaine B30.39 In the case of tetrabranched EPs, this
redox-triggered reorganization of the molecular structure is
assumed to be fluxional in nature, hence the observed electro-
chemical reversibility. The characterization of this dynamic
distortion is currently underway.
With respect to branched EPs, even though the first two
monoelectronic reductions are merged into an apparent bielec-
tronic process when experimentally investigated, it is worth
clarifying the identity of the actual site of the first reduction in
view of the possible use of these compounds as electron-acceptor
components within polyad assemblies.23,24 Theory shows that
the first electron added to the molecule is essentially located
on the pyridinium core within species 1B and 2B, whereas it is
delocalized over both pyridinium fragments within 1BMe and
2BMe, with the pendant one adopting the characteristic quino¨ıdal
(33) Takahashi, K.; Nihira, T.; Akiyama, K.; Ikegami, Y.; Fukuyo, E.
J. Chem. Soc., Chem. Commun. 1992, 620–622.
(34) Hu¨nig, S.; Gross, J. Tetrahedron Lett. 1968, 21, 2599–2604. 1968,
21, 4139.
(35) Gosztola, D.; Niemczyk, M. P.; Svec, W.; Lukas, A. S.; Wasielewski,
M. R. J. Phys. Chem. A 2000, 104, 6545–6551.
(36) Dubois, D.; Kadish, K. M.; Flanagan, S.; Haufler, R. E.; Chibante,
L. P. F.; Wilson, L. J. J. Am. Chem. Soc. 1991, 113, 4364–4366.
(37) Frontana, C.; Vazquez-Mayagoitia, A.; Garza, J.; Vargas, R.; Gonzalez,
I. J. Phys. Chem. A 2006, 110, 9411–9419.
(38) (a) Evans, D. H.; Hu, K. J. Chem. Soc., Faraday Trans. 1996, 92,
3983–3990. (b) Evans, D. H. Chem. ReV. 2008, 108, 2113–2144.
(39) Catalan, J.; Garcia de Paz, J. L.; Reichardt, C. J. Phys. Chem. A 2010,
114, 6226–6234.
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Designing Multifunctional Expanded Pyridiniums
A R T I C L E S
Figure 3. Determination of the number of electrons (n) involved in the first reduction processes of branched and fused pyridiniums, and assessment of
electrochemical reversibility. (Left) Dyad 2B-Fc (c ) 3.0 × 10-4 M in MeCN): (a) cyclic voltammetry (V ) 0.1 V s-1) and (b) rotating disk electrode
voltammetry (V ) 0.01 V s-1; rotation rate: ) 2 × 103 rpm). (Right) Dyad 2F-Fc (c ) 6.0 × 10-4 M in DMSO): (c) cyclic voltammetry (V ) 0.1 V s-1
)
and (d) rotating disk electrode voltammetry (V ) 0.01 V s-1; rotation rate ) 2 × 103 rpm).
pattern for reduced pyridinium, as expected (Figure 4). This
result is straightforwardly explained by the well-established
critical contribution to the LUMO of the carbon atom at position
4 of pyridinium rings (see Figure S-55, Supporting Information).
For monocationic EPs, the core pyridinium bears a phenyl or
biphenyl group at this pivotal site (Chart 3), whereas for dicationic
branched EPs, which comprise two pyridinium motives, the pendant
one bears the electron-withdrawing pyridinium core as the 4-sub-
stituent, hence its greater electron-accepting strength (Figure 1).
The same holds for pericondensed species, including with regard
to the fully developed quinoidal signature attached to the peripheral
(pendant) N-pyridinium (see Figure 4). Interestingly, the reduction
site remains localized on the pyridinium motives regardless of the
degree of extension of the π-conjugated systems (branched or
fused). This is in accordance with our previous findings derived
from the comparative study of EPs based on a, single, pyridinium
core (Chart 1).16,17
trochemical properties points to the enforced coplanar layout
of the two heteroaromatic fragments within fused molecular
scaffolds as the most likely explanation of these potential
variations (rather than a hypothetical lowering of the energy of
the LUMO upon extending the π-conjugated systems). Similar
impacts of the change of the interannular angle on the redox
properties of bipyridinium, namely, the anodic shift of the first
reduction potential upon decreasing this twist angle, have already
been reported in the literature.6,40 This issue is further substanti-
ated by the observation that the first reduction potential of both
methylviologen (MV) and affiliated planar diazapyrenium
(DMDAP; see Chart 3) amount to very close values (within
0.04 V; see Table 1), bearing in mind that pyridinium rings are
not far from coplanarity within MV.
(40) (a) Benniston, A. C.; Harriman, A.; Li, P.; Rostron, J. P.; Harrington,
R. W.; Clegg, W. Chem.sEur. J. 2007, 13, 7838–7851. (b) Beer, P. D.;
Chen, Z.; Grieve, A.; Haggitt, J. J. Chem. Soc., Chem. Commun. 1994,
2413–2414. (c) Kodaka, M.; Kubota, Y. J. Chem. Soc., Perkin Trans.
1999, 2, 891–894. (d) Willner, I.; Ayalon, A.; Rabinovitz, M. New
J. Chem. 1990, 14, 685–688. (e) Chen, P.; Curry, M.; Meyer, T. J.
Inorg. Chem. 1989, 28, 2271–2280. (d) Thummel, R. P.; Lefoulon,
F.; Chirayil, S.; Goulle, V. J. Org. Chem. 1988, 53, 4745–4747.
Last, the observation that the first reduction process occurs
at less negative potentials within fused EPs than in their
branched analogues remains to be commented on: -0.93 V for
1B versus -0.84 V for 1F and -0.60 V for 1BMe versus -0.43
V for 1FMe for representative instances (Table 1). The above-
established relevance of the localized description of the elec-
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A R T I C L E S
Fortage et al.
accordingly the energy of the HOMO. The absorption band is
red shifted on moving from the monopyridiniums (1B and 2B)
to the bipyridiniums (1BMe and 2BMe). This not only agrees with
the above ICT hypothesis but also indicates that electron-
acceptor moieties present in the latter EPs are significantly
stronger than in the former, in accordance with redox properties
(see Table 1).
The fused species (F series) exhibit much more complex
absorption spectra than their corresponding branched counter-
parts (Figure 5, Table S-1, Supporting Information). The spectra
are more structured and red shifted, and apparently more
transitions contribute to the absorption features. Structured bands
may be justified if fused species are considered to exhibit more
rigid structures than the corresponding branched pyridiniums.
In such a case, the nuclear configurations of ground and excited
states are more similar, and therefore, radiative transitions
between these states yield structured spectra (including for
absorption). Moreover, a pronounced vibronic effect ascribable
to the coupling of the ICT transition that involves the dangling
(bi)phenyl terminus and its torsional motion15-17 also contributes
to the absorption features. This effect is clearly visualized at
the red edge of the absorption spectra of 1F and 1FMe (Figures
5 and 6).
Figure 4. Illustration of the expansion of the site of first reduction upon
quaternarization (N-alkylation). Difference in total density computed
between the monoreduced and native forms (at the geometry of the reduced
species, isocontour ) 0.0025 au) of 1B (a), 1BMe (b), 1F (c), and 1FMe (d).
Green lobes indicate an increase of total electron density (red lobes indicate
a depletion).
Upon pericondensation, a densification of MOs close to the
HOMO-LUMO gap was computed and revealed the increasing
complexity of the absorption spectra when going from branched
to fused chromophores (Figure 6). In parallel, there occurred
two concomitant phenomena that explain the observed reduction
of the HOMO-LUMO gap. On the one hand, the LUMOs are
computed to lie at lower energy in the fused species than in the
corresponding branched EPs. This finding is in agreement with
the evolution of the first reduction potentials experimentally
measured (e.g., compare the first reduction potential of 2B,
-0.91 V, with that of 2F, -0.83 V, or the first reduction
potential of 2BMe, -0.58 V, with that of 2FMe, -0.42 V; see
Table 1). This variation has been ascribed to different values
for the critical twist angle between the pyridinium core and its
N-pyridyl(ium) influential fragment, that is, differential elec-
tronic delocalization within (i.e., intra) htt-bipyridin(ium) sub-
systems (see above). On the other hand, the computed energies
of the HOMOs are higher for the fused EPs than for the
corresponding branched ones, which is a direct electronic
consequence of pericondensation.16 As a result, the HOMO-
LUMO gaps of the fused systems are smaller than those of the
corresponding branched EPs, thereby explaining the observed
3.2. Expanded Bipyridiniums as Chromophores: Absorp-
tion Spectra. The absorption spectra of branched compounds
1B/2B-1BMe/2BMe (Figure 5 and Table S-1, Supporting Infor-
mation) exhibit an intense band in the 280-440 nm region. This
can be assigned to the lowest energy spin-allowed π-π*
transitions on the basis of molar absorption coefficients and band
shapes. The absorption spectrum of reference Rf (lacking the
N-pyridinio aryl group; see Chart 3) closely resembles that of
1B. This is not surprising since TD-DFT calculations tell us
that the electronic transitions present in this spectral region
essentially originate from intramolecular charge transfers (ICT)
involving electron-releasing aryl substituents at positions 2, 6,
and 4 only, on the one hand, and the electron-withdrawing
pyridinium core, on the other (see Supporting Information).41
The main band is red shifted on passing from 1B and 1BMe to
2B and 2BMe, respectively, when a biphenyl subunit is present.
This observation suggests, in agreement with theoretical calcula-
tions (see TD-DFT outcomes in Supporting Information), that
for the species that contain a biphenyl substituent the ICT
character is further enhanced. The biphenyl fragment is a better
electron-releasing group than a mere phenyl, thereby raising
Figure 5. Electronic spectra of branched (left) and fused (right) derivatives (acetonitrile solutions). (Left) Solid blue line, 1B; dashed blue line, 1BMe; solid
red line, 2B; dashed red line, 2BMe; black line, Rf. (Right) Solid blue line, 1F; dashed blue line, 1FMe; solid red line, 2F; dashed red line, 2FMe. For related
data, see Table S-1 in the Supporting Information.
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Designing Multifunctional Expanded Pyridiniums
A R T I C L E S
Figure 6. Superimposed experimental absorption spectra and calculated electronic transitions.42
red-shifted absorption on moving from 1B/2B and 1BMe/2BMe
to 1F/2F and 1FMe/2FMe
increase of intensity associated to the first band experimentally
observed when going from 1F and 1FMe to 2F and 2FMe
.
.
It is also interesting to note that the presence of the biphenyl
subunit in 2F and 2FMe does not translate into a clear red shift
of the lowest-energy band for the fused systems, conversely to
branched analogues (Figure 5 and Table S-1, Supporting
Information). Indeed, inspection of the MO involved in the first
electronic transition in the case of both 2F and 2FMe confirms
its ICT character from a HOMO (essentially localized on the
biphenyl substituent) to a LUMO centered on the core pyri-
dinium (2F) (Supporting Information) or pendant pyridinium
(2FMe) (Figure 7). The ICT character correctly translates in an
Nevertheless, since the HOMO of 2F and 2FMe (localized on
the biphenyl) is computed to be very close in energy to the
HOMO-1 (π orbital localized on the fused scaffold and
identical to the HOMO of the corresponding 1F and 1FMe
(42) In this context it is worth reminding that the intensity (/energy) of the
computed CT transition can be slightly overestimated (/underestimated)
when using a TD-DFT approach even in conjunction with a hybrid
functional (such as in the present case), see, for instance: (a) Jacquemin,
D.; Perpete, E. A.; Ciofini, I.; Adamo, C. Acc. Chem. Res. 2009, 42,
326–334. (b) Dreuw, A.; Head-Gordon, M. J. Am. Chem. Soc. 2004,
126, 4007–4016. This effect is amplified and can yield completely
unphysical results when the overlap between the orbitals involved in
the excitation is small or nonexistent, as is the case for through-space
CT. In the case of the chromophores studied here, a sizable overlap is
present for all ICT considered. Thus, only a modest overestimation
of oscillator strength associated with a slight shift in energy is expected
for these transitions. This is indeed observed in the sensitive instances
of 2B, 2BMe, 2F, and 2FMe (Figure 6).
(41) Similar assignment also holds for analogous 1N,2,4,6-tetraarylpyri-
diniums bearing a (substituted) phenyl group at the 1N position instead
of the pyridyl fragment; see ref 14 and also: (a) Kharlanov, V. A.;
Knyazhansky, M. I. J. Photochem. Photobiol. A 1999, 125, 21–27.
(b) Knyazhansky, M. I.; Kharlanov, V. A.; Tymiansky, Ya. R. J.
Photochem. Photobiol. A 1998, 118, 151–156.
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A R T I C L E S
Fortage et al.
Figure 8. Luminescence spectra of 1B (solid line), 2F (dashed line), and
2BMe (dashed-dotted) in acetonitrile solution at room temperature.
Figure 7. Frontier MOs of 1FMe and 2FMe (isocontour value ) 0.025 au):
(top) LUMO (1FMe (a) and 2FMe (b)); (middle) HOMO (1FMe (c) and 2FMe
(d)); (bottom) HOMO-1 (1FMe (e) and 2FMe (f)).44
compounds) the transition energies are not red shifted (Figures
6 and 7).43,44
To summarize, electro-optical features of branched EPs are
essentially governed by ICT transitions, as illustrated by the
steady bathochromic shift of their dominant large absorption
band when going from the less polar chromophore (1B) to the
most polar one: 2BMe (see Figure 5). Conversely, no such
bathochromic shift of the red-edge absorption is observed for
the series of fused analogues since frontier MOs are fortuitously
near-degenerate and can therefore be interchanged on modifying
the intramolecular ICT landscape without sizably impacting the
HOMO-LUMO gap. Besides, absorptivity at the red edge is
found to increase on enhancing the overall ICT character of
the fused EPs due to additive contributions of vibronic effects
and energy densification of electronic transitions. Overall, fused
EPs have a smaller HOMO-LUMO gap than branched ones,
Figure 9. Luminescence spectra of 1B (top) and 2F (bottom) in MeOH/
EtOH 4:1 (v/v) rigid matrix at 77 K. Dashed spectra (phosphorescence)
are recorded by using a time delay of 10 ms from the excitation pulse.
which translates into a more extended absorption in the visible,
and they exhibit more rich absorption spectra with larger
integrated absorptivities.
3.3. Expanded Bipyridiniums as Luminophores: Emission
Properties. All the compounds exhibit luminescence, both in
fluid acetonitrile solutions at room temperature and in rigid
matrices at 77 K in MeOH/EtOH 4:1 (v/v), with monoexpo-
nential decay in all cases. The luminescence data of the whole
series of EP luminophores (Chart 3) along with relevant data
of reference species including methylviologen (MV) and its
diazapyrenium fused analogue (DMDAP) are collected in Tables
2 and 3; Figures 8 and 9 show representative emission spectra.
In all cases, luminescence properties are independent of excita-
tion wavelengths.
(43) In the case of molecule 2F, the HOMO and HOMO-1 are near
degenerate so that two very close transitions are computed. The first,
very intense of ICT character is computed at 395 nm and the second,
less intense corresponding to the HOMO-1 to LUMO excitation is
computed at 391 nm. In such a case, it is not excluded that the two
transitions could be inverted due to the underestimation of ICT energies
at the TD-DFT level. From the comparison with the experimental
absorption spectrum (Figure 6), this is apparently the case. In such a
case it is not excluded, in agreement with the experimental data, that
the emitting state could be the one corresponding to a HOMO-1 to
LUMO excitation.
(44) For 1FMe/2FMe, computed energies (au) of frontier MOs (Figure 7)
are, respectively, the following:-0.137 (a)/-0.133 (b) for the LUMO;-
0.278 (c)/-0.251 (d) for the HOMO, and-0.283 (e)/-0.275 (f) for
the HOMO-1.
With regard to the room-temperature luminescence of branched
EPs (B series), it is worth noting that the fluorescence quantum
yields (Φ) of three of them (2B, 1BMe, and 2BMe) are in the
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Designing Multifunctional Expanded Pyridiniums
A R T I C L E S
Table 2. Absorption (red edge) and Luminescence Data at Room Temperature in Acetonitrile
absorptiona
luminescence
kr, s-1
λ, nm (ꢀ, Μ-1 cm-1
)
λ, nm
τ, ns
Φ
knr, s-1
∆ν,b cm-1
SS,c cm-1
Rf
302 (31 000)
308 (38 300)
312 (33 600)
348 (36 600)
323 (34 100)
366 (33 900)
429 (10 000)
430 (12 700)
430 (13 700)
428 (22 200)
428 (40 700)
260 (20 000)
418 (15 000)
430
480
435
502
455
577
465
465
480
453
610
353
423
1.5
3.0
0.2
1.9
0.4
0.6
7.0
3.3
3.1
4.1
2.8
0.9
10.4
0.4
2.7 × 108
<3.3 × 104
2.5 × 108
2.6 × 108
5.5 × 108
1.7 × 108
5.0 × 107
6.3 × 107
6.8 × 107
5.4 × 107
3.6 × 107
2.2 × 107
5.8 × 107
4.0 × 108
>3.3 × 108
6.0 × 109
2.6 × 108
1.9 × 109
1.5 × 109
9.3 × 107
2.4 × 108
2.5 × 108
1.9 × 108
3.2 × 108
1.1 × 109
3.8 × 107
3900
4000
3900
3950
4000
4170
3100
3000
2820
2150
3530
9850
11 580
9060
8810
8880
9990
1800
1760
2430
1290
6970
10 130
280
RfBd
1B
<10-4
0.04
0.50
0.22
0.12
0.35
0.21
0.21
0.22
0.10
0.02
0.60
2B
1BMe
2BMe
RfFd
1F
2F
1FMe
2FMe
MVe
DMDAPe
a The lowest-energy feature of the main absorption band is reported. b Half-width of the emission band. c Stokes shift, calculated as the difference in
energy between the lowest-energy absorption feature and the emission maxima. d From ref 16. e From ref 19.
Table 3. Luminescence Data at 77 K in MeOH/EtOH 4:1 (v/v)
(SS) is quite different for the branched B species and for the
fused F compounds. In fact, EPs of the B series exhibit quite
fluorescence
max, nm
phosphorescence
large Stokes shifts, whereas the F-type species (with the partial
exception of 2FMe) exhibit much smaller ones. This is not
surprising, when one considers that the excited state(s) of B
compounds can undergo a significant structural reorganization
process that leads to flattened relaxed geometries (planarization
relaxation process).17,24a,e Another informative observation is
the difference of broadness observed for the emission bands of
the B and F series of EPs at room temperature (see values of
half-height widths, ∆ν, in Table 2). Such values are larger for
the branched B species, which confirms the larger distortion of
the excited states of the branched species compared to the fused
compounds. Indeed, adiabatic planarization is not an important
relaxation process for F species since they have fused rigid and
already largely planar overall molecular backbones.46 In other
words, the structures of branched and fused species are more
similar at the excited state than at the ground state. Conse-
quently, the excited state energy of analogous species (and as
a consequence their luminescence spectra) tends to be similar,
even if the absorption spectra look quite different. The above
particularly applies to the 1B/1F and 1BMe/1FMe couples of
compounds. However, luminescence lifetimes are definitely
shorter for B-type EPs than for F-type EPs, although emission
quantum yields do not seem to follow a general trend on passing
from the B to the F series (see Table 2). The kr and knr radiative
and nonradiative rate constants can be calculated from quantum
yields and lifetime data (Table 2): both kr and knr values are
about 1 order of magnitude larger for the branched species
compared to the fused compounds. The enhanced structure
rigidity of the fused compounds can justify their smaller
nonradiative decay rate constant, since some vibrations, which
can assist the radiationless decay in the branched systems, are
deactivated in the fused analogues. Moreover, the delocalized
excitation over the whole π-conjugated system of fused poly-
λ
τ, ns
λ
max, nm
τ, ms
Rf
1B
2B
1BMe
2BMe
1F
370
381
410
407
432
441
442
434
430
330
424
2.3
3.0
3.0
1.5
1.8
5.3
3.2
5.0
3.5
7.0
10.7
442
447
460
453
475
506
510
494
540
418
586
3240
3000
18
700
19
800
2000
430
175
3600
900
2F
1FMe
2FMe
MVa
DMDAPa
a Recorded on butyronitrile rigid matrices; data from ref 19.
0.1-0.5 range (Table 2). This is close to the quantum yield of
the reference Rf (Φ ) 0.4) and much greater than that of the
loosely fluorescent N-phenyl-substituted RfB (Φ ≈ 10-4).16 The
noticeable drop in photoluminescence quantum yield when going
from Rf to RfB is ascribed to an intervening additional decay
process due to an intramolecular charge-transfer state (ad-ICT)
that involves the phenyl Npyridinio group as the donor (not present
in Rf) and the pyridinium core as the acceptor, which promotes
radiationless decay.41,45 The dominant radiationless decay is an
adiabatic structural relaxation toward coplanarity of the phenyl
Npyridinio group with the pyridinium ring. Replacing the phenyl
Npyridinio group with electron-withdrawing pyridine (case of 1B)
leads to an intermediate situation (Φ ≈ 10-2), since it raises
the energy of the above-mentioned ad-ICT state and thus
approaches the photophysical behavior of reference Rf. Thus,
one can reasonably ascribe the observed high emission quantum
yields of the B-series species to (1) the mere suppression of
the deleterious ad-ICT upon quaternarization of the pendant
N-pyridyl group (cases of 1BMe and 2BMe) or (2) the stabilization
of the emitting ICT state by the presence of the better electron-
releasing biphenyl. This stabilization rules out the higher-lying
ad-ICT state that involves the pendant pyridine as the electron-
releasing group and the pyridinium core as the acceptor (case
of 2B).
(46) In agreement with the observed experimental trends, smaller Stokes
shifts (Table S-2, Supporting Information) and structural rearrange-
ments were computed in the case of fused systems with respect to the
corresponding branched compounds, confirming the former’s greater
rigidity. More precisely, structural relaxation in both the singlet and
the triplet excited states was computed to be mainly related to the
planarization of the aryl substituents with respect to the central
pyridinium core and proved to be larger in the case of nonfused
architectures. For instance, the root mean square deviations (rmsd)
computed between ground and singlet excited state structures for 1B
and 1F are 0.23 and 0.11, respectively (and similar results hold for
the other compounds).
Concerning room-temperature photophysical properties as a
function of the branched or fused nature of the EPs, a rough
survey of the data of Table 2 indicates that the Stokes shift
(45) Knyazhansky, M. I.; Feigelman, V. M.; Tymyanski, Ya. R.; Druzhinin,
S. I.; Uzhinov, B. M. J. Lumin. 1987, 37, 215–218.
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A R T I C L E S
Fortage et al.
cyclic EPs results in averaged distortions and reduced Franck-
Condon factors.
DMDAP. This is similar to trends observed when going from
branched EPs (of type 1) to their fused counterparts. Along the
same line, MV is a poor luminophore at room temperature19,48
whereas DMDAP is strongly fluorescent (its emission quantum
yield amounts to 60%). These supplementary observations are
also in line with our previous findings concerning EPs of the
first generation (Chart 1)16,17 and again confirm the relevance
of the picture of pyridinium properties we put to the forth in
our molecular design strategy (section 2). In the special case of
pyridiniums, extending the π-conjugated system does not lower
the energy of the LUMO accordingly (as reflected by the first
reduction potentials). The LUMO remains localized on the
pyridinium fragment(s), and electrochemistry fits a localized
description, at least for the first reduction process. The situation
is different for optical and photophysical properties. The sole
features of the LUMO no longer govern them. Therefore, they
fit a delocalized description and are sensitive to the degree of
extension of the π-conjugated systems.
To summarize, branched and fused htt-EPs are equally good
types of luminophores but, most important, are much better
luminophores than parent bare pyridinium or bipyridinium
species.
3.4. Insights into the Prerequisites of Photobiscyclization.
Finally, on the basis of the insights we gained into the redox
(section 3.1), electro-optical (section 3.2), and photophysical
(section 3.3) behaviors of branched EPs, it is now possible to
account, a posteriori, for the synthetic strategy empirically
perfected to obtain the various fused EPs. This section is more
specifically aimed at explaining observed differences in pho-
tochemical reactivity.
It is also interesting to note that the EPs that comprise a
biphenyl subunit (2B, 2BMe, 2F, and 2FMe) show emission
energies which are definitely red shifted compared to the
emissions of the other species (Table 2). As for the absorption
spectra of the branched chromophores, this observation suggests
that the presence of the biphenyl group translates into a CT
character of the excited state (and as a consequence of the
emission) of the above-mentioned species. Comparing 2B and
2BMe to the corresponding 1B and 1BMe, this effect is not
indicated by a larger Stokes shift, as is usually the case for CT
emission, since for such compounds the lowest-energy absorp-
tion features already receive a significant CT contribution. On
the contrary, for fused polycyclic luminophores that lack both
the large photoinduced structural relaxation and the bathochro-
mic shift of the absorption red edge usually associated with CT
character (see section 3.2), the CT nature of the emission is
clearly evidenced by the large Stokes shift. This is the case for
2FMe and, to a smaller extent, 2F, as revealed by the comparison
of their SS to those of their structural analogues 1F and 1FMe
,
which exhibit an essentially “fused-core”-localized π-π*
emission.47
As regards low-temperature photophysical properties, all
compounds exhibit both fluorescence and phosphorescence at
77 K in MeOH/EtOH 4:1 (v/v) (see data in Table 3 and Figure
9). All emission spectra are structured (vibrational progression
of about 1300 cm-1, corresponding to CdC and CdN stretch-
ing) and can be assigned to locally excited π-π* singlet and
triplet states. The absence of CT emission is justified if one
considers that CT states with pronounced charge redistribution
are largely destabilized in rigid matrix at low temperature, as a
consequence of large nuclear barriers related to solvent
reorganization.
Indeed, fused EPs were obtained by the photochemical
pericondensation of the proper branched EP precursors, fol-
lowing a route adapted from that described by Katritzky et al.49
for the early synthesis of 1F.49a Synthetic strategies are given
in Scheme 1 (see also Supporting Information). These ap-
proaches were conceived because photobiscyclization pathways
were found to be ineffective in the three following cases: (1)
when the pendant N-pyridyl group is quaternarized (instance
of 1BMe), (2) when a biphenyl group is appended at the C-4
position of the pyridinium core (instance of 2B), and (3) when
the branched EP is both quaternarized and derivatized with a
biphenyl substituent (instance of 2BMe). Typical 1H NMR spectra
of the crude product of test photochemical reactions are given
in the Supporting Information (see Figures S-34-S-37). The
rationale of the relevant parameters that govern photoinduced
cyclodehydrogenation is proposed below.
The variations of the computed fluorescence and phospho-
rescence emission energies were also found in good qualitative
agreement with experimental issues. While in the case of 1B/
1
1BMe and 1F/1FMe (the 1-type compounds) the (π-π*) S1
excited state has an essentially “core-localized” character,
calculations confirm the ICT character for the 1(π-π*) singlet
excited state of all the systems that bear a biphenyl substituent
(that is, 2B/2BMe and 2F/2FMe).
Let us now address the issue of the electronic features of
structurally affiliated MV and DMDAP reference species
(Tables 2 and 3) to complete the discussion of photophysical
results. With regard to the impact of the extension of the
π-conjugated system, it is worth noticing that the behavior of
these two species parallel that of respective branched and fused
EPs not only at the electrochemical level but also at both the
electronic and photophysical levels. Typically, a scarce effect
is detected for the first reduction process (see section 3.1),
whereas absorption and emission properties are significantly
different. More precisely, the electronic absorption spectrum is
extended from the UV to the near visible (red-edge band at 418
nm) while becoming structured19 on passing from MV to
Photobiscyclization is known to take place from the singlet
excited state of branched pyridiniums.50 The maps of difference
in total electron density between the relaxed lowest lying singlet
excited state and the ground state (at the same geometry) were
calculated for the four branched EPs (Figure 10 below) to
account for this lack of photoreactivity. A comparative analysis
of these maps enabled us to derive some qualitative criteria for
photoinduced cyclodehydrogenation.
(47) However, whereas the luminescence data clearly indicate that the room
temperature emissions of 2B, 2BMe, and 2FMe are of CT nature, some
doubt remains on the origin of the emission of 2F. In fact, the Stokes
shift of this compound, although larger than those of the analogues
1F and 1FMe, is less pronounced compared to that of 2FMe, a bona
fide CT emitter. For 2F, on the sole basis of the experimental data, it
is not possible to establish whether the π-π* emission essentially
originates from a “fused-core” localized or an ICT transition or even
exhibits a mixed character; see also ref 43.
(48) Peon, J.; Tan, X.; Hoerner, J. D.; Xia, C.; Luk, Y. F.; Kohler, B. J.
Phys. Chem. A 2001, 105, 5768–5777.
(49) (a) Katritzky, A. R.; Zakaria, Z.; Lunt, E.; Jones, P. G.; Kennard, O.
J. Chem. Soc., Chem. Commun. 1979, 268–269. (b) Katritzky, A. R.;
Zakaria, Z.; Lunt, E. J. Chem. Soc., Perkin Trans. 1980, 1, 1879–
1887.
(50) Knyazhansky, M. I.; Metelitsa, A. V.; Makarova, N. I.; Feygelman,
V. M.; Tymyansky, J. R. J. Photochem. Photobiol. A 2000, 132, 59–
66.
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Designing Multifunctional Expanded Pyridiniums
A R T I C L E S
Scheme 1. Synthetic Routes for the 1B, 1BMe, 1F, and 1FMe Series (left) as well as for the 2B, 2BMe, 2F, and 2FMe Series (right)a
a [Me+] is a methylating agent (see details in the Supporting Information).
contrary, severely deactivated as a result of the reductive
intramolecular charge transfer (ICT). Brought together, these
two concomitant effects can explain the photostability of 1BMe
1
(no trace of 1FMe was found in the H NMR spectrum of the
crude reaction product of the attempt of photobiscyclization;
see Figure S-35 in the Supporting Information) versus the
quantitative phototransformation observed in the cases of 1B
and RfB (see Figure S-34 in the Supporting Information).
With regard to the observed lack of photoreactivity for
biphenyl derivatives (reactivity loss of type 2, represented by
the case of 1[2B]*), calculated electron density difference maps
show that the photoactivated part of the molecule is not the
one expected to be fused but instead the rodlike tricyclic
molecular subsystem that encompasses the pyridinium core and
the terminal biphenyl branch. In fact, if the relevant excited
state is characterized by a pronounced ICT to the benefit of the
pyridinium core, the activated protons are those of the electron-
releasing phenyl terminus (decrease of electron density) rather
than those of the targeted site of pericondensation. Consistently,
a dramatic drop in the yield of photobiscyclization is expected,
which was indeed found to be less than 5% for 2B as judged
Figure 10. Electron-density difference maps of the lowest lying relaxed
singlet excited state (S1) as compared to the ground state (S0) at the same
geometry for 1B (a), 1BMe (b), 2B (c), and 2BMe (d); isocontour value )
0.001 au. The green (red) color indicates an increase (decrease) of electron
density in a given molecular region upon excitation.
As can be seen in Figure 10 for the reference case of the
photoconvertible 1B, excited-state charge redistribution within
1[1B]* corresponds to (i) an increase in electron density on the
pyridinium core and its almost coplanar phenyl substituent at
position 4 as well as on the four interannular linkages and (ii)
the correlated significant decrease in electron density on the
two phenyl branches ortho to the pyridinium nitrogen atom and,
to a lesser extent, on the pyridyl N-group. Thereby, (i) the key
planarization of the N-pyridinio group and of the two phenyl
rings at positions 2 and 6 is induced (which is necessary to
prepare subsequent pericyclic electron motion resulting in C-C
bonds formation) and (ii) protons of electron-deficient aryls,
which are destined to be abstracted during the rearomatization
step with dioxygen (yielding to water molecules), are activated
(Scheme 1). With regard to inhibition of photoinduced dehy-
drocyclization upon N-alkylation (reactivity loss of type 1,
represented by the case of 1[1BMe]*), displaced electron density
is no longer evenly redistributed around the central pyridinium
but is almost entirely withdrawn by the pendant pyridinium.
This situation is likely to favor the only planarization of the
head-to-tail bipyridinium subsystem to the detriment of other
phenyl substituents and in particular those connected at either
side of the N-pyridinio group, hence preparing a conformation
inappropriate to subsequent pericondensation. In addition, even
if protons of phenyl 2,6-substituents of the pyridinium core are
properly activated (weakening of C-H bonds upon electron-
density depletion), those of the pendant pyridinium are, on the
1
from the H NMR features of the crude photoreaction product
of this branched EP (see Figure S-36, Supporting Information).
All in all, the absence of photoinduced cyclodehydrogenation
experimentally evidenced for the quaternarized EP which also
bears a biphenyl branch as the 4-substituent, that is, 2BMe
(reactivity loss of type 3), was therefore foreseeable (see Figure
S-37, Supporting Information). As shown in Figure 10, a huge
ICT takes place along the htt-bipyridinium main molecular axis,
ultimately leading not only to the unsuitable activation of the
protons of the electron-releasing phenyl terminus but also to
the deleterious deactivation of the protons borne by the electron-
withdrawing pendant pyridinium ring.
To summarize, we have shown here that experimental findings
regarding the photochemical reactivity of branched expanded
pyridiniums and more specifically their ability to undergo
photobiscyclization into fused EPs can be correlated with maps
corresponding to change in total electron density between the
ground and the photoreactive π-π* singlet excited states.
Qualitative criteria essentially relying on ICT features of these
1(π-π*) excited states could be derived, allowing forthcoming
computer-assisted design of fused EP-based dyes, for instance.
Manipulating the photoreactivity of branched EPs will rely on
in-silico adjusting of the proper balance between various possible
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A R T I C L E S
Fortage et al.
ICT processes, which can be governed by peripheral substituents
borne by the aryl branches of EPs.
Lastly, we validated a computational approach that permits
one to derive qualitative criteria, not only to predict the
possibility to achieve photoinduced biscyclization of interest
for synthetic strategies or within the framework of the in situ
preparation of intercalating agents (photochemotherapy),56 for
instance, but also to assess the photochemical stability of
branched EPs. In this regard, it is worth noting that the most
efficient EPs as electrophores (i.e., N-alkylated derivatives like
1BMe/2BMe) do not undergo light-triggered pericondensation.
4. Conclusion
In the present work we reported the design, synthesis, and
characterization of the electronic, photophysical, and electro-
chemical properties of a series of novel expanded pyridiniums
(EPs) featuring the so-called head-to-tail arrangement for their
bipartite redox-active subsystem. We have shown that, regardless
of their branched or fused nature, the most promising prototypes
are the dicationic ones (quaternarized EPs), whether considering
their chromophoric, luminophoric, or electrophoric behavior.
Most importantly, on the basis of their performance appraisals,
(i) large absorptivities (ε > 104 M-1 cm-1) up to 450 nm, (ii)
good luminescence quantum yields at room temperature (ΦFluo
> 0.2), and (iii) easily accessible first reduction potentials (Ered
g -0.6 V vs SCE in MeCN), these EPs can be viewed as
integrated multifunctional entities of interest in their own right.
They could also serve as functional building blocks in various
types of (supra)molecular devices, following the example of
benchmark perylene bisimides.51 Herein the described expanded
htt-bipyridinium are worth considering as the first elements of
a new family since the Rn peripheral substituents (Figure 1)
can be used to further enhance or tune the three types of
properties but also to modify the solubility of the EPs or ensure
their docking (or anchoring) to other functional components,
including surfaces.52
Achievements related to electron-accepting properties of htt-
EPs deserve to be underlined. First, both branched and fused
dicationic EPs exhibit a first reduction potential well comparable
with the best electrophores commonly used (e.g., Ered ) -0.42
V vs SCE for 1FMe/2FMe). Second, the single bielectronic
reduction observed for branched EPs, quaternarized or not (e.g.,
Ered ) -0.59 V vs SCE for 1BMe/2BMe), is quite unusual and
of interest since multielectron manipulation is highly challeng-
ing, in particular in the field of artificial photosynthesis, at the
final redox-to-chemical conversion step.53,54 This ability to
undergo a single-step, two-electron reduction can be viewed as
a functional emergence,55 which is closely related to the head-
to-tail topology of electrophores. Consequently, as unusual two-
electron acceptors, branched htt-EPs could open a new path to
the study and exploitation of multiphoton/multielectron processes.
5. Experimental Section
5.1. Syntheses, Characterization, and General Experi-
mental Details. Materials, syntheses of the EPs including general
procedures to prepare pyrylium precursors57 and branched pyri-
diniums,58 along with full characterizations are provided in the
Supporting Information.
5.2. Electrochemical Measurements. The electrochemical ex-
periments were carried out with a conventional three-electrode cell
(solution volume of 15 mL) and a PC-controlled potentiostat/
galvanostat (Princeton Applied Research Inc. model 263A). The
working electrode was a platinum electrode from Radiometer-
Tacussel exposing a geometrical area of 0.034 cm2 and mounted
in Teflon. The electrode was polished before each experiment with
3 and 0.3 µm alumina pastes followed by extensive rinsing with
ultrapure Milli-Q water. Platinum wire was used as the counter
electrode and saturated calomel electrode, SCE, as reference
electrode. Electrolytic solutions, MeCN (Aldrich, anhydrous,
99.8%) containing tetrabutylammonium hexafluorophosphate 0.1
M (TBAPF6, Aldrich, +99%) as supporting electrolyte, were
routinely deoxygenated by argon bubbling. All potential values are
given versus the calomel saturated electrode SCE. The reported
numerical values (Table 1) were corrected by using dissolved Fc+/
Fc couple as an internal reference standard and by setting E1/2 (Fc+/
Fc) equal to +0.380 V vs SCE in MeCN.59 Cyclic voltammetry
experiments were conducted at scan rate of 0.1 V s-1 (except where
specified in the text). Rotating disk electrode voltammetry experi-
ments were conducted at scan rate of 0.01 V s-1 by rotating the
disk electrode at 2000 rotations per minute (rpm) (Controvit device
from Radiometer-Tacussel, France). Square wave voltammetry
experiments were performed with a potential sweep rate of 100
mV s-1 (pulse height ) 25 mV, step height ) 2 mV, and frequency
) 50 Hz).
5.3. Photophysical Properties. For steady-state luminescence
measurements, a Jobin Yvon-Spex Fluoromax P spectrofluorimeter
was used, equipped with a Hamamatsu R3896 photomultiplier, and
the spectra were corrected for photomultiplier response using a
program purchased with the fluorimeter. For the luminescence
lifetimes, an Edinburgh OB 900 time-correlated single-photon-
counting spectrometer was used. As excitation sources, a Hamamat-
su PLP 2 laser diode (59 ps pulse width at 408 nm) and the nitrogen
discharge (pulse width, 2 ns at 337 nm) were employed. Emission
(51) Wu¨rthner, F. Chem. Commun. 2004, 1564–1573.
(52) Bongard, D.; Mo¨ller, M.; Rao, S. N.; Corr, D.; Walder, L. HelV. Chim.
Acta 2005, 88, 3200–3209.
(53) (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2009, 42,
1890–1898. (b) Magnuson, A.; Anderlund, M.; Johansson, O.;
Lindblad, P.; Lomoth, R.; Polivka, T.; Ott, S.; Stensjo¨, K.; Styring,
S.; Sundstro¨m, V.; Hammarstro¨m, L. Acc. Chem. Res. 2009, 42, 1899–
1909. (c) Sala, X.; Romero, I.; Rodriguez, M.; Escriche, L.; Llobet,
A. Angew. Chem., Int. Ed. 2009, 48, 2842–2852. (d) Tinker, L. L.;
McDaniel, N. D.; Bernhard, S. J. Mater. Chem. 2009, 19, 3328–3337.
(e) Sartorel, A.; Carraro, M.; Scorrano, G.; De Zorzi, R.; Geremia,
S.; McDaniel, N. D.; Bernhard, S.; Bonchio, M. J. Am. Chem. Soc.
2008, 130, 5006–5007. (f) Balzani, V.; Credi, A.; Venturi, M.
ChemSusChem 2008, 1, 26–58. (g) Eisenberg, R.; Gray, H. B. Inorg.
Chem. 2008, 47, 1697–1699. (h) Huyhn, M. H. V.; Meyer, T. J. Chem.
ReV. 2007, 107, 5004–5064. (i) Lewis, N. S.; Nocera, D. G. Proc.
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(57) 2,4,6-Triarylpyrylium tetrafluoroborate salts were prepared by fol-
lowing a modification of the literature methods reported by Moghimi
et al. and Wizinger et al. for the synthesis of other 2,4,6-triarylpyrylium
salts, see: (a) Moghimi, A.; Rastegar, M. F.; Ghandi, M.; Taghizadeh,
M.; Yari, A.; Shamsipur, M.; Yap, G. P. A.; Rahbarnoohi, H. J. Org.
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P. HelV. Chim. Acta 1956, 39, 5–15.
(58) Tri- and tetrabranched pyridiniums were synthesized by following a
modification of the literature methods reported by Osterby et al.,
Kessler et al., and Dimroth et al. for the preparation of N-phenolate-
2,4,6-triarylpyridinium betaine dyes, see: (a) Osterby, B. R.; McKelvey,
R. D. J. Chem. Educ. 1996, 73, 260–261. (b) Kessler, M. A.; Wolfbeis,
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quantum yields for argon-degassed acetonitrile solutions of the
organic species were determined by using the optically diluted
method60 with anthracene in air-equilibrated ethanol solution (Φem
) 0.2) as quantum yield standard.61 Experimental uncertainties are
as follows: absorption maxima, 1 nm; emission maxima, 4 nm;
molar absorption values, 10%; luminescence lifetimes, 10%;
luminescence quantum yields, 15%.
5.4. Computational Methods. All calculations were carried out
at the DFT level using the development version of Gaussian.62
Unless otherwise specified, a hybrid Hartree-Fock/density func-
tional model, referred to as PBE0, was used.63 The PBE0 was
obtained by casting the PBE exchange and correlation functional64
in a hybrid DFT/HF scheme, where the HF:DFT exchange ratio is
fixed a priori to 1:3.
For the structural optimizations and calculation of the electronic
properties, all atoms were described by a double-ꢀ quality (LANL2)
basis set.65 The molecular structure of each compound was fully
optimized.
Optical transitions were computed using the time-dependent DFT
approach at the same level of theory. For clarity, only computed
transitions with non-negligible oscillator strength (f g 0.02) are
reported in the tables. In this paper only singlet-singlet (i.e., spin-
allowed) transitions are discussed. The transitions were computed
at least down to 200 nm for all the systems studied. The polarizable
continuum model (PCM) of Tomasi and co-workers66 was applied
as an implicit solvation model to simulate solvent effects. More
specifically, we used the conductor-like PCM model as implemented
in Gaussian code (CPCM). Acetonitrile was considered as the
solvent.
In order to simulate fluorescence, the first singlet excited state
was relaxed at the CIS level for the branched compounds and at
the TD/DFT PBE0 level67 for the fused analogues.68 Fluorescence
energies were then computed at the TD-DFT level (PBE0) on such
optimized structures. Phosphorescence from the triplet state was
computed by a ∆SCF approach.
Acknowledgment. I.C., C.A., J.F., F.T., and PPL are grateful
to the French National Agency for Research (ANR) “programme
blanc” (NEXUS project; No. BLAN07-1_196405). S.C., F.N., and
F.P. also acknowledge the University of Messina and PRIN projects
for funding. Finally, Dr. V. Marvaud is acknowledged for fruitful
discussions and anonymous referees for their constructive suggestions.
Supporting Information Available: Complete refs 12b and
62; experimental details regarding the syntheses and character-
izations of new compounds and precursors including 1H NMR
(500 MHz) and 13C NMR (126 MHz) spectra as well as ESI-
mass spectra; selected cyclic and rotating disk electrode volta-
mmograms; photobiscyclization control experiments; represen-
tations of frontier MOs of various EPs; experimental and
calculated (TD-DFT) electronic absorption data for EPs as well
as computed fluorescence and phosphorescence energies. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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