Expanded pyridiniums: Bis-cyclization of branched pyridiniums into their fused polycyclic and positively charged derivatives-assessing the impact of pericondensation on structural, electrochemical, electronic, and photophysical features

FULL PAPER

DOI: 10.1002/chem.201000504

Expanded Pyridiniums: Bis-cyclization of Branched Pyridiniums into Their

Fused Polycyclic and Positively Charged Derivatives—Assessing the Impact

of Pericondensation on Structural, Electrochemical, Electronic, and

Photophysical Features

Jꢀrꢁme Fortage,^[a]Fabien Tuyꢂras,^[a]Philippe Ochsenbein,^[b]Fausto Puntoriero,^[c]

Francesco Nastasi,^[c]Sebastiano Campagna,*^[c]Sophie Griveau,^[d]Fethi Bedioui,^[d]

Ilaria Ciofini,^[e]and Philippe P. Lainꢀ*^[a]

Abstract: This study evaluates the

impact of the extension of the p-conju-

gated system of pyridiniums on their

various properties. The molecular scaf-

fold of aryl-substituted expanded pyri-

diniums (referred to as branched spe-

cies) can be photochemically bis-cy-

clized into the corresponding fused

polycyclic derivatives (referred to as

pericondensed species). The represen-

1,2,3-triphenylbenzo[h]phenanthro-

[9,10,1-def]isoquinolinium (2^Mef). Com-

bined solid-state X-ray crystallography

and solution NMR experiments

showed that stacking interactions are

barely efficient when the pericon-

densed pyridiniums are not appropri-

ately substituted. The electrochemical

study revealed that the first reduction

process of all the expanded pyridini-

ums occurs at around À1 V vs. SCE,

which indicates that the lowest unoccu-

pied molecular orbital (LUMO) re-

mains essentially localized on the pyri-

dinium core regardless of periconden-

sation. In contrast, the electronic and

photophysical properties are signifi-

cantly affected on going from branched

to pericondensed pyridiniums. Typical-

ly, the number of absorption bands in-

creases with extended activity towards

the visible region (down to ca. 450 nm

in MeCN), whereas emission quantum

yields are increased by three orders of

magnitude (at ca. 0.25 on average). A

relationship is established between the

observed differential impact of the per-

icondensation and the importance of

the localized LUMO on the properties

considered: predominant for the first

reduction process compared with sec-

ondary for the optical and photophysi-

cal properties.

tative

1,2,4,6-tetraphenylpyridinium

(1^H) and 1,2,3,5,6-pentaphenyl-4-(p-tol-

yl)pyridinium (2^Me) tetra- and hexa-

branched pyridiniums are herein com-

pared with their corresponding peri-

condensed derivatives, the fully fused

Keywords: electrochemistry

·

9-phenylbenzoACHTUNGTRENNUNG[1,2]quinolizino[3,4,5,6-

fused-ring systems · luminescence ·

pyridiniums · star-shaped molecules

def]phenanthridinium (1^Hf) and the

hitherto unknown hemifused 9-methyl-

[a] Dr. J. Fortage, Dr. F. Tuyꢀras, Dr. P. P. Lainꢁ

Laboratoire de Chimie et Biochimie Pharmacologiques

et Toxicologiques

[d] Dr. S. Griveau, Dr. F. Bedioui

Laboratoire de Pharmacologie Chimique et Gꢁnꢁtique et d’Imagerie

(CNRS UMR-8151 and INSERM U-1022)

(CNRS UMR-8601), Universitꢁ Paris Descartes

45, rue des Saints Pꢀres, 75270 Paris Cedex 06 (France)

Fax : (+33)142-868-387

Universitꢁ Paris Descartes

ꢃcole Nationale Supꢁrieure de Chimie de Paris – Chimie ParisTech

11, rue Pierre et Marie Curie, 75231 Paris Cedex 05 (France)

E-mail: philippe.laine@parisdescartes.fr

[e] Dr. I. Ciofini

[b] Dr. P. Ochsenbein

Laboratoire d’ꢃlectrochimie, Chimie des Interfaces

et Modꢁlisation pour l’ꢃnergie

LECIME (CNRS UMR-7575), ꢃcole Nationale Supꢁrieure de Chimie

de Paris – Chimie ParisTech, 11, rue Pierre et Marie Curie

75231 Paris Cedex 05 (France)

Laboratoire de Cristallographie, Sanofi-Aventis Recherche

371 rue du Professeur Blayac, 34184 Montpellier Cedex 04 (France)

[c] Dr. F. Puntoriero, Dr. F. Nastasi, Prof. Dr. S. Campagna

Dipartimento di Chimica Inorganica

Chimica Analitica e Chimica Fisica

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/chem.201000504. Details of the synthe-

sis and characterization of new compounds and precursors including

¹H (500 MHz) and ¹³C NMR (126 MHz) spectra as well as ESI-mass

spectra, ORTEP drawings of various X-ray structures, selected cyclic

and square wave voltammograms, study of the impact of concentration

on the electrochemical behavior of 1^Hand 1^Hf, electronic absorption

spectra of pyrylium precursors.

and Centro Interuniversitario per la Conversione

Chimica dell’Energia Solare

Universitꢂ di Messina, Via Sperone 31, 98166 Messina (Italy)

Fax : (+39)090-393756

E-mail: campagna@unime.it

Chem. Eur. J. 2010, 16, 11047 – 11063

ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

11047

Introduction

approach remains underexploited for pyridiniums,^[37]wheth-

er for electrochemical,^[19]electronic, or photophysical prop-

erties.

In the framework of our research devoted to the con-

trolled formation of charge-separated states within well-de-

fined photosensitizer–acceptor covalent dyad assemblies, we

have developed monocationic tetraarylpyridinium (“TP⁺”)

groups (1^R2in Scheme 1) as the electron-acceptor compo-

The present work assesses the extent to which pericondensa-

tion^[1]of the branched molecular scaffold of aryl-substituted

pyridiniums can improve^[2]not only their electrochemical

features as electrophores^[3]but also their electronic and pho-

tophysical properties as chromophores and luminophores,

respectively.

In their various forms, pyridinium salts are ubiquitous,

whether taking part in natural processes, as in the case of

the NAD⁺/NADH cofactor,^[4]or serving as functional ele-

ments in the multifaceted fields of chemistry.^[5]Beyond their

widespread use as a result of their cationic charge, pyridini-

ums are essentially valued 1) for their electron-withdrawing

properties (“pull” effect) as part of chromophores (espe-

cially solvatochromic dyes)^[6,7]and NLO-phores (push–pull

assemblies)^[8]and 2) for their electron-accepting (redox)

properties as electrophoric subunits.^[9–11]However, with the

notable exceptions of zwitterionic pyridinium N-phenoxide

dyes (Reichardtꢅs salts)^[6a]and other heterocyclic mesomeric

betaines,^[12]the electronic and redox^[13]properties of pyridi-

niums themselves are rarely finely tuned with substituents.

With regard to the electrochemical properties of pyridini-

ums, the most popular strategy for their enhancement in-

volves arranging their oligomers and especially dimers in

close covalent assemblies to take advantage of their mutual

electronic and electrostatic influences.^[14–18]Fine-tuning of

the electrochemical properties of the dimers can be ach-

ieved by varying the following parameters:^[18a,19]1) The in-

terannular dihedral angle (that is, the degree of p delocali-

zation),^[20]2) the connection scheme of the pyridinium rings,

3) the nature of the N_pyridiniosubstituent (alkyl or aryl),^[14]

and 4) the nature of the intervening linker (if any),^[21]which

can even be switchable.^[22]The same approaches also hold

for higher oligomers, even incorporating intervening

spacers^[19]like star-shaped tris-^[23]and hexakis(p-N-alkylpyri-

dinium)benzene^[24]or other discrete entities like linear poly-

pyridinium wires,^[25,26]polyviologen dendrimers^[27]and rod-

like (1D) polymers.^[28,29]

Interestingly, another strategy exists for manipulating the

redox features of certain classes of molecules and polymers.

This approach is aimed at promoting extended p systems to

enhance p delocalization.^[30]Typically, molecules like ben-

zene or pyromellitic diimide can be expanded towards hexa-

peri-benzocoronenes and graphenes^[31]for the former and

perylene-, terrylene-, quaterrylene diimides,^[32,33]and higher

extended rylenebis(dicarboximide)s^[34]for the latter. In the

case of polyimide acceptors, it is known that the first reduc-

tion potential becomes more positive (that is, more easily

accessible) as the “core aromatic ring system becomes

larger”.^[33,35,36]Whether the same trend is observed or not

for the pyridinium series deserves verification in the context

of efforts directed towards the improvement^[2]of their effi-

ciency as electrophores. Enhancing p extension and delocali-

zation usually also has an impact on the electronic and pho-

tophysical properties of expanded fused molecules (as chro-

mophores and luminophores, respectively). However, this

Scheme 1. General representation of the tetra-branched pyridiniums

[R⁰ph-TPR¹R²]⁺and the photochemically generated fused [R⁰-fTPR¹-

2

phR²]⁺species. The two expanded pyridiniums investigated here (R⁰, R¹,

R²=H) are labeled as 1^Hand 1^Hf.

nent.^[38]We are now interested in the fused (“f”) derivatives

of these tetraarylpyridiniums (1^R2f in Scheme 1). First re-

ported by Katritzky and co-workers three decades ago,^[39]or-

ganic species of the type 1^R2f have not attracted the atten-

tion deserved. This is surprising as examples of fused poly-

cyclic molecular scaffolds comprised of a centrally posi-

tioned cationic charge are quite rare in the literature.^[37,40,41]

As a first step towards the use of pericondensed species

(1^R2f) as electron and/or energy acceptors within covalent

assemblies, we have undertaken the detailed study of the

simplest representative element of the series, namely the 9-

phenylbenzoACTHNUGRTNEUNG[1,2]quinolizino[3,4,5,6-def]phenanthridinium

(noted 1^Hf in Scheme 1), which was compared with its

parent branched precursor 1,2,4,6-tetraphenylpyridinium

(noted 1^H).

We also synthesized and investigated the simplest repre-

sentative element of the hitherto unknown class of pericon-

densed and hemifused (“hf”) species (i.e., 2^R2f), that is, 9-

methyl-1,2,3-triphenylbenzo[h]phenanthro[9,10,1-def]isoqui-

nolinium (labeled 2^Mef in Scheme 2). This molecule is photo-

chemically derived from hexaaryl-substituted pyridiniums

(“XP⁺” type in Scheme 2) and more precisely from

1,2,3,5,6-pentaphenyl-4-(p-tolyl)pyridinium (noted 2^Mein

Scheme 2).

In this paper we report on the intrinsic properties of these

two classes (branched and fused) of expanded pyridiniums

with a view to optimizing their future use. It is important to

note that in contrast to the tetra-branched pyridiniums

(1^R2),^[42]virtually nothing is known about the electrochemi-

cal, electronic, and photophysical properties of hexa-

branched pyridiniums (2^R2)^[43]and pericondensed (1^R2f and

2

^R2f) expanded pyridiniums. The propensity of fused species

to undergo stacking has also been assessed because aggrega-

tion in solution is likely to complicate the proper determina-

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Chem. Eur. J. 2010, 16, 11047 – 11063

Expanded Pyridiniums

FULL PAPER

À

Aryl aryl bond formation within branched pyridiniums

was performed photochemically (Scheme 1 and 2) following

the procedure originally perfected by Dorofeenko^[50]and

Katritzky^[39]and their co-workers, who were the first to syn-

thesize 1^Hf in the late seventies along with numerous other

[R⁰-fTPR¹-phR²]⁺derivatives (R⁰, R¹, and R²in Scheme 1).

2

Basically the rings were fused by a photochemical cyclode-

hydrogenation process rationalized by Mallory^[51]and

Katz^[52]and their co-workers for the photocyclization of stil-

benes into fused phenanthrene derivatives. This photocon-

version requires the presence of an oxidizing agent such as

molecular dioxygen (present case) or iodine (I₂). There also

exists a purely chemical route to fused polycyclic aromatic

hydrocarbons (PAH) deprived of positively charged heter-

oatoms, which essentially relies on the so-called Scholl reac-

tions popularized by Mꢆllen and co-workers^[53]for the syn-

thesis of benchmark hexa-peri-hexabenzocoronene (HBC)

derivatives.^[54]The widely used oxidative version of this

chemical approach was found to be ineffective in our hands.

For example, among other trials, we treated reference spe-

cies 1^Hand 2^Mef (to further fuse the molecular scaffold) ac-

cording to the well-established procedure based on the use

of anhydrous FeCl₃in a mixture of CH₃NO₂and CH₂Cl₂at

room temperature,^[55]but we finally only recovered the start-

ing material with exchanged counteranions (see the crystal

Scheme 2. General representation of the hexa-branched pyridiniums

ACTHNUTRGNEUNG

[R⁰ph-XPR¹R³R²]⁺along with the hemifused derivatives [R⁰ph(R¹ph)₂-

2

hfXPR³R²]⁺. The four expanded pyridiniums investigated here (R⁰, R¹,

2

R³=H and R²=H, Me) are labeled as follows: 2^H, 2^Hf and 2^Me, 2^Mef.

Compounds 2^Me, 2^Mef, and 2^Hf are new species.

tion of the genuine molecular properties of expanded pyridi-

niums beyond the potential interest of this phenomenon for

the construction of self-assembled hierarchical molecular

materials.^[44,45]

Here we show that pericondensed pyridiniums are not

better electrophores than the parent branched species. From

a more general viewpoint, depending on whether a given

molecular property better fits a delocalized or localized de-

scription, extending the p system of a pyridinium by peri-

condensing its aryl branches will naturally impact or not this

property. We demonstrate that the presence of the cationic

heteroatom (i.e., the charged iminio site) imposes the reac-

tivity typical of the pyridinium ring as a prevailing pattern,

that is, a localized description as far as the electrochemistry

is concerned, regardless of the degree of p extension. This is

not the case for the electronic and photophysical features.

In addition, the hitherto unknown properties of hexa-

branched pyridinium (2^R2) are disclosed.

ACTHNUTRGNEUNG

structures of [2^Mef](FeCl₄) and [2^Mef]Cl in the Supporting In-

formation). The complexity of these reactions based on the

combined use of Lewis and/or Brønsted acids, oxidants (or

reducing agents), and so forth has been highlighted in a few

theoretical studies.^[56,57]

Note, when adding two aryl groups at the meta positions

of the pyridinium core, that is, going from tetra-branched

(1^R2, Scheme 1) to hexa-branched (2^R2, Scheme 2), the site

of photoinduced cyclodehydrogenation is shifted from the

N_pyridinio-aryl group towards the tolyl terminus at the 4-posi-

tion of the pyridinium ring. Although this finding is, at first

glance, in contradiction with the conclusions of the studies

of Katritzky and co-workers, which suggest that electron-do-

nating substituents disfavor (or deactivate) photochemical

cyclodehydrogenation,^[39]it is, however, in line with another

particular observation by the same author that “where the

possibility exists of alternative cyc1izations to a phenyl or a

p-tolyl group, the latter seems to be preferred”.^[39b]This is

the case for 2^Mef, which is by far the main photoproduct as

Results and Discussion

Synthesis: N_pyridinio-Aryl-substituted (R⁰-ph) polyarylpyridini-

ums, including the subclasses of tetraaryl- ([R⁰ph-TPR¹₂R²]⁺)

and hexaaryl- ([R⁰ph-XPR¹R³R²]⁺) expanded pyridiniums

2

(1^R2and 2^R2in Schemes 1 and 2), were obtained by treating

conveniently substituted arylamine (R⁰ph-NH₂) with the ap-

propriately decorated triaryl- and pentaaryl-substituted

pyrylium salts [TPyrR¹R²]⁺and [PPyrR¹R³R²]⁺, respec-

1

judged from the H NMR spectrum of the reactionꢅs crude

(see Figure S-VIIa in the Supporting Information). Note

that here the methyl substituent is first used as an internal

2

tively (see the Supporting Information). Procedures for the

synthesis of the pivotal pyrylium precursors in general are

1

probe for H NMR characterization. A control experiment

well documented.^[46]Triarylpyryliums ([TPyrR¹R²]⁺) were

performed on hexaphenylpyridinium 2^H(i.e., the hexa-

branched pyridinium deprived of methyl as the R²substitu-

ent) also yielded the corresponding hemifused species 2^Hf as

the major, and even only, identified photoproduct (see the

2

readily synthesized in one step from the appropriately sub-

stituted benzophenone (R¹) and benzaldehyde (R²) in the

presence of phosphorus oxychloride. The pentaaryl salt

[PPyrR¹R³R²]⁺required a two-step process: Synthesis of

1

informative H NMR (Figure S-IXa) and mass spectra (Fig-

2

the diketone precursor^[47]followed by intramolecular dehy-

dration. The latter dehydrogenative cyclization was per-

formed in the presence of triphenylcarbenium as hydride ab-

stractor (see the Supporting Information).^[48,49]

ure S-XXIg) of the photoreaction crude in the Supporting

Information). Thus, the contribution of the methyl as a

weak inductive electron donor is ruled out. Also note that

ring fusion does not proceed for all of the six pendant aryl

Chem. Eur. J. 2010, 16, 11047 – 11063

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S. Campagna, P. P. Lainꢁ, et al.

À

groups, but stops after two C C bonds have been formed

Ground-state structural characterization—insights from a

solid-state study (X-ray analysis): The ORTEP diagrams in

Figures 1 and 2 show the salient structural features of the

two types of pericondensed pyridiniums 1^R2f and 2^R2f.

around the aryl-R²terminus (whether R²is Me or H,

Scheme 2). In the following, we refer to these partly fused

products as hemifused species 2^R2f. Incomplete periconden-

sation is rationalized in terms of competing processes of ul-

trafast intramolecular energy transfers and cyclodehydroge-

nation on the one hand and existing intramolecular con-

straints in the hemifused species on the other (see General

Comments below).

Crystals of [1^Hf]

solvated structure, [1^Hf]

AHCTUNGTRENNUNG

ACHTUNGTRENNUNG

be termed a pseudopolymorph of the structure recently re-

ported by Mꢆllen and co-workers^[45](compound 1 in the ref-

erence, grown from methanol) for the same, but nonsolvat-

À

ed, BF₄salt. The crystal structure of this nonsolvated BF₄

Expanded pyridiniums as molecular platforms—insights into

the ground-state structural features of pericondensed spe-

cies: The precise determination of the structural features of

molecules is of importance not only when dealing with mol-

ecules in the ground state, for instance, to assess their ability

to self-assemble within supramolecular architectures, but

also when exploring their electrochemical, electronic, and

photophysical properties. Conformational changes following

reduction or excited-state charge or spin-density redistribu-

tion can be huge^[58,59]and must be taken into account for

semi-rigid molecules like those investigated herein. Induced

structural changes can alter the electrochemical behavior of

electrophores, whether the reversibility of redox processes

or the driving force of photoinduced electron transfer

(PET), when the reorganization energy is large. Structural

fluctuations can also appear as vibronic contributions likely

to impact the absorption spectra of the chromophores as

well as significant Stokesꢅ shifts or changes in emission

quantum yields (relative efficiency of nonradiative decay

pathways) subsequent to photoexcitation, which are key fea-

tures of luminophores. These various and inter-related as-

pects are investigated below.

salt (the crystal of 1) consists of “tail-to-tail” stacks of

planar 1^Hf molecules, which are organized as layers.^[45]On

the other hand, the orthorhombic structure of the acetoni-

À

trile-solvated BF₄salt [1^Hf]

G

displays columnar packing made up of 1^Hf molecules

stacked in a cofacial “head-to-head” manner (Figure 1).

This packing closely resembles that of the crystal structure

of the methanol-solvated benzenesulfonate (PhSO₃) salt of

1^Hf, also reported by Mꢆllen and co-workers^[45](compound 2

in the reference). In [1^Hf]

ACTHNUGTRENNUG(BF₄)·CH₃CN, the columnar stacks

are indeed arranged in a herringbone^[60]fashion rather simi-

lar to that found in the PhSO₃structure, albeit the angle

between the stacks is less pronounced in the latter case. In

À

contrast to the PhSO₃salt,^[45]but similarly to the BF₄non-

solvated pseudopolymorph (compound 1),^[45]no bending of

the molecular scaffold is observed in our case in the solid

state for reasons of crystal symmetry. Indeed, the main mo-

À

lecular axis lying along the C C bond connecting the fused

polycyclic fragment of 1^Hf and its phenyl terminus is parallel

to the [001] crystal axis.

To summarize, the structural analysis shows that [1^Hf]-

AHCTUNGTRENNUNG

Figure 1. ORTEP diagrams of [1^Hf]

(BF₄)·CH₃CN with thermal ellipsoids (50% probability). a) Top and b) side views of the 1^Hf molecular unit and c) top

ACHTUNGTRENNUNG

and d) side views of the cofacial “head-to-head” layout of two p-stacked molecules.

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Expanded Pyridiniums

FULL PAPER

used for crystallization (e.g., methanol versus acetonitrile)

and the level of solvation and regardless of the nature of the

À

counteranion (whether the same, here BF₄, or different, for

À

example, PhSO₃). This finding makes questionable the rele-

vance of the recently reported strategy^[45]for controlling the

crystal packing of naked 1^Hf molecules by relying on the

nature of counteranions.

Note also that the interplanar distance (3.39 ꢇ) between

two stacked 1^Hf fused molecules does not significantly vary

on passing from the cofacial “head-to-head” layout to the

cofacial “tail-to-tail”^[45]layout, falling in the usual range for

both the pure, neutral PAHs^[61,62](ca. 3.4–3.5 ꢇ) and the

planar carbeniums of the TOTA type^[41](ca. 3.3–3.4 ꢇ).

Therefore if the coulombic repulsion between adjacent cat-

ionic N_pyridinioatoms (which are 4.249 ꢇ apart)^[63]is operative

within the “head-to-head” columnar stacks, its destabilizing

contribution is more than compensated by stabilizing forces

such as p–p interactions involving six fused rings per mole-

cule (versus the only two rings of pendant phenyl and pyri-

dinium for the “tail-to-tail” dimer) and cation–p interac-

tions^[64](not present in the “tail-to-tail” dimer). This is all

the more the case as the pericondensed pyridiniums are seg-

regated from their fluorinated counteranions (see the Sup-

porting Information) thereby hindering the shielding role of

Figure 2. Selected ORTEP diagrams representing the structures of 2^Mef:

top) 2^Mef molecular unit and associated counteranion (from [2^Mef]

ACHTUNGTRENNUNG

À

and bottom) the head-to-head dimer (from [2^Mef]

ACHTUNGTRENNUNG

BF₄with respect to the abovementioned electrostatic inter-

action.

With regard to the hemifused species 2^Mef we solved three

X-ray structures (all monoclinic) with different counteran-

À

ions (BF₄, FeCl₄, and Cl^À), which allowed us to derive

some general structural trends (Figures 2 and 3 see also the

Supporting Information).

The planarity of the fused core noted for 1^Hf is lost in the

case of the hemifused species 2^Mef, which is strongly distort-

ed. In fact, only slight conformational variations of the three

pendant aryl rings are observed whereas the bending defor-

mation of the fused polycyclic fragment remains virtually

the same for all four characterized 2^Mef motives,^[65]regard-

less of the crystal packing, the nature of the counteranions,

and the presence of co-crystallized solvent molecules

(Figure 3). This finding strongly suggests that the origin of

this deformation is related to intramolecular structural con-

straints, which include the steric hindrance caused by the

dangling rings.

Figure 3. Superimposed molecular backbones of four independent 2^Mef

units issued from the three structures solved with different counteran-

ions: BF₄^À, FeCl₄^À, and Cl^À. Hydrogen atoms and counteranions have

been omitted for clarity.

The sandwich-herringbone^[60]packing of the hemifused

species does not appreciably change as a function of the

nature of the counteranions or the co-crystallized solvent

molecules when present. However, contrary to the case of

Ground-state structural characterization—insights from a

solution NMR study: Concentration measurably impacts on

the NMR spectra of pericondensed species like 1^Hf, as

shown in Figure 4. We therefore investigated the effect of

concentration not only to allow direct comparison of various

pyridiniums without bias, but also to gain insights into possi-

ble preferential interactions (e.g., self-assembling processes)

that could take place, as has already been reported for this

type of molecule.^[40,44]

[1^Hf]

ACHTUNGTRENNUNG(BF₄)·CH₃CN, no columnar p–p stacking was found in

any of the three structures solved, but one-dimensional pro-

todimeric associations instead (Figure 2). In the crystal

structures of both [2^Mef](BF₄)·1.64H₂O and [2^Mef]

ACTHNUGTRENNUGN ACHTUNGTRENNNUG

2

The associated molecules mutually exhibit opposite curva-

tures because they are actually symmetry-related through an

inversion center with their counteranions lying between mo-

lecular assemblies.

At first glance the marked and similar sensitivities of the

protons A3-H, C6-H, C3-H, B3-H, and B4-H towards solu-

tion concentration (Figure 4) is compatible with the impact

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S. Campagna, P. P. Lainꢁ, et al.

sors), which are both essentially

present at high concentrations.

Whether the cationic charge

embedded within fused polycyc-

lic fragments is sufficient to de-

stabilize the p–p stacks (elec-

trostatic repulsion) or the pres-

ence of a ball-shaped counter-

anion in the direct (apical) vi-

cinity of the N_pyridinioatom

merely precludes large aggrega-

tion remains unclear.

To summarize, no massive

self-assembly is directly ob-

served for bare 1^Hf molecules,

contrary to the cases reported

by Mꢆllen and co-workers^[40,44]

with 1^Hf molecular platforms

bearing long alkyl chains as the

N_pyridinio-R⁰substituent (R⁰=

C₆H₁₃and C₁₄H₂₉; Scheme 1).

Beyond the nature of both the

solvent (methanol instead of

acetonitrile) and the counteran-

ions, the presence of these lipo-

philic decorations is crucial for

the formation of supramolec-

ular assemblies. On the other

hand, a poor propensity to par-

ticipate in intermolecular inter-

1

Figure 4. Concentration (c, mm; in CD₃CN) dependence of the H NMR (500 MHz) spectra of [1^Hf]

ACTHNUTRGENUN(G BF₄).

of the formation in solution (at high concentrations) of cofa-

cial “head-to-head” or “tail-to-tail” dimers or oligomer as-

semblies according to the layouts evidenced in the solid-

state, whether in the present work (see Figure 1) or in the

recent report by Mꢆllen and co-workers.^[45]However, from

homonuclear ¹H,¹H NOESY measurements, only intramo-

lecular and no intermolecular through-space nuclear Over-

hauser effects (NOEs) related to existing p-stacked dimers

or oligomers could be detected in the favorable case of con-

centrated (and even saturated) acetonitrile solutions (Fig-

ure S-IVb in the Supporting Information).^[66]Note also that

the signs of the NOE cross-correlations are positive and that

no sign inversion or change in the relative intensities is ob-

served for the whole range of concentrations investigated

(Figures S-IVc and IVd in the Supporting Information). This

finding indicates that if aggregation takes place in solution,

the weight of assemblies should be less than 1000 Da (for a

500 MHz proton Larmor frequency at 300 K),^[67]which cor-

responds to p stacks made up of three 1^Hf molecules at the

maximum. Finally, the absence of variation of line-broaden-

ing with concentration is also consistent with the absence of

significant aggregation in solution. Therefore the marked

concentration dependence of the chemical shifts of certain

protons may be due to the combined effects of some loosely

defined p–p interactions (and slight aggregation) as well as

to some kind of ion-pairing impacting on the interannular

dihedral angle (similar to those observed for unfused precur-

actions in solution facilitates the proper (safe) characteriza-

tion of the intrinsic properties of fused species as model ac-

ceptors (electrophores) and chromophores/luminophores.

With respect to the 2^Mef hemifused species, intramolecular

NOEs involving protons of the terminal methyl and vicinal

protons D3-H of the fused moiety allowed the identity of

the molecule to be confirmed (Figure S-VIId of the Support-

ing Information), in particular, with regard to the site of bis-

cyclization (R²side instead of the R⁰side, as in 1^Hf). As in

the previous case, no intermolecular NOEs could be detect-

1

ed although a sizable dependence of the H NMR pattern

upon concentration was observed (Figure S-XXVI in the

Supporting Information).

Expanded pyridiniums as electrophores—ground-state redox

properties: The main question assessed in this section is

whether or not reduction potentials become more accessible

(anodic shift) upon pericondensation of branched pyridini-

ums. In other words, the aim is to determine whether or not

the LUMOs attached to expanded pyridiniums lie at lower

energies^[37]than in the parent branched molecules. The re-

versibility of the first reduction process(es), which is also a

key parameter for electrophores, is also discussed for each

pyridinium.

The electrochemical properties of the various pyridinium

derivatives (branched and fused) synthesized in this work

are presented in Table 1.^[68,69]

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Chem. Eur. J. 2010, 16, 11047 – 11063

Expanded Pyridiniums

FULL PAPER

ic reductions of the parent branched precursors 1^Hand 2^Me

more or less merge at around À1 V. On the basis of the

analogous electrochemical behavior of the two couples (1^R2/

Table 1. Electrochemical data and assignment of redox processes for the

examined pyridiniums in dry acetonitrile with 0.1m TBAPF₆at a plati-

num electrode.^[a]

Electrophore^+/0

Electrophore^0/À

1

^R2f) and (2^R2/2^R2f), it is inferred that the two reduction pro-

Entry

1

E ₌[V]

n

E ₌[V]

n

cesses recorded for 2^Mef are most likely monoelectronic

(Table 1). The number of electrons (n=1) was confirmed by

using data from the rotating disc electrode voltammograms

and their logarithmic treatments, assuming that the reduc-

tion process is governed by the Nernst equation.^[70]

2

1^H

À1.00; rev.

1

À1.16; rev.

1

1^Hf

À1.00; rev.

À1.77; rev.

2^Me

À1.17; quasi-irrev.

1^[b]

1^[c]

À1.17; quasi-irrev.

1^[b]

1^[c]

2

^Mef

À1.04 (E_pc); irrev.

À1.75; irrev.

1

[a] E ₌(vs. SCE) is calculated as (E_pa+E_pc)/2 in which E_paand E_pcare the

2

With respect to the different degrees of electrochemical

reversibility observed for the first reduction process(es) of

various species, one should make a basic distinction between

the behavior of branched and pericondensed electrophores.

Regarding the branched species, distinct electrochemical

reversibilities are observed depending on whether the pyri-

dinium core is tetra-branched 1^R2(TP⁺type, see Scheme 1)

or hexa-branched 2^R2(XP⁺type, see Scheme 2), as illustrat-

ed in Figure 5. The only significant difference between these

two types of molecules resides in the level of steric encum-

brance at the periphery of the electroactive pyridinium core.

In fact, the two monoelectron reductions are energetically

anodic and cathodic peak potentials, respectively, measured by cyclic vol-

tammetry at 0.1 Vs^À1. n is the number of electrons involved in the redox

process. [b] Determined with respect to the monoelectronic oxidation of

ferrocene (Fc) in the reference complex Fc-ph-2^Me(Figure 5). The elec-

trophore^+/0cathodic wave merges with the electrophore^0/À(bi-electronic

1+1 electron unresolved wave). [c] For determination of this n value, see

the text.

The number of electrons (n) involved in the various redox

processes was essentially derived from the electrochemical

study of the corresponding inorganic dyads made up of a co-

valently linked coordination compound (as the one-electron

oxidizable internal reference)

and the pyridinium moiety

under consideration. Typically,

Ru^IIand Os^IIbis-terpyridine

complexes were used for the

1^H[9,38]and 1^Hf electrophores

(Figure S-XXIIIa-c in the Sup-

porting Information) and ferro-

cenyl-based dyads (Figure 5),

specifically synthesized for the

purpose, were used for the

tetra-

methyl-substituted

and

hexa-branched

electro-

phores 1^Meand 2^Me. Rotating

disc electrode experiments un-

ambiguously showed that two

electrons are involved in the

quasi-irreversible

reduction

process of 2^Me, whereas the two

reduction processes observed at

À1.00 and À1.16 V were con-

firmed as monoelectronic in the

case of 1^Me(Figure 5). With

regard to 2^Mef, its electrochemi-

cal behavior was found to par-

allel that of 1^Hf (except for the

reversibility), as judged from a

comparison of the outcomes of

cyclic voltammetry (see Table 1

and the Supporting Informa-

tion). Indeed, two successive re-

duction processes occur at

roughly the same potentials (ca.

À1 and À1.75 V, Table 1) in the

two fused species 1^Hf and 2^Mef

Figure 5. Determining the number of electrons (n) involved in the first reduction processes of branched pyridi-

niums and assessing the related electrochemical reversibilities. Left) Dyad Fc-ph-1^Me(c=2.5ꢈ10^À4m): a) Cyclic

voltammetry (v=0.1 Vs^À1

)

and b) rotating disc electrode voltammetry (v=0.01 Vs^À1; rotation rate: 2ꢈ

10³rpm). Right) Dyad Fc-ph-2^Me(c=2.8ꢈ10^À4m): c) Cyclic voltammetry (v=0.1 Vs^À1) and d) rotating disc

whereas the two monoelectron- electrode voltammetry (v=0.01 Vs^À1; rotation rate: 2ꢈ10³rpm).

Chem. Eur. J. 2010, 16, 11047 – 11063

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11053

S. Campagna, P. P. Lainꢁ, et al.

equally demanding in both cases (occurring at ca. À1 V),

but only in the case of [XP]^Àis back transformation (reoxi-

dation) to the starting native XP⁺impeded. Changing the

scan rate [range: 0.025–1 Vs^À1] impacts sizably on the

degree of irreversibility of the process: Reoxidation is found

to be more effective at high scan rate. As far as we know,

no coupled chemical reaction is expected to occur for the

XP⁺molecule subsequent to reduction. Given that the reox-

idation process requires the retrieval of electrons from the

reduced pyridinium core, its shielding by the six pendant

aryl groups through a probable structural reorganization

might hinder its accessibility to the electrode surface. There-

fore a huge structural reorganization resulting from the rear-

rangement of bonds within the reduced pyridinium could be

the limiting factor partly circumvented when the scan rate is

high. To clarify the nature of the possible redox-induced

structural distortion of the molecular backbone of [2^Me]^À, ab

initio calculations were undertaken. Structural optimization

was carried out in the absence of constraints and by taking

into account solvent effects (namely acetonitrile) for the

hexa-branched molecule in its native monocationic [2^H]⁺,

neutral monoreduced, [2^H]⁰, and negatively charged two-

electron reduced [2^H]^Àstates. It was found that the propel-

ler-shaped geometry is retained for the one-electron-re-

duced species. The structural relaxation is apparent in the

overall flattening of the molecular backbone resulting from

the planarization of the branches with respect to the pyridi-

nium core (the interannular twist angles decrease on aver-

age from 70.4 to 65.98 upon reduction).^[71]Addition of a

second electron induces a dramatic boat-shaped distortion

of the pyridinium core correlated with pyramidalization of

its nitrogen atom (see Figure 6). This distorted conformation

is calculated to be more stable than that of the bi-reduced

species frozen in the flattened propeller-shaped geometry by

around 1.43 eV (33.06 kcalmol^À1). This unexpected skeletal

distortion accounts for the lack of reversibility observed for

the reduction of 2^Me

.

On the other hand, the difference in the electrochemical

reversibility noted for the first reduction of the expanded

pyridiniums 1^Hf and 2^Mef is instead ascribed to differences in

the hindrance experienced by the carbon atom para to the

N_pyridinioatom (that is, the 4-position of the pyridinium motif

A, noted A4 in Figure 4 (inset); see also the Supporting In-

formation). Thus, when exposed, the site A4 (and to a lesser

extent the sites A2 and A6), with its recognized peculiar

(chemical) reactivity,^[37]is potentially subject to nucleophilic

attack (pseudobase reactivity yielding addition prod-

ucts),^[40,72–75]reductive dimerization,^[17b,76]or pimerization.^[69b]

Appending bulky substituents capable of hindering the ap-

proach of a potential reactant at the site A4 is a widely used

Figure 6. Computed geometries for a) native [2^H]⁺, b) flattened monoreduced [2^H]⁰, and c) top and d) side views of the distorted bi-reduced [2^H]^À.

11054

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Chem. Eur. J. 2010, 16, 11047 – 11063

Expanded Pyridiniums

FULL PAPER

strategy for overcoming these detrimental effects. Aryl

groups are typically used for these protective substitu-

ents,^[77,78]as illustrated by the reversible first reduction of

1^H; bare N-methylpyridinium as a reference shows an irre-

versible reduction.^[17]This behavior is also reminiscent of

that of acridinium: Its reduction is normally an irreversible

process,^[79,80]which becomes reversible for the N-methyl-9-

phenylacridinium^[80b]bearing a pendant phenyl group at the

A4 position of the central pyridinium motif, as expected.^[81]

Thus, simple inspection of the molecular structures

(Schemes 1 and 2) allows us to infer that, amongst the fused

species studied, 2^Mef should be the most prone to undergo

such

a chemical reaction, as experimentally verified

(Table 1).

Note here that accounting for the electrochemical proper-

ties of the whole series of electrophores, whether

“branched” or “fused” (including acridinium derivatives),

basically involves a discussion of the properties of the pyri-

dinium motif as the key electroactive fragment. When com-

paring the electron-accepting abilities of “branched” and

“fused” expanded pyridiniums, it is found that their first re-

duction potential does not substantially change upon peri-

condensation of the branched molecular scaffold. More pre-

cisely, these reduction potentials do not become more posi-

tive (anodic shift) when the “core aromatic ring system be-

comes larger”, as in the case of the benchmark perylene dii-

mide family.^[35,36]The first reduction potential of all four 1^H,

1^Hf, 2^Me, and 2^Mef electron acceptors lies at roughly À1 V (vs.

SCE). Differences between the “branched” and “pericon-

densed” pyridiniums emerge for the second reduction pro-

cess. This second monoelectronic reduction occurs at a po-

tential very close to the first monoelectronic reduction for

both “branched” species (within ca. 0.2 V, see Table 1),

whereas it occurs at a significantly higher potential for

“fused” species (at ca. À1.75 V vs. SCE for both 1^Hf and

Figure 7. Typical electronic spectra of the native and fused derivatives

ACTHNUTRGNEUNG

(acetonitrile solutions). Top) Native [1^H](BF₄) (dotted) and fused [1^Hf]-

A

]ACTHNGUETRN(NUG BF₄) (dotted) and fused

AHCTUNGTRENNUNG

The native and fused derivatives exhibit significantly dif-

ferent electronic properties. The [R⁰ph-TPR¹R²]⁺branched

2

acceptors (see Scheme 1) usually display only one main ab-

sorption band at about 310 nm^[10a,38]whereas the correspond-

ing fused acceptors have much more complicated featur-

es^[39b,72]that moreover extend towards the visible domain

(1^Hf is bright yellow whereas 1^His colorless). Of note, also,

is the marked increase in the molar extinction coefficients

of the absorption bands upon pericondensation. The same

trend is observed on going from the hexa-branched 2^Me(dis-

playing an ill-defined and featureless absorption spectrum)

to the hemifused 2^Mef (Figure 7). With regard to the absorp-

tion spectrum of branched species of the XP⁺type, it is

worth noting not only the absence of the strong absorption

band at around 310 nm, but also the fact that the two chro-

mophores (TP⁺and XP⁺) show similar integrated absorp-

tions in the investigated spectral window whereas one could

reasonably expect a significantly larger integrated absorp-

tion for the hexaaryl chromophore (XP⁺type) compared

with the tetraphenyl one (TP⁺type). This observation can

be explained as follows: Previous detailed studies of the

electronic properties of TP⁺-like chromophores showed that

the strong absorption at around 310 nm is basically charge-

transfer (CT) in nature,^[42]comprising two p–p* intramolec-

ular CT components between the electron-donating periph-

2

^Mef). This finding strongly suggests that, contrary to the

first reduction, the active molecular orbitals (MOs) should

show a pronounced delocalized nature. In other words, for

the first reduction process, extending the p system does not

lead to an enhancement of the p delocalization, whereas it

does for the second reduction. Ongoing theoretical investi-

gations will tell us whether the primary dominant pattern of

the pyridinium (electronic) entity is lost after the first reduc-

tion.

Expanded pyridiniums as chromophores and lumino-

phores—insights into the electronic features of the ground

and excited states: Pyridiniums are usually considered for

their electron-withdrawing and -accepting properties but

rarely for their electronic or photophysical properties. We

examine here the extent to which pericondensation can pro-

vide pyridiniums with appealing chromophoric and lumino-

phoric features likely to renew their interest and use.

Electronic absorption spectra: The absorption spectra of the

branched and corresponding fused chromophores are given

in Figure 7 and the data are collected in Table 2.

Chem. Eur. J. 2010, 16, 11047 – 11063

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11055

S. Campagna, P. P. Lainꢁ, et al.

Table 2. Electronic absorption data for expanded pyridiniums (branched and pericondensed).^[a]

solution at room temperature

l_max[nm] (e [10⁴m^À1cm^À1])

(RT) and in

a butyronitrile

1^H

242.5sh

(1.40)

239.0sh 278.5sh

(4.54)

258.0

(1.88)

264.5

(3.57)

308.5

(3.83)

302.0

(6.78)

304.0

(1.10)

317.0

(4.44)

rigid matrix at 77 K (low tem-

perature, LT; see Table 3 and

Figure 8).

Because of the energies of

the excited states, the lifetimes,

the room-temperature quantum

yields, and the spectral shapes,

1^Hf 226.5 (5.12)

2^Me222.5sh (5.19)

327.0sh

(2.28)

348.0

(2.41)

340.5sh

(0.67)

359.0sh 383.0sh

(2.00) (0.40)

407.5

(0.82)

429.0

(1.00)

(4.13)

2

^Mef 227.0 (8.46)

280.5

(3.37)

352.0sh 370.0sh

(1.27) (0.73)

402.5

(0.71)

423.5

(0.98)

the

luminescent

properties

[a] Acetonitrile solutions; sh: shoulder; band maxima are given as a function of their energy, not on the basis

of their hypothetical nature or affiliation.

(fluorescence at room tempera-

ture, both fluorescence and

phosphorescence at 77 K) are

eral 2,6-aryls and the 4-aryl on the one hand, and the pyridi-

nium core on the other.^[82]Therefore, on passing from TP⁺

to the burdened propeller-shaped XP⁺chromophore in

which enforced askew-stacked aryl rings are tilted almost

perpendicularly out of the plane of the central pyridinium

platform, it is expected that both the molar absorptivity and

the energy of the merged intramolecular CT bands will be

heavily affected. This is indeed the case. Similar changes

have been reported for the electronic absorption of the two

rotamers of the N-methyl-2,4,6-triphenylpyridinium salt, the

phenyls at the 2,6 positions being conformationally locked

almost coplanar or perpendicular to the pyridinium ring: On

going from the former to the latter, the absorbance of the

CT band is significantly reduced and also blueshifted (ca.

1500 cm^À1).^[82b]

attributed to excited states with a dominant p–p* nature.^[84]

Comparison of the branched and pericondensed expanded

pyridiniums: With respect to the fluorescence emission at

Table 3. Photophysical properties (Fl.: fluorescence; Ph.: phosphorescence;

l/nm) of the luminophores at both room temperature (RT) in deoxygenated

acetonitrile fluid solution and 77 K (LT) in a butyronitrile rigid matrix.^[a]

Entry

Luminescence, RT

Luminescence, LT

t_Fll_Pht_Ph

[nm] [ns] [nm] [ms]

l_Fl

F_Fl

t_Flk_r

k_nr

[s^À1

l_Fl

[nm]

[ns]

A

]

E

]

1^H[b]

1^Hf

480

465

455

479

<10^À43.0 <3.33ꢈ10⁴>3.33ꢈ10⁸390

5.1 450 107

0.35 7.0

0.48 8.0

0.13 6.7

5.00ꢈ10⁷

6.00ꢈ10⁷

1.94ꢈ10⁷

9.28ꢈ10⁷439 10.0 494 4500

2^Me

6.50ꢈ10⁷395

1.30ꢈ10⁸430

5.0 455 2500

7.7 498 3945

2

^Mef

[a] No phosphorescence emission was detected at room temperature for any of

the species. [b] From ref. [38].

With respect to the fused species, a salient feature shared

by 1^Hf and 2^Mef is the two-band pattern present at the red

edge of their absorption spectra, which is also found in the

spectra of other members of the family.^[39b]The energy dif-

ference between these two bands remains virtually the same

for both 1^Hf and 2^Mef (ca. 0.152 eV/1230 cm^À1) as well as for

room temperature, the values of the Stokesꢅ shifts (SS) are

very different depending on the degree of fusion in the mo-

lecular scaffolds, the largest (and even in the range of un-

usually large Stokesꢅ shifts, 5000–15000 cm^À1)^[85]being for

the branched luminophores and especially for the less

crowded of them, that is, 1^H(SS=11580 cm^À1, which com-

pares with ca. 7390 cm^À1for 2^Me), as expected. The excited-

state structural relaxation responsible for this phenomenon

is planarization,^[86]which involves aryl rings connected at

both the 2,6+4^[82]and the N^[42,87]pyridinio positions. Thus,

upon photoinduced excitation, the whole molecular back-

bone flattens according to adiabatic torsional motions

around interannular bonds.^[42,82,87]Regarding the SS values

of the pericondensed pyridiniums, which are small, they

follow the expected trend with the larger value for the fused

pyridinium bearing three pendant phenyl rings (2^Mef: SS

ꢀ2740 cm^À1) and the smaller for the single phenyl-substitut-

ed pyridinium (1^Hf: SSꢀ1800 cm^À1).

all the [R⁰-fTPR¹-phR²]⁺class of chromophores^[39b]regard-

2

less of the nature of the solvent, hence the inference of a

pronounced vibronic contribution. Such a vibronic effect has

already been characterized in the case of 1^Hf^[83]and is also

confirmed at the photophysical level.

Thus, the impact on the electronic features of pyridiniums

that we observe upon pericondensation is as follows: On

going from the branched to the pericondensed pyridiniums,

there is a marked increase in both the number of absorption

bands around the red edge of their electronic absorption

spectra and their extinction coefficients. These observations

are indicative of a densification of molecular orbitals (MOs)

around the HOMO–LUMO gap, which is also reduced.

Given that electrochemistry tells us that the energy of the

LUMO remains virtually unchanged upon ring fusion (the

first reduction potential is the same in corresponding

branched and fused pyridiniums), the reduction of the

HOMO–LUMO gap should result from an increased energy

of the HOMO within pericondensed chromophores relative

to the branched parents.

Insights into the deactivation pathway(s) can also be

gained from the analysis of the variation of the blueshifts

(BS) of the fluorescence emission energy observed on pass-

ing from room-temperature fluid solutions to low-tempera-

ture rigid matrices. The blueshift reflects the energy destabi-

lization of the (emissive) lowest-lying singlet state when sol-

vent reorganization, which normally accompanies excited-

state intramolecular charge redistribution and subsequent

Luminescence properties: All the expanded pyridiniums in-

vestigated here are photoluminescent, both in acetonitrile

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Expanded Pyridiniums

FULL PAPER

orders of magnitude (i.e., the k_rvalue of the fused species is

1.5ꢈ10³larger than that of the branched parent, see

Table 3). It follows that despite their chemical parentage,

these two luminophores are basically different in nature. In

other words, from the viewpoint of their photophysical prop-

erties, there is no obvious parentage between the branched

and chemically related pericondensed pyridiniums.^[88]

Comparing tetra- and hexa-branched expanded pyridiniums:

Of note is the intriguing photophysical behavior of the pro-

peller-shaped pyridinium luminophore 2^Me. The connection

between the 1^Hand 2^Meexpanded pyridiniums, whether con-

sidering the chemical, electrochemical (Table 1), electronic

(Table 2), or photophysical (i.e., low-temperature fluores-

cence and phosphorescence emission energies in Table 3)

properties, is not a matter of debate: They are both

branched pyridiniums. However, it appears that k_nris re-

duced by a factor of five (at least) on going from the tetra-

to the hexa-branched luminophore, whereas the related

emission quantum yield is increased by a factor of 4.8ꢈ10³

(i.e., the value of k_rfor 2^Meis 1.8ꢈ10³larger than that of 1^H,

see Table 3). This discrepancy can be explained as follows:

The weak fluorescence of 1^Hat room temperature is as-

cribed to excited-state CT between the N-phenyl group and

the pyridinium core.^[42,87,89]Although the two p systems are

geometrically and electronically decoupled in the ground

state,^[42]the CT in the excited-state is accompanied by a

Figure 8. Luminescence spectra of 1^Hf (top) and

2

^Mef (bottom). The

+

À

large torsional motion around the interannular C N bond.

dotted spectra are room-temperature emission spectra in acetonitrile, the

solid spectra are emission spectra recorded in butyronitrile at 77 K in the

steady-state mode, and the dashed-dotted spectra are luminescence spec-

tra recorded in butyronitrile at 77 K with a delay of 10 ms from excitation

pulse (gate time 1 ms). In the latter case, fluorescence is suppressed and

only phosphorescence is evidenced.

As a consequence, the Franck–Condon factor for radiation-

less decay of the excited state increases accordingly, hence

the nonradiatively short-circuited fluorescence emission. In

2

^Me, on the other hand, not only is the overall molecular

backbone greatly rigidified, but more importantly, planariza-

tion of the N-aryl group is largely hindered because of intra-

molecular steric crowding. Similarly to the impact of the en-

forced perpendicular layout of the determining 2,6-aryl rings

on the electronic absorption properties, the steric constrain-

ing of the N-phenyl ring sharply out of the pyridinium plane

heavily affects the energy of the excited-state CT, as reflect-

ed by the blueshifted room-temperature fluorescence emis-

sion (455 nm) relative to that of the more flexible tetra-

branched 1^H(480 nm). This short-wavelength shift is also

translated into a markedly reduced Franck–Condon factor,

structure (here conformation) changes, is impeded (i.e.,

frozen). It is expected that the larger the intramolecular re-

organization (reflected by Stokesꢅ shifts) the larger should

be the blueshift of the emission. In fact, this correlation is

indeed observed because the same trends as for the Stokesꢅ

shifts are found: 1) For branched pyridiniums, the BS value

for 1^H(BS=4810 cm^À1) is larger than that for 2^Me(BS=

3340 cm^À1) and 2) for the pericondensed pyridiniums, the BS

values are smaller than those of the related branched lumi-

nophores and moreover the BS value for 2^Mef (BS=

2380 cm^À1) is larger than that for 1^Hf (BS=1270 cm^À1).

These observations are in line with changes in the emis-

sion quantum yields on passing from 1^Hto 1^Hf, which sug-

gests a contribution of intramolecular torsional motions to

the nonradiative deactivation pathways of the equilibrated

singlet excited state. However, structural relaxation is not

the dominant factor as far as the luminescence properties

are concerned in these compounds, as revealed by the analy-

sis of the relative values of the radiative (k_r) and nonradia-

tive (k_nr) decay rate constants. Indeed, the k_nrrate constant

decreases by a factor of 3.6 on going from 1^Hto 1^Hf, whereas

the emission quantum yield increases by more than three

hence the dramatically improved emission of 2^Me

.

Finally, the vibronic (fine) structures of 1^Hf and 2^Mef iden-

tified in the electronic absorption spectra (Figure 7)^[83]are

further substantiated by the structured emission spectra re-

corded at low temperature (Figure 8). The “two-band” pat-

tern apparent in both the fluorescence and phosphorescence

spectra of the compounds at 77 K is due to vibrational pro-

gressions but not to the different emissions, as is clearly

demonstrated by the same lifetimes measured for these two

vibronic bands. Moreover, the energy gaps between these

two bands derived from low-temperature fluorescence emis-

sion (1^Hf: ca. 1150 cm^À1; 2^Mef: ca. 1175 cm^À1) correlate well

with the energy gaps derived from the absorption spectra

(1230 cm^À1, see above).^[90]

Chem. Eur. J. 2010, 16, 11047 – 11063

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S. Campagna, P. P. Lainꢁ, et al.

To summarize, the photophysical behavior of the pyridini-

ums as luminophores is greatly improved^[2]upon pericon-

densation of their molecular scaffolds by bis-cyclization. The

extension of the p system with a correlative reduction of the

HOMO–LUMO energy gap, already inferred from the elec-

tronic absorption properties, is further confirmed here by

low-temperature fluorescence measurements. The fluores-

cence emission energy is indeed redshifted from 390–395 nm

for the branched pyridiniums to 430–440 nm for the pericon-

densed ones. Beyond the positive impact^[2]on the lumines-

cence of the increased overall molecular rigidity resulting

from pericondensation, it is a matter of fact that the photo-

bis-cyclization of branched pyridiniums gives rise to a basi-

cally new and different type of luminophore. Thus, poor lu-

minophores like N-phenyl-substituted tetra-branched pyridi-

niums are transformed into good luminophores and thereby

are in the top range of 0.1–1 for emission quantum yields.

been shown that ultrafast intramolecular singlet–singlet

energy transfer occurred and affected the photochemical cy-

clodehydrogenation process so that once the first TP⁺-like

moiety is transformed into the fused fTP⁺-like fragment,

further photocyclization of the remaining TP⁺-like sub-

system is inhibited.^[91]The photochemical behavior of photo-

sensitizer–acceptor (P–A) inorganic dyads designed to un-

dergo PET to form charge-separated states provides another

illustration of inhibited photocyclization by competing

energy transfer. For a decade now, we have been studying

these dyads based on Ru^II/Os^IIcomplexes of oligopyridines

as the colored (abs. at ca. 500 nm) P component covalently

linked to 2,4,6-triarylpyridinio groups (TP⁺) as the colorless

(absorption in the UV) electron acceptor A and we never

observed the formation of the bis-cyclized photoproduct

fTP⁺.^[38]

3) Further photocyclization processes are also likely to be

impeded by the formation of sterically hindered high-energy

intermediates that show detrimental structural constraints,

as also observed in the case of chemical cyclization.^[92]In

fact, the molecular backbone of 2^Mef exhibits a marked cur-

vature of its fused part (Figure 3) indicative of intramolecu-

lar structure constraints, which might also appear as con-

strained torsional motions for the pendant phenyl rings at

the 1-, 2-, and 6-positions (i.e., rings B and C; see nomencla-

ture in the Supporting Information).

Also of interest is the peculiar case of the pyridinium 2^Me

,

which becomes highly fluorescent upon peraryl substitution,

another valuable approach for expanding the pyridiniumꢅs

scaffold.

General Comments

Photo-bis-cyclization: Photochemical cyclodehydrogenation

is not driven to completion in the case of the hexa-branched

pyridinium 2^Me, but stops after two cyclizations (that is, two

These three factors together can explain the selective bis-

cyclization of the 2^R2hexa-branched pyridinium solely into

the 2^R2f hemifused species (R²=H, Me; see Scheme 2 for

structural formulae), that is, without the formation of any

other hemifused species or a mixture of different bis- or

higher-cyclized products, including the fully pericondensed

iminio analogue of hexa-peri-hexabenzocoronene.

C C bond formations) to give the hemifused species 2^Mef as

À

the major photoproduct. To account for this observation,

one may invoke the following three points.

1) Even though photo-bis-cyclization can take place

around the N_pyridinio-aryl group, as demonstrated by the pho-

totransformation of 1^Hinto 1^Hf, the opposite pyridinium site

located para to the N_pyridinioatom is apparently favored when

the pivotal ph-R²aryl is surrounded by two adjacent phenyls

(R³=H), as in the case of 2^R2, irrespective of whether the

R²substituent (Scheme 2) is a methyl group (2^Me) or a

simple proton (2^H). In the latter case of selective hemifusion

within the hexaphenylpyridinium, one can reasonably

expect that the dissymmetry attached to the core pyridinium

platform is most likely the basic origin of the differential re-

activity of the various possible sites of photo-bis-cyclization,

which are actually only two, with the one at the 4-position

of the pyridinium preferred, other things being equal, that

is, R⁰=R¹=R²=R³=H (Scheme 2).

2) Once the bis-cyclized and nicely luminophoric

[hfXPH₂R²]⁺fragment (see structure in Scheme 2) is

formed around the branch para to the N_pyridinioatom, it be-

haves as a well in which the energy subsequently absorbed

by the whole chromophore is funneled instead of being in-

volved in the induction of further electrocyclic rearrange-

ments required for further cyclizations. This scenario is in

line with previous observations concerning the photochemis-

try of molecular assemblies made up of two TP⁺-like subu-

nits covalently linked through a bridging spacer (of variable

length) connected to the N_pyridinioposition. In this case it has

The particular case of the propeller-shaped hexa-branched

pyridinium: First, it is interesting to note that in spite of the

respectable overall rigidity of the propeller-shaped molecu-

lar skeleton of 2^Me, large-amplitude structural rearrange-

ments can take place upon the two-electron reduction of the

electroactive pyridinium core (Figure 6). Such a large distor-

tion is the result of the conflicting contributions of intramo-

lecular steric hindrance and electronic factors associated

with the necessary rearrangement of bonds (i.e., electronic

relaxation) that accompanies bi-electronic reduction. Here,

the positive side-effect of correlated (and even concerted)^[93]

reduction processes and structural changes is the merging of

the two separate monoelectronic reductions observed for

tetra-branched species (see Figure 5) into a single bi-elec-

tronic process. So-called “potential inversion”^[94]is most

likely responsible for this intriguing electrochemical behav-

ior. We are currently investigating this phenomenon and ex-

ploring 1) the way the reduction potential of the bi-electron-

ic process can be shifted towards more positive values and

2) the conditions by which the electrochemical reversibility

of this two-electron process can be restored and even made

switchable by monitoring the structure of the electrophore.

11058

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ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2010, 16, 11047 – 11063

Expanded Pyridiniums

FULL PAPER

Secondly, it is worth noting that, on the one hand, the

nonluminescent bare N-methylpyridinium becomes fluores-

cent at room temperature upon aryl substitution (e.g., N-

methyl-2,4,6-triphenylpyridinium), but that, on the other

hand, replacing the N_pyridinio-methyl group with an aryl group

dramatically alters the emission properties, as exemplified

by 1^H. Interestingly, bright luminescence is restored upon

Another unexpected finding is the fact that extending the

p-conjugated system to enhance p delocalization is not an

efficient approach to ameliorate the electron-accepting abili-

ty of electrophores in the special case of pyridiniums, as re-

flected by the virtually unchanged value of the first reduc-

tion potential. The LUMO remains localized on the pyridi-

nium core despite polycyclic ring fusion, that is, in spite of

pericondensation. On the other hand, when the LUMO is

not the only MO involved, the properties are markedly

modified upon pericondensation. This is indeed the case for

both the electronic absorption and, above all, the photolu-

minescence, as expected. These key issues allow derivation

of the relevant guidelines for the future improvement^[2]of

pyridiniums as multifaceted molecular objects of interest in

their own right, following the example of benchmark pery-

lene bisimide dyes,^[101]even though a strategy alternative to

the extension of the p-conjugated system^{[30,33,35,36]}must be

used to improve^[2]the electrochemical features. Typically,

the well-established approach based on the synergistic close

coupling of several pyridiniums will be applied to enhance

the electron-accepting properties (anodic shift of reduction

potentials). Such a strategy should also fruitfully apply to

promising 2^R2propeller-shaped hexaarylpyridiniums.

peraryl substitution of the pyridinium core (e.g., 2^Hand 2^Me

)

as a result of the combined contributions of 1) geometry-re-

lated energy destabilization of the detrimental charge trans-

fer from the N-aryl group to the pyridinium core and 2) the

overall rigidification of the molecular backbone. Thus, there

is an interest in expanding the pyridinium core not only by

pericondensation but also by peraryl substitution to obtain

pyridiniums that show good luminophoric features.

Conclusion

From a simple inspection of the molecular structures of peri-

condensed expanded pyridiniums, p–p stacking in the solid

state and aggregation in solution are naturally expected by

analogy with their PAH counterparts. However, the picture

we obtain from experimental cross studies, and in particular

from solid-state X-ray crystallography and fluid solution

NMR analyses, is that such interactions are not prominent

and not even determining. Although not directly evidenced,

coulombic repulsion between cationic pericondensed species

could partly explain this finding, for instance, contributing

to make the sharply different “head-to-head” and “tail-to-

tail” cofacial layouts energetically comparable, hence the

pseudopolymorphism observed for the tetrafluoroborate salt

of bare 1^Hf. The features of the counteranion alone (e.g.,

size, shape) are not that determining for the columnar or-

ganization of pericondensed pyridiniums like 1^Hf and other

secondary interactions such as hydrophobic interactions

seem preferable and even mandatory for the rational control

of supramolecular assembly. As demonstrated by Mꢆllen

and co-workers, long alkyl chain(s), whether borne as deco-

ration(s) by the pericondensed expanded pyridinium

itself^[40,44]or provided by the associated counteranion,^[45]are

indeed particularly efficient in this regard. Flat or curved

(bowl-shaped)^[95]fused polycyclic pericondensed molecules

are more appealing if they display additional features such

as (cationic) charge and electroactivity and if the molecular

properties can be tuned, including with respect to self-as-

sembly. In fact, 1^R2f and 2^R2f (see Schemes 1 and 2) are ver-

satile types of expanded pyridiniums that meet these crite-

ria. On this basis they could serve, for instance, as new molec-

ular building blocks of advanced multifunctional materials, in-

cluding mesogens for discotic liquid crystalline materi-

als,^{[40,44,45,96]}as novel molecular platforms in the emerging field

of interfacial system chemistry,^[97]for (semi)rigid well-defined

and shape-persistent organic dendrimers,^[98]or as smart mole-

cules of pharmacological importance,^[99]following the example

of the TOTA^[41]species as a model pharmacophore for DNA

binding,^[100]among other potential applications.

The whole aim is to obtain advanced expanded pyridini-

ums, that is, integrated multifunctional pericondensed and

star-shaped branched bipyridiniums, which is the subject of

ongoing work.

Experimental Section

Syntheses and characterization: Experimental procedures, characteriza-

tion data for new compounds, and complete details for the synthesis of

new compounds are available in the Supporting Information.

X-ray crystal structure determinations: Intensities were collected on a

Bruker-Nonius Kappa CCD diffractometer (Mo_Ka, l=0.71069 ꢇ, graph-

ite monochromator) with sample-to-detector distances of 40 mm. The

frames were integrated and corrected for Lorentzian and polarization ef-

fects using DENZO and the scaling and refinements of crystal parame-

ters were performed by SCALEPACK. All structures were solved by

direct methods (SIR97)^[102]and refined on F²with SHELXTL.^[103]Non-

hydrogen atoms were refined anisotropically, hydrogen atoms were re-

fined isotropically with a riding model except for [1^Hf]

ACTHNUGTRENNG(U BF₄) for which

À

the non-methyl C H distances were freely refined using SADI restraints

(s=0.03).

CCDC-744709 ([2^Mef](Cl)), -744710 ([2^Mef]

G

ACHTUNGTRENNUNG(FeCl₄)),

and À744712 ([1^Hf]

ACHTUNGTRENNUNG

data for this paper. These data can be obtained free of charge from The

Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_

request/cif.

ACTHNUTRGNEUNG

Crystal data for [1^Hf](BF₄): [(C₂₉H₁₈N)⁺, BF₄^À]·CH₃CN, M=508.31, or-

thorhombic, Pnna, a=7.263(2), b=15.147(3), c=21.543(4) ꢇ, a=90, b=

90, g=908, V=2370(1) ꢇ³, Z=4, 1_calcd=1.425 gcm^À3, m=0.10 mm^À1, T=

150 K, 18315 measured, 3767 independent, 2660 observed reflections

(q<31.08), R1 [I>2s(I)]=0.0614, wR2 (all data)=0.2008, S=1.247 for

206 parameters and 28 restraints, residual electron density +0.43/

À0.32 eꢇ^À3

.

ACTHNUTRGNEUNG

Crystal data for [2^Mef](BF₄): [(C₄₂H₂₈N)⁺, BF₄^À]·1.64H₂O, M=659.75,

monoclinic, P2₁/n, a=19.648(3), b=8.320(2), c=20.745(4) ꢇ, a=90, b=

97.96(3), g=908, V=3358(1) ꢇ³, Z=4, 1_calcd=1.303 gcm^À3

m=

0.09 mm^À1, T=123 K, 32460 measured, 6253 independent, 4303 observed

,

Chem. Eur. J. 2010, 16, 11047 – 11063

ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

11059

S. Campagna, P. P. Lainꢁ, et al.

reflections (q<26.058), R1 [I>2s(I)]=0.0609, wR2 (all data)=0.1464,

S=1.17 for 486 parameters and 21 restraints, residual electron density

+0.26/À0.22 eꢇ^À3. A two-fold orientational disorder was evidenced for

two of the fluorine atoms of the BF₄anion, the population coefficients

refine to 0.50/0.50 and 0.60/0.40, respectively. A strongly disordered sol-

vent region was found by Fourier synthesis, which, given the crystal

growth conditions (only small needles could be obtained from MeOH/

H₂O), was attributed to water molecules of variable occupancy. These

molecules were refined using constrained atomic displacement parame-

ters.

functional model, referred to as PBE0, was used,^[108]which was obtained

by casting the PBE exchange and correlation functional^[109]in a hybrid

HF/DFT scheme, in which the HF/DFT exchange ratio is fixed a priori to

1:3. The molecular structure of each compound was fully optimized and

the nature of each stationary point was defined by a subsequent frequen-

cy calculation. All atoms were described by a double z quality basis

set.^[110]Solvent effects were taken into account in all calculations by ap-

plying an implicit solvation model, that is, the polarizable continuum

model (PCM) perfected by Tomasi and co-workers.^[111]More specifically,

we used the conductor-like PCM model as implemented in the Gaussian

development version Revision G.01.^[112]Acetonitrile was considered as

solvent.

Crystal data for [2^Mef]

oclinic, P2₁/c, a=16.511(3), b=12.521(2), c=16.984(3) ꢇ, a=90, b=

m=

ACHTUNGTRENNUNG

(FeCl₄): [(C₄₂H₂₈N)⁺, (Fe^IIICl₄)^À], M=659.75, mon-

95.56(3), g=908, V=3494(1) ꢇ³, Z=4, 1_calcd=1.415 gcm^À3

,

0.77 mm^À1, T=123 K, 54242 measured, 9001 independent, 6773 observed

reflections (q<28.728), R1 [I>2s(I)]=0.0478, wR2 (all data)=0.1446,

S=1.18 for 434 parameters, residual electron density +0.58/À0.38 eꢇ^À3

.

Acknowledgements

Crystal data for [2^Mef](Cl):

[(C₄₂H₂₈N)⁺, Cl^À]·2

2ACHTUNGTREUNNNG ACHTUNRTGE(NGNNU H₂O)·0.75ACHTNUGTREN(NUGN CH₃OH),

J.F., F.T., I.C., and P.P.L. are grateful to the French National Agency for

Research (ANR) “programme blanc” (NEXUS project; No. BLAN07-

1 196405) for financial support. S.C. and F.P. also acknowledge the Uni-

versity of Messina (PRA projects) for funding. The authors also would

like to thank Dr. G. Bertho for his help regarding the NMR studies, S.

Saouane for his technical assistance in crystallogenesis, and C. Peltier for

preliminary calculations. Finally, Prof. C. Adamo, Dr. N. Pietrancosta and

Dr. V. Marvaud are acknowledged for fruitful discussions.

M=1219.23, monoclinic, P2₁/n, a=15.650(3), b=16.078(3), c=

25.796(5) ꢇ, a=90, b=106.43(3), g=908, V=6226(2) ꢇ³, Z=4, 1_calcd

=

1.301 gcm^À3, m=0.16 mm^À1, T=123 K, 90121 measured, 12866 independ-

ent, 4303 observed reflections (q <26.58), R1 [I>2s(I)]=0.1008, wR2

(all data)=0.2497, S=1.00 for 834 parameters and 646 restraints, residual

electron density +0.52/À0.30 eꢇ^À3. In spite of their well-shaped mor-

phology and their acceptable dimensions, the crystals are rather poorly

diffracting. It might be that the beam damages them because attempts to

collect data at room temperature were not successful. Owing to the lack

of observed data, similarity displacement parameters restraints were ap-

plied to the ligand (s=0.03), atomic displacement parameters constraints

and bond length restraints (s=0.03) were applied to the carbon and

oxygen methanol solvent, and isotropic displacement parameters re-

straints were applied to the disordered water molecules (s=0.02).

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15 mL) and a PC-controlled potentiostat/galvanostat (Princeton Applied

Research Inc. model 263 A). The working electrode was a platinum elec-

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11060

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Chem. Eur. J. 2010, 16, 11047 – 11063

Expanded Pyridiniums

FULL PAPER

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À

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11062

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ꢄ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2010, 16, 11047 – 11063

Expanded Pyridiniums

FULL PAPER

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