Borazino-Doped Polyphenylenes

Article abstract of DOI:10.1021/jacs.7b01477

The divergent synthesis of two series of borazino-doped polyphenylenes, in which one or more aryl units are replaced by borazine rings, is reported for the first time, taking advantage of the decarbonylative [4 + 2] Diels-Alder cycloaddition reaction between ethynyl and tetraphenylcyclopentadienone derivatives. Because of the possibility of functionalizing the borazine core with different groups on the aryl substituents at the N and B atoms of the borazino core, we have prepared borazino-doped polyphenylenes featuring different doping dosages and orientations. To achieve this, two molecular modules were prepared: a core and a branching unit. Depending on the chemical natures of the central aromatic module and the reactive group, each covalent combination of the modules yields one exclusive doping pattern. By means of this approach, three- and hexa-branched hybrid polyphenylenes featuring controlled orientations and dosages of the doping B₃N₃ rings have been prepared. Detailed photophysical investigations showed that as the doping dosage is increased, the strong luminescent signal is progressively reduced. This suggests that the presence of the B₃N₃ rings engages additional deactivation pathways, possibly involving excited states with an increasing charge-separated character that are restricted in the full-carbon analogues. Notably, a strong effect of the orientational doping on the fluorescence quantum yield was observed for those hybrid polyphenylene structures featuring low doping dosages. Finally, we showed that Cu-catalyzed 1,3-dipolar cycloaddition is also chemically compatible with the BN core, further endorsing the inorganic benzene as a versatile aromatic scaffold for engineering of molecular materials with tailored and exploitable optoelectronic properties.

Full text of DOI:10.1021/jacs.7b01477

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Article
Borazino-doped polyphenylenes
Davide Marinelli, Francesco Fasano, Btissam Najjari, Nicola Demitri, and Davide Bonifazi
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b01477 • Publication Date (Web): 09 Mar 2017
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Borazino-doped polyphenylenes
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Davide Marinelli,†,‡ Francesco Fasano,†,‡ Btissam Najjari,§ Nicola Demitri,# and Davide
Bonifazi†,§,*
[∗]
†
D. Marinelli, F. Fasano, Prof. Dr. D. Bonifazi
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School of Chemistry, Cardiff University, Park Place Main Building, Cardiff, CF10 3AT, United Kingdom.
E-mail: bonifazid@cardiff.ac.uk.
Dr. B. Najjari, Prof. Dr. D. Bonifazi
Department of Chemistry, University of Namur (UNamur), Rue de Bruxelles 61, Namur 5000, Belgium.
#
Dr. N. Demitri
Elettra – Sincrotrone Trieste, S.S. 14 Km 163.5 in Area Science Park, 34149 Basovizza, Trieste, Italy
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KEYWORDS: borazines, polyphenylenes, cycloadditions, BN, heteroatom doping, boron and nitrogen, emission.
ABSTRACT: The divergent synthesis of two series of borazino-doped polyphenylenes, in which one or more aryl units are
replaced by borazine rings, is reported for the first time taking advantage of the decarbonylative [4+2] Diels-Alder cy-
cloaddition reaction between ethynyl and tetraphenylcyclopentadienone derivatives. Owing to the possibility of function-
alizing the borazine core with different groups on the aryl substituents at the N and B atoms of the borazino core, we have
prepared borazino-doped polyphenylenes featuring different doping dosages and orientations. To achieve it, two molecu-
lar modules were prepared: a core and a branching unit. Depending on the chemical nature of the central aromatic mod-
ule and of the reactive group, each covalent combination of the modules yields one exclusive doping pattern. Indulging
this approach, three- and hexa-branched hybrid polyphenylenes featuring controlled orientation and dosages of the dop-
ing B3N3-rings have been prepared. Detailed photophysical investigations showed that upon increasing the doping dosage,
the strong luminescent signal is progressively reduced. This suggests that the presence of the B3N3-rings engages addi-
tional deactivation pathways possibly involving excited states with an increasing charge separate character that are re-
stricted in the full-carbon analogues. Notably, a strong effect of the orientational doping on the fluorescence quantum
yields was observed for those hybrid polyphenylene structures featuring low doping dosages. At last, we showed that Cu-
catalyzed 1,3-dipolar cycloaddition is also chemically compatible with the BN core, further endorsing the inorganic ben-
zene as a versatile aromatic scaffold to engineering molecular materials with tailored and exploitable optoelectronic
properties.
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Introduction
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Replacing carbon by isostructural atoms is developing as a versatile functionalization strategy to tailor the op-
toelectronic properties of polycyclic aromatic hydrocarbons without significant structural perturbation of the
nanostructure.1,2 Among the different dopants,1–4 the substitution of C=C bonds by isoelectronic BN covalent
couples5–12 leads to isostructural molecular13–25 and polymeric mimics26–33 bearing strong local dipole
moments.34 This imparts a series of physical properties to the molecule, as wider HOMO-LUMO gap31,33,35–39
,
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and peculiar self-assembly behavior, favoring head-to-tail stacks at the solid state40–43 and vdW-driven assem-
blies on metal surfarces.44 In this respect, borazines12,45 have recently renewed their interest as components for
preparing optoelectronically-active materials45–47 and as precursors for preparing BN-doped graphitic
nanostructures.48,49 The latest examples include the isolation of the first BN-doped coronene through pyroly-
sis50,51 and the synthesis of flower-like networks by surface-assisted polymerization.49 However, due to the sus-
ceptibility of the BN core to undergo hydrolysis in the presence of moisture and the difficulty to prepare hy-
brid boron-nitrogen-carbon (BNC) materials with controlled doping patterns, the potential structural and op-
toelectronic diversity of borazine and its derivatives remain limited both in organic and materials chemistry.12
For their part, dendritic polyphenylenes conquered a pivotal role in nanoscience52–55 as shape-persistent nano-
particles, that depending on the peripheral chemical functions, could be used as UV emitters,56 light-
harvesting antennae,57 and chemical precursors for the bottom-up synthesis58,59,4 of graphitic nanostructures.
In view of these applications, hybrid polyphenylenes scaffoldings exposing borazine cycles placed in selected
positions represent an unprecedented class of π-extended molecular boron-nitrogen-carbon (BNC) hybrid
molecular materials. Further, it is expected that tuning the relative borazine orientation and the BN/C ratio
one can tailor the materials properties. Given the emission properties47 of hexaphenyl borazines and of their
derivatives, it is anticipated that the replacement of the aromatic six-member rings with the BN-analogues
will affect the optoelectronic properties of the polyphenylene skeleton. In this work we describe the prepara-
tion of the first hybrid BNC polyphenylenes featuring doping-dependent emissive properties.
General doping descriptors. Owing to the possibilities of introducing borazino units in different ratios, orien-
tations, and positions, three different doping parameters should be considered when replacing benzene rings
with the borazino analogues (Figure 1): the doping dosage (ρ); the doping orientation (o), and the doping vec-
tor (di). The doping dosage is defined as the percentage of the aryl units that have been substituted with the
borazine analogues (Figure 1A). Instead, the orientational parameter o defines the relative orientation be-
tween the doping borazine rings (Figure 1B). The o descriptor is
α
β
when the relevant BN ring displays the
when rotated of 60° (Figure 1B). Finally,
same orientation as that of the reference borazine ring, otherwise
the doping vector di describes the position of each doping units in a two-dimensional architecture following
the X and Y coordinate axes originating from a reference BN ring defined as the (0,0) point (Figure 1C). Each
coordinate is measured by the number of six-membered rings encountered along the two axes (y,x). Taken all
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together, the three descriptors
ρ, di and o allow the precise depiction of the borazine-doping pattern of any
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hybrid hexagonal polycyclic aromatic structure. Although not planar, polyphenylenes can be also looked as
hexagonal structures if projected in a two-dimensional space; thus the very same doping descriptors can be
used. In this first work, we will focus on the preparation of hybrid BNC polyphenylenes localizing doping bo-
razine rings uniquely at the (0,0), (-10,5), (-5,-5), (5,-10), (10,-5), (-5,10) and/or (5,5) positions. To facilitate the
reading, in this manuscript the (5,5), (5,-10) and (-10,5) doping positions are indicated as the B-sites, whereas
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the (-5,-5), (-5,10) and (10,-5) as the N-sites. Thus, a simplified nomenclature can be used in this paper: οΝ
(βΝ
or Ν) and οB βB or B) when describing hybrid polyphenylenes bearing branching borazine units (with the
α
(
α
60° or 0° orientation, respectively) at the N and B positions of a given borazine reference.
Figure 1. Parameters describing the doping pattern of borazino-doped hexagonal structures. Color code: blue: N, red: B.
Molecular design and divergent synthetic strategy. Considering that the borazine core can be made inert to-
ward hydrolysis when aromatic substituents bearing ortho Me groups are connected to the B atoms,12,47,60 it
can be inferred that Müllen’s groundbreaking approach61 to prepare all-carbon polyphenylenes through cy-
cloaddition reactions, would also lead to branched borazino-doped polyphenylene analogues. Indulging this
synthetic protocol, we take advantage of the decarbonylative Diels-Alder cycloaddition reaction to divergently
construct the first BNC polyphenylenes from reactive borazine modules that are alternatively functionalized
with peripheral ethynyl or tetraphenylcyclopentadienone groups (Figure 2). Owing to the possibility of func-
tionalizing the borazine core with different groups on the aryl substituents at the N and B atoms, we have en-
visaged to prepare borazino-doped polyphenylenes featuring different doping dosage
ρ values and orienta-
tions o. To achieve it, two molecular modules were prepared: a core and a branching unit (Figure 2). The cores
bear ethynyl moieties, whereas the branching units expose tetraphenylcyclopentadienone dienes. Depending
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on the chemical nature of the central aromatic module and of the reactive group, each covalent combination
of the modules yields one exclusive doping pattern featuring controlled doping dosages and orientations.
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Figure 2. Envisaged doping patterns for the borazino-doped polyphenylenes investigated in this work. Color code: blue, substit-
uent at the N; red, substituent at the B.
Results and Discussion
Synthesis of the borazine-derived ethynyl core modules. The borazine-derived cores used for the bottom-up
synthesis of the borazino-polyphenylenes have been prepared following a modified protocol 44,47,62 of the orig-
inal Groszos’ procedure.63 For the core aryl-substituted borazine (Scheme 1) we started from TIPS-protected
aniline 1 that was reacted with BCl3 under refluxing conditions to give intermediate TIPS-protected B-
trichloro-N-triphenylborazine. This, upon subsequent treatment with mesityllithium (MesLi) affords borazine
core 2 in 54% yield (Scheme 1). Analogously, when intermediate TIPS-protected B-trichloro-N-
triphenylborazine is reacted with the TMS-protected aryllithium derived from 4, hexasilyl protected borazine
core 5 could be prepared in 14% yield. Similarly, when B-trichloro-N-triphenylborazine is prepared from ani-
line, TMS-protected borazine core 9 could be also prepared upon addition of TMS-protected aryllithium de-
rivative 4 in 34% yield. Removal of the silyl protecting groups of molecules 2 and 9 with TBAF yields borazines
3 and 10 exposing terminal phenylacetylene moieties at the N- and B-sites, respectively. As far as molecule 5 is
concerned, addition of TBAF led to the full deprotection of the silyl groups, affording borazine core 6 exposing
six terminal acetylene groups in 67% yield (Scheme 1). On the other hands, K2CO3 in MeOH allows the or-
thogonal removal of the TMS groups, thus affording tri-protected borazine core 7 in 69% yield (Scheme 1).
Bearing three terminal ethynyl moieties at the boron centers, the latter borazine allows an orthogonal func-
tionalization at the N- and B-sites (see below).
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Scheme 1. Synthesis of borazino-based core modules: a) BCl3, toluene, reflux, 18 h; b) MesBr, tBuLi, THF, -84 °C, 16 h, 54%; c)
TBAF, THF, 0 °C, 2 h, 77%; d) BCl3, toluene, reflux, 18 h; e) 13, tBuLi, THF, -84 °C, 16 h, 14%; f) TBAF, THF, 0 °C, 2 h, 67%; g)
K2CO3, MeOH/THF (1:1), r.t., 16 h, 69%; h) BCl3, toluene, reflux, 24 h; i) 4, tBuLi, THF, -84 °C, 16 h, 34%; l) TBAF, THF, 0 °C, 1 h,
96%. TBAF: tetrabutylammonium fluoride.
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Synthesis of the borazine-derived tetraphenylcyclopentadienone branching modules. We conjectured that bo-
razine-derived tetraphenylcyclopentadienones might be easily obtained from Suzuki cross-coupling reaction
between an organoboron derivative of tetraphenylcyclopentadienone and a borazine exposing a suitable aryl
halide or related electrophile (i.e., ArOTf) either at the N- or B-sites.
Scheme 2, above. Synthesis of borazino-derived cylopenta-2,4-dienone branching module 16: a) BCl3, toluene, reflux, 18 h; b) 11,
tBuLi, THF, -84 °C, 2 h, 78%; c) TBAF, THF, 0 °C, 2 h; d) Tf2O, pyridine, r.t., 16 h, 78%; e) PhB(OH)2, [Pd(PPh3)4], K2CO3, diox-
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ane/H2O (5:1), 105 °C, 15 h, 39% (14a) and 8% (14b); f) 15, [Pd(PPh3)4], K2CO3, dioxane/H2O (5:1), 105 °C, 15 h, 71%. Below. Synthe-
sis of borazino-derived cylopenta-2,4-dienone branching module 20: g) BCl3, toluene, reflux, 18 h; h) MesBr, tBuLi, THF, -84 °C, 2
h, 62%; i) PhB(OH)2, [Pd2(dba)3], K3PO4, dioxane/H2O (5:1), 150 °C, 16 h, 27% (19a) and 25% (19b); l) 15, [Pd2(dba)3], K3PO4, diox-
ane/H2O (5:1), 150 °C, 20 h, 23%. Tf2O: trifluoromethanesulfonic anhydride.
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Thus, starting from aniline 8 (Scheme 2), we could synthesize TBDMS-protected borazine 12 as molecular pre-
cursor for preparing a suitable electrophilic partner to be reacted in the Suzuki cross-coupling reaction
scheme. In particular, through the removal of the TBDMS protecting group with TBAF and successive esterifi-
cation reaction with Tf2O in pyridine, borazine 12 could be converted into tris-triflate borazine 13 in 78% yield.
Suzuki cross-coupling between molecule 13 and phenylboronic acid (two equivalents), led to bis-phenyl and
tris-phenyl borazine derivatives 14a and 14b in 39% and 8%, respectively (Scheme 2). Final cross-coupling re-
action between monotriflate borazine 14a and cylopenta-2,4-dienone-derived boronic ester 15 gave targeted
borazino-derived 16 in 71% yield. As we have anticipated potential vulnerability of the TBDMS protecting
group under the acidic BCl3 reaction conditions, a different synthetic strategy was undertaken for installing a
suitable electrophile on the N-aryl group. Thus, a decision was made to equip the N-aryl substituent with a
chloride functional group. Following these considerations, tris-chloride borazine 18 was firstly synthesized in
62% yield starting from 4-chloroaniline following the BCl3 route (Scheme 2). Analogously to the case of bora-
zines 14a and 14b, Suzuki cross-coupling reactions between phenylboronic acid (two equivalents) and bora-
zine 18 led to bisphenyl and triphenyl derivatives 19b and 19a in 27% and 25% yield, respectively. Final graft-
ing of cylopenta-2,4-dienone-derived functionality occurred through a successive Suzuki cross-coupling reac-
tion between boronic ester 15 and borazine monochloride 19a, affording targeted molecule 20 bearing a cylo-
penta-2,4-dienyl group on the N-aryl substituent.
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Synthesis of the all-carbon core and branching modules. For the synthesis of the full-carbon modules, the Co-
catalyzed cyclotrimerization strategy was used.64 Whereas the cyclotrimerization of bis-TIPS-protected di-
phenylacetylene 24 in the presence of Co2(CO)8 gave hexasubstituted benzene core 25 in 35% yield, the reac-
tion with TIPS-protected diphenylacetylene 21 gave a mixture of two regioisomers (see ESI) that were separat-
ed by column chromatography yielding relevant hexaphenyl benzene 22 in 16% (Scheme 3). Removal of the
TIPS group using TBAF gave tris-acetynyl hexaphenyl benzene 23 and hexaphenylacetynyl benzene 26 in very
high yield. Regarding full-carbon branching unit 31, a similar strategy to that undertaken for preparing core 23
was pursued. Specifically, cyclotrimerization of methoxydiphenylacetylene 27 in the presence of Co2(CO)8 in
dioxane followed by treatment with BBr3 gave 1,3,5-tri-(4-hydroxy)-2,4,6-triphenylbenzene 28 in 14% yield. Re-
action of 28 with Tf2O in pyridine gave tri-triflate derivative 29 in 90% yield. Suzuki cross-coupling reaction
between phenylboronic acid and 29 gave bis-phenyl and tris-phenyl derivatives 30a and 30b in 20% and 9%
yield, respectively. Final Suzuki cross-coupling reaction between monotrilfate 30a and boronic ester 15 yielded
full-carbon branching module 31 in 68% yield (Scheme 3).
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R
R
1
TIPS
TIPS
2
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R
R
R
R
a)
c)
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R
R
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22 R= TIPS
23 R= H
CC
21
b)
TIPS
R
24
25 R= TIPS
d)
26 R= H
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CC
OMe
RO
OR
Ph
Ph
e), f)
h)
OR
OTf
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30a
28 R = H
29 R = Tf
g)
+
O
Ph
Ph
i)
Ph
30b
Ph
Ph
Ph
O
CC
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Scheme 3, above. Synthesis of hexaphenylbenzene cores 22 and 25: a) Co2CO8, dioxane, reflux, 18 h, 16%; b) TBAF, THF, 0 °C, 3
h, 66%; c) Co2CO8, dioxane, reflux, 48 h, 35%; d) TBAF, THF, 0 °C, 3 h, 96%. Below. Synthesis of hexaphenylbenzene-cylopenta-
2,4-dienone branching module 31: e)Co2CO8, dioxane, reflux, 18 h, f)BBr3, CH2Cl2, -84 °C, 3 h, 14%; g) Tf2O, pyridine, r.t., 18 h,
90%; h) PhB(OH)2, [Pd(PPh3)4], K2CO3, dioxane/H2O (5:1), 105 °C, 15 h, 20% (30a) and 9% (30b); i) 15, [Pd(PPh3)4], K2CO3, diox-
ane/H2O (5:1), 105 °C, 15 h, 68%.
Synthesis of the three-branched borazino-doped polyphenylenes. To endorse the chosen synthetic plan, the
thermal stability of the borazine core under the cycloaddition reaction conditions was first investigated.
When molecule 3 is reacted with tetraphenylcylopenta-2,4-dienone (CPD) in Ph2O at 180 °C, borazino-
oligophenylene molecule 32 decorated with three penta-phenylbenzene branches at the N-sites could be ob-
tained in 56% (Scheme 4). To unambiguously prove the chemical structure, a small transparent crystal of 32,
suitable for X-ray diffraction, was obtained by solvent evaporation from a CHCl3 solution. As expected, the X-
ray molecular structure shows the presence of the penta-phenylbenzene peripheries at the N atoms of the bo-
razine core (Figure 3). Given these premises, the synthesis of the different borazino-doped polyphenylenes
was undertaken following the divergent covalent branching approach through decarbonylative [4+2] Diels-
Alder cycloaddition reactions under heating conditions in Ph2O containing the relevant core and branching
units (Schemes 4-6). All molecules were purified through column chromatography and recycling gel permea-
tion chromatography (Rec-GPC) using CHCl3 as eluent. To commence, cycloaddition reaction between bora-
zines 3 and 16 (Scheme 4) gave molecule 33 (ρ = 8.9%) in which three peripheral borazine units are attached to
the N-position of the central borazino core with a αN orientational doping pattern.
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Fig. 3. ORTEP representation of borazine-polyphenylene 32 as determined by X-ray diffraction (atom colors: blue N, pink B,
white C; atomic displacement parameters, obtained at 223 K, are drawn at the 50% probability level). Space group: P21/c. B-N
bond lengths: 1.449; 1.451; 1.418; 1.444, 1.441 and 1.418 Å.
Me
Me
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Me
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BN
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Me Me
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b)
c)
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BN
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Me
3
d)
BN
BN
BN
BN
Me
Me
Me Me
N
Me
N
Me
B
N
B
N
B
N
N
B
Me
Me
Me
Me
B
B
Me
Me
Me
Me
Me
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CC
BN
CC
CC
Me
Me
Me Me
N
B
N
B
N
Me
Me
B
Me
Me
Me
35
Scheme 4. Synthesis of borazine doped frameworks 32, 33, 34 and 35: a) tetraphenylcyclopentadienone (CPD), Ph2O, 180 °C, 2 h,
56%; b) 16, Ph2O, 180 °C, 18 h, 55%; c) 20, Ph2O, 200 °C, 16 h, 27%; d) 31, Ph2O, 190 °C, 18 h, 54%.
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Following the same line, when borazine core 3 is reacted with cyclopenta-2,4-dienone 20, branched bora-
zylenes 34 (ρ = 8.9%) could be synthesized in 27%. Although the three peripheral borazine units are also at-
tached to the N-position of central BN core, they are organized in a βN orientational doping pattern. Finally,
when borazine 3 is reacted with cyclopenta-2,4-dienone 31, branched molecule 35 featuring low doping dosage
(ρ = 2.08%) with only a central doping borazine unit bearing all-carbon branches at the N-sites, is obtained in
54% yield. On the other hands, when borazine dienophile core 10 is used, branched hybrid polyphenylenes
bearing the substituents exclusively at the B-sites could be prepared (e.g., B-branched penta-phenylbenzene
reference borazine 36). Capitalizing on this building block, cycloaddition reactions of core 10 with borazine
dienes 16 and 20 (Scheme 5) gave branched borazylenes 37 (ρ = 8.9%) and 38 (ρ = 8.9%) featuring βB and αB
orientational doping patterns, respectively. In a similar manner, replacing the borazine core dienophile with
full-carbon congener 23, branched polyphenylenes 39 and 40 displaying an intermediate doping dosage (ρ =
6.5%) and full-carbon reference 41 could be synthesized using dienes 20, 16 and 31, respectively (Scheme 6).
Notably, branched polyphenylenes 39 and 40 develop the peripheral borazine units at the N- and B-site, all
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
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30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
displaying
α orientational doping pattern (i.e., αN and αB).
Scheme 5. Synthesis of borazine doped frameworks 36, 37 and 38: a) tetraphenylcyclopentadienone (CPD), Ph2O, 180 °C, 6 h,
83% b) 16, Ph2O, 190 °C, 18 h, 22%; c) 20, Ph2O, 210 °C, 16 h, 40%.
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40
41
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43
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45
46
47
48
49
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51
52
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54
55
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58
59
60
Scheme 6. Synthesis of borazine doped frameworks 39, 40 and 41: a) 20, Ph2O, 230 °C, 18 h, 27%; b) 16, Ph2O, 190 °C, 18 h, 81%; c)
31, Ph2O, 190 °C, 18 h, 51%.
Synthesis of the hexa-branched borazylenes. To extend further the number of borazine units within a poly-
phenylene architecture, hexa-branched derivatives were also prepared starting from the hexaethynyl borazine
cores. As we have anticipated a potential steric hindrance deriving from the hexasubstitution at the hexa-
substituted borazino core, the thermal conditions in Ph2O of the cycloaddition reaction were first investigated
using borazine core 6 in the presence of tetraphenylcylopenta-2,4-dienone (CPD). As expected, borazino-
oligophenylene 42 decorated with six pentaphenylbenzene branches could be obtained in good yield (48%)
only when heated at least at 250 °C (Scheme 7). Following this plan, eight hybrid polyphenylenes displaying
different doping dosage and orientations were prepared cross-reacting borazine dienophile cores 6, 7, 26 with
branching dienes 16, 20 and 31. As discussed for the three-branched derivatives, also in this case the products
were purified by column chromatography over silica followed by Rec-GPC using CHCl3 as eluent.
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Me Me
N
B
N
B
N
Me
Me
BN
B
Me
Me
7
8
9
10
11
12
Me
B
Me
Me Me
N
Me
B
N
B
N
N
B
N
Me
Me
Me
Me
Me
Me
B
B
N
42
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
N
a)
Me
Me
Me
Me
Me
Me
Me
13
B
B
N
N
B
N
B
N
B
N
B
N
B
N
B
N
B
N
B
N
Me
Me
Me
Me
Me
Me
Me
Me
B
B
14
15
16
17
18
19
20
21
N
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
B
Me
Me Me
N
N
B
N
B
B
N
B
N
b)
c)
B
N
B
N
B
N
Me
Me
Me
Me
Me
N
B
Me
Me
Me
Me
Me
Me
Me
B
Me
B
Me
Me
Me
N
Me
Me Me
N
Me
Me
N
B
6
B
N
B
N
B
N
B
N
N
B
N
B
N
Me
Me
Me
Me
Me
Me
Me
Me
B
B
N
B
N
Me
Me
Me
Me
Me
Me
Me
Me
d)
Me
Me
Me
Me
22
23
24
25
Me
Me
Me
Me
Me
Me
B
N
N
B
N
B
N
B
B
N
B
N
Me
Me
Me
Me Me
Me
Me
BN
BN
26
27
28
29
BN
BN
BN
BN
BN
BN
BN
Me
44
43
BN
BN
BN
BN
BN
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CC
CC
CC
CC
Me Me
N
B
N
B
N
Me
Me
BN
CC
B
Me
Me
CC
45
Scheme 7. Synthesis of borazine-doped frameworks 42, 43 (βNαB), 44 (αNβB) and 45: a) tetraphenylcyclopentadienone, Ph2O, 250
°C, 16 h, 48% b) 20, Ph2O, 230 °C, 16 h, 36%; c) 16, Ph2O, 260 °C, 72 h, 35%; d) 31, Ph2O, 260 °C, 30 h, 46%.
Specifically, when borazine core 6 is reacted with branching units 20, 16 and 28, hexa-branched polyphe-
nylenes 43, 44 and 45 were obtained in 36%, 35% and 46% yield, respectively (Scheme 7). Molecules 43 and 44
exposes seven borazine rings and displays the same doping dosage (ρ = 8.33%) but orientational doping pat-
terns βNαB and αNβB, respectively. On the other hands, polyphenylene 45 only bears a central borazine unit,
thus featuring low doping dosages (ρ = 1.1%). Taking advantages of the orthogonal deprotection of the two si-
lyl groups in molecule 5, a stepwise [4+2] cycloaddition strategy was undertaken to prepare heptaborazine de-
rivatives (ρ = 8.33%) featuring βNβB and αNαB orientational doping patterns (Scheme 8).
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40
41
42
43
44
45
46
47
48
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52
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Scheme 8. Synthesis of borazine doped frameworks 48 (βNβB) and 51 (αNαB): a) 16, Ph2O, 200 °C, 18 h, 55%; b) TBAF, THF, 0 °C,
2 h, 94%; c) 20, Ph2O, 260 °C, 24 h, 54%; d) 20, Ph2O, 240 °C, 18 h, 27%; e) TBAF, THF, r.t., 2 h, 84%; f) 16, Ph2O, 230 °C, 16 h,
48%.
When triacetylene borazine core 7 is reacted with cyclo-2,4-dienone borazine 16, tetraborazine intermediate
46 is obtained. Cleavage of the TIPS groups with TBAF followed by a second [4+2] cycloaddition reaction with
diene 20, gave targeted heptaborazine derivative 48 featuring a βNβB doping pattern. Conversely, when invert-
ing the cycloaddition reaction sequence, namely reacting dienophile 7 with diene 20 to give intermediate 50
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followed by [4+2] cycloadition with 16, heptaborazine 51 (αNαB) was obtained. Replacing the borazine core
dienophile with corresponding full-carbon analogue 26, branched hexaborazine polyphenylenes 52 and 53 dis-
playing ρ value of 7.06% could be obtained (Scheme 9). As for the three-branched derivatives, branched poly-
5
6
7
8
9
phenylenes 52 and 53 develop the peripheral borazine units at the N- and B-site, all displaying
(i.e., αN and B). Finally, reference full-carbon polyphenylene 54 was prepared in 49% from hexaphenyl ben-
zene core 26 and diene 31.
α orientation
α
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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26
27
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31
32
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35
36
37
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39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 9. Synthesis of borazine doped frameworks 52, 53 and 54: a) 20, Ph2O, 230 °C, 16 h, 19% ; b) 16, Ph2O, 250 °C, 18 h, 70%;
c) 31, Ph2O, 260 °C, 30 h, 49%.
The molecular structures of all cycloadducts were unambiguously identified by NMR spectroscopy (ESI) and
mass spectrometry through the detection of the peak corresponding to the molecular ion (M+). As examples,
+
HRMS-MALDI mass spectrograms for 32 (C135H108B3N3+, calc.: 1803.8822, found: 1803.8802,), 33 (C297H252B12N12 ,
+
calc.: 4118.1344, found: 4118.1251), 35 (C297H216B3N3+, calc.: 3856.7393, found: 3856.7273), 44 (C546H450B21N21 ,
+
calc.: 7532.8144, found: 7532.8324), and the MS-MALDI analysis for 53 (C546H438B18N18 , calc.: 7445.7, found:
7445.7), and 54 (C546H368+, calc.: 6926.9, found: 6926.8) are displayed in Figure 4.
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Fig. 4. Selected HRMS-MALDI mass analysis of 32 (a), 33 (b), 35 (c), and MS-MALDI mass analysis of 44 (d), 53 (e), and 54 (f) in
the positive ion mode (matrix: DCTB, N2-laser: 337 nm). Insets: zoomed calculated (above) and experimental (below) spectra.
Me
Me
N
Me
Me
Me
B
N
N
B
N
B
N
B
N
Me
Me
Me
B
Me
B
Me
Me
Me
Me
Me
Me
BN
BN
55
56
57
Scheme 10. Reference 1,2,3,4,5.pentaphenylbenzene 55, and pentaphenylbenzenyl borazines 56 and 57.
For reference purposes, pentaphenyl benzene 55 and borazines 56 and 57, bearing pentaphenylbenzene moie-
ties respectively at the nitrogen and boron positions have been also synthesized (see ESI) by cycloaddition re-
action between dienophilles CPD, 19a, and 14a with phenylacetylene, respectively.
Steady-state UV-vis absorption and emission studies in solution.
Comparative absorption spectra of the hybrid polyphenylenes in CH2Cl2 at 25 °C are shown in Figures 5 (see
also ESI), with the key absorption data resumed in Table 1. In general, the absorption spectra for both three-
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and hexa-branched derivatives cover the UV spectral window in the ∼200-325 nm range, as shown in Figure 5.
Both a broadening of the band and an enhancement of the UV-vis extinction coefficient ( ) at the maximum
ε
of absorption was observed upon increasing the number of borazine units (Figure 5): ~6×104 for the reference
5
6
7
borazine monomers, 1-3×105 for the three-branched derivatives and 3-6×105 for the hexa-branched conjugates
8
9
(Table 1).
Table 1. Steady-state absorption and emission properties for the BN-doped and all-carbon polyphenylene derivatives in CH2Cl2.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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32
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46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Absorption
Emission
[d]
77K
77K
Comp.
kv+kISC+kCS
λ
max [nm]
λ
max [nm]
λmax
[nm]
λmax,PH
λecc [nm]
τf[ns][b] kf[c] [ns-1]
(ε, M-1 cm-1)
(Φem)a
[ns-1]
[nm]
(ρ)
30b
(0%)
266
264
355
(9%)
0.3
0.7
2.7
0.3
0.28
0.12
3.03
344
322
319
-
(7.1×104)
14b
(1%)
269
(5.9×104)
263
273
327
(20%)
1.14
476, 451e,
478e, 505e
19b
(1%)
267
(6.7×104)
322
(34%)
0.25
478
268
41
(0%)
372
(76%)
274
279
266
272
272
272
274
275
280
268
0.6
0.8
0.5
0.6
0.5
0.5
0.2
0.2
0.5
0.6
1.26
0.8
0.40
0.45
1.1
371
371
501, 423e
489, 432e
490, 406e
499
(3.9×105)
266
35
373
(64%)
(2.08%) (9.9×104)
40
267
(4×105)
373
(45%)
0.9
367
355
365
351
(
(
6.5%
)
39
268
(3.1×105)
373
(44%)
0.73
0.66
0.58
0.55
0.35
1.54
1.18
0.93
1.34
1.42
4.45
4.65
0.46
0.48
6.5%
)
33
268
(1.7×105)
370
(33%)
484
(
(
(
(
8.9%
)
)
)
)
34
265
(1.9×105)
370
(29%)
477
8.9%
38
267
(2.7×104)
373
(11%)
362
353
376
372
508
8.9%
37
269
(2.3×105)
372
(7%)
482
8.9%
266
(6.1×105)
54
(0%)
376
(77%)
510, 423e
420, 504e
265
(4.7×105)
45
(1.1%)
372
(71%)
53
269
370
(62%)
263
266
0.8
0.8
0.77
0.71
0.48
0.54
373
373
498, 423e
419
(
7.06%
)
(5.3×105)
52
267
375
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(57%)
374
(
7.06%
)
)
)
)
)
(4×106)
44
263
(3.3×105)
275
274
271
0.8
0.5
0.4
0.4
0.32
0.5
0.93
1.5
365
366
369
365
521, 414e
519, 420e
504, 402e
504, 417e
(
(
(
(
8.33%
(26%)
43
266
(5.8×105)
373
(25%)
8.33%
9
48
267
(4.9×105)
377
(22%)
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49
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52
53
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58
59
60
0.55
0.45
1.95
2.05
8.33%
51
267
(1.7×105)
373
(18%)
272
8.33%
[a] Emission quantum yield (Φem); [b] life time at 295 nm; [c] radiative rate constant kf = Φem / τf; [d] total no-radiative constant (1/ τf)-kf; [e] shoulder phos-
phorescence peaks.
Figure 5. Absorption (—) at 298K, emission (---) at 298K, emission (---) at 77K, phosphorescence (…) at 77K spectra in CH2Cl2 of poly-
phenylenes (left) 33-41 and (right) 43-45, 48, 51-54.
While the highly doped polyphenylenes show a unique broad absorption bands, polyphenylenes with either
low doping dosages or full-carbon content display a combination of two bands resulting in two peak maxima.
Luminescence studies of air-equilibrated CH2Cl2 solutions of all borazine derivatives showed significant radia-
tive UV emission profiles (centered in the interval between 263 and 280 nm) and very short lifetimes (τ = 0.4 –
0.8 ns) consistent with the presence of excited singlet radiative deactivation. As expected, the excitation spec-
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tra fully match those of the corresponding absorption profiles confirming the all part of the molecules con-
tribute to the emission. No significant changes in the emission profiles were observed at 77 K. All solutions
showed significant quantum yields (Φem), the intensity of which revealed to be dosage-dependent (Φem = 7% –
76%; Table 1) for the branched derivatives, with the congeners displaying high doping dosages being the less
emissive.
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Monomeric structures and molecular references. Starting with reference borazine molecules 14b and 19b, one
can notice that the presence of the phenyl substituents on the aryl rings linked to either the N- or B-sites
clearly enhances the emission quantum yields (Φem = 20% and 34% for borazines 14b and 19b, respectively)
but reduces the life-time values (
τ
= 0.7 ns and 2.7 ns for borazines 14b and 19b, respectively) if compared to
= 7.1 ns in CH2Cl2).47 In any
the fluorescence intensity of B-trimesityl-N-triphenylborazine (Φem = 7.7% and
τ
circumstances, these monomeric borazine derivatives are stronger emitters than their all-carbon congeners,
like molecule 30b (Φem = 9%). The observed emission enhancement could be ascribed to the presence of the
ortho-methyl groups that, hampering rotational movements, rigidifies the hexasubstituted borazine scaffold
compared to that of hexaphenylbenzene derivatives. This consequently causes a decrease of the vibrational
relaxation pathway. When branched, the emission intensity of the borazine scaffolds progressively diminishes
upon increasing the number of the penta-phenylbenzene spokes (Φem = 20%, 16%, 2% and 0.8% for 56, 57, 32
and 36, respectively, see Table 1S, ESI), with reference hexa-substituted penta-phenylbenzene borazine 42 be-
ing one of the least emissive of the series (Φem = 1%). Given the negligible emissive nature of the penta-
phenylbenzene moieties (Φem < 1% for 55), the observed weakening of the emission intensity for all reference
molecules can be realistically attributed to non-radiative vibrational deactivations triggered by the penta-
phenylbenzene spokes. Notably, molecules bearing the functionalities exclusively at the N-sites are slightly
more emissive than those functionalized at the B-sites (i.e., Φem(19b) > Φem(14b); Φem(56) > Φem(57) and
Φem(32) > Φem(36)), suggesting that the functionalization pattern at the borazine core is not as innocent as one
could think in the first place. In fact, being the observed double-bond character lower than 20% for the BN
bonds (for h-BN the double bond character is ~22%),65 it can be considered that the B and N atoms are electri-
cally neutral.66 Thus, this suggests that each N atom retains the major electron bulges, while each B atom
keeps their electronic deficiencies. This hypothesis is further confirmed by the calculated electron surface po-
tential plots (ESPs, ESI), which show a charge polarization of the sp2-B3N3 surface, with the N and B atoms
negatively and positively charged, respectively. As previously reported for boron-nitrogen containing donor
acceptor dendrimers,67–70 photoinduced short-living charge separated (CS) states undergoing dynamic pro-
cesses could take place between electron-donating and electron-accepting moieties, deactivating the emissive
excited states.
Three-branched polyphenylenes. Moving to the three-branched polyphenylene series, one can appreciate a
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significant decrease of the emission intensity when increasing the doping dosage (Table 1, Figure 6). The fluo-
rescence quantum yield of the all-carbon polyphenylene is measured as the strongest value (76%), whereas
the progressive replacement of aryl rings with B3N3 surprisingly lessens the emission intensities (Figure 6),
with the tetraborazine derivatives being the weakest emitters.
8
9
10
11
12
13
14
15
16
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19
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Emission quantum yield as a function of the dosage and orientational doping for the three-branched polyphenylenes series.
This behavior is rather unexpected if one considers that monomeric borazine molecules 14b and 19b are more
emissive compared to their hexaphenylbenzene analogue 30b.47 The observed emission enhancement for mol-
ecule 35 could be reasonably attributed to an overcrowding effect that, generated by the simultaneous pres-
ence of multiple branching units,56,71–74 rigidifies the dendritic structure and enhances the emission intensity
as previously observed with other full carbon polyphenylenes.75 Interestingly, while the insertion of the B3N3
ring at the (0,0) position seems to have a minor effect on the emissive properties (Φem = 67% for 35), a signifi-
cant decrease of the fluorescence intensity is obtained when borazine units are present at the periphery (Φem
=
~45% for both 39 and 40). Notably, no effects of the borazine N- or B-site is observed for molecules 39 and 40,
as both conjugates essentially display the same Φem values. As anticipated above, the major fluorescence
quenching is observed for the four tetra-borazine derivatives (Φem < 33%), bearing both inner and peripheral
borazine rings. Among those, molecules 37 and 33 featuring βB and αB orientational doping patters display the
weakest emissions (Φem = 7% and 11%, respectively), whereas derivatives 34 and 38 display the highest values
(Φem = 29% and 33%, respectively). When the measurements are performed in O2-free solutions, the Φem val-
ues are essentially the same as those measured in air-equilibrated solutions. Calculation of the radiative (kf)
and total non radiative (knr = kv + kISC + kCS) rate constants (Table 1) allowed us to shed further light on the ef-
fect of the doping on the deactivation pathways. As it clearly appears from the derived rate constants values,
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increasing the BN dosages accelerate a non-radiative deactivation rate of the singlet-excited state through a
possible combination of ISC, increase of the CS character of the excited states and vibrational relaxation.
Among all, we can reasonably consider that the ISC and photoinduced CT are the most contributing pathways
given the emissive behavior of the dendritic structures compared to that of molecular references 32, 36, 55-57.
The quenching of the emission is particularly high when the peripheral borazines branches out through the B-
sites (i.e., molecule 37). A total non-radiative deactivation 12 times faster than that of the full-carbon analogue
was observed. When cooled to 77 K, non-negligible phosphorescence emission have been observed (Figure 5)
showing double-peak emission profiles, with the borazine-doped molecules generally featuring the highest-
energy triplet emission (Table 1), in line with previously-studied BN-containing polycyclic aromatics.41 Repre-
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sentative phosphorescence lifetime measurements (Figure 188S) showed long-lasting emission profiles (τPhos
=
2.6, 2.1 and ~2 s for 14b, 36, 33) for the borazine-containing derivatives compared to that (τPhos = 1.1 s for 41) of
the full carbon analogues.
Figure 7. Emission quantum yield as a function of the dosage and orientational doping for the hexa-branched polyphenylenes series.
Hexa-branched polyphenylenes. Similarly to the three-branched polyphenylenes, also the hexa-branched pol-
yphenylenes show a clear decrease of the emission intensity upon increasing the doping dosage (Table 1, Fig-
ure 7). With the all-carbon polyphenylene 54 showing the strongest emission (77%), the replacement of either
the central or the six aryl rings with the B3N3 analogues causes a modest quenching of the emission intensities,
with hexaborazino derivatives 52 and 53 being the weakest fluorophores (57%-62%). As observed for the three-
branched architecture, only the simultaneous presence of the central and peripheral borazine units (i.e., hep-
taborazine derivatives 43, 44, 48 and 51) provokes a significant quenching of the fluorescence signal (18%-
26%). In contrast to the tetraborazine architectures, in this case no significant effect of the orientational dop-
ing was observed for the heptaborazine derivatives, with the four patterns αNαB
,
βNαB
,
αNβB, and βNβB display-
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ing very similar Φem values. Together with the calculated radiative and total deactivation rate constants (Table
1), these data further advocate the observation for which increasing BN dosages enhance the deactivation rate
of the singlet excited state of these BNC polyphenylenes. However, at these high dosages negligible effect on
the emission intensities was observed when changing the orientation of the doping units. This suggests that
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the οN
/
οB parameter does not play a major role in affecting the excited states when the BNC polyphenylenes
features high
ρ
ratios. Probably, at high doping dosages, multiple electronic interactions take place in the hy-
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brid BNC polyphenylene averaging the emission intensity. As observed for the three-branched derivatives, al-
so the hexa-branched polyphenylenes present non-negligible a two-peak phosphorescence emission profile at
77 K.
Synthesis of a multichromophoric BNC polyphenylene
To further probe the energy harvesting properties of BNC polyphenylenes, a multichromophoric derivative
constituted by a tetra-borazino polyphenylene B architecture interring a red-absorbing dye at its central core
.
β
has been synthesized (Scheme 11). The blue naphthalene diimide (B-NDI) tag76,77 was selected for its comple-
mentary lowest-energy UV-Vis absorption profile that, covering the red region, does not overlap with that of
the tetraborazino-doped framework (TBOZ). Taking advantages of the orthogonal cleavage of the protecting
silyl groups in molecule 5, we have decided to further enrich the reactivity portfolio pertinent to the borazine
core attempting a Cu-catalyzed 1,3-dipolar cycloaddition reaction to give a triazole unit (Scheme 11).
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Scheme 11. Synthesis of NDI-bearing tetraborazine 59: a) 58, DIPEA, AcOH, CuI, CH2Cl2, r.t., 96 h, 58%.
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Starting from triethynyl tetraborazine intermediate 47 (Scheme 8), and subjecting it to Cu-catalyzed 1,3-
dipolar cycloaddition with an azido-derived naphthalene diimide chromophore 58 (see ESI for the
synthesis),78 blue-colored tetraborazine polyphenylene 59 was obtained. Despite the presence of the resident
oligophenylenes branches, remarkable high yields were obtained for the cycloaddition and no decomposition
of the borazine core was observed under the given Cu-catalyzed reaction conditions. Once again, the product
was purified by silica gel chromatography and recycling GPC (RGPC, see ESI). The molecular structure was
unambiguously identified by HR-MALDI through the detection of the peak corresponding to the molecular
+
ion (M+) at m/z 6152.02 (C417H357B12N33 , calc.: 6151.97; see ESI). Similarly to the other BNC polyphenylenes, we
have evaluated the steady-state absorption and emission properties of molecule 59. The dyad mainly absorbs
in two regions with distinct peaks at 603, 364, 347, 262 nm, characteristic of the TBOZ framework and of the
B-NDI unit, respectively (Figure 8). By choosing
λexc = 272 nm, it was possible to predominantly excite (74%,
according to the extinction coefficient of the model compounds) the tetraborazylene unit. At
λ
exc = 272 nm,
BOZ
the emission spectra of the TBOZ moiety is essentially quenched (≈ 98%) in CH2Cl2 (residual Φem
at em = 357 nm, see ESI).
= 1.98%
λ
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Figure 8. Extinction coefficient (—, εmax = 4.8×105 L mol-1 cm-1 at λ = 262 nm), emission (…...
nm) of blue-labelled 59 in CH2Cl2 at 25 °C.
, λexc = 263 nm) and excitation profiles (---, λem = 650
However, a B-NDI-centered luminescence is clearly visible, with a ΦemB-NDI value of ~0.5%. Given the low val-
ues and the experimental error, the Φem is virtually the same to that of the NDI model compound obtained
upon direct excitation (at λexc = 560 nm). This suggests that the photophysical behavior of the NDI chromo-
phores is not affected by the burying borazino-doped polyphenylene architecture. The excitation spectrum of
59 (Figure 8) further supported the ET hypothesis, showing a strong contribution of the TBOZ scaffold to the
emission of the lowest-energy acceptor B-NDI. The energy transfer efficiency (
ΦET) has been estimated follow-
ing the method by Melinger et al.79 In particular, by using the equation ET = EcTBOZ/AcTBOZ , where ExTBOZ and
Φ
ATBOZ are fluorescence intensity and absorption of the donor TBOZ scaffold corrected by residual absorption
of the NDI moiety (see ESI), a remarkable energy transfer efficiency of 83% has been estimated (λexc= 262 nm).
This suggests the presence of a fast energy transfer (ET) process leading to an efficient TBOZ→B-NDI sensiti-
zation. Taken all together, these data advocate these BNC hybrids suitable antenna systems, as previously ex-
tensively documented for the all-carbon analogues developed by Müllen and co-workers.80–82
Conclusions
In conclusion, in this paper we have reported the first example about the use of borazine as a versatile modu-
lar building blocks to prepare hybrid BNC polyphenylenes featuring doping borazino cores at selected posi-
tions, given ratios and orientation patterns. Owing to the possibility of functionalizing the borazine core with
different groups on the aryl substituents at the N and B atoms, we have prepared BNC polyphenylenes taking
advantage of the decarbonylative [4+2] Diels-Alder cycloaddition reaction. To achieve it, two molecular mod-
ules were prepared: a core and a branching unit. Depending on the chemical nature of the central aromatic
module and of the reactive group, each covalent combination of the modules could yield one exclusive doping
pattern. As far as the borazine building blocks are concerned, di-ortho-methyl substituted aryl moieties were
used to protect the electrophilic boron atoms. Indulging this approach, we have prepared the first three- and
hexa-branched hybrid polyphenylenes featuring controlled orientation and dosages of the doping B3N3-rings.
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Detailed photophysical investigations, showed that upon increasing the doping dosage, the strong lumines-
cent signal is progressively reduced (Φem = 76% – 7%). This observation suggests that the presence of B3N3-
rings possibly increase the CS character of the excited states favouring a non-radiative deactivation of the sin-
glet-excited states. Notably, the effect of the borazine substitution pattern on the emission quantum yields
was only observed for those hybrid BNC polyphenylenes featuring low doping dosages. To further probe the
energy harvesting properties and chemical versatility of BNC polyphenylenes, a multichromophoric derivative
constituted by a tetra-borazino polyphenylene bearing three blue-colored dyes was also prepared through Cu-
catalyzed 1,3-dipolar cycloaddition.
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Taken all together, these results suggest that sterically-protected borazines can be used as versatile aromatic
platforms just like benzene and its derivatives. The ability of acquire ‘at-will’ substitution and doping patterns,
make these BNC polyphenylene architectures versatile molecular precursors for synthesizing planarized gra-
phitic nanostructures featuring controlled doping patterns. However, the lack of a suitable planarization pro-
tocol allowing the formation of C-C bonds in the presence of BN-type aromatics still limits the bottom-up ap-
proach for the preparation of regularly BN-doped molecular graphenes. This will be the synthetic challenge of
our future endeavors in the molecular BNC field.
ASSOCIATED CONTENT
Supporting Information
Synthetic protocols and spectroscopic characterisations. Crystallographic data (excluding structure factors) for the
structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. 1418730.
This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
bonifazid@cardiff.ac.uk
Author Contributions
‡These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENT
D.B. gratefully acknowledges the FRS-FNRS (FRFC contract n° 2.4.550.09), the Wallon Region for the ‘Flycoat’ and
‘TUBOLED’ POC projects, the Science Policy Office of the Belgian Federal Government (BELSPO-IAP 7/05 project).
We thank the ETHZ mass service for the HR-MALDI measurements for compounds 33 and 44. We thank Dr. An-
drea Fermi (Cardiff University) for the help in recording the life-time values and interpreting the photophysical
characterization. We thank Mr. Nicolas Biot (Cardiff University) for the plotting of the ESP surface potential.
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