Tailoring Colors by O Annulation of Polycyclic Aromatic Hydrocarbons

Article abstract of DOI:10.1002/chem.201604866

The synthesis of O-doped polyaromatic hydro- carbons in which two polycyclic aromatic hydrocarbon sub units are bridged through one or two O atoms has been achieved. This includes high-yield ring-closure key steps that, depending on the reaction condition

Full text of DOI:10.1002/chem.201604866

A Journal of
Accepted Article
Title: Tailoring colors by O-annulation of polycyclic aromatic
hydrocarbons
Authors: Tanja Miletić, Andrea Fermi, Ioannis Orfanos, Aggelos
Avramopoulos, Federica De Leo, Nicola Demitri, Giacomo
Bergamini, Paola Ceroni, Manthos G. Papadopoulos, Stelios
Couris, and Davide Bonifazi
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To be cited as: Chem. Eur. J. 10.1002/chem.201604866
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Tailoring colors by O-annulation of polycyclic aromatic
hydrocarbons
Tanja Miletić,[a,b] Andrea Fermi,[b,c] Ioannis Orfanos,[d,e] Aggelos Avramopoulos,[f] Federica De Leo,[c]
Nicola Demitri,[g] Giacomo Bergamini,[h] Paola Ceroni,[h] Manthos G. Papadopoulos,[f] Stelios Couris[d,e]
and Davide Bonifazi*[a,b,c]
Abstract: The synthesis of O-doped polyaromatic hydrocarbons, in which two polycyclic aromatic hydrocarbon subunits are bridged through
one or two O atoms, has been achieved. This includes high-yielding ring-closure key steps that, depending on the reaction conditions, yield
the formation of either furanyl or pyranopyranyl linkages through intramolecular C-O bond formation. Comprehensive photophysical
measurements in solution showed that these molecules feature exceptionally high emission yields and tunable absorption properties
throughout the UV-vis spectral region. Electrochemical investigations showed that in all cases the O-annulation increases the electron donor
capabilities by raising the HOMO energy level with the LUMO energy level being less affected. Moreover, third-order NLO measurements of
solutions or thin films containing the dyes displayed very good second hyperpolarizibility values. Importantly, PMMA films containing the
pyranopyranyl derivatives displayed weak linear absorption and NLO absorption compared to the nonlinearity and NLO refraction,
respectively, revealing to be exceptional organic materials for photonic devices.
Introduction
Amongst the plethora of organic semiconductors available, polycyclic aromatic hydrocarbons (PAHs) have certainly attracted an
increasing attention.[1–6] With respect to infinite graphene, PAHs display non-zero tunable bandgaps and are thus of use as
chromophores in antennae[7–12] or emissive molecular architectures[13–19] and in general in all optoelectronic applications requiring a
tunable semiconducting material.[6,20] Exploiting the organic synthetic tools,[21,22] one can tune the molecular HOMO-LUMO gap[8]
either by: i) changing the size and edge of the carbon-based aromatic framework; ii) varying the molecular planarity upon insertion of
bulky substituents or bridging chains; iii) changing the aromatic properties of the constituting monomeric units; iv) varying the
peripheral functionalization through the insertion of electron-donating or -withdrawing substituents; v) inclosing structural ‘‘defects’’;
vi) promoting supramolecular interactions between individual molecules governing their organization into a condensed phase or vii)
replacing selected carbon atoms by isostructural and isoelectronic analogues (i.e., doping). In particular, the heteroatom doping
approach[23,24] is increasingly becoming important, as significant perturbation of the optoelectronic properties can be obtained without
a substantial structural modification.[25–36]
[a]
[b]
[c]
Dr. T. Miletić, Prof. Dr. D. Bonifazi
Bottom-up covalent synthesis can be exploited to access
structurally defined heteroatom-doped graphene fragments with
precise control over the size, periphery, substitution pattern,
doping ratio and position.[36,37] In this respect, peri-
xanthenoxanthene (PXX),[38,39] the O-doped analogue of
anthanthrene, can be conceptualized as a building unit for
engineering a new class of O-doped PAHs. Substituted PXX
derivatives are characterized by excellent carrier transport and
injection properties, as well as easy processability, chemical
inertness and high-thermal stability.[40,41] Due to these properties,
PXX has proven exceptional performance when used as active
organic semiconductor (OSC) in transistors for rollable
OLEDs.[42,43] In particular, it has been proven that the good
performances are triggered by (i) the good charge transport
properties and (ii) the good thermal and chemical stability against
parasitic oxidations occurring at the periphery of the π-
conjugated system.[41,44]
Capitalizing on these hetero-doped structures, we have reported
very recently the synthesis of a series of O-doped isosteres of
benzorylenes, in which peripheral carbon atoms are replaced by
oxygen atoms at the armchair edges exploiting the Pummerer-
modified Cu(I)-catalyzed ring-closure reaction, involving
intramolecular C-O bond formation as the planarization
reaction.[45] Aiming at studying the effect of the π-extension of the
Department of Chemical and Pharmaceutical Sciences, INSTM UdR
Trieste, University of Trieste, Piazzale Europa 1, 34127 Trieste
(Italy)
Dr. T. Miletić, Dr. A. Fermi, Prof. Dr. D. Bonifazi
School of Chemistry, Cardiff University
Park Place, CF10 3AT, Cardiff (United Kingdom)
E-mail: bonifazid@cardiff.ac.uk.
Dr. A. Fermi, Dr. F. De Leo, Prof. Dr. D. Bonifazi
Department of Chemistry, University of Namur (UNamur), 61 Rue de
Bruxelles, Namur 5000 (Belgium)
G. Orfanos, Prof. Dr. S. Couris
Department of Physics, University of Patras, 26504 Patras (Greece)
G. Orfanos, Prof. Dr. S. Couris
Institute of Chemical Engineering Sciences (ICE-HT), Foundation for
Research and Technology-Hellas (FORTH), P.O. Box 1414, Patras
26504 (Greece)
Dr. A. Avramopoulos, Dr. M. G. Papadopoulos
Institute of Biology, Medicinal Chemistry and Biotechnology,
National Hellenic Research Foundation, 48 Vas. Constantinou
Avenue, Athens 11635 (Greece)
[d]
[e]
[f]
[g]
[h]
Dr. N. Demitri
Elettra – Sincrotrone Trieste, S.S. 14 Km 163.5 in Area Science
Park, 34149 Basovizza – Trieste (Italy)
Dr. G. Bergamini, Prof. Dr. P. Ceroni
Department of Chemistry «Giacomo Ciamician», University of
Bologna, Via Selmi 2, 40126 Bologna (Italy)
Supporting information for this article is given via a link at the end of
the document.
carbon framework, here we report on the preparation of π-extended PXX derivatives through the fusion of two PAH subunits (Figure
1). In our approach, we have considered the extended PXX derivatives reflecting bishydroxyPAHs with 2-hydroxyperylene and 2-
hydroxynaphthalene moieties as the key constituting units. At the synthetic planning level, we contemplated the oxidative Cu(I)-
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catalyzed planarization reaction developed by us[45] to form the pyranopyranyl motif (Figure 1). Indulging this synthetic protocol, the
easily prepared bishydroxyPAHs precursors[46–55] give us the opportunity to alternatively fuse the PAHs through furanyl linkages[56]
following an acid-catalyzed cyclization strategy (Figure 1). Photophysical and electrochemical characterizations showed that
complementary spectroscopic and redox properties can be tailored through a fine tuning of both the π-extension of the carbon
scaffold and the oxygen linkages (i.e., furanyl vs pyranopyranyl rings), ultimately covering the visible absorption and emission
spectral region. All derivatives display high absorption values and strong emissions. Non-linear optical (NLO) responses of all O-
doped polyaromatics were also investigated both in solution and as thin-films by Z-scan technique, employing 35 ps, 532 nm (Vis)
laser excitation. All showed strong second hyperpolarizabilities values, as completely predicted by theoretical calculations performed
at the CAM-B3LYP/6-31+G** level of theory. To ease the description of the different substitution patterns around the two fundamental
furanyl and pyranopyranyl cores, the nomenclature depicted in Figure 1 will be used throughout this manuscript. Being the
fundamental cores of the two molecular families composed of either a binaphthofurane or a PXX, each subnaphthyl ring can be
differently π-extended. In accordance with this general scheme, a labeling nomenclature is proposed, where the n and m indexes
indicate the possible π-extension of the carbon framework expressed as a number of C-C fused naphthyl rings.
Figure 1. Fusion of PAHs through furanyl (right) and pyranopyranyl (left) cyclization strategies (n and m indexes indicate the possible π-extension of the carbon
framework expressed as a number if C-C fused naphthyl rings).
Results and discussion
Synthesis. The synthesis commenced with the preparation of hydroxyl-perylene 2 (Scheme 1). Friedel-Craft alkylation of perylene,
following the protocol by Pillow et al.[57] in the presence of a large excess of tBuCl,[58–60] leads to an inseparable mixture of di-, tri- and
tetra substituted tBu-perylene derivatives. The unpurified perylene mixture was subsequently submitted to selective C-H borylation
reaction in the presence of 10 mol% of [Ir(COD)(OMe)]2 catalyst and 20 mol% of dtbpy and B2pin2 in n-hexane at 80 °C for 24 h,[61]
allowing the isolation of tetra-tert-butylperylene 1a, mono- and bis-boronic esters 1b and 1c (see X-ray structure Section S6, SI) in 33,
20 and 30% yield, respectively. Oxidation of boronic esters 1b using H2O2 and NaOH in THF at r.t.,[62] yielded perylenol derivative 2 in
80-85%. Following the literature synthetic routes for preparing BINOLs,[46–52,54,55,63] we turned our attention toward the Cu-TMEDA-
based oxidative C-C bond formation[64,65] as dimerization reaction. Thus, homodimerization of perylenol 2 was performed in the
presence of [Cu(OH)Cl•TMEDA] catalyst under air at 20 °C in CH2Cl2, to give bisperylenol 4 in 71% yield. Small transparent crystals
of 4 were obtained by vapor diffusion of MeOH into a solution of 4 in CH2Br2. The asymmetric unit of the crystals contains one
independent molecule, which is H-bonded to two MeOH molecules (Figure 2). The molecular structure depicted in Figure 2a, nicely
reveals the non-planar arrangement of the two perylene scaffolds with an interplanar angle of 62.8° with the two hydroxyl groups
adopting a syn conformation. The crystal packing shows the formation of dimeric species held by H-bonding interactions bridged by
solvent (MeOH) molecules. Similarly, perylenol 2 was cross-coupled with 2-naphthol to give naphthalenylperylene derivative 3 in 26%
yield (homodimers bisperylenol 4 and BINOL were also formed and thus separated by column chromatography). It should be noted
that bishydroxyPAHs 3 and 4 were prepared and used as scalemic mixtures. Following the oxidative protocol recently developed by
us,[45] the dihydroxy species were subsequently cyclized into the relevant pyranopyran derivatives. Specifically, Cu-catalyzed
oxidative intramolecular etherification of dihydroxy derivatives 3 and 4 (CuI and PivOH in DMSO at 140 °C in the air) afforded
pyranopyran derivatives 5Pp and 6Pp, in 57% and 84% yield, respectively. The structures of all intermediates and products were
1
unambiguously identified by HR-MALDI through the detection of the peak corresponding to the molecular mass (M) and by H- and
13C-NMR, UV-Vis, and IR spectroscopy (see Section S3, SI). In particular, HR-MALDI showed the peak related to the molecular mass
(M) at m/z 574.2889 (C42H38O2, calc.: 574.2872) and 866.5059 (C64H66O2, calc.: 866.5063), for extended pyranopyran 5Pp and 6Pp
,
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respectively. Unfortunately, 13C-NMR could not be recorded, as both pyranopyranyl derivatives seem to display limited solubility,
likely triggered by their great tendency to undergo strong aggregation.
Scheme 1. Synthetic pathway for compounds 1-6Fur/Pp. Reagents and conditions: a) 1. AlCl3, t-BuCl, ODCB, 0°C to r.t., 24h; 2. [Ir(COD)(OMe)]2, 10 mol%, dtbpy,
20 mol%, B2Pin2, n-hexane, 80°C, 24h; b) NaOH, H2O2 aq. sol. 35 wt%, THF, r.t., 2h; c) 2-naphthol, [Cu(OH)(Cl)TMEDA], air, CH2Cl2, 20°C, 2h; d)
[Cu(OH)(Cl)TMEDA], air, CH2Cl2, 20°C, 1h; e,g) p-TsOH, Toluene, reflux, Ar, 4h; f,h) CuI, (CH3)3CCOOH, DMSO, 140 °C, 2h.
To qualitatively support this assumption, a 2 mM toluene solution of 6Pp was drop-casted to a silicon wafer. After solvent evaporation,
SEM imaging of the remaining powder show the presence of microscale brittle stick-like morphologies (Figure S28, SI). Alternatively,
when dihydroxy precursors 3 and 4 were reacted in the presence of p-TsOH in a refluxing toluene solution,[66,67] extended furanyl
1
derivatives 5Fur and 6Fur could be obtained in 56% and 90% yield. Again, their structures were unambiguously identified by H- and
13C-NMR, UV-Vis, and IR spectroscopies, with the HR-MALDI showing the peak corresponding to the molecular mass at m/z
560.3079 (C42H40O, calc.: 560.3079) and 852.5268 (C64H68O, calc.: 852.5270) for extended furanes 5Fur and 6Fur, respectively.
Reference compounds 7Fur/Pp and 8Fur/Pp (Scheme 1) were also prepared following similar synthetic strategies to those developed for
compounds 5 and 6 (Scheme S1, SI). To further corroborate the chemical structure of the cyclized derivatives, crystals suitable for X-
ray diffraction analysis were obtained either by vapor diffusion or slow solvent evaporation of solutions containing the relevant
product (Figures 3-5, for the detailed crystallization procedures refer to SI). Despite the many attempts, no suitable crystals were
obtained for the pyranopyran derivatives, and only those obtained for the furanyl derivatives will be discussed in this paper.
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Figure 2. a) Top-view and b,c) side-views of the crystal structure and packing of bisperynol derivative 4 (space group: P-1; atom colors: red O, grey C; hydrogens
omitted for clarity). Notably, the hydroxyl groups of the bisperynols are engaged in H-bonding interactions through two bridging MeOH solvent molecules forming
dimeric species at the solid state.
Figure 3. a,d) Top-views and b,c) side-views of the crystal structure and π-π packing arrangement of furanyl derivative 5Fur (center, space group: P 21/c; atom
colors: red O and grey C; hydrogens omitted for clarity). Crystals obtained from slow vapor diffusion of MeOH to a C6D6 solution.
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Figure 4. a,d) Top-views and b,c) side-views of the crystal structure and π-π stacking arrangement of furanyl derivative 6Fur (bottom, space group: P 2/c; atom
colors: red O and grey C; hydrogens omitted for clarity). Crystals obtained from slow evaporation of a C6D6/hexane solution.
Figure 5. a,d) Top-views and b,c) side-views of the crystal structure and π-π packing arrangement of furanyl derivative 7Fur (space group: P 212121; atom colors:
red O and grey C; hydrogens omitted for clarity). Crystals obtained from slow evaporation of a CH2Cl2 solution.
All the X-ray structures of 5Fur-7Fur confirm the presence of the furanyl ring. Looking at their organization at the solid state (Figures 3-
5c,d), one can clearly evidence the presence of columnar arrangements in which the molecules are organized in π-π stacks, with a
similar average interplanar spacing of ~3.6 Å for derivatives 5Fur, 6Fur and 7Fur. As expected, the presence of the five-membered cycle
induces a significant distortion of the PAH moieties due to C-H…H-C repulsive interactions (see also Figure 1), triggering interplanar
angles of ~18.1, ~16.7 and ~12.9° for 5Fur, 6Fur and 7Fur, respectively. Notably, the presence of the highly hindering tBu groups in
molecules 5Fur and 6Fur further raises the interplanar angle. While furane 5Fur antiparallely stacks into a columnar arrangement with no
lateral off-set, molecules 6Fur and 7Fur form off-set solid-state organizations. In particular, bisperylene furane 6Fur organizes through
perylene-perylene stacks with a lateral off-set of 3.5(1) Å, whereas phenanthrenenaphtyl furane 7Fur undergoes off-set π-π stacking
involving the naphthalene and phenanthrene moieties. The thermal stability of compounds 4, 6Fur, 6Pp were investigated by
thermogravimetric analysis (TGA) and compared with reference compound 8Pp. While molecule 8Pp starts to sublimate at 188°C
reaching its maximum at 265°C (see also DTG profile, Figure S35a, SI), the TGA profiles of compounds 4, 6Fur and 6Pp showed very
high thermal stability under N2, with sublimation temperatures centered at 320, 340 and 400°C, respectively (Figures S35b-d, SI). The
thermal stability of extended pyranopyran derivative 6Pp was also evaluated in the air with a scan rate of 10°C min-1. The TGA profile
(Figure S36, SI) only displays a significant weight loss above 370°C, suggesting that molecule 6Pp can stand the critical
environmental conditions typical of a fully-operative device, namely under O2 and high temperatures.
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Absorption and emission spectroscopic investigations. The effects of increasing the conjugation for both furanyl and
pyranopyranyl derivatives have been evaluated by steady-state UV-Vis absorption and emission spectroscopies, and the key data
are gathered in Table 1. In general, all compounds show high molar absorption coefficients (up to 105 M-1cm-1) spanning from
the blue to the red region of the visible spectrum, with luminescence lifetimes consistent with singlet radiative deactivations
(τ=2–6 ns). Quantum yields are remarkably high across the visible spectrum, with the pyranopyranyl derivatives showing
lower emission quantum efficiencies (average Ф value of ~0.5) with respect to the homologous furanyl molecules (average
Ф value of ~0.8).
Table 1. Optical properties for compounds O-doped PAHs in toluene solutions.
Molecule
λ (nm)[a], ε (M-1 cm-1)
352, 26900
446, 14100
445, 25600
452, 33700
463, 65 000
477, 47300
556, 36300
534, 97 400
639, 66400
λem (nm)[b]
Ф
τ (ns)
5.7
4.8
4.9
3.7
2.7
3.0
3.8
3.0
2.2
7Fur
7Pp
2
371
0.43
0.48
0.55
0.66
0.88
0.84
0.50
0.80
0.52
452
463
3
508
4
472
5Fur
5Pp
6Fur
6Pp
494
569
547
649
[a]UV-vis absorption maximum of the lowest energy band in toluene. [b]Emission maximum in
toluene at 25 °C.
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Taking into account singly-bonded naphthol-perylenol 3 and conjugated derivatives 5Fur and 5Pp, clear bathochromic shifts
both in the absorption and in the emission spectra are evidenced when passing from the non-planar to the planarized
derivatives (Figure 6a), with the relevant pyranopyranyl molecule displaying the larger red shifts if compared to the furanyl
analogue. An equivalent trend is also observed with perylenol 2, its singly-linked dimer 4 and molecules 6Fur and 6Pp,
showing largely tunable absorption and emission energies that fall in the red spectral region for the O-annulated derivatives
(Figure 6b). Comparing these results with those reported in the literature with all-carbon PAHs displaying comparable
visible absorption energies,[68-71] our steady-state studies suggest that, although providing significant bathochromic shifts,
the C-O planarization does not dramatically suppress the emission quantum yields as opposed to the formation of the C-C
bonds in planar rylenes, the latters usually displaying strong absorptivities but faint luminescence outputs.[72,73]
Figure 6. a) Normalized absorption (—) and emission (…..) spectra in toluene at 25°C of a) naphthol-perylenol series: 3 (dark yellow), 5Fur (green) and 5Pp (purple)
and b) bis-perylenol series: 2 (cyan), 4 (orange), 6Fur (red) and 6Pp (blue).
To probe the solid-state emissive properties of all conjugates,[56] different molding solvents[74] were screened to reproducibly obtain
solid morphologies of defined shape. Furanes 5Fur and 6Fur and pyranopyrans 5Pp and 6Pp were initially investigated. Among the
different conditions, we found that the slow addition of MeOH to a THF solution of the relevant dye gives rise to a colloidal cloudy
solution that, upon prolonged times, undergoes precipitation leading to crystalline powdery solids. SEM imaging analysis (Figure 7) of
the dried powder, shows the formation of well-defined and reproducible structures at the microscale featuring different morphologies
depending on the chemical structure of the crystallizing molecule. Specifically, compound 5Fur leads to the formation of elongated
hexagonal prisms (Figures 7a-b), 1 µm large and wide, with a length in the order of 5-10 µm. It is noteworthy to underline that needle-
like structures, longer than the average prisms, were also present in the sample. On the other side, molecule 5Pp forms 3-10 µm long
stick-like morphologies, featuring thinner diameters (Figures 7c-d). Passing to the bisperyleno derivatives, compound 6Fur arranges in
disk-like structures with a diameter in the micrometer range (Figures 7e-f), while 6Pp forms rod-like shapes (Figures 7g-h) also
featuring microscopic dimensions (for more images see also Figure S29, SI).
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Figure 7. SEM images of the organic nanostructures obtained from a THF solution upon addition of MeOH for molecules 5Fur (a, b), 5Pp (c, d), 6Fur (e, f) and 6Pp
(g, h).
The recorded solid-state emission spectra for compounds 5Fur, 6Fur and 7Fur are depicted in Figure 8. Both model compounds 8Fur and
8Pp show appreciable luminescence upon excitation of the powders at room temperature: while 8Fur displays a blue emission with a
structured spectrum featuring maximum at around 400 nm, its pyranopyranyl derivative 8Pp appears as a yellow-green emitter with a
broad spectrum with a maximum at around 545 nm (Figure S44, SI). Owing to the narrowing effect of the pyrano conjugation, also
the solid-state emission spectrum of pyrano 8Pp displays a marked red-shifted profile if compared to that of 8Fur. The extension of the
π-surface of molecule 8Pp negatively affects the solid-state emissive properties, namely the luminescence outputs of solids obtained
from molecules 5Pp and 6Pp are hardly detectable; differently, the increase of the conjugation length of furanyl compounds 5Fur and
6Fur does not dramatically affect the solid-state luminescence. Specifically, molecule 5Fur intensely emits a yellow-orange color,
showing a structured emission spectrum with a maximum around 560 nm, whereas 6Fur strongly luminesces, displaying a vibrational
structure and a sharp peak at 645 nm. Although the emission spectra of the amorphous powders of 5Fur, 6Fur and 7Fur resulted in
some extent sensitive to prolonged UV excitation, and we cannot therefore determine precisely the value of each luminescence
quantum yield, these solid-state emission findings are in agreement with literature data reporting high quantum yield for the
naphthofuran derivatives.[56,67] Differences in the emissive properties between the naphthofurans and pyranopyranyl-derivatives at the
solid are probably correlated with the structural properties of the respective crystal lattices (see above). For instance, considering that
molecule 8Pp (PXX) is self-organized at the solid-state through π-π stacks,[44] we can hypothesize that all pyranopyranyl derivatives
likely undergo similar face-to-face arrangements, the latter organizations most favoring non-radiative decays of the emissive excited
states.[15,75,76]
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Figure 8. a) emission spectra of 7Fur (blue line), 5Fur (orange line), and 6Fur (red line) in solid samples at r.t. Excitation wavelengths: 360 nm. b) calculated CIE
diagram for solid state emissions of 7Fur (triangle), 5Fur (square) and 6Fur (circle). Dotted lines are plotted to display the color gamut accessible by combinations of
the three solid emitters.
Electrochemical Investigations. Cyclic voltammetry (CV) in 1,2-dichlorobenzene (ODCB) was used to get further information about
the redox properties of compounds 5Fur/Pp-8Fur/Pp. Their redox potentials vs. Fc/Fc+ are summarized in Table 2.
Figure 9. Cyclic voltammetry of a) 8Fur (0.80 mM, solid line) and 7Fur (0.75 mM, dashed line); b) 8Pp (PXX) (0.81 mM, solid line) and 7Pp (0.43 mM, dashed line).
Half-wave oxidation potentials of 8Fur and 8Pp (PXX) are evidenced by vertical dotted lines in plots a) and b) respectively, for comparison purposes. Scan rate: 50
mV/s. Supporting electrolyte: TBAPF6. Ferrocene is used as internal reference standard.
Table 2. Half-wave potentials calculated vs. the Fc/Fc+ couple.
Molecule
8Fur
E1/2 ox1 (V)
0.89 (117)
0.30 (111)
E1/2 ox2(V)
E1/2 red1(V)
E1/2 red2 (V)
nd
nd
nd
nd
nd
nd
8Pp (PXX)
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7Fur
0.84 (100)
0.25 (81)
nd
nd
nd
7Pp
4b
nd
nd
nd
0.30 (133)
0.37 (128)
0.00 (133)
0.20 (127)
-0.15 (100)
0.52 (128)
0.90 (140)
0.56 (122)
0.57 (127)
0.32 (107)
-2.37[a] (irr)
-2.20 (90)
-2.25 (101)
-2.15 (112)
-2.16 (100)
-2.50[a] (irr)
nd
5Fur
5Pp
6Fur
6Pp
nd
-2.32 (123)
-2.26 (120)
Half-wave potentials are calculated as E1/2=(Epa+Epc)/2 considering anodic (Epa) and cathodic
(Epc) peak potentials, unless differently specified. Values in parenthesis are referred to the
peak separation (Epa-Epc, in mV) of each reversible process; "nd" stands for "not detected" and
“irr” for an “irreversible process”. [a]Determined considering the cathodic peak of an irreversible
reduction process.
As depicted in Figures 9a-b, reference molecules 7Fur-8Fur and 7Pp-8Pp each show only a 1-e-, reversible redox couple at similar
positive potentials for the furanyl (0.89 and 0.84 V for 8Fur and 7Fur) and pyranopyranyl derivatives (0.30 and 0.25 V for 8Pp and 7Pp),
suggesting that the addition of an extra benzo[a]ring in the molecular structure marginally affects the oxidation potential of the
fundamental O-annulated bisnaphthyl derivatives. Notably, no reductions were observed in the electrochemical window of
investigation in ODCB. Interestingly, direct comparison of voltammetric behaviors of compounds 8Fur and 8Pp evidences the strong
ox1
electron-donor character of the pyranopyranyl moiety compared to that of furanyl ring, with a significant lowering of the E1/2
value
of ~0.60 V. Concerning the naphthofurane family, the expansion of carbon-based π-surface as in molecules 5Fur (m = 0, n =
1) and 6Fur (m = 1, n = 1) dramatically affects the voltammetric behavior compared to that of the reference molecules
(Figure 10a-b). Specifically, two monoelectronic and reversible oxidation peaks for 5Fur appear at 0.37 and 0.90 V as a
consequence of the Coulombic interactions between the first and second oxidation holes. Curiously, a first 1-e- reduction
wave also appears as a separated, reversible couple at -2.20 V. As expected, with respect to molecule 8Fur, the lateral
extension of the π-surface through the introduction of a naphthyl unit (m = 0, n = 1) makes molecule 5Fur a stronger
electron-donor. The trend is further evidenced with molecule 6Fur, featuring symmetrical substitution with two perylenyl units
(m = n = 1), which displays even lower oxidation potentials (at 0.20 and 0.57 V for the first and the second wave,
respectively); at the same time, two reversible reductions are recorded at similar potentials as those observed for reducing
molecule 5Fur
.
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Figure 10. Cyclic voltammetry of molecules a) 5Fur 0.64 mM, b) 6Fur 0.63 mM, c) 5Pp 0.61 mM, d) 6Pp 0.63 mM, e) 4b 0.53 mM. Scan rate: 50 mV/s. Supporting
electrolyte: TBAPF6. Ferrocene is used as internal reference standard.
As expected, in the case of the PXX-derivatives, the trend is very similar to that described for the naphthofurans family:
molecule 5Pp (m=0, n = 1) features lower oxidation potentials (at ca. 0 V vs Fc/Fc+) if compared to model compound 8Pp,
while a monoelectronic reduction process appears at -2.25 V (Figure 10c). The symmetrical substitution on both sides of
the PXX core by two perylenyl units (m = n = 1) makes molecule 6Pp electron-richer and easier to oxidize than 8Pp, shifting
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the first oxidation potential at -0.15 V (Figure 10d). To gain insight on the electronic role of oxygen atoms in the conjugated
system of the PXX-based derivatives, cyclic voltammetry of dimethoxy-biperylene 4b (see Section S2, SI) was also performed
(Figure 10e). Two reversible oxidation processes are detected at 0.30 and 0.52 V, reflecting the weaker electron-donating nature of
this structure when compared to that featuring the two oxygen atoms in the π-conjugated pyranopyranyl motif, as in the case of 6Pp
.
Furthermore, we can evidence that the inclusion of the two oxygen atoms in the aromatic system affects significantly the HOMO-
LUMO energy gap values (EgCV, Table 3), which has been experimentally estimated to be 2.67 and 2.01 eV for 4b and 6Pp
,
respectively.
Table 3. Estimated HOMO-LUMO Energy gap (Eg) for compounds 5Fur/Pp-8Fur/Pp as determined from the optical (E00
)
[a], electrochemical (EgCV) and
T
theoretical (Eg
)
[b] studies.
CV
T
T
CV
E00
Eg
Eg
Eg
Eg
E00
Molecule
Molecule
(nm, eV)
(eV)
(eV)
(eV)
(eV)
(nm, eV)
8Fur
7Fur
5Fur
6Fur
361, 3.43
371, 3.34
494, 2.51
547, 2.27
---
3.95
---
3.32
---
---
449, 2.76
452, 2.74
569, 2.18
649, 1.91
8Pp
7Pp
5Pp
6Pp
---
---
2.57
2.35
2.83
2.48
2.50
2.20
2.25
2.01
[a]Calculated from the wavelength of the emission maximum in toluene. [b]Calculated bandgap value at the B3LYP/6-31G** gas phase optimized
geometry.
From these data it becomes apparent that while the lateral π-extension of the PAHs substructures with a naphthyl unit accounts for a
decrease of the EgCV value of approximately 0.30 V - cf the furanyl (left) and pyranopyranyl (right) families in Table 3. Similarly, the O-
cyclization of the bishydroxyPAHs strongly affect the EgCV value, systematically shrinking the gap of ca. 0.30 and 0.6 V for the furanyl
and pyranopyrayl cycles, respectively (cf the furanyl and pyranopyranyl analogues, Table 3 and Figure 11). Notably, an excellent
CV
accord between the electrochemical Eg and optical Eg values is clearly observable, with the latter being calculated from the λmax of
lowest-energy electronic transition. The rainbow collection in Figure 11 perfectly illustrates how the HOMO–LUMO gaps decrease
mainly because of the increase of the HOMO energy levels as a consequence of the progressive π-extension and the inclusion of
donating furanyl and pyranopyranyl cores. By playing with the type of the O-ring and the number of fused naphthyl rings one can
cover the primary colors, moving from yellow furanyl 5Fur, to orange bisperylenylfurane 6Fur to pink 5Pp and blue 6Pp pyranopyranyl
derivatives. Compared to the tunable core-substituted naphthalenediimides (cNDIs),[77-78] whose color tailoring is achieved through a
combination of electron donating substituents in the core leading to push–pull chromophores with the electron withdrawing imide
groups,[79] the O-doped PAHs investigated in this work can be considered as valid alternatives for applications in which electronically
rich (i.e., high HOMO energy levels) and strongly emissive chromophores are required.[80-82]
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Figure 11. Frontier orbital energies for compounds analyzed by cyclic voltammetry in 1,2-dichlorobenzene (dashed lines corresponding to calculated energies
using the optical E00, in red). The formal potential of the Fc/Fc+ redox couple, taken as a reference, is assumed to be -5.1 eV vs. vacuum.
Theoretical modeling. To shed further light on the electronic structure and optical properties of the O-annulated derivatives, the
electronic properties of the HOMO and LUMO levels were calculated performing Density Functional Theory (DFT) calculations using
the Gaussian 09 package.[83] The results are summarized in Tables 3 and 4. As the tBu substituents have a small effect on the EHOMO
and ELUMO energy levels, some of the calculations were performed without the alkyl substituents. Each molecule was modeled in its
neutral state performing a geometry optimization and a single point calculation using the Restricted Becke’s three-parameter
exchange functional,[85] the Lee–Yang–Parr correlation functional[84] (B3LYP/6-31G** level of theory). The crystal structures, when
available, were considered as the starting geometries. The HOMO and LUMO orbitals were plotted using the Avogadro software.[86]
Other orbitals up to HOMO-4 and LUMO+4 were also calculated (Figures S49-56, SI). It transpires that the molecular HOMO and
LUMO orbitals are located on the entire π–surface of the molecules, with the oxygen atoms contributing to the given orbitals
differently, depending on the nature of the cyclic linkage. In particular, for the furanyl derivatives the O atom does not significantly
contribute to the HOMO orbital, whereas a significant involvement of the O atoms to the HOMO of the pyranopyranyl derivatives is
clearly observable (Figure 12). On the contrary, a non-negligible contribution of the O atoms to the LUMO orbitals is noticeable for
both O-annulated derivatives. As observed with the electrochemical characterization, the O-cyclization greatly affects the HOMO
energy levels (EHOMO), with the pyranopyranyl ring inducing the greatest enhancement of the EHOMO values and thus impacting the
most both optical and electrochemical Eg values. On the other hand, the π-extension of the all-carbon aromatic units (m = 0, n = 1
and m = n = 1) affects both EHOMO and ELUMO energy levels, ultimately triggering the decrease of the Eg value as typically observed for
π-extended PAHs.[87-89] Notably, the solvent effect (toluene) on the wavelength (λ) for the first allowed electronic transition was also
considered in the simulation (Table 4).
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Figure 12. Molecular orbitals for the furanyl (left) and pyranopyranyl (right) derivatives calculated at the B3LYP/6-31G** level of theory.
For both furanyl and pyranopyranyl derivatives, λ increases with n, with the absorption of the pyranopyranyl derivatives being more
red-shifted with respect to the furanyl analogues (λPp > λFur). For 8Pp a maximum is observed at 400 nm, which lies closely to the
absorption (λ = 394.90 nm), whereas the maximum value of 8Fur is located at λ about 330 nm, which is close to the absorption (λ =
323.09 nm). In Figure S65, the first allowed electronic transitions computed with the aid of natural transition orbital pairs[90] for
compounds 8Fur, 8Pp, 5Fur, 5Pp, 6Fur and 6Pp are presented. As one can notice, all electronic transitions occur through π→π*
excitations, with the lowest energy absorption band assigned to the HOMO→LUMO transition (see SI for the simulated UV-vis
spectra along with the computed bands of the allowed electronic transitions for derivatives 8Fur and 8Pp).
[a]
[b]
Table 4 Computed EHOMO
,
ELUMO
,
EHOMO-ELUMO (EgT), absorption wavelength λ (nm), and the excitation energy Eexc (eV) for
the first allowed electronic transitions. The values were calculated with the CAM-B3LYP/6-31+G**method with explicit solvation
(toluene).
EHOMO & ELUMO
(eV)
Molecule
EgT (eV)
λ (nm)
Eexc (eV)
Type of excitation
8Fur
-5.36 & -1.42
3.95
323.09
3.84
HOMO → LUMO (92%)
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-4.87 & -2.07
2.80
2.83
446.22
2.78
2.79
5Fur
HOMO → LUMO (96.9%)
-4.73[b] & -1.90[b]
444.87[b]
-4.71 & -2.23
2.48
2.48
496.40
2.50
6Fur
8Pp
5Pp
HOMO → LUMO (90.3%)
HOMO → LUMO (95%)
HOMO → LUMO (95.8%)
-4.54[b] & -2.07[b]
494.02[b]
2.51[a]
-4.71 & -1.39
3.32
394.9
3.14
-4.49 & -2.01
2.48
2.50
502.67
2.47
-4.38[b] & -1.88[b]
499.96[b]
2.48[a]
-4.38 & -2.20
2.18
565.23
2.19
6Pp
HOMO → LUMO (93.0%)
-4.25[b] & -2.04[b]
2.20[b]
566.72[b]
2.19[a]
HOMO→LUMO (50%)
HOMO→LUMO+1 (20%)
4
-4.71[b] & -1.96[b]
2.75[b]
438.05[b]
2.83
[a] B3LYP/6-31G**; [b] with tBu group.
Nonlinear Optical (NLO) studies. With these findings in hand, the nonlinear optics (NLO) properties of molecules 4 and
5Fur/Pp, 6Fur/Pp and 8Fur/Pp have been systematically investigated using the Z-scan technique, employing visible (532 nm), 35 ps
laser pulses from a mode locked Nd:YAG laser. By performing measurements on solutions at different concentrations
under various incident laser excitation energies, the nonlinear absorption coefficient, β, and the nonlinear refractive
parameter, γ′, have been determined (Table S2; for more details on the dependence between β, γ′ and the third-order
nonlinear susceptibility, χ(3), see section S12, SI). Most of the O-doped PAHs exhibit negligible NLO absorption and
significant NLO refraction. Specifically, only molecules 8Pp and 5Pp, displaying significant NLO absorption, show reverse
saturable absorption (RSA, β>0) and saturable (SA, β<0) absorption behaviors, respectively (Figure 13). As RSA materials
show lower transmission upon increasing the incident laser intensity (i.e., the sample becoming less transparent) and SA
materials become progressively more transparent at higher incident laser intensities, both types of NLO absorption
behaviors are of great interest for a variety of photonic and opto-electronic applications (e.g., optical limiters, saturable
absorbers, etc.). Concerning the NLO refraction, most of toluene solutions of the O-annulated PAHs exhibited self-
defocusing (i.e., negative sign nonlinear refractive index parameter γ′ values, see Table 5) with the exception of pyranyl
derivatives 5Pp and 6Pp, which displayed a self-focusing behavior (i.e., positive γ' values, see Figure S57, SI). In order to
better rationalize these findings, the UV-Vis-NIR absorption spectra of toluene solutions containing the relevant O-annulated PAHs
were compared to those obtained from calculations (Figures S60 and S61, SI). Thus, from the comparison between the energy of the
lower energy electronic transitions of the different O-doped PAHs studied here and the energy of the laser excitation photons, it
becomes evident that different degree of resonant enhancement is expected to occur. In particular, going from molecule 8Fur, to 5Fur
and then to 6Furthe lower energy electronic transitions shift to longer wavelengths, getting closer to the laser excitation wavelength
and resulting in more efficient resonance enhancement as shown in Table S2, SI. Specficially, the lowest energy electronic transition
of molecule 8Fur, occurring at 357 nm, shifts to 477 nm in the case of 5Fur and to 534 nm in the case of 6Fur. Similar observations have
been evidenced for the pyranyl counterparts as well. These energy shifts of the lower energy electronic transition give rise to
significant resonant enhancement of the NLO response, in line with the general rule according to which the closer is the lowest
energy electronic transition to the energy of the laser excitation photon (532 nm), the higher is the degree of resonant enhancement
of the NLO response. Therefore, considering the lowest-energy transitions centered at 357, 477 and 534 nm for molecules 8Fur, 5Fur
and 6Fur, respectively, the excitation of furanyl derivative 6Fur gives rise to the highest NLO response. Furthermore, as the sign of the
NLO refraction depends on the relative position of the excitation wavelength with respect to the molecular absorption band,[91-93] one
can obtain NLO refractions with opposite signs. In particular, as for molecule 5Fur the excitation occurs at a longer wavelength
compared to its lowest-energy absorption band, molecules 5Fur and 5Pp exhibit opposite NLO refractions. Along this line, molecules
8Fur, 8Pp and 4 were also found to exhibit negative NLO refractions as the excitations take place at longer wavelengths than the
lowest-energy absorption bands of the relevant colorant.
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Figure 13. “Open-aperture” Z-scans of solutions containing 8Pp (a) and 5Pp (b) measured under 35 ps, 532 nm laser excitation.
The third-order susceptibility χ(3) and the second hyperpolarizability γ have been then deduced (see Tables 5 and S2, SI), identifying
a trend within each family of O-annulated compounds. Specifically, starting from model compounds 8Fur and 8Pp, we can observe an
increase of the γ values of two orders of magnitude by extending the molecular π-surface, i.e. for molecules 5Fur/Pp and 6Fur/Pp. To
shed further light on this NLO response, we have calculated the electronic static (i.e., when the excitation frequency of the incoming
laser beam tends to zero, with ω→0) and the frequency-dependent (i.e., ω≠0) second hyperpolarizabilities (Table 6), denoted as
γ(0;0,0,0) and γ(-ω;ω,-ω,ω) respectively. As one can easily discern from Table 6, γ(0;0,0,0) and γ(-ω;ω,-ω,ω) are larger for the
pyranopyranyl derivatives compared to those of the furanyl compounds. This is in full agreement with the experimental observations.
An exception to this response was found for γ(0;0,0,0) of molecules 8Pp and 8Fur, for which an opposite behaviour was observed,
namely γFur(0;0,0,0) > γPp(0;0,0,0). As one can observe (see Figure 14a) in the spectral region where the excitation occurs (532 nm),
the theoretical γ(-ω;ω,-ω,ω) value for pyranopyranyl derivative 8Pp is significantly larger compared to its furanyl analogue 8Fur, again
in agreement with the experimental findings (see above); furthermore, the large increase of γ(-ω;ω,-ω,ω) value for pyranopyranyl 8Pp
at about 250 nm, is associated with the significant light absorption of the molecule in the given spectral region. Similar results are
evidenced for molecule 5Pp, which shows a clear enhancement of the γyyyy(-ω;ω,-ω,ω) value at ca. 500 nm (see Figure 14b).
Corroborating the experimental findings, theoretical calculations indicate that γ(0;0,0,0) and γ(-ω;ω,ω,ω) are strongly affected by the
type of O-annulation, with the pyranyl derivatives displaying the larger values. Finally, the NLO response of the O-doped PAHs
studied here was found to be of comparable magnitude with that of some other organic conjugated materials, reported recently and
being also promising for photonic and opto-electronic applications.[98,99]
Table 5. Second hyperpolarizability (γ) values of O-doped PAHs dissolved in toluene, determined under 35 ps, 532 nm laser excitation.
Reγ
Imγ
γ
γ
Imγ
Reγ
[a]
a
Molecule (λmax
)
Molecule (λmax)
(×10-31 esu)
(×10-31 esu)
(×10-31 esu)
(×10-31 esu)
(×10-31 esu)
(×10-31 esu)
8Fur (357 nm)
5Fur (477 nm)
6Fur (534 nm)
4 (463 nm)
-0.0015±0.0003
-0.11±0.05
-
-
-
-
0.0015±0.0003
0.11±0.05
0.013±0.002
0.14±0.04
1.34±0.20
0.0058±0.002
-0.012±0.002
0.094±0.001
1.34±0.20
8Pp (444 nm)
5Pp (556 nm)
6Pp (639 nm)
-0.099±0.003
-0.14±0.05
0.14±0.05
-
-0.025±0.005
0.025±0.005
[a] Wavelength of the lowest-energy electronic transition in toluene.
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Figure 14. Calculated wavelength dependence γyyyy(-ω;ω,-ω,ω) profile for 8Pp and 8Fur (a) and 5Pp (b). The values were computed with CAM-B3LYP/6-31+G**
method at the gas phase.
In order to study the effect of the dielectric environment (solvent effect) on the NLO response, we have also computed the ρ value,
defined as the ratio between the static second-hyperpolarizability values in solution γ(0;0,0,0)sol and in the gas phase γ(0;0,0,0)gas
(Table 6). As one can observe from Figure 15a, the ρ value increases with the extension of the molecular π-surface; also, larger ρ
values are found for the planar pyranopyranyl derivatives (ρPp>ρFur). These data suggest that the more extended is the π-surface
and the planarity of the molecule, the greater is the effect of the solvent on the NLO response. Moreover, in Figure 15b the
T
dependence between γ(0;0,0,0) and Eg for derivatives 5Fur/Pp, 6Fur/Pp and 8Fur/Pp is depicted. It clearly appears that the decrease of
T
Eg leads to an increase of γ(0;0,0,0), suggesting that for these molecular scaffolds the tuning of the HOMO and LUMO energy
levels greatly affects their NLO response. It is important to emphasize here that the difference of one order of magnitude between the
NLO responses of 8Fur and 8Pp can be attributed to the change of the degree of molecular planarity, as their lower absorption bands
are lying at 357 and 444 nm, respectively, well below the excitation wavelength at 532 nm.
Figure 15. a) The average value of the static second-hyperpolarizability of the studied molecules computed in the gas phase and in toluene solution. b) Variation
of the static average second-hyperpolarizability γ(0;0,0,0) for the O-annulated PAHs with the EgT. The CAM-B3LYP/6-31+G** method was used in the presence of
toluene as solvent.
Table 6. The average values of static (γ(0)) and dynamic second-hyperpolarizabilities (γ(-ω;ω,-ω,ω)) of pyranopyranyl and furanyl derivatives. The
reported data were calculated at the B3LYP/6-31G**gas phase optimized geometry, by using the CAM-B3LYP/6-31+G** method
Property
8Fur
8Pp
5Fur [b]
5Pp [b]
6Fur [b]
6Pp [b]
4 [b]
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ρ=γsol/γgas [d]
1.68
1.72
1.94
2.07
2.09
2.18
1.74
γ(0;0,0,0)
104
96.4
457.5
889[d]
476.6
989[d]
1712
1842
627
[×103] (a.u.)
175[d]
166[d]
3578[e]
4010[e]
1094[d]
γ(-ω;ω,-ω,ω)
[×103] (a.u.)[a]
140
2010
-4264
-15770
-116933
NC[f]
-
9110
235[e]
3457[e]
8272[e]
-32640[e]
245559[e]
15860[e]
0.00145
0.0133
0.113
0.14±0.04[i]
-0.16[e,h]
0.138
1.335
0.025
γ(-ω;ω,-ω,ω)
[×10-31] (esu)
±0.00030[g]
±0.0020[g]
±0.050[g]
±0.050[g]
±0.200[g]
±0.005[g]
0.0012[e,h]
0.017[e,h]
0.04[e,h]
-1.2[e,h]
-
0.079[e,h]
[a]Frequency-dependent value, λ=532 nm. [b]The tBu groups are substituted with H. [c]γsol: second hyperpolarizability computed in the presence of
the solvent (toluene); γgas: second hyperpolarizability computed in the gas phase. [d]Value computed in the presence of toluene. [e]Computed by
multiplying the gas-phase value with the scaling factor, ρ, presented in the first line, in order to get an estimation of the property in solution. [f]Non-
convergence. [g]The experimental value was measured by using the Z-Scan technique, with 35 ps laser excitation at 532 nm (solvent: toluene).
[h]The computed value was converted to esu by using the conversion factor 1 a.u.=5.0367 x 10-40 esu.
NLO responses in thin films: towards devices. Driven from the promising NLO results obtained in solution, thin films of PMMA
containing the relevant dye were prepared by spin coating (thickness of 350-550 nm as measured by a Dektak XT™ stylus
profilometer). The results are gathered in Table 7 (details in section S12, SI).[94-95] As thin films present a different dielectric
environment compared to that of a liquid solution, different behaviors of NLO responses are observed.[96] The Z-scan measurements
were performed under 532 nm, 35 ps laser pulses. The UV-Vis-NIR optical absorption spectra of the prepared thin films are shown in
Figures S62-63, SI. While the absorption spectra of the thin films containing the furanyl derivatives were found to closely match those
taken from toluene solutions, the absorption profiles of the films containing the pyranopyranyl derivatives (with the exception of those
containing molecule 8Pp) exhibited significant broadening of the main absorption bands, suggesting a non-negligible aggregation of
the molecules in PMMA. Concerning the general properties of the thin films, it is important to note that the NLO responses of all films
are exclusively dominated by NLO refraction (for the range of incident laser intensities used, i.e. up to 30-35 GW/cm2). No evidences
of nonlinear absorption have been observed for laser intensities as high as the damage threshold of the films. These results are a
very promising for practical applications as they show high damage thresholds, while the absence of absorption reduces significantly
the drawbacks of any thermal effects.[97] As for the magnitude of the NLO refraction (i.e., the real part of the third-order susceptibility,
Reχ(3)), thin films containing furanyl derivatives 8Fur and 5Fur show very similar values to those obtained from the corresponding
toluene solutions (i.e., about 10-13 esu). On the other hand, pyranyl derivatives 5Pp, 6Pp and 8Pp values are lower, most probably
because of aggregation (see broadening of the UV-Vis absorption profile for the pyranyl molecules in PMMA displayed in Figures
S62-63). To assess the exploitability and suitability of the thin-films NLO response for engineering optoelectronic devices, two figures
of merit are usually considered: T and W. These parameters are defined as follows:[94,96]
T=βλ/γ′<1 and W=Δn/αλ>1
where β is the nonlinear absorption coefficient, λ is the excitation wavelength, γ′ is the nonlinear refractive parameter, Δn = γ′I is the
induced index change and α is the linear absorption coefficient (in cm-1). The first parameter (T < 1) infers that the NLO absorption
must be weak compared to the NLO refraction, while the second (W > 1) suggests that the linear absorption must be relatively weak
compared to the nonlinearity. In the present case, the T value is always < 1, thus fulfilling the necessary requirements of a negligible
NLO absorption of the films (i.e., β ≈ 0). High W values ranging between ~390 and 10 are estimated for the less π-extended
derivatives 8Fur/Pp and most π-extended 6Pp, respectively, assuming an intermediate incident laser from those employed. These W
values are very high and thus very encouraging to further exploit thin films containing pyranopyranyl derivatives in waveguiding
devices and optical couplers.[94-96]
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Table 7. Thickness (L), nonlinear refractive index parameters (γ' and Reχ(3)) and figure of merit (W) of PMMA thin films containing the relevant O-doped PAHs
on glass.
γ'
Reχ(3)
Reχ(3)
γ'
Molecule
L (nm)
W
W
L (nm)
Molecule
(×10-18 m2/W)
(×10-13 esu)
(×10-13 esu)
(×10-18 m2/W)
8Fur
5Fur
6Fur
367
377
380
-0.43±0.02
-1.12±0.02
-
-0.62±0.03
-1.59±0.03
-
340
200
-
390
20
-0.37±0.,01
-0.94±0.04
-0.70±0.03
-0.26±0.01
-0.66±0.03
-0.49±0.02
361
391
383
8Pp
5Pp
6Pp
10
Conclusions
In conclusion, in this paper we have described the synthesis of O-doped polyaromatic hydrocarbons, in which two polycyclic aromatic
hydrocarbon substructures are fused through furanyl or pyranopyranyl cycles. Starting from the bishydroxyPAHs, acid- and Cu-
catalyzed O-annulation reactions allowed the planarization of the molecules through the formation of furanyl or pyranopyranyl rings.
Comprehensive photophysical measurements in solution showed that these molecules feature high emission yields (Φ = 0.5 – 0.9)
and tunable absorption properties throughout the UV-vis spectral region. Complementary solid-state photophysical studies of the
dyes organized in microscopic morphologies showed that only those prepared from the furanyl derivatives retain the emissive
molecular properties. Electrochemical investigations showed that in all cases the O-annulation increases the electron donor
capabilities by raising the HOMO energy level with the LUMO energy level being less affected. This ultimately causes a shrinking of
HOMO-LUMO energy gaps, with the pyranopyranyl planarization triggering the narrower gaps and thus the lowest-energy emissive
species. Finally, third-order NLO measurements of solutions containing the relevant dyes displayed significant second
hyperpolarizibility, the extent of which depends on the i) molecular planarity and ii) the HOMO-LUMO energy gap. Theoretical
computation of the optoelectronic properties performed with the CAM-B3LYP/6-31+G** method provides reliable data for predicting
the excitation spectra, energy gaps and second hyperpolarizability values, for which the pyranopyranyl derivatives display the larger
second hyperpolarizability values. In this respect, PMMA films containing the pyranopyranyl derivatives displayed weak linear
absorption and NLO absorption compared to the nonlinearity and NLO refraction, respectively, revealing to be prime materials for
engineering photonic devices, such as waveguiding and optical couplers or fluorescent probes in lipid bilayer membranes.[100] From
these results, it is clear that the potential to build a broad variety of new colorful molecules is apparent. Thus, further investigations
will be now centered on the study of different heteroatoms, like the other chalcogens such S, Se and Te atoms, the polarizability of
which is expected to further affect the NLO response and finely tune the HOMO-LUMO gap.
Experimental Section
Full experimental details and characterization data, spectroscopic measurments, cyclic voltammograms, computational studies and
NLO measurement details are gathered in the Supporting Information. CCDC 1424424-1424428 contain the supplementary
crystallographic data for compounds 1c, 4, 5Fur, 6Fur and 7Fur. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgments
D.B. gratefully acknowledges the EU through the ERC Starting Grant “COLORLANDS” project, the Science Policy Office of the
Belgian Federal Government (BELSPO-IAP 7/05 project), MIUR through the FIRB Futuro in Ricerca “SUPRACARBON” (contract no.
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10.1002/chem.201604866
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RBFR10DAK6), the "SACS" project (grant number 310651) and the Cardiff University. We thank Ms. Francesca Vita for the SEM
analyses, Ms. Maria Mercedes Lorenzo Garcia for the help with ATR analyses, Dr. Caroline A. Ahad Hadad for some TGA
measurements and Dr. Domenico Milano for the artwork used in the TOC image. The authors also acknowledge Dr. Simon J. A.
Pope for the help with some of the fluorescence lifetime measurements and Panagiotis Aloukos for the precious suggestions and
help with the NLO analyses on the thin films.
Keywords: PAHs, O-annulation, heteroatom doping, chromophores, nonlinear optics.
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Entry for the Table of Contents (Please choose one layout)
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Staring at the rainbow. The synthesis
of a new π-extended O-doped PAHs is
achieved thanks to high-yielding ring-
closure key steps that, depending on
the reaction conditions, lead to the
Tanja Miletić, Andrea Fermi, Ioannis
Orfanos, Aggelos Avramopoulos,
Federica De Leo, Nicola Demitri,
Giacomo Bergamini, Paola Ceroni,
Manthos G. Papadopoulos, Stelios
Couris and Davide Bonifazi*
formation
of
either
furanyl
or
pyranopyranyl
frameworks.
Photophysical and electrochemical
characterization showed that these
Page No. – Page No.
Tailoring colors by O-annulation of
polycyclic aromatic hydrocarbons
molecules
exhibit
finely
tunable
optoelectronic properties, while third-
order NLO measurements displayed
very good second hyperpolarizibility
values, making them exceptional
candidates for photonic applications.
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