Exciton coupling in molecular salts of 2-(1,8-naphthalimido)ethanoic acid and cyclic amines: Modulation of the solid-state luminescence

Article abstract of DOI:10.1039/c3ce41651h

In this study we have purposely altered the solid state luminescence properties of 2-(1,8-naphthalimido)ethanoic acid (NEaH) (0) via molecular salts formation with cyclic amines such as 1,4-diazabicyclo[2.2.2]octane (DABCO), quinuclidine (ABCO), 3-quinuclidinol (OH-ABCO), and piperazine (PIP). All crystalline materials have been characterized in the solid state via single-crystal and variable temperature powder X-ray diffraction and thermal methods; luminescence spectra in the solid state have been recorded. Exciton interactions have been determined with quantum-chemical calculations for all molecular organic salts, and tuning of their magnitude in response to changes in the crystal packing has been demonstrated. It is suggested that the variations in photoluminescence can be interpreted on the basis of the different excitonic interactions amongst the naphthalimide moieties.

Full text of DOI:10.1039/c3ce41651h

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Exciton coupling in molecular salts of
2-(1,8-naphthalimido)ethanoic acid and cyclic amines:
modulation of the solid-state luminescence†
Cite this: CrystEngComm, 2013, 15,
10470
a
Simone d'Agostino, Fabrizia Grepioni, a Dario Braga,a Daniele Moreschi,a
*
b
a
a
a
*
Valeria Fattori, Francesca Delchiaro, Simone Di Motta and Fabrizia Negri
In this study we have purposely altered the solid state luminescence properties of 2-(1,8-naphthalimido)ethanoic
acid (NEaH) (0) via molecular salts formation with cyclic amines such as 1,4-diazabicyclo[2.2.2]octane (DABCO),
quinuclidine (ABCO), 3-quinuclidinol (OH-ABCO), and piperazine (PIP). All crystalline materials have been character-
ized in the solid state via single-crystal and variable temperature powder X-ray diffraction and thermal methods;
luminescence spectra in the solid state have been recorded. Exciton interactions have been determined with
quantum-chemical calculations for all molecular organic salts, and tuning of their magnitude in response to
changes in the crystal packing has been demonstrated. It is suggested that the variations in photoluminescence
can be interpreted on the basis of the different excitonic interactions amongst the naphthalimide moieties.
Received 4th July 2013,
Accepted 9th October 2013
DOI: 10.1039/c3ce41651h
www.rsc.org/crystengcomm
Introduction
dyes.6 Their photophysical properties are useful in many
applications as organic light-emitting diodes,8 fluorescent
Crystal engineering1 is aimed to the design and preparation
of crystalline materials with desired properties, starting from
a knowledge of the supramolecular bonding capacity of
molecular components that self-assemble in the solid. Multi-
component crystals, such as co-crystals and molecular salts,
are attracting much interest because of the potential new
dyes for polymers and textile materials,6,9 but also as sensors,
liquid crystals displays, and logic gates.10 It has been recently
reported that 2-(1,8-naphthalimido)ethanoic acid (NEaH)
exhibits a blue fluorescence in the solid state.11
In this work 2-(1,8-naphthalimido)ethanoic acid NEaH,
chosen as the starting photoactive component, has been
reacted with the cyclic amines reported in Scheme 1: the
organic bases were chosen with the aim of purposely altering
the exciton interactions, therefore the luminescent properties
of the resulting crystalline material, by altering the packing
pattern of the NEaH chromophore. Beside the tuning of exci-
ton interactions also the tendency to form excimers, already
known for NEaH derivatives, may be affected by packing and
can therefore influence the photoluminescence. In view of
our successful experience with the use of mechanochemical
methods,12 all crystalline materials were prepared via
kneading,12a i.e. grinding of the starting materials in the
presence of a catalytic amount of solvent.
solid state properties deriving from
a combination of
molecular components with the structure of the final supra-
molecular aggregate. Excellent examples can be found in
solid state templated photoreactions2 or in cases where Active
Pharmaceutical Ingredients (APIs) are turned into multiple
crystal forms such as co-crystals, ionic co-crystals, coordina-
tion compounds and molecular salts to improve properties
such as thermal stability, morphology, dissolution rate, etc.3
Most recently there has been a surge of interest in the use of
co-crystals as a means to modify fluorescence4 and phospho-
rescence5 properties of emitting molecules such as organic
dyes.6 Solid state luminescent materials are of great interest
and are studied for their potential applications in the
field of optoelectronic devices.7 Among them, N-substituted
naphthalimides derivatives are an attractive class of organic
Results and discussion
For the purpose of this study (NEaH) was mechanically
reacted with each aliphatic base (DABCO, ABCO, OH-ABCO,
and PIP) in the presence of a catalytic amount of solvent
(kneading), under a UV lamp (365 nm). When the samples
were kneaded in the presence of water, a change in the lumi-
nescence colour was noticed. The blue luminescence of NEaH
(0) changed to violet when the solid was kneaded in the pres-
ence of half an equivalent of 1,4-diazabicyclo[2.2.2]octane
a Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2,
40126 Bologna, Italy. E-mail: fabrizia.grepioni@unibo.it, fabrizia.negri@unibo.it
b Istituto per la Sintesi Organica e la Fotoreattività (ISOF) – CNR, Via Gobetti
101, 40129 Bologna, Italy
† Electronic supplementary information (ESI) available: Solid-state structural
characterization, DSC and TGA, computational details. CCDC 941379–941382.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3ce41651h
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Fig. 1 Pictures, taken under UV light (365 nm), of fluorescent crystals of pure
[NEaH] (0) and of the molecular salts) [DABCOH2][NEa]2 (1), [ABCOH][NEa]·4H2O (2),
[OH-ABCOH][NEa]·1.5H2O (3) and [PIPH2][NEa]2·4H2O (4), obtained via
recrystallization from solution.
Scheme
1
The photoactive component, 2-(1,8-naphthalimido)ethanoic acid
(NEaH), and the bases utilized in the present study: 1,4-diazabicyclo[2.2.2]octane
(DABCO), quinuclidine (ABCO), 3-quinuclidinol (OH-ABCO), and piperazine (PIP).
(DABCO) or piperazine (PIP), and to green when kneading
was carried out in the presence of one equivalent
of quinuclidine (ABCO) or 3-quinuclidinol (OH-ABCO)
(see Table 1).
Fig.
2
Discrete hydrogen bonded units in crystalline [DABCOH2][NEa]2 (1).
H
CH omitted for clarity.
The formation of new crystal forms (see Table 1) was con-
firmed by comparing X-ray powder diffraction patterns of the
solid products with those calculated on the basis of single
crystals of the corresponding compounds, obtained via
recrystallization from solution‡ (see Fig. S1†). Under irradia-
tion with a suitable UV lamp (λ = 365 nm) the crystals
obtained from solution exhibited the same luminescence
colours of the corresponding solid–solid reaction products
(see Fig. 1).
Fig. 3 A 2D-layer of π-stacked hydrogen bonded trimeric units in crystalline 1 (a);
The structure of NEaH (0) was reported recently, and for
its description the reader is addressed to reference.11 Single
crystal X-ray diffraction shows that all compounds are in the
form of molecular salts containing completely deprotonated
2-(1,8-naphthalimido)ethanate anions and, depending on the
number of N-atoms on the base used as a reagent, mono/
di-protonated cations. The reaction with DABCO yields the
anhydrous form [DABCOH2][NEa]2 (1) constituted of discrete
units held together by hydrogen bonds between a fully pro-
tonated DABCO unit, hence in the form of the dication
DABCOH22+, and the carboxylate groups on the deprotonated
NEa− units. The hydrogen bonding distance is typical of a
a second layer (in blue) fills in the voids (b). HCH atoms not shown for clarity.
The reaction between NEaH and ABCO also yields a
molecular salt via proton transfer from acid to base; the
product is a tetrahydrate, of formula [ABCOH][NEa]·4H2O (2).
The water molecules are organized in hydrogen bonded
hexamers, sharing two opposite edges with adjacent rings,
thus forming an infinite tape [T6(2) motif,14 Ow⋯Ow
115.62(9)–
=
2.7509(2)–2.829(2) Å, Ow⋯Ow⋯Ow angles
=
126.5(1)°]; the angle between the planes defined by the
hexamers is of ca. 127 deg. The water hexamers, in turn,
interact through a net of hydrogen bonds with the [NEa]−
+
“charge assisted” hydrogen bond [OO−⋯ON–H = 2.534(2) Å],
see Fig. 2.13
−
The discrete, overall neutral trimeric units interact via
π-stacking and are organized in zig-zag ribbons in the crys-
tal. Adjacent ribbons form 2D-layers parallel to the ac-plane
(see Fig. 3a); a second layer is shifted with respect to the
first (see blue spheres in Fig. 3b) and is accommodated in
the empty cavities of the first layer.
anions [Ow⋯OCO/C–O = 2.718(2)–2.720(3) Å] and with the
[ABCOH]+ cations [Ow⋯NN–H = 2.672(3) Å] (see Fig. 4).
+
Differently from what observed in crystalline 1, no close
contact is observed between cation and anion; all interactions
between the two moieties are mediated by the water mole-
cules, which act as a “neutral glue”, thus forming a layer
Table 1 List of the solid compounds discussed in this study
Compound
Code
Fluorescence coloura (UV-lamp, λ = 365 nm)
NEaHb
0
1
2
3
4
Blue
[DABCOH2][NEa]2
[ABCOH][NEa]·4H2O
[OH-ABCOH][NEa]·1.5H2O
[PIPH2][NEa]2·4H2O
Violet
Green
Green
Violet
a
b
All compounds are colourless under visible light. Known structure11 (CSD refcode: UBAGOV).
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Fig. 5 (Left) Side view, along the b-axis, of the hydrogen-bonded layer extending
parallel to the ab-plane in crystalline 2, with anions and cations “glued” together
via the water hexamers; (right) on both sides, additional layers (in blue) “click” to
the central one via π-stacking interactions. HCH atoms not shown for clarity.
Fig. 4 (Left) Space filling representation of the T6(2) water motif in crystalline (2);
(right) water hexamers interacting through hydrogen bonds with the [NEa]− anions
and with the [ABCOH]+ cations. HCH omitted for clarity.
extending parallel to the ab-plane. The aromatic moieties
stick out of the layer (see Fig. 5, left); two adjacent layers
“click” to the central one via π-stacking interactions (see
Fig. 5, right).
The reaction of NEaH with OH-ABCO also yields
a
hydrated phase of formula [OH-ABCOH][NEa]·1.5H2O (3).
Through the carboxylate function the [NEa]− anions interact
with both the protonated N-atom and the hydroxyl group on
Fig. 6 Hydrogen bonding interactions in crystalline [(NEa)(OH-ABCOH)·1.5H2O] (3).
the [OH-ABCOH]+ cationic units [OCOO−⋯NN–H = 2.668(7)–
+
H
CH atoms not shown for clarity.
2.691(8) Å; OCOO−⋯OOH= 2.775(7) Å, see Fig. 6], which in turn
also interact with water molecules [OOH⋯Ow = 2.756(8)–
2.892(9) Å, not shown in the picture]. [NEa]− anions interact
also with water molecule by using either the carboxylate and
the imido carbonyl group [OCOO−⋯Ow = 2.728(9)–2.825(8) Å;
OCO⋯Ow = 2.871(1) Å]. The water molecules interact with
each other via hydrogen bonds, forming trimeric units
[Ow⋯Ow = 2.90(1)–2.72(1) Å, see Fig. 6]. The resulting packing
is reminiscent of the one observed in crystalline 2, with
adjacent layers forming π-stacking interactions via inter-
penetration of the aromatic moieties, in a velcro-like fashion
(see Fig. 7).
Fig. 7 (a) A layer (side-view) of hydrogen bonded cations, anions and water
molecules in crystalline 3; (b) on both sides, additional layers (in blue) are linked to
the central one via π-stacking interactions.
In crystalline [PIPH2][NEa]2·4H2O (4) there is a complex
network of hydrogen bonding interactions between the
carboxylate function on the [NEa]− anions and the water
−
molecules [Ow⋯OC–O = 2.747(4)–2.988(4) Å] and the [PIPH2]2+
+
dication [Ow⋯NN–H = 2.746(4) Å], which in turn interacts
+
with the imide carbonyl [OCO⋯NN—H = 2.795(4) Å] (see
Fig. 8). The water molecules also interact with each other via
dimer formation [Ow⋯Ow = 2.90(1)–2.749(4) Å] (see Fig. 8).
The packing of crystalline 4 markedly differs from those
described above, in that no π-stacking interactions are at
work in this solid: the aromatic units are arranged in a
herring-bone fashion, as can easily be seen in Fig. 9.
Fig.
8
Hydrogen bonding interactions in crystalline [PIPH2][NEa]2·4H2O (4).
H
CH atoms not shown for clarity.
Since the solid state photophysical behavior depends
strongly on the interactions between the chromophores, it is
important to see if the organization of the molecular ions in
the solid state has led to any recognizable pattern in the crys-
tal packings and if these reflect to any extent on the proper-
ties. The focus is on patterns involving π-stacking, as these
are more relevant to photophysical properties. (Of course this
does not necessarily imply that these are the most relevant in
keeping the crystal together, i.e. the most relevant in terms of
‡ All solvents and chemicals were bought from Sigma-Aldrich and used with-
Fig. 9 Van der Waals spheres representation of the herring-bone arrangement of
the aromatic units in crystalline 4. HCH atoms not shown for clarity.
out further purification; doubly-distilled water was used.
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Table 2 Types of dimers observed in crystalline 1–4, together with their
energy, as in crystals made of molecular ions interactions can
still be fairly energetic at large distances).
relevant geometrical parameters and codes used for the computational study
Scheme 2 shows how geometrical parameters were evalu-
ated for the “dimers” present in the solids; calculated values
for all compounds are reported in Table 2, together with
graphical representation of all relevant dimers also used for
the computational investigation. For compounds 1–3 the
naphthalimide moieties are oriented parallel to each other in
a columnar fashion; different π-stacking modes are detected.
Crystalline 4 differs from the other solids, as an edge-to-face
interaction is at work between the naphthalimide moieties
(see Fig. 9 and Table 2).
Compound and dimer
Side view
Top view
(1)
DIMER A
ID 3.45 Å
CD 3.62
offset = 1.09 Å
(1)
DIMER B
ID = 3.12 Å
CD = 6.31 Å
offset = 5.49 Å
(1)
Thermal behaviour
DIMER C
ID = 3.31 Å
CD = 4.99 Å
offset = 3.73 Å
Thermal behaviour of the hydrated compounds 2, 3 and 4
was also investigated, in order to detect a possible additional
effect on luminescence colour change due to water removal
from the crystal lattice. Thermal gravimetric analysis for
compound [ABCOH][NEa]·4H2O (2) indicates a weight loss
of 15.6% in the range 37–100 °C, corresponding to the
release of 4 water molecules per formula unit. The DSC trace
shows an endothermic peak at 65 °C (peak temperature,
ΔH = 310.51 J mol−1) in the same region (see Fig. S2†).
Variable temperature powder diffraction experiments were
particularly useful to investigate the dehydration processes
(see Fig. 10a). Formation of a crystalline anhydrous phase
(2_anhyd) following dehydration is observed upon heating a
polycrystalline sample of 2 up to 100 °C. The transformation
is fully reversible: when the sample is cooled back to room
temperature, 2_anhyd reverts back to the starting solid 2 via
water uptake from the atmosphere (this process is acceler-
ated if the powder sample is ground).
(2)
DIMER A
ID = 3.66 Å
CD = 3.92 Å
offset = 1.40 Å
(2)
DIMER B
ID = 3.52 Å
CD = 4.57 Å
offset = 2.92 Å
(3)
ID = 3.46 Å
CD = 3.84 Å
offset = 1.67 Å
(4)
dC–H⋯π = 2.69–3.44 Å
or dC⋯π = 3.51–4.06 Å
In the case of compound [OH-ABCOH][NEa]·1.5H2O (3)
the thermogram indicates a weight loss of 7.2% in the range
35–110 °C, consistent with the release of 1.5 water molecules
per formula unit. The DSC trace shows an endothermic peak
at 69 °C (peak temperature, ΔH = 113 J mol−1) in the same
interval (see Fig. S3†). The dehydration process was also
followed with variable temperature powder diffraction
(Fig. 10b). Heating a sample up to 120 °C results in the trans-
formation to an anhydrous phase (3_anhyd). As observed
with compound 2, the change is fully reversible, and after
cooling back to room temperature 3_anhyd reverts back to
the starting solid 3 via water uptake from the atmosphere;
again, the process is accelerated if the powder is ground.
Fig. 10 Comparison of the experimental VT-XRPD patterns for the hydrated phase at
RT (black) and the anhydrous phase at 100 °C (red) for crystalline 2 (a), 3 (b) and 4 (c).
An analogous behavior can be observed for crystalline
[PIPH2][NEa]2·4H2O (4). A TGA trace indicates a weight loss
of 11% in the range 50–100 °C, due to the release of 4 water
molecules per formula unit. The anhydrous phase is stable
up to ca. 200 °C, then decomposition is observed. The DSC
trace shows an endothermic peak at 79 °C (peak temperature,
ΔH = 243 J mol−1) (see Fig. S4†). Variable temperature powder
diffraction experiments for this compound are presented in
Fig. 10c. Heating up to 100 °C results in the formation of a
Scheme 2 ID = interplanar distance; CD = distance between the geometrical
centres of the aromatic systems; offset = shift of the geometrical centres, in
projection.
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new anhydrous phase, denoted as 4_anhyd. Even in this case,
when the sample is cooled back to room temperature, the
anhydrous phase reverts to the starting solid (4) via water
uptake from atmospheric moisture; as noted for the previous
systems, the process is accelerated via grinding.
For compounds 2, 3, and 4 no apparent colour changes,
under day light or UV light (365 nm), have been detected dur-
ing or after dehydration at high temperature. Unfortunately,
it was not possible to carry out photophysical measurements
on the anhydrous phases 2_anhyd, 3_anhyd, and 4_anhyd,
because upon cooling to room temperature they rapidly
reverted to their hydrated forms.
dichloromethane, respectively) and the second at ca. 330 nm
(λmax = 333 nm, ε = 16 900 M−1 cm−1 and λmax = 335 nm, ε =
11 000 M−1 cm−1 in methanol and dichloromethane, respect-
ively). Fig. 11b shows the fluorescence spectra of NEaH (0).
As the excitation wavelengths we choose the maximum
absorption wavelengths at 330 nm. In dichloromethane the
compound shows a structured band with a maximum at
385 nm (Φem = 0.19), while in methanol the same band is
less resolved and slightly red-shifted, with a maximum at
387 nm (Φem = 0.075).16 The methanol solution shows both
a blue shifted absorption and a red shifted emission
resulting in a slightly increased Stokes shift with respect to
the dichloromethane solution, together with structure
burring of the emission band and luminescence quenching.
These effects can be associated to hydrogen bonding with
the protic solvent. The electronic transitions from the
singlet excited electronic state in 1,8-naphthalimide deriva-
tives are affected by a triplet state very close in energy and
having n,π* character,17 therefore, the interactions of the
carbonyl with a protic solvent affect the inter-system cross-
ing processes and non-radiative deactivations.18
Photophysics
We examined the optical properties of NEaH (0) in solution
and in the solid state, and the luminescence properties of the
crystalline materials [DABCOH2][NEa]2 (1), [ABCOH][NEa]·4H2O
(2), [OH-ABCOH][NEa]·1.5H2O (3), and [PIPH2][NEa]2·4H2O (4).
Electronic absorption spectra for NEaH (0) were recorded
in methanol and dichloromethane solutions (Fig. 11a), and,
as expected from previous solution studies of substituted
1,8-naphthalimides,15 the spectra show two absorption max-
ima, one at ca. 230 nm (λmax = 232 nm, ε = 55 000 M−1 cm−1
and λmax = 236 nm, ε = 38 000 M−1 cm−1 in methanol and
Photoluminescence spectra recorded on polycrystalline sam-
ples of NEaH (0), [DABCOH2][NEa]2 (1), [ABCOH][NEa]·4H2O
(2), [OH-ABCOH][NEa]·1.5H2O (3), and [PIPH2][NEa]2·4H2O (4)
together with excitation spectra are shown in Fig. 12. Emission
and excitation maxima for all samples are reported in Table 3
together with the wavelengths chosen as excitation and fluores-
cence quantum yields.
For all compounds both luminescence and excitation
spectra are red-shifted compared to solution, while no effects
of hypo/hyperchromism are present in the solid state (lumi-
nescence quantum yields are of the same order of magnitude
in solution and in the solid state). Clearly, the spectral fea-
tures in compounds (0)–(4) arise from the different
intermolecular excitonic, van der Waals and π–π interactions
at work in their crystals between the naphthalimide moieties,
that stabilize the extended excited state (exciton), more than
the ground state.11,19
While the excitation spectra are relatively similar for all
the compounds (all of them have a maximum or shoulder in
the 385–390 nm range), the emission spectra clearly show
distinct behaviours. The large differences in emission spectra
and their marked red shift seem to indicate the presence of
excimeric emission, which has been already reported for
naphthalimide derivatives,20 originated by the face-to-face
interaction between two naphthalimide moieties.
Quantum-chemical modelling
The observed photophysical changes of the crystals result
from the interplay of intramolecular vibronic activity,
intermolecular interactions, among which excitonic interac-
tions21 play a relevant role.
To investigate the modulation of exciton couplings
induced by the different crystal packing and to discuss their
possible influence on the photophysics we carried out
Fig. 11 (a) Electronic absorption spectra of NEaH (0) in methanol (blue) and
dichloromethane (black) solution, and (b) normalized fluorescence spectra of NEaH
(0) in methanol (blue) and dichloromethane (black) solution.
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Fig. 13 Schematic representation of the potential energy surfaces for the ground
and excited states of NEaH, showing the vertical absorption and emission transitions
(red arrows), the normal mode displacement ΔQ, and the relaxation energies λexcited
and λground
.
reorganization energy values in Table S1,† amounts to
0.36 eV (0.52 eV) at B3LYP/6-31+G* (CAM-B3LYP/6-31+G*)
and compares well with the experimental measurements in
dichloromethane shown in Fig. 13 (ca. 0.48 eV) and with the
data from literature for similar systems.
Different crystal packing may lead to different patterns of
excitonic interactions. Therefore we identified the most rele-
vant pairs of interacting chromophores (hereafter indicated
as dimers) for some of the investigated crystals (compounds
0–2) and evaluated the excitonic couplings.
The dimers were determined by analyzing portions of crys-
tals and selecting the relevant dimers according to the
intermolecular distances between centers of mass. This pro-
cedure, already employed to investigate charge transport
properties of crystalline organic semiconductors23 shows that
there are two relevant dimers for 0, three for 1, and two for 2,
whose center of mass distances are shorter than 7 Å. A sum-
mary of distances, orientations of the molecules in the dimers
and labelling of the dimers are collected in Table S2,† Table 2
(for compounds 1 and 2) and Fig. S6† for compound 0.
Under the assumption that the excited states of molecular
aggregates are weakly influenced by inter-molecular forces,
they can be approximated by a linear combination of the
excited states localized on the isolated molecule. Considering
a simple dimer formed by to identical molecules, due to the
presence of the Vexciton term, the localized excited states of
the isolated molecules give rise to two delocalized excitonic
states (Frenkel states) for the dimer, which are in phase (state
Fig. 12 (a) Normalized solid state excitation spectra recorded at RT for compounds
0, 1, 2, 3, and 4; (b) normalized solid state luminescence spectra recorded at RT for
compounds 0, 1, 2, 3, and 4.
quantum-chemical calculations on the chromophores of com-
pounds 0, 1 and 2. We first considered the isolated chromo-
phore NEaH and evaluated the Stokes Shift (SS) between
absorption and emission. To this end we determined the
reorganization energies associated with relaxation in the
ground and excited electronic states, within the adiabatic
potential (AP) approach22 as schematically shown in Fig. 13.
The equilibrium structure of the ground and excited states
were determined and the four energy values, indicated in Fig.
S5† and collected in Table S1,† were employed to evaluate
reorganization energies. The computed SS, sum of the two
Table 3 Emission and excitation maxima and photoluminescence quantum yields in the solid state for the hydrated compounds 0, 1, 2, 3, and 4
Compound
λex (nm)
λem (nm)
Φem
Fluorescence colour (λ = 365 nm)
NEaH (0)
386
390
346
357
346
440
425
470
491
416
0.13
0.11
0.14
0.13
0.06
Blue
[DABCOH2][NEa]2 (1)
[ABCOH][NEa]·4H2O (2)
[OH-ABCOH][NEa]·1.5H2O (3)
[PIPH2][NEa]2·4H2O (4)
Violet
Green
Green
Violet
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Table 4 TDHF/6-31+G* Computed excitonic interactions (with the PDA and
supramolecular approaches) for the most relevant dimers of compounds 0–2
labelled +) and out of phase (state labelled −) combinations
of the molecular excited states. The excitation energies E+
and E− of the Frenkel exciton states are split with respect to
the energy of the isolated molecule E0, as a result of the exci-
Dimer
PDA (cm−1
)
Supramolecular (cm−1
)
Compound
A
B
A
B
C
A
B
351
321
291
4
205
855
435
620
536
201
155
130
211
918
0
0
1
1
1
2
2
tonic interaction Vexciton
.
The excitonic coupling can be shown to be composed of
two dominant contributions:24
Vexciton ¼ VCoulomb þ VExchange
ð1Þ
of which the first term shows a long range dependence which
dominates the coupling between singlet excited states, while
the second is a short range contribution which becomes rele-
vant at short intermolecular distances.
The above discussion shows that the exciton coupling can
be estimated in a supramolecular approach (Vsupra) by com-
puting the exciton energies E+ and E− 25 and the difference Δε
between the excitation energies of the isolated molecules,
when these are not identical (and assuming negligible polari-
zation driven site energy splitting), from which the exciton
coupling can be derived as
Fig. 14 Direction of the computed transition dipole moment for NEaH (left) and
NEa (right).
anions, the transition dipole moment is directed along the
short naphthalene molecular axis, as shown in Fig. 14.
2
1
V supra

E  E  ()2
ð2Þ


Notably, the investigated dimer configurations of com-
pounds 1 and 2 correspond to arrangements of the mono-
mers typical for H aggregates28 except for the dimer B of 2.
In this latter case, the J character deduced according to the
PDA approximation turns into H character when the less
approximated supramolecular approach is considered. As for
the dimers of compound 0, these can be classified as non-
planar transition dipoles according to the PDA classification
discussed in reference28 and are neither J nor H pure aggre-
gates. Inspection of Table 4 shows that very large variations
occur when moving to the PDA approach to the supramolecu-
lar approach as a result of their different level of approxima-
tion. This is particularly evident for dimer B of compound 1
whose exciton coupling is almost zero, at PDA level, since the
angle formed by the direction of the molecular transition
dipole moments and the intermolecular distance vector (see
Table S2†) is close to the limit angle of 54.728 which corre-
sponds to zero excitonic interaction. Moving to the less
2
We also estimated the Coulombic contribution to the exci-
ton coupling according to the point dipole approximation
(PDA):
1  2
1
V PDA

k
ð3Þ
40
r3
12
where k, the orientational factor, is expressed in terms of
unitary vectors
˘
˘
˘
˘
˘
˘
2
k      3   r   r
2  12  12 
ð4Þ


1
1
and μ1 and μ2 are the transition dipole moments of the two
molecules forming the dimer and r12 is their distance.
While the PDA approach is valid for distances larger than
the molecular dimension and provides an estimate of the
Coulombic term in the excitonic interaction, in the supramo-
lecular approach both the Coulombic and exchange terms
approximated supramolecular approach leads to a non-
negligible interaction as shown in Table 4. A similar drastic
change in computed interactions concerns those of com-
pound 2. Moving from the PDA to the supramolecular
approach, the magnitudes of the interactions of the two
investigated dimers exchange each other. Interestingly, how-
ever, this compound, among the three computationally inves-
tigated, is the one displaying the largest interactions for both
approaches. We note that the two molecules forming the
dimers are, in this case, identical and equivalent by symmetry,
therefore the results from the supramolecular approach are
deemed reliable. As a result of the comparably larger exci-
tonic interactions, we expect in this case stronger
photophysical changes: since the aggregation is of H type, a
larger SS is generally expected. The tendency to form
are included in the indirect evaluation of Vsupra 25,26
In addi-
.
tion, in the supramolecular approach the Coulombic term is
correctly estimated in contrast with the PDA approach where
it is obtained by a truncated multipolar approximation.
The exciton interactions, computed with the two
approaches outlined above and collected in Table 4, show the
typical magnitudes reported for other conjugated chromo-
phores in crystals.27 According to the PDA approach, we can
classify the dimers of the investigated crystals as J or H aggre-
gates by considering the direction of the molecular transition
dipole moments. For the NEaH chromophores and their
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excimers should also be considered in the discussion of the
observed spectroscopic changes. However, the largest
observed changes between absorption and emission for
compound 2 can be reasonably accounted for by the largest
computed excitonic interactions.
solution of glycine (0.55 g, 7.3 mmol) in H2O (8 ml) 0.37 g
(6.6 mmol) of solid KOH were added, and the solution was
stirred for 20 minutes. Then 1.3 g (6.6 mmol) of naphthalene
monoanhydride (NMa) suspended in ca. 25 mL of EtOH were
added. The cream-coloured suspension was left under reflux
for 6–8 h; the resulting clear solution was cooled to RT and
concentrated HCl (37%) was added dropwise until a white
precipitate was obtained. The product was recovered by filtra-
tion and washed with cold H2O (5 × 2 mL) and with cold
EtOH (5 × 2 mL). The solid obtained was dried overnight in a
desiccator [yield = 76% (1.35 g, 5 mmol)]; it was then
suspended in ca. 10 mL of methanol and boiled until a clear
solution was obtained. Colourless, needle-shaped crystals of
NEaH (identified by X-ray powder diffraction) grew from this
solution in 24 h.
Conclusions
In this paper we have reported on the crystalline molecular salts
formed by reaction of 2-(1,8-naphthalimido)ethanoic acid
[NEaH] (0) with various cyclic amines, namely [DABCOH2][NEa]2
(1), [ABCOH][NEa]·4H2O (2), [OH-ABCOH][NEa]·1.5H2O (3), and
[PIPH2][NEa]2·4H2O (4). All solids have been structurally and
thermally investigated by a combination of solid-state tech-
niques, including differential scanning calorimetry (DSC),
thermogravimetric analysis (TGA), single crystal and variable
temperature powder X-ray diffraction (VT-XRPD). All compounds
should be described as molecular salts and not as co-crystals, as
in all cases proton transfer takes place from NEaH to the base;
the difference between the two types of multicomponent crystals
is, however, rather semantic and it has been amply demon-
strated29 that in hydrogen bonded systems there is almost a con-
tinuum of situations, also dependent on temperature, whereby
the hydrogen is transferred from the acid to the base, and that
the bulk physical properties of the materials (solubility, dissolu-
tion rates, melting point) do not seem to depend on the degree
of proton transfer.
Synthesis of [DABCOH2][NEa]2 (1)
A suspension of NEaH (32.4 mg, 0.12 mmol) and DABCO
(10.9 mg, 0.097 mmol) in ca. 10 mL of H2O–EtOH 3 : 7 was
boiled until a clear solution was obtained. Colourless, prism-
like crystals grew from this solution in ca. 2 days.
Synthesis of [ABCOH][NEa]·4H2O (2)
A suspension of NEaH (31.9 mg, 0.12 mmol) and ABCO
(12.4 mg, 0.11 mmol) in ca. 10 mL of H2O was boiled until a
clear solution was obtained. Colourless, prism-like crystals
grew from this solution in ca. 2 days.
Upon heating, water loss is observed for all compounds,
followed by formation of crystalline anhydrous phases
(2_anhyd, 3_anhyd, and 4_anhyd), which revert back to their
hydrate form upon cooling to room temperature.
Synthesis of [OH-ABCOH][NEa]·1.5H2O (3)
Photoluminescence properties of solid 1, 2, 3, and 4 have
also been investigated and compared with those of 0, both in
solution and in the solid state. For all compounds fluores-
cence spectra in the solid state are red-shifted with respect to
those in solution; luminescence in the solid state is observed
at different wavelengths, from violet (ca. 420 nm for 1 and 4)
to green (ca. 490 nm for 2 and 3) passing from blue (ca. 460 nm
for 0). These variations can be interpreted on the basis of the
different intermolecular van der Waals, π–π and excitonic
interactions amongst the naphthalimide moieties.
The modulation of excitonic interactions as a function of
crystal packing has been computed with quantum-chemical
methods. The largest excitonic interactions are indeed com-
puted for compound 2 displaying the largest photophysical
changes. Thus, the choice of the amine used as base/co-for-
mer has a marked effect on the supramolecular interactions
formed in the solid state and results in a modification of the
NEaH solid state fluorescence.
A suspension of NEaH (29.6 mg, 0.11 mmol) and OH-ABCO
(14 mg, 0.11 mmol) in ca. 10 mL of ethanol–H2O 7 : 3 was
boiled until a clear solution was obtained. Slow evaporation
of this solution yielded a white powder. Colourless, prismatic
crystals were obtained upon recrystallization of the powder
from n-propanol.
Synthesis of [PIPH2][NEa]2·4H2O (4)
A suspension of NEaH (60.8 mg, 0.22 mmol) and PIP (9.7 mg,
0.11 mmol) in ca. 10 mL of ethanol–H2O 6 : 4 was boiled until
a clear solution was obtained. Slow evaporation of this solu-
tion yielded a white powder. Colourless, prismatic crystals
were obtained upon recrystallization of the powder from
n-propanol.
Syntheses in the solid state
Work is in progress to further investigate and design the
luminescent properties of multicomponent systems (salts
and co-crystals) of luminescent dyes in the solid-state.
In the solid-state reactions NEaH was manually ground in an
agate mortar for ca. 10 min with DABCO, ABCO, OHABCO, or
PIP in 1 : 1 molar ratio. In all the kneading experiments a few
drops of solvent (H2O or EtOH) were added to the grinding
mixture. Formation of the solid products 1, 2, 3 and 4 was
confirmed by comparison of the experimental XRPD patterns
with those calculated on the basis of single crystal structures.
Synthesis of NEaH (0)
The 2-(1,8-naphthalimido)ethanoic acid was synthesized
according to a previously reported procedure.11,30 To a
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Crystal structure determination
on the band maxima is 2 nm. Luminescence quantum yields
were determined using air equilibrated solutions of naphtha-
lene in cyclohexane (Φ = 0.036).
All measurements on solid samples were performed on
finely ground powder placed inside two quartz slides (sand-
wich mode). Emission and excitation spectra of solid samples
were acquired on a Perkin-Elmer LS 50 spectrofluorimeter;
the uncertainty on the band maxima is 2 nm.
Absolute photoluminescence quantum yields were mea-
sured according to the method of deMello32 by using a cus-
tom designed integrating sphere inside a SPEX Fluorolog
fluorometer and checking the precision of the measurements
with reference samples in the same experimental setup.33
Single-crystal data for all compounds were collected at RT on
an Oxford X'Calibur S CCD diffractometer equipped with a
graphite monochromator (Mo-Kα radiation, λ = 0.71073 Å).
Data collection and refinement details are listed in Table S5.†
All non-hydrogen atoms were refined anisotropically; HOH
and HNH atoms were either directly located or added in calcu-
lated positions; HCH atoms for all compounds were added in
calculated positions and refined riding on their respective
carbon atoms. SHELX9731a was used for structure solution
and refinement on F2. The program PLATON31b was used
to calculate hydrogen bonding interactions. CYLview,31c
Mercury31d and Schakal9931e were used for molecular
graphics. CCDC 941379–941382† contain the supplementary
crystallographic data for this paper.
Computational details
The ground state and excited state potential energy surfaces
were computed at both CAM-B3LYP/6-31+G* and B3LYP/
6-31+G* levels of theory. The basis set was chosen in view of
the excited state switching predicted for basis sets not includ-
ing diffuse functions (see ref. 34). The computed optimized
geometries of the ground and excited states of NEaH are
collected in Fig. S4.†
Transition dipole moments were evaluated with time
dependent (TD) DFT calculations carried out on the mole-
cules forming the dimers. The TDHF/6-31+G* level of theory
was employed in this case and for the following evaluation
with the supramolecular approach to avoid mixing between
Frenkel and charge transfer exciton states as already pointed
out in previous studies.35
In the supramolecular approach the excitonic interaction
is indirectly estimated by the excitation energies of the two
excitonic states computed for molecular dimers. Therefore
the following steps have been followed: (a) calculation of exci-
tation energies for molecular dimers, (b) identification of the
two Frenkel excitations of the dimer and (c) evaluation of the
exciton coupling V. In keeping with their molecular salts
nature, when evaluating exciton interactions with PDA and
supramolecular approaches the anionic form, [NEa]−, was
considered for compounds 1 and 2.
Powder diffraction measurements
X-ray powder diffractograms in the 2θ range 5–40° (step size,
0.02°; time per step, 20 s; 0.04 rad soller; V × A 40 × 40) were
collected in Bragg–Brentano geometry on a Panalytical X'Pert
PRO automated diffractometer equipped with an X'Celerator
detector, using Cu Kα radiation without a monochromator.
The program Mercury31d was used for calculation of X-ray
powder patterns on the basis of single crystal data. The iden-
tity between bulk materials and single crystals was always ver-
ified by comparing calculated and experimental powder
diffraction patterns (see Fig. S1†). For variable temperature
experiments the diffractometer was equipped with an Anton
Paar TTK 450 system for measurements at controlled temper-
ature. All data were collected in open air.
Differential scanning calorimetry (DSC)
Calorimetric measurements were performed with a Perkin-
Elmer DSC-7 equipped with a PII intracooler. Temperature
and enthalpy calibrations were performed using high-purity
standards (n-decane, benzene and indium). Heating of the
aluminium open pans containing the samples (3–5 mg) was
carried out at 5 °C min−1 in the temperature range 40–350 °C.
Thermogravimetric analysis (TGA)
Acknowledgements
Thermogravimetric analyses were performed with a Perkin-
Elmer TGA-7. Heating was performed in a nitrogen flow
(20 cm3 min−1) using a platinum crucible at the rate of
5 °C min−1 up to decomposition. The sample weights were in
the range 5–10 mg.
We thank the University of Bologna and MIUR for financial
support. We thank Dr. Massimo Sgarzi for photophysics
measurements in solution.
Notes and references
Photophysics
Solvents used for photophysical determinations were of spec-
troscopic grade (C. Erba). UV-Vis absorption spectra were
recorded at room-temperature with a Perkin-Elmer Lambda
45 spectrophotometer. Quartz cuvettes with optical path
length of 1 cm were used. Fluorescence spectra were recorded
with a Perkin-Elmer LS 50 spectrofluorimeter, the uncertainty
1 (a) G. R. Desiraju in Crystal Engineering: The Design of
Organic Solids, ed. G. R. Desiraju. Elsevier, Amsterdam,
1989; (b) D. Braga, F. Grepioni and A. G. Orpen, in Crystal
Engineering: from Molecules and Crystal to Materials, ed.
D. Braga, F. Grepioni and A. G. Orpen, Kluwer Academic
Publishers, Dordrecht, 1999; (c) G. R. Desiraju, J. Chem. Sci.,
10478
|
CrystEngComm, 2013, 15, 10470–10480
This journal is © The Royal Society of Chemistry 2013

View Article Online
CrystEngComm
Paper
2010, 122, 667–675; (d) D. Braga, F. Grepioni and L. Maini,
Chem. Commun., 2010, 46, 6232–6242; (e) C. B. Aakeröy,
N. R. Champness and C. Janiak, CrystEngComm, 2010, 12,
22–43.
9 (a) T. Peters and M. J. Bide, Dyes Pigm., 1985, 6, 349–375; (b)
T. Konstantinova, P. Meallier and I. Grabtchev, Dyes Pigm.,
1993, 22, 191–198.
10 (a) J. Gan, K. Chen, C.-P. Chang and H. Tian, Dyes Pigm.,
2003, 57, 21–28; (b) Z. Jin, X.-B. Zhang, D.-X. Xie, Y.-J. Gong,
J. Zhang, X. Chen, G.-L. Shen and R.-Q. Yu, Anal. Chem.,
2010, 82, 6343–6346; (c) T. Gunnlaugsson, P. E. Kruger,
P. Jensen, J. Tierney, H. D. P. Ali and G. M. Hussey, J. Org.
Chem., 2005, 70, 10875–10878; (d) I. Grabchev, I. Moneva,
V. Bojinov and S. Guittonneau, J. Mater. Chem., 2000, 10,
1291–1296; (e) V. F. Pais, P. Remon, D. Collado,
J. Andeasson, E. Perez-Inestrosa and U. Pischel, Org. Lett.,
2011, 13(20), 5572–5575.
2 (a) M. Nagarathinam, A. M. P. Peedikakkal and J. J. Vittal,
Chem. Commun., 2008, 5277–5288; (b) L. R. MacGillivray,
J. Org. Chem., 2008, 73, 3311–3317; (c) Y. Sonoda, Molecules,
2011, 16, 119–148; (d) D. Braga, S. d'Agostino and
F. Grepioni, Cryst. Growth Des., 2012, 12, 4880–4889.
3 (a) V. Andre, M. T. Duarte, D. Braga and F. Grepioni, Cryst.
Growth Des., 2012, 12, 3082–3090D. Braga, F. Grepioni,
L. Maini, D. Capucci, S. Nanna, J. Wouters, L. Aerts and
L. Quéré, Chem. Commun., 2012, 48, 8219–8221.
4 (a) J. D. Wuest, Nat. Chem., 2012, 4, 74–75; (b) G.-G. Hou,
H.-J. Zhao, J.-F. Sun, D. Lin, X.-P. Dai, J.-T. Han and H. Zhao,
CrystEngComm, 2013, 15, 577–585; (c) Y. Abe, S. Karasawa
and N. Koga, Chem.–Eur. J., 2012, 18, 15038–15048; (d)
L. Quan, Y. Chen, X.-J. Lv and W.-F. Fu, Chem.–Eur. J., 2012,
18, 14599–14604; (e) S. Chen, N. Chen, Y. L. Yan, T. Liu,
Y. Yu, Y. Li, H. Liu, Y. S. Zhao and Y. Li, Chem. Commun.,
2012, 48, 9011–9013; (f) D. Yan, A. Delori, G. O. Lloyd,
B. Patel, T. Friŝćiĉ, G. M. Day, D.-K. Bučar, W. Jones, J. Lu,
M. Wei, D. G. Evans and X. Duan, CrystEngComm, 2012, 14,
5121–5123; (g) S. Philip Anthony, S. Varughese and
S. M. Draper, J. Phys. Org. Chem., 2010, 23, 1074–1079; (h)
T. Hinoue, Y. Shigenoi, M. Sugino, Y. Mizobe, I. Hisaki,
M. Miyata and N. Tohnai, Chem.–Eur. J., 2012, 18,
4634–4643; (i) D. Yan, A. Delori, G. O. Lloyd, T. Friŝćiĉ,
G. M. Day, W. Jones, J. Lu, M. Wei, D. G. Evans and X. Duan,
Angew. Chem., Int. Ed., 2011, 50, 12483–12486.
5 (a) O. Bolton, K. Lee, H.-J. Kim, K. Y. Lin and J. Kim, Nat.
Chem., 2011, 3, 205–210; (b) X. P., H. Wang, X. R. Zhao and
W. J. Jin, CrystEngComm, 2013, 15, 2722–2730; (c) H. Y. Gao,
X. R. Zhao, H. Wang, X. Pang and W. J. Jin, Cryst. Growth
Des., 2012, 12, 4377–4387.
6 P. Bamfield and M. G. Hutchings in Chromic Phenomena
Technological Applications of Colour Chemistry, 2nd edn, ed.
P. Bamfield and M. G. Hutchings, Royal Society of
Chemistry, Cambridge, 2010.
11 D. L. Reger, A. Debreczeni, J. J. Horger and M. D. Smith,
Cryst. Growth Des., 2011, 11, 4068–4079.
12 (a) D. Braga, L. Maini and F. Grepioni, Chem. Soc. Rev.,
2013, 42, 7638–7648; (b) D. Braga, S. d'Agostino,
E. Dichiarante, L. Maini and F. Grepioni, Chem.–Asian J.,
2011, 6, 2214–2223; (c) S. L. James, C. J. Adams, C. Bolm,
D. Braga, P. Collier, T. Friŝćiĉ, F. Grepioni, K. D. M. Harris,
G. Hyett, W. Jones, A. Krebs, J. Mack, L. Maini, A. G. Orpen,
I. P. Parkin, W. C. Shearouse, J. W. Steed and D. C. Waddell,
Chem. Soc. Rev., 2012, 41, 413–447.
13 (a) D. Braga and F. Grepioni, Acc. Chem. Res., 2000, 33,
601–608; (b) P. Gilli and G. Gilli, in Supramolecular Chemistry:
From Molecules to Nanomaterials, ed. P. A. Gale and J. W.
Steed, WILEY-VCH, Weinheim, 2012, pp. 2829–2867.
14 (a) L. Infantes and S. Motherwell, CrystEngComm, 2002, 4,
454–461; (b) L. Infantes, J. Chisholm and S. Motherwell,
CrystEngComm, 2003, 5, 480–486; (c) M. Mascal, L. Infantes
and J. Chisholm, Angew. Chem., Int. Ed., 2006, 45, 32–36.
15 (a) S. M. Alexiou, V. Tychopoulos, S. Ghorbanian,
J. H. P. Tyman, R. G. Brown and P. I. Brittain, J. Chem. Soc.,
Perkin Trans. 2, 1990, 837–842; (b) H. Cao, V. Chang,
R. Hernandez and M. D. Heagy, J. Org. Chem., 2005, 70,
4929–4934.
16 V. Wintgens, P. Valat, J. Kossanyi, L. Biczok, A. Demeter and
T. Berces, J. Chem. Soc., Faraday Trans., 1994, 90(3),
411–421.
7 (a) R. H. Friend, R. W. Gymer, A. B. Holmes, J. H. Burroughes,
R. N. Marks, C. Taliani, D. D. C. Bradley, D. A. Dos Santos,
J. L. Brédas, M. Lögdlund and W. R. Salaneck, Nature, 1999,
397, 121–128; (b) C. A. Strassert, C. Chien, M. D. G. Lopez,
D. Kourkoulos, D. Hertel, K. Meerholz and L. De Cola, Angew.
Chem., 2011, 123, 976–980; (c) C. A. Strassert, C. Chien,
M. D. G. Lopez, D. Kourkoulos, D. Hertel, K. Meerholz and
L. De Cola, Angew. Chem., Int. Ed., 2011, 50, 946–950; (d)
J. You, G. Li, R. Wang, Q. Nie, Z. Wang and J. Li, Phys. Chem.
Chem. Phys., 2011, 13, 17825–17830; (e) K. Watanabe,
T. Sakamoto, M. Taguchi, M. Fujiki and T. Nakano, Chem.
Commun., 2011, 47, 10996–10998.
8 (a) D. Kolosov, V. Adamovich, P. Djurovich, M. E. Thompson
and C. Adachi, J. Am. Chem. Soc., 2002, 124, 9945–9954; (b)
F. Cacialli, R. H. Friend, C. M. Bouche, P. Le Barny,
H. Facoetti, F. Soyer and P. Robin, J. Appl. Phys., 1998, 83,
2343–2356.
17 (a) P. Berci, V. G. Toscano and M. J. Politi, J. Photochem.
Photobiol., A, 1988, 43(1), 51–58; (b) K. Matsubayashi and
Y. Kubo, J. Org. Chem., 2008, 73(13), 4915–4919.
18 (a) L. Biczok, T. Berces and H. Linschitz, J. Am. Chem. Soc.,
1997, 119(45), 11071–11077; (b) O. V. Prezhdo, B. V. Uspenskii,
V. V. Prezhdo, W. Boszczyk and V. B. Distanov, Dyes Pigm.,
2007, 72(1), 42–46.
19 Y. Mizobe, T. Hinoue, A. Yamamoto, I. Hisaki, M. Miyata,
Y. Hasegawa and N. Tohnai, Chem.–Eur. J., 2009, 15(33),
8175–8184.
20 (a) T. C. Barros, P. Berci, V. G. Toscano and M. J. Politi,
J. Photochem. Photobiol., A, 1995, 89(2), 141–146; (b)
D. W. Cho, M. Fujitsuka, K. H. Choi, M. J. Park, U. C. Yoon
and T. Majima, J. Phys. Chem. B, 2006, 110(10), 4576–4582;
(c) D. W. Cho, Fujitsuka, M. A. Sugimoto and T. Majima,
J. Phys. Chem. A, 2008, 112(31), 7208–7213.
21 F. C. Spano, Acc. Chem. Res., 2010, 43, 429–439.
This journal is © The Royal Society of Chemistry 2013
CrystEngComm, 2013, 15, 10470–10480 | 10479

View Article Online
Paper
CrystEngComm
22 J.-L. Bredas, D. Beljonne, V. Coropceanu and J. Cornil, Chem.
Rev., 2004, 104, 4971–5003.
31 (a) G. M. Sheldrick, SHELX97, Program for Crystal Structure
Determination, University of Göttingen. Göttingen, Germany,
1997; (b) A. L. Speck, PLATON Acta Crystallogr., Sect. A:
Found. Crystallogr., 1990, 46, C34; (c) C. Y. Legault, CYLview,
1.0b, Université de Sherbrooke, Sherbrooke, Canada, 2009;
(d) C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington,
P. McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor,
J. van de Streek and P. A. J. Wood, J. Appl. Crystallogr.,
2008, 41, 466–470; (e) E. Keller, SCHAKAL99, Graphical
Representation of Molecular Models, University of Freiburg,
Freiburg, Germany, 1999.
23 (a) S. Di Motta, M. Siracusa and F. Negri, J. Phys. Chem. C,
2011, 115, 20754–20764; (b) E. Di Donato, R. P. Fornari,
S. Di Motta, Y. Li, Z. Wang and F. Negri, J. Phys. Chem. B,
2010, 114, 5327–5334; (c) S. Di Motta, E. Di Donato,
F. Negri, G. Orlandi, D. Fazzi and C. Castiglioni, J. Am.
Chem. Soc., 2009, 131, 6591–6598.
24 G. D. Scholes and K. P. Ghiggino, J. Phys. Chem., 1994, 98,
4580–4590.
25 R. Fink, J. Pfister, A. Schneider, H. Zhao and B. Engels,
Chem. Phys., 2008, 343, 353–361.
26 D. Beljonne, J. Cornil, R. Silbey, P. Millié and J.-L. Bredas,
J. Chem. Phys., 2000, 112, 4749–4758.
27 D. Fazzi, C. Quarti and M. Del Zoppo, Phys. Chem. Chem.
Phys., 2011, 13, 18615–18625.
28 M. Kasha, H. R. Rawls and M. Ashraf El-Bayoumi, Pure Appl.
Chem., 1965, 11, 371–392.
29 (a) D. Braga, L. Maini, G. De Sanctis, K. Rubini, F. Grepioni,
M. R. Chierotti and R. Gobetto, Chem.–Eur. J., 2003, 9,
5538–5548; (b) S. Childs, G. P. Stahly and A. Park, Mol.
Pharmaceutics, 2007, 4, 323–338.
32 J. C. deMello, H. F. Wittmann and R. H. Friend, Adv. Mater.,
1997, 9, 230–232.
33 More information about the theory and practice of
integrating spheres can be found in the Technical Notes “A
Guide to Integrating Sphere, Theory and Applications”,
available at http://www.labsphere.com, and “Integrating
Sphere, Design and Applications”, available at http://www.
sphereoptics.com.
34 D. Jacquemin, E. A. Perpete, G. Scalmani, M. J. Frisch,
I. Ciofini and C. Adamo, Chem. Phys. Lett., 2007,
448, 3–6.
30 D. L. Reger, J. J. Horger, A. Debreczeni and M. D. Smith,
Inorg. Chem., 2011, 50, 10225–10240.
35 R. F. Fink, J. Pfister, H. M. Zhao and B. Engels, Chem. Phys.,
2008, 346, 275–285.
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