Stokes shift/emission efficiency trade-off in donor-acceptor perylenemonoimides for luminescent solar concentrators

Article abstract of DOI:10.1039/c5ta01134e

Perylenediimides (PDIs) are among the best performing organic luminescent materials, both in terms of emission efficiency and chemical and photochemical stability because of their rigid, symmetric and planar structure; however, they exhibit very small Sto

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Stokes shift/emission efficiency trade–off in donor-acceptor perylene
monoimides for luminescent solar concentrators.
Riccardo Turrisi,a,c Alessandro Sanguineti,a Mauro Sassi,a Brett Savoie,c Atsuro Takai,c Giorgio E.
Patriarca,a Matteo M. Salamone,a Riccardo Ruffo,a Gianfranco Vaccaro,a Francesco Meinardi,a Tobin J.
Marks,c Antonio Facchettib and Luca Beverinaa,*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Perylenediimides (PDIs) are among the most performing organic luminescent materials, both in terms of
emission efficiency and chemical and photochemical stability thanks to their rigid, symmetric and planar
10 structure. However, they exhibit very small Stokes shifts. The sizeable reabsorption of the emitted light
limits perylenediimides performances for example in imaging applications and luminescent solar
concentrators. Perylenemonoimides (PMIs) having an electron donating substituent in one of the free peri
positions feature larger Stokes shift values, while retaining high chemical stability. Selection of the most
appropriate donor, both in terms of electron donating capabilities and steric demand, boosts emission
15 efficiency and limits reabsorption losses. The synthesis, optical spectroscopy, molecular orbital
computations, UPS, electrochemical, spectroelectrochemical, and multinuclear NMR investigation of a
series of PMI derivatives functionalized with donors having different electronic characteristics and steric
demands are discussed. Results are relevant for the fabrication of single layer, plastic Luminescent Solar
Concentrators (LSC).
(PMMA), although in specific cases other materials can be
used.33,34 The embedded luminescent molecules absorb sunlight
and emit light inside the slab. If the refractive index of the slab is
significantly higher than that of the air, most of the emitted light
50 is trapped by total internal reflection. The emitted light will travel
to the slab edges and there be collected in a small area where a
standard silicon solar cell is located. The advantages of such
strategy are: 1) The LSC is a light collector where diffuse light
over a large area is concentrated at the slab edges; this is usefl,
55 since Silicon PV cells need a certain light intensity threshold to
convert light into electricity. 2) Strong reduction in the amount of
silicon in the cell since it is required only to cover the LCS slab
edges. 3) The slabs can be easily integrated with buildings due to
wide colour tuning capabilities. If properly engineered, a LSC
60 can be at the same time a structural component (for example in
sunroofs and windows), an active energyꢀproducing device and a
decorative element.
The main limitation of the LCS concept is the reꢀabsorption of
the emitted light due to incomplete spectral separation between
65 the dye absorption and emission spectra.35 This effect strongly
limits the slab maximum collecting surface. Recently Currie et al.
demonstrated that the use of biꢀlayer structures consisting of a
thin film of organic dyes vacuum deposited on a highꢀrefractiveꢀ
index glass efficiently reduces reꢀabsorption losses.36 Further
70 optimizations of the LSC structure have also been recently
proposed.31,37ꢀ39 Low cost, plastic, single layer LSCs require the
design of efficient luminophores, having complete spectral
separation between absorption and emission (i.e. large Stokes
20 Introduction
Perylene dyes are among the most successful πꢀconjugated
organic derivatives for optoelectronic applications. Their most
relevant features include flexibility in the chemical structure,
tuning of electrical, optical and optoelectronic properties, low
25 toxicity, high absorption and emission efficiencies as well as
unrivalled photo, thermo and chemical stability.1ꢀ5 As such,
perylene dyes have found applications in diverse research fields
such as organic field effect transistors,6ꢀ10 dye lasers,11 donorꢀ
acceptor dyads,12,13 sensors,14,15 imaging and bioimaging,16ꢀ18
30 nonlinearoptics19ꢀ21 and dye sensitized22ꢀ25 and bulk
heterojunction solar cells.26,27 Perylenedimidies (PDIs), under the
brand name of Lumogen, are also the core active component of
luminescent solar concentrators (LSCs),
a class of light
concentrating devices introduced in the early ‘70s and recently
35 revisited in view of their applicability in building integrated
photovoltaics.28ꢀ31
The LSC concept was introduced to reduce production costs and
overcome some limitations of standard siliconꢀbased
photovoltaics without changing the basic photonꢀtoꢀcurrent
40 conversion technology (silicon single junction cells). Moreover,
these devices possess building integration opportunities even
greater than those of large area dye sensitized solar cells. In their
most common embodiment, LSCs are slabs of transparent, highꢀ
quality optical materials doped with luminescent molecules.32
45 The host material is typically poly(methylmethacrylate)
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Chromophore’s Design and Synthesis.
shifts). Moreover, the dye should absorb the largest possible
portion of the solar spectrum, efficiently emit in the slab as well
as withstand direct exposure to solar light and possibly extreme
weather conditions for years. Among various chromophore
60 Figure 1 shows the chemical structure of the PMI derivatives
investigated in this study along with that of the planar/rigid
derivative PMI-qs68 used here for comparison. Derivatives 1ꢀ5
share the same PMI core and differ for the donor group at the peri
position. Aiming at establishing structureꢀproperty relationships
65 ruling emission efficiency and extent of the Stokes shift, we
selected a series of donor groups sharing, except for derivative 1,
the same donating centre, a nitrogen atom featuring an available
lone pair, embedded in very diverse electronic structures.
5
classes
for
LSCs
(rhodamines28
and
coumarines,40
oligothiophenes,33 phycobilisomes,41 lanthanide chelates42ꢀ47 and
more recently quantum dots38,48ꢀ50) perylenediimides represent the
stateꢀofꢀthe art in LSC materials, even though their Stokes Shift is
very small.51,52
10 Molecules featuring a large Stokes shift pertain to two main
classes, having in common a large change in the molecular
structure upon optical excitation (Figure S1, Supporting
Information): a) donorꢀacceptor derivatives (DꢀA) and b) twisted
(TW) structures. In DꢀA derivatives the optical transition
15 involves a redistribution of the electron density from an electronꢀ
rich group (the donor) to an electronꢀpoor one (the acceptor),
through a conjugated bridge.53ꢀ56 This is by far the broader class
of large Stokes shift derivatives, finding applications for example
in fluorescence bioꢀimaging.57,58 Efficient DꢀA fluorophores
20 feature rigid, planar and easily polarizable conjugated bridges
enabling for large changes in the electronic distribution upon
optical excitation.
D =
N
N
O
N
O
1
2
O
N
O
N
4
N
3
N
N
D
N
Conversely, TW molecules possess conjugated bridges having
substantial deviation from planarity. The pꢀterphenyl molecule
25 represents a good example of this class of materials (see Figure
S1). Optical excitation involves a transition from the aromatic
and twisted ground state structure to a quinoidal excited state
structure having formal double bonds connecting the neighboring
benzene rings. The resulting major variations in the molecular
30 electronic structure translates into a large Stokes shift.59,60 While
the PDI core cannot be manipulated to fit in any of two such
PMI-qs
5
70
Figure 1.
In details, molecule 1 features an indolizine donor where the
nitrogen lone pair involved in the substituent π orbital makes the
pentatomic ring πꢀexcessive, and thus electron donating. The
molecule is connected to the PMI core through its 1ꢀposition.
75 This is the only member of this series featuring a carbonꢀcarbon
bond between the donor and the perylene core. Molecule 2
possesses a carbazole donor directly connected to the perylene
core through the nitrogen atom. Since the central ring of
carbazole is aromatic, the nitrogen lone pair, involved in the
80 carbazole π orbital, is not particularly prone to delocalization
towards the PMI electronꢀwithdrawing end. Derivatives 3 and 4
have already been described in the literature as DꢀA
molecules.53,64,65 In terms of donating capabilities, it can be
anticipated that a dialkylamine will be a stronger donor due to the
85 lack of delocalization of the nitrogen lone pair over aryl
substituents rather than the perylene core.69 Moreover, derivative
4 is the only member of the series that can be described as a
purely DꢀA molecule without TW contribution. Finally,
derivative 5 features a dibenzoazepine donor, directly connected
90 with the perylene core through its nitrogen atom. The central ring
of dibenzoazepine features 8 π electrons and therefore, as per
Hückel’s definition, it is antiaromatic. Thus, the nitrogen lone
pair should be extremely prone to delocalize, since donation of
the electron pair would provide aromatic stabilization. It should
95 be noted that Hückel rule applies for monocyclic, planar
compounds. The case of dibenzoazepine is different, and thus
deviations from a purely antiaromatic behaviour are expected.70
The synthesis of derivatives 1ꢀ5 is reported in Scheme S1 of the
Supporting Information. The key intermediate for all of the
100 compounds is the unsubstituted perylene monoimide 7. The latter
can be prepared, according to the method of Langhals, by the
condensation/decarboxylation reaction of perylendianhydride 6
and 3,5ꢀditertbutylaniline.71 This reaction was carried out using
classes, perylenemonomides (PMIs) can display
a rather
pronounced DꢀA character, provided that they carry a strong
electron donating substituent in one or both free peri
35 positions.53,61ꢀ66 Also, substitution at the same positions with
bulky arenes leads to TWꢀtype structures having relevant Stokes
shifts, as demonstrated by the PMI dimers prepared by the
Langhals group.67
In specific cases, i.e. when the arene introduced at one of the free
40 peri positions is bulky and is also a donor group, the resulting
PMI will behave according to a combination of the DꢀA and TW
governed regimes. The capability to control the interplay of the
TW vs. DꢀA contributions in PMIs could provides a tool for
further optimization of these compounds for LSC applications. In
45 fact, while twisted structures usually feature high fluorescence
efficiency and modest molar absorptivity, the opposite occurs for
donorꢀacceptor compounds.
The present paper aims at studying the influence of the donor
residue electronic and steric characteristics on the Stokes shift
50 and emission efficiency in a series of PMIs (1ꢀ5, Figure 1) for
single layer LSC. These derivatives were investigated by steadyꢀ
state UVꢀVis absorption and emission spectroscopies, UVꢀVis
transient
spectroelectrochemistry,
absorption
spectroscopy,
UPS and
electrochemistry,
multinuclear NMR
55 measurements to establish general structureꢀproperties
relationships. We will then show how the molecular properties
dominate single layer LSC efficiencies, with particular emphasis
on reabsorption losses.
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Table 1. Optimized HOMO and LUMO geometries (B3LYP/6ꢀ31G**),
energies, torsional angle formed between perylene core and Donor residue
and ground state dipole moment for derivatives 1ꢀ5 and PMI-qs.
imidazole as the solvent and in the presence of Zn(AcO)2 in a
steel autoclave at 190°C for 24 h. Pure 6 can be isolated in 33%
yield after chromatographic purification. Regioselective
bromination of 6, according to Nagao procedure, gives the
bromide 8 in 91% yield after chromatographic purification.72
HOMO (up)
Torsional
angle (°)
ꢀ
5
PMI
Derivative
1
was prepared by direct arylation of 2ꢀ
LUMO (down)
(Debye)
methylindolizine with bromide 8. This type of reactions are
becoming relevant due to the lack of toxic/difficult to remove
organometallic intermediates and the generally high yields, once
10 the reaction conditions are optimized.73 We obtained the best
results working under Fagnou conditions74,using DMAc at 100
°C with Pd(AcO)2/P(cychlohexyl)3HBF4 as the catalyst in the
presence of pivalic acid and K2CO3 as a base. Different from all
other compounds in the series, derivative 1 proved to be air
15 unstable. The BuchwaldꢀHartwig amination of 8 with 4,7ꢀdiꢀtertꢀ
butylcarbazole, diphenylamine, dibutylamine and dibenzoazepine
gives derivatives 2 (41%), 3 (70%), 4 (70%) and 5 (75%),
respectively. The reaction conditions were the same for all of the
compounds: we used a Pd(dba)2/P(tBu)3 catalyst in refluxing
20 toluene with tertꢀBuONa as the base. We carried out these
reactions for 6ꢀ8 h under microwave irradiation. The conversion
was complete in all of the cases while the reaction yields
reflected the nucleophilicity of the donor nitranion.
61
62
45
ꢀ
5.93
4.75
6.70
8.14
1
2
3
4
25 Molecular Orbital Computations.
In order to obtain insights into the electronic structure and
geometry of the PMI derivatives, particularly molecular planarity
upon donor variation, we carried out molecular orbital
computations.75 All DFT calculations were performed using the
30 QꢀChem software suite, details are discussed in the Supporting
Information.76 Table
1 shows the calculated ground state
geometries, HOMO and LUMO energies, torsional angle between
the donor and perylene core and the dipole moments of
derivatives 1ꢀ5 and of reference molecule PMI-qs. Derivatives 1
35 and 2 featuring the planar and rigid indolizine and carbazole
donating groups, respectively, exhibit remarkably large torsional
angles with respect to the perylene core (61° and 62°,
respectively). MO calculations give a ground state dipole moment
higher for derivative 1 (5.93 Debye) than for derivative 2 (4.75
40 Debye). Derivative 3 having a diphenylamine donating group is
also considerably twisted (45°) and shows a calculated dipole
moment of 6.70 Debye, which is larger than that of both 1 and 2.
Finally, derivative 5 structure shows that first of all the central
ring of the dibenzoazepine residue is sizably bended (its two
45 benzene rings forming an angle of 122°), a data consistent with
the reported Xꢀray structure.70 Moreover, the whole
dibenzoazepine residue is almost perpendicular with respect to
the perylene plane (80° of torsional angle). Derivative 5
calculated dipole moment is 5.98 Debye, lower than that of 3, as
50 expected due to the severe deviation from planarity. With the
exclusion of derivative 4 (dipole moment 8.14 Debye), a purely
DꢀA compound, all other molecules can be described as the
combination of the donorꢀacceptor and twisted structures. In fact,
all of them show a sizeable deviation from planarity (like in the
55 case of pꢀterphenyl discussed in the Introduction and shown in
Figure S1).
80
5.98
3.51
5
0
PMI-qs
60
At the same time, the variation in the ground state molecular
dipole moment as a function of the type of donor group
(increasing in the order 2<1<5<3<4) indicates the presence of a
DꢀA behaviour (similar to that of DCM, figure S1).
65 Electrochemical, Spectroelectrochemical, UPS and Transient
Absorption Characterizations.
The inspection of the differential pulse voltammetry (DPV) plots
of a DꢀA compound family possessing the same acceptor and
conjugated core enables to rank the donating capabilities of a
70 donor series. In this study we used DPV instead of the more
commonly employed cyclic voltammetry (CV) since some of our
PMIs do not exhibit reversible oxidations (Figures 2a,b show the
DPV plots. Figure S2 of the Supporting Information shows the
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corresponding CV plots for all compounds). Donating
substituents are expected to increase the electrochemicallyꢀ
derived HOMO energies according to their specific donating
strength. Furthermore, if the donor and the acceptor ends are
nitrogenꢀbased donor as its nitrogen lone pair can be fully
delocalized over the perylene bridge, not being shared by any
other conjugated residue. The electrochemical LUMO levels
(Table 2) show the same trend of the corresponding HOMOs,
5
efficiently coupled, an increasing donating strength should also 50 even though the differences in the corresponding energies are less
increase the LUMO energy as the accepting end, where most
likely the LUMO is localized, becomes harder to reduce. Thus,
when a particular donor mostly affects the HOMO energy
without altering the LUMO, its coupling with the acceptor (and
pronounced, as expected by the calculated LUMOs, whose major
contributions come from the perylene core.
We compared DPV results with solventꢀindependent UPS data.
We included in the experiment the strongly electronꢀdeficient
10 the extent of the πꢀconjugation) may be considered weak. The 55 derivative PMI-qs, which was previously used for luminescent
likely reasons for such behaviour are excessive bridge
conjugation length and/or the presence of a sizeable torsional
angle between the donor and the acceptor (the acceptor being, at
least in our case, forced to be coplanar with the bridge). As such,
solar concentrators. Figure 3c shows the highꢀbinding energy
cutoff region of the normalized photoemission spectra at ꢀ9V bias
for compounds 1,2,3,5 and PMI-qs. The full photoemission
spectra are reported in Figure S9 of the Supporting Information.
15 the simultaneous inspection of both reduction and oxidation 60 The ionization potential of all solids is equal to the difference
processes in a molecule series enables to rank the donor strength
in terms of electron density effectively transferred towards the
acceptor. It is worthwhile noting that DPV is not a direct vertical
ionization technique, consequently, solvent stabilization of
20 charged species as well as the reorganization energy always
impact the electrochemicallyꢀderived HOMO and LUMO
levels.69 Thus, in parallel with electrochemical techniques we also
employed ultraviolet photoemission spectroscopy (UPS) ꢀ a
vertical ionization technique – and compared the results (Figure
25 2cꢀd). All DPV, UPS, and the resulting molecular orbital energies
are collected in Table 2, along with corresponding calculated
values.
between the highꢀbinding (low kinetic) cutꢀoff energies and the
lowꢀbinding (highꢀkinetic) Fermi edge onsets. Figure 2d shows
the lowꢀbinding energy cutoff region highlighting the difference
in the first ionization shoulder (vide infra).
Table 2. Comparison between the calculated, electrochemical and UPS
estimates of the HOMO and LUMO levels for derivatives 1ꢀ5 and PMI-
30 qs.
1
2
3
4
5
PMI-
qs
Epcred (V vs
Fc+/Fc)
ꢀ1.32
0.25
ꢀ1.29 ꢀ1.33 ꢀ1.41
0.79 0.52 0.29
ꢀ1.45
0.36
ꢀ1.84
EpcOx(V vs
ꢀ
Fc+/Fc)
65
Electrochemical
HOMO (eV)
Electrochemical
LUMO (eV)
UPS HOMO
(eV)
Calculated
HOMO
Calculated
LUMO
ꢀ5.48
ꢀ3.65
ꢀ5.20
ꢀ5.09
ꢀ2.80
ꢀ6.02 ꢀ5.75 ꢀ5.52
ꢀ3.62 ꢀ3.66 ꢀ3.74
ꢀ5.59
ꢀ3.78
ꢀ5.60
ꢀ5.47
ꢀ2.80
ꢀ
Figure 2. Electrochemical and UPS characterization of derivatives 1ꢀ5.
Reduction (a) and oxidation (b) DPV plots for derivatives 1ꢀ5 in CH2Cl2
with tetrabutylammonium pꢀtoluenesulfonate as the supporting
electrolyte. c) Normalized photoemission spectra under a ꢀ9V bias for
70 compounds 1,2,3,5 and the reference derivative PMI-qs. Highꢀbinding
energy cutoff region. The onset for all of the spectra has been moved to ꢀ
8.92 eV in order to evidence the arising difference in ionization
potentials. d) Lowꢀbinding energy cutoff region highlighting the
difference in the first ionization shoulder.
ꢀ4.17
ꢀ5.95
ꢀ5.71
ꢀ3.18
ꢀ5.60 ꢀ5.45
ꢀ
ꢀ5.36 ꢀ5.17 ꢀ5.20
ꢀ2.88 ꢀ2.75 ꢀ2.64
75 Table 2 shows the comparison between UPS ionization potentials
and the corresponding calculated and electrochemical HOMO
levels. The trend for derivatives 1, 2 and 3 is consistent with the
DPV data. The deviations between the datasets possibly are
ascribed to solvent/aggregation effects associated to the DPV
80 experiments. However, in our case, the UPS and DPV datasets
are almost superimposable for derivative 5. Moreover, 5 and 2
UPSꢀ and oxidation potentialꢀderived HOMO energies are the
same. Additionally, if we examine the lowꢀbinding energy region
of the UPS spectra (Figure 2d) one can notice that while 1, 2 and
85 3 show a shoulder attributed to the first ionization process, both
PMI-Qs and 5 show a weaker signal. While the lack of a peak is
expected for PDI-Qs – an all acceptor compound – this result is
unexpected for 5. As observed previously for the naphthalene
DPV plots (Figures 2a,b) show that derivative 1, the only
compound exhibiting poor air stability, features the lowest
oxidation potential in this series (+0.25 V vs Fc+/Fc), with those
35 of 2 (+0.79 V), 3 (+0.52 V), and 4 (+0.29 V) located at higher
potentials. Thus, derivative 1 electrochemicallyꢀderived HOMO
energy (ꢀ5.48 eV) is higher than those of derivatives 2ꢀ4 (2: ꢀ6.02
eV; 3: ꢀ5.75 eV; 4: ꢀ5.52 eV). Amongst the nitrogen based
donors, the dibutylamine is the strongest one (derivative 4)
40 followed by the diphenylamine (derivative 3) and next by the
carbazole. Interestingly, the use of dibenzoazepine (derivative 5),
a supposedly very strong donor, leads to an electrochemical
HOMO (ꢀ5.59 eV) intermediate between those of 3 and 4. This
result can be rationalized by the peculiar geometry of compound
45 5 (see previous paragraph). Dibutylamine remains the stronger
4
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analogue of 5, the molecule is so severely twisted that DꢀA 30 The transient spectra of 5 in the 500ꢀ700 nm region show intense
charge transfer becomes strongly impaired.77 In the solid state
derivatives 2 and 5 share the same deep HOMO level (ꢀ5.60 eV),
closer to that of the allꢀacceptor derivative PMI-qs (ꢀ5.95 eV).
absorption bands at 566 and 638 nm (Figure 3). Bleaches
(negative ꢁA) at λ < 520 nm and λ > 700 nm are due to ground
state depletion and stimulated emission, respectively. The decay
time of the absorption bands at 566 and 638 nm is in the order of
35 a few nanoseconds. The absorption band at 638 nm decays with a
firstꢀorder rate constant of 3.7 × 1010 s–1 (Figure 4a), while the
change in the absorption at 566 nm occurs in two steps with firstꢀ
order rate constants of 3.7 × 1010 s–1 and 2.3 × 108 s–1 (see Figures
S11 and S12 of the Supporting Information). The initial fast
40 process may be attributed to a conformational change of (5)*,
(e.g. increased planarity and sp3ꢀ>pseudoꢀsp2 geometry of the
linking dibenzoazepine nitrogen).78
A direct excitation of the dibenzoazepine moiety and the
consequent energy transfer from (PMIꢀ1dibenzoazepine*) to
45 (1PMI*ꢀ dibenzoazepine) should be excluded because the
dibenzoazepine moiety does not absorb in the 480 nm region (i.e.
excitation wavelength). Afterwards the signal decays according to
a chargeꢀtransfer process from the dibenzoazepine donor to the
PMI acceptor, in agreement with a DꢀA behaviour. These spectra
50 correspond to the sum of the reduced (radical anion) and oxidized
0,05
5 ps
40 ps
200 ps
2000 ps
7000 ps
0,04
0,03
0,02
0,01
0,00
-0,01
-0,02
-0,03
566 nm
638 nm
500
600
700
800
900
Wavelength (nm)
5
Figure 3. Differential transient absorption spectra of 5 (1.5 × 10–5 M) for 7
to 7000 ps after laser pulse irradiation at 480 nm in deaerated
dichloromethane at 298 K.
(radical cation) forms of
5
as obtained by
a
spectroelectrochemical experiment (Figure 4). The rate constant
of the charge transfer process (2.3 × 108 s–1; 4.3 ns) is consistent
with the fluorescence lifetime of analogous PMI derivatives (~3
Conversely, the DPV data place derivative 5 HOMO energy well
55 ns).79
10 above those of derivatives 2 and 3. This inconsistency questions
the real charge transfer nature of the HOMOꢀLUMO transition of
derivative 5. Indeed the inspection of the orbital densities of 5 in
the gas phase suggests a marginal role of the donor group in both
the ground and the first excited state electron densities (Table 1).
15 Such conflicting data can be explained by taking into account a
likely different geometry of 5 in solution with respect to both the
solid state and the gas phases. However, to better characterize the
nature of the HOMOꢀLUMO transition of 5 in solution, we
carried out timeꢀresolved transient absorption experiments in
20 deaereated dichloromethane at room temperature and we
compared the results with a spectroelectrochemical analysis in the
same solvent.
Multinuclear NMR investigation
1
Multinuclear H and 13C NMR spectroscopies can be of further
aid in characterizing the effective electron donating capabilities
of our donors. In fact, the inspection of the chemical shift of
60 selected positions across the perylene bridge provides qualitative
information about the amount of charge that a given donor is
conveying towards the acceptor.
Ar
N
Ar
N
Ar
N
Ar
N
O
O
O
O
O
O
O
O
2
1
5
6
3
4
7
8
6b
9
D
Energy (eV)
D
D
D
2,4 2,3
2,2
2,1
2
1,9
0.0 V
1,8
1,7
0,010
0,008
0,006
0,004
0,002
0,000
aromatic form
quinoidal form (1)
quinidal form (2)
quinidal form (3)
0,90
0,75
0,60
0,45
0,30
0,15
0,00
+0.6 V
-1.5 V
transient absorption 5000 ps
65 Figure 5. Top: canonical representations of a general PMI highlighting
delocalization of donor charge on positions 6 and 6b. Bottom: ranking of
the donating capabilities of different donors according to the 13C chemical
shift of carbon 6b.
500 525 550 575 600 625 650 675 700 725 750
Wavelength (nm)
Figure 4. Comparison between the transient absorption spectrum at 5000
ps of a dichloromethane solution of 5 and the corresponding
spectroelectrochemical absorption spectra of a CH3CN solution of the
same compound at no applied bias (solid line), ꢀ1.5 V (dotted line) and +
0.6 V (dashed line). Potentials are reported versus the Fc/Fc+ redox
couple.
This effect is distinct from and complementary to the HOMO
70 energy increase evidenced by electrochemical and UPS
techniques. NMR will monitor an increase in the charge transfer
character through the chemical shift variations of the perylene
core. We considered particularly meaningful the positions 6 and
25
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6b, highlighted in the quinoidal forms (2) and (3) of the top part
very little DꢀA character. Upon optical excitation, this molecule
of Figure 5. In fact, according to the relative contribution of the 60 undergoes a quinoidal distortion enforcing molecular planarity.
aromatic versus quinoidal canonical description of the PMI
ground state, the selected positions will show an upfield shift
proportional to the donating strength of the employed
donor.69,80,81
The resulting major difference between the ground and first
excited electronic states is responsible for the particularly large
Stokes shift.
5
Table 3. UVꢀVis molar extinction coefficients, absorption and emission
65 maxima, luminescence quantum yields and Stokes shifts for derivatives 1ꢀ
5 in toluene and CHCl3 solutions. Luminescence quantum yields in
PMMA slabs. Derivative 1 was not embedded in PMMA slabs for
stability reasons.
1
Table S1 of the Supporting Information summarizes the H and
13C NMR data for the positions 5,6,7,8 and 6b of the perylene
core for 1ꢀ5. The detailed inspection of all the data show the same
10 trend, more or less pronounced, for all positions: an upfield shift
of the 1H and 13C NMR signals in the order 2<1<3<4<5. We will
discuss in particular the data referring to the 13C signals of
positions 6b. This particular carbon is quaternary and unaffected
by throughꢀspace effects. Moreover, as it is shown in the
15 quinoidal form (3) of Figure 5, in one of the canonical
representation of the general structure of our PMIs the charge
residing on the Donor can be delocalized in that position. Thus,
the 13C signal of position 6b for derivative 2 (130.40 ppm) is
essentially the same as that of the corresponding signal for
20 derivative 1 (130.26 ppm). This result reflects the inefficiency of
indolizine to act as a π Donor, even though such residue is
strongly electron rich (and thus easily oxidized) in
electrochemical terms. Derivative 3 signal (127.64 ppm) is
sizably upfield shifted, according to the documented donating
25 capabilities of aromatic amines. Derivative 4 is even more upfield
shifted (124.10 ppm). Exactly as in the case of the
electrochemical ranking of donating capability, the NMR data
confirm that an alkyl amine is a stronger donor with respect to an
aryl one.69 Finally, derivative 5 signal experiences the highest
30 upfield shift in the series (123.81 ppm) confirming that, at least in
solution, the dibenzoazepine residue is a stronger donor with
respect to a standard aromatic amine and a comparable one with
λmax
Abs
(nm)
502
513
517
λmax
Fluo
(nm)
541
578
643
Stokes Shift
(cmꢀ1)
ε
(L molꢀ1
cmꢀ1)
Compound
φi
1 (CHCl3)
2 (toluene)
2 (CHCl3)
2
< 5 %
99 %
43 %
25600
32000
33000
1436
2192
3790
93 %
(PMMA)a
3 (toluene)
3 (CHCl3)
3
550
560
629
676
95 %
56 %
31500
31000
2284
3064
73 %
(PMMA)a
4 (toluene)
4 (CHCl3)
4
530
550
654
696
39 %
33 %
28000
29000
3577
3814
40 %
(PMMA)a
5 (toluene)
5 (CHCl3)
5
562
577
646
677
73 %
36 %
30000
29000
2314
2560
70 %
(PMMA)a
a Absorption maxima of the slabs are not reported as the LSC
70 demonstrators were too absorptive for UVꢀVis spectrometers. Emission
maxima depend on the distance with respect to the excitation point (see
next section).
Derivatives 1 and 2 exhibit similar torsional angles; with the
notable difference that indolizine in 1 is a stronger donor. This
75 has profound consequences in terms of optical properties and
photostability. The close inspection of derivative 1 absorption
spectrum shows that its band is made of two contributions, a high
energy absorbing, vibrationally structured one peaking at 504 nm
and a low energy absorbing shoulder around 570 nm. The Stokes
80 shift is unusually small (1436 cmꢀ1).
1
respect to aliphatic amines. The inspection of both H and 13C
signals of positions 6 and 8 shows the same trend, with the only
35 exception of position 8 of derivative 4, sizably upfield shifted
with respect to the other compounds in the series. The deviation
can be associated with the fact that this derivative is the only one
featuring an alkyl substituent and thus unaffected by through
space shielding effects that are instead affecting the position 8 of
40 all other compounds.
Also, the vibrationally resolved component of both the absorption
and the emission bands of 1 closely resembles that of the
unsubstituted (Donor = H) PMI 7 (see structure in Scheme S1).
Interestingly, Figure S8 of the Supporting Information shows that
85 upon exposure to direct sunlight in an airꢀequilibrated solution,
the spectrum of 1 becomes superimposable to that of 7. We
believe that the low energy component of derivative 1 absorption
corresponds to a throughꢀspace photoinduced charge transfer
from the indolizine residue to the PMI core with consequent
90 formation of an airꢀunstable indolizine radical cation and a PMI
radical anion. While the anion can be reversibly quenched by
molecular oxygen, the radical cation undergoes irreversible
degradation.82 Such mechanism is likely to apply for all electronꢀ
rich and strongly twisted substituent. Figure S13 shows a cartoon
95 highlighting the tentative mechanism for the observed
degradation.
UV-Vis absorption and emission spectroscopy.
Figure 5 shows the absorption (left) and emission (right) spectra
of 1ꢀ5 in CHCl3 and Table
4 summarizes all UVꢀVis
characterization data. Literature data shows that DꢀA PMIs are
45 characterized by a broad and featureless absorption band peaking
at 500ꢀ650 nm, sizeable Stokes shifts (3000ꢀ4000 cmꢀ1) and
generally low emission efficiencies (< 40%).53,64
This holds true for derivatives 3 and 4, with 4 outperforming 3 in
terms of Stokes Shift by 750 cmꢀ1. The absorption spectrum of 5
50 is also broad but with a distinguishable vibronic structure. The
latter feature is connected with a less pronounced charge transfer
behavior with respect to both 3 and 4, as corroborated by the
smaller Stokes shift (2560 cmꢀ1). In contrast to all other PMI,
derivative 2 possesses a vibrationally structured absorption and a
55 broad and featureless emission. Also, its Stokes shift (3790 cmꢀ1)
is nearly as large as that of 4 (3814 cmꢀ1), even though carbazole
hardly qualifies as a strong donor. Likewise, 2 is the only
derivative in the series behaving mostly as a TW derivative with
Analysis of the emission efficiencies is also particularly
meaningful. First of all, derivative 4 is the only compound whose
(low) emission efficiency does not change appreciably on going
6
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from toluene to chloroform solutions. This is essentially a DꢀA
chromophore and thus it exhibits low emission efficiency (39%
and 33% in toluene and CHCl3 respectively) and a large Stokes
Shift. For all other compounds the emission efficiency is
segregation. The slab was characterized in terms of the solid state
quantum yield and reꢀabsorption losses as a function of the
distance between the excitation point and the slab edge where
emitted light was collected. It is worthwhile to note that
5
significantly higher in toluene (99, 95 and 73% for derivatives 2, 50 derivative 2 in PMMA matrix is almost as efficient (93%) as the
3 and 5 respectively) than in chloroform (43, 56 and 36% for 2, 3
and 5 respectively).
reference Lumogen f 240 orange dye (peryleneꢀ3,4,9,11ꢀ
tetracarboxylic acid bisꢀ(2’,6’ꢀ diisopropylanilide), quantum yield
99%).68
Figure 6. Normalized absorption (left) and emission (right) spectra of
derivatives 1ꢀ5 in CHCl3
10 Indeed, solvent polarity influences the interplay between DꢀA and
TW regimes, as a polar solvent (chloroform) will enhance the
former contribution, while a low polarity one (toluene) will do
the opposite. Coherently, for all compounds possessing sizeable
torsional angles (see Table 1) between the donor and the
15 acceptor, low polarity solvents leads to higher luminescence
quantum yields. For the purpose of the proposed application, the
data in PMMA are particularly meaningful. In this case the
emission efficiency will depend on polarity, as in the case of
solutions, but also on viscosity. In fact, molecules possessing two
20 conjugated fragments connected by a single bond having
restricted rotational freedom, frequently behave as “molecular
rotors”. The most distinctive characteristic of such class of
compounds is an increase of the luminescence quantum yield
upon increase of the local viscosity.60 In particular, we recently
25 introduced the naphthalene analogue of derivative 5 as a very
efficient molecular rotor capable to probe the nanostructure of
coreꢀshell nanoparticles obtained through the selfꢀassembly of
amphiphilic block copolymers.77
Indeed, with the sole exception of derivative 4 (a further
30 confirmation of the purely DonorꢀAcceptor nature of this
compound) all emission quantum yields in PMMA are
systematically higher than the values in chloroform and generally
comparable with those in toluene, even though the polarity of
PMMA is even higher than that of chloroform. The value is
35 particularly high for derivative 2, again not surprisingly, as this is
almost a purely twisted compound and thus a molecular rotor. As
derivative 2 combines a very high Stokes Shift and the higher
quantum yield in the series, we tested its performances as the
luminophore in a LSC prototype.
55 Figure 7. a) PL spectra excited at 405 nm at increasing distance (from 1 to
10 cm) between the excitation spot and detection at LSC edge for
derivative 2; b) Integrated PL output signal as a function of the distance
from the excitation spot for derivatives 2. For comparison also the
scattering losses, obtained from the diffusion of a laser beam at 820 nm
60 travelling through a pure PMMA slab (c), is reported. d) Total PL output
for a linear growth of the slab size for a LSC based on the derivative 2
and for a reference LSC based on Lumogen f 240. The blackꢀline
indicates the “hypothetic” PL output increase for an ideal lossless sample.
Several processes including light scattering at the interfaces or
65 inside the slab itself, and photoluminescence reꢀabsorption affect
the LSC efficiency. In order to isolate the optical losses due to the
reꢀabsorption, strictly related to the active dye efficiency, we
plotted the evolution of the normalized emission spectrum as a
function of the distance between the detection at the LSC edge
70 and the excitation spot (laser operating @ 405 nm). The
normalization was carried out on the lowꢀenergy tail of the
spectrum (λ > 730 nm) where the reꢀabsorption is negligible
(Figure 7a). The PL spectrum shifts towards lower energies when
increasing the distance, but the overall intensity is only slightly
75 attenuated as clearly shown in Figure 7b. Thus, for an optical
path of 10 cm the LSC based on the derivative 2 retains about
80% of its initial intensity. Figure 7b also reports the scattering
losses obtained from the attenuation of a laser @ 820 nm (photon
energy below the band gap) traveling through the slab (figure 7c).
80 The scattering contribution is generally very small and constant
throughout the sample, additional evidence that any further
improvement of LSC performances will mostly depend on the
optimization of the luminophore properties in terms of Stokes
shift and luminescence quantum yield.
40 Luminescent Solar Concentrator Devices.
A 10ꢀ4 M solution in PMMA/MMA syrup of the derivative 2 was
heated in a cell cast immersed in a water bath at 56°C for 48 h.
The cell cast was then annealed at 100°C for 12 h (see Supporting
Information). During the whole process the sample remained
45 homogeneous, without showing any sign of precipitation/phase
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Conclusions
Our findings demonstrate that derivative 2 represents an optimal
trade off as it shows both a very large Stokes shift and a high
emission quantum yield. This compound represents a significant
entry in the field of luminescent materials for LSC and is a valid
alternative to the traditional Lumogen dyes for the fabrication of
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5
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uncertain even though derivative 2 luminescence quantum yield
15 is sizably smaller. The use of the latter is in any case
advantageous as its red shifted emission spectrum corresponds to
the region of maximum External Quantum Yield of silicon solar
cells.
80
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20 a Department of Materials Science, Università di Milano-Bicocca, via
Cozzi 55, 20125, Milano, Italy E-mail: luca.beverina@unimib.it
b Polyera Corporation, 8045 Lamon Avenue, Suite 140 ,Skokie, IL 60077.
c Department of Chemistry and the Argonne-Northwestern Solar Energy
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