“N-alkyl diketopyrrolopyrrole-based fluorophores for luminescent solar concentrators: Effect of the alkyl chain on dye efficiency”

Article abstract of DOI:10.1016/j.dyepig.2016.03.036

We report on the preparation of luminescent solar concentrators (LSCs) made of poly(methyl methacrylate) (PMMA) thin films doped with six new diketopyrrolopyrrole (DPP) fluorophores obtained in good yields by using simple N-alkylation and direct C[sbnd]H

Full text of DOI:10.1016/j.dyepig.2016.03.036

Accepted Manuscript
“N-alkyl diketopyrrolopyrrole-based fluorophores for luminescent solar concentrators:
effect of the alkyl chain on dye efficiency”
Jonathan Lucarelli, Marco Lessi, Chiara Manzini, Pierpaolo Minei, Fabio Bellina,
Andrea Pucci
PII:
S0143-7208(16)30104-8
10.1016/j.dyepig.2016.03.036
DYPI 5161
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 27 January 2016
Revised Date: 17 March 2016
Accepted Date: 19 March 2016
Please cite this article as: Lucarelli J, Lessi M, Manzini C, Minei P, Bellina F, Pucci A, “N-alkyl
diketopyrrolopyrrole-based fluorophores for luminescent solar concentrators: effect of the alkyl chain on
dye efficiency”, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.03.036.
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ACCEPTED MANUSCRIPT

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“N-alkyl diketopyrrolopyrrole-based
fluorophores for luminescent solar
concentrators: effect of the alkyl chain on
dye efficiency”
Jonathan Lucarelli,a Marco Lessi,a Chiara Manzini,a Pierpaolo Minei,a Fabio Bellina,a,b Andrea
Puccia,b,*
(a) Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Moruzzi 13, 56124 Pisa,
Italy
(b) INSTM, UdR Pisa, Via Moruzzi 13, 56124 Pisa, Italy
* Corresponding author: andrea.pucci@unipi.it;

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Abstract
We report on the preparation of luminescent solar concentrators (LSCs) made of poly(methyl
methacrylate) (PMMA) thin films doped with six new diketopyrrolopyrrole (DPP) fluorophores
obtained in good yields by using simple N-alkylation and direct C-H arylation synthetic strategies.
Spectroscopic investigations in solution and in PMMA thin films combined with photocurrent
measurements revealed that the branched alkyl chains were efficient in preventing DPP segregation
from the PMMA matrix thanks to their higher steric hindrance. The aromatic substituent was found
to expand DPP conjugation but favoured DPP adverse aggregation, thus affecting fluorescence
emission and photocurrents of PMMA films. The worthwhile combination of the appropriate alkyl
chain and aromatic moieties assured to achieve optical efficiency of 6.8% that was comparable to
that of PMMA LSCs of the state-of-the art (7.2%).
Keywords
Diketopyrrolopyrrole fluorophores, poly(methyl methacrylate), dye dispersion, optical efficiency,
luminescent solar concentrators
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1. Introduction
Since the dawning of solar power production, concentration of solar radiation has been proposed as
a solution to decrease the price of photovoltaic energy. Solar concentration is achieved by collecting
the sun radiation incident on a large surface and redirecting it on a smaller area, thus allowing to
reduce the amount of photoactive materials, while maintaining the same power output.[1-3] The
most convenient example of this approach is represented by luminescent solar concentrators
(LSCs), which show several advantages such as the ability to work with diffuse light, light weight,
reduced costs, and transparency.[4-7] These last features make LSCs very well suited to be
implemented in modern building architectures, which make use of plenty of coloured windows and
panels.[8] LSCs are thin, flat or bulk plates of highly fluorescent materials that absorb sunlight and
concentrate part of the resulting fluorescence to their edges by internal reflection according to the
refractive index of the host material. The photoactive elements are organic dyes, luminescent
nanoparticles or Eu3+ complexes dispersed in a transparent polymer matrix such as poly(methyl
methacrylate) (PMMA).[8-10] The solar radiation is thus conveniently transmitted to PV cells at
their edges, even with a cloudy sky. Nevertheless, current LSC-PV devices achieve too low power
conversion efficiencies, being strongly plagued by a multitude of unfavourable processes that
hinder their ability to deliver light to PV cells, in particular re-absorption of light emitted by
luminescent species. [8, 11] [5, 12-21] Notably, the maximum power conversion efficiency of 7.1%
was recorded for a LSC-PV device based on the perylene derivative Lumogen F Red 305, whose
recently quotation (~7,500 €/kg, BASF) might limit the worldwide distribution of the LSC
technology.[21]
Although LSC-PV devices that exploit the optical features of organic luminophors are well
documented,[5, 12, 14, 16, 22-24] alternative and effective red-emitting fluorophores to perylene
derivatives are not available yet. Recent research has been focused on the use of
(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran
(DCJTB)
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fluorophore (Figure 1),[17, 25-27], but the optical performances of the derived PMMA-based LSC
appeared still lower than the state-of-the art.[26]
<Figure 1 near here>
Surprisingly, fluorophores based on the diketopyrrolopyrrole (DPP) core structure are, to the best of
our knowledge, still ignored as dopants for LSC. DPPs are structurally based on the π-conjugated
2,5-dihydropyrrolo[4,3-c]pyrrolo-1,4-dione dilactam moiety, which is easily functionalized by
electrophilic and nucleophilic reactions. Functional DPPs are well-known compounds with high
fluorescence quantum yields and exceptional thermal and photostability, which make them
excellent building blocks for many applications.[28, 29] For example, DPPs are widely used as
pigments in paints, varnishes, and high-quality printing[30, 31] and exhibit outstanding
semiconducting properties for advanced organic field-effect transistors (OFETs) and organic light-
emitting diodes (OLEDs).[32-34]
Notable is the fact that the modification of the DPP structure, both on the nitrogen atoms and on the
aryl groups on 3 and 6 positions, has a great impact on the solubility of these compounds and on
their absorption and fluorescence properties.[35] For example, the introduction of alkyl substituents
onto the nitrogen atoms of DPP results in significant improvement in the DPP solubility, as they are
no longer able to form intermolecular hydrogen bonds.[36] The modification of aryl groups at
positions 3 and 6 of the DPP core causes significant bathochromic shifts in the absorption and
emission maxima and provides intense fluorescence bands near 600 nm.[28] Recent papers
demonstrated that the position, length, and bulkiness of these alkyl chains strongly affect the
aggregation behaviour of these materials in solid state, thus impact their electronic properties.[37-
39] However, the influence of DPP aggregation on material fluorescence has not yet been addressed
fully and it merits further study.
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Herein, we report on the impact of different N-alkyl chains on the optical behaviour in solution and
in PMMA film of six diketopyrrolopyrrole-based fluorophores, i.e., compounds 1a-c and 2a-c
(Figure 2) for the preparation of high performance LSC.
<Figure 2. near here>
Thin-film LSC devices were prepared by the dispersion of the synthesized fluorophores in
poly(methyl methacrylate) (PMMA) films coated over high optical quality glass slab. The LSC
optical efficiencies were discussed in terms of the effect provided by the three different alkyl chains
in relation to the aryl substitution of the DPP core, and compared to that measured for LSC devices
with the same geometry and containing Lumogen F Red 305.
2. Experimental part
2.1 Materials
Unless otherwise stated, all reactions were performed under argon by standard syringe, cannula and
septa techniques. 3-Ethyl-1-iodoheptane (4b) was prepared according to a literature procedure.[40]
All the other commercially available reagents and solvents were used as received. Poly(methyl
methacrylate) (PMMA, Aldrich, Mw = 350,000 g/mol, acid number <1 mg KOH/g).
2.2. 2,5-Dioctyl-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (1a)
This compound was synthesized according to a literature procedure.[41] 3,6-Di(thiophen-2-
yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (3) (0.45 g, 1.5 mmol) and K2CO3 (0.83g, 6.0 mmol)
were added to the reaction vessel. The reaction vessel was fitted with a silicon septum, evacuated,
and back-filled with argon. This sequence was repeated twice. Anhydrous DMF (16 mL) was added
under a stream of argon, and the resulting mixture was stirred at 120 °C for 1h. Then, 1-iodooctane
(4a) (1.44 g, 6.0 mmol) was added in one portion, and the mixture was stirred at 120 °C for 24 h.
After cooling to room temperature, the reaction mixture was poured into ice-cold water (100 mL),
and extracted with CH2Cl2 (4x25 ml). The organic fractions were collected, dried over Na2SO4,
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filtered, and concentrated at reduced pressure. The residue was purified by flash chromatography on
silica gel with a mixture of CH2Cl2 and petroleum ether (1:1) as eluent. The chromatographic
fractions containing the required compound were collected and concentrated at reduced pressure to
1
give 1a as a dark pink solid (0.46 g, 58 % yield): mp 141-143 °C [Lit: mp 143 °C].[41] H NMR
(400 MHz CDCl3) ꢀ (ppm) 8.93 (dd, J = 3.9, 1.2 Hz, 2H), 7.63 (dd, J = 5.1, 1.2 Hz, 2H), 7.28 (dd, J
= 5.1, 3.9 Hz, 2H), 4.07 (m, 4 H), 1.74 (m, 4 H), 1.33 (m, 20 H), 0.87 (t, J = 6.1 Hz, 6H). The
spectral properties of this compound are in agreement with those previously reported.[41]
2.3. 2,5-Bis(3-ethylheptyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (1b)
This compound was synthesized in a similar fashion to compound 1a, substituting 3-ethyl-1-
iodoheptane (4b) for 1-iodooctane (4a). The crude reaction mixture was purified by flash
chromatography on silica gel with a mixture of CH2Cl2 and petroleum ether (1:1) as eluent. The
chromatographic fractions containing the required compound were collected and concentrated at
reduced pressure to give 1b as a dark pink solid (0.46 g, 55 % yield): mp 134-136°C. 1H NMR (400
MHz, CDCl3) ꢀ (ppm) 8.90 (dd, J = 3.9, 1.2 Hz, 1H), 7.65 (dd, J = 5.1, 1.1 Hz, 1H), 7.30 (dd,
13
J=5.1, 3.9 Hz, 2 H), 4.10 (m, 4 H), 1.70 (m, 4 H), 1.38 (m, 18 H), 0.91 (t, J = 6.9 Hz, 12H). C
NMR (100 MHz, CDCl3): δ = 161.5, 140.2, 135.2, 130.6, 129.9, 128.7, 108.1, 41.0, 37.6, 33.6,
32.9, 29.0, 26.0, 23.2, 14.3, 11.0 ppm. HRMS (ESI) m/z found [M + H]+ 553.2919; C32H44N2O2S2
requires [M + H]+ 553.2917.
2.4. 2,5-Bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (1c)
This compound was synthesized in a similar fashion to compound 1a, substituting 2-ethylhexyl
bromide (4c) for 1-iodooctane (4a). The crude reaction mixture was purified by flash
chromatography on silica gel with a mixture of CH2Cl2 and petroleum ether (1:1) as eluent. The
chromatographic fractions containing the required compound were collected and concentrated at
1
reduced pressure to give 1c as a dark pink solid (0.19 g, 24 % yield): mp 127 °C. H NMR (400
MHz, CDCl3) ꢀ (ppm) 8.91 (dd, J = 3.9, 1.2 Hz, 2H), 7.65 (dd, J = 5.0, 1.2 Hz, 2H), 7.29 (dd,
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J=5.0, 3.9 Hz, 2 H), 4.05 (m, 4 H), 1.88 (m, 2 H), 1.33 (m, 16 H), 0.89 (m, 12 H). The spectral
properties of this compound are in agreement with those previously reported.[42]
2.5. General procedure for the palladium-catalyzed direct arylation of 2,5-dialkyl-3,6-di(thiophen-
2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-diones 1a-c with 4-bromoanisole
According to a modified literature procedure,[43] a 2,5-bis-alkyl-3,6-di(thiophen-2-yl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (1a, 1b or 1c) (0.12 mmol), anhydrous K2CO3 (0.3 mmol,
41 mg), pivalic acid (0.036 mmol, 3.67 mg), Pd(OAc)2 (0.006 mmol, 1.35 mg) were suspended into
anhydrous DMA (2 mL) in a 25 mL two-neck round flask and degassed for 10 minutes under argon
atmosphere. Then, 1-bromo-4-methoxybenzene (0.3 mmol, 56,1 mg) was added, and the resulting
dark purple mixture was stirred at 110 °C for 24h. It was cooled to room temperature, then washed
with water (100 ml) and extracted with CH2Cl2 (3x100 ml). The organic fractions were collected,
dried over Na2SO4, filtered and concentrated at reduced pressure. The residue was purified by flash
chromatography on silica gel. This procedure was used to prepare compounds 2a–c.
2.5.1. 3,6-Bis(5-(4-methoxyphenyl)thiophen-2-yl)-2,5-dioctyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-
diones 2a
The crude reaction product, which was obtained by Pd-catalyzed reaction of 1a with 4-
bromoanisole, was purified by flash chromatography on silica gel with a mixture of CH2Cl2 and
petroleum ether (8:2) as eluent to give 2a (44 mg, 50 %) as a dark purple solid: mp 250–252 °C. 1H
NMR(400 MHz, CDCl3): δ = 8.94 (d, J = 4.2 Hz, 2H), 7.62 (m, 4H), 7.3 (d, J = 4.2 Hz, 2H), 6.95
13
(m, 4 H), 4.11 (m, 4H), 3.86 (s, 6H), 1.79 (m, 4H), 1.37 (m, 20 H), 0.87 (m, 6H) ppm. C NMR
(100 MHz, CDCl3): δ = 161.5, 169.5, 150.1, 139.5, 136.9, 128.1, 127.7, 126.2, 123.8, 114.8, 108.0,
55.6, 42.4, 32.0, 30.2, 29.4, 27.1 (2C), 22.8, 14.2 ppm. HRMS (ESI) m/z found [M + H]+ 737.3410;
C44H52N2O4S2 requires [M + H]+ 737.3441.
2.5.2.
2,5-Bis(3-ethylheptyl)-3,6-bis(5-(4-methoxyphenyl)thiophen-2-yl)-2,5-dihydropyrrolo[3,4-
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c]pyrrole-1,4-diones 2b
The crude reaction product, which was obtained by Pd-catalyzed reaction of 1b with 4-
bromoanisole, was purified by flash chromatography on silica gel with a mixture of CH2Cl2 and
petroleum ether (7:3) as eluent to give 2b (50.5 mg, 55 %) as a dark purple solid: mp 215–217 °C.
1H NMR(400 MHz, CDCl3): δ = 8.96 (d, J = 4.2 Hz, 2H), 7.63 (m, 4H), 7.39 (d, J = 4.2 Hz, 2H),
6.97 (m, 4 H), 4.15 (t, J = 8.4 Hz, 4H), 1.76 (m, 4H), 1.42 (m, 18 H), 0.83 (m, 12H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 161.5, 160.1, 150.0, 139.6, 136.8, 128.0, 127.7, 126.3, 123.8, 114.8, 108.1,
55.6, 41.2, 37.8, 33.7, 33.0, 29.2, 26.1, 23.3, 14.3, 11.1 ppm. HRMS(ESI) m/z found [M+Na]+
787.3558; C46H56N2O4S2 requires [M+Na]+ 787.3574.
2.5.3.
2,5-Bis(2-ethylexyl)-3,6-bis(5-(4-methoxyphenyl)thiophen-2-yl)-2,5-dihydropyrrolo[3,4-
c]pyrrole-1,4-diones 2c
The crude reaction product, which was obtained by Pd-catalyzed reaction of 1c with 4-
bromoanisole, was purified by flash chromatography on silica gel with a mixture of CH2Cl2 and
petroleum ether (8:2) as eluent to give 2b (41 mg, 45 %) as a dark purple solid: mp 214–216 °C. 1H
NMR(400 MHz, CDCl3): δ = 8.94 (d, J = 4.1 Hz, 2 H), 7.62 (m, 4 H), 7.36 (d, J = 4.1 Hz, 2 H),
6.96 (m, 4 H), 4.08 (m, 4 H), 3.86 (s, 6 H), 1.95 (m, 2 H), 1.34 (m, 16 H), 0.89 (m, 12H). The
spectral properties of this compound are in agreement with those previously reported.[44]
2.6 Preparation of polymer films for optical studies
Different dye/PMMA thin films were prepared by drop casting, i.e. pouring 0.8 mL chloroform
solution containing 30.5 mg of the polymer and the proper amount of dye to obtain concentrations
in the range 0.1–2.0 wt.% on 35x50 mm area over a glass surface. The glass slides were cleaned
with chloroform and immerged in 6 M HCl for at least 12 h, then they were rinsed with water,
acetone and isopropanol and dried for 8 h at 120 °C. Solvent evaporation was performed on a warm
hot plate (about 30 °C) and in a closed environment. The film thickness was measured by a Starrett
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micrometer to be 25±5 µm. The PMMA films were easily removed with a spatula after immersion
in water so that they can be stored for successive measurements and comparison by attaching them
on 50x50x3 mm optically pure glass substrate (Edmund Optics Ltd BOROFLOAT window 50x50
TS) with a high-purity silicone oil with a refractive index comparable to PMMA and glass (i.e.,
poly(methylphenyl siloxane), 710 fluid, Aldrich, refractive index n = 1.5365). Absorption and
emission properties of such devices showed negligible differences with the freshly prepared ones.
2.7 Apparatus and Methods
Melting points were recorded on a hot-stage microscope (Reichert Thermovar). Fluka precoated
silica gel PET foils were used for TLC analyses. Purifications by flash chromatography were
performed using silica gel Merck 60 (particle size 0.040-0.063 mm). NMR spectra were recorded at
room temperature at 400 MHz (1H) and 100 MHz (13C) and were referred to TMS or to the residual
protons of deuterated solvents.
ESI-Q/ToF flow injection analyses (FIA) were carried out using a 1200 Infinity HPLC (Agilent
Technologies, USA), coupled to a Jet Stream ESI interface (Agilent) with a Quadrupole-Time of
Flight tandem mass spectrometer 6530 Infinity Q-TOF (Agilent Technologies). Two sets of eluents
were used: 100% MeOH or 85% H2O and 15% acetonitrile, both added with 1% formic acid. The
flow rate was 0.2 mL/min. Injection volume: 3 µL. The ESI operating conditions were: drying gas
(N2, purity >98%): 350 °C and 10 L/min; capillary voltage 4.5 KV; nebuliser gas 35 psig; sheath
gas (N2, purity >98%): 375 °C and 11 L/min.
Absorption spectra were recorded at room temperature on a Perkin-Elmer Lambda 650
spectrometer. Fluorescence spectra were measured at room temperature on a Horiba Jobin-Yvon
Fluorolog®-3 spectrofluorometer and equipped with a 450 W xenon arc lamp, double-grating
excitation and single-grating emission monochromators. The fluorescence of dye/PMMA films
were recorded by using the Solid-Sample Holder and collecting the front-face emission at 30°.
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The fluorescence quantum yield (Φf) in chloroform was determined at room temperature relative to
Rhodamine B (Φsf = 0,5 in ethanol) using the following relation:[45]
Where the subscripts ST and X are standard and dye respectively, Grad the gradient from the plot of
integrated fluorescence intensity vs absorbance for different solutions of standard and dyes. In order
to minimise re-absorption effects absorbances never exceed 0.1 at and above the excitation
wavelength. η is the refractive index of the solvent, i.e. Refractive indexes of ethanol and
chloroform were assumed 1.361 for ethanol and 1.446 for chloroform.[46]
Photocurrent measurements
A proper apparatus was build and composed by a plywood wooden box 15x15x30 cm with walls
1.5 cm thick. A removable cover hosting a housing for a solar lamp is present at the top. During the
measurement a solar lamp TRUE-LIGHT® ESl E27 20W was used. Two 50x3 mm slits were
carved out at 5 cm from the bottom of the box to exactly fit the LSC systems (dimensions 50x50x3
mm) so that the minimum amount of light would come out during the measurement conditions. On
the outer side of the slit, a set of three 1x1 cm photodiodes (THORLABS FDS1010 Si photodiode,
with an active area of 9.7 x 9.7 mm and high responsivity (A/W) in the spectral range of 400–1100
nm) connected in parallel fashion was placed and coupled to a multimeter (KEITHLEY Mod. 2700)
for photocurrent measuring. In order to collect a more intense and stable output signal, both the
photodiodes and the multimeter are connected to an amplifier, realized in laboratory, following the
specifications given by THORLABS. The measurement procedure comprehends a 20 minutes
warm-up for the lamp, in order to reach its maximum power output. After this time, the current
intensity can be measured every minute, for a total of five minutes.
Efficiency measurement using a PV-cell
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A different set of LSC samples was prepared to measure the concentration efficiency attaching a Si-
PV cell (IXYS SLMD121H08L mono solar cell 86x14 mm, with a solar cell efficiency of 22% and
a fill factor > 70%) to one edge of the sample. This set of samples was made covering the full
50x50 area of the previously introduced optically pure glass slabs with a 24±5 µm dye/PMMA
thick film. One edge of the LSC was connected to a Si–based PV cell masked to cover just the LSC
edge (50x3 mm) using silicone grease while the remaining edges were covered with an aluminum
tape. These devices where then placed over a white poly(ethylene terephthalate) scattering sheet
(Microcellular® MCPET reflective sheet, ERGA TAPES Srl) and placed about 20 cm under a solar
lamp (TRUELIGHT® ESL E27 20W, with a correlated color temperature of 5500 K). The
efficiency is reported as ηopt, which is the ratio between the short circuit current of the PV cell
attached the LSC edges under illumination of a light source (ILSC) and the short circuit current of the
bare cell put perpendicular to the light source (ISC).
3. Results and discussion
3.1 Synthesis of DPP-based fluorophores
The six DPP-based fluorophores 1a–c and 2a–c were prepared by known synthetic procedures
starting from commercially available 2,5-dihydro-1,4-dioxo-3,6-dithienylpyrrolo [3,4-c]-pyrrole
(3). At first, 3 was reacted with the appropriate alkyl halide 4a–c in DMF solution at 120 °C in the
presence of K2CO3 for 24h. The resulting deep red-colored 2,5-dialkylsubstituted DPP derivatives
1a–c were then regioselectively arylated at the C5 position of the two thiophene rings with 4-
methoxyphenyl bromide in the presence of Pd(OAc)2 as the precatalyst, K2CO3 as the base, and
pivalic acid (PivOH) in DMA.[44, 47] The required DPP fluorophores 3a–c were isolated after 24 h
at 110 °C in 50, 55, and 45 % yield, respectively (Scheme 1).
<Scheme 1. Near here>
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3.2 Optical properties of DPP-based fluorophores
The optical properties of the prepared DPP compounds were examined by UV-vis absorption and
photoluminescent spectroscopies, and the main results were collected in Table 1.
Table 1. Spectroscopic data of DPP compounds: absorption and emission maxima (λmax), molar
extinction coefficients (εmax), and fluorescence quantum yields (Φf)
Figure S1 shows the absorption spectra of all DPP compounds in CHCl3 solution. 1a-c fluorophores
display similar absorption profiles with strong low energy bands at around 513 and 550 nm
attributed to donor-acceptor charge transfer transitions, and high energy bands near 320 nm due to
localised π−π transitions.[44] Fluorophores 2a-c show the same band structure, but
bathochromically shifted to between 500 and 650 nm, with the two maxima positioned around 570
and 607 nm, respectively. The introduction of the 4-methoxyphenyl substituent on the thiophene
rings lowers in energy the onset of the absorption and promotes the molar extinction coefficient
rising according to the enlargement of the π-electron system in these molecules. [48] The different
alkyl chain attached on the nitrogen atoms does not significantly affect the absorption profile in
CHCl3 solution. Conversely, the ethylheptyl moiety in compounds 1b and 2b contributes in
amplifying the molar extinction coefficient, possibly due to the increased solubility of the
compound in the solvent (Table 1).[39, 49]
The fluorescence of the DPP compounds in CHCl3 solution exhibits a mirror image relation with
respect to the corresponding absorption (Figure S2). Fluorophores 1a-c display a similar profile
with a large fluorescence characterised by a main emission at 564 nm and a smaller contribution at
around 610 nm due to the vibronic structure. Fluorophores 2a-c show a significant bathochromic
shift of the emission maximum, with a larger band centred at 631 nm, and the vibronic contribution
at around 680 nm. This phenomenon confirms the effect provided by the 4-methoxyphenyl donor
group on the fluorophore π-conjugation and emission characteristics. Quantum yields higher than
50% were calculated in CHCl3 and no significant differences between these values are observed
(Table 1).
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3.3 Optical properties of DPP-based fluorophores in PMMA
Owing to the aforementioned properties, that is good quantum yields and emission shifted to the red
portion of the electromagnetic spectrum of light, DPP compounds were dispersed at different
concentrations (0.1–2.0 wt.%) in a transparent and totally amorphous polymer matrix such as
PMMA. These concentrations were selected to realize homogeneous polymer films of a thickness
of 24 ± 5 µm with optical and morphological features that are not impacted by spurious effects
stemming from high fluorophore content such as phase-separation and dye segregation at the film
surface. Aim of this approach was the determination of the optical properties of the derived thin
films in terms of their potential use as LSC.
In detail, the linear n-octyl chain on compound 1a was the less efficient in preventing the
aggregation of the fluorophore molecules, thus the limit concentration for a homogeneous film was
set at 1.4 wt.%. The two branched ethylheptyl and ethylhexyl chains allowed to increase this limit,
i.e. 1.8 wt.% for 1b and 2.0 wt.% for 1c, respectively, thanks to their higher steric hindrance which
makes interactions between DPP cores more difficult to occur. This trend was also confirmed for
the 2a-c fluorophores (0.4, 0.8 and 1 wt.%, respectively), even if the stronger π-π stacking
interactions between DPP molecules due to the higher conjugation rendered the effect of the
branched alkyl functionalities less effective.
The optical properties of 1a-c and 2a-c fluorophores dispersed in PMMA films follow a similar
trend as a function of dye content. In figure 3, the main absorption and emission characteristics of
1c/PMMA and 2c/PMMA films are displayed as the most representative, whereas reporting in
figures S3 and S4 those of the other samples.
<Figure 3. near here>
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The absorption spectra of 1c/PMMA and 2c/PMMA films show large absorption bands with two
maxima positioned at 513-550 nm and at 570-607 nm, respectively. These values are equivalent to
those collected in CHCl3 solution due to the similar polarity indexes of the dispersing media.[50]
Differently from solution, the absorption spectra of PMMA films level off at the highest
concentration possibly due to saturation phenomena, which start causing dye segregation from the
polymer matrix but without promoting the emersion of any significant band due to DPP aggregates.
Conversely, the emission spectra of 1c/PMMA and 2c/PMMA films strongly differ from those in
solution, particularly in the case of 1a-c compounds (see also figure S4). According to literature,
DPP-based molecules with thiophene rings display the typical monomeric emission band at around
560 nm flanked by an intense non-structured emission band at longer wavelengths (>600 nm),
which emerges with fluorophore concentration.[51] This behaviour, together with the absence of
typical ground-state aggregates in the absorption spectra, is ascribed to the formation of excimers
(excited dimers) that originate from interactions between the excited state of the molecule and the
ground state of the same molecule.[52] The excimers stabilise the excited state causing photons
emission at lower energy, thus overall reducing the spectral overlap between the absorption and
emission bands. Being the excimer band predominant over the monomeric emission, Stokes shift
increases (i.e., from about 15 nm to 60 nm) and brings the emission maximum to wavelengths
higher than 600 nm. The progression of the excimer band at the expenses of the monomer
contribution was clearly observed by plotting the ratio between excimer and monomer emission
intensities (I605/I560) with 1c content (Figure S5). Nevertheless, the excimer increasing with dye
content occurred up to a threshold concentration (i.e. 1.4 wt.% for 1c/PMMA film), above which
the emission experienced a substantial decreasing being higher the probability to find quenching
centres through energy transfer between closely spaced pairs.[53]
In stark contrast to the aforementioned case, the 2c/PMMA film shows emission features equivalent
to those collected in CHCl3 solution with fluorescence bands pointed at 630 and 680 nm and Stokes
14

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shift of about 25 nm. The emission intensity increased with fluorophore content up to 0.7 wt.%,
above which fluorescence quenching occurred. For highly conjugated DPP fluorophores such as
2a–c, the photoluminescence is effectively quenched in the solid state due to the aggregation among
molecules. These strong interactions provide the sites for non-radiative recombination and
significantly reduce the probability of radiative emission. In other words, aggregates in PMMA
from the 2a–c serie appeared non luminescent with respect to those from 1a–c, which in turn
impacted on the overall fluorescence emission of the derived films. To confirm such behaviour, the
fluorescence emission intensity was plotted vs. absorbance for 1c/PMMA and 2c/PMMA films, in
order to reveal possible loss of the absorbed photons via non-radiative pathways (Figure S6).[54] In
detail, the emission intensity of PMMA films was found to increase linearly with the absorbance up
to 0.8 wt.% and 0.7 wt.% of 1c and 2c, respectively, indicating a negligible effect of dissipation
phenomena. Conversely, when the concentration was increased further, a deviation from linearity
was observed, suggesting that dissipative phenomena occur. It is worth noticing that for 1c/PMMA
films the fluorescence emission intensity levels off up to 1.2 wt.% of 1c, before dropping to
minimum values. Conversely, for 2c/PMMA films, an evident decrease of the luminescent response
ccurs just after the linear regime (i.e., for 2c content higher than 0.8 wt.%), thus confirming the non-
emissive nature of the 2c aggregates.
3.4 Photocurrent measurements and data analysis of DPP-based fluorophores in PMMA
In order to assess the performances as Luminescent Solar Concentrators (LSCs), an optically pure
50x50x3 mm glass was coated with DPP fluorophore/PMMA films with a thickness of 15 ± 5 ꢀm.
In Figure 3, an example of LSC obtained by coating a thin layer of 1c/PMMA (Figure 4a) and of
2c/PMMA (Figure 4b) are reported.
<Figure 4. near here>
15

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Notably, 1c/PMMA and 2c/PMMA LSCs emit light colors such as yellow and red, respectively, in
agreement with the position of the fluorescence bands in the photoluminescent spectra reported in
figure 3.
Photocurrent measurements were accomplished with a home-built apparatus[55] (see experimental
part) by using a set of three 1x1 cm photodiodes assembled in parallel fashion. Photodiodes are
ideal for measuring light sources in LSC emission range by converting the optical power to an
electrical current, allowing for a fast, precise and reproducible response even with different sets of
samples. This approach was used to study the best working conditions for different DPP/PMMA
LSC systems since the response curves of the photodiodes and the utilized PV module do not differ
significantly. For example, the photocurrents measured for a set of samples based on 1c/PMMA and
2c/PMMA thin film are reported in figure 5.
<Figure 5. near here>
The current intensity is found to increase up to a maximum peaked at about 1.2 wt.% of 1c and 0.5
wt.% of 2c, after that a slow decreasing occurs due to the presence of dissipative phenomena of the
emission response. This result agrees well with the fluorescence behaviour of the respective
1c/PMMA and 2c/PMMA thin films (Figure 3 and S6), being the latter less emissive than the
former at the highest fluorophore content.
Notably, the photocurrent behaviour fits quite well with eq. 1:
ꢁꢂꢃꢄ = ꢅ′ ∙ ꢆ ∙ ꢇꢈꢉ ∙ꢍ + ꢎ (eq. 1)
ꢊꢋꢌ
where ηopt is a term proportional to the current generated by photodiods, c is the concentration of
the dye in wt.%, and ε' and ꢀopt are two empirical constants defined as:
̅
ꢅ′ ∝ ℎ ∙ ꢇꢈꢏ (eq. 2)
̅
ꢐꢂꢃꢄ ∝ ꢐ′′(ꢑꢒ, ꢓ) ∙ ꢔ (eq. 3)
where h is the thickness of the thin film, ꢔ is the mean path length of the radiation in the optical
̅
system and
µ'' is a term depending on both QY and the probability of fluorescence re-absorption
16

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(p), being greater at high p and low QY. D is an empirical constant added since even an empty
system of transparent material (c = 0) is capable of trapping some light by means of surface and
bulk defects due to scattering phenomena. Eq. 1 was inspired by the work of Goezberger[11] who
proposed in 1977 an effective method to evaluate LSC efficiency. Both ε’ and µopt must be
considered as completely empirical since even the most accurate estimations require strong
approximations. Nevertheless, the determination of how they affect the final ηopt is straightforward
for determining the LSC performances. Notably, ε’ is a coefficient related to the absorption
properties of the dye/polymer system, whereas ꢀopt combines all the fluorescence quenching
mechanisms due to the dye. An optimal dye/polymer system should therefore present a high ε’ and
a small µopt so that the maximum efficiency is shifted to higher concentrations and the curve
steadily rises under the influence of the linear part (eq. 1). A complete and exhaustive determination
of eq. 1 was recently reported in literature by our group.[55]
The fitting parameters, reported in table 2, were compared to those recently gathered for PMMA
films with the same thickness but containing the Lumogen Red F 305 (LR, Figure 1), selected as
reference as it is considered the state-of-the-art in dyes for LSC applications.[56]
17

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Table 2. Fitting parameters of the photocurrent data measured for 1a-c/PMMA, 2a-c/PMMA and
LR/PMMA[56] films.
The fitting parameters collected in table 2 show an increase in the ε’ values going from the 1a-c
fluorophores, to the 4-methoxyphenyl functionalised 2a-c compounds. This fact indicates that the
functionalised fluorophores are capable to reach the highest photocurrent values at a lower
concentration regime, thanks to their higher molar extinction coefficient due to the enlarged π-
electron system (see table 1). These values are comparable and often superior to those reported for
LR/PMMA systems, thus indicating that DPP compounds have great potentialities as fluorophores
for LSC applications.
Conversely, the smaller µopt values gathered from 1a-c with respect to 2a-c are addressed to the
larger Stokes shifts of the former fluorophores, i.e. 62 against 25 nm, which is a fundamental
feature that prevents dye self-absorption and efficiency losses. Moreover, it is clear the role
provided by the branched 2-ethylhexyl functionalities in reducing dissipative phenomena thanks to
its superior ability to maintain 1c and 2c fluorophores better dispersed within PMMA. Indeed, the
3-ethylheptyl chains reveals less performing in preventing efficiency losses caused by dye
aggregation, possibly due to the larger distance between alkyl ramification and DPP nucleus. It is
worth noting that 1c/PMMA shows µopt value close to that gathered from the LR/PMMA system
(i.e., 0.81±0.02 against 0.45), thus confirming the use of DPP dyes as effective substitutes of
perylene-based fluorophores in LSC.
On the other hand, D values resulted to be quite similar for all the dye/PMMA systems, thus
suggesting that the contribution of non–fluorescent trapping is more or less the same for samples
with the same thickness.
1a-c/PMMA and 2a-c/PMMA films with the highest photocurrents were analysed by using a Si-
based PV cell attached to one edge of the concentrator, as described in the experimental section.
The optical efficiency ηopt (Table 3) was evaluated from the concentration factor C, which is the
18

ACCEPTED MANUSCRIPT
ratio between the short circuit current measured in the case of the cell over the LSC edge (ILSC) and
short circuit current of the bare cell when perpendicular to the light source (ISC) (eq. 4):
I
LSC
ηopt= I
(eq. 4)
∙G
SC
where G is the geometrical factor (in our case, G = 13.3), which is the ratio between the area
exposed to the light source and the collecting area. The data collected were compared to that
gathered from LSC based on LR in the same range of fluorophore concentration and geometrical
factor.
Table 3. Optical efficiencies (ηopt) calculated for 1a-c/PMMA, 2a-c/PMMA and compared to that
of LR/PMMA with similar dye content and geometrical factor[56]
Notably, ηopt increases for both DPP series 1 and 2 on going from fluorophores with linear n-octyl
chains to compounds bearing branched 2-ethylhexyl moieties, in agreement with the photodiodes
measurements. The 2-ethylhexyl group helps in keeping DPP nuclei well separated, thus acting as
an effective compatibilizing tool for DPP fluorophores in PMMA.
As far as the effect of the 4-methoxyphenyl group is concerned, expanded π-electron system would
favour higher efficiency values being the fluorophore emission superimposes the wavelenght range
of PV cell maximum efficiency. Conversely, 2a-c show lower values than 1a-c, possibly due to the
larger Stokes shifts of the latter fluorophores which helps in reducing dissipative disexcitation
phenomena. Accordingly, the 1c/PMMA system shows the ηopt maximum of 6.8 that is comparable
to that of LSC based on LR/PMMA thin film (ηopt = 7.2%) collected for similar fluorophore content
(1.4 wt.%).[56]
The capability to retain the fluorescence intensity after continuous light irradiation is a fundamental
requisite for fluorophores in LSC applications. Photostability tests were eventually carried out by
continuously irradiating a 0.25 cm2 area of 1c/PMMA and 2c/PMMA films at their excitation
wavelengths (i.e. 550 nm and 607 nm, respectively) with a 450 W Xe arc lamp under aerobic
conditions.[35] Notably, 1c/PMMA and 2c/PMMA films retained respectively the 95% and 99% of
19

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their fluorescence, after 60 min of continuous excitation, thus indicating great photostability,[28]
which makes DPP fluorophores a viable alternative to perylene derivatives for LSC applications.
Conclusions
We have shown that DPP fluorophores once embedded into PMMA, confers to the resulting thin
films optical efficiencies, which make them suitable for the preparation of LSCs. DPP fluorophores
based on the donor-acceptor-donor skeleton were prepared by N-alkylation and direct C-H
arylation. Linear n-octyl chain and the aromatic substituent (4-methoxyphenyl) on the thiophene
rings were found to adversely affect DPP fluorescence due to their tendency to promote DPP
aggregation, which in turn limited fluorophore dispersion within PMMA. By contrast, the branched
2-ethylhexyl chains and the unsubstituted thiophene moieties allowed to obtain the best performing
DPP fluorophore in PMMA thin film in terms of dye compatibility, fluorescence emission and
photocurrent intensities. In light of these special features, DPP/PMMA LSC system yielded
maximum optical efficiencies (ηopt) of 6.8%, which were found comparable to that gathered from
LSC based on LR in the same range of fluorophore concentration and geometrical factor. The
performances were attributed to the larger Stokes shift of DPP bearing unsubstituted thiophene
moieties that prevented loss of efficiencies due to self-absorption. Future approaches for ηopt
enhancement should adopt new synthetic strategies aimed at limiting DPP extensive aggregation
while maintaining the emission maxima in the range of the highest quantum efficiency of the PV
cell (> 600 nm). Considering the easy preparation and excellent photostability, all findings
consistently support the effective use of the DPP fluorophores in the realization of high
performance LSCs.
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Acknowledgements
The research leading to these results has received funding from MIUR-FIRB (RBFR122HFZ) and
in part from the Università di Pisa under PRA 2015 (project No. 2015_0038). ERGA TAPES Srl
and BASF Italia S.p.A are kindly acknowledged for providing some free samples of
“Microcellular® MCPET reflective sheet” and Lumogen Red F 305, respectively.
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Table 1. Spectroscopic data of DPP compounds: absorption and emission maxima (λmax), molar
extinction coefficients (ε
), and fluorescence quantum yields (Φf) max
Absorption
λmax (nm)
549
Fluorescence
λmax (nm)
563
10-4 εmax (M-1cm-1)
5.4
Φf (%)
48
52
56
52
Compound
1a
1b
1c
2a
2b
2c
549
548
605
568 and 605
568 and 606
9.0
5.0
10.0
13.0
10.2
564
563
630
630
53
55
631
Table 2. Fitting parameters of the photocurrent data measured for 1a-c/PMMA, 2a-c/PMMA and
LR/PMMA[56] films.
Entry
ε'
ꢀopt
D
1a/PMMA
1b/PMMA
1c/PMMA
2a/PMMA
2b/PMMA
2c/PMMA
LR/PMMA
138±6 1.07±0.01 14±1
213±10 1.29±0.02 28±2
142±7 0.81±0.02 25±2
297±10 3.11±0.07 14±1
413±31 3.09±0.14 8±3
208±8 1.89±0.03 39±1
140
0.45
20
Table 3. Optical efficiencies (ηopt) calculated for 1a-c/PMMA, 2a-c/PMMA and compared to that
of LR/PMMA with similar dye content and geometrical factor[56]
Entry
wt.%
0.8
0.9
1.2
0.4
0.3
0.5
1.4
ηopt (%)
6.1
1a/PMMA
1b/PMMA
1c/PMMA
2a/PMMA
2b/PMMA
2c/PMMA
LR/PMMA
5.9
6.8
5.3
5.4
6.1
7.2
25

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Figure 1. Chemical structures of Lumogen F Red 305 and DCJTB
Figure 2. Chemical structures of the synthesized fluorophores: 2,5-dioctyl-3,6-di(thiophen-2-yl)-
2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (1a); 2,5-bis(3-ethylheptyl)-3,6-di(thiophen-2-yl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (1b); 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)-2,5-
dihydropyrrolo[3,4-c]pyrrole-1,4-dione (1c); 3,6-bis(5-(4-methoxyphenyl)thiophen-2-yl)-2,5-
dioctyl-2,5-dihydro pyrrolo[3,4-c]pyrrole-1,4-dione (2a); 2,5-bis(3-ethylheptyl)-3,6-bis(5-(4-
methoxyphenyl)thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (2b) and 2,5-bis(2-
ethylhexyl)-3,6-bis(5-(4-methoxyphenyl)thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione
(2c)
26

ACCEPTED MANUSCRIPT
Scheme 1. Synthesis of DPP fluorophores 1a–c and 2a–c
27

ACCEPTED MANUSCRIPT
Figure 3. 1c/PMMA films: absorption (up, left) and emission (down, left) spectra as a function of
dye content (wt.%); 2c/PMMA films: absorption (up, right) and emission (down, right) spectra as a
function of dye content (wt.%). Film thickness = 15 ± 5 µm; λexc = 525 nm
(a)
(b)
Figure 4. Optically pure 50x50x3 mm glass coated with (a) 1c/PMMA film and with (b) 2c/PMMA
film. Film thickness = 15 ± 5 µm; λexc = 366 nm
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