Colourless p-phenylene-spaced bis-azoles for luminescent concentrators

Article abstract of DOI:10.1016/j.dyepig.2016.07.005

We report on the synthesis of new push-pull imidazole-benzothiazole and thiazole-benzothiazole fluorophores obtained in good yields by using direct C-H arylation synthetic strategies. Spectroscopic investigations in CHCl₃ solution evidenced that the 1,4-phenylene-spaced imidazole-benzothiazole bearing an electron-donating dimethylamino group achieved a trade-off between fluorescence maximum (516 nm), Stokes shift (165 nm) and a quantum yield higher than 0.4. Dispersions of the selected fluorophore in poly (methyl methacrylate) (PMMA) thin films mostly maintained the optical features in solution with significant light transmittance in the visible region (90% at 440 nm), and a brilliant green emission at 500 nm. Photocurrent experiments performed on PMMA thin films coated over high purity transparent glasses promoted their use in the development of colourless luminescent solar concentrators with optical efficiencies close to 6%.

Full text of DOI:10.1016/j.dyepig.2016.07.005

Accepted Manuscript
Colourless p-phenylene-spaced bis-azoles for luminescent concentrators
Fabio Bellina, Chiara Manzini, Giulia Marianetti, Cristofer Pezzetta, Elisabetta
Fanizza, Marco Lessi, Pierpaolo Minei, Vincenzo Barone, Andrea Pucci
PII:
S0143-7208(16)30309-6
10.1016/j.dyepig.2016.07.005
DYPI 5339
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 31 May 2016
Revised Date: 1 July 2016
Accepted Date: 3 July 2016
Please cite this article as: Bellina F, Manzini C, Marianetti G, Pezzetta C, Fanizza E, Lessi M, Minei P,
Barone V, Pucci A, Colourless p-phenylene-spaced bis-azoles for luminescent concentrators, Dyes and
Pigments (2016), doi: 10.1016/j.dyepig.2016.07.005.
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ACCEPTED MANUSCRIPT
Colourless p-phenylene-spaced bis-azoles
for luminescent concentrators
Fabio Bellina,a,b Chiara Manzini,a Giulia Marianettic Cristofer Pezzettaa,c Elisabetta Fanizza,d,e
Marco Lessi,a Pierpaolo Minei,a Vincenzo Barone,c 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
(c) Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa (Italy)
(d) Dipartimento di Chimica, Università degli Studi di Bari, Via Orabona 4, 70126 Bari, Italy
(e) CNR-Istituto per i Processi Chimico Fisici, UOS Bari, Via Orabona 4, 70126 Bari, Italy
* Corresponding author: andrea.pucci@unipi.it;
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Abstract
We report on the synthesis of new push-pull imidazole-benzothiazole and thiazole-benzothiazole
fluorophores obtained in good yields by using direct C-H arylation synthetic strategies.
Spectroscopic investigations in CHCl3 solution evidenced that the 1,4-phenylene-spaced imidazole-
benzothiazole bearing an electron-donating dimethylamino group achieved a trade-off between
fluorescence maximum (516 nm), Stokes shift (165 nm) and a quantum yield higher than 0.4.
Dispersions of the selected fluorophore in poly(methyl methacrylate) (PMMA) thin films mostly
maintained the optical features in solution with significant light transmittance in the visible region
(90 % at 440 nm), and a brilliant green emission at 500 nm. Photocurrent experiments performed on
PMMA thin films coated over high purity transparent glasses promoted their use in the
development of colourless luminescent solar concentrators with optical efficiencies close to 6%.
Keywords
push-pull azole-based fluorophores, near-UV absorption, poly(methyl methacrylate), dye
dispersion, luminescent solar concentrators
2

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1. Introduction
Sunlight concentration is a promising path to cost-effective photovoltaic (PV) technologies. 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, which has the
largest impact on the final costs.[1-3] There are mainly two kinds of solar concentrators, one type is
based on geometrical optics[4] and another category resides on luminescent components.[3, 5]
Compared to standard concentrators based on geometrical optics, luminescent solar concentrators
(LSCs) show several advantages: low weight, high theoretical concentration factors, ability to work
well with diffuse light and no needs of sun tracking or cooling apparatuses. LSCs demonstrate an
entirely new standard for very large area and highly adoptable solar windows that can translate into
improved building efficiency, enhanced UV-barrier layers, and lower cost solar harvesting systems.
LSCs consist in a slab of transparent material doped with a fluorophore able to absorb the solar
spectrum.[6] The higher refractive index of the host compared to the environment allows to trap a
fraction of the emitted photons by means of total internal reflection. Photons are then collected at
the edges of the device to produce electric power by means of PV cells. Moreover, the use of
commodity plastics such as poly(methyl methacrylate) (PMMA) and polycarbonate (PC) and well
consolidated and economic industrial processes for the preparation of LSCs offer encouraging
means to include solar energy to the built environment.
However, conventional LSCs are often plagued by a multitude of unfavourable processes that
hinder their ability to deliver light to PV cells, in particular fluorescence quenching due to
aggregation phenomena between luminescent species.[6, 7] These issues triggered a great flurry of
research in the field, leading to a large number of solutions that took into account fluorophore
features, polymer hosts and their effective combinations.[6] In the recent years, the research on PV
devices based on LSC technology has been focusing on achieving high power conversion
efficiencies.[8-18] A simple approach for higher concentrations is to enhance the spectral window
of absorption of the LSC, therefore increasing the number of available photons. To this end,
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multiple dye systems have long been proposed to cope for the narrow absorption characteristic of
organic dyes as well as new design solutions.[14, 19, 20] Sloff et al.[18] described a stacked device
with a power conversion efficiency of 7.1%, which is, at best of our knowledge, the highest
efficiency ever reported for LSC-PV systems. Conversely, a dye mixture in a single slab offers the
possibility of cascading of emission via non-radiative processes such as the fluorescence resonance
energy transfer (FRET).[19, 21] Nevertheless, the maximum efficiencies for LSCs were recorded
for PMMAs device embedding perylene-based fluorophores[8, 10, 22]. Lumogen F Red 305 is a
red-emitting perylene fluorophore, which is considered the state-of-the-art in dyes for LSC
applications[6] since it shows a quantum yield (QY) of about 1 even at high concentration (>300
ppm) in polymers [23] and a good photostability.[24]
However, the strong absorption in the visible spectrum of perylene-based fluorophores leads to a
large degree of colored tinting. In order to overcome this issue, visible-transparent LSCs based on
NIR-emissive cyanine salts have been recently proposed as innovative devices with transparency of
near 90% in the visible spectrum.[12] This technology is considered very promising and it has
received much attention in the scientific community due to LSCs potential application as
architectural windows.[12, 25, 26]
In connection with these findings, we exploited the potentiality offered by the new 1,4-phenylene-
spaced azoles as near-UV absorbing fluorophores characterized by large Stokes shifts (SS) and
green emission to be used in colorless LSC with optical efficiencies close to 6. Mixed imidazole-
benzimidazole analogues were already proposed by us as fluorophores with SS near 100 nm, very
high Φ, and with a bright blue-green emission well retained in the solid state. Notwithstanding their
excellent emission features, their fluorescence peaked at λ < 430 nm did not match the working
window of a Si-based PV cell (λ ≥ 500 nm),[27] thus impeding their application in LSCs
technology.[28] Therefore, new push-pull imidazole-benzothiazole and thiazole-benzothiazole
fluorophores, 1a-f and 2a-c (Scheme 1), were efficiently synthesized aimed at retaining the features
of imidazole-benzimidazole analogues (3a-c), while shifting their emission to longer wavelengths.
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Notably, high Φ is still required to maximize the LSC efficiency. Indeed, increasing SS of the
fluorophore reduces reabsorption but radiationless internal conversion processes become more
probable when the electronic excitation energy decreases, relative to the energy of molecular
vibrations.[29] Therefore, an effective trade-off between SS and Φ should be achieved [30, 31]
Scheme 1. Chemical structure of compounds 1a-f, 2a-c, and 3a-c
Thin-film LSC devices were then prepared by dispersing fluorophores in poly(methyl methacrylate)
(PMMA) films coated over high optical quality glass slab. 1d and 1f 1,4-phenylene-spaced azoles
were selected being their emission higher than 500 nm. The LSCs optical efficiencies were
discussed and compared to that measured for LSCs with the same geometry and containing
Lumogen F Red 305.
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2. Experimental part
2.1 Materials
Unless otherwise stated, all reactions were performed under argon by standard syringe, cannula and
septa techniques. 2-(4-(5-(4-Methoxyphenyl)-1-methyl-1H-imidazol-2-yl)phenyl)-1-methyl-1H-
benzo[d]imidazole
benzo[d]imidazole
(3a),
(3b),
1-methyl-2-(4-(1-methyl-5-(p-tolyl)-1H-imidazol-2-yl)phenyl)-1H-
4-(1-methyl-2-(4-(1-methyl-1H-benzo[d]imidazol-2-yl)phenyl)-1H-
imidazol-5-yl)benzonitrile (3c), and 2-(4-bromophenyl)-1-methyl-1H-benzo[d]imidazole (6) were
prepared as previously described by us.[28] 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 General procedures for the Palladium-catalysed direct 5-arylation of 1-methyl-1H-imidazole
(8) and thiazole (9) with aryl bromides 10a-c.[28, 32]
Pd(OAc)2 (11.2 mg, 0.05 mmol), Bu4NOAc (0.60 g, 2.0 mmol), and aryl bromide 10 (1.5 mmol), if
a solid, were placed in a flame-dried reaction vessel. The reaction vessel was fitted with a silicon
septum, evacuated, and back-filled with argon. This sequence was repeated twice more. DMA (5
mL), aryl bromide 10 (1.5 mmol), if a liquid, and the appropriate azole 8 or 9 (1.0 mmol) were then
added successively under a stream of argon by syringe at room temperature. The resulting mixture
was stirred under argon for 24h at 70 °C when thiazole (9) was employed, or at 110 °C when 1-
methyl-1H-imidazole (8) was the coupling partner. After cooling to room temperature, the reaction
mixture was diluted with AcOEt, filtered through a plug of Celite and eluted with additional AcOEt
and CH2Cl2. The filtrate was concentrated under reduced pressure and the residue purified by flash
chromatography on silica gel. This procedure was used to prepare compounds 4a-c and 5a-c.
2.2.1 5-(4-Methoxyphenyl)-1-methyl-1H-imidazole (4a)
The crude reaction product, which was obtained by Pd-catalysed reaction of 1-methyl-1H-imidazole
(8) with 4-bromoanisole (10a), was purified by flash chromatography on silica gel with a mixture of
CH2Cl2 and MeOH (96:4) as eluent to give 4a (0.15 g, 82 %) as a light-yellow solid, m.p. 106-108
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1
°C. H NMR (200 MHz, CDCl3): δ 7.48 (s, 1H), 7.30 (m, 2H), 7.02 (s, 1H), 6.96 (m, 2H), 3.83 (s,
3H), 3.61 (s, 3H) ppm. 13C NMR (50.3 MHz, CDCl3): δ 159.1, 138.3, 132.3, 129.6 (2C), 127.1,
121.8, 113.9 (2C), 55.1, 32.1 ppm. MS (EI): m/z (%) = 189 (12), 188 (100), 174 (10), 173 (82), 145
(17). The spectral properties of this compound are in agreement with those previously reported.[33]
2.2.2 1-Methyl-5-(p-tolyl)-1H-imidazole (4b)
The crude reaction product, which was obtained by Pd-catalysed reaction of 1-methyl-1H-imidazole
(8) with 4-bromotoluene (10b), was purified by flash chromatography on silica gel with a mixture
1
of CH2Cl2 and MeOH (96:4) as eluent to give 4b (0.14 g, 81 %) as a yellow oil. H NMR (200
MHz, CDCl3): δ 7.49 (s, 1H), 7.26 (m, 2H), 7.25 (m, 2H), 7.07 (s, 1H), 3.63 (s, 3H), 2.39 (s, 3H)
13
ppm. C NMR (50.3 MHz, CDCl3): δ 138.7, 137.6, 133.3, 129.3 (2C), 128.3 (2C), 127.6, 126.7,
32.4, 21.2 ppm. MS (EI): m/z (%) = 173 (13), 172 (100), 171 (17), 144 (14), 130 (16). The spectral
properties of this compound are in agreement with those previously reported.[34]
2.2.3 4-(1-Methyl-1H-imidazol-5-yl)benzonitrile (4c)
The crude reaction product, which was obtained by Pd-catalysed reaction of 1-methyl-1H-imidazole
(8) with 4-bromobenzonitrile (10c), was purified by flash chromatography on silica gel with a
mixture of CH2Cl2 and MeOH (96:4) as eluent to give 4c (0.13 g, 69 %) as a light-yellow solid,
m.p. 148-151 °C. 1H NMR (200 MHz, CDCl3): δ 7.73 (m, 2H), 7.59 (s, 1H), 7.53 (m, 2H), 7.22 (s,
13
1H), 3.75 (s, 3H) ppm. C NMR (50.3 MHz, CDCl3): δ 140.5, 134.3, 132.5 (2C), 131.6, 129.8,
128.3 (2C), 118.5, 111.2, 33.0 ppm. MS (EI): m/z (%) = 159 (12), 158 (100), 130 (16), 116 (13),
103 (13), 89 (12). The spectral properties of this compound are in agreement with those previously
reported.[32]
2.2.4 5-(4-Methoxyphenyl)thiazole (5a)
The crude reaction product, which was obtained by Pd-catalysed reaction of thiazole (9) with 4-
bromoanisole (10a), was purified by flash chromatography on silica gel with a mixture of toluene
1
and AcOEt (92:8) as eluent to give 5a (0.13 g, 67 %) as a bright-yellow solid, m.p. 89-92 °C. H
NMR (200 MHz, CDCl3): δ 8.68 (s, 1H), 7.97 (s, 1H), 7.48 (m, 2H), 6.92 (m, 2H), 3.82 (s, 3H)
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13
ppm. C NMR (50.3 MHz, CDCl3): δ 159.8, 151.2, 139.2, 138.0, 128.2 (2C), 123.6, 114.5 (2C),
55.4 ppm. MS (EI): m/z (%) = 191 (100), 176 (58), 149 (20), 148 (27), 121 (20). The spectral
properties of this compound are in agreement with those previously reported.[35]
2.2.5 5-(p-Tolyl)thiazole (5b)
The crude reaction product, which was obtained Pd-catalysed reaction of thiazole (9) with 4-
bromotoluene (10b), was purified by flash chromatography on silica gel with a mixture of toluene
and AcOEt (90:10 + 0.1 % Et3N) as eluent to give 5b (0.11 g, 65 %) as a pale-yellow solid, m.p.
1
82-84 °C. H NMR (200 MHz, CDCl3): δ 8.69 (s, 1H), 8.02 (s, 1H), 7.44 (m, 2H), 7.18 (m, 2H),
13
2.35 (s, 3H) ppm. C NMR (50.3 MHz, CDCl3): δ 151.4, 139.3, 138.4, 138.3, 129.6 (2C), 128.1,
126.7 (2C), 21.2 ppm. MS (EI): m/z (%) = 176 (12), 175 (100), 148 (32), 147 (43), 115 (17). The
spectral properties of this compound are in agreement with those previously reported.[36]
2.2.6 4-(Thiazol-5-yl)benzonitrile (5c)
The crude reaction product, which was obtained by Pd-catalysed reaction of thiazole (9) with 4-
bromobenzonitrile (10c), was purified by flash chromatography on silica gel with a mixture of
toluene and AcOEt (90:10) as eluent to give 5c (0.11 g, 61 %) as a yellow solid, m.p. 101-102 °C.
1H NMR (200 MHz, CDCl3): δ 8.86 (s, 1H), 8.19 (s, 1H), 7.70 (m, 4H) ppm. 13C NMR (50.3 MHz,
CDCl3): δ 153.7, 140.5, 137.2, 132.7, 135.3, 127.1, 118.2, 111.6 ppm. MS (EI): m/z (%) = 187 (%),
186 (100), 160 (10), 115 (10), 114 (12). The spectral properties of this compound are in agreement
with those previously reported.[37]
2.3 N,N-Dimethyl-4-(1-methyl-1H-imidazol-5-yl)aniline (4d)
Pd(OAc)2 (11.2 mg, 0.05 mmol), P(2-furyl)3 (23.2 mg, 0.1 mmol), K2CO3 (0.28 g, 2.0 mmol), and
4-bromo-N.N-dimethylanyline 10d (0.30 g, 1.5 mmol) were placed in a flame-dried reaction vessel.
The reaction vessel was fitted with a silicon septum, evacuated and back-filled with argon. This
sequence was repeated twice more. DMA (5 mL) and 1-methyl-1H-imidazole (8) (80 ꢀL, 82.1 mg,
1.0 mmol) were then added successively under a stream of argon by syringe at room temperature.
The resulting mixture was stirred at 110 °C under argon for 48h. After cooling to room temperature,
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the reaction mixture was diluted with EtOAc, filtered through a plug of Celite, and eluted with
additional EtOAc and CH2Cl2. The filtrate was concentrated under reduced pressure, and the crude
product was purified by flash chromatography on silica gel with a mixture of CH2Cl2 and MeOH
1
(95:5) as eluent to give 4d (0.15 g, 76 %) as a brown crystalline solid, m.p. 133-137 °C. H NMR
(400 MHz, CDCl3): δ 7.49 (s, 1H), 7.28 (m, 2H), 7.02 (s, 1H), 6.79 (m, 2H), 3.65 (s, 3H), 3.02 (s,
6H) ppm. 13C NMR (50.3 MHz, CDCl3): δ 150.1, 137.6, 134.0, 129.5 (2C), 124.9, 116.3, 112.1
(2C), 40.3 (2C), 32.6 ppm. MS (EI): m/z (%) = 202 (14), 201 (100), 200 (42), 186 (13), 185 (17).
Elemental analysis calcd (%) for C12H15N3: C 71.61, H 7.51; found: C 71.68, H 7.49.
2.4 2-(4-Bromophenyl)benzo[d]thiazole (7a)[38, 39]
A three-necked 500 mL flask equipped with air condenser and magnetic stirrer was charged with 2-
aminothiophenol (0.67 mL, 0.78 g, 6.25 mmol), 4-bromobenzaldehyde (0.93 g, 5 mmol) and
DMSO (150 mL). The mixture was stirred under air at 190 °C. The progress of the reaction was
monitored by GLC and after 2 h the conversion was found complete. The reaction mixture was
poured into water and the resulting white-greenish precipitate was recovered by filtration, purified
by crystallization in ethanol giving 7a as a white solid (1.34 g, 92 %), m.p. 51-53 °C. 1H NMR (200
MHz, CDCl3): δ 8.09-8.05 (m, 1H), 8.00-7.89 (m, 3H), 7.66-7.60 (m, 2H), 7.51 (dt, J1 = 8 Hz, J2 =
1.4 Hz, 1H), 7.40 (dt, J1 = 8 Hz, J2 = 1.4 Hz, 1H) ppm. MS (EI): m/z (%) = 291 (100), 290 (20),
289 (99), 210 (30), 108 (19). The spectral properties of this compound are in agreement with those
previously reported.[39]
2.5 2-(4-Bromophenyl)-6-methylbenzo[d]thiazole (7b)[40, 41]
According to a reported procedure,[40] in a three-necked 500 mL flask equipped with mechanic
stirrer and dropping funnel were added 125 mL of glacial acetic acid that were cooled at 5 °C. A
mixture of KSCN (5.83 g, 60 mmol) and p-toluidine (6.43 g, 60 mmol) was added to the precooled
acetic acid giving a brownish solution. Then, a bromine solution (60 mmol of Br2 in 60 mL glacial
acetic acid) was added very slowly with constant stirring so that temperature did not rise above 5
°C. The reaction mixture was still stirred for additional 3 h. The hydrobromide salt obtained was
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filtered, the solid was dissolved in 150 mL of hot water and filtered once more. The solution was
neutralized with ammonia and white crystals formed of 6-methylbenzo[d]thiazol-2-amine were
collected. Afterwards, the intermediate obtained was refluxed with an aqueous solution of NaOH (5
times by weight of benzothiazole) until its own complete depletion. The solution was filtered and
neutralized with glacial acetic acid. The separated yellowish semisolid formed was extracted with
diethyl ether and dried over Na2SO4. The evaporation of the solvent and crystallization from
MeOH/H2O afforded 2,2’-disulfanediyilbis(4-methylaniline) as yellowish needle-shaped crystals
(3.92 g, 47 %), m.p. 84-85 °C. 1H NMR (400, CDCl3): δ 6.98-6.95 (m, 4H), 6.64 (d, 2H, J = 8 Hz),
13
4.18 (bs, 4H, NH2), 2.13 (s, 6H, CH3) ppm. C NMR (100 MHz): δ 146.3, 137.14, 132.4, 127.6,
119.0, 115.4, 20.1 ppm. MS (EI): m/z (%) = 277 (8), 276 (48), 139 (24), 138 (100), 94 (24), 77 (8).
2,2’-Disulfanediyilbis(4-methylaniline) (3.59 g, 13 mmol), bromobenzaldehyde (4.88 g, 26.4
mmol) and sodium metabisulfite (2.52 g, 13.26 mmol) were dissolved in DMSO (100 mL). The
resulting orange reaction mixture was stirred at 120 °C and the formation of the desired product
monitored by TLC and GC analyses. After 4 h, the reaction mixture was cooled at room
temperature, water was added (200 mL) and the resulting precipitate was collected by vacuum
filtration. Afterwards, the solid was well washed with excess water (150 mL) in order to remove
residual DMSO, re-dissolved in CH2Cl2 and finally washed with brine. The organic layer was dried
over Na2SO4 and the solvent removed in vacuo giving compound 7b as a yellow solid (5.14 g, 64
%), m.p. 175-178 °C. 1H NMR (400, CDCl3): δ 7.95-7.93 (m, 3H), 7.69 (s, 1H), 7.62 (d, 2H, J = 8
13
Hz), 7.31 (dd, 1H, J1 = 8 Hz, J2 = 2 Hz), 2.50 (s, 3H) ppm. C NMR (100 MHz, CDCl3) 165.6,
152.2, 135.6, 135.2, 132.7, 132.2, 129.1, 128.2, 125.1, 123.3, 121.4, 21.6 ppm. MS (EI): m/z (%) =
306 (17), 305 (100), 304 (36), 303 (95), 302 (21). The spectral properties of this compound are in
agreement with those previously reported.[41]
2.6 2-(4-Bromophenyl)benzo[d]thiazole-6-carbaldehyde (7c)
A two-necked 1 L flask equipped with condenser was charged with 2-(4-bromophenyl)-6-
methylbenzo[d]thiazole (7b) (5 g, 16.4 mmol), N-bromosuccinimide (6.12 g, 34.4 mmol) and AIBN
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(0.43 g, 2.6 mmol). The reaction vessel was fitted with a silicon septum, then evacuated and back-
filled with argon for three times. After that, degassed benzene (320 mL) was added and the reaction
mixture was left to reflux 6 h. The reaction was filtered and the solvent was removed in vacuo. The
solid obtained was dissolved in CH2Cl2, washed with water (3 times) and brine, thus dried over
Na2SO4. The crude product was a mixture of monobrominated and dibrominated compound (30:70,
1
respectively) as shown by H NMR. The mixture of brominated products obtained (7.52 g) was
added without further purification to a two-necked 500 mL flask equipped with a condenser and
magnetic stirrer. The reaction vessel was evacuated and back-filled with argon for three times.
AgNO3 (6.27 g, 36.9 mmol) and 200 mL anhydrous 1,2-dimethoxyethane were then added and the
reaction mixture was allowed to reflux until the complete disappearance of the precursor.
Anhydrous triethylamine (23 mL, 164 mmol) was finally added and the reaction was refluxed 2 h
more. Afterwards, CH2Cl2 was poured to the reaction mixture that was filtered on a layer of Celite.
The solvent was removed and the crude product was dissolved again in CH2Cl2, washed with water
(3 times) and brine and dried over Na2SO4. The solid obtained after evaporation of the solvent was
recrystallized from CH2Cl2/hexane giving product 7c as an off-white solid (3.28 g, 63 %), m.p. 305-
1
309 °C. H NMR (400, CDCl3): δ 10.12 (s, 1H, CHO), 8.45 (d, 1H, J = 2 Hz), 8.17 (d, 1H, J = 8
Hz), 8.03 (dd, 1H, J1 = 8 Hz, J2 = 2 MHz), 7.99 (m, 2H), 7.67 (m, 2H) ppm. 13C NMR (100 MHz,
CDCl3): δ 191.1, 171.3, 158.1, 135.8, 133.6, 132.6 (2C), 132.1, 129.3 (2C), 127.6, 126.6, 124.5,
123.9 ppm. MS (EI): m/z (%) = 319 (100), 318 (79), 317 (94), 316 (64), 209 (32). Elemental
analysis calcd (%) for C14H8BrNOS: C 52.85, H 2.53; found: C 52.89, H 2.52.
2.7 General procedures for the Pd-catalysed C-2 arylation of 5-aryl-imidazoles (4a-d) and 5-aryl-
thiazoles (5a-c) with bromides 7a and 7c
Method A:[42-44]
A two-necked flask equipped with a magnetic stirrer and a reflux condenser was charged with the
appropriate 5-aryl-1-methyl-1H-imidazole 4a-c (1 mmol) or 5-aryl-thiazoles 5a-c (1 mmol), 2-(4-
bromophenyl)benzo[d]thiazole (7a) (1.1 mmol), Pd(OAc)2 (11.2 mg, 0.05 mmol) and CuI (381 mg,
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2 mmol). The apparatus was fitted with a silicon septum, evacuated and back-filled with argon three
times. Anhydrous DMA (5 mL) was added by syringe against a positive pressure of argon. The
reaction mixture was stirred at 160 °C. The reaction evolution was followed by GLC and GLC-MS
analyses. After cooling at room temperature, the reaction mixture was diluted with CH2Cl2 (200
mL) and poured into a saturated aqueous NH4Cl solution to which 5 mL of concentrated aqueous
ammonia had been added. The resulting mixture was stirred in the open air for 30 minutes and then
extracted with CH2Cl2. The organic extracts were washed with saturated brine, dried, concentrated
under reduced pressure, and the resulting crude reaction product was purified as described below.
This procedure was used to prepare compounds 1a, 1b, 1c, 2a, 2b, and 2c.
Method B:[32]
A two-necked flask equipped with a magnetic stirrer and a reflux condenser was charged with the
5-aryl-1-methyl-1H-imidazole 4a, or 4d (1 mmol), 2-(4-bromophenyl)benzo[d]thiazole (7a) or 2-
(4-bromophenyl)benzo[d]thiazole-6-carbaldehyde (7c) (1.1 mmol), Pd(OAc)2 (11.2 mg, 0.05
mmol), Bu4NOAc (0.60 g, 2 mmol) and CuI (380.9 mg, 2 mmol). The apparatus was fitted with a
silicon septum, evacuated and back-filled with argon three times. Anhydrous DMA (5 mL) was
added by syringe against a positive pressure of argon. The reaction mixture was stirred at 110 °C
for 24 h. The reaction evolution was followed by TLC analysis. After cooling at room temperature,
the reaction mixture was diluted with CH2Cl2 (100 mL) and poured into a saturated aqueous NH4Cl
solution to which 5 mL of concentrated aqueous ammonia had been added. The resulting mixture
was stirred in the open air for 30 minutes and the organic layer was extracted. The solvent was
removed and the solid crude product was well washed with water in order to remove the excess of
Bu4NOAc, filtered, re-dissolved in CH2Cl2, washed with saturated brine, dried and concentrated
under reduced pressure. The resulting crude reaction product was purified by crystallization. This
procedure was used to prepare compounds 1d and 1e.
2.7.1 2-(4-(5-(4-Methoxyphenyl)-1-methyl-1H-imidazol-2-yl)phenyl)benzo[d]thiazole (1a).
The crude reaction product obtained by Pd- and Cu-mediated reaction of 4a and 7a according to
12

ACCEPTED MANUSCRIPT
Method A was purified by treatment with a hot mixture of toluene and AcOEt (70:30), followed by
filtration. The solid recovered (0.09g) resulted to be pure 1a, while the filtrate was further purified
by flash chromatography on silica gel using a mixture of toluene and AcOEt (70:30) as eluent. The
chromatographic fractions containing the product were collected, concentrated and added to the
solid previously recovered by filtration. In this way pure 1a was obtained as a yellow solid (0,14 g,
1
66 %), m.p. 244-248 °C. H NMR (200 MHz, CDCl3): δ 8.21 (m, 2H), 8.10 (d, J = 8.0 Hz, 1H),
7.93 (d, J = 8.0 Hz, 1H), 7.86 (m, 2H), 7.55-7.37 (m, 4H), 7.20 (s, 1H), 7.01 (m, 2H), 3.87 (s, 3H),
3.71 (s, 3H) ppm. 13C NMR (50.3 MHz, CDCl3): δ 167.2, 159.5, 154.1, 147.9, 135.9, 135.1, 133.4,
133.3, 130.2, 129.0, 127.7, 127.5, 126.4, 125.3, 123.3, 122.3, 121.7, 114.3, 55.4, 34.0 ppm. MS
(EI): m/z (%) = 398 (30), 397 (100), 396 (27), 382 (15), 198 (13). Elemental analysis calcd (%) for
C24H19N3OS: C 72.52, H 4.82; found: C 72.49, H 4.80.
2.7.2 2-(4-(1-Methyl-5-(p-tolyl)-1H-imidazol-2-yl)phenyl)benzo[d]thiazole (1b)
The crude reaction product obtained by Pd- and Cu-mediated reaction of 4b and 7a according to
Method A was purified by flash chromatography on silica gel with a mixture of toluene and AcOEt
(60:40) as eluent to give 1b as an iridescent yellow-greenish solid (0.12 mg, 66 %), m.p. 204-206
°C. 1H NMR (200 MHz, CDCl3): δ 8.21 (m, 2H), 8.10 (dd, J1 = 8.0 Hz, J2 = 0.6 Hz, 1H), 7.93 (m,
1H), 7.86 (m, 2H), 7.56-7.23 (m, 8H), 3.74 (s, 3H), 2.42 (s, 3H) ppm. 13C NMR (50.3 MHz,
CDCl3): δ 167.2, 154.1, 148.1, 138.0, 136.2, 135.1, 133.4, 133.2, 129.5, 129.0, 128.8, 128.6, 127.7,
127.0, 126.4, 125.3. 123.3, 121.6, 34.0, 21.3 ppm. MS (EI): m/z (%) = 383 (9), 382 (30), 381 (100),
380 (49), 190 (14). Elemental analysis calcd (%) for C24H19N3S: C 75.56, H 5.02; found: C 75.59,
H 5.03.
2.7.3 4-(2-(4-(Benzo[d]thiazol-2-yl)phenyl)-1-methyl-1H-imidazol-5- yl)benzonitrile (1c)
The crude reaction product obtained by Pd- and Cu-mediated reaction of 4c and 7a according to
Method A was purified by flash chromatography on silica gel with a mixture of toluene and AcOEt
(40:60) as eluent to give 1c as a yellow solid (0.15 g, 83 %), m.p. 217-220 °C. 1H NMR (200 MHz,
CDCl3): δ 8.24 (m, 2H), 8.11 (d, J = 7.4 Hz, 1H), 7.94 (d, J = 8.4 Hz, 1H), 7.86 (m, 2H), 7.77 (m,
13

ACCEPTED MANUSCRIPT
2H), 7.60 (m, 2H), 7.53-7.37 (m, 3H), 3.78 (s, 3H) ppm. 13C NMR (50.3 Hz, CDCl3): δ 166.9,
154.1, 150.0, 135.1, 134.4, 134.3, 134.0, 132.7, 132.4, 129.7, 129.3, 128.5, 127.8, 126.5, 125.5,
123.4, 121.7, 118.5, 111.4, 34.4 ppm. MS (EI): m/z (%) = 394 (9), 393 (27), 392 (100), 391 (66),
196 (10). Elemental analysis calcd (%) for C24H16N4S: C 73.45, H 4.11; found: C 73.50, H 4.09.
2.7.4 4-(2-(4-(Benzo[d]thiazole-2-yl)phenyl)-1-methyl-1H-imidazol-5-yl)-N,N-dimethylaniline (1d)
The crude reaction product obtained by Pd- and Cu-mediated reaction of 4d and 7a according to
Method B was purified by crystallization from toluene to give 1d as a yellow solid (0.19 g, 46 %),
m.p. 220-225 °C. 1H NMR (400 MHz, CDCl3): δ 8.19 (m, 2H), 8.09 (d, J = 8 Hz, 1H), 7.92 (d, J =
8 Hz, 1H), 7.86 (m, 2H), 7.51 (t, J = 8 Hz, 1H), 7.40 (t, J = 8 Hz, 1H), 7.33 (m, 2H), 7.16 (s, 1H),
6.80 (m, 2H), 3.71 (s, 3H), 3.01 (s, 6H) ppm. 13C NMR (50.3 MHz, CDCl3): δ 167.5, 154.3, 150.3,
147.7, 137.0, 135.2, 133.7, 133.3, 129.9, 129.0, 127.8, 127.1, 126.5, 125.4, 123.4, 122.4, 121.8,
117.6, 112.4, 40.5, 34.0 ppm. HRMS (ESI) m/z found [M+H]+ 411.1645; C25H22N4S requires
[M+H]+ 411.1638.
2.7.5 2-(4-(5-(4-Methoxyphenyl)-1-methyl-1H-imidazol-2-yl)phenyl)benzo[d]thiazole-6-
carbaldehyde (1e)
The crude reaction product obtained by Pd- and Cu-mediated reaction of 4a and 7c according to
Method B was purified by crystallization from AcOEt to give 1e as a yellow solid (0.44 g, 68 %),
1
m.p. 254-257 °C. H NMR (400 MHz, CDCl3): δ 10.13 (s, 1H), 8.48 (d, J = 2 Hz, 1H), 8.25 (m,
2H), 8.20 (d, J = 8 Hz, 1H), 8.03 (dd, J1 = 8 Hz, J2 = 2 Hz, 1H), 7.90 (m, 2H), 7.40 (m, 2H), 7.20 (s,
1H), 7.01 (m, 2H), 3.88 (s, 3H), 3.73 (s, 3H) ppm. 13C NMR (100 MHz, CDCl3): 191.2, 171.9,
159.8, 158.2, 147.8, 136.3, 135.8, 134.3, 133.5, 132.9, 130.3, 129.2, 128.1, 127.8, 127.6, 124.6,
123.9, 114.5, 55.5, 34.1 ppm. HRMS (ESI) m/z found [M+H]+ 426.1276; C25H19N3O2S requires
[M+H]+ 426.1271.
2.7.6 2-(4-(5-(4-Methoxyphenyl)thiazol-2-yl)phenyl)benzo[d]thiazole (2a)
The crude reaction product obtained by Pd- and Cu-mediated reaction of 5a and 7a according to
Method A was treated with hot AcOEt and filtered. The solid recovered by filtration was further
14

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treated with hot CH2Cl2, filtered and concentrated. In this way a first aliquot of 2a was recovered
(26 mg). The solution obtained from the first filtration was purified by flash chromatography on
silica gel with a mixture of CH2Cl2 and AcOEt (96:4) as eluent. The chromatographic fractions
containing the product were collected, concentrated and added to the first aliquot of 2a, to give the
required product as an iridescent yellow solid (0.14 g, 72 %), m.p. 196-199 °C. 1H NMR (200 MHz,
CDCl3): δ 8.20-8.05 (m, 5H), 7.97-7.91 (m, 2H), 7.58-7.37 (m, 4H), 6.96 (m, 2H), 3.86 (s, 3H)
13
ppm. C NMR (100 MHz, acquired at 40 °C, CDCl3): δ 167.2, 165.1, 160.2, 154.5, 140.3, 138.9,
136.1, 135.4, 134.8, 128.3, 126.9, 126.6, 125.6, 124.1, 123.6, 121.8, 114.8, 55.6 ppm. MS (EI): m/z
(%) = 400 (100), 281 (22), 207 (38), 149 (32), 44 (24). Elemental analysis calcd (%) for
C23H16N2OS2: C 68.97, H 4.03; found: C 68.89 and H 4.02.
2.7.7 2-(4-(5-(p-Tolyl)thiazol-2-yl)phenyl)benzo[d]thiazole (2b)
The crude reaction product obtained by Pd- and Cu-mediated reaction of 5b and 7a according to
Method A was purified adding a mixture of toluene and AcOEt (95:5), filtrating an insoluble
residual and performing a flash chromatography on silica gel of the filtrate to give 2b as a yellow
solid (47 mg, 25 %), m.p. 207-210 °C. 1H NMR (200 MHz, CDCl3): δ 8.21-8.16 (m, 2H), 8.12-8.06
(m, 3H), 8.03 (s, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.56-7.40 (m, 4H), 7.24 (m, 2H), 2.40 (s, 3H) ppm.
13C NMR (100 MHz, CDCl3): δ 167.2, 165.5, 154.4, 140.5, 139.3, 138.8, 136.0, 135.3, 134.8,
130.0, 128.5, 128.3, 126.9, 126.8, 126.7, 125.6, 123.5, 121.8, 21.4 ppm. MS (EI): m/z (%) = 385
(26), 384 (100), 192 (12), 148 (26), 147 (31). Elemental analysis calcd (%) for C23H16N2S2: C
71.84, H 4.19; found: C 71.78, H 4.20.
2.7.8 4-(2-(4-(Benzo[d]thiazol-2-yl)phenyl)thiazol-5-yl)benzonitrile (2c)
The crude reaction product obtained by Pd- and Cu-mediated reaction of 5c and 7a according to
Method A was treated with boiling CH2Cl2 for 2h, filtered and concentrated, to give 2c as a yellow
solid (96 mg, 53 %), m.p. 233- 235 °C. 1H NMR (200 MHz, acquired at 40 °C, CDCl3): δ 8.23-8.08
(m, 6H), 7.93 (d, J = 7.6 Hz, 1H), 7.71 (s, 4H), 7.56-7.38 (m, 2H) ppm. 13C NMR (100 MHz,
acquired at 40 °C, CDCl3): δ 168.0, 166.9, 154.4, 141.5, 138.0, 135.9, 135.7, 135.4, 133.13, 128.4,
15

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127.3, 127.2, 126.7, 125.8, 123.7, 121.9, 118.5, 112.1 ppm. MS (EI): m/z (%) = 397 (12), 396 (29),
395 (100), 197 (8), 159 (52). Elemental analysis calcd (%) for C23H13N3S2: C 69.85, H 3.31; found:
C 69.79, H 3.29.
2.8 Ethyl 2-cyano-3-(2-(4-(5-(4-methoxyphenyl)-1-methyl-1H-imidazol)phenyl)benzo[d]thyazol-6-
yl)acrylate (1f)
According to a reported procedure,[45] a two-necked 10 mL flask equipped with a condenser was
charged with compound 1e (64 mg, 0.15 mmol) and imidazole (2 mg, 0.03 mmol). The reaction
vessel was fitted with a silicon septum, then evacuated and back-filled with argon for three times.
Then, dichloromethane (1 mL) and ethyl 2-cyanoacetate (16 ꢀL, 0.15 mmol) were added and the
reaction mixture was heated to reflux overnight. The yellow crude product was worked-up by
diluting with CH2Cl2 and washing with water three times. The organic layer was dried over Na2SO4,
the solvent was evaporated and the crude product was purified by washing with 5 mL of diethyl
ether, giving compound 2f as a yellow-orange solid (70 mg, 90%), m.p. 231-235 °C. 1H-NMR (200
MHz, CDCl3): 8.67 (d, J = 2 Hz, 1H), 8.36 (s, 1H), 8.24 (m, 2H), 8.16 (d, J = 8 Hz, 1H), 8.09 (dd,
J1 = 8 Hz, J2 = 2 Hz, 1H), 7.89 (m, 2H), 7.40 (m, 2H), 7.20 (s, 1H), 7.02 (m, 2H), 4.42 (q, J = 8 Hz,
13
2H), 3.87 (s, 3H), 3.73 (s, 3H), 1.42 (t, J = 8 Hz, 3H) ppm. C-NMR (100 MHz, CDCl3): 171.8,
162.7, 159.9, 157.3, 154.3, 147.8, 136.4, 136.3, 134.4, 132.9, 130.4, 129.8, 129.3, 128.6, 128.2,
127.8, 124.9, 124.0, 122.4, 115.9, 114.5, 102.7, 63.0, 55.5, 34.1, 14.3 ppm. HRMS (ESI) m/z found
[M+H]+ 521.1646; C30H24N4O3S requires [M+H]+ 521.1642.
2.9 Preparation of polymer films for optical studies
Different fluorophore/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 fluorophore 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
16

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performed on a warm hot plate (about 30 °C) and in a closed environment. The film thickness was
measured by a Starrett 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.
2.10 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. GLC analyses were performed on a Dani GC 1000
instrument equipped with a PTV injector using two types of capillary columns: an Alltech AT-35
bonded FSOT column (30 m x 0.25 mm i.d.) and an Alltech AT-1 bonded FSOT column (30 m x
0.25mm i.d.). Purifications by flash chromatography were performed using silica gel Merck 60
(particle size 0.040-0.063 mm). EI-MS spectra were recorded at 70 eV by GLC-MS, performed on
an Agilent 6890N gas-chromatograph interfaced with Agilent 5973N mass detector. NMR spectra
were recorded at room temperature at 400 MHz (1H) and 100 MHz (13C), or 200 MHz (1H) and 50.3
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 mL. 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.
17

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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 fluorophore/PMMA
films were recorded by using the Solid-Sample Holder and collecting the front-face emission at 30°.
The fluorescence quantum yield (Φf) in CHCl3 was determined at room temperature relative to
quinine sulphate (Φsf = 0.54 in 0.1 M H2SO4) using the following relation:[46]
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. 1.45 for CHCl3 and 1.333 for water.[47] The
emission quantum yields of the solid films were obtained by means of a 152 mm diameter "Quanta-
phi" integrating sphere coated with Spectralon® and mounted in the optical path of the
spectrofluorimeter, using as an excitation source a 450 W Xenon lamp coupled with a double-
grating monochromator for selecting wavelengths.
2.10 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 (Figure S1). 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, Figure S2) connected in parallel fashion was placed and coupled to a
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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.
2.11 Efficiency measurement using a PV-cell
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%, Figure S3) 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 25±5 µm
fluorophore/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 colour
temperature of 5500 K, Figure S1). 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).
2. Results and discussion
3.1 Synthesis of mixed azole-benzazole-based dyes
Imidazole-benzothiazole dyes 1 and thiazole-benzothiazole analogues 2 were prepared according to
the retrosynthetic pathway reported in Scheme 2. This approach resembles that adopted by us for
the synthesis of the already reported imidazole-benzimidazole dyes 3.[28]
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Scheme 2. Retrosynthetic pathway for compounds 1, 2, and 3
This two-step sequence involves at first a regioselective direct C5 arylation of 1-methyl-1H-
imidazole (8) or thiazole (9) with aryl bromides 10a-d. This coupling, catalysed by Pd(OAc)2 and
promoted by Bu4NOAc or K2CO3,[43-45] allowed the preparation of the required 5-arylazoles 4a-d
and 5a-c in 61-92% isolated yield (Scheme 2). Azoles 4a-c and 5a-c were then reacted with 2-(4-
bromophenyl)-benzothiazole (7a) according to our protocol for the base-free and ligandless direct
C2 arylation of azoles,[42-44] mediated by Pd and Cu, in DMA at 160 °C for 72h (Method A,
Scheme 2). In this way, the required fluorophores 1a-c and 2a-c were isolated in 25-83% yield.
However, the base-free protocol proved to be unsuitable for the synthesis of compounds 1d and 1e,
due to the high reaction temperature. With our delight, these two derivatives were successfully
prepared using Bu4NOAc as an additive, which allowed us to carry out the coupling at 110 °C
instead of 160 °C, in DMA for 24h.[32] According to this procedure, 1d and 1e were obtained in 46
and 68% isolated yield, respectively, by Pd/Cu-mediated direct C2 arylation of 5-arylimidazoles 4d
and 4a with bromobenzothiazoles 7a and 7b (Scheme 3). All the intermediates and products were
characterized by NMR spectroscopy and mass spectrometry, with satisfactory results.
20

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Scheme 3. Synthesis of compounds 4a-d, 5a-c, 2a-e and 3a-c
Finally, compound 1f was obtained in 90% isolated yield by Knoevenagel condensation of 1e with
ethyl cyanoacetate in CH2Cl2 (Scheme 4).[45]
Scheme 4. Synthesis of compound 1f
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3.2 Optical properties in CHCl3 solutions
All the optical features of compounds 1a-f and 2a-c are summarized in Table 1 and compared to
that of imidazole-benzimidazole analogues (3a-c) recently published by our group.[28]
Table 1 Spectroscopic properties of fluorophores 1a-f, 2a-c and 3a-c dissolved in CHCl3
SS [nm]
λabs [nm]
345
ε [M-1cm-1]
32,000
32,000
42,000
32,700
19,000
38,000
38,800
43,000
47,500
36,000
31,000
39,000
λem [nm]
457
Φfa
112
100
83
165
143
147
88
76
64
107
100
87
0.90
0.81
0.80
0.42
0.50
0.11
0.61
0.59
0.55
0.90
0.88
0.83
1a
1b
1c
1d
1e
1f
2a
2b
2c
3a
3b
3c
342
343
351
363
385
372
367
372
442
426
518
506
532
460
443
436
320
316
328
427
416
415
aFluorescence quantum yield (Φ) was determined relative to quinine sulphate in 0.1M H2SO4 (Φ =
0.54).
The absorption features of the imidazole-benzothiazole fluorophores 1a and 1b were basically the
same, with absorption bands peaked around 342-345 nm. Compound 1c absorbed in the same range
too, but showed higher extinction coefficient as analogously reported for imidazole-benzoimidazole
fluorophores 3a-c.[28] This characteristic was also reflected in fluorescence since the spectrum
becomes progressively red-shifted in the order 1c, 1b, 1a, and the Stokes shift (SS) thus increased
in the same direction (Figure 1). The fluorescence quantum yields (Φ) of the three fluorophores
were also determined to have a quantitative picture (Table 1). All molecules resulted strong emitters
with quantum yields comprised between 0.8 and 0.9. The lowest Φ corresponds to 1c that showed
the lowest SS (83 nm) due to the highest overlapping between absorbance and emission (Figure 1a).
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1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1a
1b
1c
(a)
(b)
(c)
300
400
500
600
wavelength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
2a
2b
2c
300
400
500
600
wavelength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1d
1e
1f
300
400
500
600
700
wavelength (nm)
Figure 1. Normalized absorption and emission spectra of 10-5 M CHCl3 solutions of 1a-c ((a),
λexc.= 330 nm), 2a-c ((b), λexc.= 355 nm) and 1d-f ((c), λexc.= 363 nm)
The same trend was also observed for 1,4-phenylene-spaced thiazole-benzothiazole fluorophores,
that were prepared with the primary aim of increasing the emission wavelength while maintaining
23

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the SS. Notably, the introduction of the thiazole moiety contributed to red-shift the absorption
maxima of more than 25 nm for all 2a-c, but a lower shift resulted in emission (Figure 1b). This
behaviour caused the decreasing of SS to less than 80 nm and the significant reduction of Φ below
0.6. Moreover, the emission interval between 436 and 460 nm shown by fluorophores did not match
the range of maximum efficiency of a photovoltaic cell, thus excluding these fluorophores for final
application in LSCs.[27, 48]
In connection with these findings, 1,4-phenylene-spaced imidazole-benzothiazole 1d-f were
designed with the aim to red-shift their fluorescence emission above 500 nm. Notably, 1e and 1f
were functionalized at the benzothiazole unit with aldehyde and α-cyano ester electron-
withdrawing groups, respectively, whereas 1d was decorated with the electron-donating
dimethylamino group. It is worth noting that the introduction of the electron-withdrawing groups
red-shifted the emission maximum of 50 (1e) and 80 nm (1f), respectively, while maintaining the
absorption maxima below 390 nm (Figure 1c). This phenomenon contributed to the enhancement in
SS to values higher than 140 nm, but adversely affected Φ, whose values were calculated to be 0.50
and 0.11 for 1e and 1f, respectively. This is not surprising since de-excitation by internal conversion
is more efficient when the energy difference between the ground and excited states is small.[29]
Indeed, red-emitting organic fluorophores strongly quenched their own fluorescence both in
solution and in condensed phases.[49]
Compound 1d was then utilized to achieve a trade-off between longer emission wavelengths and Φ
higher than 0.3-0.4, in order to comply with the requirements of fluorophores in LSCs.[6] The
introduction of the dimethylamino group in the 1,4-phenylene-spaced imidazole-benzothiazole
conjugated structure provided an absorption band peaked at 351 nm, a fluorescence maximum at
516 nm (SS = 165 nm) and a Φ higher than 0.4. Moreover, the extinction coefficient was calculated
to be similar to the highest reported in Table 1. Notably, the combination of optical properties, i.e.
no significant absorption for wavelengths > 400 nm, largest SS, emission ≥ 500 nm and Φ of 0.42,
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allowed 1d to be investigated in PMMA films for applications as colourless LSC. Compound 1f,
characterised by emission > 500 nm was also analysed for comparison purposes.
Optical characterization of the 1d/PMMA films
Owing to the aforementioned opto-electronic properties, 1d was also investigated when dispersed in
the transparent and totally amorphous polymer matrix of PMMA. PMMA is 100% amorphous,
transparent, cheap and commercially available, characteristics that make this polymer a perfect
candidate for large-scale LSC applications.[50]
The optical characteristics of 0.5 wt.% of 1d in PMMA films were reported in figure 2 with
maximum absorption and emission bands found at 359 nm and 500 nm, respectively, with a SS of
141 nm.
1.8x106
0.8
0.7
0.6
0.5
0.4
0.3
1.6
1.4
1.2
1.0
0.8
0.6
0.2
0.1
0.0
0.4
0.2
0.0
300 350 400 450 500 550 600 650 700
wavelength (nm)
Figure 2. Absorption (solid line) and emission spectra (dotted line, λexc.= 360 nm) of a 0.5 wt.%
1d/PMMA film with a thickness of 25±5 µm.
1d showed absorptions in the near-UV region with a negligible contribution in the visible range of
the light spectrum (at 440 nm, about 90% of transmitted light, Figure 3a)) also in the solid state
when dispersed in the polymer matrix. SS of 1d in PMMA was smaller than that recorded in CHCl3
solution, possibly due to the blue-shifted emission caused by the different dielectric constant (for
PMMA = 2.6-2.8; for CHCl3 = 4.81). Nevertheless, a SS larger than 135 strongly limited any
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adverse auto-absorption phenomena among 1d fluorophores and contributed to a brilliant green
emission from the PMMA film (Figure 3b).
100
90
80
70
(a)
60
50
40
400
420
440
460
480
500
wavelength (nm)
(b)
Figure 3. (a) UV-Vis transmittance spectrum in the 400-500 nm region of a 1.6 wt.% 1d/PMMA
film with a thickness of 25±5 µm coated over an optically pure 50x50x3 mm glass and picture of
the same film under (inset) visible light and (b) a long-range UV lamp at 366 nm
The absolute fluorescent quantum yield (QY) of 1d in PMMA reached an average value of 28 %,
which slightly decreased at the highest concentration investigated (i.e. 22 % for 1.8 wt.%
1d/PMMA film). This value is lower than that of CHCl3 solution (42 %) due to the self-quenching
that usually accompanies fluorophores in the solid state,[49] but it results still interesting and
promising for optical applications in polymers.
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The performances of 1d PMMA films as LSC were determined on optically pure 50x50x3 mm
glass. Photocurrent measurements were accomplished with a home-built apparatus[48] 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 1d/PMMA films since the
response curves of the photodiodes and the PV module do not differ significantly. The data
acquired were compared to that collected from 1f/PMMA systems, in order to relate the molecular
structures to the device performances. 1f was selected as reference since it displayed high SS (147
nm) but lower Φ (0.11 vs 0.50) in solution, and their optical features resulted mostly maintained in
the PMMA matrix.
Figure 4. Photocurrent of 1d/PMMA and 1f/PMMA films of a thickness of 25±5 µm with
increasing dye concentration (wt.%). Photocurrents were fitted with eq. 1 (grey curve) with
parameters listed in Table 2 (see below)
The data followed a peculiar trend, i.e. photocurrent increasing with 1d content and levelling off at
the highest concentration investigated (>1.6 wt.%). In detail, photocurrent was found to increase
steadily with 1d concentration up to 1.4 wt.% of fluorophore, indicating a negligible effect of self-
quenching on fluorescence efficiency. Conversely, when the concentration was further increased
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adverse dissipative phenomena prevailed. The same trend was also found for 1f/PMMA films, even
if much reduced photocurrents were observed for all the range of concentration investigated. This
result was ascribed to the lower Φ of 1f, which did not compensate the lost in excitation energy due
to the large SS, and introduced substantial loss of emission trapping efficiency.
Notably, the photocurrent behaviour fits quite well with eq. 1:
ꢀꢁꢂꢃ = ꢄ′ ∙ ꢅ ∙ ꢆꢇꢈ ∙ꢌ + ꢍ
(eq.1)
ꢉꢊꢋ
where ηopt is the optical efficiency a term proportional to the current generated by photodiodes, c is
the concentration of the dye in wt.%, and ε' and ꢀopt are two empirical constants defined as:
̅
ꢄ′ ∝ ℎ ∙ ꢆꢇꢎ
(eq. 2)
(eq. 3)
̅
ꢏꢁꢂꢃ ∝ ꢏ′′(ꢐꢑ, ꢒ) ∙ ꢓ
̅
where h is the thickness of the thin film, ꢓ is the mean path length of the radiation in the optical
system and ꢏ′′ is a term depending on both QY and the probability of fluorescence re-absorption
(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 recently determined inspired by the work of Goezberger[7] 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.[48]
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The fitting parameters, reported in Table 1, were compared to those recently gathered for PMMA
films with the same thickness of 25±5 µm but containing Lumogen Red F305 (LR),[48, 51]
selected as reference as it is considered the state-of-the-art for LSC applications.[6]
Table 2. Fitting parameters of the photocurrent data measured for 1d/PMMA, 1f/PMMA and
LR/PMMA[48] films. For both systems, the film thickness was 25±5 µm
Entry
ε'
67
66
ꢀopt
0.49
0.80
0.45
D
1d/PMMA
1f/PMMA
LR/PMMA
35
25
20
140
The fitting parameters of 1d/PMMA films were found to be different from those of LR/PMMA only
for ε’. The smaller ε’ is a result of the lesser absorption window of 1d compared to that of LR in
PMMA, being ε’ related to the light absorption properties of the system. This data is not surprising
since 1d was actually designed to have limited absorption in the visible region of light spectrum. It
is worth noting that the fluorophores showed identical µopt values, notwithstanding the lower QY of
1d compared to that of LR (28% against about 1, respectively).[48] This phenomenon was ascribed
to the limited fluorescence quenching mechanisms experienced by 1d at high concentration, which
conferred PMMA films good emission efficiencies. 1f/PMMA and 1d/PMMA films showed
identical ε’ due to the similar extinction coefficients, i.e. 38,000 M-1cm-1 for 1f and 32,700 M-1cm-1
for 1d. Conversely, µopt values appeared quite different for 1f/PMMA films, due to the much–
reduced emission efficiency caused by the very low QY. This result confirms that to maximize the
emission trapping in a LSC, it is more effective to prepare fluorophores having the highest
achievable QY, rather than very large SS.
Notably, 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.
The 1d/PMMA films with the highest photocurrent, i.e. the those containing the 1.4 and 1.8 wt.% of
1d, were analysed by using a Si-based PV cell attached to one edge of the concentrator, as
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