One pot synthesis of low cost emitters with large Stokes' shift

Article abstract of DOI:10.1016/j.dyepig.2016.09.056

A series of 1,3-diarylated imidazo[1,5-a]pyridine derivatives were synthesized in high yields by a one-pot, three components, condensation of phenyl(pyridin-2-yl)methanone with several aldehydes in the presence of ammonium acetate. These compounds were characterized by spectroscopic and crystallographic techniques and their optical properties were discussed in relation to their chemical structures. Absorption and fluorescence spectra generally show two absorption maxima around 310 nm and 350 nm and an emission maximum between 460 and 550 nm with a remarkable Stokes' shift range of 90–166 nm. Moreover, depending on the chemical structure of the substituent in position 3, we were able to tune the quantum yields (Φ) in solution from 6.4% to 38.5%. Finally, the 1-phenylimidazo[1,5-a]pyridine substituted with a 3-(2-methoxyphenyl) group (large Stokes' shift, high Φ) was dispersed in a transparent thermosetting polyurethane resin giving a luminescent low cost material.

Full text of DOI:10.1016/j.dyepig.2016.09.056

Accepted Manuscript
One pot synthesis of low cost emitters with large Stokes' shift
G. Volpi, G. Magnano, I. Benesperi, D. Saccone, E. Priola, V. Gianotti, M. Milanesio,
E. Conterosito, C. Barolo, G. Viscardi
PII:
S0143-7208(16)30821-X
10.1016/j.dyepig.2016.09.056
DYPI 5508
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 10 June 2016
Revised Date: 22 September 2016
Accepted Date: 25 September 2016
Please cite this article as: Volpi G, Magnano G, Benesperi I, Saccone D, Priola E, Gianotti V, Milanesio
M, Conterosito E, Barolo C, Viscardi G, One pot synthesis of low cost emitters with large Stokes' shift,
Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.09.056.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
One pot synthesis of low cost emitters with large Stokes’ shift
G. Volpi,a G. Magnano,a I. Benesperi, a D. Saccone, a E. Priola, a,c V. Gianotti, b M. Milanesio,b,c
E. Conterosito,b C. Barolo,* a,d G. Viscardia
a
Dipartimento di Chimica, NIS Interdepartmental Centre and INSTM Reference Centre , Università di Torino, Via
Pietro Giuria 7, 10125, Torino, Italy. Fax: +39 011 670 7591; Tel: +39 011 670 7596; E-mail:
claudia.barolo@unito.it
b Università del Piemonte Orientale Dipartimento di Scienze e Innovazione Tecnologica Viale T. Michel 11, I-15121
Alessandria, Italy
c CrisDi Interdepartmental Center for Crystallography
d ICxT Interdepartmental Centre
* corresponding author
Keywords: imidazo[1,5-a]pyridine, fluorescence, large Stokes’ shift, quantum yield, down-
shifting.
Abstract
A series of 1,3-diarylated imidazo[1,5-a]pyridine derivatives were synthesized in high yields by
a one-pot, three components, condensation of phenyl(pyridin-2-yl)methanone with several
aldehydes in the presence of ammonium acetate. These compounds were characterized by
spectroscopic and crystallographic techniques and their optical properties were discussed in
relation to their chemical structures. Absorption and fluorescence spectra generally show two
absorption maxima around 310 nm and 350 nm and an emission maximum between 460-550 nm
with a remarkable Stokes’ shift range of 90-166 nm. Moreover, depending on the chemical
structure of the substituent in position 3, we were able to tune the quantum yields (Φ) in solution
from 6.4% to 38.5%. Finally, the 1-phenylimidazo[1,5-a]pyridine substituted with a 3-(2-
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methoxyphenyl) group (large Stokes’ shift, high Φ) was dispersed in a transparent thermosetting
polyurethane resin giving a luminescent low cost material.
1. Introduction
Large Stokes’ shift organic emitting molecules are acquiring a great interest due to their potential
application as low-cost and eco-friendly active compounds in down-shifting luminescent layers
both in lighting [1] and photovoltaic (PV) technologies [2]. Moreover, luminescent solar
concentrators (LSC) based on thin-film organic molecules represent a convenient technology to
improve power conversion efficiency of PV systems [3].
Two of the most important prerequisites for a good candidate molecule for down-shifting and
LSC applications are a large Stokes’ shift (to avoid re-absorption) and a high quantum yield [4].
It is of paramount importance that absorbed photons are re-emitted and not dissipated as heat. In
previous studies regarding down-shifting technologies, the choice of fluorophores has been
focused on molecules with a very high quantum yield (95%), but a very small Stokes’ shift [5].
While the high quantum yield ensures that almost every absorbed photon is then re-emitted, the
very small Stokes’ shift leads to a very high re-absorption. In these conditions, a photon may be
re-absorbed several times before it can reach the working layer of the photoelectric device. Since
the emission of the molecule is isotropic and not focused, a high photon loss is unavoidable. In
this work we focused our attention on imidazo[1,5-a]pyridine derivatives, a class of UV
absorbers that is well known for biological activities [6-11], but which also shows unique
photophysical properties. These compounds have been applied in a large variety of fields,
including organic thin-layer field effect transistors (OFETs) [12] and organic light-emitting
diodes (OLEDs) [13]. This class of compounds is known for its large Stokes’ shift, but these
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dyes generally show too low quantum yields for practical applications [14-15]. A deep synthetic
effort, in conjunction with a solid photophysical and structural characterization [16], is therefore
needed in order to understand which are the moieties able to improve the emission behavior
without decreasing the Stokes’ shift properties. For this purpose, we synthesized and
characterized eleven 3-substituted-1-phenylimidazo[1,5-a]pyridines (Figure 1). Concerning the
synthetic approach, several methods have been recently reported involving the use of Pd-
catalyzed [14] and Cu-catalyzed reactions on different substrates [17]. Less recently, most
reactions were conducted starting from amines and ketones or aldehydes either in the presence of
a sensitive Lewis acid [18] or of a stoichiometric amount of elemental sulphur (S8) as an oxidant
[15], or from the oxidation of a Schiff base [19]. Unfortunately, all these methods require two or
three steps from the initial reagents [20] and careful processing due to the use of highly sensitive
reagents, thus hampering an easy scale up of the synthesis towards industrial application.
Nonetheless, an efficient synthetic method for the preparation of the imidazo[1,5-a]pyridine
moiety has also been proposed via a three component condensation reaction using
pyridylketones, aldehydes and ammonium acetate [21]. In the present work, we selected this last
synthetic approach, due to its simplicity and low cost of reagents, with promising scale up
perspectives. This method allowed us to introduce different substituents, enhancing the
conjugation length and introducing electron-donating or electron-withdrawing moieties in
position 3 of the 1-phenylimidazo[1,5-a]pyridine skeleton. In compounds 3 and 10 an OH group
in the ortho position of the substituent was inserted in order to verify the possibility for the
molecules to undergo an Excited State Intramolecular Proton Transfer (ESIPT) [22] fluorescence
mechanism. In compound 8 a triphenylamine group was added, which is known to be a thermally
stable electron-donating group [23] and a charge-transporting species used in nonlinear optics
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[24], two-photon absorption [25] and OLED materials [26]. This structural diversity should also
imply photofunctional differences making the 1-phenylimidazo[1,5-a]pyridines an interesting
series of easily tunable photofunctional materials.
Finally, the most promising compound 4 (Figure 1) was also dispersed in a thermosetting
polyurethane resin, and its photophysical properties were investigated in the solid state.
Please insert here Figure 1.
2. Experimental details
2.1 Materials and techniques
All solvents and raw materials were used as received from commercial suppliers (Sigma-Aldrich
and Alfa Aesar) without further purification. The two precursors of the polyurethane resin used
for the down-shifting tests, a polyol with product name LCR 540RT–DH and an isocyanate with
product name DK 100LV, were acquired from S.E. Special Engines Srl. TLC was performed on
Fluka silica gel TLC-PET foils GF 254, particle size 25 nm, medium pore diameter 60 Å.
Column chromatography was performed on Sigma-Aldrich silica gel 60 (70-230 mesh ASTM).
13
1H and C NMR spectra were recorded on a Bruker Avance 200 spectrometer at 200 MHz and
50 MHz, respectively, in CDCl3 or CD3OD. Mass spectra were recorded on a Thermo-Finnigan
Advantage Max Ion Trap Spectrometer equipped with an electrospray ion source (ESI) in
positive or negative ion acquiring mode for 1, 2, 6 and 9. High-resolution mass spectra were
recorded on an LTQ Orbitrap Hybrid Mass Spectrometer for compounds 3-5, 7, 8, 10 and 11.
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2.2 Procedure for the preparation of compounds 1-11
Four compounds of this series (1, 2, 6 and 9) were previously reported with different synthetic
approaches based on the use of a metal catalyst, a Lewis acid or a strong oxidant, as reported in
Table 1. Very recently [17b, 17c] also compound 4 has been reported.
Please insert here Table 1.
2.2.1 General synthetic procedure
A mixture of phenyl(pyridin-2-yl)methanone (800 mg, 4.37 mmol, 1 eq), aldehyde (6.55 mmol,
1.5 eq), and ammonium acetate (1704 mg, 21.85 mmol, 5 eq) in glacial acetic acid (25 mL) was
stirred at 110 °C under inert atmosphere. After 5 h, the reaction mixture was cooled to room
temperature and the acetic acid was removed by evaporation under reduced pressure. The
obtained solid was dissolved in a saturated aqueous solution of Na2CO3 and the mixture extracted
with CH2Cl2. The organic layer was separated, dried and the solvent evaporated under vacuum.
The obtained crude product was purified via column chromatography on silica gel (CH2Cl2-
CH3OH 98:2).
3-methyl-1-phenylimidazo[1,5-a]pyridine (1)
712 mg, yield = 78.2%
1H NMR (200 MHz, CDCl3): δ 2.50 (s, 3H), 6.36 (t, J = 6.6 Hz, 1H), 6.54 (dd, J = 6.4 Hz, 9 Hz,
1H), 7.13-7.44 (m, 5H), 7.60 (d, J = 9.2 Hz, 1H), 7.79 (d, J = 7.4 Hz, 1H)
13C NMR (50 MHz, CDCl3): δ 12.06, 112.18, 118.34, 118.50, 120.70, 125.79, 125.97, 126.02,
128.40, 129.25, 134.56, 134.84
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MS (ESI): m/z 209.17[M+1]+
1,3-diphenylimidazo[1,5-a]pyridine (2) [17]
956 mg, yield = 81.1%; solid, m.p.:120°C
1H NMR (200 MHz, CDCl3): δ 6.54 (ddd, J = 1 Hz, 6.4 Hz, 7.2 Hz, 1H), 6.76 (ddd, J = 0.8 Hz,
6.4 Hz, 9.2 Hz, 1H), 7.27-7.36 (m, 1H), 7.40-7.58 (m, 5H), 7.80-7.87 (m, 3H), 7.93-7.99 (m,
2H), 8.21 (dt, J = 1 Hz, 7.2 Hz, 1H)
13C NMR (50 MHz, CDCl3): δ 113.35, 119.13, 119.83, 121.76, 126.61, 126.85, 127.68, 128.35,
128.78, 128.89, 129.06, 130.06, 131.90, 134.89, 138.06
MS (ESI): m/z 271.33[M+1]+
2-(1-phenylimidazo[1,5-a]pyridin-3-yl)phenol (3)
792 mg, yield = 50.7%; solid, m.p.:187°C
1H NMR (200 MHz, CDCl3): δ 5.07 (s, 1H), 6.43 (ddd, J = 1.2 Hz, 6.4 Hz, 7.3 Hz, 1H), 6.60
(ddd, J = 1 Hz, 6.4 Hz, 9.2 Hz, 1H), 6.77 (ddd, J = 1.4 Hz, 7.2 Hz, 7.8 Hz, 1H), 6.96 (ddd, J =
0.4 Hz, 1.4 Hz, 8.2 Hz, 1H), 7.03-7.15 (m, 2H), 7.21-7.30 (m, 2H), 7.51 (ddt, J = 0.4 Hz, 1.6 Hz,
7.8 Hz, 1H), 7.58-7.68 (m, 3H), 8.22 (dt, J = 0.8 Hz, 7.4 Hz, 1H)
13C NMR (50 MHz, CDCl3): δ 113.94, 114.31, 117.94, 119.14, 119.42, 120.65, 122.63, 124.67,
126.81, 127.03, 127.17, 128.94, 129.83, 130.17, 133.65, 135.66, 156.56
HRMS (ESI): m/z calculated for C19H15N2O [(M + H+)] 287.1184; found 287.1173.
HRMS (ESI): m/z calculated for C19H13N2O [(M – H] 285.1028 ; found 285.0944.
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3-(2-methoxyphenyl)-1-phenylimidazo[1,5-a]pyridine (4) [17b,c]
810 mg, yield = 61.8%; solid, m.p.:106°C
1H NMR (200 MHz, CDCl3): δ 3.80 (s, 3H), 6.53 (ddd, J = 1.2 Hz, 6.4 Hz, 7.2 Hz, 1H), 6.80
(ddd, J = 1 Hz, 6.4 Hz, 9.2 Hz, 1H), 7.05 (dd, J = 0.8 Hz, 8.4 Hz, 1H), 7.13 (td, J = 1 Hz, 7.5 Hz,
1H), 7.24-7.33 (m, 1H), 7.42-7.52 (m, 3H), 7.59 (dt, J = 1 Hz, 7.2 Hz, 1H), 7.70 (ddd, J = 0.4
Hz, 1.8 Hz, 7.6 Hz, 1H), 7.86 (dt, J = 1.2 Hz, 9.4 Hz, 1H), 7.95-8.00 (m, 2H)
13C NMR (50 MHz, CDCl3): δ 55.61, 111.25, 112.15, 118.64, 119.05, 119.67, 121.29, 123.59,
126.35, 126.81, 127.47, 128.70, 130.97, 131.41, 132.88, 135.19, 136.20, 157.47
HRMS (ESI): m/z calculated for C20H17N2O [(M + H+)] 301.1341; found 301.1330.
4-(1-phenylimidazo[1,5-a]pyridin-3-yl)benzonitrile (5)
1070 mg, yield = 81.6%; m.p.:228°C
1H NMR (200 MHz, CDCl3): δ 6.70 (ddd, J = 1.4 Hz, 6.5 Hz, 7.2 Hz, 1H), 6.89 (ddd, J = 1 Hz,
6.4 Hz, 9.2 Hz, 1H), 7.29-7.37 (m, 1H), 7.44-7.53 (m, 2H), 7.79 (ddd, J = 1.4 Hz, 2 Hz, 8 Hz,
2H), 7.86-7.94 (m, 3H), 8.01 (ddd, J = 1.4 Hz, 1.9 Hz, 8.1 Hz, 2H), 8.28 (dt, J = 1 Hz, 7.2 Hz,
1H)
13C NMR (50 MHz, CDCl3): δ 111.85, 114.65, 118.70, 119.58, 120.85, 121.58, 127.03, 127.25,
128.27, 128.76, 128.96, 132.86,133.23, 134.14, 134.22, 135.77
HRMS (ESI): m/z calculated for C20H14N3 [(M + H+)] 296.1187; found 296.1177.
N,N-dimethyl-4-(1-phenylimidazo[1,5-a]pyridin-3-yl)aniline (6) [14b]
758 mg, yield = 55.3%; m.p.:138°C
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1H NMR (200 MHz, CD3OD ): δ 3.04 (s, 6H), 6.66 (t, J = 6.8 Hz, 1H), 6.83 (d, J = 7.8 Hz, 1H),
6.91 (d, J = 8.3 Hz, 2H), 7.31 (t, J = 7.3 Hz, 1H), 7.46 (t, J = 7.3 Hz, 3H), 7.62 (d, J = 8.4 Hz,
2H), 7.81 (d, J = 7.6 Hz, 3H), 8.22 (d, J = 7.3 Hz, 1H).
13C NMR (50 MHz, CD3OD )): δ 152.61, 140.53, 135.86, 131.98, 130.57, 129.77, 128.31,
127.99, 127.66, 123.23, 121.27, 119.75, 117.89, 114.58, 113.49, 40.49.
MS (ESI): m/z 314.40[M+1]+
3-(1H-indol-3-yl)-1-phenylimidazo[1,5-a]pyridine (7)
341 mg, yield = 25.3%; m.p.:139°C
1H NMR (200 MHz, CD3OD): δ 4.54 (s, 1H), 6.53 (t, J = 6.4 Hz, 1H), 6.67 (t, J = 7.4 Hz, 1H),
7.25-7.03 (m, 3H), 7.47-7.35 (m, 3H), 7.59 (d, J = 7.4 Hz, 1H), 7.83 -7.67(m, 4H), 7.98 (d, J =
6.8 Hz, 1H).
13C NMR (50 MHz, CD3OD): δ 137.95, 136.03, 131.83, 129.78, 128.11, 127.87, 127.55, 127.37,
126.63, 126.59, 123.81, 123.53, 121.33, 121.21, 121.00, 119.68, 114.25, 112.99, 105.39.
HRMS (ESI): m/z calculated for C21H16N3 [(M + H+)] 310.1344; found 310.1332.
N,N-diphenyl-4-(1-phenylimidazo[1,5-a]pyridin-3-yl)aniline (8)
195 mg, yield = 34.8%; m.p.:155°C
1H NMR (200 MHz, CDCl3): δ 6.54-6.62 (m, 1H), 7.79 (broad, 1H), 7.03-7.34 (m, 14H), 7.47 (t,
J = 7.4 Hz, 2H), 7.70 (d, J = 8.6 Hz, 2H), 7.84 (dt, J = 1.2 Hz, 9.2 Hz, 1H), 7.95 (d, J = 7.4 Hz,
1H), 8.24 (broad, 1H)
13C NMR (50 MHz, CDCl3): δ 119.39, 120.03, 122.08, 122.16, 123.06, 123.19, 123.75, 125.16,
126.92, 127.03, 127.11, 127.22, 127.41, 128.89, 129.45, 129.57, 138.02, 147.37, 148.87
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HRMS (ESI): m/z calculated for C31H24N3 [(M + H+)] 438.1970; found 438.1953.
3-(naphthalen-1-yl)-1-phenylimidazo[1,5-a]pyridine (9)
1095 mg, yield = 78.2%; m.p.:160°C
1H NMR (200 MHz, CDCl3): δ 6.49 (t, J = 6.8 Hz, 1H), 6.83 (dd, J = 6.4 Hz, 9.2 Hz, 1H), 7.28-
7.66 (m, 7H), 7.73-7.82 (m, 2H), 7.90-8.04 (m, 5H)
13C NMR (50 MHz, CDCl3): δ 112.78, 118.75, 119.86, 122.02, 125.30, 125.45, 126.26, 126.38,
126.60, 126.95, 127.04, 127.11, 128.49, 128.67, 128.71, 129.88, 131.49, 131.78, 133.86, 135.03,
136.68
MS (ESI): m/z 321.13[M+1]+
1-(1-phenylimidazo[1,5-a]pyridin-3-yl)naphthalen-2-ol (10)
1053 mg, yield = 71.6%; m.p.:196°C
1H NMR (200 MHz, CDCl3): δ 6.47 (d, J = 6.7 Hz, 1H), 6.75(t, J = 6.8 Hz, 1H), 7.05 (d, J = 8.8
Hz, 1H), 7.24-7.47 (m, 6H), 7.68 (d, J = 8.4 Hz, 1H), 7.74-7.80 (m, 4H), 8.0 (d, J = 8.0 Hz, 1H).
13C NMR (50 MHz, CDCl3): δ 155.14, 148.69, 137.21, 134.20, 133.08, 131.73, 131.51, 131.11,
130.81, 128.72, 128.31, 127.12, 126.80, 126.51, 126.31, 124.78, 124.53, 123.57, 120.59, 113.20,
108.17.
HRMS (ESI): m/z calculated for C23H17N2O [(M + H+)] 337.1341; found 337.1328.
3-(anthracen-9-yl)-1-phenylimidazo[1,5-a]pyridine (11)
456 mg, yield= 28.2%; m.p.:202°C
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1H NMR (200 MHz, CDCl3): δ 6.41 (t, J = 6.8 Hz, 1H), 6.86 (dd, J = 6.4 Hz, 9.2 Hz, 1H), 7.17
(d, J = 7 Hz, 1H), 7.30-7.56 (m, 9H), 8.03 (d, J = 9.4 Hz, 1H), 8.12 (d, J = 7.8 Hz, 4H), 8.67 (s,
1H)
13C NMR (50 MHz, CDCl3): δ 113.04, 118.99, 120.09, 122.20, 122.95, 125.57, 125.64, 126.54,
126.75, 127.04, 127.10, 128.84, 129.61, 129.65, 131.51, 131.71, 131.91, 135.14, 135.17
HRMS (ESI): m/z calculated for C27H19N2 [(M + H+)] 371.1548; found 371.1536.
2.3 Spectroscopic characterization of the 1-phenylimidazo[1,5-a]pyridines
UV-Vis spectra were recorded on a Shimadzu UV-1700 spectrophotometer using different
solvents in order to investigate the solvatochromic behavior of 1-11: a stock solution (3.5·10−3
M) in acetonitrile was prepared and diluted solutions (3.5·10−5 M) in cyclohexane,
dichloromethane, acetonitrile, methanol and aqueous solution of HCl 1 M were analyzed.
Fluorescence measurements were recorded using a HORIBA Jobin Yvon Fluorolog 2 (for the
excitation wavelengths see Table 3). The wavelength range used for fluorescence emission
measurements was between 360 nm and 750 nm. Fluorescence quantum yields were determined
using the same instrument with a comparative method [30], using Rhodamine 6G (Φ = 0.94) and
Rhodamine 101 (Φ = 0.96) as standards [31-32].
2.4 Crystal Structure.
Single crystal data of compounds 2, 7 and 11 have been collected on a Gemini R Ultra
diffractometer and on a X-Calibur Sapphire3 diffractometer [33]. All data were collected using
graphite-monochromated Mo Kα radiation (λ = 0.71073). Cell parameters were retrieved using
CrysAlisPro [34] software, and the same software was used to perform data reduction with
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correction for Lorenz and polarizing effects. Scaling and absorption corrections were applied by
CrysAlisPro multi-scan technique. All structures were solved by direct methods using SHELXS-
97 [35] and SIR2014 [36] and refined with full-matrix least-squares on F2 using SHELXL-97
[35]. All non-hydrogen atoms, except disordered moieties in compound 11, have been refined
anisotropically. Hydrogen atoms have been located in the final Fourier-difference maps and
refined as riding atoms. Structural pictures have been drawn using CCDC Mercury [37]. Crystal
data and structure refinements can be found in Table 2. All the refinement data, the atomic
parameters and the tables with angles and distances can be found in SI document. CIF files of the
structures have been deposited on the CCDC database with codes 1447690, 1447691 and
1447959.
Please insert here Table 2.
2.5 Preparation of polyurethane films with compound 4
An amount of compound 4 necessary to reach a final concentration of 0.1% w/w in the resin was
dissolved in a minimum volume of dichloromethane. To this solution, the two precursors of the
polyurethane resin (the polyol, LCR 540RT-DH and the isocyanate, DK 100LV) were added in a
1:1 ratio and mechanically stirred. The mixture was placed in a vacuum bell to evaporate the
dichloromethane – leaving compound 4 dissolved in the resin – and to degas the liquid to avoid
air bubbles in the final film. To produce the film, a mold was fabricated by Scotch-taping two
aluminum bars (to facilitate the film removal) and then placing a spacer of a defined thickness on
the long sides of one of the bars to control the film thickness. Once the liquid resin was deposited
on the substrate, the substrate was placed again in the vacuum bell for a final outgassing step and
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then covered with the second aluminum bar. The mold was finally placed in an oven at 80°C for
30 min to speed up the resin polymerization.
3. Results and Discussion
3.1 Synthesis
A one-pot direct cyclization of phenyl(pyridin-2-yl)methanone with eleven different aldehydes in
the presence of ammonium acetate in boiling acetic acid was used to obtain compounds 1-11
(Scheme 1). Electron withdrawing and electron donating substituents on the aldehyde were very
well tolerated in this reaction. The optimal condition was established to have a 1.0 : 1.5 : 5 molar
ratio of ketone/aldehyde/ammonium acetate.
Please insert here Scheme 1.
The isolated yields vary from moderate (28.2% for 11) to excellent (81.6% for 5).
3.2 Optical characteristics of the compounds
Good absorption properties in a selected spectral region and efficient emission properties with
large Stokes’ shifts are fundamental requirements for any organic emitter, designed to be used in
luminescent down-shifting (LDS) layers [4]. Our substituted 1-phenylimidazo[1,5-a]pyridines
show absorption maxima below 400 nm, with a good transparency in the visible range, and a
wide variety of fluorescent emissions at wavelengths in the range of 467-562 nm, thus resulting
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in a large Stokes’ shift, as can be seen in Table 3, where the main optical parameters for all
molecules in CH2Cl2 are reported.
Please insert here Table 3.
3.2.1 Absorption properties
The main absorption peaks were observed in the wavelength range between 300 nm and 382 nm,
with almost no absorption beyond 450 nm. Usually, the compounds presented two main bands,
one in the 300-320 nm range (except compounds 3 and 5) and a second one centered at 360-380
nm. The intensity of the absorption significantly increased going from compound 1 (methyl
substituent in position 3) to 2-11, as a clear consequence of the hyperchromic effect induced by
the extension of the conjugation due to the aromatic substituent in position 3.
The various substituents had a definite effect on the absorption bands. It was evident that both
electron-donating (i.e. N(CH3)2 group, compound 6) and electron-withdrawing groups (i.e. CN
group, compound 5) on a phenyl substituent in position 3 had a strong and opposite effect on the
absorption behavior in the 360-380 nm range, compared to the parent phenyl group (Figure 2,
Top-Left). Compound 4, having a donor methoxy group instead of a hydroxyl group, showed a
different behavior in respect to compound 3.
Please insert here Figure 2.
In Figure 2 (Top-Right) the absorption spectra of 3-substituted 1-phenylimidazo[1,5-a]pyridines
with methyl (compound 1), phenyl (compound 2), naphthalene (compound 9) and anthracene
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(compound 11) groups were compared. The extension of the conjugation system was evident in
the evolution of the ratio between the intensity of the peak at high energy (308 nm), and the
second band at 360-390 nm.
Figure 2 (Bottom-Left) shows the absorption spectra of compounds 1, 2, 7, 8 to compare the
effect of different substituents in position 3. In comparison to reference dyes (1 and 2), an
evident hyperchromic effect was shown for compound 8 at low energy, while the indole group
induced a hypochromic effect over 350 nm.
The presence of the hydroxyl group in compounds 3 and 10 increased the intensity of the
absorption in respect to the analogous dyes 2 and 9 without hydroxyl group (Figure 2, Bottom-
right).
The light harvesting properties of these compounds were also studied in a series of solvents with
different polarity, to assess the effect of the chemical environment. The resulted spectra showed
very little changes with increasing solvent polarity in non protic systems, indicating that there is
practically no charge transfer effect even in the presence of a strong electron donor (i.e. OCH3,
compound 4) or of a strong electron acceptor (CN, compound 5), as stated in Table 3. In Figure
3, spectra of compound 3 in different solvents are reported as an example. This behavior is of
particular interest because it demonstrates the possibility to host this class of molecules in several
polymers without an appreciable change in their photophysical response.
Conversely, in the case of protic solvents and aqueous solutions at different pH, the absorption
profile was significantly different. This behavior was common to all studied compounds, which
lead to the conclusion that the common moiety imidazo[1,5-a]pyridine was most likely
responsible for this difference [38]. Insertion of the molecules in polar matrixes might thus result
in varied photophysical properties.
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Please insert here Figure 3.
3.2.2 Emission properties
The emission spectra of compounds 1-11 in dichloromethane solution are showed in Figures 4-6
and their emission wavelengths are reported in Table 3. Most of the products showed an
emission band centered between 470 nm and 500 nm. The emission of compound 11 bearing an
anthracene moiety, which occurred 80 nm bathocromically shifted (Figure 4a), respect to the
other compounds, resulted particularly interesting. In the case of a substitution with an indole
(compound 7) or a triphenylamine (compound 8), emission data showed moderate bathochromic
shifts (15-20 nm) compared to the reference dyes 1 and 2 (Figure 4b).
Please insert here Figure 4.
The presence of a substituent on the phenyl moiety (compounds 2-6) had little influence on the
emission spectra. These results showed that electron-donating group strength and position did
not modify the emission maximum, while a small bathochromic shift was observed in compound
6, bearing and electron donor group (Figure 5).
Please insert here Figure 5.
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Stokes’ shifts of compounds 1-11 were strongly influenced by the group bonded to the
imidazo[1,5-a]pyridine ring. The extension of the conjugation system in the side group increased
the Stokes’ shift from 99 nm for the phenyl in compound 2, to 129 nm for the naphthalene in
compound 9 and to 166 nm for the anthracene in compound 11. There was also a clear effect of
the substituent on the phenyl ring. Compound 5, bearing a CN group, showed the smallest
Stokes’ shift, while compound 6, bearing a dimethylamino group in the same position, showed a
Stokes’ shift up to 123 nm. The substituent influenced the fluorescence intensities. The 2-
methoxyphenyl group (compound 4) showed the maximum quantum yield (Φ = 0.385), which, to
our knowledge, is also the highest reported in literature among the imidazo[1,5-a]pyridines. With
the exception of the methoxy group of compound 4, all other substituent groups in position 3 on
the phenylimidazo[1,5-a]pyridine skeleton decreased the quantum yield compared to the methyl
group of compound 1 (Φ = 0.263) with the lowest yield observed in presence of the anthracene
mojety (Φ = 0.064).
As already discussed in the introduction section, compounds 3 and 10, bearing a hydroxyl group
in the ortho position of the 3-phenyl substituent, were intentionally synthesized in order to verify
a possible ESIPT effect on the emission properties. However, the comparison among the
quantum yields and emission spectra of compounds 2, 3 and 9, 10 highlighted the absence of
interactions between the hydroxyl group in the o-cresol and the naphthalen-2-ol substituents and
the 2-nitrogen of the imidazo[1,5-a]pyridine. The emission spectrum of the 2-naphthol group
(compound 10) was shifted of about 20 nm compared to the naphthalene substituent (compound
9), while the comparison between the phenyl (compound 2) and the phenol (compound 3) group
indicated no difference (Fig. 6).
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Please insert here Figure 6.
The solvent effect on the fluorescence behavior of all compounds was studied. Spectra of
compound 3 are reported as an example (Figure 7). Regarding the emission spectra, all
compounds showed a structured emission in cyclohexane solution around 450 nm with its
vibronic bands at about 480 nm and 510 nm. No significant differences were observed in the
emission peak maxima in both protic and aprotic solvents, while the acid aqueous solution
showed a significantly different profile. This interesting behavior is promising for a possible use
of this class of molecules as pH sensors [38].
Please insert here Figure 7.
Please insert here Table 4.
Please insert here Table 5.
3.4 Crystal Structure Analysis
Compound 2 crystallized after slow evaporation of acetone in the orthorhombic non-
centrosymmetric space group Cmc21 (see Fig. 8). This non-centrosymmetric space group was
rather unexpected, since compound 2 does not have any chiral center. It is worth pointing out
that, we attempted the solution of the structure both in symmetries lower than orthorhombic and
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in the centrosymmetric orthorhombic Cmcm but no solution was found. The R value was rather
good in Pn21a, similarly to Cmc21and the correlation coefficients and thermal factor showed no
anomalies, typically observed in the case of a wrong space group. The higher symmetry non-
centrosymmetric space group (i.e. Cmc21) with good agreement values was finally chosen. It
must be noticed that a non-centrosymmetric group is sometimes observed in non-chiral
molecules due to the crystal packing, as already observed for this class of compounds [21e]. The
asymmetric unit contains only half molecule; the entire molecule is generated by a symmetry
plane passing through the N2 atom and the N1-C3 bond. This symmetry implies an overall
positional disorder in the molecule, with two different positions for the N1/C3 atoms,
constrained to the same probability and both labelled in the same position in Figure 11a. The
overall crystal packing is substantially driven by Van der Waals interactions, since steric
hindrance of the phenyl moieties does not permit the π-π stacking of the aromatic systems.
Compound 4, used for dispersion in the polymeric resin, is very similar (only a methoxy group is
added) to compound 2, and it can be inferred that similar weak interactions characterize its
packing and clustering behavior. The presence of weak and non-polar interactions explains its
easy dispersion into the polyurethane resin, to produce a photoactive polymer. The main
molecule-polymer interactions should consist of the same weak contacts involving the aromatic
rings. However, looking at the molecular structure of 2 (Figure 8a), the nitrogen atom should be
accessible enough to accept weak CH•••N bonds when 4 is dispersed into polyurethane resin.
Please insert here Figure 8.
Please insert here Figure 9.
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Please insert here Figure 10.
Compound 7 crystallized from dichloromethane in yellow crystals in the monoclinic space group
P21/c. Two different non-equivalent molecules of the product and two disordered molecules of
the solvent co-crystallized during the crystal growth, as can be seen in the asymmetric unit
(Figure 9a). This disorder might explain the weak diffracting nature of this crystal. The overall
structure is built up by chains based on hydrogen bonds through the N-indolic hydrogen and the
imidazolic nitrogen of the imidazopyridine central backbone of the vicinal molecules (Figure
9b). These chains are isolated by the presence of the solvent molecules that seem not involved in
weak NH•••Cl hydrogen bonds. The presence of the phenyl group generates a separation of the
chains in layers bonded by dispersion forces only, without the presence of π-π stacking due to the
relevant steric hindrance. These relatively strong H-bonds are not perturbed by dichloromethane
and presumably also by other polar solvents. It might be inferred that the NH•••Cl interaction is
maintained also in solution of different polarity, thus explaining the similar position of emission
maxima in polar solvents (see Table 5) for compound 7.
Crystals of compound 11 were grown by slow evaporation of a dimethyl ether solution. All
tested crystals resulted twinned. In the best dataset, the presence of three individuals was
recognized (Table 2S) and twinning was treated by CrysAlisPro. In the measured sample one
individual was largely predominant and the simultaneous refinement of the four individuals with
the HKLF 5 option resulted unstable, so this method was discarded. The refinement was thus
completed on the extracted HKLF4.hkl file of the predominant individual. The structure was
initially solved in Pn, then an additional 2-fold screw axis was found by PLATON [39] and the
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space group changed in the more common centrosymmetric P21/n. The phenyl ring is free to
oscillate, as indicated by the presence of disorder that was treated using the PART command.
Compound 11 crystallized in a cell with two angles of 90° and the β angle close to 90° (Fig.10).
The crystal system is thus monoclinic but almost orthorhombic, with a lattice parameter
a≈1/2b≈1/2c. The individuals grew by exchanging “b” and “c”, which were similar, and
occasionally doubling the packing along “a”, as can be inferred from the orientation matrices
reported in table 2S in the SI file. The twinning, occurring through the above mentioned axes
exchange, was thus favored by the absence of directional interactions in the packing, held only
by weak interactions such as T-shape CH-π contacts. These contacts should also be favored in
polar solvent, inducing a molecular clustering, while in apolar solvents they should be
overwhelmed by solute-solvent interaction, with a more efficient dispersion. Such different
dispersion efficiency might contribute to the explanation of the different spectroscopic behavior
in different solvents. In fact, as can be seen in Table 4 and 5, the different solvents induce
different positions of the emission maximum.
3.5 Optical properties in a polymer host
Based on the data in Table 3, compound 4 was selected as a possible candidate for down-shifting
applications, due to the much higher quantum yield compared to the other molecules of the
series. This approach has also been recently proposed on another class of low cost fluorophores
[40].
To assess the photophysical properties of compound 4 for down-shifting applications, films with
the compound uniformly dissolved inside a polymeric medium were produced, in order to
evaluate its properties in the solid state. A commercial polyurethane resin already used for
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photovoltaic and lighting devices was used to produce a luminescent polymer (see Fig.12).
Spectra of such films are reported in Figure 11 and show that the absorption in the solid state did
not change compared to the dichloromethane solution, while the emission was blue-shifted of
about 15 nm when compared to the solution. Despite this blue shift, the overlap between the
absorption and emission peaks in the solid state resulted still negligible, which means that there
will be very little re-absorption of the emitted light from the compound itself.
Please insert here Figure 11.
Please insert here Figure 12.
4. Conclusions
Eleven 1,3-diarylated imidazo[1,5-a]pyridine derivatives were synthesized by means of the easy
to scale up one-pot condensation of phenyl(pyridin-2-yl)methanone with several aldehydes and
ammonium acetate. Reaction yields were moderate to excellent (from 28.2% for 11 to 81.6% for
5). All the phenylimidazo[1,5-a]pyridines obtained exhibited interesting optical properties in
solution, with a wide variety of fluorescent emissions (467-548 nm). These compounds showed
large Stokes’ shifts and, depending on the chemical structure of the substituent in position 3,
fluorescence quantum yields (Φ) in solution were easily tuned from 6.4% (compound 11) to
38.5% (compound 4). From the results of the optical studies we excluded the ESIPT fluorescence
mechanism in all the products. The crystal structures of 2, 7 and 11 were also solved, in order to
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gain insights about the intermolecular interactions, and their effect on the optical properties in
solution and when dispersed into a polymeric resin.
Finally, compound 4, showing the best compromise between stoke shift and high Φ, was
dissolved in a polyurethane resin to investigate its properties in the solid state. Despite a blue
shift of 15 nm when compared to a dichloromethane solution, the overlap between the absorption
and emission spectra was negligible, proving that it is an interesting low cost candidate for
down-shifting applications, also thanks to its large solubility and homogeneous dispersion in the
polymer.
On the basis of these promising emission properties, further studies are in progress to test these
phenylimidazo[1,5-a]pyridines derivatives as tunable low-cost fluorescent materials.
Acknowledgements
This work was supported by Piedmont Region FINPIEMONTE POR-FESR 2007/2013 project
“Dye Hard”. Dott. A. Menozzi and S.E. are acknowledged for the collaboration, fruitful
discussion and providing of polyurethane resin.
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