New unsymmetrical squaraine dyes derived from imidazo[1,5-A]pyridine

Article abstract of DOI:10.1016/j.dyepig.2012.10.006

The reaction of squaric acid with excess 3-methylimidazo[1,5-A]pyridine afforded almost exclusively the corresponding semisquaraine. The later underwent condensation with several heterocyclic compounds possessing active methyl groups, forming a novel class of unsymmetrical squaraines. The spectral properties of the obtained dyes were investigated and quantum-chemical calculations were performed to examine the dependence of the electronic structure and electron transitions on the molecular constitution of the dyes. The oxygen-generation ability of the new dyes was accessed by the 1,3-diphenylisobenzofuran (DPBF) quenching method, envisioning their potential use as sensitizers for photodynamic therapy (PDT).

Full text of DOI:10.1016/j.dyepig.2012.10.006

Accepted Manuscript
New unsymmetrical squaraine dyes derived from imidazo[1,5-a]pyridine
Yaroslav Prostota, Oleksiy D. Kachkovsky, Lucinda V. Reis, Paulo F. Santos
PII:
S0143-7208(12)00292-6
10.1016/j.dyepig.2012.10.006
DYPI 3751
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 9 August 2012
Revised Date: 3 October 2012
Accepted Date: 5 October 2012
Please cite this article as: Prostota Y, Kachkovsky OD, Reis LV, Santos PF, New unsymmetrical
squaraine dyes derived from imidazo[1,5-a]pyridine, Dyes and Pigments (2012), doi: 10.1016/
j.dyepig.2012.10.006.
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ACCEPTED MANUSCRIPT
O
1.0
0.5
0.0
1
O2
O
Se
+
N
N
N
Et
Me
O
300
400
500
600
700
800
Wavelength (nm)

ACCEPTED MANUSCRIPT
Highlights.
•
•
A semisquaraine derived from 3-methylimidazo[1,5-a]pyridine nuclei was obtained.
New unsymmetrical squaraine dyes containing the imidazo[1,5-a]pyridine ring were
synthesized.
•
•
Quantum-chemical calculations were performed for the dyes prepared.
The photostability of the dyes was assessed by simple fading tests.
•
All dyes possess inherent singlet oxygen generation ability.

ACCEPTED MANUSCRIPT
New unsymmetrical squaraine dyes derived from imidazo[1,5-a]pyridine
a
b
a
a*
Yaroslav Prostota , Oleksiy D. Kachkovsky , Lucinda V. Reis , Paulo F. Santos
a
Centro de Química - Vila Real, Universidade de Trás-os-Montes e Alto Douro, 5001-801 Vila Real, Portugal.
Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5 Murmanska str., 03660 Kyiv, Ukraine
b
Abstract
The reaction of squaric acid with excess 3-methylimidazo[1,5-a]pyridine afforded almost
exclusively the corresponding semisquaraine. The later underwent condensation with several
heterocyclic compounds possessing active methyl groups, forming a novel class of unsymmetrical
squaraines. The spectral properties of the obtained dyes were investigated and quantum-chemical
calculations were performed to examine the dependence of the electronic structure and electron
transitions on the molecular constitution of the dyes. The oxygen generation ability of the new dyes
was accessed by the 1,3-diphenylisobenzofuran (DPBF) quenching method, envisioning their
potential use as sensitizers for photodynamic therapy (PDT).
Keywords: unsymmetrical squaraines, imidazo[1,5-a]pyridine, semisquaraines, singlet oxygen,
PDT
1
. Introduction
Squarylium dyes, also called squaraines, are zwitterionic donor-acceptor-donor (D-A-D)
compounds bearing a central acceptor squaric ring 1,3-disubstituted with electron-rich aromatic or
heteroaromatic nuclei. Compounds from this class of dyes typically display sharp and intense
5
-1
-1
absorption bands (ε ~ 10 M cm ) in the red to near-infrared region [1-3]. Their unique
photophysical and photochemical properties turns them highly appropriated for a number of
applications [2, 3], both in the technological (e.g. photoreceptors in copiers [4], photoconductors in
organic solar cells [5-7], IR absorbers in organic optical disks [8]) and biological fields (e.g. metal
ion sensors [9], long wavelength fluorescent labels [10-12], reagents for the detection of thiols
*
Corresponding author: email address: psantos@utad.pt. Tel. (351)259350257. Fax. (351)259350480.

ACCEPTED MANUSCRIPT
[
13]). The intense absorption in the so-called phototherapeutic window (600-850 nm), due to the
intramolecular charge-transfer (CT) character of the S0-S1 electronic transition, combined with an
extended conjugated π-electron network, also makes squaraines promising sensitizers for
photodynamic therapy (PDT) [14].
The imidazo[1,5-a]pyridines are an important class of 10π-electron fused nitrogen-
containing heterocyclic compounds due to their biological activity [15-18] and special
photophysical properties. The later have turned them, for example, potential materials for organic
light-emitting diodes (OLED) [19] and organic field-effect transistors (OFET) [20]. Their use as
metal ligands [21, 22] and as precursors of N-heterocyclic carbenes [23, 24] has also been reported.
To the best of our knowledge neither polymethine nor squaraine dyes based on this
heterocyclic system have hitherto been described. Earlier, Pagani and co-workers reported the
synthesis of squaraine 1 (Fig. 1) based on the indolizine (pyrrolo[1,2-a]pyridine) ring, a structural
analogue of the imidazo[1,5-a]pyridine system, and find it to be a potential candidate as non-
porphyrinic photosensitizer for PDT [25]. We anticipated that imidazo[1,5-a]pyridine-derived
squaraines may likewise exhibit interesting singlet oxygen generation ability and envisioned the
synthesis of several squaraine dyes starting from 3-methylimidazo[1,5-a]pyridine.
Figure 1
2
. Experimental
2
.1. General
All reagents were purchased from Aldrich and used as received. Solvents were of analytical
grade. Acetic anhydride was distilled prior to use. The quaternary heterocyclic slats were prepared
according to the usual methods described in the literature [26]. All reactions were monitored by
thin-layer chromatography on aluminium plates coated with Merck silica gel 60 F254 (0.25 mm).
1
13
Column chromatography was performed on Merck silica gel 60 (70-230 mesh). H and C NMR
spectra were recorded at 298 K on a Bruker ARX400 spectrometer (at 400.13 and 100.62 MHz).

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Chemical shifts (δ) are reported in ppm and coupling constants (J) in Hz. IR spectra were obtained
on a Unicam Research Series FTIR spectrometer using KBr pellets. Wavenumbers (νmax) are
-1
reported in cm . UV-Vis spectra were recorded on a Perkin-Elmer Lambda 25 spectrophotometer in
spectral grade acetonitrile and dichloromethane. High resolution electrospray ionization time-of-
flight (ESI-TOF) mass spectra and electron impact time-of-flight (EI-TOF) mass spectra were
measured with a VG AutoSpec M spectrometer. Melting points were determined in open capillary
tubes in a Buchi 530 melting point apparatus and are uncorrected. Light fluence rate was measured
with an International Light IL 1400 radiometer equipped with a SEL033 probe.
2
2
.2. Synthesis of compounds
.2.1. 3-Methylimidazo[1,5-a]pyridine (3).
A solution of 2-picolylamine (2) (5.00 g, 4.63 mmol) and p-toluenesulfonic acid monohydrate (8.34
g, 4.63 mmol) in acetic anhydride (25 mL) was heated at 100 ºC for 3h. After cooling to room
temperature, the reaction mixture was poured into water (100 mL), made alkaline with 20%
aqueous NaOH and the resulting mixture was extracted with CH Cl (3x35 mL). The combined
2
2
2 4
organic layers were washed with water (50 mL), dried over anhydrous Na SO and evaporated to
dryness under reduced pressure to afford 3 (5.54 g, 91%), as a sticky yellow-greenish residue which
solidified on standing. Mp 54-56 ºC (dec.). IR νmax: 663, 732, 782, 1000, 1359, 1399, 1489, 1640.
1
H NMR (CDCl
3
3
) δ: 2.54 (3H, s, CH ), 6.39–6.43 (1H, m, ArH), 6.51–6.55 (1H, m, ArH), 7.25–
1
3
7
1
.30 (2H, m, ArH), 7.53 (1H, d, J = 7.1, ArH). C NMR (CDCl
3
) δ: 12.2, 111.8, 117.1, 117.9,
+
18.0, 120.2, 129.9, 134.5. MS (EI-TOF) m/z (%BPI): 64 (13), 104 (7), 127 (5), 132 (100, M ).
2
.2.2. 3-Hydroxy-4-(3-methylimidazo[1,5-a]pyridin-1-yl)cyclobut-3-ene-1,2-dione (5).
Squaric acid (0.43 g, 3.8 mmol) in a mixture of butan-1-ol/toluene (1/1, v/v) (25 mL) was heated
under reflux, with azeotropical removal of water, for 30 min. Then a solution of 3 (1.00 g, 7.6
mmol) in butan-1-ol/toluene (1/1, v/v) was added and the reaction mixture was maintained at 100
2
ºC for 5 h. The solid residue formed on cooling was filtered off and washed with Et O (15 mL) to

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o
afford 5 (0.30 g, 40%), as a yellow solid. Mp > 300 C. IR νmax: 755, 863, 1091, 1226, 1590, 1775,
1
3
036, 3417. H NMR (DMSO-d
6
) δ: 2.86 (3H, s, CH
3
), 7.15 (1H, t, J = 6.5, ArH), 7.20-7.24 (1H,
1
3
m, ArH), 8.37 (1H, d, J = 7.2, ArH), 8.67 (1H, d, J = 9.2, ArH). C NMR (DMSO-d
6
) δ: 10.6,
1
2
14.7, 117.5, 121.3, 123.3, 123.7, 126.3, 137.2, 164.3, 193.3, 212.0. HRMS (ESI-TOF) m/z:
+
9 2 3
29.06011 ([M+H] ; calc. for C12H N O : 229.06077).
2
.2.3. 4-[(1,3,3-Trimethylindol-1-ium-2-yl)methylidene]-2-(3-methylimidazo[1,5-a]pyridin-1-yl)-3-
oxocyclobut-1-en-1-olate (6a).
A solution of 5 (0.034 g, 0.15 mmol) and 1,2,3,3-tetramethylindolium iodide (0.045 g, 0.15 mmol)
in a mixture of butan-1-ol/pyridine (10/1, v/v) (10 mL) was heated under reflux for 5 h. After
cooling to room temperature, the mixture was evaporated to dryness under reduced pressure. The
3
resulting residue was purified by column chromatography (CHCl /MeOH, 9/1, v/v) to afford 6a
º
(
0.038 g, 67%), as a blue solid. Mp 254-256 C. IR νmax: 632, 756, 796, 972, 1059, 1258, 1444,
1
1
521, 1602, 3415. H NMR (CDCl
3 3 2 3
) δ: 1.82 (6H, s, C(CH ) ), 2.74 (3H, s, CH ), 3.73 (3H, s,
NCH
3
), 6.17 (1H, s, CH=C), 6.87 (1H, t, J = 6.5, ArH), 7.13-7.16 (2H, m, ArH), 7.25-7.28 (1H, m,
1
3
ArH), 7.36-7.42 (2H, m, ArH), 7.83 (1H, d, J = 7.0, ArH), 9.18 (1H, d, J = 9.0, ArH). C NMR
(CDCl
3
) δ: 12.8, 26.5, 29.6, 31.8, 50.5, 89.3, 110.5, 115.4, 122.1, 122.4, 123.1, 123.7, 124.3, 125.5,
+
1
28.1, 136.1, 141.2, 142.3, 142.4, 170.7, 175.3, 187.5. HRMS (ESI-TOF) m/z: 384.17053 ([M+H] ;
22 3 2
calc. for C24H N O : 384.17065).
2
.2.4. 4-[(3-Ethylbenzothiazol-3-ium-2-yl)methylidene]-2-(3-methylimidazo[1,5-a]pyridin-1-yl)-3-
oxocyclobut-1-en-1-olate (6b).
A solution of 5 (0.15 g, 0.68 mmol) and 3-ethyl-2-methylbenzothiazolium iodide (0.23 g, 0.68
mmol) in a mixture of butan-1-ol/pyridine (10/1, v/v) (25 mL) was heated under reflux for 5 h.
After cooling to room temperature the mixture was evaporated to dryness under reduced pressure.
3
The residue was purified by column chromatography (CHCl /MeOH, 9/1, v/v) to afford 6b (0.13 g,
º
3
8%), as a greenish-blue solid. Mp 273-275 C. IR νmax: 750, 1082, 1264, 1449, 1516, 1602, 3426.

ACCEPTED MANUSCRIPT
1
H NMR (DMSO-d
NCH CH ), 6.32 (1H, s, CH=C), 6.90 (1H, t, J = 6.9), 7.09 (1H, t, J = 6.6, ArH), 7.51 (1H, t, J =
.8, ArH), 7.64 (1H, t, J = 8.3, ArH), 7.92 (1H, d, J = 8.5, ArH), 8.16 (1H, d, J =7.9, ArH), 8.22
6 2 3 3
) δ: 1.37 (3H, t, J = 7.2, NCH CH ), 2.64 (3H, s, CH ), 4.61 (2H, brq, J = 7.2,
2
3
7
1
3
(
1H, d, J = 7.1, ArH), 8.92 (1H, d, J = 9.1, ArH). C NMR (DMSO-d
6
) δ: 12.3, 12.9, 42.2, 89.5,
1
1
3
14.1, 114.3, 121.2, 122.3, 122.6, 123.3, 125.9, 128.1, 128.9, 132.7, 139.6, 140.0, 161.8, 164.4,
+
78.2, 179.1, 184.4. HRMS (ESI-TOF) m/z: 388.11085 ([M+H] ; calc. for C22
H
18
N
3
O
2
S:
88.11142).
2
3
.2.5. 4-[(3-Ethylbenzoselenazol-3-ium-2-yl)methylidene]-2-(3-methylimidazo[1,5-a]pyridin-1-yl)-
-oxocyclobut-1-en-1-olate (6c).
A solution of 5 (0.10 g, 0.44 mmol) and 3-ethyl-2-methylbenzoselenazolium iodide (0.17 g, 0.44
mmol) in a mixture of butan-1-ol/pyridine (10/1, v/v) (25 mL) was heated under reflux for 5h. After
cooling to room temperature, the mixture was evaporated to dryness under reduced pressure. The
3
residue was purified by column chromatography (CHCl /MeOH, 9/1, v/v) to afford 6c (0.11 g,
º
5
1
8%), as a greenish-blue solid. Mp 194-196 C. IR νmax: 746, 978, 1076, 1258, 1313, 1444, 1516,
1
590, 3412. H NMR (DMSO-d
CH ), 6.39 (1H, s, CH=C), 6.90 (1H, t, J = 6.4, ArH), 7.09-7.13 (1H, m, ArH), 7.41
1H, t, J = 7.6, ArH), 7.58 (1H, t, J = 7.3, ArH), 7.84 (1H, d, J = 8.4, ArH), 8.18 (1H, d, J = 7.3,
6 2 3 3
) δ: 1.36 (3H, t, J = 7.0, NCH CH ), 2.64 (3H, s, CH ), 4.59 (2H,
q, J = 7.0, NCH
2
3
(
1
3
ArH), 8.23 (1H, d, J = 7.0, ArH), 8.90 (1H, d, J = 9.0, ArH). C NMR (DMSO-d
6
) δ: 12.1, 12.7,
4
1
2.8, 92.9, 114.1, 115.6, 121.1, 122.3, 122.6, 123.2, 125.5, 126.3, 127.7, 130.6, 132.8, 139.6, 141.4,
+
62.0, 169.7, 178.8, 179.1, 184.8. HRMS (ESI-TOF) m/z: 436.05478 ([M+H] ; calc. for
C
22
H
18
N
3
O
2
Se: 436.05597).
2
.2.6.
4-[(3-Ethylquinolin-3-ium-4-yl)methylidene]-2-(3-methylimidazo[1,5-a]pyridin-1-yl)-3-
oxocyclobut-1-en-1-olate (6d).
A solution of 5 (0.10 g, 0.44 mmol) and 1-ethyl-4-methylquinolinium iodide (0.13 g, 0.44 mmol) in
a mixture of butan-1-ol/pyridine (10/1, v/v) (25 mL) was heated under reflux for 5h. After cooling

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to room temperature the mixture was evaporated to dryness under reduced pressure. The residue
was purified by column chromatography, eluting sequentially with CHCl
3
/MeOH, 9/1 (v/v),
CHCl
3
/MeOH, 2/1 (v/v) and finally CHCl /MeOH, 1/1 (v/v), to afford 6d (0.060 g, 36%), as a
3
º
green solid. Mp > 300 C. IR νmax: 678, 755, 854, 1076, 1168, 1221, 1249, 1380, 1435, 1527, 1576,
1
3
7
425. H NMR (DMSO-d
6
) δ: 1.50 (3H, t, J = 7.1, NCH
2 3 3
CH ), 2.63 (3H, s, CH ), 4.71 (2H, q, J =
.1, NCH
2
CH
3
), 6.72 (1H, s, CH=C), 6.83 (1H, t, J = 7.3, ArH), 6.96-7.00 (1H, m, ArH), 7.77 (1H,
t, J = 7.6, ArH), 8.02 (1H, t, J = 8.6, ArH), 8.13 (1H, d, J = 7.1, ArH), 8.22 (1H, d, J = 8.8, ArH),
8
6
1
.63 (1H, d, J = 8.0, ArH), 8.76 (1H, d, J = 7.0, ArH), 8.88 (1H, d, J = 9.0, ArH), 9.33 (1H, d, J =
13
6
.9, ArH). C NMR (DMSO-d ) δ: 12.2, 14.8, 50.1, 99.1, 113.8, 118.1, 120.8, 121.2, 122.8, 123.1,
25.3, 126.0, 127.4, 131.2, 133.6, 137.6, 138.7, 143.5, 150.8, 155.7, 180.0, 181.2. HRMS (ESI-
+
TOF) m/z: 382.15552 ([M+H] ; calc. for C24
H
20
N
3
O
2
: 382.15500).
2
.3. Evaluation of the singlet oxygen generation ability
-5
-6
Solutions of DPBF (5ꢀ10 M) and dyes 6a-c (5ꢀ10 M) in dichloromethane were prepared and
saturated with oxygen for 3 min. Each oxygenated solution was placed into a 1cm quartz cell and
irradiated under gentle magnetic stirring with a 24V, 250 W halogen lamp (Osram) for different
time intervals. The light was filtered with an Oriel 59494 optical filter (λ > 515 nm) and the fluence
-2
rate at the surface of the solution was adjusted to 6.3-6.8 mWcm .
3
. Results and discussion
3
.1. Synthesis
The starting 3-methylimidazo[1,5-a]pyridine (3) was readily obtained by cyclocondensation
of commercially available 2-picolylamine (2) with acetic anhydride, in the presence of an equimolar
amount of p-toluenesulfonic acid monohydrate [27].
Aiming at obtaining squaraine dye 4, the condensation of 3 with half molar equivalent of
squaric acid was carried under the reaction conditions commonly employed for the preparation of
symmetrical squaraine dyes (e.g. reflux in butan-1-ol/toluene or butan-1-ol/pyridine) [3].

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Unexpectedly, all attempts to obtain dye 4 resulted in the formation of semisquaraine 5 as the major
reaction product, while the desired compound was detected only in trace amounts (Scheme1).
Scheme 1
The usual synthesis of symmetrical squarylium dyes involves the one-pot condensation of
electron-rich aromatic or heterocyclic compounds with squaric acid in a solvent mixture containing
butan-1-ol. Butylsquarate (3-butoxy-4-hydroxy-1,2-dioxocyclobut-3-ene) was proposed as the
reactive intermediate [28, 29], firstly reacting with one molecule of the (hetero)aromatic compound
to give an intermediate semisquaraine that subsequently undergoes a second condensation with
another molecule of the same (hetero)aromatic compound to yield the final squaraine dye. The
success of the reaction depends primarily on the nucleophilicity of the (hetero)aromatic species [30,
3
1]. In the present case the electron-donating ability of the saturated nitrogen atom in the
imidazo[1,5-a]pyridine nucleus rends the squaric ring less reactive towards an additional molecule
of 3, which, in turn, is not nucleophilic enough to complete the reaction, that consequently stops at
the semisquaraine formation. On the basis of the possible resonanse structures for the imidazo[1,5-
a]pyridine nucleus, a discrete electron density is expected both on the imidazolic nitrogen and on
the carbon close to it, but only the constitutional isomer 3 was isolated and unambiguously
confirmed structurally.
A similar failure in obtaining a squaraine dye under the usual conditions employed for the
synthesis of symmetrical squaraines has been observed in the reaction of 5-hydroxy- and 5-
ethoxyquinaldinium iodides [32] with squaric acid. However, therein the semisquaraine was
isolated as the butoxy-substituted 1,3-regioisomer [33]. The isolation of a simple unsubstituted
semisquaraine intermediate under the reaction conditions used for the preparation of symmetrical
squaraine dyes is very uncommon and, to the best of our knowledge, has only been briefly
mentioned in the condensation of antipyrine and 3-aminopyridine with squaric acid [28]. Such type
of intermediate is found in the conventional procedure for the synthesis of unsymmetrical squaraine

ACCEPTED MANUSCRIPT
dyes, resulting from the deliberated basic hydrolysis of the intermediate semisquaraine alcoxy-
substituted 1,2-regioisomer, obtained from the addition of one molar equivalent of the aromatic
nucleophile to a starting dialkylsquarate [34, 35].
Semisquaraine 5 was found to react easily with the nucleophilic enamines generated from
2
,3,3-trimethylindolinium, 2-methylbenzothiazolium, 2-methylbenzoselenazolium and 4-
methylquinolinium iodides in butan-1-ol/pyridine - 10/1 (v/v), forming the corresponding
unsymmetrical dyes 6a-d, which were isolated in good to moderate yields (Scheme 1).
Dyes 6a-d are the first representatives of a new type of skeleton of unsymmetrical squaraines.
In addition to the unprecedented presence of an imidazo[1,5-a]pyridine end group, these dyes
possess one unsaturated heterocyclic moiety directly connected to the central squaric ring whilst the
other heterocyclic nucleus is bonded through an usual methyne bridge.
3
.2. Structure and molecular modeling
Quantum-chemical calculations were performed for all the synthesized dyes in order to study
the dependence of the electronic structure and electronic transitions on the molecular constitution.
The equilibrium geometry of the dye molecules in the ground state was optimized by the non-
empirical HF/6-31G(d,p) and DFT/6-31G(d,p)/B3LYP methods (package Gaussian 03 [36]). The
electron transition characteristics were calculated by the TD DFT and semi-empirical ZINDO
methods. Although there is not a complete concurrence between calculated and experimental data, it
is adequate to a correct analysis of the nature of the electronic transitions. The calculations of the
equilibrium molecular geometry showed the conjugated part of all molecules to be essentially
planar, a typical feature of π-electron systems. Only the methyl groups of the indolenine fragment
in dye 6a are out of the plane of the main chromophore (Fig. 2). The optimized lengths of the C-C
bonds in the central four-member ring are somewhat longer than those of the C-C bonds in the open
chain.
Figure 2

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Because of the unsymmetrical constitution of the dyes, the lengths of the bonds involving
squaric C2 and C4 atoms and the adjacent carbons (bonds 1 and 2) differ from each other.
According to the HF method, the difference between these bond lengths, ꢁl (1-2), increases upon
increasing the end groups basicity in the order: indolenine (6a) > benzoselenazole (6c) >
benzothiazole (6b) > quinoline (6d) (Table 1). The DFT method, by taking into consideration the
electron correlation, gives lower values for ꢁl (1-2). Using this approach, the dependency of the
asymmetry parameter ꢁl (1-2) on the basicity of the end nuclei is not confirmed. The two
approximations (HF and DFT) show different trends for the lengths of the neighboring bonds 2 and
3
, what points out to low asymmetry in the polymethine chain. It can be proposed that the
difference in electronic transitions is mainly determined by the chemical constitution of the end
groups and hence by their electron levels positions.
Table 1
3
.3. Spectral properties and the nature of the electronic transitions
The absorption spectra of the obtained dyes were measured in acetonitrile and methylene
chloride, an all exhibit intense bands in the range 600-700 nm, as typical for squaraines. When
compared to the correspondent symmetrical analogues [34], compound 6a displays absorption at
approximately the same wavelength, while that of 6b and 6c is shifted hypsochromically (38 and 36
nm, respectively), a characteristic behavior of unsymmetrical squaraine dyes [34, 35] (Table 2).
Table 2
It was established earlier that end groups possessing highly localized orbitals produce the
main contributions to the highly occupied MOs of cyanine dyes with relatively short polymethine
chains [37-40]. Among short chain cationic cyanine dyes and their squaraine analogues, the local

ACCEPTED MANUSCRIPT
MOs of the end groups are positioned higher than the highest delocalized orbital of the polymethine
chain. It was ascertained that the two highest occupied MOs can be considered to be generated
mainly from the highest MOs of both end residues conjugated with the delocalized π-electron
system of the polymethine chain and, therefore, these MOs can be treated as donor orbitals [37-40].
The donor MOs φ (D
1 2
) and φ (D ) interact with each other to produce two new MOs resulting from
the symmetrical (i) and unsymmetrical (ii) linear combinations:
-1/2
-1/2
ϕ1 = (2) {φ (D ) + φ (D )} (i)
ϕ2 = (2) {φ (D ) – φ (D )} (ii)
1 2
1
2
Figure 3 shows schematically the splitting of the donor levels both in a symmetrical and an
unsymmetrical dye. As the energies of the donor levels in unsymmetrical dyes should be dissimilar
1 2
(D ≠ D ), the difference in the positions of the initial donor orbitals of the two end groups should
determine the degree of asymmetry. Taking into consideration that an interaction of the energy
levels is inversely proportional to the distance between them, the splitting energy (∆Espl) decreases
in unsymmetrical dyes. In squaraines 6a-d, the imidazo[1,5-a]pyridine nucleus is the less basic of
2
the end groups and, consequently, its donor level is positioned lower (D ).
Figure 3
The scheme in figure 3 is strongly simplified. The initial donor MOs interact with the
delocalized MOs of both the open chain and the squaric ring. As a result, the direct quantum-
chemical calculations for the unsymmetrical dyes gave considerable delocalization of both the
highest occupied MO (HOMO) and the next orbital (HOMO-1). The increase of the basicity of the
end groups is accompanied by shifting up of the splitting levels (HOMO and HOMO-1).
Besides the delocalized MOs, local orbitals in the unsymmetrical squaraine dyes also exist. It
is shown that the two n-orbitals (HOMO-2 and HOMO-4) are located nearer to the energy gap
(these orbitals may take part in the electronic transitions responsible for the appearance of the long-
wavelength absorption bands) (Fig.4). The orbitals situated at the 4-membered ring (LUMO+5 in
dye 6a and LUMO+6 in dye 6d) are positioned too high to be involved in the lowest electron

ACCEPTED MANUSCRIPT
transitions. At the same time, the levels of the n-MOs are positioned not far from the energy gap
and hence could be involved in electron transitions which are connected to the absorption spectra of
dyes in the visible and/or UV.
It was shown that the lowest electron transitions in symmetrical and unsymmetrical
polymethine dyes and squaraines possessing the typical Brooker’s groups (indolenine,
benzothiazole, pyridine, quinoline, etc.) are described mainly by single excited configurations [37-
4
3]. The calculations confirmed this regularity for the unsymmetrical squaraines 6a-d. Some lowest
single excited transitions involving the frontier and nearest levels are presented in Figure 4 and
Table 3. All lowest transitions are practically “pure”, e.i., they are described by one single excited
configuration.
Figure 4
Table 3
There is a reasonable good agreement between the experimental position of the maximum of
the longest wavelength absorption band, λmax, and the calculated ZINDO value of λ for the first
electron transitions, whereas the non-empirical TD DFT method shows considerable divergence
between calculated and spectral data. The analysis of the agreement between calculated and
experimental results for the higher transitions is turned difficult by the poorly defined bands in the
short wavelength spectral region. Nevertheless, the spectra of dyes 6a and 6d (Fig. 5) demonstrate
the considerable distance between the intense long-wavelength band (625 and 694 nm, respectively)
and the weaker broad band in the short-wavelength region (approximately between 300 and 400
1 2
nm), what is confirmed by the calculations for the π→π* transitions (ꢁλ12 = λ - λ ).
Figure 5

ACCEPTED MANUSCRIPT
The ZINDO method gives ꢁλ12 ≈ 250-300 nm, while the DFT method gives lower values:
λ12 ≈ 140-180 nm. Both approximations point out correctly the ratio of the oscillator strengths for
ꢁ
the first and second π→π* transitions, what agrees with the high intensity of the first spectral band
and with the low intensity of the short-wavelength one.
An important result is connected to the fact that n→π* electron transitions involve the lone
electron pair of the oxygen atoms. The TD DFT method predicts the n→π* transition energy to be
even lower than the energy of the first π→π* transition in dyes 6a, 6b and 6d. In contrast, the semi-
empirical ZINDO method envisages that the energy of the n→π* transition exceeds significantly
the energy of the lowest π→π* transition for all dyes, what should be more realistic because the
non-empirical approximation points to a considerable divergence between the calculated energy of
the first transition and the experimental value obtained from the absorption spectra. It was recently
found that the increase of the n-MOs energies and hence the decrease of the n→π* transitions
energy, in squaraines, can be reached by the substitution of oxygen by sulfur atoms [43].
The calculations concerning the lowest electron transitions of dyes 6a-d (Table 3) confirm the
assumption about the comparatively high transition energies involving the MO localized in the π-
0 5
system flanked by the oxygen atoms of the central squaric ring [S → S transition in dyes 6a and
6
0 6
b; S → S transition in dye 6d (ZINDO method)].
3
.4. Photochemical stability
In order to be useful for a number of applications a dye must display good photochemical
stability. Thus, the photostability of all prepared dyes in acetonitrile was investigated. The
quinoline-derived dye 6d showed to undergo decomposition quite rapidly (complete decomposition
was achieved within 5 h) and, for that reason, the fading tests were performed only for dyes 6a-c.
The dyes absorption at the corresponding λmax was measured after 60, 96 and 120 h of exposure to
indirect sun light and it was compared to that of the unexposed dye (table 4). It was observed that
the lightfastness of squaraines 6a-c decreases in the order: indolenine (6a) > benzothiazole (6b) >

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benzoselenazole (6c) >> quinoline (6d), which is in agreement with previously described data [34,
4
4].
Table 4
3
.5. Evaluation of singlet oxygen generating ability
Having addressed the absorption and the photochemical stability issues, the evaluation of the
singlet oxygen generation efficiency of the synthesized dyes was next undertaken using the 1,3-
diphenylisobenzofuran (DPBF) quenching method. DPBF forms a colorless endoperoxide upon
cycloaddition with singlet oxygen, resulting in the disappearance of its characteristic absorption
-5
-6
band at 415 nm [45]. Thus, a solution of DPBF (5ꢀ10 M) and photosensitizers (5ꢀ10 M) in
dichloromethane was irradiated with a filtered light (λ > 515 nm), for different time intervals, until
a significant decrease of the quenchers and/or dyes absorption was attained.
In the case of 6a the absorption did not significantly changed within 60 min, while about half
of DBPF was decomposed. Further irradiation led only to the slow simultaneous degradation of
both the dye and DPBF. For dye 6b irradiation over the same period of time resulted in a noticeable
degradation of both squaraine and quencher; additional irradiation caused essentially the complete
decomposition of the dye. Squaraine 6c promoted the complete decay of DBPF absorption within
2
6 min, but a significant self degradation was also observed (Fig.6).
The decay of dyes absorption is likely to result from the reaction with the singlet oxygen
produced upon irradiation, which turns out to be more probable as the sensitizers and DPBF
concentrations become comparable.
Figure 6
The decrease of DPBF absorption at 415 nm as a function of the irradiation time for each dye
compared to that promoted by 3,7-bis(N,N-dimethylamino)phenothiazin-5-ium chloride (Methylene

ACCEPTED MANUSCRIPT
Blue), a well-known and very efficient long-wavelength singlet oxygen generator [46], shows that
squaraines 6a-c do possess singlet oxygen generation ability (Fig. 7). The later is more acquitted for
the benzoselenazole-derived dye 6c, due to the inherent heavy atom effect.
Figure 7
4
. Conclusions
Attempts to obtain a symmetric squarylium dye derived from 3-methylimidazo[1,5-
a]pyridine led almost exclusively to the formation of the corresponding semisquaraine. This was
found to undergo base-catalysed condensation with quaternary salts derived from 2,3,3-
trimethylindolenine, 2-methylbenzothiazole, 2-methylbenzoselenazole and 4-methylquinoline to
yield the corresponding unsymmetrical squaraines, possessing a new skeleton among this family of
dyes.
Quantum-chemical calculations of the obtained dyes shown that the asymmetry degree in
the charge distribution and bond lengths is negligible due to the influence of the central squaraine
(acceptor) fragment. The nature of the lowest two electron transitions is the same as in the
correspondent cationic dicarbocyanines, both involving two HOMOs and the same LUMO.
By performing simple fading tests it was found that the photostability of the dyes in
acetonitrile strongly depends on the end group, decreasing in the order: indolenine > benzothiazole
>
benzoselenazole >> quinoline.
The evaluation of the singlet oxygen sensitization ability of the dyes by the DBPF
quenching method showed that all dyes are capable of producing singlet oxygen upon irradiation.
The singlet oxygen generation efficiency is particularly interesting for dye 6c, when compared to
that of Methylene Blue.
The sharp and intense long-wavelength absorption bands and the inherent singlet oxygen
generation capacity, make some of the synthezised dyes potential candidates as new non-
porphyrinic photosensitizers for PDT.

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Acknowledgements
FCT (Portugal’s Foundation for Science and Technology) and FEDER are acknowledge for
financial support through the research unit Centro de Química - Vila Real (POCTI-SFA-3-616),
program Ciência 2008. The authors are also grateful to FCT for financing through the research
project PTDC/QUI-QUI/100896/2008.
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O
+
N
N
O
N
N
O
X
+
N
N
X
4
_
HO
HO
O
I
R
N
N
+
(
trace amounts)
O
HOTs
O
n-BuOH : Py
R
10 : 1
Ac O, 1000
2
C
6a-c
N
+
N
2
n-BuOH : PhMe
NH2
N
a: R = Me, X = CMe
2
1 : 1
b: R = Et, X = S
c: R = Et, X = Se
HO
3
O
N
N
O
5
+
O
N
_
I
N
n-BuOH : Py
0 : 1
1
N
O
+
N
6
d
Scheme 1. Synthesis of squaraine dyes.

ACCEPTED MANUSCRIPT
Table 1
Bond lengths in the polymethine chain of dyes 6a-d (bond numeration showed in Fig. 2).
Dye
Bond 1
HF
Bond 2
HF
∆l (1-2)
HF
Bond 3
∆l (2-3)
DFT
1.407
1.407
1.408
1.408
DFT
1.395
1.383
1.387
1.388
DFT
0.012
0.024
0.021
0.020
HF
DFT
1.388
1.392
HF
0.009
DFT
6
6
a
1.419
1.419
1.420
1.424
1.391
1.386
1.384
1.381
0.018
0.033
0.036
0.043
1.382
1.384
1.385
1.395
0.007
b
0.002 -0.009
6
c
1.394 -0.001 -0.007
1.405 -0.014 -0.017
6
d

ACCEPTED MANUSCRIPT
Table 2
Absorption maxima (λmax) and molar extinction coefficient (ε) of squaraines 6a-d (c = 2⋅10 M).
-5
CH
3
CN
CH
2
Cl
2
Dye
-1
-1
-1
-1
λ
max (nm)
ε (M cm )
151000
81000
λ
max (nm)
ε (M cm )
157000
81500
a
6
6
a
578 (sh), 625
620
584 (sh), 631 [637]
a
b
632 [670]
a
6
c
632
89500
642 [678]
72000
b
b
6
d
584 (sh), 668
43000
694
31500
a
Absorption maxima of the parental symmetrical squaraines (based on indolenine, benzothiazole and benzoselenazole
b
2 2
residues) [34]. In CH Cl /MeOH – 9/1 (v/v) due to low solubility.

ACCEPTED MANUSCRIPT
Table 3
Absorption maxima (λmax) and oscillator strengths (f) of the lowest electron transitions of dyes 6a-d
calculated by the TD DFT and ZINDO methods.
TD DFT
ZINDO
f
Dye Transition
Main
Main
configuration
0.96 H→L>
λmax (nm)
f
λmax (nm)
configuration
a
a
a
c
S
S
S
0
0
0
→ S
→ S
→ S
1
2
3
575
0.000
1.182
0.014
0.99 H-1→L> 644 (631) 1.3334
a
a
b
a
b
a
542
416
393
357
331
0.99 H→L>
424
0.0001
0.0065
0.0024
0.0002
0.0706
0.96 H-2→L>
0.97 H→L+1> 387
0.93 H→L+1>
0.93 H-4→L>
0.89 H-2→L+6>
0.68 H-1→L>
0.96 H→L>
6
a
S0 → S4
0.0001 0.99 H-3→L> 374
b
b
S
0
0
→ S
5
6
0.166
0.000
0.000
1.105
0.006
0.85 H-2→L> 349
S
→ S
0.98 H-1→L+1> 342
a
a
a
c
S0 → S1
587
0.99 H-1→L> 652 (632) 1.4019
a
a
S
S
S
0
0
0
→ S
→ S
→ S
2
3
4
564
425
394
360
344
568
0.99 H→L>
433
0.0000
0.0068
0.0033
0.0001
0.0378
0.97 H-2→L>
0.97 H→L+1> 386
0.92 H→L+1>
0.92 H-4→L>
0.89 H-2→L+5>
0.75 H-1→L>
6
b
0.0002 0.99 H-3→L> 381
b
b
S0 → S5
0.007
0.028
1.062
0.006
0.002
0.85 H-2→L> 352
0.98 H-2→L+1> 345
S
0
S
0
S
0
→ S
→ S
→ S
6
1
2
c
0.98 H→L>
(642)
a
a
a
560
0.99 H-1→L>
S0 → S3
424
385
351
350
0.97 H→L+1>
d
6
c
S
0
0
→ S
4
5
0.0004 0.99 H-3→L>
S
→ S
0.010
0.010
0.000
1.076
0.020
0.93 H→L+2>
S0 → S6
0.86 H-2→L>
a
a
a
S
0
S
0
S
0
→ S
→ S
→ S
1
2
3
623
0.99 H-1→L> 718 (694) 1.4780
0.96 H→L>
a
a
a
622
444
412
406
0.98 H→L>
433
0.0000
0.0220
0.0031
0.0223
0.0002
0.95 H-2→L>
0.97 H→L+1> 402
0.75 H→L+1>
0.93 H-4→L>
0.85 H-1→L>
6
d
S0 → S4
0.0002 0.98 H-3→L> 380
S
0
0
→ S
5
6
0.002
0.127
0.91 H→L+2> 360
0.96 H→L+3> 357
b
b
b
S
→ S
380
0.71 H→L+5>
a
b
c
Data for n→π* transitions. Data for π→π* (perpendicular system) transitions. Experimental value of the dyes
d
absorption maxima in CH
2
Cl
2
.
The semi-empirical ZINDO method does not operate with parameter for Se atom.

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Table 4
Remaining absorbance of dyes 6a-c in CH
3
CN after different exposure periods to ambient light.
Dye
Abs (%) in CH CN after
3
6
0 h
96 h
43
11
-
120 h
6
6
a
55
26
9
36
6
b
6
c
-

ACCEPTED MANUSCRIPT
O
+
N
N
O
1
Fig. 1. Indolizine-derived squaraine 1.

ACCEPTED MANUSCRIPT
Fig. 2. Molecular geometry of dye 6a (bond lengths in Å) (HF method).

ACCEPTED MANUSCRIPT
Fig. 3. Schematic disposition of delocalized and donor (D
1 2
and D ) levels, as well as the first two
electron transitions, in a symmetrical (a) and unsymmetrical dye (b) (∆E - splitting energy).
spl