Photophysical and electrochemical studies of highly fluorescent pyrazole and imidazole containing heterocycles

Article abstract of DOI:10.1016/j.dyepig.2016.09.030

Highly congested pyrazole and imidazole ring containing heterocycles were studied for their photophysical and electrochemical properties. TDDFT calculations of the molecules were carried out using B3LYP/G(d,p) level theory in DMF medium and the results we

Full text of DOI:10.1016/j.dyepig.2016.09.030

Accepted Manuscript
Photophysical and electrochemical studies of highly fluorescent pyrazole and
imidazole containing heterocycles
Hardesh K. Maurya, Ch. Pavan Kumar, M. Chandrasekharam, Atul Gupta
PII:
S0143-7208(16)30720-3
10.1016/j.dyepig.2016.09.030
DYPI 5482
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 15 June 2016
Revised Date: 5 September 2016
Accepted Date: 12 September 2016
Please cite this article as: Maurya HK, Kumar CP, Chandrasekharam M, Gupta A, Photophysical and
electrochemical studies of highly fluorescent pyrazole and imidazole containing heterocycles, Dyes and
Pigments (2016), doi: 10.1016/j.dyepig.2016.09.030.
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Graphical Abstract:
ACCEPTED MANUSCRIPT
Photophysical and electrochemical studies of highly fluorescent pyrazole and imidazole containing
heterocycles
Hardesh K. Maurya, CH. Pavan Kumar, M. Chandrasekharam,* Atul Gupta*

ACCEPTED MANUSCRIPT
Photophysical and electrochemical studies of highly fluorescent
pyrazole and imidazole containing heterocycles
Hardesh K. Mauryaa, Ch. Pavan Kumarb,c, M. Chandrasekharam*b,c, Atul Gupta*#a
a Medicinal Chemistry Department, Central Institute of Medicinal and aromatic Plants, PO. CIMAP,
Kukrail Road, Lucknow-226015, India
b CSIR-Indian Institute of Chemical Technology, I&PC Division, Uppal Road, Tarnaka, Hyderabad -
500 607, India.
cAcademy of Scientific and Innovative Research, CSIR-IICT, Hyderabad - 500 607, India
Keywords: pyran-2-one, benzo[h]chromene, thiochromeno[4,3-b]pyran, fluorescence, TDDFT
Corresponding Author
*Atul Gupta, Tel.: 0915222718556, E-mail: atisky2001@yahoo.co.in (A. G.),

ACCEPTED MANUSCRIPT
Abstract
Highly congested pyrazole and imidazole ring containing heterocycles were studied for their
photophysical and electrochemical properties. TDDFT calculations of the molecules were carried out
using B3LYP/G(d,p) level theory in DMF medium and the results were in good agreement with the
experimental values. The imidazole derivatives showed a red shifted absorption peak (λmax, 455-474 nm)
and higher molar extinction coefficient than pyrazole derivatives. In the case of increasing π-conjugation
from pyrazoles to imidazoles red shift in the emission maxima (537 nm-565 nm) was observed. These
preliminary results of absorption and emission spectroscopy illustrate that these molecules may have
potential organic electronic applications.

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1 INDRODUCTION
Fluorescent organic compounds are of immense interest in many areas such as functional
materials in organic light-emitting devices (OLEDs) [1–3], photovoltaics [4,5], structure, dynamics,
dyes for polymers and textile [6], These molecules can also be used as functions of biomolecules studies
[7,8], labels and probes for coloring of malignancies in image-guided surgery [9,10]. The multifarious
uses of fluorophores arouse the demand and interest for the synthesis of newer fluorescent compounds
with desired properties.
Based on their applicability in the natural light harvesting systems, porphyrins have been extensively
explored as effective photofluorescent agents. It was realized that for photophysical activity of these
molecules, structural planarity and dominant π-electron density (highly conjugated system) is essentially
required. Based on these observations, various other photofluorescent organic compounds incorporating
coumarin, indoline, phenothiazine, fluorene, carbazole, pyrazole, imidazole, triphenylamine and
tetrahydroquinoline nucleus have been studied as bio-imaging agents [11], fluorescent probes [12],
chemosensors [13], electroluminescent materials for organic electronics [14] light harvesting agents
[15], transistor devices [16], photovoltaic cells [17] and OLEDs [18]. Development of Sulforhodamine
B [19] (i, a bioimaging dye), bodipy [20] (ii, visible fluorescence chemosensor), PTPOBS [21] (iii, hole-
transporters in OLEDs) and tris(8-hydroxyquinolinato)aluminum (iv) [22] (blue luminescent in OLEDs)
are some notable examples in this regards Fig. 1.
As a part of on going program to design and develop fluorescent sensitizers, we have studied
photophysical and electrochemical properties of highly congested pyrazole and imidazole based
heterocycles. Most of the compounds possess rod like configuration with high π-electron charge density
in conjugation with nitrogen atoms usually present in the organic dyes.
<<Insert Figure-1>>

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2. Experimental details
The reagents and solvents used in this study were of analytical or laboratory grade and used without
further purification unless stated otherwise. All the reactions were monitored on Merck aluminium silica
gel thin layer chromatography (TLC, UV254nm) plates. Column chromatography was carried out on silica
gel (100-200 mesh). The melting points were determined on Buchi melting point M560 apparatus in
open capillaries and are uncorrected. 1H (300 MHz) and 13C NMR (75 MHz) spectra were recorded on a
Bruker WM-300 using CDCl3 as reference solvent. Chemical shifts (δ-value) are reported in parts per
million (ppm) Signal patterns are indicated as s, singlet; bs, broad singlet; d, doublet; dd, double doublet;
t, triplet; m, multiplet. Coupling constants (J) are given in Hertz. Infrared (IR) spectra were recorded on
a Perkin-Elmer AX-1 spectrophotometer in KBr disc and reported in wave number (cm-1). ESI-MS mass
spectra were recorded on Shimadzu LC-MS and/or LC-MS-MS APC3000 (Applied Biosystem). IUPAC
nomenclature was recorded on Chem Draw-15.
2.1. General procedure for the synthesis of 7-imino-6-oxo-10-aryl-7,9,13,14-tetrahydro-6H-
benzo[7,8]chromeno[4,3-d]pyrazolo[1,5-a]pyridine-12-carbonitriles (8a-c)/ 7-imino-6-oxo-10-aryl-7,9-
dihydro-6H,13H-pyrazolo[1,5-a]thiochromeno[3',4':5,6]pyrano[4,3-d]pyridine-12-carbonitrile (8d-h)
[23]
A mixture of lactone 3 (1.0 mmol), 2-(5-aryl-1H-pyrazol-3-yl)acetonitrile (7, 1.0 mmol) and
powdered KOH (1.2 mmol) in DMF (9 mL) was stirred at room temperature for 5-8 h. The reaction was
monitored using silica gel TLC. After consumption of 3, the crude residue was poured onto crushed ice
with vigorous stirring. The aqueous suspension was neutralized with dil. HCl (if required) and the
precipitate obtained was filtered, washed with water and dried. Residue was purified by silica gel
column chromatography using chloroform/methanol (0-30%);

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7-Imino-6-oxo-10-phenyl-7,9,13,14-tetrahydro-6H-benzo[7,8]chromeno[4,3-d]pyrazolo[1,5-
a]pyridine-12-carbonitrile (8a)
Yellow solid; yield 67%; mp: 303-304ºC; Rf 0.38 (CHCl3); IR (KBr): 1577, 1611, 1692 (C=O), 2203
(CN), 3259 and 3370 (NH) cm-1; 1H NMR (300 MHz, DMSO-d6): δ 2.94 (t, 2H, J=7.2 Hz, CH2), 3.21 (t,
2H, J=7.8 Hz, CH2), 6.61 (s, 1H, CH), 7.28-7.48 (m, 7H, Ar-H & NH), 7.64 (m, 1H, Ar-H), 8.02 (d, 3H,
J=7.5 Hz, Ar-H); MS: 403.1 (M-H) +; Anal. Calcd (C25H16N4O2): C, 74.25; H, 3.99; N, 13.85 Found: C,
74.12; H, 3.96; N, 13.78; HRMS (ESI): calc. for C25H17N4O2: 405.1346 (M++H); found: 405.1368.
7-Imino-6-oxo-10-(4-methoxyphenyl)-7,9,13,14-tetrahydro-6H-benzo[7,8]chromeno[4,3-
d]pyrazolo[1,5-a]pyridine-12-carbonitrile (8b)
Yellow solid; yield: 60%; mp: 280-281°C; Rf 0.32 (DCM); IR (KBr): 1591, 1696 (C=O), 2206 (CN),
3292 and 3409 (NH) cm-1; 1H NMR (300 MHz, DMSO-d6): δ 2.94 (t, 2H, J=7.2 Hz, CH2), 3.16 (t, 2H,
J=7.2 Hz, CH2), 3.78 (s, 3H, OMe), 6.54 (s, 1H, CH), 6.95 (m, 3H, Ar-H & NH), 7.30 (m, 3H, Ar-H),
7.64 (s, 1H, Ar-H), 7.94 (d, 2H, J=8.1 Hz, Ar-H), 8.27 (s, 1H, NH); MS: 433.1 (M-H) +; Anal. Calcd
(C26H18N4O3): C, 71.88; H, 4.18; N, 12.90 Found: C, 71.76; H, 4.16; N, 12.84; HRMS (ESI): calc. for
C26H19N4O3: 435.1451 (M++H); found: 435.1472.
7-Imino-3-methoxy-6-oxo-10-phenyl-7,9,13,14-tetrahydro-6H-benzo[7,8]chromeno[4,3-
d]pyrazolo[1,5-a]pyridine-12-carbonitrile (8c)
Yellow solid; yield: 79%; mp: 305-306ºC; Rf 0.80 (CHCl3); IR (KBr): 1572, 1693 (C=O), 2203 (CN),
3286 and 3429 (NH) cm-1; 1H NMR (300 MHz, DMSO-d6): δ 2.91 (t, 2H, J=7.5 Hz, CH2), 3.17 (t, 2H,
J=7.5 Hz, CH2), 3.79 (s, 3H, OMe), 6.58 (s, 1H, CH), 6.88 (m, 3H, Ar-H & NH ), 7.42 (m, 3H, Ar-H),
7.64 (s, 1H, Ar-H), 7.55 (m, 1H, Ar-H), 8.01 (d, 1H, J=7.2 Hz, Ar-H), 8.27 (s, 1H, NH); MS: 433.2 (M-

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H) +; Anal. Calcd (C26H18N4O3): C, 71.88; H, 4.18; N, 12.90 Found: C, 71.80; H, 4.15; N, 12.86; HRMS
(ESI): calc. for C26H19N4O3: 435.1451 (M++H); found: 435.1475.
7-Imino-6-oxo-10-phenyl-7,9-dihydro-6H,13H-pyrazolo[1,5-a]thiochromeno[3',4':5,6]pyrano[4,3-
d]pyridine-12-carbonitrile (8d)
Yellow solid; yield: 68%; mp: 290ºC (start to decompose); Rf 0.46 (CHCl3); IR (KBr): 1586, 1690
1
(C=O), 2208 (CN), 3305 and 3436 (NH) cm-1; H NMR (300 MHz, DMSO-d6): δ 4.43 (s, 2H, SCH2),
6.65 (s, 1H, CH), 7.30-7.45 (m, 7H, Ar-H & NH), 7.74 (d, 1H, J=7.5 Hz, Ar-H), 8.02 (d, 3H, J=7.5 Hz,
Ar-H); 13C NMR (75 MHz, DMSO-d6): δ 25.05, 68.76, 88.12, 95.59, 106.96, 121.48, 126.08, 127.02,
127.28 (2C), 127.87, 128.55, 129.35, 129.43 (2C), 131.17, 133.32, 134.88, 140.56, 143.05, 150.56,
153.38, 153.75, 161.72; MS: 421.1 (M-H) +; HRMS (ESI): calc. for C25H15N4O2S: 423.0916 (M++H);
found: 423.0964.
7-Imino-6-oxo-10-(4-methoxyphenyl)-7,9-dihydro-6H,13H-pyrazolo[1,5-
a]thiochromeno[3',4':5,6]pyrano[4,3-d]pyridine-12-carbonitrile (8e)
Yellow solid; yield: 61%; mp: 280°C (start to decompose); Rf 0.14 (CHCl3); IR (KBr): 1586, 1690
1
(C=O), 2209 (CN), 3310 and 3425 (NH) cm-1; H NMR (300 MHz, DMSO-d6): δ 3.85 (s, 3H, OMe),
4.53 (s, 2H, SCH2), 6.56 (s, 1H, CH), 7.10 (m, 4H, Ar-H & NH), 7.44 (d, 2H, J=7.5 Hz, Ar-H), 7.80 (m,
+
1H, Ar-H), 8.02 (d, 2H, J=7.5 Hz, Ar-H), 9.52 (bs, 1H, NH); MS: 451.1 (M-H) ; Anal. Calcd
(C25H16N4O3S): C, 66.36; H, 3.56; N, 12.38 Found: C, 66.25; H, 3.51; N, 12.31; HRMS (ESI): calc. for
C25H15N4O3: 451.0859 (M++H); found: 451.0880.
7-Imino-6-oxo-10-(4-fluorophenyl)-7,9-dihydro-6H,13H-pyrazolo[1,5-
a]thiochromeno[3',4':5,6]pyrano[4,3-d]pyridine-12-carbonitrile (8f)

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Yellow solid; yield: 63%; mp: 310°C (start to decompose); Rf 0.11 (CHCl3); IR (KBr): 1602, 1691
1
(C=O), 2207 (CN), 3303 and 3422 (NH) cm-1; H NMR (300 MHz, DMSO-d6): δ 4.30 (s, 2H, SCH2),
6.50 (s, 1H, CH), 7.10-7.28 (m, 6H, Ar-H & NH), 7.59 (d, 1H, J=7.2 Hz, Ar-H), 7.92 (d, 2H, J=5.7 Hz,
Ar-H), 9.38 (bs, 1H, NH); MS: 439.1 (M-H) +; Anal. Calcd (C24H13N4O3FS): C, 65.45; H, 2.97; N, 4.31
Found: C, 65.39; H, 2.94; N, 4.28; HRMS (ESI): calc. for C24H13N4O3FS: 441.0816 (M++H); found:
441.0816.
7-Imino-6-oxo-10-(3-(trifluromethyl)phenyl)-7,9-dihydro-6H,13H-pyrazolo[1,5-
a]thiochromeno[3',4':5,6]pyrano[4,3-d]pyridine-12-carbonitrile (8g)
Yellow solid; yield: 76%; mp: 309-310°C; Rf 0.13 (CHCl3); IR (KBr): 1615, 1692 (C=O), 2210 (CN),
3305 and 3418 (NH) cm-1; 1H NMR (300 MHz, DMSO-d6): 4.31 (s, 2H, SCH2), 6.71 (s, 1H, CH), 7.18-
7.29 (m, 3H, Ar-H & NH), 7.54-7.62 (m, 3H, Ar-H), 8.16 (s, 1H, Ar-H), 8.19-8.21 (m, 2H, Ar-H), 9.45
(bs, 1H, NH); MS: 489.1 (M-H) +; Anal. Calcd (C25H13F3N4O2S): C, 61.22; H, 2.67; N, 11.62 Found: C,
61.12; H, 2.65; N, 11.57; HRMS (ESI): calc. for C25H14F3N4O2S: 491.0784 (M++H); found: 491.0815.
7-Imino-6-oxo-10-(3,4,5-trimethoxyphenyl)-7,9-dihydro-6H,13H-pyrazolo[1,5-
a]thiochromeno[3',4':5,6]pyrano [4,3-d]pyridine-12-carbonitrile (8h)
Yellow solid; yield: 65%; mp: 275°C (start to decompose); Rf 0.84 (Hexane: EtOAc, 1:1); IR (KBr):
1
1606, 1695 (C=O), 2202 (CN), 3294 and 3407 (NH) cm-1; H NMR (300 MHz, DMSO-d6): δ 3.74 (s,
9H, 3xOMe), 4.30 (s, 2H, SCH2), 6.60 (s, 1H, CH), 7.14 (s, 2H, Ar-H), 7.18-7.28 (m, 4H, Ar-H & NH),
7.60 (d, 1H, J=4.9 Hz, Ar-H), 9.38 (bs, 1H, NH); MS: 511.1 (M-H) +; Anal. Calcd (C27H20N4O5S): C,
63.27; H, 3.93; N, 10.93 Found: C, 63.6; H, 3.89; N, 10.88; HRMS (ESI): calc. for C27H21N4O5S:
513.1227 (M++H); found: 491.1233.

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2.2. General procedure for the Synthesis of 9-amino-8-oxo-1,2-dihydro-8H-benzo[4,5]imidazo[1,2-
a]benzo[7,8]
chromeno[4,3-d]pyridine-16-carbonitrile
(10a-b)
/
9-amino-8-oxo-1H,8H-
benzo[4,5]imidazo[1,2-a]thiochromeno[3',4':5,6]pyrano[4,3-d]pyridine-16-carbonitrile (10c-d) [23].
A mixture of lactone 3 (1.0 mmol),2-cyanomethyl-1H-benzimidazole (9, 1.0 mmol) and powdered
KOH (1.2 mmol) in DMF (9 mL) was stirred at room temperature for 5-8 h. The reaction was monitored
using silica gel TLC. After consumption of 3, the residue was poured onto crushed ice with vigorous
stirring. The aqueous suspension was neutralized with dil. HCl (if required) and the precipitate obtained
was filtered, washed with water and dried. The crude residue was purified by silica gel column
chromatography using chloroform/methanol (0-30%);
9-Amino-8-oxo-1H,8H-benzo[4,5]imidazo[1,2-a]thiochromeno[3',4':5,6]pyrano[4,3-d]pyridine-16-
carbonitrile (10a)
Orange solid; yield: 76%; mp: >315°C (start to decompose); Rf 0.42 (CHCl3:MeOH, 10:1); IR (KBr):
1563, 1612, 1692 (C=O), 2208 (CN), 3173 and 3440 (NH) cm-1; 1H NMR (300 MHz, DMSO-d6): δ 2.93
(bs, 2H, CH2), 3.22 (bs, 2H, CH2), 7.11 (m, 1H, Ar-H), 7.32 (m, 4H, Ar-H), 7.49 (m, 1H, Ar-H), 7.64
+
(m, 1H, Ar-H), 7.89 & 9.93 (bs, 2H, NH2), 8.79 (m, 1H, Ar-H); MS: 377.1 (M-H) ; Anal. Calcd
(C23H14N4O2): C, 73.01; H, 3.73; N, 14.81 Found: C, 72.92; H, 3.70; N, 14.73; HRMS (ESI): calc. for
C23H15N4O2: 379.1189 (M++H); found: 491.1197.
9-Amino-5-methoxy-8-oxo-1,2-dihydro-8H-benzo[4,5]
d]pyridine-16-carbonitrile (10b)
imidazo[1,2-a]benzo[7,8]chromeno[4,3-
Orange solid; yield: 98%; mp: >315°C (start to decompose); Rf 0.40 (CHCl3:MeOH, 10:1); IR (KBr):
1
1612, 1690 (C=O), 2204 (CN), 3181 and 3446 (NH) cm-1; H NMR (300 MHz, DMSO-d6): δ 2.94 (t,
2H, J=6.6 Hz, CH2), 3.26 (t, 2H, J=6.6 Hz, CH2), 3.83 (s, 3H, OCH3), 6.92 (s, 2H, Ar-H), 7.14 (t, 1H,

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J=7.2 Hz, Ar-H), 7.28 (t, 1H, J=7.2 Hz, Ar-H), 7.54 (d, 1H, J= 8.1 Hz, Ar-H), 7.63 (d, 1H, J= 9.3 Hz,
+
Ar-H), 8.82 (d, 1H, J=7.8 Hz, Ar-H), 9.96 (bs, 2H, NH2); MS: 407.1 (M-H) ; Anal. Calcd
(C24H16N4O3): C, 70.58; H, 3.95; N, 13.72 Found: C, 70.46; H, 3.92; N, 13.63; HRMS (ESI): calc. for
C24H17N4O3: 409.1295 (M++H); found: 409.1294.
9-Amino-8-oxo-1H,8H-benzo[4,5]imidazo[1,2-a]thiochromeno [3',4':5,6]pyrano[4,3-d]pyridine-16-
carbonitrile (10c)
Orange solid; yield: 84%; mp: 309°C (start to decompose);; Rf 0.64 (CHCl3:MeOH, 10:1); IR (KBr):
1556, 1603, 1691 (C=O), 2208 (CN), 3177 and 3426 (NH) cm-1; 1H NMR (300 MHz, DMSO-d6): δ 4.48
(s, 2H, SCH2), 7.14 (t, 2H, J=7.5 Hz, Ar-H), 7.25-7.41 (m, 5H, Ar-H & NH2), 7.55 (d, 1H, J=7.8 Hz,
Ar-H), 7.74 (dd, 1H, J= 1.5 & 7.5 Hz, Ar-H), 8.80 (d, 1H, J=8.1 Hz, Ar-H); 13C NMR (75 MHz,
DMSO-d6): δ 25.24, 69.31, 88.97, 107.11, 115.55, 117.50, 117.64, 121.08, 124.37, 126.29, 127.02,
127.90, 128.56, 131.32, 132.86, 135.22, 144.50, 145.41, 151.28, 151.59, 154.97, 161.31; MS: 395.1
(M+-1); Anal. Calcd (C22H12N4O2S): C, 66.66; H, 3.05; N, 14.13 Found: C, 66.51; H, 3.01; N, 14.03;
HRMS (ESI): calc. for C22H13N4O2S: 397.0753 (M++H); found: 397.0763.
9-Amino-5-chloro-8-oxo-1H,8H-benzo[4,5]imidazo[1,2-a]thiochromeno[3',4':5,6]pyrano[4,3-
d]pyridine-16-carbonitrile (10d)
Orange solid; yield: 76%; mp: 312oC (start to decompose); Rf 0.85 (CHCl3:MeOH, 10:1); IR (KBr):
1562, 1606, 1694(C=O), 2217 (CN), 3190 and 34.27 (NH) cm-1; 1H NMR (300 MHz, DMSO-d6): δ 4.49
(s, 2H, SCH2), 7.38-7.43 (m, 5H, Ar-H & NH2), 7.58 (d, 1H, J=7.8 Hz, Ar-H), 7.66 (d, 1H, J= 1.8 Hz,
Ar-H), 8.84 (d, 1H, J=8.1 Hz, Ar-H), 9.95 (bs, 1H, NH2); 13C NMR (75 MHz, DMSO-d6): δ 25.21,
69.75, 88.94, 107.96, 117.52, 117.68, 120.98, 121.05, 124.31, 125.53, 129.44, 130.08, 130.74, 131.41,
132.88, 134.05, 144.04, 145.44, 149.95, 151.45, 154.88, 160.91; MS: 429.1 (M-H)+; HRMS (ESI): calc.

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for C22H12ClN4O2S: 431.0369 (M++H); found: 431.0394.
3. Computational Details
All the calculations have been performed using the Gaussian 09 program package [24]. Geometrical
optimization was performed in vacuo using B3LYP [25,26] exchange-correlation functional and a 6-
311G(d, p) basis set [26]. The optimized geometries were then used to obtain frontier molecular orbitals
(FMOs). To simulate the optical spectra, the lowest spin allowed singlet–singlet transitions were
computed on the ground state geometry. TDDFT calculations of the lowest singlet–singlet excitations
were performed in DMF solution, on the optimized geometries using the B3LYP/6-311G (d, p) level of
theory. The integral equation formalism polarizable continuum model (PCM) [27] within self-consistent
reaction field (SCRF) theory, has been used to describe the solvation of the molecules. The software
GaussSum 2.2.5 [28] was utilized to simulate the major portion of absorption spectrum and to analyze
the nature of transitions.
4. Results and discussion
4.1 Chemistry
The proposed pyrazoles (8a-h) and imidazoles (10a-d) derivatives were synthesized using earlier
reported procedures following the reaction of suitably substituted benzo[h]chromene (3a-b, 4a-b) and
thiochromeno[4,3-b]pyrans (3c-d, 4c-d) with active methylene group containing 2-(5-aryl-1H-pyrazol-
3-yl)acetonitrile (7) or 2-imidazolyl acetonitrile (9) under basic reaction conditions (Scheme 1 and 2)
[23].
The 3a-d and 4a-d were selected owing to their interesting chemistry and versatile applicability as a
synthon for the synthesis of diverse heterocycles[15]. Structurally, benzo[h]chromene (3a-b, 4a-b) and
thiochromeno[4,3-b]pyrans (3c-d, 4c-d) have three electrophilic centers. Due to the presence of three
electrophilic centers, they produce different products when reacted with nucleophiles under different

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reaction conditions and in situ yields cyclic arenes and heteroarenes.
<<Insert Figure-2>>
<<Insert Figure-3>>
<<Insert Scheme 1>>
In brief, the starting compounds 3a-b [29] were prepared from the reaction of 1-tetralone (2a-b) with
methyl 2-cyano-3,3-dimethylthioacrylate (1) and 4-methylthio-2-oxo-2,5-dihydrothiochromeno-[4,3-
b]pyran-3-carbonitriles (3c-d) [30], from the reaction of thiochromen-4-ones (2c-d) with methyl 2-
cyano-3,3-dimethylthioacrylate (1). Subsequent amination of 3a-d were carried out using piperidine in
boiling ethanol, to afford corresponding 4a-d (Fig. 2). Further, active methylene containing pyrazoles (7)
were synthesized using hydrazine and 2H-pyran-2-ones (6) as precursors Fig. 3 [30,31]. Compound 6
was finally synthesized from the reaction of aryl methyl ketone (5) with methyl 2-cyano-3,3-
dimethylthioacrylate (1) [31].
<<Insert Scheme 2>>
Finally the pyrazole (8a-h) and benzimidazole (10a-d) derivatives were synthesized using an equimolar
mixture of lactone (3 or 4) and pyrazole (7) or imidazole (9) by the attack of nucleophile at C4 position
of 3 or 4 in KOH/DMF at room temperature (Scheme 1-2) in good yields.
4.2. Optical properties
The photophysical and electrochemical data of compounds such as λmax of their UV-Vis
absorption and photoluminescence (PL) spectra, fluorescence quantum yield, HOMO/LUMO energy
levels, and the electrochemical band gap (Eg) were investigated. The fluorescence quantum yields in

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DMF were determined by the optically dilute method using fluorescein (Φfl = 0.79 in ethanol) as a
reference.
The UV-Vis absorption spectra of pyrazoles (8a-h) and imidazoles (10a-d) are recorded in DMF
solution as shown in Fig. 4. The various photophysical data of the compounds are shown in Table 1. A
broad band between 270 and 350 nm observed in the absorption spectra in DMF was attributed to the π-
π* transition. The imidazole moiety (10a-d) showed a red shifted absorption peak (λmax, 455-474 nm)
and higher molar extinction coefficient compared to that of pyrazole (8a-c) moieties resulting from
increased π-conjugation. Interestingly, when the sulfur atom was introduced into the chromene unit to
construct 8d-h, the λmax was red-shifted by ≈14 nm [32]. A similar trend was also observed in the case of
imidazole molecules.
The normalized UV-Vis and PL spectra of synthesized molecules are shown in Fig. 5. The emission
properties are summarized in Table1. The PL maxima of pyrazoles (8a-h) showed strong green
fluorescence at 493-511 nm with a moderate to good fluorescence quantum yield up to 78% [33]. In the
case of increasing π-conjugation from pyrazoles to imidazoles (10a-d) red shift in the emission maxima
(537 nm-565 nm) was observed. The emission peak was not strongly affected by the hetero atom (s) in
the pyrazole moieties (≈ 495 nm for 8a & 8b and ≈ 508 nm for 8d-h).
<<Insert Table 1>>
<<Insert Figure-4>>
Interestingly, compounds 10c and 10d showed PL maxima in the red region (562 nm, 565nm
respectively). The imidazoles (10a-d), exhibited relatively large Stokes shifts (80-91 nm) compared to
pyrazoles (53-70 nm) possibly due to high intramolecular charge-transfer excitation of an electron from
HOMO level to the LUMO level of the molecules.

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In order to provide in-depth insight into the electronic structures, we have chosen one molecule from
each category (8a, 8d, 10a and 10c) for further theoretical study. We have carried out TDDFT (Time
Dependent Density Functional Theory) studies in DMF using B3LYP/6-311G(d,p) level of theory.
Structures of optimized geometries of these molecules at B3LYP/6-311 G(d,p) level theory were shown
in Fig 6. The TDDFT calculations with B3LYP are in nearly good agreement with experimental UV-Vis
spectra. The spectra have been produced by convoluting Gaussian functions with full width at half
maximum (FWHM) = 0.03 eV. The normalized plots of simulated and experimental UV-Vis spectra of
these molecules are shown in Fig. 7. The wavelengths of the excitations with the largest oscillator
strengths (f) within these bands are given in Table 2. Based on TDDFT calculations, the intense singlet
transition at around 407 nm (8a, f = 0.334), 418 nm (8d, f = 0.453), 467 nm (10c, f = 0.383) respectively
is attributed to the HOMO to LUMO transition, whereas for the 10a at around 400 nm (f=0.606)
attributed to transition from HOMO to LUMO+2 (Table 2) The optimized geometry parameters, at the
B3LYP/6-311 G(d,p) level of theory are shown in Table 3.
<<Insert Figure-5>>
<<Insert Table 2>>
4.3. Electrochemical Properties
Electrochemical studies using cyclic voltammetry (CV) were performed to evaluate the redox behavior
of the pyrazoles (8a-h) and imidazoles (10a-d). All measurements were performed in a three-electrode
cell setup using Ag/AgCl as reference electrode and a Pt electrode as the working electrode, using a 0.1
M of the electrolyte tetrabutylammoniumhexafluorophosphate (Bu4NPF6) dissolved in DMF. The DPV
voltammograms obtained were employed to evaluate the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) energy levels [34]. As shown in Fig. 8, the first
oxidation potential (Eox) corresponds to the HOMO level of the molecule. The HOMO level was readily

ACCEPTED MANUSCRIPT
estimated using the equation EHOMO = -(Eox+4.8) eV. The LUMO could be calculated from the equation
Eox-E0-0 where E0–0 is the zeroth-zeroth transition value obtained from the intersection of the normalized
absorption and emission spectra as shown in Fig. 5 and tabulated in Table 1. The Eox of these
compounds were calculated relative to the ferrocene/ferrocenium (Fc/Fc+) 0.53 V vs Ag/AgCl electrode
[35]. The HOMO and LUMO levels of the new molecules were found to be in the range of -5.64 eV to -
5.43 eV and -2.61 ev to -2.95 eV, respectively. The imidiazoles exhibited higher HOMO level compared
to pyrazoles, attributed from increase in the π-conjugation.
We have plotted the isosurfaces (isovalue=0.02) of the HOMO and LUMO Fig. 10 as well as the
nearest frontier orbitals of 8a, 8d, 10a and 10c which are involved in transitions with strong
contributions to the first excitation as well as to next two excitations with strong oscillator strength. The
HOMO and LUMO energies of the pyrazole molecules (8a, 8d) located over the chromene and pyrazole
moiety, whereas for imidazole moieties (10a, 10c) the HOMO is distributed over the total molecule.
There is no clear trend in the LUMO energies of the imdiazole molecules.
The calculated HOMO, LUMO energies and the HOMO-LUMO gap (HLG) at a B3LYP/6-311g (d,p)
level in DMF solvent is given in Fig. 9. The computationally and experimentally determined HOMO,
LUMO energies are in good agreement. The HOMO-LUMO gaps are shown in Table 2.
<<Insert Table 3>>
<<Insert Figure-6>>
<<Insert Figure-7>>
<<Insert Figure-8>>
<<Insert Figure-9>>
<<Insert Figure-10>>

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3. Conclusions:
In summary, we have synthesized and successfully measured the photophysical and electrochemical
properties of highly congested pyrazole and imidazole ring containing heterocycles. The UV-Vis spectra
of these molecules showed that the imidazole bearing molecules exhibited red shift in the absorption
band higher molar extinction coefficient compared to pyrazole containing molecules. This work
demonstrates that these molecules having strong, broad absorption and appropriate energy levels, could,
with further development, be employed as materials for organic electronics as well as sensitizers.
Notes
The authors declare no competing financial interest.
Acknowledgements
AG and HKM acknowledge SERB, DST New Delhi, India for financial support as DST Fast Track
Young Scientist Fellowship. Ch. P.K. thanks UGC, New Delhi for a senior research fellowship.
Authors are thankful to Director CSIR-CIMAP, CSIR-CDRI, Lucknow and CSIR-IICT, Hyderabad.
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[35] Initially, before measuring redox potential of synthesized compounds, referance elcetrods Ag/AgCl
was calibrated in DMF using a ferrocene/ferrocenium (Fc/Fc+) redox couple as an external standard.

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Ligend for firures
Fig. 1. Some notable organic dyes applied for different purposes.
Fig. 2. Synthesis of the substituted 5,6-dihydro-2-oxo-2H-benzo[h] chromene-3-carbonitriles (3a-b,4a-
b) and 2,5-dihydro-2-oxo-thiochromeno[4,3-b]pyran-3-carbonitriles (3c-d,4c-d).
Fig. 3. Synthesis of pyran-2-one (3) and 2-(5-aryl-1H-pyrazol-3-yl) acetonitrile (4).
Fig. 4. UV-vis spectra of 8a-h and 10a-d in DMF.
Fig 5. Electronic absorption & emission spectra of 8a-h and 10a-d in DMF.
Fig. 6: Structures of optimized geometries of 8a, 8d, 10a and 10c at B3LYP/6-311 G(d,p) level theory
Fig 7. Comparison between experimental (black lines) and calculated (red lines) UV-Vis absorption
spectra of the 8a, 8d, 10a and 10c molecules in DMF solution. Red vertical lines represent the calculated
singlet excitation energies in GaussSum 2.2.5.
Fig. 8. Cyclic voltammograms of 8a, 8d, 10a and 10c in DMF; scan rate 100 mV s-1; supporting
electrolyte: tetrabutylammonium hexafluorophosphate (NBu4PF6).
Fig. 9. Calculated HOMO–LUMO gaps at B3LYP/6-311G(d, p) level of theory and respective HOMO
and LUMO orbital pictures at B3LYP/6-311G(d, p)level of 8a, 8d, 10a and 10c in DMF solvent.
Fig. 10. Molecular orbitals of 8a, 8d, 10a and 10c in B3LYP functional, involved in transitions that
contribute to the first excitation and to the next high absorbance excitation

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Ligend for Tables
Table 1. Photophysical properties of compounds 8a-h and 10a-d in DMF
Table 2. Calculated properties of the 8a, 8d, 10a and 10c using B3LYP/6-311G (d,p).
Specifically HOMO and LUMO energies (eV), HOMO–LUMO gap (eV), HLG, with
corresponding oscillator strengths, f, the wavelengths of the first excitation and excitations with
the largest oscillator strengths and the dipole moment (D), µ.
Table 3. Optimized geometry parameters of 8a, 8d, 10a and 10c calculated at B3LYP/6-311
G(d,p) level theory.
Ligend for Shemes
Scheme
1.
Synthesis
of
7-imino-6-oxo-10-aryl-7,9,13,14-tetrahydro-6H-
benzo[7,8]chromeno[4,3-d]pyrazolo[1,5-a]pyridine-12-carbonitrile (8a-c)/ 7-imino-6-oxo-10-
aryl-7,9-dihydro-6H,13H-pyrazolo[1,5-a]thiochromeno[3',4':5,6]pyrano[4,3-d]pyridine-12-
carbonitrile (8d-h).
Scheme
a]benzo[7,8]chromeno[4,3-d]pyridine-16-carbonitrile
benzo[4,5]imidazo[1,2-a]thiochromeno[3',4':5,6]pyrano[4,3-d]pyridine-16-carbonitrile (10c-d).
2.
Synthesis
of
9-amino-8-oxo-1,2-dihydro-8H-benzo[4,5]imidazo[1,2-
(10a-b) 9-amino-8-oxo-1H,8H-
/

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Figure 1
Figure 2

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Figure 3
Figure4

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Figure 5
Figure 6
8a
8d
10c
10a

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Figure 7

ACCEPTED MANUSCRIPT
Figure 8
Figure 9

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Figure 10
8d
10a
10c
8a
LUMO+2
LUMO+1
LUMO
HOMO
HOMO-1
HOMO-2

ACCEPTED MANUSCRIPT
Table 1
b
c
aλabs
(nm)
gΕ0−0
h∆ν
(nm)
iφ
ε (M-1cm-1)
Compound
dEox
eHOMO
fLUMO
λem
(nm)
493
495
507
508
508
511
508
508
545
537
562
428
428
428
442
444
441
441
444
455
457
467
474
13,000
10,470
17,003
9,792
10,591
12,133
10,227
8,927
20,833
20,781
15,790
11,249
0.64
0.68
0.76
0.72
0.76
0.84
0.64
0.60
0.73
0.70
0.63
-5.44
-5.48
-5.56
-5.52
-5.56
-5.64
-5.44
-5.40
-5.53
-5.50
-5.43
-5.44
-2.61
-2.63
-2.71
-2.77
-2.81
-2.88
-2.68
-2.64
-2.95
-2.89
-2.90
2.83
2.85
2.85
2.75
2.75
2.76
2.76
2.76
2.58
2.61
2.53
2.54
65
67
53
66
64
70
67
64
90
80
98
91
0.55
0.66
0.78
0.34
0.41
0.38
0.29
0.22
0.17
0.32
0.05
0.03
8a
8b
8c
8d
8e
8f
8g
8h
10a
10b
10c
10d
565
0.64
-2.90
b
c
d
aλabs (nm): Absorption wavelength, λem (nm): Emission wavelength, ε: Molar extinction coefficient, Eox: The formal oxidation potentials in
e
f
DMF were internally calibrated with ferrocene (0.53 V vs Ag/AgCl), HOMO = -(Eox+4.8)eV, LUMO = Calculated with the expression of
LUMO = HOMO - E0-0. gE0-0: Band gap, was derived from the intersection between the absorption and emission spectra ∆ν: Stokes shifts, Φ:
h
i
fluorescence quantum yield.
Table 2
a
λmax
(nm)
λmax
Compound
HOMO
LUMO
HLG
F
Main Contribution
ꢀ (D)
(nm)
428
442
455
467
407
418
400
437
-5.83
-5.75
-6.07
-5.95
-2.56
-2.59
-3.83
-2.70
3.27
3.16
2.24
3.25
0.334
0.453
0.606
0.383
H → L (98%)
H → L (96%)
H → L+2 (94%)
H → L (89%)
6.95
5.96
3.58
5.54
8a
8d
10a
10c
aabsorption values obtained from experimental data.
Table 3
8a
8d
10a
10c
Bond lengths (°A)
C16-N29
N17-N20
C30-N31
C21-C23
1.27
1.38
1.15
1.46
O10-C11
C10-O14
C15-N28
N16-N19
1.41
1.19
1.27
1.39
O11-C12
C12-O15
C24-N25
C16-N23
1.37
1.21
1.15
1.32
C9-S41
1.82
C7-O10
C11-O14
C23-N24
1.36
1.21
1.15
Dihedral angles (°)
N29-C16-N17-N20
O15-C12-C13-C16
C18-N17-N20-C21
N31-C30-C19-C18
N20-C21-C23-C28
O15-C12-O11-C7
C22-C21-C23-C24
-1.45
4.11
5.78
N28-C15-C12-C11
O14-C11-O10-C7
N28-C15-N16-N19
C7-C8-S45-C9
N19-C20-C22-C27
O10-C7-C5-C4
12.27
177.56
-1.409
-31.19
-25.33
22.30
C7-O11-C12-O15
O11-C7-C5-C4
-176.18
15.16
-1.58
-0.15
-3.27
-19.05
4.45
O14-C11-O10-C7
O14-C11-C12-C15
C8-C9-S41-C6
N22-C15-N16-C32
C19-N31-C17-C18 -177.24
-176.41
-0.58
51.70
2.05
O15-C12-C13-C16
N23-C16-N17-C35
N34-C18-C19-C24
S25-C24-C19-C18
C19-C14-C8-C9
-4.70
25.17
-178.05
26.59
N24-C23-C18-C17
N31-C17-C18-C23
-35.79
-4.03
C17-N16-C15-N28
168.14