Photophysical studies of donor, acceptor substituted tetrahydrodibenzo[a,i]phenanthridines

Article abstract of DOI:10.1016/j.dyepig.2016.07.036

Highly luminescent, 5?aryl?7,8,13,14-tetrahydrodibenzo[a,i]phenanthridines (THDPs) in Donor(D)?π?Acceptor(A), A?π?A, A?π?A?π?A₁ configurations were synthesized in good to moderate yields by simple, two step reactions. All the phenyl rings were

Full text of DOI:10.1016/j.dyepig.2016.07.036

Dyes and Pigments 134 (2016) 409e418
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Photophysical studies of donor, acceptor substituted
tetrahydrodibenzo[a,i]phenanthridines
Balijapalli Umamahesh a, Udayadasan Sathiskumar b, George Augustine c,
,
Shanmugam Easwaramoorthi b *, Niraikulam Ayyadurai c,
,
**
Kulathu Iyer Sathiyanarayanan a
a School of Advanced Sciences, Vellore Institute of Technology University, Vellore, India
b Chemical Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai, 600020, India
c Biotechnology Division, CSIR-Central Leather Research Institute, Adyar, Chennai, 600020, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly luminescent, 5ꢀarylꢀ7,8,13,14-tetrahydrodibenzo[a,i]phenanthridines (THDPs) in Donor(D)ꢀ
Received 1 June 2016
Received in revised form
12 July 2016
Accepted 27 July 2016
Available online 31 July 2016
p
ꢀAcceptor(A), Aꢀ
p
ꢀA, Aꢀ
p
ꢀAꢀ ꢀA1 configurations were synthesized in good to moderate yields by
p
simple, two step reactions. All the phenyl rings were found to be in near-coplanar configuration due to
the enforced planarity by the ethylene bridge. As a result, efficient -conjugation was maintained
p
throught the molecule. UVevisible absorption and fluorescence spectra of THDP became shifted to longer
wavelengths upon substitution of different moieties at 5-Aryl position. A maximum of 200 nm red-
shifted fluorescence with respect to parent compound was observed for the nitro substituent. Howev-
er, tradeoffs in spectral shifts were noticed when two strongly electron withdrawing groups such as nitro
and cyano groups were present simultaneously. Presence of one or more acceptors induced the intra-
molecular electron transfer processes, and the photophysical properties of THDP core was tuned by
different substituents. Application of this fluorophore has also been demonstrated by means of DNA
interaction studies.
Keywords:
Phenanthridines
DNA intercalation
Photophysical studies
Fluorescence spectra
UV spectra
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
density with available positions for the introduction of functional
groups to tune the electronic properties, only limited attention has
Phenanthridine, a well-known polycyclic aromatic moiety has
been the basic structural moiety for natural products, biologically
and therapeutically active compounds and organic materials [1e5].
Indeed, phenanthridine derivatives such as ethidium bromide and
propidium iodide have widely been explored for their role as
fluorescent probe for biomolecules, owing to their high sensitivity
towards hydrophobic and hydrophilic environments [6e12].
Considering their versatility, influence of heteroaryl ring fusion,
and ease in substituting diverse functional groups, a variety of
derivatives based on phenanthridine skeleton have been developed
for different applications [13e15]. Most of the reports on phenan-
thridine derivatives have been studied for pharmaceutical and
been paid in understanding their electronic properties [18]. Indeed,
molecules having donorꢀ ꢀacceptor configurations (Dꢀ ꢀA)
p
p
were found to show desired properties for multitude of applica-
tions ranging from organic electronics [19e21], fluorescent sensors
[22e25], fluorescent probes for biological studies [26,27], and
nonlinear optics [28]. The flexible nature of donor-acceptor systems
offers exceptional tunability by diversifying the choice of electron
donating and withdrawing substituents to produce the desired
properties [29e34]. While polyaromatic compounds such as pyr-
ene, fluorene, anthracene, and phenothiazine have been widely
studied [35e37], not much work has been done on phenanthridine
skeleton despite its synthetic advantage. In this work, we report the
synthesis and photophysical studies of tetrahydrodibenzo[a,i]phe-
nanthridine (THDP) substituted with electron donating and with-
drawing groups. Recently, we have developed 5-aryl THDPs bearing
fashionable functionalities [38,39]; however, the detailed fluores-
cence properties were not reported. The chromophoric unit re-
sembles 2,3,5-triphenyl pyridines [39]. The presence of ethane
other biological applications [16,17]. Despite the rich
pꢀelectron
* Corresponding author.
** Corresponding author.
E-mail addresses: moorthi@clri.res.in (S. Easwaramoorthi), sathiya_kuna@
hotmail.com (K.I. Sathiyanarayanan).
http://dx.doi.org/10.1016/j.dyepig.2016.07.036
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

410
B. Umamahesh et al. / Dyes and Pigments 134 (2016) 409e418
bridge ensures the coplanarity of all aromatic rings, thereby aiding
the extensive -electron delocalization throughout the molecule.
3123, 4690 cmꢀ1 respectively, which is an indication of stronger
structural perturbations between the ground and excited state
geometries in presence of 5ꢀphenyl substituent. Indeed, substitu-
tion of 5-phenyl (5ae5d) and cyano groups on phenanthridine
p
2. Results and discussion
(6ae6d) respectively in Dꢀ
p
ꢀA, Aꢀ
p
ꢀDꢀ ꢀA format significantly
p
altered the absorption and emission spectra characteristic to the
nature of the substituents, as shown in Fig. 2. The spectral prop-
erties, summarized in Table 1 and Fig. 2, suggest that electron
donating OMe (5a), N(Me)2 (5b), N(Et)2 (5c) and electron with-
drawing NO2 (5d) groups in 5ꢀphenyl ring cause red-shifted ab-
sorption and fluorescence respectively by 10e52 nm, and
57e201 nm compared to 4. Surprisingly, 5d with nitro substitution
showed nearly 200 nm red-shifted (lfl ¼ 560 nm) fluorescence with
respect to the parent compound 4 (lfl ¼ 360 nm). It should be noted
that the spectral shift was larger in magnitude for fluorescence than
that of the absorption bands. The Stokes shift value, calculated from
the lowest energy absorption and the highest energy emission, was
found to be 4690, 7110, 7560, 6180, 11130 cmꢀ1 respectively for 4,
5a, 5b, 5c, and 5d. The larger Stokes shift values and the significant
substituent effect on fluorescence revealed strong intramolecular
charge transfer interactions between the phenanthridine and
substituted 5-phenyl moieties. Indeed, combinations of strong
electron donating, ꢀNR2 and electron accepting, cyano group
further red-shifted emission at least by 44 nm in 6b, 6c when
compared to 5b, 5c. On the other hand, the compound with elec-
The 7,8,13,14-tetrahydrodibenzo[a,i]phenanthridines (Scheme
1) were synthesized in a one-pot reaction in good yields, accord-
ing to the reported procedure [38,39]. Similar protocol was fol-
lowed
for
the
synthesis
of
2,10-dibromo-7,8,13,14
tetrahydrodibenzo[a,i] phenanthridines (Br-THDP, 5ee5h) which
was then converted to dicyano derivatives (6ae6d) in moderate
yields using CuCN in DMF (Scheme 1). The compounds were
characterized using FTIR, 1H NMR, 13C NMR, HRMS and single
crystal X-ray crystallographic techniques. The single crystal of 5b
was grown in 1:1 ratio of ethanol and tetrahydrofuran mixture, and
was then crystallized as triclinic system without solvation. The
ORTEP diagram of 5b given in Fig. 1 indicates that the dihedral
angle 32.12ꢁ respectively, is in fact smaller than that of the simple
biphenyl (44 1.2ꢁ). The significant decrease in dihedral between
the aromatic units viz. C3C8C9C21, C19C20C22C23, and
C10C19C18C13 was measured to be 24.69ꢁ, 37.30ꢁ and the angle
was attributed to the enforced coplanarity caused by ethylene
bridge that connected 2, 20 positions of both the phenyl groups.
The UVeVisible absorption spectra of the parent compound 3,
given in Fig. 2, shows two well-resolved peaks in acetonitrile sol-
vent with the peak maxima at 294, 322 nm. The fluorescence
emission having mirror image relationship with lowest energy
absorption band was observed at 358 nm. While going from 3 to 4,
by introducing the phenyl group at the 5th position of tetrahy-
drodibenzo[a,i]phenanthridine ring, absorption spectra became
blue shifted by 14 nm and the emission maximum was found to be
slightly red shifted. Overall Stokes shift for 3 and 4 corresponded to
tron withdrawing nitro substituent, 6d in Aꢀ
p
ꢀDꢀ(
p
ꢀA)ꢀ ꢀA1
p
format shows fluorescence at 518 nm, which is blue shifted by
43 nm when compared to 5d. These features suggest that the
photophysical properties could be altered efficiently by the judi-
cious choice of donor and acceptor combinations.
The fluorescence quantum yields (ffl) measured with reference
to quinine sulphate in 0.5 M H2SO4 is given in Table 1. It reveals that
5-substituted tetrahydrodibenzo[a,i]phenanthridines.
Scheme 1. Synthetic route for Dꢀ
pꢀA, Aꢀ
pꢀA, Aꢀ
p
ꢀAꢀ ꢀA1 phenanthridines.
p

B. Umamahesh et al. / Dyes and Pigments 134 (2016) 409e418
411
Fig. 1. ORTEP diagram of compound 5b with ellipsoid shown at the 40% contour percent probability level (CCDC-1446308).
from the intramolecular charge transfer (ICT) interactions between
the electron rich and deficient groups. Solvent polarity induced
red-shifted fluorescence was observed for all the compounds,
however to a different extent, which indicates the ICT interactions.
Representative fluorescence spectra of compounds 5b, 5c, 6b, and
6c in solvents of different polarity are given in Fig. 3, and the data
are compiled in Table 2 (see the Supporting information, Fig. S2).
The fluorescence spectrum became gradually red-shifted with an
increase in solvent polarity due to the larger, solvent polarity
induced dipole moment changes between the ground and excited
states. It should be noted that the introduction of electron with-
drawing cyano group in the phenanthridine moiety induced
significantly enhanced charge transfer character, as can be seen
from large spectral shifts for 6b, 6c than 5b and 5c. This feature can
be ascribed to the presence of both-electron donating and with-
drawing groups connected through
pꢀconjugation. Indeed, the
magnitude of ICT character could also be understood from the slope
value of the Lippert-Mataga13 plot, an indication of the difference in
stabilization of ICT states by the substituents (see the Supporting
information, Fig. S3). For instance, electron donating groups ꢀNR2,
exhibited larger slope value ~9000 cmꢀ1 while that of ꢀOCH3 was
calculated to be 3650 cmꢀ1. Further, the lifetime of the parent
compound 3 was found to remain constant irrespective of the
solvent polarity, the donor-acceptor substituted one show pro-
nounced enhancement in fluorescence lifetime in polar solvents.
The corresponding change in fluorescence lifetime spectra with
respect to solvent polarity for compounds 5b, 5c, 6b and 6c is
depicted in Fig. 4. This feature can be ascribed to the emission from
the charge transferred state. Further, it can be expected that the
N,N-dialkylamino groups would form highly conjugated quinonoid
like charge transferred state having intervening double bonds be-
tween the phenyl rings as depicted in Scheme 2.
Interestingly, the presence of two cyano groups in 6 offers
possibly, two different CT states having ortho and or para quinonoid
structures. It is really difficult to discriminate between the CT states
experimentally, as both of them are expected to show sol-
vatochromism in fluorescence. To explore this further, molecular
orbitals were calculated at B3LYP/6-31G level using Gaussian 03
software [40,41] using the geometry optimized structure at the
same level of theory and corresponding molecular orbitals was
given in Fig. 5 (see the Supporting information, Fig. S4). While the
Fig. 2. UVeVisible absorption (ꢀ) and fluorescence (—) spectra of compounds 3, 4,
5ae5d and 6ae6d measured in acetonitrile solvent.
the ffl becomes enhanced, by the factor of 180, 320 times respec-
tively by ꢀN(CH3)2 (5b, 6b) andꢀN(C2H5)2 (5c, 6c) when compared
to the parent compound 4. To get more insights, fluorescence life-
times of the samples in solution state were measured using time
correlated single photon counting technique by exciting the sam-
ples at 375 nm. The decay profiles of the samples monitored at
respective emission wavelength and the corresponding data are
summarized in Table 1 (see the Supporting information, Fig. S1).
The fluorescence lifetime of 4, 5a, 5b, 5c, and 5d was measured to
be 1.39, 1.44, 1.19, 0.68, 2.05 ns respectively in acetonitrile solvent.
Longer fluorescence lifetimes were observed for 6b, 6c with
Aꢀ
p
ꢀDꢀ ꢀD1 system than that of the corresponding D-A systems
p
such as compounds 5b and 5c. This feature would have originated

412
B. Umamahesh et al. / Dyes and Pigments 134 (2016) 409e418
Table 1
Photophysical properties of tetrahydrodibenzophenanthridines in acetonitrile solvent.
Comp
labs nm (ε 10ꢀ1 Mꢀ1 cmꢀ1
)
lfl nm
Stokes shift cmꢀ1
aFfl
btfl ns
Kr [108 sꢀ1
]
Knr [108 sꢀ1
]
3
4
257 (0.42), 283 (0.18), 294 (0.12), 322 (0.16)
263 (0.36), 291 (0.18), 308 (0.15)
266 (0.34), 282 (0.26), 293 (0.22), 318 (0.19)
255 (0.31), 292 (0.16), 342 (0.20)
263 (0.37), 299 (0.17), 360 (0.22)
258 (0.33), 274 (0.30), 344 (0.14)
237 (0.27), 283 (0.40), 328 (0.28)
279 (0.34), 302 (0.28), 337 (0.18)
278 (0.38), 310 (0.25), 366 (0.18)
251 (0.22), 288 (0.28), 345 (0.12)
358
360
417
463
463
561
432
507
525
518
3123
4690
7110
7560
6180
11130
7220
9350
8730
9610
0.690
0.001
0.007
0.182
0.351
0.037
0.036
0.191
0.339
0.039
3.66
1.39
1.44
1.19
0.68
2.05
ꢂIRFc
3.56
3.77
1.11
0.189
0.001
0.005
0.153
0.516
0.018
e
0.085
0.719
0.709
0.688
0.954
0.470
e
5a
5b
5c
5d
6a
6b
6c
6d
0.054
0.090
0.035
0.227
0.175
0.862
a
Fluorescence quantum yields were measured with reference to quinine sulfateFfl ¼ 0.546 in H2SO4 0.5 M, lfl ¼ 310 nm.
The decay profiles were monitored at respective fluorescence maximum.
IRF-Instrument response function.
b
c
5c
5b
0.9
Hexane
Toluene
Ethylacetate
Dichloromethane
Tetrahydrofuran
Methanol
Acetonitrile
Dimethylsulfoxide
0.6
0.3
0.0
0.9
6c
6b
0.6
0.3
0.0
420 480 540 600 660
420 480 540 600 660
Wavelength (nm)
Fig. 3. Fluorescence spectra for the compounds 5b, 5c, 6b and 6c measured in solvents of variable polarities.
electron donating groups destabilized the highest occupied mo-
lecular orbitals (HOMO) at least by 0.54 eV with respect to parent
compound, the lowest unoccupied molecular orbital (LUMO) did
not show any significant changes with substitution. On the con-
trary, the HOMO was stabilized by 0.18, 0.80 eV respectively for 5d
and 6d when compared to 4. Overall decrease in HOMO-LUMO gap
was observed when both the donor-acceptor moieties were pre-
sent. The theoretically calculated HOMO-LUMO values were in
good agreement with the experimental values (see Table 3). The
electron density, mainly localized on 5-phenyl substituent, became
redistributed in phenanthridine core during transition from HOMO
to LUMO. In contrast, compound 5d with nitro substituent showed
complete shift in electron density from phenanthridine core to
nitrophenyl ring during transition from HOMO to LUMO orbital.
The frontier orbitals did not distinguish the ortho and para quino-
noid forms and hence both might contribute to the charge trans-
ferred state. Similar behaviour was observed for the compound 6d
that had both nitro and cyano groups. Among them, nitro group
was found to be strong acceptor.
(dsDNA), which is a powerful method for many molecular biology
experiments, was carried out. Hence, the intercalating efficiency of
compound 3 to 6d was investigated with bacterial genomic DNA,
PCR product, and DNA ladders. Efficient separations and repro-
ducibility of migration time of dsDNA fragments were obtained for
all the compounds. The linear range and sharp intensity of dsDNA
ladder mix was comparable with the commonly used ethidium
bromide. Further, the PCR product and genomic DNA interaction
indicated that detection, or intercalating efficiency of all the com-
pounds were high and well suited for DNA electrophoresis or other
dsDNA quantification analysis (Fig. 6). On the other hand, it has
been well documented that DNA intercalating agents are not only
used for labelling studies but also plays an essential role in devel-
oping several clinically used anticancer and antibiotic drugs. In this
connection much attention has been focused to rational designing,
new natural products and synthesis of potential synthetic and
efficient DNA-targeting cytotoxic intercalators. Our results on
compound 3 to 6d intercalating efficiency encourage us to believe
that it can also be used as a potential drug for cancer therapy.
However, to prove our state of concept, further studies may be
needed for better understanding of the DNA intercalating
To evaluate the applications of THDP derivatives and the role of
substituents, labelling and detection of double strand DNA

B. Umamahesh et al. / Dyes and Pigments 134 (2016) 409e418
413
Table 2
Optical data measured in different solvents.
Compound
Solvent
labs. nm
lfl. nm
Stoke's shift cmꢀ1
Ffl
tfl ns
Kr [108 sꢀ1
]
Knr [ 108 sꢀ1
]
3
Hexane
Toluene
259, 282, 294, 323
295, 323
358
362
363
356
358
361
358
360
363
361
361
362
359
362
360
360
389, 408
419
424
420
425
405, 428
416
415
422
436
449
452
449
460
463
470
425
437
449
452
449
461
463
470
e
3027
3335
3508
2870
3813
3163
3123
2991
4814
4350
3746
4325
3992
3822
4690
3868
6546
7384
7471
7246
8729
8082
7114
7154
5205
5554
6547
6612
7314
7501
7556
7295
4640
4972
5739
5887
6630
6717
6180
6424
-
0.149
0.513
0.291
0.665
0.567
0.141
0.690
0.227
0.001
0.002
0.010
0.009
0.030
0.002
0.001
0.003
0.015
0.011
0.051
0.027
0.362
0.017
0.007
0.012
0.050
0.112
0.269
0.396
0.164
0.080
0.182
0.215
0.112
0.123
0.446
0.437
0.321
0.106
0.351
0.494
-
3.14
3.24
3.67
3.33
3.46
3.96
3.66
3.88
0.26
<IRF
<IRF
<IRF
<IRF
0.29
1.39
0.51
0.45
<IRF
0.25
<IRF
<IRF
0.36
1.44
0.67
<IRF
0.27
0.80
0.91
0.75
<IRF
1.19
2.13
<IRF
0.80
1.22
1.06
0.85
0.68
0.68
1.02
e
0.047
0.158
0.079
0.199
0.163
0.035
0.188
0.058
0.004
e
0.271
0.150
0.193
0.101
0.125
0.217
0.084
0.199
3.842
e
Dichloromethane
Tetrahydrofuran
Ethyl acetate
Methanol
Acetonitrile
Dimethylsulfoxide
Hexane
266, 293, 322
256, 283, 295, 323
274, 285, 315
261, 281, 294, 324
255, 282, 293, 322
263, 285, 297, 325
264, 291, 309
312
266, 294, 318
266, 293, 313
265, 292, 314
265, 292, 318
263, 291, 308
266, 294, 316
268, 282, 294, 322
294, 320
267, 282, 322
267, 280, 294, 322
258, 273, 285, 310
266, 280, 291, 318
267, 282, 295, 321
268, 282, 294, 320
255, 273, 289, 346
292, 351
258, 292, 347
253, 291, 348
283, 338
256, 291, 342
255, 290, 343
293, 350
259, 291, 355
292, 359
262, 293, 357
255, 292, 357
283, 346
261, 291, 352
259, 298, 360
263, 292, 361
251, 276, 339
294, 351
261, 278, 353
250, 277, 350
267, 339
258, 270, 334
255, 274, 345
266, 352
245, 282, 336
295, 334
286, 333
287, 332
276, 324
282, 326
283, 329
293, 331
270, 348
301, 344
4
Toluene
Dichloromethane
Tetrahydrofuran
Ethyl acetate
Methanol
Acetonitrile
Dimethylsulfoxide
Hexane
e
e
e
e
e
e
0.007
0.001
0.006
0.033
e
3.441
0.719
1.955
2.188
e
5a
5b
5c
5d
6a
6b
6c
Toluene
Dichloromethane
Tetrahydrofuran
Ethyl acetate
Methanol
Acetonitrile
Dimethylsulfoxide
Hexane
0.204
e
3.796
e
e
e
0.047
0.004
0.017
2.730
0.689
1.474
e
Toluene
0.414
0.336
0.435
0.218
e
3.288
0.913
0.663
1.114
e
Dichloromethane
Tetrahydrofuran
Ethyl acetate
Methanol
Acetonitrile
Dimethylsulfoxide
Hexane
0.152
0.100
e
0.687
0.368
e
Toluene
0.154
0.366
0.412
0.378
0.156
0.516
0.484
e
1.096
0.454
0.531
0.799
1.315
0.954
0.496
e
Dichloromethane
Tetrahydrofuran
Ethyl acetate
Methanol
Acetonitrile
Dimethylsulfoxide
Hexane
Toluene
e
-
-
e
e
e
Dichloromethane
Tetrahydrofuran
Ethyl acetate
Methanol
Acetonitrile
Dimethylsulfoxide
Hexane
540
e
9810
-
-
0.059
-
-
1.13
e
0.052
e
0.832
e
e
e
e
e
e
-
-
e
e
e
560
563
420
422
425
426
423
426
430
435
444
469
482
500
489
488
505
516
455
474
493
499
494
525
525
541
11128
10647
5952
6243
6501
6646
7224
7201
7139
7223
6213
7748
8493
9070
9580
9629
9353
9690
5122
6077
7264
7432
7690
8730
8730
8182
0.037
0.085
0.048
0.007
0.066
0.050
0.188
0.048
0.036
0.039
0.193
0.125
0.573
0.379
0.183
0.116
0.191
0.214
0.209
0.427
0.842
0.740
0.565
0.148
0.339
0.470
2.05
1.09
<IRF
<IRF
<IRF
<IRF
<IRF
<IRF
<IRF
0.27
0.60
1.40
1.68
2.93
2.55
1.13
3.56
1.79
1.07
1.48
1.53
3.19
2.74
1.77
3.77
2.42
0.018
0.078
0.470
0.839
Toluene
Dichloromethane
Tetrahydrofuran
Ethyl acetate
Methanol
Acetonitrile
Dimethylsulfoxide
Hexane
0.144
0.322
0.089
0.341
0.129
0.072
0.103
0.054
0.200
0.195
0.289
0.550
0.232
0.206
0.084
0.090
0.194
3.559
1.345
0.625
0.254
0.212
0.320
0.782
0.227
0.734
0.739
0.387
0.103
0.082
0.159
0.481
0.175
0.219
Toluene
Dichloromethane
Tetrahydrofuran
Ethyl acetate
Methanol
Acetonitrile
Dimethylsulfoxide
Hexane
281, 303, 342
278, 303, 344
271, 293, 333
279, 332
256, 290, 343
282, 304, 344
273, 302, 369
304, 368
274, 305, 363
273, 303, 364
264, 296, 358
273, 360
260, 298, 360
275, 306, 375
Toluene
Dichloromethane
Tetrahydrofuran
Ethyl acetate
Methanol
Acetonitrile
Dimethylsulfoxide
(continued on next page)

414
B. Umamahesh et al. / Dyes and Pigments 134 (2016) 409e418
Table 2 (continued )
Compound
6d
Solvent
labs. nm
l
fl. nm
Stoke's shift cmꢀ1
Ffl
tfl ns
Kr [108 sꢀ1
]
Knr [ 108 sꢀ1
]
Hexane
Toluene
280, 350
299, 343
284, 345
283, 331
274, 329
282, 333
288, 349
282, 360
413
443
475
469
452
475
525
510
4358
6581
7933
8890
8271
8977
9606
8170
0.300
0.014
0.292
0.043
0.015
0.010
0.039
0.073
0.64
0.88
1.46
1.36
1.11
0.84
1.11
1.24
0.515
0.015
0.200
0.031
0.013
0.011
0.035
0.058
1.046
1.120
0.484
0.703
0.887
1.178
0.865
0.747
Dichloromethane
Tetrahydrofuran
Ethyl acetate
Methanol
Acetonitrile
Dimethylsulfoxide
Fig. 4. Fluorescence lifetime decay profile for the compounds 5b, 5c, 6b and 6c measured in solvents of variable polarities.
Scheme 2. Feasible charge transfer states in A-p-D system (compounds 6b and 6c).
mechanisms and cytotoxicity efficiency, which will facilitate the
improvement in selectivity to become a potent DNA intercalator.
Current studies, including understanding the mechanism along
with those lines of research, are on-going in our laboratories.
phenanthridines with fashionable useful functionalities and
exceptional fluorescence properties. The electronic properties of
phenanthridine were significantly altered in a desired way by
introducing suitable substituents. The combinations of electron
donating and withdrawing groups as well as two different electron
withdrawing groups were also found to induce the intramolecular
charge transfer interactions, however, to a different extent. Indeed,
the intercalating efficiency of all these compounds is also
3. Conclusions
In conclusion, we have developed tetrahydrodibenzo[a,i]

B. Umamahesh et al. / Dyes and Pigments 134 (2016) 409e418
415
Fig. 5. Frontier molecular orbitals of phenanthridines at B3LYP/6-31G level of theory using Gaussian 03 software.
Table 3
Theoretical and electrochemical properties.
Compound
aEox (V)
aEred (V)
bEHOMO (eV)
bELUMO (eV)
cEg (eV)
dHOMO (eV)
dLUMO (eV)
eEg (eV)
fDE00 (eV)
3
4
1.68
1.64
1.52
1.45
1.25
1.75
1.64
0.99
0.97
1.67
ꢀ1.51
ꢀ1.52
ꢀ1.87
ꢀ1.89
ꢀ1.93
ꢀ1.37
ꢀ1.75
ꢀ1.82
ꢀ1.80
ꢀ1.40
ꢀ6.08
ꢀ6.04
ꢀ5.92
ꢀ5.85
ꢀ5.65
ꢀ6.15
ꢀ6.04
ꢀ5.39
ꢀ5.37
ꢀ6.07
ꢀ2.89
ꢀ2.88
ꢀ2.53
ꢀ2.51
ꢀ2.47
ꢀ3.03
ꢀ2.65
ꢀ2.58
ꢀ2.60
ꢀ3.00
3.19
3.16
3.39
3.34
3.18
3.12
3.39
2.81
2.77
3.07
ꢀ5.89648
ꢀ5.89644
ꢀ5.35470
ꢀ4.74162
ꢀ4.73944
ꢀ6.07853
ꢀ5.88124
ꢀ5.18408
ꢀ5.15360
ꢀ6.69569
ꢀ1.06751
ꢀ1.06750
ꢀ1.19513
ꢀ1.03431
ꢀ1.04275
ꢀ2.73477
ꢀ2.13665
ꢀ1.99407
ꢀ1.99407
ꢀ3.12389
4.83
4.82
4.16
3.71
3.69
3.34
3.74
3.18
3.16
3.57
3.58
3.57
3.51
3.16
3.03
3.06
3.39
2.90
2.80
3.08
5a
5b
5c
5d
6a
6b
6c
6d
a
The redox potential of compounds obtained from cyclic voltammetry using glassy carbon as working electrode with reference to Fc/Fc þ couple. 0.1 M Tetrabuty-
lammonium perchlorate was used as a supporting electrolyte.
b
EHOMO/LUMO HOMO and LUMO energy levels calculated from the redox potentials.
c
d
e
f
Eg ¼ electrochemical HOMOeLUMO energy gap.
HOMO, LUMO energy calculated using Gaussian 03 programme at B3LYP/6-31G level.
Computed HOMOeLUMO energy gap.
Optical HOMOeLUMO energy gap (ZeroeZero transition energy estimated from the point of intersection of normalized absorption and emission spectra in acetonitrile).
comparable with that of the standard molecule ethidium bromide.
This work provides basic understanding about the electronic
properties of phenanthridine derivatives, and its spectral proper-
ties can be extended to NIR region and the work is in progress.
4. Experimental section
4.1. General information
All organic chemicals and solvents were purchased from Sigma
Aldrich, SDFine and were used without further purification. 1H and
13C NMR spectra were taken on Bruker 400 MHz using CDCl3 as the
solvent with TMS as an internal standard. Melting points were
measured on Microprocessor based melting point apparatus, and
were not corrected. HRMS values were obtained on Joel GC Mate II
GC- Mass Spectrometer. FTIR spectra of the synthesized organic
compounds were recorded using a Jasco-4100 spectrometer in-
strument. UVeVisible spectra were taken using Shimadzu UV-1800
spectrophotometer. Fluorescence spectra in solution and solid
states were measured using CARY-Eclipse fluorescence spectro-
photometer. The fluorescence quantum yields (Ffl) were measured
with quinine sulphate (Ffl) 0.546 in 0.5 N H2SO4 solution as a
standard, lex ¼ 310 nm. Analytical Thin-layer chromatography
(TLC) was performed on pre-coated plates (Merck, silica gel 60
F254). Silica gel (60e120 mesh) was used for column
Fig. 6. The intercalating efficiency of compound 3 with different types of dsDNA. (A)
DNA ladder with linear range of dsDNA product, (B) Bacterial genomic DNA (c) PCR
product.
chromatography. Single crystal X-ray diffraction data were taken on
bruker kappa APEXII. The structures were solved by direct methods.
Fluorescence life time measurements were recorded using the IBH
fluorescence lifetime spectrometer. Electrochemical measurements

416
B. Umamahesh et al. / Dyes and Pigments 134 (2016) 409e418
were made using a CH instruments CH 600E electrochemical ana-
lyser. A conventional three-electrode configuration, consisting of a
glassy carbon working electrode, Pt-wire counter electrode, and Ag/
AgCl reference electrode, was used. 0.1 M [Bu4N]ClO4 was used as
supporting electrolyte.
158.1; HRMS for C27H21N Calculated [Mþ] m/z 359.1674, Found
359.1660.
4.3.3. 5-(4-methoxyphenyl)-7,8,13,14-tetrahydrodibenzo[a,i]
phenanthridine (5a)
Pale yellow solid; Melting point: 246e248 ꢁC; IR (KBr): 3049,
3032, 2993, 2900, 2833, 2326, 1745, 1604, 1543, 1508, 1396, 1290,
1230, 1105, 1029, 879, 833, 746, 717, 586 cmꢀ1; 1HNMR (400 MHz,
CDCl3)ppm: 2.77e2.74 (t, J ¼ 6.0 Hz, 2H), 2.95e2.92 (t, J ¼ 6.0 Hz,
2H), 3.08e3.06 (t, J ¼ 4.0 Hz, 2H), 3.13e3.09 (t, J ¼ 8.0 Hz, 2H), 3.82
(s, 3H), 6.87e6.85 (d, J ¼ 8.0 Hz, 2H), 6.93e6.89 (t, J ¼ 8.0 Hz, 1H),
6.97e6.95 (d, J ¼ 8.0 Hz, 1H), 7.13e7.09 (t, J ¼ 8.0 Hz, 1H), 7.35e7.23
(m, 4H), 7.40e7.38 (d, J ¼ 8.0, 2H), 7.50e7.48 (d, J ¼ 8.0, 1H); 13C
NMR (100 MHz, CDCl3)ppm:29.3, 29.5, 29.6, 33.2, 55.3, 113.8, 125.7,
126.0, 126.8, 127.1, 127.5, 127.8, 128.6, 128.7, 129.5, 131.1, 133.2, 133.3,
134.5, 138.7, 139.6, 145.7, 153.6, 158.1, 159.4; HRMS for C28H23NO
Calculated [Mþ] m/z 389.1780, Found 389.1780.
4.2. General procedure for the synthesis of 5-phenyl-7,8,13,14-
tetrahydrodibenzo[a,i] phenanthridine derivatives (3, 4, 5ae5d)
A
mixture of acetaldehyde/substituted benzaldehydes (2)
(10 mmol) and ammonium acetate (3) (11 mmol) was taken in a
100 ml conical flask containing 10 ml absolute ethanol at room
temperature. It was sealed and warmed using water bath for 5 min
until the dissolution of the solid contents. After bringing the reac-
tion mixture to room temperature, 1 equivalent of 2-tetralone (1)
(20 mmol) was added and sealed. Then, the mixture was warmed
for 5 min and the reaction mixture was kept aside for 24 h in open
air. After the completion of the reaction, as monitored by TLC, the
resulting product was purified by column chromatography over
silica gel (60e120 mesh) using n-hexane and ethyl acetate mixture
(9:1) as eluent to give the compounds (3, 4, 5aed). Thus, the ob-
tained solid was further purified by recrystallizing in 1:1 ethanol
and tetrahydrofuran mixture.
4.3.4. N,N-dimethyl-4-(7,8,13,14-tetrahydrodibenzo[a,i]
phenanthridin-5-yl)aniline (5b)
Yellow solid; Melting point: 216e218 ꢁC; IR (KBr): 3053, 3017,
2937, 2829, 2326,1764,1579,1506,1454,1392,1361,1230,1123, 997,
943, 742, 673, 663 cmꢀ1; 1HNMR (400 MHz, CDCl3)ppm: 2.77e2.74
(t, J ¼ 6.0 Hz, 2H), 2.95e2.92 (t, J ¼ 6.0 Hz, 2H), 3.96 (s,
6H),3.08e3.06 (t, J ¼ 4.0 Hz, 2H), 3.13e3.09 (t, J ¼ 8.0 Hz, 2H),
6.68e6.66 (d, J ¼ 8.0 Hz, 2H), 6.96e6.92 (t, J ¼ 8.0 Hz, 1H) 7.13e7.08
(m, 2H), 7.35e7.23 (m, 6H), 7.50e7.48 (d, J ¼ 8.0, 1H); 13C NMR
(100 MHz, CDCl3)ppm: 29.3, 29.6, 29.6, 33.2, 40.5, 112.2, 125.6,
125.9, 126.5, 126.6, 126.7, 127.3, 127.8, 128.2, 128.6, 129.5, 130.0,
130.8, 133.3, 133.7, 138.5, 139.6, 145.6, 150.3, 154.2, 158.0; HRMS for
4.3. General procedure for the synthesis of 5-phenyl-7,8,13,14-
tetrahydrodibenzo [a,i]phenanthridine-2,10-dicarbonitrile
derivatives (6ae6d)
A mixture of 5-phenyl-7,8,13,14-tetrahydrodibenzo[a,i]phenan-
thridine derivatives (1.0 mmol) and CuCN (10 mmol) in dry DMF
(50 mL) was heated to reflux under N2 for 72 h. The mixture was
cooled to room temperature, water (100 mL) was added, and the
resulting white precipitate was collected by filtration. The solid was
then washed with a 30% aqueous solution of ethylenediamine until
the washings were colorless. The solid was treated with dichloro-
methane, and the suspension was washed with 30% aqueous eth-
ylenediamine and then with cold water, and finally with brine
solution. The organic phase was dried over anhydrous MgSO4 and
filtered, and volatiles were then removed by evaporation under
reduced pressure. Column chromatography on silica gel (petroleum
C
29H26N2 Calculated [Mþ] m/z 402.2096, Found 402.2090.
4.3.5. N,N-diethyl-4-(7,8,13,14-tetrahydrodibenzo[a,i]
phenanthridin-5-yl)aniline (5c)
Yellow solid; Melting point: 178e180 ꢁC; IR (KBr): 3059, 3027,
2968, 2893, 2835, 2326, 1712, 1606, 1517, 1427, 1398, 1371, 1267,
1195, 1180, 1076, 1006, 798, 746, 623 cmꢀ1 1HNMR (400 MHz,
;
CDCl3)ppm: 1.17e1.13 (t, J ¼ 8.0 Hz, 6H), 2.77e2.74 (t, J ¼ 6.0 Hz,
2H), 2.95e2.92 (t, J ¼ 6.0 Hz, 2H), 3.12e3.06 (m, 4H), 3.39e3.33 (q,
J ¼ 8.0 Hz, 4H), 6.63e6.61 (d, J ¼ 8.0 Hz, 2H), 6.96e6.92 (t, J ¼ 8.0 Hz,
1H) 7.16e7.09 (m, 2H), 7.34e7.23 (m, 6H), 7.50e7.48 (d, J ¼ 8.0, 1H);
13C NMR (100 MHz, CDCl3)ppm:12.6, 29.3, 29.6, 29.6, 33.3, 44.4,
111.7,125.6,125.9,126.4,126.6,126.7,127.3,127.8,128.1,128.6,128.8,
129.5, 130.9, 133.4, 133.9, 138.5, 139.6, 145.6, 147.6, 154.3, 158.0;
HRMS for C31H30N2 Calculated [Mþ] m/z 430.2409, Found 430.2400.
ether
and
CH2Cl2)
afforded
5-(4-phenyl)-7,8,13,14-
tetrahydrodibenzo[a,i]phenanthridine-2,10-dicarbonitrile
rivatives as yellow colour solids.
de-
4.3.1. 7,8,13,14-tetrahydrodibenzo[a,i]phenanthridine (3)
White solid; Melting point: 78e80 ꢁC; IR (KBr): 3076, 3002,
2999, 2962, 2801,1739, 1704, 1643, 1501, 1345, 1273, 1201, 1021, 859,
832, 759, 701, 635 cmꢀ1; 1HNMR (400 MHz, CDCl3)ppm: 3.08e3.04
(t, J ¼ 8.0 Hz, 4H), 3.20e3.17 (t, J ¼ 6.0 Hz, 4H), 7.41e7.32 (m, 6H),
7.85e7.83 (d, J ¼ 8.0 Hz, 2H), 8.38 (s,1H); 13C NMR (100 MHz, CDCl3)
ppm: 27.5, 28.64, 29.0, 30.9, 123.5, 125.9, 126.1, 127.2, 127.3, 127.8,
127.9, 128.0, 128.2, 128.4, 128.7, 128.7, 129.0, 132.8, 136.7, 136.9,
155.6; HRMS for C21H17N Calculated [Mþ] m/z 283.1361, Found
283.1366.
4.3.6. 5-(4-nitrophenyl)-7,8,13,14-tetrahydrodibenzo[a,i]
phenanthridine (5d)
Yellow solid; Melting point: 230e232 ꢁC; IR (KBr): 3051, 3021,
2947, 2883, 2841, 2326, 1745, 1691, 1593, 1556, 1508, 1394, 1340,
1290, 1132, 765, 744, 663 cmꢀ1 1HNMR (400 MHz, CDCl3)ppm:
;
2.82e2.79 (t, J ¼ 6.0 Hz, 2H), 2.99e2.96 (t, J ¼ 6.0 Hz, 2H), 3.10e3.07
(t, J ¼ 6.0 Hz, 2H), 3.19e3.16 (t, J ¼ 6.0 Hz, 2H), 6.80e6.78 (d,
J ¼ 8.0 Hz, 1H), 6.94e6.90 (t, J ¼ 8.0 Hz, 1H), 7.20e7.16 (t, J ¼ 8.0 Hz,
1H), 7.39e7.26 (m, 4H), 7.54e7.52 (d, J ¼ 8.0, 1H), 7.66e7.64 (d,
J ¼ 8.0 Hz, 2H), 8.21e8.19 (d, J ¼ 8.0 Hz, 2H); 13C NMR (100 MHz,
CDCl3)ppm:29.1, 29.4, 29.5, 33.1, 123.6, 126.0, 126.2, 127.3, 127.7,
128.0, 128.1, 128.5, 128.8, 129.5, 129.6, 130.9, 132.1, 132.6, 138.9,
139.7, 146.11, 147.28, 148.7, 151.1, 158.6; HRMS for C27H20N2O2
Calculated [Mþ] m/z 404.1525, Found 404.1526.
4.3.2. 5-Phenyl-7,8,13,14-tetrahydrodibenzo[a,i]phenanthridine (4)
White solid; Melting point: 223e225 ꢁC; IR (KBr): 3066, 3034,
2960, 2929, 2831, 2326, 1737, 1544, 1487, 1425, 1396, 1290, 1089,
945, 835, 759, 744, 617 cmꢀ1 1HNMR (400 MHz, CDCl3)ppm:
:
2.79e2.75 (t, J ¼ 8.0 Hz, 2H), 2.97e2.93 (t, J ¼ 8.0 Hz, 2H),
3.16e3.08 (m, 4H), 6.88e6.87 (d, J ¼ 4.0 Hz, 2H), 7.13e7.09 (m, 1H),
7.36e7.24 (m, 7H), 7.46e7.43 (dd, J ¼ 16.0, 4.0 Hz, 2H), 7.52e7.50
(d, J ¼ 8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3)ppm: 29.2, 29.5, 29.6,
33.2,125.6,126.0,126.9,126.9,127.5,127.7,127.8,127.9,128.4,128.7,
128.8, 129.7, 129.8, 133.0, 133.1, 138.7, 139.7, 142.1, 145.7, 154.0,
4.3.7. 5-(4-methoxyphenyl)-7,8,13,14-tetrahydrodibenzo[a,i]
phenanthridine-2,10-dicarbonitrile (6a)
Pale yellow solid; Melting point: 218e220 ꢁC; IR (KBr): 3047,
3022, 3001, 2983, 2900, 2854, 2327, 1755, 1607, 1547, 1501, 1386,

B. Umamahesh et al. / Dyes and Pigments 134 (2016) 409e418
417
1295, 1230, 1115, 1039, 897, 833, 749, 714, 645, 576 cmꢀ1
;
1HNMR
Acknowledgements
(400 MHz, CDCl3)ppm: 2.78e2.75 (t, J ¼ 6.0 Hz, 2H), 2.94e2.90 (t,
J ¼ 8.0 Hz, 2H), 3.05e3.02 (m, 4H), 3.78 (s, 3H), 6.83e6.81 (d,
J ¼ 8.0 Hz, 2H), 6.99e6.97 (d, J ¼ 8.0 Hz, 1H), 7.16e7.13 (dd, J ¼ 16.0,
4.0 Hz, 1H), 7.31e7.25 (m, 2H), 7.56e7.48 (m, 4H); 13C NMR
(100 MHz, CDCl3)ppm: 14.1, 28.9, 29.7, 55.4, 114.2, 128.9, 129.5,
130.0, 130.6, 131.2, 160.2; HRMS for C30H21N3O Calculated [Mþ] m/z
439.1685, Found 439.1690.
B.U thanks CSIR for providing Senior Research Fellowship (CSIR-
SRF, Acknowledgement No-09/844(0018)/2013EMR-I). S. E and U. S
thank DST (Grant No.SERB/F/4232/2013-15) India. The work is also
supported by CSIR suprainstitutional, XIIth five year plan project
STRAIT. We also thank VIT-SIF, NCUFP-University of Madras, and
SAIF-IIT Madras for NMR, lifetime and single crystal X-ray diffrac-
tion facilities respectively. The authors thank Dr. R. Srinivasan, SSL-
VIT for language editing.
4.3.8. 5-(4-(dimethylamino)phenyl)-7,8,13,14-tetrahydrodibenzo
[a,i]phenanthridine-2,10-dicarbonitrile (6b)
Yellow solid; Melting point: 216e218 ꢁC; IR (KBr): 3066, 3010,
2951, 2900, 2888, 2857, 2329, 1759, 1504, 1499, 1464, 1431, 1429,
1390, 1301, 1214, 1105,1088, 955, 801, 741 cmꢀ1; 1HNMR (400 MHz,
CDCl3)ppm: 2.66e2.63 (t, J ¼ 6.0 Hz, 2H), 2.82e2.79 (t, J ¼ 6.0 Hz,
2H), 2.88 (s, 6H), 2.96e2.93 (m, 4H), 6.58e6.56 (d, J ¼ 8.0 Hz, 2H),
6.83e6.81 (d, J ¼ 8.0 Hz, 1H)), 6.96e6.94 (d, J ¼ 8.0 Hz, 1H),
7.29e7.19 (m, 3H), 7.33e7.29 (m, 2H), 7.39 (s, 1H); 13C NMR
(100 MHz, CDCl3)ppm: 23.7, 23.9, 27.5, 34.2, 106.8, 115.2, 123.4,
123.7, 124.3, 124.6, 125.3, 125.6, 135.2, 136.3, 145.1, 152.2; HRMS for
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2016.07.036.
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phenanthridine-2,10-dicarbonitrile (6c)
Yellow solid; Melting point: 212e214 ꢁC; IR (KBr): 3109, 3017,
2974, 2960, 2838, 2333, 1885, 1756, 1636, 1539, 1477, 1423, 1389,
1350, 1228, 1197, 1169, 1085, 999, 851, 831, 619, 588 cmꢀ1 1HNMR
;
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126.5, 127.4, 128.1, 128.8, 129.5, 130.2, 130.3, 130.4, 130.7, 131.8,
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4.3.10. 5-(4-nitrophenyl)-7,8,13,14-tetrahydrodibenzo[a,i]
phenanthridine-2,10-dicarbonitrile (6d)
Yellow solid; Melting point: 208e210 ꢁC; IR (KBr): 3110, 3011,
2984, 2875, 2812, 1783, 1537, 1471, 1388, 1345, 1210, 1171, 1148,
1102,1095,1066, 870, 827, 793, 636; 1HNMR (400 MHz, CDCl3)ppm:
2.61e2.58 (t, J ¼ 6.0 Hz, 2H), 2.77e2.74 (t, J ¼ 6.0 Hz, 2H), 2.93e2.86
(m, 4H), 6.47e6.45 (d, J ¼ 8.0 Hz, 1H), 6.89e6.87 (d, J ¼ 8.0 Hz, 1H),
7.08 (s, 1H), 7.19e7.17 (d, J ¼ 8.0 Hz, 1H), 7.29e7.27 (d, J ¼ 8.0 Hz,
1H), 7.34 (s, 1H), 7.40e7.38 (d, J ¼ 8.0 Hz, 2H), 7.48e7.46 (d,
J ¼ 8.0 Hz, 2H); 13C NMR (100 MHz, CDCl3)ppm: 23.5, 23.8, 23.9,
27.5, 106.5, 116.4, 116.8, 122.4, 123.3, 123.9, 124.1, 124.8, 125.0, 125.3,
125.6, 125.7, 125.8, 126.2, 127.0, 135.5, 136.5, 140.3, 140.8, 153.2;
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