Carbazole-based π-conjugated 2,2′-Bipyridines, a new class of organic chromophores: Photophysical, ultrafast nonlinear optical and computational studies

Article abstract of DOI:10.1016/j.dyepig.2020.108932

The developmen of donor-acceptor (D-A) architecture based organic chromophores with large two-photon absorption (2 PA) cross-sections are essential for myriad of applications, ranging from nonlinear microscopy to biomedical imaging. Here, we present the results from a comprehensive study of 2 PA cross-sections of a new series of carbazole mono substituted-π-conjugated-2,2′-bipyridine derivatives with D-π-A architecture, possessing carbazole as the donor moiety and bipyridine core as an acceptor. Further, we have extended the π-conjugation by introducing the phenyl and butoxy substituted phenyl linkers. The fluorescence properties of these D-A chromophores are highly sensitive to solvent polarity and alteration of electron donor functionalities. The relevant computation studies support our experimental results, e.g., a shift in emission maxima and band gaps of the molecules. The third-order nonlinear optical (NLO) properties of the title donor-acceptor (D-A) chromophores demonstrated that 2 PA cross-section values are in the range of 6–39 GM, obtained using ~50 fs (fs) laser pulses at a wavelength of 800 nm. Further, the nonlinear refractive indices (n₂) of these chromophores were found to be ~10^?14 cm²/W, rendering them potential optical switching candidates since the coefficients were obtained with kHz, fs pulses extracting pure electronic nonlinearities. Based on our experimental findings and theoretical calculations presented in this study, we believe that carbazole-based π-conjugated 2,2′-bipyridines, demonstrated in this work, would be potential ligands to obtain a series of transition metal coordination complexes of interesting physical properties (for example, superior NLO behaviour).

Full text of DOI:10.1016/j.dyepig.2020.108932

Journal Pre-proof
Carbazole-Based π-Conjugated 2,2'-Bipyridines, a New Class of Organic
Chromophores: Photophysical, Ultrafast Nonlinear Optical and Computational Studies
Ramakrishna Bodapati, Gaurav Ranjan Dey, Gunjan R. Ramteke, Katturi Naga
Krishnakanth, S. Venugopal Rao, K. V. Jovan Jose, Samar K. Das
PII:
S0143-7208(20)31629-6
DOI:
https://doi.org/10.1016/j.dyepig.2020.108932
DYPI 108932
Reference:
To appear in: Dyes and Pigments
Received Date: 24 August 2020
Revised Date: 8 October 2020
Accepted Date: 8 October 2020
Please cite this article as: Bodapati R, Dey GR, Ramteke GR, Krishnakanth KN, Rao SV, Jose KVJ,
Das SK, Carbazole-Based π-Conjugated 2,2'-Bipyridines, a New Class of Organic Chromophores:
Photophysical, Ultrafast Nonlinear Optical and Computational Studies, Dyes and Pigments, https://
doi.org/10.1016/j.dyepig.2020.108932.
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Graphical Abstract
Text for TOC.
Twelve carbazole-based 2,2’-bipyridine chromophores with D-π-A architecture are synthesized
and characterized. Their photophysical properties are highly sensitive to solvent polarity and
alternation of electron donor functionalities. The third-order nonlinear optical (NLO) properties
of these chromophores demonstrate 2PA cross-section values that are in the range of 6-39 GM.
Computations studies corroborate the experimental findings of electronic absorption and
emission studies.

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Carbazole-Based π-Conjugated 2,2'-Bipyridines, a New Class of
Organic Chromophores: Photophysical, Ultrafast Nonlinear
Optical and Computational Studies
†
†
†
Ramakrishna Bodapati
,
Gaurav Ranjan Dey
,
Gunjan R. Ramteke, Katturi
‡
‡
†
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Naga Krishnakanth, S. Venugopal Rao, K. V. Jovan Jose,* and Samar K.
†
Das*
†
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School of Chemistry
‡
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Advanced Centre of Research in High Energy Materials (ACRHEM)
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University of Hyderabad, Central University P.O., Hyderabad 500 046, Telangana, India.
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E-mail: skdas@uohyd.ac.in; samar439@gmail.com
* To whom correspondence should be addressed
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Abstract
The development of donor-acceptor (D-A) architecture based organic chromophores with
large two-photon absorption (2PA) cross-sections are essential for myriad of applications,
ranging from nonlinear microscopy to biomedical imaging. Here, we present the results from
a comprehensive study of 2PA cross-sections of a new series of carbazole mono substituted-
π-conjugated-2,2’-bipyridine derivatives with D-π-A architecture, possessing carbazole as the
donor moiety and bipyridine core as an acceptor. Further, we have extended the π-
conjugation by introducing the phenyl and butoxy substituted phenyl linkers. The
fluorescence properties of these D-A chromophores are highly sensitive to solvent polarity
and alteration of electron donor functionalities. The relevant computation studies support our
experimental results, e.g., a shift in emission maxima and band gaps of the molecules. The
third-order nonlinear optical (NLO) properties of the title donor-acceptor (D-A)
chromophores demonstrated that 2PA cross-section values are in the range of 6‒39 GM,
obtained using ~50 femtosecond (fs) laser pulses at a wavelength of 800 nm. Further, the
-
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2
nonlinear refractive indices (n ) of these chromophores were found to be ~10 cm /W,
2
rendering them potential optical switching candidates since the coefficients were obtained
with kHz, fs pulses extracting pure electronic nonlinearities. Based on our experimental
findings and theoretical calculations presented in this study, we believe that carbazole-based
π-conjugated 2,2'-bipyridines, demonstrated in this work, would be potential ligands to obtain
a series of transition metal coordination complexes of interesting physical properties (for
example, superior NLO behaviour).
Keywords: Donor-acceptor;
Carbazole;
2,2’-Bipyridine derivatives; Photophysical
properties; NLO properties; Computation studies.
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1. Introduction
The ever-increasing energy demands in recent times have drawn special attention to
renewable energy sources, particularly dye-sensitized solar cells (DSSCs) [1-6]. Dye-
sensitized solar cells (DSSCs) generate electricity from sunlight by utilizing ruthenium
bipyridine complexes and other photosensitizers with excellent efficiency. Recent years have
seen great strides and in-depth investigations in this field of DSSCs because of their low cost
in productivity, ease of fabrication, and comparatively high solar energy conversion
efficiencies. In this context, the π-extended donor-acceptor chromophores have been explored
extensively not only because of their exclusive photophysical properties but also due to their
interesting NLO (non-linear optics) properties [7-10] and their importance in organic field-
effect transistors (OFETs), organic light-emitting diodes (OLEDs) [11-14] etc. These
advancements, to a large extent, stand on the backbone of π-conjugated functionalities,
especially the donor-acceptor moieties having diverse push-pull chromophores. Bipyridine-
based donor-acceptor systems are of special interest in this regard, for example, some
extensively conjugated π systems with bipyridine as an acceptor are suitable for light-
harvesting systems [1,15-18] and with an electron-donating moiety are suitable organic
semiconductors [19,20].
Moreover, rich and extensive absorption features of bipyridine
systems in the visible region offer an effective use of the input solar spectrum [21-22].
Therefore, electron D-A (donor-acceptor) compounds involving bipyridine (as an acceptor)
have been explored extensively by various research groups in the contexts of demonstrating
organic light-emitting diodes (OLEDs), electrogenerated chemiluminescence (ECL) [23-24],
dye-sensitized solar cells (DSSCs), and fluorescent sensors [25-29]. The following paragraph
explains of why a bipyridine derivative is an integral part of many D-A systems of
photophysical importance.
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An important aspect of these D-A molecules is to tune the bandgap due to extended
π-conjugation by monitoring the energetics of intramolecular charge transfer (ICT) from
electron-rich donor to electron-deficient acceptor moieties [30-33]. And these ICT molecules
have the ability to respond to the electric field of light nonlinearly, thereby possessing first
and second-order hyperpolarizabilities. Some donor-acceptor (D-A) type ICT molecules,
indeed, display high two- and three-photon absorption cross-sections [34,35]. Such nonlinear
optical (NLO) and electronic properties of D-A systems can be exploited by modifying donor
and acceptor moieties through proper designing of experiments and performing theoretical
studies on the designed molecules. Careful tuning of the ICT from D to A can result in
emission in the desired range of the electromagnetic spectrum of the designed D-A system.
The resulting excited singlet state is usually stabilized in polar solvents, which can be
realized by the large Stokes shift in the resulting fluorescence emission spectrum [36,37]. In
other words, the ‘push-pull’ D–A molecules can be designed by considering the electronic
association (for example, through π conjugation) between the donor and acceptor mesomeric
units in a chromophore system, which, in turn, is linked with diverse functionalities for
spontaneous charge redistribution. Nitrogen-containing heterocycles, that are generally
emissive, are potential building blocks of such D–A systems. Practically, 2,2'-bipyridines, in
this regard, are promising acceptor systems; when they are π-conjugated, particularly, with
donor functionalities, the energy of LUMO (lowest unoccupied molecular orbital) of pyridine
ring of bipyridine gets lowered than that of ‘per se’ benzene LUMO, which makes bipyridine
an acceptor component of the π-conjugated bipyridine derivatives. This effectively means
that a bipyridine can be made an efficient donor-acceptor (D-A) system if a π-conjugation is
extended from the bipyridine ring by introducing various donor end-capped functionalities.
These functionalities can tune the optical properties, e.g., fluorescence, of the resulting D-A
systems. Fluorescence is an important analytical tool to determine the ‘efficiency’ of these π-
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conjugated D-A materials and the extent of their applicability in real life by monitoring their
quantum yield and the region of the pertinent spectra where they are emissive. Indeed,
conjugated bipyridine derivatives, that are fluorescent molecules are potential candidates for
electroluminescent and optical switching devices owing to their high non-linear
susceptibilities [38-40]. Thus over the years, various research groups across the globe have
actively been engaged in the study of 2,2'-bipyridine dyads. Le Bozec’s research group have
extensively studied substituted bipyridine systems and have commented on the versatile
tuning of electronic absorption properties of such systems, achieved through simple
modifications of the capping end-groups and π linkers [41-45]. Such modifications,
expectedly, control the fluorescence emission, band shifts and NLO properties of the relevant
D-A systems. Beer [46-50] and Abbotto groups [7, 51-53] have used 2,2'-bipyridine based
molecules to construct metal complexes that have found applications in DSSCs and in ion
sensing.
Over the years, our research group has reported 2,2’-bipyridine-based D–A systems
that exhibit strong fluorescence and can act as selective metal ion sensors [54-58]. In the
present work, efforts have been made to synthesize, characterize and investigate the
photophysical properties of 2, 2’-bipyridine-based unsymmetrical systems of the A-A-π-D
archetype, where the A-A unit represents the 2,2'-bypyridine acceptor core, which is attached
to different donor termini through π-conjugation. The target molecules (chromophores 4-15,
chart I) have been prepared using the Horner-Wordsworth-Emmons (HWE) reaction to
introduce electron-donating groups onto the bipyridine acceptor core unit because this
method is known to give high yields in addition to the E-selectivity. Generally, both amino-
and alkoxy-groups have high electron-donating ability and all previously reported relevant
studies were associated with either amino- or alkoxy-substitutes (not both together) in the D-
A molecule. But our group introduced both amino as well as alkoxy donors in the same
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molecule and tuned the photophysical properties of the resulting symmetrically and
unsymmetrically substituted donor-acceptor chromophores [54,55]. In the present study, we
have used carbazole as an amino donor to tune the optical properties of the bipyridine based
twelve D-A molecules (chromophores 4-15), as illustrated in Chart 1. Push-pull molecules
having carbazole as an electron dornor are known to have rich photophysical behaviour,
efficient charge-transporting properties and chemosensor ability [19,20,27]. We have
described detailed photophysical properties of the title compounds using UV-visible
spectroscopy and fluorescence studies. We have reported the NLO properties of all the
chromophores; the relevant nonlinear refractive indices (n ) of these chromophores are found
2
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2
to be ~10 cm /W, demonstrating that these D-A chromophores will have potential optical
switching applications. We could describe a single-crystal structure analysis of one the
chromophores as the representative one. We have also performed computation studies for all
the chromophores to corroborate with the experimental photophysical (experimentally
obtained) results. We must mention that there have been some reports on similar molecular
structures of the chromophores, we have described in this work (but not identical) [27,59,60];
however these reports were lack of detailed photophysical properties, including NLO studies
and any crystallographic analysis.
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Chart 1. Chromophores 4-15, synthesized and studied in the present work.
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53
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2. Results and Discussion
2.1. Synthesis of bipyridine phosphonate ester, aldehydes, and precursors
The bipyridine phosphonate precursor, appropriate aldehydes and their precursors were
synthesized according to available literature reports [25,26,54,61-64]. Initially, the mono
derivatized chloromethylbipyridine compound 2 (Scheme 1) was synthesized following a
method, developed by Fraser and co-workers [64] that involves the use of 1.0 equivalent of
base, i.e. LDA (instead of potassium tert-butoxide), which deprotonates proton from one of
7

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the methyl groups in 1 (see Scheme 1), thereby allowing in-situ Knoevenagel-type reactions
to be done in order to achieve the unsymmetrically substituted chloromethyl derivative 2.
1
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58
Scheme 1. The synthetic approach of targeted A-A-π-D bipyridine chromophores.
The mono-phosphonate intermediate 3 was then easily derivatized from the corresponding
chloromethyl bipyridine derivative 2 through an Arbuzov reaction.To synthesize the targeted
donor-acceptor chromophores, the bipyridine phosphonate precursor was treated with the
appropriate aldehydes. The bipyridine phosphonate, the aldehydes and its precursors were
prepared according to previous literature reports [64,65], while the target molecules have
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been synthesized using an efficient HWE reaction protocol (Scheme 1). The HWE reaction
enjoys a number of advantages over the traditional Wittig approach, notably, high yielding,
stereoselective E-olefination of aldehydes and the generation of water-soluble phosphate salt
that can be removed by aqueous workup procedure. Thus the problems associated with the
separation of the Wittig by-product, triphenylphosphine oxide, could be avoided. Fraser and
co-workers have developed novel methods for the preparation of mono-chloromethyl
derivatives using LDA in place of tert-butoxide, to completely deprotonate the starting
material 1 in a selective manner and thus allowing in-situ Knoevenagel type reactions to take
place. This protocol allows high yielding synthesis of our target molecules with desired
symmetry and has garnered much acclaim owing to its ease and efficiency of synthesizing π-
conjugated bipyridine chromophores. The molecular structures of all the resulting
1
13
chromophores (4-15) were determined through NMR ( H and C) and mass spectroscopy
including CHN analysis. The target N-alkylated carbazoles and their aldehyde precursors
were then prepared by the synthetic route, shown in Scheme 1. N-alkylation (ethyl, butyl,
hexyl and octyl) of the carbazoles followed by mono-formylation afforded the carbazole
aldehyde (18a-d, Scheme 1). The alkylation of the carbazole, taken place in DMF as the
solvent system, afforded corresponding N-alkylcarbazoles in high yields and thereafter, they
were mono-formylated using the Vilsmeier-Haack reaction with an excellent mono–
selectivity. In addition, the π–conjugated benzaldehydes (21a-h) with different substituents
were synthesized via a two-step synthetic methodology as shown in Scheme 1 [54,62]. The
N-alkylcarbazole aldehydes (18a-d) were treated with the corresponding bromo substituted
phenyl phosphonate ester (19a-b, Scheme 1) using the HWE reaction; thereafter, the resultant
extended conjugated carbazole compounds were treated with nBuLi to obtain the π–
conjugated benzaldehydes (21a-h, Scheme 1). The physical nature of the synthesized pull-
push chromophores are colored solids and, in some cases, they are sticky gums.
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2.2. Photophysical Properties
The photophysical properties of the synthesized chromophores (4-15, chart 1) were
investigated in the solution phase, in three different solvents based on the solvent polarity,
such as toluene, dichloromethane (DCM) and acetonitrile (ACN) at room temperature and the
corresponding photophysical data are summarized in Table 1. The investigation of the
absorption and emission properties reveals the strong correlation between the nature and
position of the donor functional groups and the polarity of the solvent used.
a
b
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400 500 600 700 800
Compound 13
c
Compound
4
d
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Toluene
DCM
ACN
Toulene
DCM
ACN
0
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3
00 400 500 600 700 800
300
400
500
600
Wavelength (nm)
1
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96
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Fig. 1. Normalized UV−visible absorption and emission spectra: (a) and (b), chromophores 4-15
(above) in DCM solvent; (c) and (d), molecules 4 and 13 (below) showing the solvatochromic effect.
1
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00
The lowest energy absorption bands in the range of 352-414 nm in all the chromophores are
attributed to intramolecular charge transfer (ICT) π→π* electronic transition. The results are
consistent with computational results (vide infra) and in agreement with ICT electronic
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tranistions from donor-based molecular orbitals to acceptor molecular orbitals, which were
confirmed from their sensitivity to solvent polarity. The molar extinction coefficient values of
the compounds have been measured in DCM solvent and presented in Table 1. The alkyl
substituents (ethyl, butyl, hexyl and octyl) on the carbazole moiety have little effect on the
absorption behaviour of the resulting donor-acceptor chromophores. Extension of the
conjugation length by the introduction of the phenyl ring results in the shift of the absorption
maxima by about 40 nm. A comparison of the absorption trends of 4 vs 8 or 5 vs 9 or 6 vs 10
or 7 vs 11 in DCM (shown in Table 1) reveals that the absorption maxima values get red-
shifted (bathochromic shift) with the increase in the conjugation length and donating strength
of the targeted compounds. On the addition of electron-donating butoxy groups to the
extended conjugated styrylcarbazoles, the absorption maxima are shifted about 22 nm.
In the case of emission, the trend is similar to the absorption behaviour: emission
maxima of the target molecules have been shifted with an increase in the conjugation length.
The change in the alkyl chain substituents (ethyl, butyl, hexyl and octyl) of the carbazole
does not affect the emission maxima values. In the emission spectra, the λmax of 4 is observed
at 449 nm, and later on extending the π conjugation through a phenyl ring, as in 8 and 12, the
λmax shift to 588 and 585 nm, respectively. To increase the donor strength to compound 12,
two electron-donating butoxy moieties were attached to the phenyl ring. Similar trends are
observed for 5, 9 and 13, where 5 exhibits maxima at 450 nm, 9 at 592 nm, and 13 at 588 nm.
The increase in conjugation due to the incorporation of the linking phenyl ring in
chromophores 8-11 accounts for the 155 nm shift.
The absorption and emission behaviour of the targeted chromophores in DCM solvent
have been shown in Fig. 1a and 1b. The quantum yields (QYs) of the simple carbazole
substituted chromophores (4-7) fall in the range of 0.07-0.1. On the other hand, when the
conjugation is extended by substituting the phenyl ring to the carbazole moiety, the resulting
1
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chromophores (8-15) exhibit high quantum yields, fall in the range of 0.5 to 0.79 in DCM
solvent medium as shown in Table 1. All the compounds exhibit a solvatochromic effect
during emission but in the case of absorption, it is negligible. As shown in Fig. 1c and 1d,
compounds 4 and 13 exhibit solvatochromic phenomenon (spectral changes), which are
presented as representative ones for the series. The polarized excited states of the molecules
are stabilized by the dipole moment of the polar solvents, which reorient the molecules and
lower the energy of the excited system leading to a redshift in the resulting emission spectra.
Since the solvents have a negligible effect on the absorption maxima, it clearly indicates the
absence of solvatochromic effect in the ground state of the molecule. Needless to say, the
emissive solvatochromic behaviour is in line with that expected from such push-pull
fluorophores [66–68]. An important point to note is that in certain cases (for example,
compound 14), the QY values are fairly similar in different solvents. QY depends on the ratio
of the area of curves under the absorption and emission spectra. The area in turn depends on
the intensity and broadness of the curve. Either of these factors can offset the other, leading to
similar areas for different spectra. Consequently, similar QY values are observed inspite of
solvatochromism [27].
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Table 1. Photophysical data of the A−A−π−D asymmetrical bipyridine carbazole chromophores
4−15.
*
Compound
Solvent
λmax
ε
λem
Stokes shift
Δν̅ (cm )
Φ
Solution
state
-1
-1
-1
(nm)
(M cm ) (nm)
in DCM
4
5
6
7
8
Toluene
DCM
ACN
Toluene
DCM
ACN
Toluene
DCM
ACN
Toluene
DCM
ACN
352
352
352
352
352
352
352
352
352
352
352
352
390
403
3595
6137
7087
3717
6187
7132
3778
6187
7132
3779
6187
7223
6809
0.102
0.085
0.090
0.092
0.099
0.124
0.112
0.112
0.114
0.110
0.078
0.091
0.518
15400
15200
23400
27600
449
469
405
450
470
406
450
470
406
450
472
531
Toluene
1
2

DCM
ACN
Toluene
DCM
ACN
Toluene
DCM
ACN
Toluene
DCM
ACN
Toluene
DCM
ACN
Toluene
DCM
ACN
Toluene
DCM
ACN
Toluene
DCM
391
384
390
391
387
390
391
386
390
29800
32000
33200
22400
29000
28600
24000
28200
588
604
507
592
620
480
583
585
477
575
583
515
585
606
508
586
594
506
583
580
505
576
604
8569
9485
5917
8684
9711
4808
8423
8812
4677
0.755
0.817
0.483
0.792
0.803
0.578
0.696
0.680
0.435
0.696
0.651
0.459
0.607
0.657
0.476
0.604
0.636
0.475
0.606
0.603
0.443
0.567
0.659
9
1
0
1
2
3
4
5
1
1
1
1
1
391
384
414
8184
8889
4737
7061
7889
4470
7090
7555
4392
7002
7149
4353
6793
7775
414
410
414
414
410
414
414
410
414
414
411
ACN
2
2
47
48
The QY data (Table 1) present an interesting trend, which can be used to correlate the effect
of structure on QY. Compounds 4-7 have very low QY values, presumably due to the limited
conjugated chain; and variation in alkyl chain length (connected to carbazole N) has no
observable effect. For compounds 8-11, an extension of the chain by conjugated phenyl
groups and for compounds 12-15, the introduction of electron-donating butoxy groups to the
phenyl ring, resulting in a marked increase in the QY values. This can be attributed to the fact
that the electron delocalization through the extended conjugated moieties (molecules 8-15)
significantly stabilizes the system. Moreover, while similar donor-acceptor backbone
molecules have been reported before with low QY values [27], rationalized modification of
the conjugated backbone of these molecules, presented in this work, results in a significant
increase in QY values. The curious case of similar QY values for compound 6, inspite of
notable differences in solvatochromic emission values, is due to the nature of the spectra
(where the intensity of the peak is offset by the broadness of the curve), as explained earlier.
2
2
2
2
2
2
2
2
2
2
2
2
49
50
51
52
53
54
55
56
57
58
59
60
1
3

2
2
2
2
2
2
2
2
2
2
2
2
2
61
62
63
64
65
66
67
68
69
70
71
72
73
Thus the QY, which is a ratio of the areas under the absorption and emission spectra, result in
similar values [27].
2.3. Crystallography: Crystal Structure of Compound 4
Single crystal X-ray diffraction (SCXRD) quality crystals were obtained by slow evaporation
of solution of 4 in acetonitrile (ACN) solvent at room temperature, which yielded orange-red
colored crystals after one day. Compound 4 crystallized in monoclinic 'P2 /c' space group
1
and the relevant asymmetric unit contains a full molecule of this compound. The ORTEP
diagram of the same is shown in Fig. 2a at a thermal ellipsoidal probability factor of 30%.
The relevant crystal data and structure refinement parameters are summarized in Table S1
(Supporting information). The bond lengths and bond angles present in the crystal structure
of compound 4, are given in Table S2 (Supporting information). Crystal structure analysis of
compound 4 shows intermolecular C‒H···N hydrogen bonding interactions as shown in Fig.
2b.
2
2
2
74
75
76
Fig. 2 (a) Thermal ellipsoidal plot of molecule 4 (30% probability). (b) Molecular packing diagram in
the crystal structure of compound 4 showing intermolecular hydrogen bonding interactions.
1
4

2
2
2
2
2
2
2
2
2
77
78
79
80
81
82
83
84
85
2.4. Solid-state emission studies
The solid-state absorption and emission properties of the targeted chromophores have been
investigated. The photoluminescence spectra were recorded by coating the compound on the
quartz plate. As shown in Fig. S81 (SI), the relevant compounds absorbed in the spectral
range of 380-440 nm, and we observed that there is a redshift in the absorption maxima in
going from solution to solid-state, which signifies the presence of intermolecular interactions
between the molecules. In the case of emission, all the chromophores 4-15 exhibit a broad
emission feature in the region of 550-685 nm (Fig. S81, SI). As shown in Fig. S81, the
majority of the compounds exhibit a bathochromic shift in their solution state.
2
2
2
2
2
2
2
2
2
2
2
2
2
2
3
3
86
87
88
89
90
91
92
93
94
95
96
97
98
99
00
01
2.5. NLO Properties
The third-order NLO properties of the compounds 4-15 were investigated using open
and closed-aperture Z-scan techniques. The experiments were performed in the femtosecond
(fs) laser regime using a Ti: sapphire laser (Libra, M/s Coherent) delivering ∼50 fs pulses at
800 nm wavelength with a repetition rate of 1 kHz. A broadband oscillator generating ∼40 fs
pulses (at 800 nm with a bandwidth of 35 nm and a repetition rate of 80 MHz) was the seed
for the amplifier. The output from the amplified laser system was ∼50 fs pulses delivered
with a maximum of ∼4 mJ energy centered at 800 nm (a FWHM of ∼28 nm). The laser
pulses were focused using a plano-convex lens (focal length of 15 cm). The sample was
positioned on a translation stage (with 0.1 µm resolution) and data was recorded using a Si-
photodiode, the output of which was fed to a lock-in amplifier (Signal recovery 7265). All the
instruments and the data collection were controlled by a home-made LabVIEW program. All
the samples in solution form were placed in a 1-mm cuvette. The beam waist (ω ) size at the
0
focus was estimated to be ~35 µm with a corresponding Rayleigh range of ~4.8 mm,
satisfying the thin sample limit (required for the implementation of normal Z-scan analysis in
the present case). The typical input power, used in the experiments to measure the
1
5

3
3
3
3
3
3
3
3
3
3
02
03
04
05
06
07
08
09
10
11
nonlinearities, was ~0.2 mW. The complete details of these experiments have been presented
in our previous works [57,58,69]. The closed aperture Z-scans were performed by placing an
aperture in front of the detector and at input peak intensities, where the contribution from the
higher-order NLO effects are negligible. The Z-scan data for the solvent (DCM) was also
recorded and found that the NLO coefficients (β and n ) were negligible (at least an order of
2
magnitude lower than those obtained for the solvent+solute) at the peak intensities used in
these experiments. The two-photon absorption (2PA) values are calculated for 1mM
concentration. The errors in the NLO coefficients, arising from the uncertainties in the
estimation of beam waist (and thereby input peak intensity), input laser fluctuations and
fitting errors, were estimated to be ±5%.
3
3
3
3
3
3
3
3
12
13
14
15
16
17
18
19
The open aperture Z-scan data of all the samples is investigated in the present study,
along with their theoretical fits, are illustrated in Fig. 3. The transmissions were symmetric
with respect to the focus. The molecules were found to exhibit strong reverse saturation
absorption (RSA) as the absorption was found to be maximum at the focus (Z=0). Further, it
is estimated that all the samples undergo simultaneous two-photon absorption (2PA) since
one-photon energy (at 800 nm) is found to be less than the energy difference between the
excited state and ground state. From the fits to the open aperture Z-scan data, nonlinear
absorption coefficient (β) was estimated using the following relation (eqn. 1)
3
20
(1),
3
3
3
3
3
21
22
23
24
25
where T(z) is the normalized transmittance, I is the peak intensity at the focus (~218
0
2
GW/cm ) and L is the effective length of the sample. The calculated 2PA coefficient (β)
eff
-13
values of all the samples were ~10 cm/W and the values are tabulated in Table 2. It is
-13
observed that compound 4 had a maximum magnitude of the coefficient (9.40×10 cm/W)
as compared to those of others. Further, the β values of compound 5 and compound 10 were
1
6

-
13
-13
3
3
3
26
27
28
found to be 7.0×10 cm/W and 8.68×10 cm/W, respectively, which are relatively lower
than that of compound 4 and greater than those of the rest.
3
29
3
3
30
31
Fig. 3. Open aperture Z-scan data of molecules 4–15. Open symbols (circles) are the experimental
data points, while the solid lines represent the theoretical fits.
3
3
3
3
3
32
33
34
35
36
The closed aperture Z-scan curves of the compounds 4-15 are shown in Fig. 4. The
peak followed by valley indicates a negative nonlinear refractive index n observed due to
2
self-defocusing effect, i.e., an increase in the transmitted intensity (due to refraction) as the
sample approaches the focal point (Z = 0) followed by a decrease in intensity as it moves
away from the focal point and towards the detector.
1
7

3
3
37
38
The nonlinear refraction (n ) was calculated using the closed aperture curves by fitting with
2
the relation,
3
39
(2)
3
40
where, ∆Φ is the nonlinear phase shift and X=Z/Z . The nonlinear refractive index n can be
0
0
2
3
3
3
3
3
3
3
3
3
3
41
42
43
44
45
46
47
48
49
50
calculated by using the relation ∆Φ = kn I L . The calculated values of the nonlinear
0
2 0 eff
-14
2
refractive index for all the compounds were ~10 cm /W. The n value of compound 9 was
2
-14
2
found to be the largest (9.07×10 cm /W) when compared to those of others. On the other
hand, compound 8 was found to possess the smallest n than that of compound 9 and larger
2
than those of the rest. Since the Z-scans were performed with kHz repetition rate (time
between two pulses being 1 ms, before which all the thermal processes die down), we believe
that the observed nonlinearity is purely electronic in nature. This was further strengthened
from the fact that the distance of ∆ZP-V was ~1.7×Z in all the cases, which clearly suggests
0
the presence of a Kerr type of nonlinearity. Such strong and fast refractive nonlinearities have
applications in all-optical switching.
3
3
3
51
52
53
The magnitude of the third-order nonlinear optical susceptibility decides the NLO efficiency
of the materials [70]. The real, imaginary and magnitude of the susceptibility can be
determined by using the following relations,
3
3
54
55
3
3
56
57
Where, c is the velocity of light, n is the linear refractive index and ω is the angular
0
(3)
3
58
frequency. The χ values of compound 9 and compound 8 are found to be (7.05 and
1
8

-
12
3
3
3
59
60
61
4.67×10 esu, respectively) larger than those of other samples (Table 2). The rest of the
-12
samples are found to be in the range of 1.95 – 3.2×10 esu.
3
3
62
63
Fig. 4. Closed aperture Z-scan curves of the compounds 4-15. Open symbols (circles) are the
3
64
experimental data points, while the solid lines represent the theoretical fits.
3
65
−
16
3
3
3
3
3
3
66
67
68
69
70
71
We recently reported the nonlinear refractive index (n ) values of (0.9−1.9)×10
2
2
cm /W for mononuclear Ru(II) complexes of arene and asymmetrically substituted 2,2′-
bipyridine ligands [58]. Obviously, the present molecules possess an order of magnitude
-15
2
higher n values (~10 cm /W), rendering them potential materials for optical switching
2
applications. Further, the 2PA cross-sections for our earlier Ru(II) coordination complexes
were in the range of 3-28 GM [58], whereas in the present work, we observed slightly higher
1
9

3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
values, that are in the range of 6-39 GM. Compounds 4 and 10 have the highest nonlinear
absorption coefficient values while compound 9 has the highest nonlinear refractive index.
We have attempted to find a correlation in the NLO coefficients and the structures by
breaking them into a series of molecules (4-7, 8-11, 12-15) but could not find any. There are
several reasons for the nonlinearities in such molecules, including the pi-electron strength,
electron donor-acceptor capability, orientation, planarity of the molecule etc. The obtained
NLO data are insufficient to come up with a clear structure-property correlation. Further
detailed studies at different wavelengths, including the femtosecond transient absorption data
would provide a deeper insight into the mechanisms causing the nonlinearity (both nonlinear
absorption and nonlinear refractive index).
Jędrzejewska et al. have reported 2PA values of 490–2600 GM from uncharged push-
pull oxazol-5(4H)-ones dyes and the effective 2PA cross-sections were found to be increased
with the branching of the triphenylamine core [71]. Sadowski et al. have reported the 2PA
cross-section value of 2840-5180 GM from novel dyes, based on the
dipyrrolonaphthyridinedione core and based on double and triple bond linkage to the core to
peripheral units, the molecules showed strong 2PA cross-section values [72]. Furthermore,
they observed that the change in HOMO energies by push-pull effect influenced the strength
of 2PA transitions. Also, Derkowska-Zielinska et al. reported 2PA cross-section values of
~20 GM from heterocyclic azo dyes [73]. Clearly, the present molecules investigated in this
study, exhibit moderate 2PA cross-section values (a maximum of 39 GM). Bhattacharya et
−
16
2
al. have reported the nonlinear refractive index values of order ~10 cm /W from Zinc(II)
phthalocyanine molecule [2,10,16,24-tetrakis(9-phenyl-9H-carbazol-2-yl)-phthalocyanine]
[69] and it is evident that the current molecules have exhibited superior n values.
2
Furthermore, Teran et al. investigated twisted thiophene-based chromophores and obtained n2
-16
2
values of ~10 cm /W at 800 nm using ~160 fs laser pulses and at 800 nm. Our molecules
2
0

3
3
3
97
98
99
have superior coefficients of n compared to these donor-acceptor π-systems [74]. The
2
(3)
-12
overall magnitude of the χ for these molecules was ~10 esu, finding applications in
photonic devices and optoelectronics.
4
4
4
00
01
02
Table 2. Summary of the third-order NLO coefficients of the compounds 4-15 obtained from
femtosecond Z-scan measurements.
(
3)
(3)
2
(3)
(3)
β
n2
Re χ
Im χ
Total χ
Total χ
(2)
(2)
Compound
σ
(GM) σ /MW
-13
-14
2
-20
2
2
-24
2
-20
2
2
-12
1
0
cm/W
10 cm /W 10 m /V 10 m /V 10 m /V 10 esu
4
5
6
7
8
9
9.40
39
29
6
0.1
2.50
2.76
3.04
3.57
6.00
9.07
4.12
3.30
3.00
2.80
3.57
3.60
2.71
3.00
3.30
3.88
6.52
9.85
4.47
3.58
3.26
3.04
3.88
3.91
6.50
1.9
2.72
2.99
3.30
3.87
6.52
9.84
4.47
3.58
3.26
3.04
3.87
3.90
1.95
2.14
2.36
2.77
4.67
7.05
3.20
2.56
2.33
2.17
2.77
2.80
7.00
1.53
2.68
1.40
1.78
8.68
2.10
3.30
2.63
2.40
2.10
0.07
0.013
0.023
0.012
0.013
0.065
0.015
0.022
0.016
0.014
0.012
1.05
1.85
0.97
1.23
6.00
1.45
2.28
1.81
1.65
1.45
11
6
7
1
1
1
1
1
1
0
1
2
3
4
5
36
9
14
11
10
9
4
4
4
4
4
4
4
4
03
04
05
06
07
08
09
10
2.6. Computational studies
The geometries of all the molecules (4-15) in chart 1 were optimized at MPW1PW91
density functional theory (DFT), with the 6-31G(d) basis function with the Gaussian 09 suite
of packages [75]. The stationary points were characterized using harmonic vibrational
analysis and the local minimum was characterized by the absence of imaginary frequencies
for all the structures. The optimized geometries have been employed to calculate the highest
and lowest occupied molecular orbitals’ (HOMO and LUMO, respectively) bandgaps and are
depicted in Fig. 5. Further, the UV-VIS absorption emission spectra of molecules were
2
1

4
4
4
4
11
12
13
14
calculated using time-dependent (TD)-DFT functionals with CAM-B3LYP hybrid functionals
and 6-311g(d) basis function. The effect of the solvent is incorporated using the polarization
continuum model (CPCM) with dichloromethane (DCM) as the solvent, as implemented in
the Gaussian 09 suite of packages [75].
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
We have classified these twelve molecules (4-15, chart 1) into three sets, based on
the substituents on the donor moiety as (A) 4-7 compounds, (B) 8-11 compounds and (C) 12-
15 compounds. The set A compounds were designed by adding alkyl-group substitution on
the chromophore without changing the conjugation length of the chromophore. The alkyl
substituents on carbazole do not participate in intermolecular charge transfer within the
compounds. While, in the next set of compounds, set B, the bandgap was tuned further by
extending the length of the conjugation by inserting a phenyl ring along with the alkyl
substitution on carbazole functional group. This combination promotes the intermolecular
charge transfer from electron-rich carbazole to electron-deficient bipyridine, which results in
decreasing the bandgap. Further, inset C, the substitution on the phenyl ring was performed
with two butoxy moieties to increase the donor strength of the phenyl group. Fig. 5
demonstrates a visual representation of HOMO-LUMO frontier molecular orbitals of all the
molecules. This functional group substitution on the chromophores destabilizes the HOMO
and stabilizes the LUMO [76]. The difference in HOMO energies (destabilization energies)
of sets A and B compounds is ~0.218 eV while that of sets B and C is ~0.264 eV. This results
in a significant narrowing in the bandgap energy [77]. The theoretical absorption spectra,
shown in Fig. 6a-c, corroborates the experimental results (displayed in Fig. 1). An analysis of
the absorption spectra, depicted in Fig. 6, shows an increasing bathochromic shift with the
length of the π-conjugation, which in turn, improves the emission properties progressively.
As a preliminary theoretical analysis, we have plotted the iso-density plots of HOMO-LUMO
frontier molecular orbitals (Fig. 5).
2
2

4
36
4
4
4
37
38
39
Fig. 5. A visual representation of HOMO-LUMO frontier molecular orbitals of molecules calculated
at MPW1PW91/6-31G(d): (a) molecules 4-9 and (b) molecules 10-15. The depicted HOMO-LUMO
energies and bandgap energies of molecules are in electron volts (eV).
2
3

4
4
4
4
4
40
41
42
43
44
Table 3. Absorption spectra calculated at CAM-B3LYP/6-311G(d) level of theory in the solvent
phase using C-PCM implicit solvent model (solvent = dichloromethane) for the molecules depicted in
Chart 1. The columns report the structure number, transition in energy levels, the wavelength of
absorption (nm), oscillator strength (f) and bandgap (in eV).
Compound
MO level
H → L
H → L
H → L
H → L
H → L
H → L
H → L
H → L
H → L
H → L
H → L
H → L
Wavelength (nm)
337.24
Oscillator strength
1.2919
Band Gap (eV)
4.050
4
5
6
7
8
9
337.49
1.2996
4.041
337.47
1.3029
4.040
337.55
1.3079
4.037
382.05
2.2327
3.447
382.54
2.3417
3.438
10
11
12
13
14
15
383.10
2.3414
3.432
382.63
2.3541
3.435
394.91
2.1087
3.375
394.91
2.1253
3.369
395.63
2.1347
3.365
395.09
2.1313
3.369
4
4
4
4
4
4
4
4
4
4
45
46
47
48
49
50
51
52
53
54
The HOMO, depicted in Fig. 5, is sited with the donor moiety, carbazole core in all the
chromophores, while LUMO is sited on the acceptor bipyridine moiety. Table 3 has
described the electronic excitation energies (in nm) between energy levels and the
corresponding oscillator strength (a.u.) at CAM-B3LYP/6-311G(d) level of theory. For the
first set of compounds 4-7, the absorption maximum is at 337 nm and emission at 419 nm.
While, in the compounds 8-11, the maximum in absorption spectra is at 382 nm and the
emission at 485 nm. In the last set, 12-15 compounds, the maximum in absorption is at 394-
395 nm and emission at 491 nm. The bandgap ranges from 4.00-4.13 eV for the first four
molecules 4-7. Thus, an increase in alkyl branching (going from 4 to 7) on molecules makes
2
4

4
4
4
4
4
4
4
4
4
55
56
57
58
59
60
61
62
63
no significant differences in the bandgap (absorption maxima at 337 nm). For the second set
of compounds 8-11, the insertion of a phenyl ring reduces the bandgap to 3.42-3.45 eV (when
the absorption maximum is shifted to a higher wavelength, 382-383 nm). For the next four
compounds, 12-15, branching of the butoxy group in the donor moiety of compounds
decreases the bandgap further to 3.37-3.38 eV with an increased absorption maximum at 395
nm. Vertical TD-DFT calculation also yielded oscillator strength which can be related to the
experimental molar absorption coefficient and thus to the intensities of measured absorption
bands [78]. Compounds 8-11 show an increase in the transition probability compared to
those of the other sets in both absorption as well as emission spectra.
4
4
4
4
64
65
66
67
Table 4. Emission spectra calculated at CAM-B3LYP/6-311g(d) level of theory in the solvent phase
using CPCM (solvent = DCM) for the molecules depicted in Fig. 1. The columns report the structure
number, transition in molecular orbital (MO) levels, the wavelength of absorption (nm) and oscillator
strength (f).
Molecule
Molecular orbital level
Wavelength (nm)
Oscillator strength (f)
4
5
6
7
8
9
H ← L
H ← L
H ← L
H ← L
H ← L
H ← L
H ← L
H ← L
H ← L
H ← L
H ← L
H ← L
418.88
419.14
419.17
419.29
485.72
485.89
485.99
485.92
491.75
492.00
491.98
491.99
1.574
1.584
1.587
1.588
2.496
2.506
2.508
2.510
2.394
2.403
2.241
2.407
10
11
12
13
14
15
4
68
2
5

4
4
4
4
4
4
4
4
4
69
70
71
72
73
74
75
76
77
The relative photophysical properties of the compounds, depicted in Fig. 6, follow a regular
trend while increasing the strength of the donor moiety. From this observation, we can
conclude that a successful enhancement in conjugation length increases the intermolecular
charge transfer. This, in turn, decreases the bandgap and increases the absorbance, and emits
in higher wavelength. The calculated maximum and the relative shifts in the UV-vis
absorption and emission spectra corroborate well with the experiment. Further, the relative
shifts in the calculated absorption maxima show a fair correlation with the bandgap of the
compounds. The theoretical studies enable in understanding the intramolecular electronic
push-pull effects and engineer the UV-vis absorption and emission spectra of compounds.
4
78
4
4
4
4
4
4
4
79
80
81
82
83
84
85
Fig 6. The UV-VIS absorption and emission spectra of molecules (4-15) at CAM-B3LYP/6-311g(d)
level of theory in the solvent phase using CPCM (solvent=DCM) for the (A) set of molecules 4-7 (B)
molecules 8-11 and (C) molecules 12-15. The half-width of the fitted line is 0.1 eV. The solid
continuous lines represent the absorption spectra, while the dashed lines represent the emission
spectra. The experimentally obtained UV-VIS absorption and emission spectra of these molecules
have been depicted in Fig. 1.
2
6

4
4
4
4
4
4
4
4
4
4
4
4
4
4
5
5
5
5
86
87
88
89
90
91
92
93
94
95
96
97
98
99
00
01
02
03
The emission in the visible region of the electromagnetic spectrum makes these compounds
good dye-sensitized solar cell (DSCC) and OLED materials. As shown in Fig. 6, the
calculated absorption and emission spectra of several molecules appear to be blue-shifted,
and the maximum spectral values are close, and hence the spectral lines overlap each other.
The calculated absorption and emission maxima values are given in Table 3.
2.7. Electrochemistry
The present A-A-π-D system consists of bipyridine (acceptor A-A)- and carbazole (D)-
moieties, both of which are electrochemically active. The electrochemistry of bipyridines, as
expected, would be dominated by reduction chemistry [79] and that of carbazoles would be
mainly oxidation chemistry [80]. As described in the previous section, the present conjugated
A-A-π-D system (12 molecules, 4-15, chart 1) can be classified into three sets A (molecules
4-7), B (molecules 8-11) and C (molecules 12-15) based on the substituents on the donor
moiety. We have performed electrochemical studies of three molecules 7, 9 and 13 as
representative ones, representing the sets A, B and C, respectively. The cyclic
voltammograms of compounds 7, 9 and 13 are shown in Fig. S82 (Supporting information).
As shown in Fig. S82, all three representative molecules exhibit oxidative responses due the
presence of carbazole moiety [80] and reductive responses due to the presence of bipyridine
moieties [79] in the title chromophores.
5
5
04
05 3. Experimental Section
5
5
5
5
5
06
07
08
09
10
3.1. General Procedures.
All reactions were carried out under ambient conditions unless otherwise stated. Column
chromatography was performed on silica gel (100-200 mesh). Unless stated otherwise, all
reagents were purchased from commercial sources and used without additional purification.
1
13
H NMR and C NMR spectra were recorded either on a Bruker AV 400 or on a 100 MHz
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machine in CDCl as solvent with TMS as reference. FLASH EA series 1112 CHNS analyzer
3
has been used for elemental analysis. Infrared (IR) spectra were recorded by using KBr
pellets on an FT/IR spectrometer. HRMS spectra were taken from a Bruker Maxis
spectrophotometer (ESI positive mode). Absorbance spectra were recorded on a Shimadzu
model UV-3100 or Cary 100 Bio UV-Visible spectrophotometer and fluorescence emission
spectra were recorded on a Jobin Yvon Horiba model Fluoromax-3 spectrofluorometer.
Synthesis of (E)-9-ethyl-3-(2-(4'-methyl-[2,2'-bipyridin]-4-yl)vinyl)-9H-carbazole (4).
°
Potassium tert-butoxide (0.22 g, 2 mmol) was added at 0 C to a solution of mono
phosphonate (3, 0.32 g, 1 mmol) and the aldehyde (18a, 0.22 g, 1 mmol) in THF (20 mL)
under nitrogen atmosphere; the ice bath was subsequently removed, and the reaction mixture
was allowed to stir at room temperature for 1h. The reaction mixture was quenched with 10
ml of ammonium chloride solution, and the aqueous layer was extracted with
dichloromethane (2×30 mL). The resulting mixture was washed several times with water and
then with brine. The resultant material, obtained, was washed with ether and dried in air and
further purified over silica gel (100−200 mesh) column using methanol/dichloromethane 2:98
v/v as the eluent to obtain compound 4 as light green solid. Yield: 0.29 g (77 %). IR (neat):
1
nmax: 2958.79, 2923.7, 2854.5, 1592.5, 1466.5, 1375.5, 1236.3, 741.8. H NMR (400 MHz,
CDCl ): δ 8.64 (d, J = 5 Hz, 1H), 8.60 (d, J = 5 Hz, 1H), 8.57 (s, 1H), 8.28 (d, J = 8 Hz, 2H),
3
8.14 (d, J = 8 Hz, 1H), 7.70 (d, J = 9 Hz, 1H), 7.65 (d, J = 16 Hz, 1H), 7.50 (t, J = 8 Hz, 1H),
7.42-7.37 (m, 3H), 7.30-7.26 (m, 1H), 7.16 (s, 1H), 7.14 (d, J = 16 Hz, 1H), 4.64 (q, J = 7 Hz,
1
3
2H), 2.46 (s, 3H), 1.44 (t, J = 7 Hz, 3H). C NMR (100 MHz, CDCl ): δ 156.5, 156.1, 149.4,
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149.0, 148.1, 146.4, 140.4, 140.3, 134.3, 127.5, 126.0, 124.8, 124.7, 123.4, 123.3, 122.9,
122.0, 120.7, 120.5, 119.6, 119.3, 118.0, 108.8, 108.7, 37.7, 21.2, 13.8. HRMS: m/z calcd.
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8