Deciphering the photophysical role of conjugated diyne in butadiynyl fluorophores: Synthesis, photophysical and theoretical study

Article abstract of DOI:10.1021/jp404809g

The present work focuses on the current interest in diyne bridged chromophores necessitating a clearer understanding of the photophysics of such molecules. The significance of the diyne moiety in the photophysics has been investigated by synthesizing simp

Full text of DOI:10.1021/jp404809g

Article
pubs.acs.org/JPCA
Deciphering the Photophysical Role of Conjugated Diyne in
Butadiynyl Fluorophores: Synthesis, Photophysical and Theoretical
Study
Avik Kumar Pati,† Monalisa Mohapatra,† Pokhraj Ghosh,† Santosh J. Gharpure,*,†,‡
and Ashok K. Mishra*,†
†Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, Tamil Nadu, India
‡Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
S
* Supporting Information
ABSTRACT: The present work focuses on the current interest in diyne bridged chromophores necessitating a clearer
understanding of the photophysics of such molecules. The significance of the diyne moiety in the photophysics has been
investigated by synthesizing simple substituted diphenyl butadiynyl derivatives following a quick and efficient microwave assisted
Eglinton coupling of terminal alkynes. Emission of the fluorophores is observed from the usual locally excited (LE) state and
intramolecular charge transfer (ICT) state. Separation of pure ICT emission from pure LE emission has been carried out by
Gaussian/Lorentzian curve fitting. The vibronic coupling in the local transitions appears to be confined to the normal mode
involving the C−C triple bond stretching of the diyne moiety. This implies that the LE transition involves the diyne moiety, a
conclusion supported by quantum chemical calculations. The resolved ICT emission follows double linear dependence on
ET(30) solvent polarity scale. The important role of the diyne moiety in the photophysics of this class of molecules is clearly
discernible in this study.
structure−property relationship of butadiyne conjugated
carbazole dimers has been described by Nakamura et al.35
These studies not withstanding, the photophysics of the
butadiyne moiety itself is not completely unravelled. This is
because most of these studies either include the butadiyne
moiety attached to inherent fluorophores/bulkier aromatic
groups or the butadiyne moiety is overloaded with extended
alkyne conjugation, thereby masking the original photophysical
contribution from the butadiyne part. Thus an understanding of
the contribution of the butadiyne moiety to the photophysics of
such molecular systems assumes importance. The diyne moiety
has even found its applications in nanoscience,36 molecular
rotors,37 liquid crystals,38−41 and antiviral activities42 providing
an impetus for contemporary research. Intriguingly, the
ubiquity of this moiety in several biologically active natural
INTRODUCTION
■
Recent years have seen an upsurge in the development of novel
organic fluorophores exhibiting intramolecular charge transfer
(ICT) owing to their efficacious applications in field effect
diodes,1,2 light emitting diodes,3,4 chemical sensors,5−7 and in
many other photochemical and photobiological processes.8−10
Often these fluorophores are found to contain alkyne units in
conjugation with different aromatic moieties. Thus the
chemistry of triple bond attached fluorophores is very rich.
Surprisingly, a deeper photochemical understanding of
conjugated two triple bonds, that is, butadiyne moiety, remains
scarce in the literature from a long time ago. Nevertheless,
some efforts have been made in recent years.11−35 Bryce et al.
have explored the photophysics of 2,5-diphenyl-1,3,4-oxadia-
zole (OXD) containing butadiynyl derivatives.13,14 Otera and
Orita et al. have exploited the photochemistry of diphenylami-
no substituted phenylene-(poly)ethynylene derivatives.15,16
The photochemistry of a butadiyne conjugated pyrene dimer
has been disclosed by Benniston et al.22 Very recently, a
Received: May 16, 2013
Revised: July 2, 2013
Published: July 3, 2013
© 2013 American Chemical Society
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Scheme 1. Retrosynthesis of Diyne 1
Table 1. Synthesis of Diyne Derivatives 1aa−cd
a
Isolated yields of purified product.
products43 attracts special attention to look at its fundamental
photophysical properties.
RESULTS AND DISCUSSION
■
Synthesis. A number of methods for coupling reactions of
At this juncture, we envisaged that the role of the diyne
moiety in controlling photophysical behavior of a butadiynyl
derivative could be properly investigated only when the
aromatic moieties attached to the periphery of the diyne are
as simple as phenyl ring such that they do not perturb the
photophysical properties of core diyne moiety. Thus, simply
substituted 1,3-diphenyl butadiynes could be fundamentally
important representatives to study the effect of the diyne
moiety in deciding photochemical behavior of a fluorophore.
The objectives of this work are (1) to develop a synthetic root
for the quick access of diynes, especially photophysically
important hetero diynes despite synthetic challenges, (2) to
study a detailed and systematic photophysical behavior of the
derivatives in a series of solvents of different polarities, (3) to
find a uniform model to understand the photophysical
properties, and (4) to understand better the electronic structure
of diynes through theoretical calculations.
terminal alkynes are documented in the literature. Among all of
them, Glaser,44 Eglinton,45 Hay,46,47 Cadiot-Chodkiewicz48,49
and Sonogashira50,51 reactions are well-known.52 Recently, a
fast, solvent free, microwave assisted heterogeneous homocou-
pling of terminal alkynes on alumina surface is reported.53 The
protocol was limited to only homocoupling in heterogeneous
phase. Here, we wish to report a microwave assisted
homogeneous Eglinton coupling of terminal alkynes for the
quick access to both homo- and heterocoupling products. The
retrosynthetic analysis for the synthesis of 1,3-diynes 1 is
depicted in Scheme 1. Commercially available phenyl acetylene
was directly used for reaction purposes, and the other alkynes
were prepared from the corresponding aldehydes using
literature reported procedures.54−56 With the requisite alkynes
in hand, we turned our attention toward the synthesis of 1,3-
diynes 1aa−cd via microwave assisted Cu(II) catalyzed
homogeneous Eglinton reaction of alkynes. Cu(OAc)2 was
used as salt, and piperidine was used as base.57 To test the
feasibility of the reaction, phenyl acetylene 2a was first
subjected to microwave assisted homogeneous homocoupling
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Figure 1. (a) Absorption spectra of 1aa−cd and (b) emission spectra of 1aa−bc in cyclohexane (10−5 M, λex = 335, 340, 330, 350, 340, 350 nm for
1ab, 1bb, 1ac, 1cc, 1ad, 1bc, respectively).
Figure 2. UV−vis spectra of (a) 1aa, (b) 1ab, (c) 1bb, and (d) 1ad in various solvents (10−5 M).
reaction to give the corresponding diyne 1aa in excellent yield
(Table 1, entry 1). The identity of the compound was
ascertained from its spectral data. Encouraged by this result,
various alkynes were subjected to homo- and heterocoupling
reactions under the same reaction conditions, and various 1,3-
diynes were obtained in good to excellent yields (Table 1, entry
2−8). The identities of the compounds were confirmed by
NMR spectral analysis which was further confirmed by mass
spectral analysis. Heterocoupling reaction was carried out using
two different terminal alkynes in the ratio of 1:5. It is pertinent
to mention that each hetero coupling reaction ended up with
the formation of homocoupling products of the alkynes along
with a major heterocoupling derivative. The required product
was purified by extensive column chromatography to ascertain
the absence of impurities. Each compound was finally filtered
through silica column using HPLC grade hexane and
recrystallized prior to photophysical studies.
and emission spectra of all compounds were recorded in
cyclohexane. A closer look at the onset of electronic absorption
spectra reveals that eight molecules here under study can be
divided into four groups as if the spectral features among the
molecules are distributed over 1:3:3:1 pattern (Figure 1a). The
onset of electronic absorption shifts bathochromically from
parent molecule 1aa to methoxy substituted derivatives 1ab,
1bb and nitrile substituted 1ad to dimethyl amino substituted
derivatives 1cc, 1ac, 1bc to donor dimethyl amino and acceptor
nitrile substituted derivative 1cd. This gradual red shift
indicates the presence of ICT through the butadiynyl moiety.
The parent chromophore 1aa was found to be non
fluorescent25,26 under our experimental conditions. Emission
spectra of all other derivatives in cyclohexane were found to be
structured with long band/tail extended to the longer
wavelength (Figure 1b). Fluorescence intensity was found to
be higher for dimethyl amino substituted derivatives while
comparing with methoxy derivatives. Owing to the structural
emission spectrum in non polar cyclohexane, we were keen to
Absorption and Emission Spectra in Cyclohexane.
Cyclohexane being a nonpolar, noninteractive solvent, UV−vis
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Figure 3. UV−vis spectra of (a) 1ac, (b) 1cc, (c) 1bc, and (d) 1cd in various solvents (10−5 M).
observe its effect in polar solvents. At this junction, we
envisioned that a detailed solvatochromic investigation of UV−
vis and emission spectra might reveal some important features
of the butadiyne moiety.
moderately polar to highly polar solvent, indicating the
presence of strong ICT. The diyne 1cd attracts special eyes
when it is compared with its butadiene analogue, dimethyla-
mino-cyano-diphenylbutadiene (DCB).59 The derivative 1cd
absorbs at λmax 385 nm in acetonitrile while DCB absorbs at
402 nm in acetonitrile. The data annotates the shorter
molecular axis of 1cd compared with the diene DCB.
Absorption Properties in Solvents of Different Polar-
ities. Absorption maxima of the derivatives did not change
much with changing solvents of different polarities (Figure 2
and 3). The spectra even remained almost unaltered for the
molecule 1cd which has a strong donor dimethyl amino group
and strong acceptor nitrile moiety at the terminals of the
butadiyne bridge, ruling out the possibility of its ground state
dipole formation. The observations corroborate that all the
fluorophores are presumably exposed to the same local
environment in both ground and excited state, being
completely noninteractive with the solvent environment within
very short femtosecond absorption time scale. We were also
highly elated to note the same UV−vis spectral trend among
molecules in a particular solvent, discussed in an earlier section,
not only for cyclohexane but also for all other solvents of
different polarities. The parent molecule 1aa absorbs at λmax
323 nm in cyclohexane while the donor−acceptor based
molecule 1cd absorbs at λmax 396 nm in cyclohexane. Thus
simple tuning of electronic environment at the periphery of the
butadiyne moiety allows opening a large absorption window.
The absorption band spread over around 315 to 415 nm could
be assigned to the S0→S1 transition whereas the region around
250 to 300 nm could be due to the S0→S2 transition. The
derivatives 1ab, 1bb, and 1ad have very high S0→S2 transition
intensity along with the S0→S1 transition. On the other side,
the S0→S1 transition for 1cc and 1bc intensifies at the cost of
the absorption intensities of S0→S2 transition. Similar
observation was found in some substituted amino containing
butadiynyl derivatives reported by Shenoy et al.58 The diyne
1cd shows almost equal intensities of S0→S1 and S0→S2
transitions. Significant loss of vibrational fine structures has
been registered for 1cd on moving from non polar to
Emission Properties in Solvents of Different Polar-
ities. As our investigation proceeded, the emission spectral
features in various solvents of different polarities had continued
to puzzle us from molecule to molecule. The emission maxima
for 1ab and 1bb do not change much with changing solvents of
different polarities (Figure 4a, 4b). For example, the emission
maxima for 1ab are 441 nm in n-heptane and 457 nm in
acetonitrile while 471 and 495 nm are those for 1bb
respectively. Thus, singly substituted methoxy to doubly
substituted methoxy groups at the phenyl rings in 1ab and
1bb respectively procure a 30 nm red shift of emission maxima
for n-heptane and 38 nm for acetonitrile. A close look at the
emission spectra of 1ab and 1bb (Figure 4a, 4b) reveals a small
structural emission part at around 400 nm. The emission
maxima for 1ad lie between 397 and 410 nm from n-heptane to
acetonitrile exhibiting spectral insensitivity toward solvent
polarities (Figure 4c). The emission profile for 1ad is a little
sharper at shorter wavelength but a long tail has been tailored
to the longer wavelength. The emission maxima for 1ac are
considerably red-shifted as the solvent polarity rises. The effect
of dimethyl amino group substituted phenyl ring as strong
donor and unsubstituted phenyl ring as acceptor in 1ac has
been reflected in the shift of emission maxima from 446 nm in
n-heptane to 526 nm in acetonitrile. The emission spectrum of
1ac was observed to be very broad. The presence of two strong
dimethyl amino groups in 1cc furnished the emission spectrum
as highly structured with a long tail emission at around 525 nm
which is much more prominent for non polar n-heptane and
cyclohexane. Thus moving from one dimethyl amino group
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Figure 4. Emission spectra of (a) 1ab (λex = 330 nm), (b) 1bb (λex = 340 nm), (c) 1ad (λex = 340 nm), (d) 1ac (λex =330 nm for cyclohexane and
heptane, λex = 360 nm for other solvents), (e) 1cc (λex = 350 nm), and (f) 1bc (λex = 350 nm) in different solvents (10−5 M). Dotted circle indicates
Raman scattering.
bearing derivative 1ac to two dimethyl amino groups
substituted derivatives 1cc, a drastic difference of emission
spectral features was witnessed (Figure 4d, 4e). Widely
separated dual emission was found for 1bc where the locally
excited (LE) state is at around 425 nm and the ICT state is at
around 525 nm (Figure 4f). The fluorophore 1cd attracted
special attention owing to its unique emission spectral features
compared to other fluorophores discussed here. The spectra of
1cd were noticed to be structured in n-heptane and
cyclohexane exhibiting emission maxima at 426 and 431 nm,
respectively. The emission spectra exhibit a huge response in
changing polarities of solvents (Figure 5a). The spectra are
significantly red-shifted from non polar to polar solvents (Table
2). This implies that the excited state of 1cd is comparatively
more polar than its ground state and hence the excited state is
more stabilized by polar solvents exhibiting a huge bath-
ochromic shift. A very long emission tail extended to the higher
wavelength for n-heptane and cyclohexane whereas a short
hump came up at shorter wavelength for solvents of moderate
and high polarities in 1cd. This observation shows dual
fluorescence of 1cd confirming emissions from both equili-
brated LE and ICT states. The presence of an LE and a charge
transfer state was suggested by Khundkar et al. in donor
(MeS)−acceptor (CN) substituted butadiynyl derivative.11
Guo and Xia et al. have also theoretically shown the
involvement of a charge transfer state in donor−acceptor
substituted α,ω-diphenylpolyynes.19 There is also a preeminent
finding for 1cd in increase in Stokes’ shift which is around 7419
cm−1 between n-heptane and acetonitrile. This large Stokes’
shift is a consequence of efficient charge transfer from the
donor dimethyl amino group to the acceptor nitrile moiety. It is
also imperative to mention that quenching of fluorescence
intensity took place in highly polar solvents. The photograph
(Figure 5b) of 1cd taken under UV light clearly shows the
solvent induced Stoke’s shift. The color ranges from blue in n-
heptane (λem = 426 nm) to orange in DMF (λem = 602 nm).
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Figure 5. (a) Emission spectra of 1cd in different solvents (1 × 10−5 M, λex = 390 nm) and (b) photograph of 1cd in different solvents under UV
light, (1) n-heptane (λem= 426 nm); (2) cyclohexane (λem= 431 nm); (3) dioxane (λem = 531 nm); (4) methanol (λem= 575 nm); (5) THF (λem=
545 nm); (6) isopropanol (λem= 559 nm); (7) DMF (λem= 602 nm).
are composed of both an LE and an ICT state. Thus
simultaneous appearance of an LE and ICT state is solely the
characteristic outcome of conjugated diyne, regardless of the
electronic environment at the periphery of the diyne moiety.
The electronic environment may only affect the position and
intensity of the two states depending on the chemical
environment, thereby mystifying the emission features from
molecule to molecule. To simplify our thoughts, all the
emission spectra were fitted into multiple Gaussians. Generally,
standard emission spectral profiles have Gaussian shape30,65
while spectral profiles of Raman scattering as well as vibrational
lines resemble the Lorentzian shape.66−68 As many of the
emission spectra are structured because of vibronic spectral
lines of LEs, and in many cases Raman scattering is very close
to fluorescence maxima, Gaussian fitting was found to be
troublesome in some of the molecules. In these cases, the
problem could be circumvented by using multiple Lorentzians
fitting of the emission spectrum. Major parts of Gaussian and
Lorentzian bands are the same, and the Lorentzian curve differs
from the Gaussian curve in the fact that the Lorentzian curve
has a long tail compared to the Gaussian one as stated by
Meier.69 It is pertinent to mention that this difference really
does not hamper our study of interest. As expected, multiple
Gaussian/Lorentzian fitting has unravelled the emission spectra
by resolving it as a mixture of multiple emission bands. The
shorter wavelength bands could be assigned to a LE which was
again split into vibronic spectra whereas the longer wavelength
band or tail could be due to ICT. The derivatives 1ab and 1ad
have been chosen here as representatives to show the
Gaussian/Lorentzian fitted curves (Figure 6, 7). The presence
of Raman line was observed in all the fluorescence spectra
recorded, which was identified by the usual procedure of
recording the fluorescence spectrum at different excitation
wavelength. Since the locally excited emission band showed
vibronic structuring, it was necessary to carefully identify and
remove the Raman band for the further analysis of the spectral
profiles. Figure 8 shows that the resolved longer wavelength
ICT band for 1ab and 1ad is red-shifted with changing solvents
of increasing polarities. As for illustration, emission maxima of
Table 2. Photophysical Parameters of 1cd in Different
Solvents
λabs (longest
absorption
wavelength) nm
λem
(max)
(nm)
Stokes’
shift
quantum
yield
solvents
(cm−1
)
(1) heptane
393
396
426
431
1971
2051
0.03
0.03
(2)
cyclohexane
(3) dioxane
(4) THF
389
389
390
385
531
545
537
559
6874
7358
7019
8085
0.11
0.11
0.11
0.02
(5) DCM
(6)
isopropanol
(7) MeOH
(8) CH3CN
(9) DMF
383
385
393
575
603
602
8718
9390
8834
0.02
0.02
0.02
The quantum yield of the molecule 1cd was recorded with
respect to quinine sulfate. The data in Table 2 shows that the
value decreases with an increase in solvent polarity. The lower
quantum yield in the highly polar solvent could be due to the
formation of the ICT state which would result in nonradiative
decay in the excited state. The change in dipole moment
between the excited and ground states of 1cd was obtained
from the slope of Lippert−Mataga plot60,61 which was found to
be about 35.2 D assuming Onsager radius as 8.14 Å. The
Onsager radius was calculated as half of the distance between
nitrogen of donor dimethyl amino group and nitrogen of
acceptor nitrile moiety in the optimized geometry of the
molecule 1cd using B3LYP/6-31G* DFT62,63 geometry
optimization calculation using the Gaussian 03 computational
package.64 This result assures the high polar nature of the
fluorophore 1cd due to ICT aided by the two alkynyl bridges
between the donor dimethyl amino and acceptor nitrile
moieties.
Origin of Asymmetry of Emission Spectra. The
asymmetry of emission spectral features among various
fluorophores under study triggered us to understand its origin.
We anticipated that the emission spectra of all the fluorophores
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Figure 6. Lorentzian fitting of emission spectra of 1ab (blue peak indicates Raman scattering).
Figure 7. Gaussian fitting of emission spectra of 1ad (blue peak indicates Raman scattering).
fitted ICT band for 1ab have been found to be 462 nm in n-
heptane and 506 nm in acetonitrile whereas those are 427 nm
in n-heptane and 472 nm in acetonitrile for 1ad. The shorter
wavelength bands for 1ab and 1ad have been resolved into a
mixture of vibronic spectrum. Pleasingly, the energy gap
obtained from the difference of the maxima of two vibronic
spectra has been found to be around 2000 cm−1 which is
unambiguously assigned to C−C triple bond stretching of
butadiynyl moiety,27 comparing its value with the experimental
and theoretical values (Table 3). The LEs, wherever possible
for the derivatives, have been resolved into vibronic spectra (see
Supporting Information), and the calculated infrared stretching
frequency has been found to be almost consistent with the
former value. Thus, the observed association of C−C triple
bond stretching frequency with the electronic transition
corresponding to the LE state is important as it provides a
spectroscopic evidence for the participation of the butadiyne
moiety in electronic transition.
Our effort was further directed at finding the quantum yields
of 1ab and 1ad from the area of the Gaussian/Lorentzian fitted
emission profiles (Figure 6 and 7). The calculated quantum
yields of 1cd have been considered as standard to find out the
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Figure 8. Normalized longer wavelength emission spectra (from the fitted curves) of (a) 1ab and (b) 1ad.
follow a different trend than aprotic solvents (Figure 9). Similar
trends have been observed and attributed to the changes in the
character of emissive states.71,72 Attempts to fit the data in a
Kamlet Taft73−75 plot were not successful.
Table 3. IR Stretching Frequency for C−C Triple Bond
IR values for C−C triple bond
theoretical
experimental
value obtained from vibronic
c
a
b
derivatives value (cm−1
)
value (cm−1
)
progression (cm−1
)
Fluorescence Lifetime Studies. To attest the concurrent
existence of an LE and an ICT state, a time-resolved
fluorescence dynamic study was carried out. The fluorophore
1cd was taken here as a molecule of choice to accomplish this
study. The experiment was performed in various solvents of
different polarities, and fluorescence lifetime was collected at
different emissions which have been shown in Figure 10. Non
polar solvents like n-heptane and cyclohexane fit well in
monoexponential decay when lifetime is collected at λem 426
nm whereas they show biexponential behavior when lifetime is
collected at λem 492 and 537 nm. The amplitude of the two
states also changes accordingly while going from 426 to 492 to
537 nm (Table 5). Moderately polar solvent tetrahydrofuran
(THF) and dichloromethane (DCM) exhibit biexponential
behavior at λem 426 nm while they start showing mono-
exponential behavior from λem 492 to 537 to 605 nm. The
amplitude in THF and DCM at different emissions also
changes accordingly. On the other hand, in isopropanol and
acetonitrile the lifetimes are found to be biexponential at all the
emission points. These results confirm the excited state
equilibrium between an LE and an ICT state. The lifetime is
less than 1 ns in n-heptane and cyclohexane while in THF and
DCM it is little more than 1 ns. The lifetime is even lower in
highly polar solvents such as isopropanol and acetonitrile. The
low lifetime data undoubtedly indicates the association of an
ICT state in the excited state.
1ab
2318
2245
2317
2206
2145
2210
2137
2273
2023
1ad
2245
b
a
c
B3LYP/6-31g*. FT-IR. In cyclohexane.
quantum yields of 1ab and 1ad. The data has been tabulated in
Table 4 which shows very low quantum yield for the
derivatives. Table 4 shows that ICT has significant contribution
to the total quantum yield for both derivatives 1ab and 1ad.
The contribution of ICT to the overall quantum yield in
derivative 1ab, having one methoxy group as donor, is much
higher than that of the LE contribution. Fluorophores
containing bulkier aromatic groups at the periphery of the
diyne have been found to have higher or reasonably higher
quantum yield in the literature.13−16,22,35 Thus our data
prompted us to conclude that the diyne moiety really does
not play an integral role in enhancing quantum yield; rather it is
the peripheral moieties which control the quantum yield. Here
phenyl rings being situated at the periphery of the diyne hardly
help to increase the quantum yield value of the derivatives.
The λmax values obtained from the resolved ICT bands were
used to understand the solvation dynamics of the fluorophores
1ab and 1ad. The values were fitted well in ET(30)70 plot
where polar protic solvents like isopropanol and methanol
Table 4. Quantum Yield for 1ab and 1ad Calculated from the Area of Fitted Emission Spectra (The data in the parenthesis
indicates the contribution of ΦLocal or ΦICT to the ΦTotal
)
quantum yield (Φ × 10−3
)
1ab
1ad
solvents
ΦLocal × 10−3
ΦICT × 10−3
ΦTotal × 10−3
ΦLocal × 10−3
ΦICT × 10−3
ΦTotal × 10−3
(1) heptane
(2) cyclohexane
(3) dioxane
(4) THF
1.8 (0.27)
2.4 (0.39)
1.0 (0.26)
1.6 (0.31)
1.4 (0.30)
1.4 (0.20)
1.1 (0.19)
1.4 (0.20)
1.5 (0.24)
4.8 (0.73)
3.8 (0.61)
2.8 (0.74)
3.5 (0.69)
3.3 (0.70)
5.5 (0.80)
4.7 (0.81)
5.6 (0.80)
4.8 (0.76)
6.6
6.2
3.8
5.1
4.7
6.9
5.8
7.0
6.3
4.7 (0.52)
3.4 (0.52)
6.6 (0.63)
7.0 (0.64)
8.1 (0.70)
6.3 (0.68)
5.1 (0.58)
9.5 (0.73)
9.4 (0.69)
4.4 (0.48)
3.1 (0.48)
3.9 (0.37)
3.7 (0.35)
3.5 (0.30)
3.0 (0.32)
3.7 (0.42)
3.6 (0.27)
4.2 (0.31)
9.1
6.5
10.5
10.7
11.6
9.3
(5) DCM
(6) isopropanol
(7) MeOH
(8) CH3CN
(9) DMF
8.8
13.1
13.6
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Figure 9. ET(30) plots for (a) 1ab, and (b) 1ad using resolved ICT data.
Figure 10. Lifetime profile of 1cd at different emissions in (a) heptane, (b) cyclohexane, (c) THF, and (d) acetonitrile.
Theoretical Investigations. To gain further insights into
the electronic transition of the fluorophores theoretical
calculations using density functional theory with the B3LYP
hybrid functional and 6-31g* basis set were performed.62,63
Figure 11 represents the molecular orbitals involving the first
singlet vertical transition of 1cd as the representative. The S0→
S1 transition mainly involves transitions from the highest
occupied molecular orbital (HOMO) to the lowest unoccupied
molecular orbital (LUMO) for all the derivatives. It is noted
that the HOMO of the molecule 1cd is substantially localized
on the dimethyl amino group substituted phenyl ring and
butadiyne moiety with lesser electronic distribution on the
nitrile containing phenyl ring. On the other hand, the LUMO
of 1cd is highly concentrated on the phenyl ring having a nitrile
group and butadiyne bridge with small extension of electron
density to the dimethyl amino substituted phenyl ring. A similar
nature of the HOMO and LUMO is observed for neutral-donor
1ab, neutral-acceptor 1ad, and neutral-donor 1ac derivatives.
For the donor−donor derivatives 1bb, 1bc, and 1cc, the
HOMO is centered on both the donor substituted phenyl rings
along with butadiyne bridge whereas the LUMO is distributed
over the butadiyne moiety with less electronic distribution on
donor substituted phenyl rings. Thus a HOMO to LUMO
transition corresponds to the charge transfer state. Albeit the
HOMO to LUMO transition mainly contributes to the S0→S1
transition, the minor contribution can not be ignored given the
fact that it involves molecular orbitals with electron density
solely localized on the diyne moiety. This observation is unique
for all the butadiynyl derivatives (see Supporting Information).
Thus all electronic transitions are controlled by the same
orbitals and as a consequence, structured absorption and local
emission are observed for the butadiynyl derivatives.
The mixing up of orbitals contributing to an ICT and LE
state clearly explains our experimental observation of emissions
from both the LE and the ICT state for all the fluorophores.
Figure 12 shows that orbital contribution to the ICT state is
much higher than the LE state. Moving from neutral 1aa to the
donor−acceptor 1cd derivative, the orbital contribution to the
ICT state increases. On the other hand, moving from donor−
acceptor 1cd to neutral 1aa derivative, the orbital contribution
to the LE state starts rising. At this point, it is worth stressing
that the observed trend of the orbital contribution to the ICT
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Figure 11. Frontier orbitals of 1cd for the first singlet vertical
transition.
Figure 12. Orbital contribution for LE and ICT state of 1aa−1cd.
state from molecule to molecule (Figure 12) is almost in similar
agreement with our experimental finding of a gradual red shift
of the onset of the electronic absorption spectra (Figure 1a)
from neutral to donor−donor to donor−acceptor molecules.
The electronic distribution in LUMO+3 and HOMO-2 in 1cd
is noteworthy. Here, in 1cd, the electronic distribution is
parallel to the phenyl rings whereas it is perpendicular to the
phenyl rings for HOMO and LUMO orbitals.76 Orita and
Otera et al. found similar observation for the NH2-yne-triyne
derivative.15
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CSIR, New Delhi, for research fellowships. A.K.P. thanks Mr. V.
Prasath, IIT Bombay, for helpful discussion.
CONCLUSIONS
■
In summary, quick access to a series of diphenyl butadiynyl
derivatives has been achieved through microwave assisted
Eglinton coupling of terminal alkynes. A systematic and
detailed study solely focused on the evaluation of fundamental
photophysics of the “butadiyne” core moiety has been
accomplished. Several important findings emerged through
the evaluation of photophysical properties of these butadiynyl
fluorophores.
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ASSOCIATED CONTENT
* Supporting Information
General experimental methods, synthetic procedures and
characterization data for all the butadiynyl derivatives,
Lorentzian fitting of emission spectra, frontier orbitals for the
first singlet vertical transition of fluorophores. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
*E-mail: sjgharpure@iitb.ac.in (S.J.G.), mishra@iitm.ac.in
(A.K.M.). Phone: +91-22-2576 7171 (S.J.G.), +91-44-
22574207 (A.K.M.). Fax: +91-22-2576 7152 (S.J.G.), +91-44-
22574202 (A.K.M.).
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Monkman, A. P.; Bryce, M. R. Efficient Intramolecular Charge
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank CSIR, DBT, and DST, New Delhi, for
financial support. The high performance computing facility of
IIT Madras is greatly acknowledged. A.K.P. and M.M. thank
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