Effect of Donor-Acceptor Coupling on TICT Dynamics in the Excited States of Two Dimethylamine Substituted Chalcones

Article abstract of DOI:10.1021/acs.jpca.5b08203

Significant effect of coupling between the electron donor and acceptor groups in intramolecular charge transfer (ICT) dynamics has been demonstrated by comparing the photophysical properties of two isomeric N,N-dimethylaminochalcone derivatives (namely, DMAC-A and DMAC-B). In the case of the DMAC-B molecule, the distance between the donor (N,N-dimethylaniline or DMA) and the acceptor (carbonyl) groups is larger by one ethylene unit as compared to that in the case of DMAC-A. The excited singlet (S₁) states of both the isomers have strong ICT character but their photophysical properties are remarkably different. In polar solvents, fluorescence quantum yields (and the lifetimes of the S₁ state) of DMAC-A are more than 2 orders of magnitude lower (and shorter) than those of DMAC-B. Remarkable differences in the photophysical properties of these two isomers arise due to occurrence of the ultrafast twisting of the DMA group (or the TICT process) during the course of deactivation of the S₁ state of the DMAC-A molecule, but not in the case of DMAC-B. In the later case, because of the presence of a large energy barrier along the twisting coordinate(s), TICT is not a feasible process, and hence, the S₁ state of DMAC-B has the planar ICT structure. In the DMAC-A molecule, the strength of coupling between the donor and acceptor groups is relatively stronger because of a shorter distance between these groups. Femtosecond transient absorption spectroscopic measurements and DFT/TDDFT calculations have been adopted to establish the above aspects of the relaxation dynamics of the S₁ states of these two isomeric chalcones.

Full text of DOI:10.1021/acs.jpca.5b08203

Article
pubs.acs.org/JPCA
Effect of Donor−Acceptor Coupling on TICT Dynamics in the Excited
States of Two Dimethylamine Substituted Chalcones
Rajib Ghosh and Dipak K. Palit*
Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India
S
* Supporting Information
ABSTRACT: Significant effect of coupling between the electron donor and acceptor
groups in intramolecular charge transfer (ICT) dynamics has been demonstrated by
comparing the photophysical properties of two isomeric N,N-dimethylaminochalcone
derivatives (namely, DMAC-A and DMAC-B). In the case of the DMAC-B molecule,
the distance between the donor (N,N-dimethylaniline or DMA) and the acceptor
(carbonyl) groups is larger by one ethylene unit as compared to that in the case of
DMAC-A. The excited singlet (S1) states of both the isomers have strong ICT character
but their photophysical properties are remarkably different. In polar solvents,
fluorescence quantum yields (and the lifetimes of the S1 state) of DMAC-A are
more than 2 orders of magnitude lower (and shorter) than those of DMAC-B.
Remarkable differences in the photophysical properties of these two isomers arise due
to occurrence of the ultrafast twisting of the DMA group (or the TICT process) during
the course of deactivation of the S1 state of the DMAC-A molecule, but not in the case
of DMAC-B. In the later case, because of the presence of a large energy barrier along
the twisting coordinate(s), TICT is not a feasible process, and hence, the S1 state of DMAC-B has the planar ICT structure. In
the DMAC-A molecule, the strength of coupling between the donor and acceptor groups is relatively stronger because of a
shorter distance between these groups. Femtosecond transient absorption spectroscopic measurements and DFT/TDDFT
calculations have been adopted to establish the above aspects of the relaxation dynamics of the S1 states of these two isomeric
chalcones.
maximum charge separation between the donor and the
acceptor moieties due to the minimum orbital overlap.4
On the other hand, strong viscosity dependent fluorescence
yield of several diphenyl and triphenylmethane dyes led to the
concept of large amplitude structural relaxation to facilitate
nonradiative deactivation of the excited state. Ultrafast
spectroscopic measurements on these molecules revealed that
twisting of the N,N-dimethylaniline (DMA) group generates
nonemissive TICT excited state in pico or subpicosecond time
scale depending on the viscosity of the medium.8−13
Dimethylamino substituted aromatic ketone derivatives, such
as 4-dimethylaminobenzophenone and Michler’s ketone, also
exhibit similar ultrafast relaxation dynamics involving twisting
of the DMA group.14−16
Grabowski et al. have reviewed a large amount work to
establish the fact that enhanced charge transfer upon
photoexcitation drives the structural relaxation to the TICT
state in D-Ar-A molecules.4 On the other hand, it has also been
established that in many D-Ar-A molecules, the ICT state with
strong charge transfer character does not undergo TICT
relaxation.17−21 For example, 4, 4′-Dimethylaminocyanostil-
bene has been established to have planar ICT structure in the
1. INTRODUCTION
Photoinduced charge transfer or electron transfer processes
play the fundamental role in conversion of light energy, e.g.,
photosynthesis in plants, artificial light harvesting systems, and
in numerous existing or conceived molecular photonic
applications.1−3 Donor and acceptor substituted conjugated
organic molecular systems undergo extensive charge transfer
upon photoexcitation to the electronically excited state. The
photoinduced intramolecular charge transfer (ICT) state may
lead to quantitative charge separation by structural reorganiza-
tion, which makes the ICT process irreversible, and this is
highly desirable for solar energy conversion. Thus, the
fundamental understanding of the structure and dynamics of
the ICT process in the excited states has been of great interest,
and hence, numerous donor (D) and acceptor (A) substituted
aromatic molecules (D-Ar-A (here, Ar is the aromatic or π-
conjugated alkene bridge between D and A)) have been studied
extensively.4−13 The concept of relaxation of the excited state
via twisting of single bond has mainly evolved from the
observations of dual fluorescence of 4-dimethylaminobenzoni-
trile (DMABN) and related molecules. The higher energy
emission occurs from the ICT state, whereas the twisted
intramolecular charge transfer (TICT) state, in which the
dimethylamino (donor) group attains a dihedral angle of about
90° with respect to the molecular plane, is responsible for the
lower energy emission.4 This twisted geometry allows
Received: August 23, 2015
Revised: October 16, 2015
© XXXX American Chemical Society
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excited state without TICT relaxation.20,21 It is intuitive that the
ICT dynamics via twisting relaxation will depend on the relative
energies of the ICT and TICT states and intervening barrier
height along the twisting coordinate. It is highly desirable to
understand the structural effect on charge transfer dynamics
facilitated by large amplitude twisting motion.
In this paper, we attempt to address this issue by comparing
the spectroscopic properties and excited state dynamics of two
isomeric 4-dimethylaminochalcones (abbreviated as DMAC-A
and DMAC-B, see Scheme 1), where the DMA group and the
DMAC-B: Yield: 60%, orange crystals, mp 105−107 °C. H
1
NMR(CDCl3) δ (ppm): 3.04 (s, 6H), 6.70 (d, 2H), 7.34 (d,
1H),7.48 (t, 2H), 7.55 (m, 3H), 7.79 (d, 1H),8.01(d, 2H) 13C
NMR (CDCl3) δ (ppm): 40.2, 112.0, 117.1, 122.8, 128.4,
128.5., 130.5, 132.6, 139.2, 145.9, 152.1, 190.8.
Fluorescence excitation spectra of each of these two samples
have been compared with its ground state absorption spectrum
and excellent agreements between them ensure the absence of
any fluorescent impurity in the samples (Figure S2 in the SI
section). All solvents were of spectroscopic grade (Spectro-
chem, India) and were used as received.
Scheme 1. Molecular Structures of DMAC-A and DMAC-B
2.2. Photophysical Measurements. Steady state absorp-
tion spectra were recorded using a Thermo-Electron model
Biomate spectrophotometer. Fluorescence spectra, which were
corrected for the wavelength dependence of the instrument
sensitivity, were recorded using Hitachi model 4010 spectro-
fluorimeter. Fluorescence measurements were performed with
dilute solutions having optical density (OD) < 0.2 at the
excitation wavelength to eliminate the error due to
reabsorption. Fluorescence quantum yields of DMAC-B in
various solvents were calculated using coumarin 30 in
acetonitrile as the reference (fluorescence quantum yield, ϕF
= 0.67). The fluorescence quantum yields of DMAC-A were
calculated relative to that of DMAC-B in acetonitrile.
Uncertainty in the quantum yield measurements of DMAC-B
is within 10% whereas that for DMAC-A may be up to 20%
because of very weak emission.
2.3. Time Resolved Measurements. Fluorescence life-
times were measured either using time correlated single photon
counting (TCSPC) or fluorescence up-conversion techniques.
The TCSPC instrument is from Horiba-Jobin-Yvon, IBH
(U.K.). The samples were excited using a 408 nm diode laser (1
MHz) and fluorescence was collected at right angles to the
excitation source and detected using a microchannel plate
(MCP) detector. The IRF of the TCSPC instrument was found
to be ∼100 ps, which was measured by collecting the scattering
light from a TiO2 suspension in water.
The fluorescence upconversion spectrometer (FOG 100,
CDP, Moscow) has been described in detail elsewhere.26 In
brief, laser pulses of 400 nm wavelength of 1 nJ energy/pulse
and 100 fs duration are generated by frequency doubling the
fundamental beam at 800 nm from a self-mode-locked
Ti:sapphire oscillator (Tsunami, Spectra Physics) pumped by
a 5 W DPSS laser (Millenia, Spectra Physics) using a 1 mm
thick type I BBO crystal. These 400 nm femtosecond laser
pulses were used to excite the sample in a 1 mm thick rotating
quartz cell. The fluorescence photons emitted by the
photoexcited sample were upconverted in a nonlinear BBO
(β-barium borate) crystal of 0.5 mm thickness by mixing with
the gate pulse consist of a portion of the fundamental 800 nm
beam. Polarization of the gate pulse was fixed at the magic angle
with respect to that of the excitation pulse to eliminate
contribution of the reorientational motion of the solute to the
population dynamics of the excited state. The upconverted light
is dispersed in a double grating monochromator and detected
by a photo multiplier tube based photon counter. The
instrument response function of the present experimental set
up was determined by cross-correlation function obtained using
the Raman scattering from ethanol which has a fwhm of 300 fs.
The femtosecond fluorescence decays are fitted using a
Gaussian function of the same fwhm using as the instrument
response function.
carbonyl group act as donor and acceptor, respectively, but the
two isomers differ in the relative positions of the donor and the
acceptor groups. In the case of the DMAC-B molecule, this
distance is larger by one ethylene unit as compared to that in
the case of DMAC-A. This small difference in the structures of
the molecules leads to different photophysical behaviors
because of the difference in the strength of coupling between
the donor and acceptor groups. Photophysical properties of
DMAC-B were reported earlier in connection with the lipid
sensor activity due to prominent solvent polarity dependence of
the fluorescence quantum yield.22−24 However, DMAC-A has
not been studied so far. Herein, we show that there is
remarkable contrast in the photophysical properties and the
excited state dynamics of these two isomeric dimethylamino-
chalcones. Comparison of the steady state and time-resolved
spectroscopic properties allows us to unravel the intricate role
of the relative positioning of the donor and acceptor groups on
intramolecular charge transfer and structural relaxation
dynamics. Optimization of the excited state structures using
time dependent density functional theory (TDDFT) elucidates
the geometrical consequences of the ICT process occurring in
the first excited singlet states of these two isomers supporting
the experimental observations.
2. EXPERIMENTAL SECTION
2.1. Synthesis. DMAC-A and DMAC-B molecules were
synthesized using crossed-aldol condensation reaction following
a reported standard procedure.25 DMAC-A was prepared from
the reaction between 4-dimethylaminoacetophenone and
benzaldehyde using sodium hydroxide as base in aqueous
ethanol. For synthesis of DMAC-B, a stronger base, sodium
hydride was used and the synthesis was performed in dry
tetrahydrofuran (THF). The compounds were purified by
column chromatography followed by crystallization from
ethanol. The purified samples were characterized by proton
NMR and mass spectra analysis, the results of which are given
in the Supporting Information (SI) section (Figures S1a−f).
1
DMAC-A: Yield: 80%, yellow needles, mp 170−172 °C. H
NMR(CDCl3) δ (ppm): 3.06 (s, 6H), 6.73 (d, 2H), 7.39 (m,
3H), 7.58 (d, 1H), 7.64 (m, 2H), 7.78(d, 1H), 8.02 (d, 2H)13C
NMR (CDCl3) δ (ppm): 40.3, 111.2, 122.3, 126.4, 128.3,
128.9, 130.1, 130.9, 135.6, 142.7, 153.9, 187.9.
B
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Relaxation dynamics of the excited states in various solvents
have been measured using subpicosecond time-resolved
transient absorption techniques. The pump−probe spectrom-
eter based on a amplified laser system (Thales Optronique SA,
Elancourt, France) which provides the pulses of about 40 fs and
1 mJ energy per pulse at a repetition rate of 1 kHz. Pump
pulses of 400 nm were generated for excitation of the samples
by frequency doubling of one part (100 μJ/pulse) of the 800
nm output of the amplifier in a 0.5 mm thick BBO crystal and
another small part (∼1 μJ/pulse) of the amplifier output was
used to generate the white light continuum (480−1000 nm)
probe in a 2 mm thick sapphire plate. The direction of
polarization of the pump beam was fixed at the magic angle
with respect to that of the probe beam. The energy of the pump
beam was maintained at <5 μJ/pulse. The sample solutions
were kept flowing through a quartz cell of 2 mm path length.
To record the temporal profiles, wavelength regions of 10 nm
width were selected using pairs of interference filters. To
construct the transient absorption spectra, we recorded the
temporal profiles at different wavelengths with 10 nm interval.
Following a correction of the zero time arising due to group
velocity dispersion at different wavelengths, absorbance values
at different delay times were plotted against the wavelength.
The overall time resolution of the absorption spectrometer was
determined to be about 120 fs. The temporal profiles recorded
using different probe wavelengths were fitted with up to three
exponentially decaying or growing components by iterative
deconvolution method using a sech2 type instrument response
function with fwhm of 120 fs.
Figure 1. Normalized steady state absorption (A) and fluorescence
(B) spectra of DMAC-A in cyclohexane (CH), ethyl acetate (EA),
acetone (Actn), acetonitrile (Acn), and dimethyl sulfoxide (DMSO).
Inset of B shows the Lippert-Mataga plot.
2.4. Computational Method. Ground-state geometry was
optimized by using density functional theory (DFT) with
B3LYP hybrid functional and by using 6-311G (d, p) basis
set.27,28 Geometry optimization of the lowest excited singlet
state (S1) was carried out using same functional and basis set in
TDDFT formalism. Vibrational frequencies were then calcu-
lated on optimized structure to verify the minimum energy
geometry. All calculations were carried out using the GAMESS
software package.29 Solvent effect was incorporated using the
PCM solvation model as implemented in GAMESS.30,31
Figure 2. Normalized steady state absorption (A) and fluorescence
(B) spectra of DMAC-B in cyclohexane (CH), ethyl acetate (EA),
acetone (Actn), acetonitrile (Acn), and dimethyl sulfoxide (DMSO).
Inset of B shows the Lippert-Mataga plot.
3. RESULTS AND DISCUSSION
3.1. Steady State Absorption and Fluorescence
Studies. Steady state absorption and fluorescence spectra of
DMAC-A and DMAC-B molecules have been recorded in
solvents of varying polarities and are shown in Figures 1 and 2,
respectively. The absorption spectrum of DMAC-A shows a
strong absorption band in the 350−480 nm region, which
undergoes significant bathochromic shift in polar solvents
indicating strong ICT character of the Franck−Condon (FC)
excited state. DMAC-B also shows a similar solvent dependent
shift of its absorption spectrum revealing the ICT character of
the excited state. It is important to point out that in each of
these solvents the absorption spectrum of DMAC-B is more
red-shifted as compared to that of DMAC-A. This suggests that
extended donor−acceptor conjugation in DMAC-B results in
larger mesomeric stabilization of the ICT state.
Steady state fluorescence spectra of both the compounds
undergo large bathochromic shifts (as large as 125 nm) on
changing the solvent from nonpolar cyclohexane to polar
DMSO. Systematic increase in fluorescence Stokes shift with
solvent polarity suggests strong ICT character of the emissive
excited states of both the compounds. Indeed, large increase in
dipole moment following photoexcitation of either of these two
molecules (Δμ = 11 and 11.8 D for DMAC-A and DMAC-B,
respectively) has been calculated from the Lippert-Mataga plots
(insets of Figures 1B and 2B).
Although the emissive states of both the isomers have strong
ICT character, quantitative measurements of fluorescence
yields show astonishing results. DMAC-A is weakly fluorescent
in all solvents and the quantum yield decreases with increase in
polarity of the aprotic solvents (Table 1). In solvents of large
polarity, fluorescence yield is decreased significantly (Φf <
10−3). In contrast, the fluorescence yield of DMAC-B increases
with an increase in polarity of the aprotic solvents and the
emission yield is as large as 0.2 in acetone. Thus, in polar
aprotic solvents, emission quantum yield of DMAC-A is more
than 2 orders of magnitude less than that of DMAC-B, in spite
of similar ICT character of the excited states of both the
molecules (Δμ = μe−μg ∼ 11 D). Weaker fluorescence of
DMAC-A suggests involvement of an additional nonradiative
relaxation channel in polar solvents. Another intriguing
observation is the difference in the widths of the emission
bands of these two compounds (Table 1).
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Table 1. Photophysical Parameters of the ICT States of DMAC-A and DMAC-B in Various Solvents
DMAC-A
DMAC-B
fl. band fwhm
Φf
τf
(ps)
kr (108
knr (1011
s−1
fl. band fwhm
Φf
τ
f b
kr (108
s−1
knr (109
a
b
solvents (π*)
(cm−1
)
(10−2
)
s−1
)
)
(cm−1
)
(10−2
)
(ps)
)
s−1
)
cyclohexane (0.00)
ethyl acetate (0.45)
acetone (0.62)
3480
5789
5739
5780
5644
1.2
8.0
3.1
2.4
1.4
4.1
9
5
5
1.25
3.2
4.1
7.1
2.4
3400
2580
2560
2554
2544
0.22
7.7
5.2
4.2
1.2
1.4
2.6
1.6
192
0.16
600
1400
380
880
1.54
0.56
2.34
0.97
0.097
0.046
0.276
19.6
10
acetonitile (0.66)
3.5
6.5
c
DMSO (1.00)
13.9
a
b
π* values are taken from ref 32. Fluorescence lifetimes measured by using fluorescence upconversion technique (for DMAC-A) or TCSPC
c
technique (DMAC-B). Dimethyl sulfoxide (Typical error in the measured parameters is about 10%).
Fluorescence spectra of both these molecules in nonpolar
solvent, cyclohexane, are characterized by well resolved vibronic
bands, suggesting a near planar structure of the excited state.
Possibly in the excited state, distribution of conformers formed
by rotations about the single bonds becomes much narrower as
compared to that in the ground state. In polar solvents,
fluorescence spectra of DMAC-A are much broader (fwhm
>5000 cm−1) as compared to those of DMAC-B (fwhm ∼2550
cm−1). Larger width of the fluorescence spectra of DMAC-A
suggests relaxation of the excited state on a shallow potential
energy surface along the reaction coordinate, representing
twisting about one or more of the single bonds (or
conformational relaxation) (vide infra). The excited states of
differently twisted conformers populated during conformational
relaxation contribute to the total fluorescence. Extremely weak
fluorescence yield in the case of DMAC-A also supports the
prediction about occurrence of ultrafast excited state conforma-
tional relaxation, which is largely absent in the excited state of
DMAC-B.
3.2. Time Resolved Fluorescence Measurements.
Fluorescence lifetime of DMAC-A has been observed to be
much shorter than the time resolution of our TCSPC set up
(100 ps). Therefore, the fluorescence lifetimes of DMAC-A in
various solvents have been measured by using fluorescence
upconversion technique. Temporal fluorescence profiles along
with the best-fit functions are shown in Figure 3A. Fluorescence
lifetimes of DMAC-B in polar solvents could be determined
using the TCSPC technique (Figure 3B). Fluorescence
lifetimes of these molecules are given in Table 1. We find
that in polar solvents the emissive excited state(s) of DMAC-A
is much shorter lived than that of DMAC-B. This is in
agreement with the results of fluorescence quantum yield
measurements for these two isomers.
Using experimentally measured fluorescence lifetimes and
quantum yields, the radiative (kr) and nonradiative (knr) rate
constants in various solvents have been calculated and the
values of these parameters are given in Table 1. Values of kr for
these two molecules are similar (∼108 s−1) and also are nearly
solvent independent. Whereas the values of knr for DMAC-A in
polar solvents are larger by more than 2 orders of magnitude
than those in the case of DMAC-B. Large nonradiative decay
rates (knr > 1011 s−1), low fluorescence yields (Φfl < 10−3), as
well as ultrashort fluorescence lifetimes (τfl < 4 ps) of DMAC-A
in polar solvents are indicative of the occurrence of ultrafast
nonradiative deactivation processes in the excited state(s). To
decipher the nature of the excited state deactivation
mechanisms, femtosecond transient absorption (TA) spectro-
scopic measurements have been performed and the results of
these measurements are described in the next section.
Figure 3. Temporal fluorescence profiles of DMAC-A measured by
fluorescence upconversion (A) and of DMAC-B measured by TCSPC
technique (B). In each case, the emission wavelength was set at 550
nm.
3.3. Transient Absorption Studies of DMAC-A. Figure
4A shows the time evolution of the transient absorption (TA)
spectra recorded following photoexcitation of DMAC-A in
acetonitrile by using 400 nm laser pulses of 50 fs duration. We
observe the appearance of an excited state absorption (ESA)
band in the 480−950 nm region at 0.15 ps delay time. With
increase in delay time up to 0.5 ps, this ESA band in the 550−
950 nm region decays leading to the development of a
stimulated emission (SE) band with the maximum at 600 nm.
During this time domain, however, we observe the rise of ESA
in the <550 nm region, thus generating an isosbestic point at ca.
550 nm. On further increase in delay time up to 5 ps, the ESA
band in the <550 nm region decays but absorbance in the 550−
950 nm region increases leading to the decay of the SE band
(Figure 4B). The broad ESA band in the entire spectral region
decays within about 25 ps delay time.
Time evolution of the TA spectra suggests population of
several excited states during the course of the relaxation process
following photoexcitation of DMAC-A. Careful analyses of the
temporal absorption profiles recorded at a few selective
wavelengths support this prediction and also help us to identify
different kinds of excited states or processes associated with the
excited state relaxation mechanism. In Figure 4B, we have
shown the temporal profiles recorded at 480, 610, and 900 nm
along with the best fit functions. Each of these profiles could be
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Figure 4. Time evolution of the TA spectra (A) and temporal TA profiles monitored at a few selective probe wavelengths (B) following
photoexcitation of DMAC-A in acetonitrile using 400 nm laser pulses of 50 fs duration. Lifetimes (in ps) associated with the components of the
mutiexponential fitting functions are given in the insets.
Table 2. Lifetimes of Three Processes Associated with the Excited State Relaxation of DMAC-A in Solvents of Varying Polarities
(ε0 and π*) and Viscosities (η)
a
a
b
c
solvent
ε0 π*
η(cp)
⟨τ⟩sol (ps)
τ1 (ps)
τ2 (ps)
τ3 (ps)
ethyl acetate
acetone
6.0, 0.45
0.4
2.5
1.4 0.2
0.7 0.2
0.3 0.1
0.9 0.2
1.9 0.2
2.5 0.3
3.1 0.2
2.4 0.2
1.35 0.1
3.2 0.2
4.2 0.2
5.5 0.2
22
9
2
1
1
1
1
1
20.0, 0.62
35.5, 0.66
36.5, 0.88
46.5, 1.00
0.35
0.44
0.82
1.99
2.66
0.58
acetonitrile
0.26−0.5
0.91−2.0
2.0
6
d
DMF
11
12
14
DMSO
e
PC
64.5, 0.83
3.0
a
b
c
d
e
Reference 32. Reference 33. Reference 34. Dimethylformamide. Propylene carbonate.
fitted using a three exponential function consisting of the rise
and decay components and the lifetimes of these components
are given in the insets of the Figure 4B. It is important to note
that each of them is associated with the time evolution of same
three processes with the lifetimes of 0.3 0.1, 1.32 0.3, and 6
ps. Association of the ultrafast component with the lifetime of
0.3 ps with the rise of intensity of the SE band in 0.5 ps time
domain leads us to assign this component to the vibrational
relaxation, which represents the lifetime of the relaxation of the
FC state to the local excited (LE) state, which is an ICT or
S1(ICT) state, as the most probable process. However,
acetonitrile being a fast dielectric relaxing solvent, contribution
of the solvation process for the ICT state in the same time
domain cannot be excluded (vide infra).
The second component with the lifetime of 1.32 ps is
associated with the decay of the SE band and hence the decay
of the S1(ICT) state. This value agrees well with the
fluorescence lifetime determined using the upconversion
technique (Table 1). Results of our steady state fluorescence
spectroscopy as well as transient absorption and fluorescence
spectroscopic techniques lead us to conclude that the S1(ICT)
state, which is weakly emissive, undergoes ultrafast nonradiative
relaxation to a nonemissive excited state. Before we confirm this
prediction from our solvent dependence study, we tentatively
assign the process associated with this lifetime to the twisting of
the DMA group (not the only N,N-dimethylamino moiety, vide
infra). Such ultrafast relaxation processes occurring via twisting
of the DMA group have already been established by our group
in dimethylaminobenzophenone derivatives, auramine, etc.15,16
In all those cases, twisting of the DMA group has been
established to be a barrierless process. The twisted ICT state,
i.e., S1(TICT) state, decays back to the ground electronic state
and has a lifetime of about 6 ps in acetonitrile.
In order to establish the assignments with conformity, we
determined the lifetimes of those three processes in solvents of
different polarities and viscosities. Temporal dynamics of
DMAC-A recorded in several solvents are shown in the SI
section (Figures S3−S4) and the lifetimes of three excited state
relaxation processes thus estimated along with the solvent
parameters are given in Table 2. Referring to the lifetime of the
ultrafast process, τ1, its value nicely correlates with the average
solvation times of the solvents. Therefore, this component
should be assigned to the solvation of the S1(ICT) state rather
than the vibrational relaxation process. The later process
correlates neither with the polarities nor the viscosities of the
solvents and the lifetime in DMSO (about 2 ps) is too long to
assign it to the vibrational relaxation process. However, rise of
the SE band also suggests that both the vibrational relaxation
and solvation processes may be taking place concurrently.
One of the important characteristics of the TICT process is
the strong viscosity dependence of the rate of this process due
to its association with the large amplitude twisting motion
about a single bond. Decrease in the relaxation rate with
increase in the viscosity of the solvent may be considered as a
direct evidence in favor of its assignment to the twisting process
and has been observed in the case of similar molecular
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Figure 5. Time evolution of the TA spectra recorded following photoexcitation of DMAC-B in acetonitrile (A) and temporal decay behaviors at a
few selective probe wavelengths (B). Lifetimes determined from mutiexponential fittings of the temporal profiles are given in insets.
systems.11−16 The viscosity dependence of the rate of an
activated barrier crossing process, like the conformational
relaxation via twisting of the DMA group about a single bond in
a TICT process, can be fitted fairly well to a power-law function
(Kramer−Smoluchowski relationship, eq 1)35−40
observed by Forster and Hoffman.39 Several possible
̈
explanations for the origin of the fractional viscosity depend-
ence of the rates have been given, such as breakdown of
Stokes−Einstein relationship,41 multidimensionality of the
potential energy surface,42 time-dependent friction,7,43,44
existence of specific solute−solvent interactions, and so on.
Therefore, the results of the subps time-resolved transient
absorption studies on DMAC-A suggest that the excited state
relaxation dynamics is mainly dominated by the large amplitude
twisting of the dimethylaniline group.
⎛
⎞
⎟
⎠
Ea
RT
k2 = Zη−αexp −
(0 < α < 1)
⎜
⎝
(1)
⎛
⎞
⎟
⎠
Ea
RT
⎜
ln(k2) = −α ln(η) + ln(Z) −
⎝
3.4. Transient Absorption Studies for DMAC-B. Larger
fluorescence quantum yields and longer fluorescence lifetimes
of DMAC-B in polar solvents as compared to those of DMAC-
A (vide supra) possibly suggest absence of the ultrafast
nonradiative deactivation channel via twisting of the DMA
group in the excited state deactivation of the former molecule.
To substantiate this conclusion, we investigated the early time
dynamics of the excited states of DMAC-B by using TA
spectroscopy in several aprotic solvents. Figure 5A shows the
time evolution of the TA spectra following photoexcitation of
DMAC-B in acetonitrile by using 400 nm laser pulses of 50 fs
duration. TA spectra recorded at 0.15 ps delay time are
characterized by a weak SE band in the 480−580 nm region
and an ESA band in the 600−750 nm region. With increase in
delay time, the intensity of the SE band rises up to about 2 ps
along with a significant dynamic red shift of the emission
maximum. During this period, an ESA band appears in the
wavelength region below 500 nm. At 2 ps delay time, the TA
spectrum is characterized by a strong SE band with the
maximum at 550 nm, which corresponds well to the steady
state emission maximum. Thus, like in the case of DMAC-A,
the time evolution of the transient spectra in sub-2 ps time
domain may be assigned to the vibrational relaxation as well as
solvation processes occurring concurrently. On further increase
in delay time up to about 30 ps, time evolution of the transient
spectra is associated with a small rise of the SE band in the
500−600 nm region. During this time period, rise of the ESA
band in the 690−750 nm region has also been observed. Due to
much smaller amplitude as that of the SE band, these spectral
evolutions are not very evident in the TA spectra shown in
(2)
where Z is a pre-exponential factor and the height of the energy
barrier, Ea, for the twisting process, may be considered a
constant, if the rates have been measured at a particular
temperature and in a set of similar kinds of solvents, in which
the energy barrier is not altered significantly. In this condition,
−1
eq 2 is likely to represent a linear relation between ln(k2 = τ2
)
and ln(η), with a slope of α. Figure S5 in the SI section presents
the Kramer−Smoluchowski plot for all the aprotic solvents,
which have been used in the present work. Considering the
error in determining the lifetimes of the transient states, we find
that a linear relationship between ln(k2) and ln(η) is obeyed
only in the polar solvents, but in less polar solvents, the rates
are much slower. Since population of the S1(TICT) state is
associated with further charge transfer leading to near
quantitative charge separation at the twisted geometry, solvent
polarity too is likely to influence the energy barrier for the
twisting process, and hence the relaxation time, significantly.
Therefore, the slower rates of the twisting process in less polar
solvents suggests a higher energy barrier in these solvents and
twisting is the barrier controlled. But in polar solvents, twisting
takes place on a nearly barrierless PES.
The value of α as obtained from the slope of the linear fit in
Figure S5 is 0.65. The fractional viscosity dependence of
barrierless isomerization reactions have been studied using the
mode coupling theory, which finds the value of the slope in the
range of 0.5−0.8.38 Therefore, our value (i.e., 0.65), determined
for the viscosity dependence of the twisting rate in polar
solvents is in close agreement with the theoretical prediction
and reveals η0.65 dependence of viscosity, as predicted and
F
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Table 3. Average Time Constants of the Relaxation Dynamics of DMAC-B in a Few Aprotic Solvents of Varying Polarities and
Viscosities
a
a
b
c
solvent
ε0
,
π*
η(cp)
<τ>sol (ps)
τ1(ps)
τ2 (ps)
τ3(ps)
ethyl acetate
acetone
6.0
0.45
0.62
0.66
0.88
1.00
0.4
2.5
2.0 0.2
0.8 0.2
0.6 0.1
1.4 0.3
1.9 0.3
12
10
12
13
11
1
1
1
1
1
600 20
1400 50
350 10
900 30
880 30
20.0
35.5
36.5
46.5
0.35
0.44
0.82
1.99
0.58
acetonitrile
DMF
0.26−0.5
0.91−2.0
2.0
DMSO
a
b
c
Reference 32. Reference 33. Reference 34
Figure 5A, but can easily be noted from the temporal profiles
shown in Figure 5B.
3.5. Exploring the Possibility of Other Relaxation
Processes. It is worth mentioning here that we have described
the excited state dynamics of two isomeric N,N-dimethylami-
nochalcone derivatives only in polar aprotic solvents, in which
specific differences in the behaviors of the excited states of
these molecules have been observed. In nonpolar solvent,
namely cyclohexane, the S1 state of DMAC-B has been
reported to undergo efficient triplet formation within 20 ps
time scale (τISC = 5 ps).45 In the case of DMAC-A too, we
observed efficient triplet formation in cyclohexane (Figures S8
and S9 in the SI section). However, the yields of the triplet
states of both these molecules are significantly small in polar
solvents. This is in agreement with the earlier observations
made from both the experimental as well as theoretical studies,
that in polar solvents, quantum yield of formation of the triplet
state via ISC process reduces significantly because of
stabilization of the S1(ICT) state with respect to the T1
state.45,46
On a longer time scale, the ESA and SE bands appearing in
the entire 480−750 nm region decay indicating deactivation of
the S1 state leading to near complete recovery of the ground
state. Temporal analyses of the kinetic profiles presented in
Figure 5B reveal three exponential dynamics. An ultrafast
component with a time constant of 0.4−0.7 ps (wavelength
dependent) appears either as decay (in the blue region) or
rising component (in the red region) of SE. A similar time
constant has been observed to be associated with the temporal
profiles recorded in the 690−730 nm region as one of the decay
components of the ESA signal. Time evolution of the SE band
reveals a dynamic red shift and the time constant of the process
essentially agrees well with the average solvation time of
acetonitrile (<τ>sol = 0.5 ps34) and hence these features suggest
its assignment to solvation dynamics.
Following solvation, an intermediate relaxation process with
the lifetime of 12 2 ps has been seen to be associated with the
temporal profiles recorded in the entire spectral region prior to
the decay of ESA or SE band with the lifetime of about 350 ps.
It can be noted that this long decay component (ca. 350 ps)
agrees well with the fluorescence lifetime of DMAC-B in
acetonitrile measured by using TCSPC technique, and thus
represents the lifetime of the emissive singlet state, which
decays populating the ground state. The intermediate
component with the lifetime of about 13 ps is possibly
associated with some kind of minor intramolecular structural
reorganization in the S1(ICT) state of the molecule to populate
structurally relaxed emissive S1(ICT′) state, but maintaining the
near planar conformation. Presence of this component has also
been evident in the transient fluorescence decay of DMAC-B
recorded in acetonitrile solvent using fluorescence upconver-
sion technique (Figure S6 in the SI section).
We have also studied the temporal dynamics of DMAC-B in
a few other aprotic solvents and the results are given in Table 3
and Figure S7 in the SI section. Transient decay behavior is
similar in all solvents. The first component (τ1) corresponds
well to solvation time of the respective solvent.34 The
intermediate component (τ2) is associated with the population
of fully relaxed S1(ICT′) state, which is long-lived (>300 ps).
Absence of viscosity dependence of the intermediate (τ2)
component clearly suggests that large amplitude twisting
motion is not associated with the relaxation dynamics of the
S1(ICT) state of the DMAC-B molecule, rather some wagging
motion including bond length and bond angle reorganization
may be associated with this process to populate the relaxed
S1(ICT′) state. Long fluorescence lifetime and large emission
quantum yield also support the conclusion that large amplitude
twisting motion to populate a nonemissive S1(TICT) state is
not involved in the photodynamics of the DMAC-B molecule.
We have also not ignored the other possible relaxation
mechanism, namely the cis−trans isomerization about the
ethylenic double bond. We observed no change in the
absorption spectra and 1H NMR spectra of both these
molecules even after prolonged photoirradiation in acetonitrile
to rule out the possibility of occurrence of cis−trans
photoisomerization reaction. Further, our TDDFT calculations
reveal that cis−trans isomerization coordinate is associated with
a large barrier (vide infra).
3.6. Quantum Chemical Calculations. Results of our
steady state and time-resolved absorption and fluorescence
studies suggest that the excited state deactivation of DMAC-A
involves a large amplitude conformational relaxation channel
but not that in the case of DMAC-B, in spite of their structural
similarities as well as similarities in the charge transfer
properties in the S1 state. To corroborate those experimental
findings, we performed DFT/TDDFT calculations to deter-
mine the structures of the molecules in the ground (S0) state as
well as in the S1 state. Optimization of the geometries of those
two isomers in the S0 and S1 states have been performed by
using DFT/TDDFT theoretical calculations, and the results are
shown in Figure S10 (in the SI section) and the bond
parameters are given in Table S1 in the SI section. DFT
optimized structures show that both the molecules are nearly
planar in the S0 state. Planar structure and extended π-
conjugation between the donor and the acceptor groups ensure
efficient charge transfer upon photoexcitation. HOMO−
LUMO compositions clearly indicate that in the S0 state
electron density is localized on the DMA group, whereas in the
S1 state, electron density is localized on the carbonyl group
(Figure S11 in the SI section). This delineates the aspect of
strong ICT character of the S1 state as inferred from the
experimental results (vide supra). The S1 ← S0 transition
energies are calculated to be 3.1 and 2.94 eV for DMAC-A and
G
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The Journal of Physical Chemistry A
Article
DMAC-B, respectively, which are in close agreement with those
values determined experimentally. Further, the calculated values
of the dipole moments of the S0 and S1 states of DMAC-A are
6.8 and 20.1 D, respectively, which support strong intra-
molecular charge transfer (Δμ = 13.3 D) following photo-
excitation. Similar results have been obtained for DMAC-B, for
which the calculated values of the diploe moments of the S0 and
S1 states are 8.4 and 21.9 D, respectively (Δμ = 13.5 D; Table
S2 in the SI section).
Starting with the optimized planar structure (i.e., FC
geometry) of the S0 state as the initial guess, geometry
optimization for the S1 state of the DMAC-A by using the
TDDFT method leads to a structure of the lowest energy, in
which the dihedral angle of the DMA group is twisted by 90°
with respect to the rest of the molecular plane (Figure S12 in
the SI section). We also constructed the potential energy
surface (PES) along the reaction coordinate involving the
twisting motion of the DMA group in acetonitrile solvent. We
find a barrierless twisting of the DMA group leading to a
potential energy minimum at the twist angle of 90° (Figure 6).
thermal energy at 298 K (0.025 eV), existing at the twist angle
of 40°. Because of this energy barrier, although the potential
energy of the fully twisted structure (i.e., with the twist angle of
90°) is very similar (2.87 eV) to that of the S1(TICT) state of
DMAC-A, the twisting process could not experimentally be
observed in the S1 state of DMAC-B. The possible reason for
the absence of this channel in DMAC-B is that π-conjugation
between the donor and the acceptor is extended by an
additional ethylene bond (as compared to DMAC-A), which
results in a weaker coupling and hence relatively larger energy
barrier along the reaction coordinate involving the twisting
motion of the DMA. We have also calculated the potential
energy change upon twisting of two other possible torsional
channels, namely twisting of dimethylamine group and twisting
of the olefinic bond (Figure S12 in the SI section). Twisting
along dimethylamine group results in an increase in PE up to
about 50°, and the barrier of this twisting is calculated to be
0.24 and 0.22 eV for DMAC-A and DMAC-B, respectively. On
the other hand, the double bond twisting appears to have even
larger energy barrier (∼0.7 eV) for both the molecules. Thus,
twisting relaxation around the dimethylamine group or the
olefin bond is less probable for both the molecules.
4. SUMMARY AND CONCLUSIONS
Both the isomers of dimethylaminochalcones studied here have
strong ICT character in the excited states owing to donor−
acceptor substitutions. In polar solvents, fluorescence behavior
of the two compounds is remarkably different. Weak emission
and short fluorescence lifetime of DMAC-A in polar solvents
suggest efficient and very fast nonradiative relaxation of the
emissive ICT state. Subpicosecond transient absorption studies
confirmed ultrafast structural relaxation to a nonemissive TICT
state in a few picosecond time scale. Excited state structure
optimization further substantiates that the structure with the
twisted DMA group has the minimum potential energy. On the
other hand, stronger fluorescence (Φf = 0.1−0.2) and longer
lifetime (350−1400 ps) of the S1 state of DMAC-B in polar
solvents are assigned to the planar ICT state and absence of the
dominant nonradiative structural relaxation channel. Large
energy gap between the S1 and S0 states is responsible for
longer lifetime of the S1 state of DMAC-B. The TA experiment
reveals that, in addition to solvation, some intramolecular
relaxation process with about 10 ps lifetime is involved in the
S1(ICT) state of DMAC-B. This process is not influenced by
the solvent viscosities, suggesting an absence of large amplitude
structural relaxation. This intermediate relaxation could not
unequivocally be characterized in the present study but
tentatively assigned to low amplitude changes in bond angles
and dihedral angles required to populate the electronically
relaxed S1(ICT′) state.
Figure 6. Theoretical potential energy surface (PES) diagrams in
acetonitrile calculated using DFT/TDDFT methods, representing the
changes in potential energies (relative to that of the ground state
minimum) along the twisting angles of the DMA group.
This delineates our prediction regarding conformational
relaxation of the S1(ICT) state of DMAC-A via twisting of
the DMA group to populate the twisted structure of lower
energy. The twisted conformation is characterized by negligibly
small oscillator strength (f = 0.001) for spontaneous emission
to the S0 state and thus suggests its nonemissive character. This
is in conformity with our experimental results, which describe
the ultrafast deactivation of an emissive state, S1(ICT), to an
intermediate nonemissive state, S1(TICT), prior to deactivation
of the S1 state to the ground state. Thus, both the experimental
results and TDDFT calculations clearly establish that large
amplitude relaxation motion via twisting of the DMA group is
an efficient channel for the nonradiative deactivation of the S1
state of the DMAC-A molecule.
On the other hand, the TDDFT optimized geometry of the
S1(ICT) state of DMAC-B is similar to that of the S0 state with
the only differences in the bond lengths and bond angles,
excluding the possibility of any large amplitude conformational
change in the excited state (Table S1 in the SI section). We
constructed the PES of twisting of the DMA group in the
excited state of DMAC-B in acetonitrile (Figure 6), and we find
a energy barrier of about 0.06 eV, which is much larger than the
The excited state properties of the two molecules are
remarkably different, in spite of the fact that both of them
possess strong ICT character. This leads to the conclusion that
large charge transfer upon photoexcitation is not the sole
criteria for TICT relaxation, rather the structure of the
molecule determines the shape of the excited state potential
energy surface and thus the excited state reaction dynamics. In
the case of DMAC-A, the potential energy surface is quasi-
barrierless (in polar solvents) along the DMA twisting
coordinate, which allows the excited state population to relax
to a TICT state on the ultrafast time scale, whereas for DMAC-
B, the TICT state is separated from the planar ICT geometry
by a significantly large energy barrier and thus relaxation to the
H
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twisted state is kinetically unfavorable. We propose that the
relative positioning of the donor and acceptor groups plays an
important role on the energetics of the ICT - TICT relaxation
process. In comparison to DMAC-A, the DMA group of
DMAC-B is in extended conjugation with the carbonyl group
by an additional olefin bond which provides significant
mesomeric stabilization to the planar ICT state of DMAC-B.
This is also evident from the absorption spectra, where DMAC-
B absorption is significantly lower in energy as compared to
that of DMAC-A in all solvents. Increase of an energy barrier
along the torsional coordinate of the excited state with
extended π-conjugation has also been reported for polymethine
cyanine dyes.47 Following the results of the above inves-
tigations, the excited state potential energy surfaces have been
schematically presented in Figure 7.
parameters, and potential energy surfaces along the
reaction coordinates of twisting about the single bonds of
both these molecules (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dkpalit@barc.gov.in.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Dr. Raghunath Chowdhury, Bio-Organic
Division, Bhabha Atomic Research Centre, Mumbai, for his
help in synthesis of the compounds and Prof. Anindya Datta,
IIT Bombay, for fluorescence up-conversion measurements for
some of the samples.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpca.5b08203.
■
S
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DOI: 10.1021/acs.jpca.5b08203
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