Femtosecond transient absorption spectroscopic study of a carbonyl-containing carotenoid analogue, 2-(all- trans -retinylidene)-indan-1,3- dione

Article abstract of DOI:10.1021/jp111313f

The photophysical properties of a carbonyl-containing carotenoid analogue in an s-cis configuration, relative to the conjugated π system, 2-(all-trans-retinylidene)-indan-1,3-dione (C20Ind), were investigated by femtosecond time-resolved spectroscopy in various solvents. The lifetime of the optically forbidden S₁ state of C20Ind becomes long as solvent polarity increases. This trend is completely opposite to the situation of S _1-ICT dynamics of carbonyl-containing carotenoids, such as peridinin and fucoxanthin. Excitation energy dependence of the transient absorption measurements shows that the transient absorption spectra in nonpolar solvents were originated from two distinct transient species, while those in polar and protic solvents are due to a single transient species. By referring to the results of MNDO-PSDCI (modified neglect of differential overlap with partial single- and double-configuration interaction) calculations, we conclude: (1) in polar and protic solvents, the S₁ state is generated following excitation up to the S₂ state; (2) in nonpolar solvents, however, both the S₁ and the ¹nπ* states are generated; and (3) C20Ind does not generate the S_1-ICT state, despite the fact that it has two conjugated carbonyl groups.

Full text of DOI:10.1021/jp111313f

ARTICLE
pubs.acs.org/JPCA
Femtosecond Transient Absorption Spectroscopic Study of a
Carbonyl-Containing Carotenoid Analogue, 2-(all-trans-Retinylidene)-
indan-1,3-dione
Toshiyuki Kusumoto,† Daisuke Kosumi,† Chiasa Uragami,† Harry A. Frank,‡ Robert R. Birge,‡
||
Richard J. Cogdell,§ and Hideki Hashimoto*,†,
†CREST/JST and Department of Physics, Graduated School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku,
Osaka 558-8585, Japan
‡Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, Connecticut 06269-3060, United States
§Institute for Molecular, Cell and Systems Biology, Glasgow Biomedical Research Centre, University of Glasgow, 120 University Place,
Glasgow G12 8QQ, Scotland, United Kingdom
The Osaka City University Advanced Research Institute for Natural Science and Technology (OCARINA), 3-3-138 Sugimoto,
Sumiyoshi-ku, Osaka 558-8585, Japan
ABSTRACT: The photophysical properties of a carbonyl-
containing carotenoid analogue in an s-cis configuration, relative
to the conjugated π system, 2-(all-trans-retinylidene)-indan-
1,3-dione (C20Ind), were investigated by femtosecond time-
resolved spectroscopy in various solvents. The lifetime of the
optically forbidden S1 state of C20Ind becomes long as solvent
polarity increases. This trend is completely opposite to the
situation of S1-ICT dynamics of carbonyl-containing carote-
noids, such as peridinin and fucoxanthin. Excitation energy
dependence of the transient absorption measurements shows that the transient absorption spectra in nonpolar solvents were
originated from two distinct transient species, while those in polar and protic solvents are due to a single transient species. By
referring to the results of MNDO-PSDCI (modified neglect of differential overlap with partial single- and double-configuration
interaction) calculations, we conclude: (1) in polar and protic solvents, the S1 state is generated following excitation up to the S2
state; (2) in nonpolar solvents, however, both the S1 and the 1nπ* states are generated; and (3) C20Ind does not generate the S1-ICT
state, despite the fact that it has two conjugated carbonyl groups.
’ INTRODUCTION
on the basis of evolution of the S1 state.4-13,16-18 Recently, it was
reported that the S1 and SICT states of fucoxanthin can be
generated distinctly by selecting two-photon excitation.19 The
situation of peridinin, however, is more complicated, and a
distinction of S1 and S1-ICT is not straightforward.9 In the case
of 30-hydroxyechinenone (heCN), its S1-ICT state was not found
to be generated in solution, but it is formed in the orange
carotenoid protein (OCP) from the cyanobacterium Arthrospira
maxima.20 The carbonyl group of heCN in OCP is locked in an
s-trans configuration to its polyene backbone.20 Furthermore,
canthaxanthin and rhodoxanthin with terminal ring structures
containing carbonyl groups do not produce an S1-ICT state in
solution because of the twisted geometry of the terminal rings
with respect to the conjugated polyene backbone.21 These
phenomena suggest that the conformation of a carbonyl group
to the π-conjugated system in polyene backbone plays an
important role in the generation of the S1-ICT state.
Carotenoids are naturally occurring chromophores containing
a long-chain polyene backbone and play an important role in
photosynthesis acting as accessory light harvesting molecules.1,2
The singlet excited states of carotenoids without carbonyls are
designated by referring to an idealized C2h point symmetry
group.2,3 The lowest optically allowed singlet excited state is
the 11Buþ state (S2), and an optically forbidden 21Ag- state (S1)
exists below S2. The presence of an intramolecular charge
transfer (SICT) state coupled to the S1 state (S1-ICT) have been
reported for carbonyl-containing carotenoids, such as peridinin,
in solution and in pigment-protein complexes.4-9 The S1-ICT
state plays a key role in the photophysical properties of carbonyl-
containing carotenoids as was shown by transient absorption
spectroscopic measurements.10-13 The lifetime of the S1 was
found to become shorter as the length of the π-conjugation
increases10-13 similar to the behavior of carotenoids without
carbonyls.14,15 In contrast, the S1-ICT state lifetimes are invari-
ant with respect to the conjugated chain length.10-13 These
experimental findings are supported by quantum-chemical cal-
culations.16-18 Models of the S1-ICT state have been proposed
Received: November 29, 2010
Revised:
January 27, 2011
Published: March 01, 2011
r
2011 American Chemical Society
2110
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ARTICLE
Figure 1. Chemical structure of 2-(all-trans-retinylidene)-indan-1,3-
dione (C20Ind).
Moreover, there is another excited singlet state that compli-
cates the photophysics of these polyenes. The presence of a low-
lying 1nπ* state is a general feature of polyenes with a carbonyl
group, such as benzophenone, trans-β-hydrindanone, and linear
polyenals.22-24 This state plays important role in intersystem
crossing from the singlet to triplet state as well as the trans to cis
isomerization.22,25 The presence of the 1nπ* state between the S1
and ground state (S0) was reported in retinal having five conju-
gated double bonds and an aldehyde-type carbonyl group.26-29
1
The lifetime of the nπ* state of retinal in protic solvents is
shorter than that in nonpolar solvents.29 However, as yet there is
no clear evidence for the presence of the 1nπ* state in carotenoids
with carbonyl groups except for the case of siphonaxanthin.30
In a previous study, we synthesized a carotenoid analogue with
two carbonyl groups, 2-(all-trans-retinylidene)-indan-1,3-dione
(abbreviated as C20Ind, see Figure 1 for chemical structure).31
The conjugation of C20Ind extended onto both the carbonyls
but not onto the indan ring. It has six carbon-carbon conjugated
double bonds attached onto a branched double bond.31 This
conjugation pattern is very similar to that of peridinin. The
structure of C20Ind has been determined by X-ray crystal-
lography.31 The carbonyl groups of C20Ind were locked in the
s-cis conformation in relation to the conjugated polyene chain.
This suggests that C20Ind represents a simplified model to help
examine the role of the carbonyl group in the generation of the
S1-ICT and/or 1nπ* states.
Figure 2. Steady-state absorption spectra of C20Ind in (a) n-hexane,
(b) toluene, (c) THF, (d) acetone, (e) acetonitrile, and (f) methanol. Pf
is the solvent polarity factor, which was determined from the dielectric
constant ε and the refractive index n of the solvents using the expression,
Pf = (ε - 1)/(ε þ 2) - (n2 - 1)/(n2 þ 2). Circles indicate the
experimental data. Solid lines indicate the results of spectral fitting based
on a Franck-Condon analysis. All the spectra are normalized at the
absorption maximum.
The aim of this present study is to investigate the formation of
1
the S1-ICT and/or nπ* states in a short carotenoid with two
carbonyl groups in an s-cis conformation. We present here a
combined experimental and theoretical study of C20Ind. Our
experimental methods include electronic and Raman spectroscopy
as well as femtosecond time-resolved absorption spectroscopy in a
variety of solvents. Our theoretical method is MNDO-PSDCI
(modified neglect of differential overlap with partial single- and
double-configuration interaction) theory including full single and
double configuration interaction (CI) within the π system.
’ EXPERIMENTAL METHODS
Sample Preparation and Solvents Used in This Study.
C20Ind was synthesized and purified as previously reported.31
Retinyl acetate was purchased from BASF (Switzerland) and
purified by silica gel column chromatography and subsequent
recrystallization from n-hexane. Indan-1,3-dione was purchased
from Tokyo Kasei (Japan) and used as received. The all-trans
retinal was prepared by the hydrolysis of retinyl acetate followed
by oxidation using MnO2 (Aldrich Chemicals, U.S.). C20Ind was
prepared by means of a modified Knoevenagel condensation of
retinal and indan-1,3-dione in methanol.
C20Ind was dissolved in nonpolar, n-hexane (Kishida Chemi-
cal, Japan) and toluene (WAKO Chemical, Japan), in polar, THF
(WAKO Chemical, Japan), acetone (Kishida Chemical, Japan),
and acetonitrile (Kishida Chemical, Japan), and in protic,
methanol (Kishida Chemical, Japan), solvents for the following
studies.
Figure 3. Spontaneous resonance Raman spectra of C20Ind in (a) n-
hexane, (b) toluene, (c) THF, (d) acetone, (e) acetonitrile, and (f)
methanol observed at 532 nm excitation. Raman lines due to solvents are
subtracted. All the spectra are normalized at the highest peak signals.
Steady-State Absorption and Resonance Raman Spectro-
scopic Measurements. The optical density of C20Ind was
adjusted to 1 at the maximum of the steady-state absorption
spectrum. The steady-state absorption spectra were measured
using a conventional spectrophotometer (V-670, Jasco, Japan). A
diode-pumped solid-state CW-laser with output at 532 nm
(SDL-532-SLM-030T, Shanghai Dream Lasers Technology,
China) was used as the excitation light-source to generate
resonance Raman spectra of C20Ind. The Raman scattering
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Table 1. Huang-Rhys Factor, 0-0 Transition Energy, Vibrational Energy of the CdC Stretching Mode, and a Full Width at Half-
Maximum (fwhm) Determined by Franck-Condon Analysis of the Steady-State Absorption Spectra of C20Ind in Various Solvents
n-hexane
toluene
THF
acetone
acetonitrile
methanol
Huang-Rhys factor
0-0 transition energy/eV
vibrational energy/eV
fwhm/eV
1.401
1.368
1.404
1.393
1.377
1.357
2.487
2.385
2.395
2.388
2.381
2.347
0.1928
0.2004
0.1918
0.2155
0.1919
0.2412
0.1908
0.2602
0.1907
0.2688
0.1905
0.2899
was detected by using a liquid nitrogen cooled CCD camera
(LN/CCD-576-E/1, Roper Scientific, Japan) attached to a
spectrometer (U1000, Horiba-Jovin-Ybon, Japan). The back-
scattering optical geometry was used, and all measurements were
performed at room temperature.
Femtosecond Transient Absorption Spectroscopic Mea-
surements. The absorbance of C20Ind was adjusted to 0.5 at the
maximum of steady-state absorption spectra in 1 mm optical
path-length quartz cell. The experimental setup of the femtose-
cond transient absorption measurements was as described in a
previous paper.32 A mode-locked Ti:Sapphire oscillator and a 1 kHz
regenerative amplifier (Hurricane-X, Spectra Physics, U.S.)
provided the excitation and probe pulses. Excitation pulses were
obtained by sum-frequency mixing the output of an optical
parametric amplifier (OPA-800CF, Spectra Physics, U.S.) with
a residual fundamental pulse in a 1.0 mm BBO crystal. The
excitation energy was adjusted to 20 nJ/pulse. A white light
continuum probe pulse, generated using a 5.0 mm sapphire plate,
was detected by a photodiode array (1024 pixels NMOS linear
image sensor S3903-1024G, Hamamatsu, Japan) through a
spectrometer (Acton SP275i, Princeton Instruments, U.S.).
The excitation pulses were modulated at 500 Hz by an optical
chopper (C-995 Optical Chopper, Terahertz Technologies Inc.,
U.S.), and the data output was synchronized with the laser
repetition of 1 kHz using home-built electronic circuitry.32
The instrumental response function of the system determined by
cross-correlation between the excitation and probe pulses was better
than 100 fs. The cross-correlation function was used to determine
the zero time delay at each probe energy. After chirp compensation,
the uncertainty in the zero time delay was less than 20 fs.
Quantum-Chemical Calculation. The ground-state equilibrium
geometry and ground-state properties of C20Ind were calcu-
lated by using Gaussian 03, the B3LYP density functional, and a
6-31G(d) basis set.33 The excited-state electronic properties
were calculated by using MNDO-PSDCI molecular orbital
theory and an AM1 Hamiltonian. Quantum-chemical calcula-
tions using MNDO-PSDCI, SAC-CI, and TDDFT methods have
been applied to a carbonyl containing carotenoid fucoxanthin.18
None of each calculation can fully explain the properties of the SICT
state of fucoxanthin, but some parts of the experimental results were
satisfactorily interpreted using the results of these calculations.
Among these three calculation methods, SAC-CI is the highest
end, but it needs sufficiently high computer resource and huge
amount of calculation time for the practical usage. Nevertheless,
calculations using this method cannot correctly predict the values of
parameter |Δμ| (change of static dipole moment of molecules upon
Figure 4. Solvent polarity dependences of the 0-0 transition energy
(top) and a full width at half-maximum (fwhm) (bottom) of the
absorption bands of C20Ind. The “O” show the experimental results.
Solid lines show the results of the least-squares fittings except for the data
points in methanol ((f) in this figure).
method. Therefore, we have adopted the MNDO-PSDCI method
in this present study. MNDO-PSDCI calculations using an AM1
Hamiltonian were applied to C20Ind whose structure was opti-
mized in the S0 ground state using DFT method. These methods
have been applied successfully to the study of several comparable
systems including carbonyl-containing carotenoids.17,18,34-43 We
carried out full single and double CI calculations within the eight
highest energy filled π orbitals, the eight lowest energy unfilled π
orbitals, and the two highest energy filled n orbitals. The MNDO-
PSDCI programs are available by contacting R. R. Birge
(rbirge@uconn.edu).
’ RESULTS AND DISCUSSION
Steady-State Absorption and Resonance Raman Spectra.
Figure 2 shows steady-state absorption spectra of C20Ind in
several solvents. In all cases, the vibronic development of the
S0 f S2 transition cannot be resolved due to inhomogeneous
broadening. However, it is clearly seen that the energy of the
maximum absorbance is red-shifted when increasing solvent
polarity. This red shift is caused by an electrostatic solvent effect
as has already been observed in other carbonyl-containing
carotenoids.4,5,10-13 Figure 3 shows resonance Raman spectra
of C20Ind in the same set of solvents. On the basis of the
assignment of the Raman bands in previous papers, the Raman
lines observed for C20Ind can be empirically assigned as
below.44-49 The Raman lines in the 959-967 spectral region
are assigned to the C-C-H out-of-plane bending vibration.
photoexcitation) that is important to discuss the property of SICT
.
The TDDFT method is useful to theoretically predict |Δμ| values,
but it does not correctly predict the ordering of the singlet excited
states of fucoxanthin. The MNDO-PSDCI method is handy to use
and can correctly reproduce the ordering of the S1 (21Ag-), S2
(11Bu-), and S3 (11Buþ) states as can be done using the SAC-CI
2112
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The Journal of Physical Chemistry A
ARTICLE
Figure 5. Femtosecond transient absorption spectra of C20Ind following excitation at 2.43 eV in (a) n-hexane, (b) toluene, (c) THF, (d) acetone, (e)
acetonitrile, and (f) methanol. Dotted lines show the steady-state absorption spectra. Dashed, solid, dashed-dotted, dashed double-dotted, and long-
dashed lines show the femtosecond transient absorption spectra recorded at 0.1, 0.5, 2.0, 5.0, and 10 ps after excitation, respectively.
Those around 1007 cm-1 are assigned to methyl in-plane rocking
vibrations. Those around 1270 cm-1 are assigned to the coupling
mode between C-C-H bendings and the carbon-carbon single
and double bond stretching vibrations. Those in the 1200-1150
and 1550-1530 cm-1 spectral regions are assigned to the
carbon-carbon single and double bond stretching vibrations,
respectively. Those in the 1375-1328 cm-1 region and around
1445 cm-1 are assigned to the symmetric and asymmetric methyl
deformation, respectively. Those in the 1730-1715 cm-1 region
are assigned to the carbonyl CdO stretchings. In all solutions, the
Raman lines of the carbon-carbon double bond stretching vibra-
tions (1550-1530 cm-1 spectral region) are the strongest as
compared to the other Raman lines. On the basis of the results of
resonance Raman spectra, the system origin and the full width at
half-maximum (fwhm) of the steady-state absorption spectra were
determined by a Franck-Condon analysis as described below.50
For transitions from the ground state, |S0æ, to the lowest
optically allowed excited state, |S2æ, with energies {E2}, the linear
absorption coefficient, R(ω), defined as the fraction of energy
absorbed in passing through an isotropic material, is
2
^
RðωÞ ꢀ ∑jÆS0jμjS2æj Flm δðω - Ω02 - l ωvÞ
3
3
3
l
Flm = |Æl|mæ|2 is a Franck-Condon factor, the square magnitude of
the instantaneous overlap of the mth nuclear wave function of the
ground state, |mæ, and the lth nuclear wave function of the excited
̂ is the transition dipole operator for the transition from
state, |læ. μ
Figure 6. The normalized transient absorption kinetics of C20Ind in
(a) n-hexane, (b) acetone, (c) THF, (d) acetone, (e) acetonitrile, and (f)
methanol. All kinetic traces were normalized using the amplitude at 3 ps
after excitation. The “O”, “0”, and “4” show the experimental data. Solid
lines show the results of fitting using multiexponential functions. The
insets show the same plots of a short time regime after excitation.
|S0æ to |S2æ. Ω02 = (E2 - E0)/p is the angular transition frequency
of the state |S2æ. ω is the angular frequency of incident light. ων is
the angular molecular vibrational frequency. Assuming that the
steady-state absorption spectra of C20Ind in solution are con-
voluted with Gaussian sub-bands, we have
!
where σ is a full width at half-maximum (fwhm) of a Gaussian
function. At the temperature of the system T = 0, the lowest
vibrational state (m = 0) of the ground state is occupied. We define
the Franck-Condon factor as
2
ðω - Ω02 - l ωvÞ
2
3
^
RðωÞ ꢀ ∑jÆS0jμjS2æj Flm exp -
3
3
2 Γ2
l
3
Sl expð-SÞ
σ
2
3
pffiffiffiffiffiffiffiffiffiffi
Γ ¼
Fl0 ¼ jÆlj0æj ¼
l!
2 2 ln 2
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The Journal of Physical Chemistry A
ARTICLE
Table 2. Solvent Dependence of Decay Time Constants Determined by the Fitting of Transient Absorption Kinetics Following
Excitation at 2.43 eV Using Multi-exponential Functions of C20Inda
time constants
solvent
probe energy
τ1/fs
τ2/ps
τ3/ps
τ4/ns
n-hexane
2.00 eV (620 nm)
2.17 eV (571 nm)
2.27 eV (546 nm)
2.08 eV (596 nm)
2.16 eV (574 nm)
2.30 eV (539 nm)
2.10 eV (590 nm)
2.23 eV (556 nm)
2.27 eV (546 nm)
2.15 eV (577 nm)
2.29 eV (541 nm)
2.39 eV (519 nm)
2.19 eV (566 nm)
2.29 eV (541 nm)
2.37 eV (523 nm)
2.17 eV (571 nm)
2.30 eV (539 nm)
2.37 eV (523 nm)
80 ( 30 (þ)
80 ( 30 (-)
1.3 ( 0.1 (þ)
1.3 ( 0.1 (þ)
1.3 ( 0.1 (þ)
2.6 ( 0.1 (þ)
2.6 ( 0.1 (þ)
2.6 ( 0.1 (þ)
1.6 ( 0.4 (þ)
1.6 ( 0.4 (-)
1.6 ( 0.4 (-)
0.82 ( 0.16 (þ)
0.82 ( 0.16 (-)
0.82 ( 0.16 (þ)
6.4 ( 0.3 (þ)
6.4 ( 0.3 (þ)
6.4 ( 0.3 (þ)
4.1 ( 0.1 (þ)
4.1 ( 0.1 (þ)
4.1 ( 0.1 (þ)
5.6 ( 0.1 (þ)
5.6 ( 0.1 (þ)
5.6 ( 0.1 (þ)
10.7 ( 0.1 (þ)
10.7 ( 0.1 (þ)
10.7 ( 0.1 (þ)
9.7 ( 0.2 (þ)
9.7 ( 0.2 (þ)
9.7 ( 0.2 (þ)
11.2 ( 0.3 (þ)
11.2 ( 0.3 (þ)
11.2 ( 0.3 (þ)
17.5 ( 0.9 (þ)
17.5 ( 0.9 (þ)
17.5 ( 0.9 (þ)
13.8 ( 0.1 (þ)
13.8 ( 0.1 (þ)
13.8 ( 0.1 (þ)
80 ( 30 (-)
100 ( 40 (-)
100 ( 40 (-)
100 ( 40 (-)
210 ( 60 (þ)
210 ( 60 (-)
210 ( 60 (-)
150 ( 30 (þ)
150 ( 30 (-)
150 ( 30 (-)
280 ( 160 (þ)
280 ( 160 (-)
280 ( 160 (-)
200 ( 60 (-)
200 ( 60 (-)
200 ( 60 (-)
toluene
THF
10 (fixed) (-)
10 (fixed) (-)
10 (fixed) (-)
10 (fixed) (-)
10 (fixed) (-)
10 (fixed) (-)
10 (fixed) (-)
acetone
acetonitrile
methanol
10 (fixed) (-)
10 (fixed) (-)
a Plus and minus signs on the right-hand side of decay time constants indicate rise and decay phases, respectively.
where S is a Huang-Rhys factor. Next, we obtain the fitting model
function of the steady-state absorption spectra, A(ω), as
Sl expð-SÞ
2 3
3
^
AðωÞ ꢀ RðωÞ ꢀ ∑jÆS0jμjS2æj
l!
l
!
2
ðω - Ω02 - l ωvÞ
3
exp -
3
2Γ2
On the basis of the observed resonance Raman spectra, the
frequency of carbon-carbon double bond stretching mode is
dominant and hence used as ων. S, Ω02, and Γ are determined
by spectral fitting.
The calculations (solid lines in Figure 2) agree well with the
experimental results. Table 1 summarizes the fitting parameters
determined by the Franck-Condon analysis. The large Huang-
Rhys factor is due to the large displacement of equilibrium
nuclear positions between ground and excited states caused by
strong electron-phonon coupling. Figure 4 shows the plots of
the 0-0 transition energy and fwhm as a solvent polarity factor,
Pf = (ε - 1)/(ε þ 2) - (n2 - 1)/(n2 þ 2). This parameter can
be derived to combine the Lippert and Mataga equation51 and
the effect of elliptical cavity formed by the surrounding solvent
molecules.52 It is appropriately applicable for the case of carote-
noids to explain the solvent-dependent energy shift of optical
absorption.10,52 It is suggested that the system origins of the
steady-state absorption spectra are blue-shifted and that the
spectral line widths are broadened as solvent polarity increases.
This trend is similar to that observed with other carbonyl-
containing carotenoids.4,5,10-13
Figure 7. Excitation energy dependence of the transient absorption
spectra of C20Ind in n-hexane (left-hand side) and acetone (right-hand
side). Parts (a) and (b) show the transient absorption spectra in n-
hexane for 2.25 and 2.58 eV excitaion, respectively. Parts (c) and (d)
show those in acetone for 2.25, 2.58 eV excitaion, respectively. Dotted
lines show the steady-state absorption spectra. Dashed, solid, dashed-
dotted, dashed double-dotted, and long-dashed lines show the femto-
second transient absorption spectra recorded at 0.1, 0.5, 2.0, 5.0, and
10 ps after excitation, respectively.
absorption spectra are observed at early delay times after excita-
tion (0.1, 0.5, and 2 ps). A typical relaxation pathway of
carotenoids in nonpolar solvent has been proposed as S2 f
hot S1 f S1 f S0.53-55 On the basis of this idea, the spectral
change observed here is assignable to the vibrational relaxation in
Solvent Dependence of Femtosecond Transient Absorp-
tion Spectra and Kinetics of C20Ind. Figure 5 shows femtose-
cond transient absorption spectra of C20Ind in several sol-
vents. In all cases, blue-shifting and narrowing of the transient
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Figure 8. Delay time dependence and spectral decomposition of the normalized transient absorption spectra of C20Ind in n-hexane (left-hand side) and
acetone (right-hand side). Parts (a) and (b) show the results of spectral decomposition in n-hexane following excitation at 2.25 and 2.58 eV, respectively.
Parts (c) and (d) show those in acetone following excitation at 2.25 and 2.58 eV, respectively. Dashed, dashed dotted, and dashed double-dotted lines
show the femtosecond transient absorption spectra recorded at 3, 5, and 10 ps after excitation, respectively. Solid lines show the results of spectral
decomposition determined by two or three Gaussian functions together with the bleaching of the steady-state absorption spectra. Dotted lines show the
Gaussian sub-bands determined by the spectral decomposition. Dashed and solid arrows in (b) indicate the lower energy (LE) and the higher energy
(HE) bands, respectively.
Table 3. Relative Intensities, Peak Energy, and Full Width at Half-Maximum (fwhm) of Gaussian Profiles of the Excited-State
Absorption Components and Relative Bleaching Intensity of the Steady-State Absorption Component Determined by the
Convolution of the Transient Absorption Spectra of C20Ind
lower energy
middle energy
highest energy
delay time/
ps
relative
peak energy/ fwhm/
relative
peak energy/ fwhm/
relative
peak energy/ fwhm/
bleaching
intensity
intensity
eV
eV
intensity
eV
eV
intensity
eV
eV
2.25 eV excitation
3
5
1
2.19
2.18
2.19
2.11
2.12
2.12
2.11
2.12
2.11
2.11
2.11
2.11
0.146
0.142
0.139
0.121
0.128
0.130
0.107
0.107
0.107
0.121
0.121
0.121
0
0
0
1
1
1
1
1
1
1
1
1
/
/
/
/
0
/
/
/
/
/
/
/
1
in n-hexane
1
/
0
/
0.77
0.46
1
10
3
1
/
0
/
2.58 eV excitation
0.446
0.612
0.656
0.078
0.083
0.083
0.104
0.107
0.113
2.25
2.26
2.26
2.26
2.26
2.26
2.26
2.26
2.26
0.133
0.125
0.119
0.084
0.083
0.082
0.090
0.087
0.086
0
/
in n-hexane
5
0
/
0.73
0.47
1
10
3
0
/
2.25 eV excitation
in acetone
0.183
0.168
0.152
0.167
0.157
0.172
2.38
2.39
2.39
2.40
2.40
2.40
0.123
0.114
0.114
0.115
0.111
0.121
5
0.86
0.51
1
10
3
2.58 eV excitation
in acetone
5
0.86
0.64
10
the S1 state. In the transient absorption spectra in polar solvents
(THF, acetone, and acetonitrile) at 5 ps after excitation (dashed
double-dotted lines in Figure 5), the peak energy of transient
absorption is blue-shifted with an increase of solvent polarity.
The spectral band shape does not show significant difference at
delay times longer than 2 ps after excitation (Figure 5c-e). A
similar trend can be seen in a protic solvent (Figure 5f). In
carbonyl-containing carotenoids such as peridinin and fucox-
anthin, a transient absorption of the S1-ICT can be seen at the
lower energy side of the S1 f Sn absorption transition band in
polar and protic solvents.4,5,10-13 In Figure 5, however, a new
transient absorption band of C20Ind in polar and protic solvents
is not detected as compared to that in nonpolar solvents. This
phenomenon suggests that the transient absorption of the S1-ICT
state of C20Ind has not been detected. The spectral behavior in
nonpolar solvents, however, is quite different from that in polar
and protic solvents. The spectral bandwidths of the transient
absorption of C20Ind in nonpolar solvents are apparently broad-
er than those in polar solvents. To gain insight into the origin of
the spectral broadening, the excitation energy dependence of the
transient absorption spectra and their kinetics was investigated,
for C20Ind in n-hexane and acetone (see next section).
Figure 6 shows the solvent dependence of the transient
absorption kinetics of C20Ind. In all cases, the kinetics are well
fitted by multiexponential functions. Table 2 summarizes the
results of the fitting. Based on the relaxation pathway of typical
carotenoids, such as β-carotene, lycopene, zeaxanthin, and
spheroidene,53-55 τ1, τ2, and τ3 correspond to the S2, hot S1,
and S1 lifetimes, respectively. 10 ns is regarded as infinity as
compared to the time scale in which we are observing the kinetics
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ARTICLE
using femtosecond time-resolved absorption spectroscopy. This
value is conventionally adopted and is fixed for the fitting of the
kinetics traces. Also, the 10 ns decay component shows good
agreement with the bleaching signal of S0 f S2 absorption in the
spectral domain. The degradation of C20Ind by direct photo-
excitation has already been reported by Fujii et al.56 Therefore,
we assigned the origin of this 10 ns decay component to the
bleaching of the S0 f S2 absorption caused by the sample
decomposition. The S1 f S0 internal conversion time constant
(τ3 in Table 2) becomes long as solvent polarity increases. This
trend is completely opposite to the situation of S1-ICT dynamics
of carbonyl-containing carotenoids, such as peridinin and
fucoxanthin.4,5,10-13 Rather, this trend is reminiscent of the S1
dynamics of a carotenoid with an aldehyde-type carbonyl
group.29
Excitation Energy Dependence of the Femtosecond Tran-
sient Absorption Spectra and Their Kinetics for C20Ind in
n-Hexane and Acetone. Figure 7 shows the excitation energy
dependence of the transient absorption spectra of C20Ind in
n-hexane and acetone. In all cases, the maxima of transient
absorptions show blue shifts between 0.1 and 2 ps. This
phenomenon is interpreted as vibronic relaxation in the excited
state.53-55 The transient absorption spectra at longer than 2 ps
after excitation in n-hexane depend strongly on excitation energy,
whereas those in acetone do not. To gain insight into the origin of
the spectral difference, spectral decomposition of the transient
absorption spectra of C20Ind was carried out.
Figure 8 shows the excitation energy dependence of the
normalized transient absorption spectra and the results of the
spectral decomposition at delay times after excitation of 3, 5, and
10 ps in both solvents. Table 3 shows the relative intensities, peak
energies, and fwhm of the transient absorptions determined by
spectral decomposition. In the cases following excitation at 2.25
and 2.58 eV in n-hexane, the transient absorption spectra are well
reproduced using one or two Gaussian functions together with a
bleaching component corresponding to bleaching of the steady-
state absorption spectrum, as shown in Figure 8a and b. In the
case of both these excitation energies in acetone, three Gaussian
functions (the lowest, middle, and the highest energies) and one
bleaching component are required to achieve satisfactory fitting
of the transient absorption spectra (Figure 8c and d). Transient
absorptions following excitation at 2.25 eV in n-hexane
(Figure 8a) show a similar spectral pattern at each delay time,
whereas with those following excitation at 2.58 eV in n-hexane
the relative intensity of each Gaussian sub-band apparently
changes depending on a delay time (see relative intensity of
the “lowest energy” and “middle energy” positions following
excitation at 2.58 eV in n-hexane in Table 3). This suggests that at
least two excited states must be involved in the dynamics of these
Figure 9. Excitation energy dependence of the kinetics of transient
absorbance change of C20Ind (a) in n-hexane following excitation at
2.25 eV, (b) in n-hexane following excitation at 2.58 eV, (c) in acetone
following excitation at 2.25 eV, and (d) in acetone following excitation at
2.58 eV. The “O”, “0”, and “4” show the experimental data. Solid lines
show the results of fitting using multiexponential functions. The insets
show the same plots of a short time regime after excitation. “HE band”
and “LE band” in (b) correspond to the higher and lower energy sub-
bands, respectively, in Figure 8b.
Table 4. Excitation Energy Dependence of Decay Time Constants Determined by Fitting of Transient Absorption Kinetics of
C20Ind Using Multi-exponential Functionsa
time constants
solvent
probe energy
τ1/fs
τ2/ps
τ3/ps
τ4/ns
At 2.25 eV (551 nm) Excitation
108 ( 17 (-)
1.4 ( 0.1 (þ)
n-hexane
2.07 eV (599 nm)
2.17 eV (571 nm)
2.30 eV (539 nm)
2.15 eV (577 nm)
2.29 eV (541 nm)
2.39 eV (519 nm)
5.0 ( 0.1 (þ)
5.0 ( 0.1 (þ)
5.0 ( 0.1 (þ)
11.4 ( 0.2 (þ)
11.4 ( 0.2 (þ)
11.4 ( 0.2 (þ)
108 ( 17 (-)
108 ( 17 (-)
110 ( 20 (þ)
110 ( 20 (-)
110 ( 20 (-)
not applicable
not applicable
0.96 ( 0.24 (þ)
0.96 ( 0.24 (-)
not applicable
10 (fixed) (-)
10 (fixed) (-)
10 (fixed) (-)
10 (fixed) (-)
acetone
At 2.58 eV (481 nm) Excitation
n-hexane
LE band
70 ( 100 (-)
60 ( 100 (-)
130 ( 40 (þ)
130 ( 40 (-)
130 ( 40 (-)
not applicable
5.4 ( 0.1 (þ)
5.0 ( 0.1 (þ)
10.3 ( 0.4 (þ)
10.3 ( 0.4 (þ)
10.3 ( 0.4 (þ)
HE band
1.1 ( 0.1 (þ)
acetone
2.15 eV (577 nm)
2.29 eV (541 nm)
2.39 eV (519 nm)
0.76 ( 0.10 (þ)
0.76 ( 0.10 (-)
0.76 ( 0.10 (þ)
10 (fixed) (-)
10 (fixed) (-)
a Plus and minus signs on the right-hand side of the decay time constants indicate rise and decay phases, respectively.
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Figure 11. Schematic drawings of the relaxation pathways of C20Ind in
n-hexane. Dashed arrows show the excitation by incident light. Thin
solid arrows indicate the vibrational relaxation in the S2 or internal
1
conversion from the vibrational excited state of S2 to the nπ* state.
Thick solid arrows show the relaxation pathways with experimentally
determined lifetimes.
that only the S2 was generated, whereas in the case following
excitation at 2.58 eV in n-hexane, both the S2 and the 1nπ* states
were generated. The S2 of carotenoids instantaneously relaxes to
the S1.53-55 Therefore, it is suggested that the HE band produced
following excitation at 2.58 eV and the excited state produced
following excitation at 2.25 eV in n-hexane can be unequivocally
assigned to the S1 f Sn transition. The LE band produced
following excitation at 2.58 eV in n-hexane can then be assigned
to the 1nπ* state f Sm (a higher lying excited state) transition.
Figure 11 shows a schematic description of the relaxation path-
ways of C20Ind in n-hexane based on the experimental results and
the calculations. In the case of excitation at 2.25 eV, the relaxation
pathway is proposed as S2 f hot S1 f S1 f S0. In the case of
excitation at 2.58 eV, two cases are considered for the relaxation
pathways of C20Ind. One is that both hot S2 fS2 fhot S1 fS1 f
S0 and 1nπ* f S0 occur simultaneously; the other is that both hot
Figure 10. (Top) A ground-state chemical structure of C20Ind deter-
mined by density functional B3LYP/6-31G(d) calculation. (Bottom)
Energy diagram of three lowest singlet excited states of C20Ind in (a)
vacuum, (b) isopentane, (c) cyclohexane, (d) diethyl ether, (e) acet-
onitrile, (f) benzene, (g) decaline, (h) 3-methylpentane, and (i) ethanol
predicted by MNDO-PSDCI calculations. Dotted, dashed, and solid
lines show the transition energy of the 21Ag-, the 11Buþ, and the 1nπ*
states of C20Ind, respectively.
transient absorption spectra at times longer than 3 ps after
excitation at 2.58 eV.
Transient absorption band shapes in acetone are independent of
the delay time and excitation energy. This suggests that the same
single excited state was generated following the relaxation from the
S2 state after excitation at 2.25 and 2.58 eV in acetone. Figure 9
shows the excitation energydependence of the transientabsorption
kinetics of C20Ind in n-hexane and acetone. The sharp peak that
appeared instantaneously following excitation has been assigned to
nonlinear optical effects.57,58 In all cases, the kinetics are well fitted
by multiexponential functions. Table 4 summarizes the results of
this fitting. The lower energy (LE) and higher energy (HE) bands
generated following excitation at 2.58 eV in n-hexane have different
decay time constants (5.4 and 5.0 ps, respectively). Furthermore,
the decay time constant of the HE band is close to that of the
excited state generated following excitation at 2.25 eV in n-hexane.
These results suggest that the HE band is the same excited state
that is generated following excitation at 2.25 eV. The LE band is
generated in a time regime shorter than the instrument response
function (∼100 fs). Therefore, it can be tentatively concluded that
the LE band was generated either by direct photoexcitation or by
relaxation from the higher vibrational excited state of the S2 state.
The rise component of the HE band is not synchronous with the
decay component of the LE band.
1
S2 f S2 f hot S1 f S1 f S0 and hot S2 f nπ* f S0 occur
simultaneously. The latter idea, the branching pathway from the S2
state, has been reported in previous papers.59,60 The lifetimes of each
excited state of C20Ind were experimentally determined (Table 4).
In the previous reports, the 1nπ* state was reported to be generated
in the relaxation process of the S1 of retinal having a short polyene
chain conjugated with an aldehyde-type carbonyl group.26-29 On
the other hand, it is reported that the nearest underlying state to the
1
S2 is the nπ* state for siphonaxanthin, a carbonyl-containing
carotenoid with longer conjugated chain length than retinal.30
The relaxation pathway of siphonaxanthin following excitation to
the S2 is proposed as S2 f 1nπ* state f S1 f S0.30 The generation
and relaxation process of the 1nπ* state of C20Ind is clearly different
from those of retinal and siphonaxanthin.
On the basis of the calculations, it is expected that both the S2
and the 1nπ* states are generated by the excitation at 2.58 eV in
acetone. However, the transient absorption spectra and kinetics
are independent of excitation energy, suggesting the production
of a single excited state. The lifetime of the 1nπ* state of retinal in
protic solvents is shorter than that in nonpolar solvents.29 By
inference, on the basis of this observation, it can be suggested that
MNDO-PSDCI Calculations of C20Ind. Figure 10 shows the
ground-state chemical structure of C20Ind determined by den-
sity functional B3LYP/6-31G(d) calculation and the solvent
dependence of the energies of the S2 (11Buþ), S1 (21Ag-), and
1nπ* states predicted by MNDO-PSDCI calculations. It should
1
the nπ* state of C20Ind in acetone is too short-lived to be
detected in the present study.
1
’ CONCLUSIONS
be noted that the energies of the S2 and nπ* states were
determined to be very close (∼0.1 eV). Consequently, in the
The steady-state and femtosecond transient absorption spec-
tra of C20Ind in various solvents were investigated. The peak
case following excitation at 2.25 eV in n-hexane, it is suggested
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The Journal of Physical Chemistry A
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shift and the broadening of the steady-state absorption spectra
with increasing solvent polarity are similar to those observed in
other carbonyl-containing carotenoids such as peridinin and
fucoxanthin.4,5,10-13 In contrast, the trend observed for the
solvent dependence of the transient absorptions was unusual.
The observed solvent dependence was similar to that observed
for the S1 state of retinal.29 The femtosecond transient absorp-
tion spectra of C20Ind following excitation at 2.58 eV in
nonpolar n-hexane were composed of the two transient absorp-
tion bands (the HE and LE bands), whereas in the case of
excitation at 2.25 eV only the HE band was observed. The HE
and LE bands have been assigned to S1 f Sn, and 1nπ* states f
Sm (higher lying excited states), respectively, based on the
MNDO-PSDCI calculations. The kinetics of the LE band
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1
indicate that the nπ* state of C20Ind was generated by either
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: hassy@sci.osaka-cu.ac.jp.
’ ACKNOWLEDGMENT
(26) Takeuchi, S.; Tahara, T. J. Phys. Chem. A 1997, 101, 3052–3060.
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H.H. thanks the Nissan Science Foundation for financial
support. R.J.C. and H.H. thank HFSP for financial support. D.K.
was supported in part by the Grant-in-Aid for JSPS Fellows (No.
20002429) from JSPS. The work by H.A.F. was supported by a
grant from the National Science Foundation (MCB-0913022)
and the University of Connecticut Research Foundation. The
work by R.R.B. was supported by the National Institutes of
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