Solvent and Structural Fluctuations Induced Symmetry-Breaking

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C: Energy Conversion and Storage; Energy and Charge Transport

Solvent and Structural Fluctuations Induced SymmetryBreaking Charge Transfer in a Porphyrin Triad Taeyeon Kim, Woojae Kim, Hirotaka Mori, Atsuhiro Osuka, and Dongho Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05363 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

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The Journal of Physical Chemistry

Solvent and Structural Fluctuations Induced Symmetry-Breaking Charge Transfer in a Porphyrin Triad Taeyeon Kim,a Woojae Kim,a Hirotaka Mori,b Atsuhiro Osuka,b,* and Dongho Kima,* a

Department of Chemistry and Spectroscopy Laboratory for Functional π-Electronic Systems,

Yonsei University, Seoul 03722, Korea b

Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto,

606-8502, Japan

AUTHOR INFORMATION Corresponding Authors [email protected], [email protected]

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT This study aims to show that the symmetry-breaking charge transfer (SBCT) in a donor– acceptor–donor (DAD) porphyrin triad occurs via solvent and structural fluctuations measured by using femtosecond broadband fluorescence up-conversion spectroscopy (FLUPS), which can directly trace the origin of the emitting state by monitoring its emission dipole moment as a function of time. While the emission dipole moment of the triad in the excited state remains nearly unchanged in nonpolar solvents such as cyclohexane and toluene, it is significantly reduced in polar solvents such as benzonitrile due to a change in the emitting state from quadrupolar (the exciton coupled state) to dipolar symmetry (the relaxed S1 state). The latter state is formed by SBCT process of DAD via a combination of solvent and structural fluctuations.


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Introduction Molecular arrays with various combinations of chromophores and linkers have been synthesized and their properties have been extensively investigated. Particularly, the photophysical properties of such systems have attracted significant attention as these are the key criteria for numerous optoelectronic applications.1–8 The photophysical properties of a system, composed of multiple chromophores, cannot be determined by a linear combination of the data for each component as there are more complicated effects, such as exciton and vibronic couplings, π–conjugation, and solvent polarity effects, among others.9,10,19,20,11–18 Thus, we can gain proper insight into the relation between the structure and photophysical properties of molecular arrays only by conducting systematic spectroscopic studies. Among many chromophore candidates, we have been intensively working with an aim of deepening our scientific understanding of the functioning of a variety of porphyrin arrays. In particular, we have mainly focused on the exciton couplings in various well-defined porphyrin arrays and revealed how their structural factors, such as the number of constituent units, linking positions, and linkers, are related to the exciton couplings.21–29 While a variety of molecular systems have already been reported to exhibit Symmetrybreaking charge transfer (SBCT), such as perylenediimide dimers,30–32 bianthryl,33,34 biperylenyl,35 polycyclic compounds,36 and quadrupolar systems,37–41 with theoretical supports42,43 recently, we observed the first example of a porphyrin charge acceptor (A)–donor (D)–acceptor (A) trimer (ADA) that exhibits SBCT in the excited state.44 The directly linked (i.e., without the use of a linker) linear porphyrin arrays exhibit strong exciton coupling strength and an enhanced quantum yield in comparison with its reference monomer ZnTPP (ФF=0.033).45 On the contrary, ADA in polar solvents exhibits a considerably low quantum yield (ФF=0.051 in CHX and ФF=0.004 in BCN) and red-shifted broadened fluorescence spectra indicating the different nature of its emitting state to that of typical exciton coupled



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state. This observation, along with information gained from time-resolved spectroscopic experiments such as transient absorption (TA), time-correlated single photon counting (TCSPC), and time-resolved IR (TRIR), provided further evidence of the SBCT dynamics present in ADA inferred by the results of porphyrin dimer (AD) and trimer (AAA) as dipolar and nonpolar reference systems, respectively. However, two limitations that are still present in the previous study are as follows: 1) Since these systems lack ideal infrared (IR) markers such as a CC triple bond40 owing to their porphyrin units being directly linked, we could only monitor the vibrational mode of the aromatic core where a large contribution by solvents exists at around 1500 cm-1. In this regard, the overall TRIR spectra were not adequate for the detailed analysis. 2) Time-resolved fluorescence (TRF) spectra were reconstructed via TCSPC, hence the time-resolution was limited to an instrumental response function (IRF) of ~ 70 ps. In the proposed study, we investigated the SBCT in the target DAD molecule (Chart 1), concentrating on TRF measurements using femtosecond broadband fluorescence up-conversion spectroscopy (FLUPS) in order to directly trace the emission dipole moments.39 The basic photophysical properties of DAD were obtained using the steady-state absorption and fluorescence measurements. Also, the overall excited-state dynamics of DAD was studied using TA measurement. Chart 1. Molecular structure of DAD (left) and its substituents (right); the pentafluorophenyl (A) and di-tertbutylphenyl (D) groups. Ar2

F N

Ar1 N NZn N

Ar2

Ar2

Zn

Ar 1 =

N Zn N N

Ar1

N

N

F F

Ar2

N

N N

F

Ar2

Ar 2 =

Ar2

DAD


F

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Results and discussion Molecular structure and dihedral angle. The target molecule (DAD) is a porphyrin trimer, where the constituent porphyrin units are directly and linearly linked in a meso-meso fashion, as shown in Chart 1. The number of substituents in D and A is 3 when it is located at the side and 2 at the center, respectively. The distance between the neighboring porphyrin units is 8.4 Å and dihedral angle is around 90° due to the presence of steric hindrance between the hydrogen atoms at the β position and the bulky substituents pentafluorophenyl and di-tertbutylphenyl subtituents. This feature was also well illustrated in the single point energy calculation by rotating one side of the porphyrin unit (Figure S1). AAA seems to be more flexible than DAD as seen from the broader energy distribution of AAA than that of DAD as a function of the dihedral angle. In addition, we calculated the optimized structure of the S1 state at a B3LYP/6-31G(d) level. The dihedral angles of the calculated structures of AAA,

CHX

Absorbance / a.u.

d

300

400

500

600

700

800

400

500

600

700

800

500

600

700

800

TOL

300

Fl. intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


BCN

300

400

Wavelength / nm Figure 1. Steady-state absorption and fluorescence spectra of DAD in cyclohexane (CHX), toluene (TOL), and benzonitrile (BCN). An arrow at 750 nm indicates expanded range of fluorescence.



ADA, and DAD are 86.7°, 82.5° and 89.0°, respectively, indicating that the excited state of DAD is relatively more rigid than that of ADA (Table S1) due to the presence of different substituents. Steady-state absorption and fluorescence spectra. The steady-state absorption and fluorescence spectra were recorded to obtain the basic optical properties of DAD in three solvents, which are cyclohexane (CHX), toluene (TOL), and benzonitrile (BCN) (Figure 1). Directly linked meso-meso linked homogeneous porphyrin arrays are well known to possess characteristic spectral features in the absorption and fluorescence spectra in that the absorption is composed of B-bands (monomeric and split bands) and Q-bands and the fluorescence features strong 0–0 and distinct 0–1 vibronic bands at around ~1500 cm-1, reflecting the Qbands in the absorption spectra.22,23,46 The steady-state absorption and fluorescence spectra of DAD in CHX are also similar to the characteristic features exhibited by directly linked porphyrin arrays except that they show some additional vibronic bands between the typical 0– 0 and 0–1 bands at around 500–800 cm-1 similar to those observed in the spectra of ADA.44 In TOL and BCN, these additional vibronic transitions between the typical 0–0 and 0–1 bands are broad and seem to be more strongly allowed. In the absorption spectra, the transitions in the Q-bands are quite complicated to analyze in detail because the Qx and Qy bands (assuming x as the long molecular axis and y as the monomeric axis) and their vibronic bands overlap in this region.27 Interpretation is much easier in the case of the fluorescence spectra since the transitions occur from the specific lowest excited state to the ground vibronic state depending on the solvent polarity. In particular, the radiative decay should correspond to the fluorescence from the exciton coupled Qx (π,π*) or symmetry-broken CT state to the ground state depending on solvent polarity. The fluorescence spectra of DAD in CHX, TOL and BCN show clear differences since the vibronic structures in TOL and BCN are totally different from that in a


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typical fluorescence spectrum of the porphyrin in CHX. Moreover, the fluorescence spectrum in BCN exhibits further red-shifting and broadened features extending to 800 nm. Relative band ratios and emission dipole moments. For a detailed analysis of the steadystate fluorescence spectra, 1) the fluorescence spectra of DAD in the three solvents were fitted by Gaussian functions to examine the relative band ratios of vibronic bands and 2) the emission dipole moments were calculated to assign the origins of the emitting states. The Gaussian function fitting results are shown in Figure S2 and their relative band ratios are listed in Table S2. The fluorescence spectra in CHX and TOL can be fitted with three Gaussian functions to reproduce the vibronic bands. While the ratios of the 0–0 band in CHX and TOL are similar (26%), the ratios of the other two 0–1 bands are different, which are in agreement with the fluorescence spectral shape resulting from the change in the Franck-Condon (FC) factor. The fluorescence spectrum in BCN can be reproduced with an additional Gaussian function with a broader width and located in the near IR region extending over 800 nm, which may correspond to the emission from a symmetry broken dipolar state.

Table 1. Photophysical properties of DAD in three solvents (CHX, TOL, and BCN)

(107s-1)

Nonrad. rate (108s1 )

Dielectric constant (ε)

1.57

3.2

6.0

2.02

2.8

2.8

0.05

1.55

3.2

6.1

2.38

2.6

2.6

0.01

1.09

0.9

9.1

26.00

0.6

0.4

QY

Lifetime (ns)

Rad. rate

DAD

CHX

0.05

TOL

BCN


μem in FC μ in relaxed em S1 (D) (D)


Secondly, the steady-state emission dipole moments of DAD in the three solvents were calculated in order to determine the nature of the emitting state. The details of calculation of the emission dipole moments can be found elsewhere.39 The magnitudes of the emission dipole moments have been calculated to be 2.8, 2.6, and 0.4 in CHX, TOL, and BCN, respectively, as shown in Table 1. The emission dipole moments are directed along the long molecular axis and are constructed to form a strongly allowed transition via head-to-tail exciton coupling between the constituent units. Thus, while in CHX and TOL DAD exhibits a modest emission dipole moment, the remarkably small emission dipole moment in BCN indicates that the origin of the emitting state may have changed. Consequently, it is conceivable that the lowest emitting state in BCN is a symmetry-broken charge transfer state. However, it is still not clear whether the relaxed S1 state is a charge transfer (CT) state or not because it is also possible that the initial photoexcited exciton coupled state, without a CT process, can be perturbed by the surrounding solvent molecules resulting in a reduction of its emisison dipole moment. To determine the correct reason, it is necessary to measure the TRF to monitor possible CT processes from the exciton coupled state. Fluorescence lifetime and related values. The fluorescence lifetime was initially measured using TCSPC and the IRF was estimated to be ~ 70 ps. The fluorescence lifetimes of the relaxed S1 state of DAD were determined to be 1.57, 1.55, and 1.09 ns in CHX, TOL, and BCN, respectively (Figure S3). Combining this kinetic information with the measured fluorescence quantum yield, the radiative and non-radiative rate constants were extracted and are listed in Table 1. Interestingly, DAD has the similar 0–0 band ratio values, quantum yields, and radiative and non-radiative rates in both CHX and TOL indicating the comparable emission probability while the spectral shape and FC factor are quite different. Unlike the single exponential decay component observed in CHX and TOL, a double exponential decay profile was observed in BCN as shown in Figure S3, which led to the conclusion that DAD has two


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1.0 0.8

fluorescence origins in BCN. This additional emitting0.6state could arise from the symmetry0.4

broken CT emission as was previously assigned to the similar SBCT features observed for ADA.44

ㅇ

600

650

700

750

800

Wavelength / nm

TOL

50 ps

1 ps 50 ps 400 ps

CHX TOL BCN

0.2 0.0 0

10

20

30

40

50

d

Time / ps

(b)

0.8 ps

1 ps 50 ps 400 ps

Center of mass / 103cm-1

(a) CHX

15.6

15.2

14.8

14.4 0

10

20

30

40

50

Time / ps 600

650

700

750

800

Wavelength / nm

BCN 1 ps 50 ps 400 ps 600

650

700

750

800

Wavelength / nm

600

650

700

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800

Total fluorescence intensity / norm.

Fluorescence intensity / arb.

1.0 0.8 0.6 0.4 CHX TOL BCN

0.2 0.0 0

10

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30

Time / ps

Wavelength / nm Center of mass / 103cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Total fluorescence intensity / norm.

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40

50

d

Figure 2. (a) Time-resolved fluorescence spectra of DAD in15.6 CHX, TOL, and BCN. The insets show the normalized fluorescence spectra at the specific times of 1, 5, and 400 ps. (b) The kinetic profiles of the spectral center of mass, and (c) the normalized total fluorescence intensity. 15.2

Fluorescence up-conversion spectroscopy. Next, to obtain more detailed insight into the 14.8

solvation and SBCT dynamics, we conducted TRF measurements using FLUPS47 ranging from 14.4

sub-picosecond to 400 ps and the full width half maximum0 (FWHM) of cross-correlation 10 20 the 30 40 50 Time / ps

function between the scattered pump pulse (550 nm) and the gate pulse (1320 nm) was measured to be 180 fs (Figure S4). In Figure 2a, the TRF spectra up to 50 ps are shown and the insets show the normalized fluorescence spectra at 1, 50, and 400 ps, respectively. The fluorescence spectra of DAD in BCN show rapid decay with an observed red-shift and broadening, while those in CHX and TOL maintain their intensity with a slight red-shift and a



spectral shape change only observed in TOL. These spectral evolutions were initially analyzed as the spectral center of mass and the total fluorescence intensity as a function of time as shown in Figures 2b and c, respectively. The extent of the red-shift in the spectral center of mass is larger in a more polar solvent while the total fluorescence intensity shows a large decrease only in BCN. Time-dependent emission dipole moment. In CHX, DAD shows features characteristic of directly linked homogeneous porphyrin arrays. The spectral shape and fluorescence intensity rarely change within 50 ps because the exciton coupled state (i.e., the lowest electronic state) is insensitive to nonpolar CHX molecules and its fluorescence lifetime of 1.57 ns is long enough for the fluorescence intensity to remain. The slightly red-shifted fluorescence feature in TOL indicates the existence of an effective solute-solvent interaction that stabilizes the emitting state, resulting in a change in the vibronic band ratio in the fluorescence spectrum despite the fact that TOL is generally considered as a nonpolar solvent. The role of TOL as a quadrupolar solvent was experimentally demonstrated by Dereka and co-authors using timeresolved IR spectroscopy.38 They observed that the quadrupolar moment of the solvent gives rise to effective solute–solvent interactions, leading to SBCT process. However, due to the

(a)

1.1

d

1.0

em / norm.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.9

CHX TOL BCN

0.8 0.7 0.6 0.5 0

10

20

30

40

50

Time / ps Figure 3. Normalized emission dipole moment of DAD in CHX, TOL, and BCN as a function of time.


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unchanged total fluorescence intensity and emission dipole moments of DAD in TOL (Figure 3), the possibility of SBCT can be excluded in this solvent. The emission dipole moments and total fluorescence intensity do not change in in CHX and TOL while, in BCN, these are greatly reduced owing to the decrease in the orbital overlap between the dipolar excited state and quadrupolar ground state.48 Therefore, the declining emission dipole moment in BCN provides direct evidence for the SBCT process in the excited state. Solvent and structural fluctuations. The kinetic profile of the total fluorescence intensity of DAD in BCN was analyzed within 50 ps to see how the SBCT process proceeds without being disturbed by the fluorescence lifetime of ~1 ns. The trace can be fitted with bi-exponential decay functions with time constants of 1.3 (33%) and 32 ps (67%), respectively (Figure 4, top). From our previous work,44 this few tens of picosecond components of ADA and AD in BCN were found to be dependent on the viscosity rather than the solvent polarity, above a certain polarity threshold. Accordingly, it can be concluded that the structural fluctuation plays a

/ norm. / norm. intensity intensity fluorescence fluorescence Total Total

crucial role in the SBCT process. In line with this, the latter time constant of 32 ps, which

Center of mass / 103cm-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.0 1.0 0.8

BCN: 1.3 / 32 ps (1:2) d

0.8 0.6 0.6 0.4

CHX TOL BCN

0.4 0.2 0.2 0.0 0.0

0

10

20

30

40

0

10

20 30 Time / ps Time / ps

40

CHX TOL BCN 50 % (4,@LG) %50 (5,@LG)

15.6

15.2

d TOL: 15 ps BCN: 2.2 / 37 ps (1:5)

14.8

14.4 0

10

20

30

40

50

Time / ps

Figure 4. Kinetic profiles of total fluorescence intensity in BCN and spectral center of mass in TOL and BCN.



induces SBCT, corresponds to the structural fluctuation and the former time constant of 1.3 ps has to be assigned due to solvent fluctuation since there is no additional driving force for the SBCT process. Likewise, the kinetic profiles of the spectral center of mass of DAD in TOL and BCN were explored. The dynamic Stokes shifts of the TRF were fitted with exponential functions as 15 ps for TOL and 2.3 (17%) and 37 ps (83%) for BCN, respectively (Figure 4, bottom). The shift seen for the molecule in TOL indicates that the structural relaxation dynamics in the excited state brings about a new electron density equilibrium where TOL can interact differently with DAD, unlike that in insensitive CHX. In BCN, since the decreased amplitude of the fast time component (the solvent fluctuation) in the dynamic red-shift in comparison with that in the total fluorescence intensity can provide us with some physical insight, we fitted the vibronic bands (1,2, and 3) and the CT band (4) with multiple Gaussian functions (Figure S5). In TOL, a decrease in the area of band 1 and an increase in the area of band 2 were observed, which is correlated to the dynamic Stokes shift with the position of each band remaining the same. In BCN, the dynamic Stokes shift is mainly dependent on the structural fluctuation while the total fluorescence intensity is sensitive to both the solvent and structural fluctuations time scale. This analysis indicates that although SBCT occurs via both solvent and structural fluctuations, the further stabilized (red-shifted) CT band can be attributed to structural fluctuation. Absolute emission dipole moments. The absolute emission dipole moments of the FC states were found to be 2.8, 2.6, and 0.6 D in CHX, TOL, and BCN, respectively (Figure S6). The values in CHX and TOL are higher and are preserved over 50 ps, which are different from that in BCN that loses 40% of its magnitude as it approaches 0.4 D. Consequently, it was inferred that the initially formed exciton coupled state in BCN must have already been under the influence of the surrounding solvent field to be able to exhibit such a low initial emission dipole moment (0.6 D) and SBCT takes place afterward to have 40% reduced emission dipole moment


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(0.4 D). This change of absolute emission dipole moment can be viewed in terms of a change in mixing coefficients in linear combination of exciton and CT configurations since this kind of strongly coupled system cannot undergo pure or complete charge transfer.43,49 In particular, FC state of DAD in BCN might already consist of a certain extent of CT configurations which subsequently evolves to asymmetric CT configurations (SBCT) to either side unlike nearly exciton configurations in CHX and TOL. 0.06 0.04

0.00

CHX

-0.02

S1->T1: 1750 ps -0.04

T1-> G: >10 ns

0.04

(b)

0.02

Signal amplitude

(a) Signal amplitude

Signal amplitude

0.02

0.02 0.00

TOL

-0.02 -0.04

S1->T1: 1350 ps

-0.06

T1-> G: >10 ns

(c)

0.00 -0.02 -0.04

BCN

-0.06

S1->CT: 28 ps

-0.08

CT->T1: 834 ps T1->G: >10 ns

-0.08

-0.06

-0.10 -0.10 -0.12

-0.08

-0.12

0.02

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0.02

0.03

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0.00

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-0.06

-0.06

-0.08

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Wavelength / nm

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-0.11

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550

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Wavelength / nm

Figure 5. (top) Evolution-associated spectra and (bottom) 2D-contour map of transient absorption (TA) spectra of DAD until 5 ns in (a) CHX, (b) TOL, and (c) BCN with pump energy of 550 nm (shaded region).

EC

Q

B

EC

Q

A,B B

Energy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B

BCN ∆μem

Solvent and Structural Fluctuations Induced Symmetry-Breaking

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