Photoinduced Electron Transfer in a BODIPY-ortho-Carborane Dyad

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A: Spectroscopy, Photochemistry, and Excited States

Photoinduced Electron Transfer in a BODIPY-ortho-Carborane Dyad Investigated by Time-Resolved Transient Absorption Spectroscopy So-Yoen Kim, Yang-Jin Cho, Ho-Jin Son, Dae Won Cho, and Sang Ook Kang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b01539 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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The Journal of Physical Chemistry

Photoinduced Electron Transfer in a BODIPYortho-Carborane Dyad Investigated by TimeResolved Transient Absorption Spectroscopy So-Yoen Kim,† Yang-Jin Cho,† Ho-Jin Son,*,† Dae Won Cho*,†,‡ and Sang Ook Kang*,† †

Department of Advanced Materials Chemistry, Korea University (Sejong), Sejong, 30019,

South Korea. E-mails: [email protected], [email protected] ‡

Center for Photovoltaic Materials, Korea University (Sejong), Sejong, 30019, South Korea. E-

mail: [email protected]

ABSTRACT We report the results of photoinduced electron transfer (PET) in a novel dyad, in which a boron dipyrromethene (BODIPY) dye is covalently linked to o-carborane (o-Cb). In this dyad, BODIPY and o-Cb act as electron donor and acceptor, respectively. PET dynamics were investigated using a femtosecond time-resolved transient absorption spectroscopic method. The free energy dependence of PET in the S1 and S2 states was examined on the basis of Marcus

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theory. PET in the S1 state occurs in the Marcus normal region. Rates are strongly influenced by the driving force (G), which is controlled by solvent polarity; thus, PET in the S1 state is faster in polar solvents than in nonpolar ones. However, PET does not occur from the higher energy S 2 state despite large endothermic G values, because deactivation via internal conversion is much faster than PET.

1. INTRODUCTION Boron dipyrromethene (BODIPY) dyes have promise in many applications, because BODIPY has large extinction coefficient, ease of synthesis, controllable modification, and high photostability.1-4 BODIPY dyes have been widely used in biological labelling and molecular sensors,5-14 as sensitizers of singlet oxygen production for photodynamic therapy,15-18 and for energy transfer cassettes and light harvesting.19-22 In particular, BODIPY has been used as a lowenergy photosensitizer to carry out efficient solar light harvesting for artificial photosynthetic systems23-25 and solar cells.26,27 The dyes possess a long-wavelength absorption band (S1←S0) with a large extinction coefficient (40 000–110 000 M-1cm-1 at ca. 500 nm) making them suitable for low-energy light harvesting. BODIPY also has a short-wavelength absorption band (S2←S0); however, this high-energy state is less useful, because of its small extinction coefficient and ultrafast dynamics compared with the lower energy state. Higher-excited states (Sn, n>2) produced by excitation of the second or higher absorption bands deactivate rapidly to the lowest-energy excited state. Therefore, most photochemical


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processes do not depend on the excitation wavelength. Most organic molecules excited to higherenergy states deactivate rapidly by internal conversion to the S1 state. This behaviour is said to conform to Kasha’s rule.28 Conversely, some chromophores (e.g. azulene,29,30 porphyrin,31 and phthalocyanine32,33 derivatives) emit anomalous fluorescence from the singlet second excited state (S2). There are many reports of S2 emission. It is well known that many energy or electron transfer processes in molecules exhibiting S2 emission can initiate from higher excited states, because the internal conversion process is slow compared to that of ordinary molecules. BODIPY dyes exhibit dual emission from the S1 and S2 states; however, investigations of the photodynamic behavior of BODIPY in the S 2 state are rare compared to the number of other compounds known to violate Kasha’s rule.34-36 BODIPY has moderate electron donating ability.37 However, the greater electron accepting ability of ortho-carborane (o-Cb) has been utilized extensively in various D-A dyads and D-A-D triads,38-42 which show strong solvent polarity effects in their emission spectra.43,44 We employed o-Cb as the electron withdrawing moiety in the electron donor-acceptor (DA) system shown in Scheme 1 to investigate the photoinduced electron transfer (PET) behaviour of BODIPY. In this work, we investigate the rapid PET dynamics in the S 1 and S2 states by femtosecond time-resolved transient absorption (fs-TA) spectroscopy. PET in the S1 state is strongly influenced by the driving force (G) that is governed by solvent polarity. Thus, PET rates in the S1 state occur in the Marcus normal region. However, we do not observe any PET events from the S2 state despite large endothermic G values, because of fast internal conversion from the S2 state compared with PET.


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Scheme 1. Molecular structures

F F B N N

F N B F N

F F B N N

BoPh

BoCb

2. RESULTS AND DISCUSSION 2.1. Steady-state absorption and emission spectroscopic properties. Figure 1 shows the absorption and fluorescence spectra of the BODIPY-phenyl reference molecule (BoPh) and the BODIPY-o-Cb dyad (BoCb) in CH2Cl2. The strong absorption band near 540 nm is attributed to the S1←S0 (,*) transition of the BODIPY moiety. The weak absorption band near 380 nm is assigned to the S2 ←S0 transition, which is partially forbidden and has a small absorption coefficient.34 The absorption spectral features of BoPh and BoCb are almost identical at overall wavelengths, where it is known that o-Cb shows a band near 220 nm.


4

1.2

Absorbance (nor.)

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


1.2

BoPh BoCb

1.0

So-S1 1.0

Flu

Abs

0.8

0.8

0.6

0.6

0.4

0.4 So-S2

0.2

0.2

0.0

Fluorescence Intensity (nor.)

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0.0

300

350

400

450

500

550

600

650

700

Wavelength (nm)

Figure 1. Steady-state absorption and fluorescence spectra of BoPh and BoCb in CH2Cl2. Excitation wavelength is 530 nm.

Fluorescence emissions were observed near 550 nm upon excitation at 530 nm (Figure 1). The emission maximum of BoCb occurred at a longer wavelength than that of BoPh. Some BODIPY derivatives show S2 emission in the high energy region. However, no S2 emissions were detected for BoPh and BoCb at shorter wavelengths. Moreover, regardless of excitation at 530 or 380 nm, which can excite BODIPY into either the S1 or S2 state, respectively, the emissions of BoPh and BoCb were detected at identical wavelengths near 550 nm, but with different intensities. This means that the principal deactivation process of the S2 state in both BODIPY derivatives is internal conversion to the S1 state. The fluorescence emission quantum yields (f) of BoPh and BoCb in CH2Cl2 were 0.62 and 0.23, respectively,37 and showed negligible excitation wavelength dependence. We also observed a small (550 nm). The negative band spans wavelengths of 460–700 nm. As shown in Figure 2a, the TA spectral decay of BoPh is not complete after 4.775 ns. However, the TA and bleaching bands of BoCb recover completely after that time, which means that the lifetime of excited BoCb is shorter than that of BoPh. The TA signal of BoPh monitored at 440 nm decays with a lifetime of 3.8 ns as shown in the temporal profiles of Figure 2c. The negative bleaching signal recovers concurrently within this timeframe. On the other hand, the decay time of BoCb is 0.72 ns in CH3CN, which is much shorter than that of BoPh. As shown in Figure 2d, the decay time constants of BoCb are 2.72 ns in nonpolar n-hexane and 1.22 ns in moderately polar CH2Cl2, which is similar to the reported fluorescence lifetime (1.3 ns).37 This decay is assigned to deactivation of the S1 state to the ground state. Interestingly, BoPh does not show a solvent polarity dependence in its TA spectra. Indeed, identical lifetimes of about 3.8 ns are found for excited BoPh in all three solvents, as shown in Fig 2c. The photodynamic parameters are listed in Table 1. The lifetime shortening for BoCb is attributed to PET from the BODIPY centre to the o-Cb moiety.


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(d) BoCb

(c) BoPh

0.02

1.0 0.00

0.025 ns 4.775 ns

-0.02

0.5

-0.06

 A (nor.)

-0.04

(a) BoPh

A

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


0.0

0.00

-0.02

-0.5

0.025 ns 4.775 ns

-0.01

n-hexane CH2Cl2 CH3CN

n-hexane CH2Cl2 CH3CN

-1.0

(b) BoCb

-0.03

400

450

500

550

600

650

700

0

1

2

3

Time (ns)

Wavelength (nm)

4

50

1

2

3

4

5

Time (ns)

Figure 2. Transient absorption spectra for (a) BoPh and (b) BoCb in CH3CN. Excitation wavelength is 380 nm. The normalized temporal decay profiles of (c) BoPh and (d) BoCb monitored at 440 and 550 nm, respectively, in n-hexane, CH2Cl2, and CH3CN. The solid green lines indicate the fitting of values to a single exponential function.

Using the lifetime of BoPh as a reference, the PET efficiency (S1) of BoCb in the S1 state is determined using eqn (1):

 S1 =

o  S1 -  S1 o  S1

(1)

where oS1 and S1 are the lifetimes of reference BoPh and dyad BoCb in the S1 state, respectively, determined by TA measurement. S1 in the S1 state is 0.68 for CH2Cl2, which is slightly smaller than the value (0.71)37 determined by fluorescence lifetime measurement. S1 is


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0.15 in n-hexane and 0.81 in CH3CN, respectively. The S1 values are large in polar solvents, which means that PET occurs efficiently under these conditions. 2.3. Driving force dependence (G) of PET. The rate constant (kPET) of PET is determined using eq (2)

kPETSn 

1

(

1

 Sno 1 /  Sn - 1

)

(2)

where n = 1 or 2 denotes the first or second excited state, respectively. For the S1 state of BoCb, kPET,S1 equals 1.13 × 109, 6.8 × 108, and 9.6 × 107 s-1 in CH3CN, CH2Cl2, and n-hexane, respectively (Table 1). The driving force (G) for electron transfer from an electron donor to an electron acceptor in the excited state can be determined using the relationship: G° = E°ox – E°red – E00. Here, E°ox and E°red are the oxidation and reduction potentials of the donor (BODIPY) and acceptor (o-Cb) molecules, respectively, and E00 is the excitation energy. To understand specific solvent effects on the rate of PET, two additional terms related to solvent behaviour must be considered as shown in eq (3).46,47

o o ΔGPET=( Eox -Ered )-E00 -

e2 e2 1 1 1 1 + ( + )( ) 4πεRDA 4πε 2rD 2rA ε εsp

(3)

e2/4RDA is the Coulombic term, e is the charge of the electron, and rD and rA are the radii of BODIPY and o-Cb, respectively. RDA is the distance from the centre of BODIPY to the adjoining carbon of o-Cb. Based on the X-ray crystallography of asymmetric BoCb, we determined short


8

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and long RDA distances of 7.251 and 9.711 Å , respectively. The value of e2/4 is 14.4 eV∙Å . sp is the static dielectric constant of the more polar solvent in which the redox potentials were determined, and  is the static dielectric constant of the less polar solvent. The oxidation potential (E°ox) of BoPh is 0.63 V. The reduction potential (E°red) of Cb is reported to be -1.64 V.48 The excitation energy (E00) of the lowest excited state is 2.22 eV (560 nm). The calculated G values for BoCb in the S1 state are -0.17 and -0.32 eV in CH2Cl2 and CH3CN, respectively. The negative G values mean that PET is feasible in these polar solvents. The PET process is slower in CH2Cl2 compared with CH3CN, which is in good agreement with the weaker driving force (less negative G). The G value for n-hexane is positive, which means that PET is endergonic in this nonpolar solvent. Contrary to the prediction of G, we observed PET even in n-hexane via the TA experiment, but S1 and kPET∙S1 were smaller than the values in polar solvents. Thus, calculation of G is useful for predicting trends in PET, but is only indicative. The kPET-S1 values in Table 1 increase with increasing G°PET∙S1. Thus, PET in BoCb lies in the Marcus normal region,49-52 where the reorganization energy is greater than G°PET, (> G°PET). The large influence of solvent on the kPET-S1 values in Table 1 indicates that the solvent contribution to  predominates over that arising from structural reorganization of the reactants. The structural change upon addition or removal of electron distributed over a large molecular framework. Therefore, the solvent contribution is lager for large molecules. For BoCb, the value of  primarily reflects solvent reorganization, because contributions from structural rearrangement of large entities such as BODIPY and o-Cb are expected to be small.


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Table 1. Photophysical parameters: decay times (S1 and S1) and rate constants (kS2 and kS1) of reference BoPh and dyad BoCb, PET rate constants (kPET∙S1), PET efficiencies (S1), and driving forces (GPET∙S1) in the S1 state Sample

BoPh

BoCb a

°S1 (S1)

k°S1 (kS1)

(ns)

(×108 s-1)

n-hexane

3.69

2.72

CH2Cl2

3.80

CH3CN

S1

kPET∙s1

G°PET∙S1

(×107 s-1)

(eV)a

-

-

-

2.63

-

-

-

3.87

2.58

-

-

-

n-hexane

2.72

3.68

0.26

9.6

0.51 (0.78)

CH2Cl2

1.22

8.20

0.68

56

-0.17 (-0.12)

CH3CN

0.72

13.9

0.81

113

-0.32 (-0.30)

solvent

The values in parentheses are calculated for the longer RDA.

2.4. Kinetics of internal conversion. We carried out TA measurements for BoPh and BoCb with excitation at 380 and 530 nm. Excitation at 380 nm corresponds to the S 2 ← S0 transition in the absorption spectrum. Figure 3a and 3b show fs-TA spectra of BoPh and BoCb in CH2Cl2 upon excitation at 380. The TA band near 435 nm increases in intensity with the delay time and is attributed to an Sn ← S1 absorption. Concurrently, the negative band at 420700 nm gradually increases in intensity. The negative band is attributed to bleaching of the ground state absorption and stimulated emission. Because these events originate in the S1 state, the growth of the negative bleaching peak is accompanied by an internal conversion process that accesses the S 1 state from a higher excited state. The temporal profiles monitored at 435 nm show a parallel


10

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increase in the BoPh and BoCb components with a time constant of 234 ± 40 fs. Results are illustrated in Figure 3c and 3d and listed in Table 2. The time constants monitored at 560 nm are 256 ± 31 fs for both BoPh and BoCb and are slightly longer than those measured at 435 nm, because the negative bleaching dynamics for ground-state absorption and induced emission may interfere with the TA at this wavelength.

0.00 -0.10 ps 2.17 ps

A

-0.02 (a) BoPh

ex = 380 nm

-0.04 0.00

-0.10 ps 2.16 ps

-0.01 -0.02

ex = 380 nm

(b) BoCb

400

450

500

550

600

650

700

Wavelength (nm) 1 (c) BoPh  A (nor.)

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


0

at 435 nm

ex = 380 nm

at 560 nm

ex = 530 nm

-1 1 (d) BoCb 0

at 435 nm

ex = 380 nm

at 560 nm

ex = 530 nm

-1 -0.5

0.0

0.5

1.0

1.5

2.0

Time (ps)

Figure 3. Chirp-corrected transient absorption spectra of (a) BoPh and (b) BoCb in CH2Cl2. Excitation wavelength is 380 nm. Temporal decay profiles of (c) BoPh and (d) BoCb monitored at 435 and 560 nm upon excitation at wavelengths of 380 and 530 nm, respectively. Solid lines indicate the fitting to an exponential function after deconvolution of the instrumental response function (IRF) of 120 fs.


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BODIPY is promoted directly to the S1 state by excitation at 530 nm. Indeed, when this moiety is excited by a 530-nm pulse, no rise or recovery is observed at early delay times, as shown in Figure 3c and 3d. Therefore, the fast dynamics observed upon excitation at 380 nm are assigned to S1 ← S2 internal conversion. The TA spectra obtained by the excitation at 530 nm were provided in Figure S1 in supporting information (SI).

Table 2. Photophysical parameters: decay times (°S2 and S2) and rate constants (k°S2 and kS2) of BoPh and BoCb in the S2 state. kPET∙S2 is the PET rate constant of the S2 state calculated from Marcus theory using eqn. 4 and the corresponding values of G°PET,S2

Sample

BoPh

BoCb

a

S2 (S2)

kS2 (kS2)

kPET∙S2

G°PET∙S2

(fs)

(×1012 s-1)

(×109 s-1)a

(eV)

n-hexane

184

5.4

-

-

CH2Cl2

234

4.3

-

-

CH3CN

244

4.1

-

-

n-hexane

184

5.4

0.58

-0.14 (0.13)

CH2Cl2

234

4.3

2.73

-0.83 (-0.78)

CH3CN

244

4.1

3.33

-0.98 (-0.96)

solvent

Values calculated by use of eqn. 4 based on the parabolic curve obtained for the S1 state and

ΔG°PET∙S2.


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It is notable that the internal conversion rate constant (kIC) of BoCb is identical to that of BoPh in CH2Cl2 and the other solvents as listed in Table 2. This result means that there is no deactivation process such as PET from the S2 state. Thus, the main deactivation process of the S2 state is internal conversion (k°S2 ≈ kS2 ≈ kIC).

2.5. Free energy analysis of PET by Marcus theory. PET can be described quantitatively by the Marcus theory of electron transfer. Marcus’ expression49-52 for the electron transfer rate constant (kPET) is:

k PET = (

4 3 h 2 k BT

1 )2

( + G o ) 2 V exp() 4k BT 2

(4)

where V is the electronic coupling matrix element between donor and acceptor, is the reorganization energy, and h and kB are the Planck and Boltzmann constants, respectively. T is the absolute temperature (300 K). kPET is controlled primarily by the environment surrounding the reactants. Indeed, the lifetime of the S1 state of BoCb shows a strong dependence on solvent polarity, which is consistent with the assignment of PET as the deactivation process. Figure 4 shows the relationship between the rate of PET and –G°PET determined by eqn. (3) in various solvents, and the fit of the data to eqn. (4) with  = 1.6 eV and V = 2 cm-1. The PET rates of BoCb in these solvents are in the normal region of the Marcus parabola (–ΔG°PET < λ). The value of λ for intramolecular PET in the S1 state is slightly smaller than the reduction potential (1.64 eV) of o-Cb in CH2Cl2.


13


In Table 2, we predict the PET rates (kPET∙S2) corresponding to each ΔG°PET∙S2 value in the S2 state. This calculation is valid under the assumption that λ in the S2 state is equal to that in the S1 state. The predicted kPET∙S2 values are in the range of 108 to 109 s-1, which is much slower than the rate of internal conversion.

1E10

-1

kPET (s )

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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

1E9

1E8 S1 (Exp) S2 (Theo)

1E7 -1.0

-0.5

0.0

0.5

1.0

1.5

2.0

-G (eV)

Figure 4. Plot of kPET vs. –G°PET for electron transfer from BODIPY to o-carborane in various solvents. The solid line is drawn based on the Marcus theory of electron transfer (eqn. 4). Orange circles are the measured values for the S1 state, and green circles are the theoretically estimated values for the S2 state based on –G°PET∙S2.

3. EXPERIMENTAL SECTION 3.1. Materials. BoPh and BoCb were synthesized according to the procedure reported previously in elsewhere.37


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3.2.

Steady-state

absorption

and

emission

measurements.

The

absorption

and

photoluminescence spectra were recorded on an Agilent Technologies Cary 5000 and a Varian Cary Eclipse, respectively. 3.3. Time-resolved absorption spectroscopic measurement. The sub-picosecond TA spectra were measured using a pump–probe transient absorption spectroscopy system (Ultrafast Systems, Helios), which is described in detail elsewhere.53 The seed pulse was generated using a titanium sapphire laser (Spectra Physics, MaiTai SP). The pump light was generated by using a regenerative amplified titanium sapphire laser system (Spectra Physics, Spitfire Ace, 1 kHz) pumped by a diode-pumped Q-switched laser. (Spectra Physics, Empower). The excitation pulses of 320–350 nm were generated from an optical parametric amplifier (Spectra Physics, TOPAS prime). A white light continuum pulse was generated by focusing the residual of the fundamental light at a 1 mm path length quartz cell containing water, which was used as a probe beam. The white light was directed to the sample cell with an optical path of 2.0 mm and detected with the CCD detector installed in the absorption spectroscopy system after the controlled optical delay. The pump pulse was chopped by a mechanical chopper synchronised to one-half of the laser repetition rate of 1 kHz, resulting in a pair of spectra with and without the pump, from which absorption changes induced by the pump pulse were estimated. All spectra were corrected for the chirp of the white-light probe. The chirp is the term to describe the temporal offset of wavelengths. When the white-light probe pulses travel through transparent medium such as solvent and cuvette, the chirp arises from the refractive index dependence on wavelength. Therefore, the different wavelength components in the probe pulse arrive at the sample with different time-delays. Actually the time-deviation by the chirp was ca. 500 fs in range of 450–800 nm. In order to adjust the zero delay for each wavelength and to get the chirp-


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corrected spectrum, we carried out the transformation process using the software program (Surface Xplore) provided from Ultrafast Systems.

4. CONCLUSIONS We have carried out a fs-TA study to clarify the PET processes in the excited states of BoPh and BoCb. As shown in Figure 2, photoexcitation at 380 nm promotes BoPh and BoCb to the S2 state. Internal conversion from the S2 state to the S1 state occurs with the same time constant in both molecules. Although PET from the S2 state is endothermic, PET is not observed because internal conversion occurs more rapidly. However, the lifetime of BoCb in the S1 state is shorter than that of BoPh due to PET. The driving force of the PET reactions is controlled by solvent polarity. Thus, the relative energy level of the charge transfer (CT) state in Figure 5 changes with this quantity. All PET processes occur in the Marcus normal region.


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Figure 5. Schematic energy diagram for the deactivation processes of BoPh and BoCb. CT indicates the charge transfer state.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] ORCID So-Yoen Kim: 0000-0001-9392-7195 Yang-Jin Cho: 0000-0003-3963-0694 Ho-Jin Son: 0000-0003-2069-1235 Dae Won Cho: 0000-0002-4785-069x Sang Ook Kang: 0000-0002-3911-7818 Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A6A1030732 and NRF-2017R1D1A3B03033085). The research was supported by the International Science and Business Belt Program through the Ministry of Science and ICT (2017K000494).


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■ ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Photophysical parameters (Table S1) and the transient absorption spectra (Figure S1) (PDF)

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85x47mm (150 x 150 DPI)


Photoinduced Electron Transfer in a BODIPY-ortho-Carborane Dyad

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