Spin Orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in

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Spin Orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in Bodipy-Phenoxazine Dyads: Effect of the Chromophore Orientation and Conformation Restriction on the Photophysical Properties Yu Dong, Andrei A. Sukhanov, Jianzhang Zhao, Ayhan Elmali, Xiaolian Li, Bernhard Dick, Ahmet Karatay, and Violeta K. Voronkova J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06170 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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The Journal of Physical Chemistry

Spin Orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in Bodipy-Phenoxazine Dyads: Effect of the Chromophore Orientation and Conformation Restriction on the Photophysical Properties Yu Dong †§ Andrey A. Sukhanov,¶§ Jianzhang Zhao *,†,‡ Ayhan Elmali, Xiaolian Li,†,* Bernhard Dick, *, Ahmet Karatay*, and Violeta K. Voronkova*,¶ †

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling Gong Road, Dalian 116024, P. R. China. E-mail: [email protected] (J. Z.); [email protected] (X. L.)

¶ Zavoisky

Physical-Technical Institute, FRC Kazan Scientific Center of Russian Academy of Sciences, Kazan 420029, Russia. E-mail: [email protected]

‡

School of Chemistry and Chemical Engineering, and Key Laboratory of Energy Materials Chemistry, Institute of Applied Chemistry, Xinjiang University, Urumqi 830046, China.



Lehrstuhl für Physikalische Chemie, Institut für Physikalische und Theoretische Chemie, Universität Regensburg, Universitätsstr. 31, D-93053 Regensburg, Germany. E-mail: [email protected]  Department

of Engineering Physics, Faculty of Engineering, Ankara University, 06100

Beşevler, Ankara, Turkey. Email: [email protected]

RECEIVED DATE (automatically inserted by publisher) §

Y. D. and A. A. S. contributed equally to this work.

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Abstract: The spin orbit charge transfer-induced intersystem crossing (SOCT-ISC) in BodipyPhenoxazine (BDP-PXZ) compact electron donor/acceptor dyads was studied. PXZ is electron donor and BDP is the electron acceptor. The molecular geometry is varied by applying different steric hindrance on the rotation about the linker between the two subunits. Charge transfer (CT) absorption bands were observed for the dyads with more coplanar geometry (electronic coupling matrix elements is up to 2580 cm1). Ultrafast charge separation (CS, 0.4 ps) and slow charge recombination (CR, 3.8 ns. i.e. SOCT-ISC process) were observed. Efficient ISC (T = 54%) and long triplet state lifetime (T = 539 s) were observed for the dyads. Notably the triplet state lifetime is 2-fold of that accessed with heavy atom effect, indicating the advantage of using a heavy atom-free photosensitizer. The low-lying CT state in the dyads was confirmed with intermolecular triplet photosensitizing method. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy show that the electron spin polarization (ESP) of the triplet state formed by the SOCT-ISC is the same to that of spin orbit-ISC (SO-ISC). 3CT and 3LE (localized excited state) states were simultaneously observed for one dyad, which is rare. Moreover, normally the CT state was observed as spin-correlated radical pair (SCRP). The dyads were used as triplet photosensitizers for triplet-triplet annihilation upconversion (TTA UC), the quantum yield is up to 12.3%. Large anti-Stokes shift (5905 cm1) was achieved by excitation into the CT absorption band, not the conventional LE absorption band.

1. INTRODUCTION Triplet photosensitizers are compounds showing strong absorption of UV or visible light, high intersystem crossing (ISC) quantum yield, and long-lived triplet excited state.15 These properties are beneficial for efficient photo-induced electron transfer and energy transfer. Triplet

2


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photosensitizers have been widely used in photocatalysis,610 photodynamic therapy,15,11 photovoltaics,11,12 and photon upconversion.4,1318 One of the pivotal issues for these areas is to design new and highly efficient triplet photosensitizers. However, the ISC is elusive for metal-free organic chromophores, and it is still a challenge to rationally design a heavy atom-free organic molecule showing efficient ISC.4,19 In general, the S1T1 ISC is a non-radiative process, a driving force rather than the electronic excitation must be applied for the electron spin flipping or rephrasing, and the force is a magnetic torque, not an electronic excitation based on this mechanism.20 The most popular method to enhanced ISC is the heavy atom effect. The atoms such as Pt, Ir, Ru, or Br and I are with large atomic mass, these atoms exert enormous attractions on the electrons approaching these atoms, then the magnetic torque generated by the orbital angular momentum will interact with the magnetic torque generated by electron spin, thus the ISC is facilitated.2025 The phosphorescent transition metal complexes are typical triplet photosensitizer based on this mechanism.2631 Another approach to enhance ISC in organic molecules is based on the n-*-* transition. In this case the electron exchange energy is small (J values), as such the S1/T1 state energy gap is small, thus the ISC is facilitated. Conservation of the angular momentum also contribute to this ISC mechanism. Aromatic carbonyl compounds are the representative triplet photosensitizers.20 Exciton coupling was also reported to be able to enhance ISC, but the examples are rare.32 Recently, the charge recombination (CR)-induced ISC has attracted much attention.3337 For instance, Verhoeven and Mataga reported charge recombination-induced ISC of D-A molecules (D stands for electron donor and A stands for electron acceptor).33,34,3843 Ziessel found that the pyridium moiety attached with boron dipyrromethene (boidpy) is able to induce ISC by charge separation/charge recombination.44 However, most of these electron donor/acceptor molecular

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hybrids are synthetically demanding and they show low triplet state quantum yields. Normally a long and rigid linker between the electron donor and acceptor is required, in order to reduce the electron–electron exchange energy (J values) of the charge separated state (CSS), thus to make the 1CT/3CT

state energy gap extremely small ( 1 cm1), to ensure the hyperfine coupling interaction

(HFI) to drive ISC.4547 These polyads are not ideal triplet photosensitizers, due to the synthetically-demanding molecular structures and the slow ISC yield. On the other hand, efficient ISC can be resulted, even with compact electron donor/acceptor dyads (with short linkers), given the molecular geometries are orthogonal for the electron donor and acceptor, the scenario is similar to the n-*-* systems, because the electron transfer (charge separation or recombination) will result in molecular orbital angular momentum change, which generates the magnetic torque for the electron spin flip or rephrasing, as such the ISC is enhanced.48,49 This is the so-called spin-orbit charge transfer ISC (SOCT-ISC). This approach is of particular interest for ISC. First, the synthesis of the molecules is feasible.20 Second, the SOCTISC is independent on heavy atom effect, thus metal-free organic triplet photosensitizers can be devised.48 Third, photo-induced charge separation and charge recombination is ubiquitous for photocatalysis and photovoltaics, thus it is also fundamentally important to investigate the fate of CR.33,34,36 However, much room is left for the investigation of SOCT-ISC. The chromophores used for SOCT-ISC are limited to acridine and anthracene.48 Visible light-harvesting chromophores, such as (BDP), were rarely used.5052 Electron donors are limited to aminophenyl,43,54 or naphthyl moieties.55 Different electron donor may offer different driving force for CS, and the resulted conformations of the dyads may be also different. All these factors may contribute to the different

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ISC efficiency of the dyads. However, the relationship between the molecular structure and the SOCT-ISC efficiency is not fully elucidated. In order to address the above challenges, herein we prepared a series of compact BDP-PXZ dyads. PXZ is selected as the electron donor. PXZ is widely used as electron donor in organic semiconductors,56 catalysts for photoredox organic reactions,57 and thermally activated delayed fluorescence (TADF) materials.58,59 The conformation restriction in the dyads is varied by using different steric hindrance for the torsion about the linker between the electron donor and acceptor, to study the relationship between the molecular structure/conformation and the SOCT-ISC efficiency of the dyads. The photophysical properties of the dyads were studied with steady state and time-resolved transient spectroscopies. Based on the different magnitude of the electronic coupling matrix elements (VDA), charge transfer absorption and emission bands were observed for some dyads. ISC of the dyads was found to be highly dependent on the solvent polarity, as well as molecular geometry and the spin density of the linker atoms. Nanosecond transient spectra confirmed the formation of triplet states with the dyads. The electron spin polarization (ESP) of the triplet state was studied with time-resolved electron paramagnetic resonance (TREPR), both 3

CT state and 3LE state were observed simultaneously.

2. EXPERIMENTAL SECTION 2.1. Materials and Equipment. All materials we used in chemical experiment are analytically pure. Solvents for synthesis were freshly dried over suitable drying agents before using. 1H and 13C

NMR spectra were recorded on a Bruker 400/500 MHz. HRMS (high resolution mass spectrum)

were recorded by MALDI-TOF-MS and ESI-MS spectrometer. Fluorescence were recorded on RF-5301PC spectrophotometer (Shimadzu Ltd, Japan). UVVis spectra were recorded on HP8453A UVVis spectrophotometer (Agilent Ltd, USA). Fluorescence lifetimes were recorded

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on OB920 luminescence lifetime spectrometer (Edinburgh Instrument Ltd, UK). The instrument response function (IRF. Time resolution) of the OB920 luminescence lifetime spectrometer is 100 ps; Repetition rates of the EPL picosecond laser are different for different lifetime ranges: 20 MHz for 50 ns, 10 MHz for 100 ns and 5 MHz for 200 ns; Count rate is 9 MHz (periodic). 2.2. Synthesis of BDP-PXZ-1. Compound 2 (534 mg, 2.0 mmol) and 2,4-dimethylpyrrole (0.4 mL, 4.0 mmol) were dissolved in dry DCM (100 mL). After the solution was degassed for 30 min by bubbling with N2, a few drops of trifluoroacetic acid were added to the solution, then the mixture was stirred at RT for 5 h. Then dicyano-5,6-dichlorobenzoquinone (DDQ) (0.4 g, 1.74 mmol) was added in one portion. Dry triethylamine (2 mL) was added after 0.5 h. Then the mixture was stirred for 15 min, BF3Et2O (2 mL) was added dropwise with ice bath cooling of the solution. After stirring at RT overnight, the mixture was extracted with DCM. The organic layers were combined, dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography (silica gel, PE/DCM = 2/1, v/v) to give BDPPXZ-1 as red solid (69 mg, yield: 6%). M.p. 243.6242.4 C. 1H NMR (400 MHz, CDCl3): δ = 6.846.80 (m, 1H), 6.696.62 (m, 3H), 6.546.51 (m, 3H), 5.98 (s, 2H), 3.52 (t, 2H, J = 8.0 Hz), 2.54 (s, 6H), 1.70 (t, 2H, J = 8.0 Hz), 1.68 (s, 6H), 1.511.44 (m, 2H), 1.03 (t, 3H, J = 6.0 Hz). 13C NMR (100 MHz, CDCl3): δ = 155.27, 145.69, 144.67, 143.07, 141.08, 134.08, 132.86, 131.69, 126.87, 123.97, 123.32, 121.26, 121.04, 115.52, 115.13, 111.56, 44.15, 29.71, 26.97, 20.15, 14.81, 14.59, 13.88. ESIHRMS: m/z [M+H]+ Calcd for C29H31BF2N3O+: m/z = 486.2529, found: m/z = 486.2527. 2.3. Synthesis of BDP-PXZ-2. Compound 2 (534 mg, 2.0 mmol) and pyrrole (15 ml) were dissolved in dry DCM (5 mL). After being degassed for 30 min, a few drops of trifluoroacetic acid were added to the solution, then the mixture was stirred at RT for 1 h. The mixture was extracted

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with DCM, the organic layers were combined, dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography (silica gel, PE/DCM = 10/1, v/v) to obtain the intermediate. 1,2,4,5-tetrachloro-benzoquinone (587 mg, 2.39 mmol) was added to the DCM solution of the intermediate. Dry triethylamine (2 mL) was added after 0.5 h. Then the mixture was stirred for 15 min, BF3·Et2O (2 mL) was added dropwise under ice bath cooling. After stirring at RT overnight, the mixture was extracted with DCM. The organic layers were combined, dried over anhydrous Na2SO4 and concentrated under reduced pressure. The crude product was purified by column chromatography (silica gel, PE/DCM = 2/1, v/v) to give BDP-PXZ-2 as green solid (300 mg, yield: 35%). M.p. 194.0196.1 C. 1H NMR (400 MHz, CDCl3): δ = 7.89 (s, 2H), 7.107.07 (m, 3H), 6.90 (d, 1H, J = 4.0 Hz), 6.84 (t, 1H, J = 8.0 Hz), 6.726.65 (m, 2H), 6.576.53 (m, 4H), 3.56 (s, 2H), 1.751.67 (m, 2H), 1.541.45 (m, 2H), 1.05 (t, 3H, J = 8.0 Hz).

13C

NMR (100 MHz, CDCl3): δ = 142.85, 130.64, 124.09, 117.99, 110.89,

20.17, 13.90. ESIHRMS: m/z [M+Na]+ Calcd for C25H22BF2N3ONa+: m/z = 452.1721, found: m/z = 452.1713. 2.4. Triplet State Quantum Yield (T). The triplet state quantum yields were determined by ground state bleaching method according to the equation 1, by using nanosecond transient absorption spectroscopy.

 ε  ΔΑ sam   Φsam  Φ std  std  ε ΔΑ std   sam 

(1)

In the equation 1, sample and standard are represented by “sam” and “std” respectively.  is the triplet quantum yield,  is the molar absorption coefficient determined by UVVis absorption spectrum and A is the optical intensity of the ground state bleaching band determined by

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nanosecond transient absorption spectroscopy. Optically matched solutions were used for the measurement. 2.5. Fluorescence Quantum Yield (F). The fluorescence quantum yields were determined by the equation 2.

Φsam

 Α  F  η   Φsam  std  sam  sam   Α sam  Fstd  ηstd 

2

(2)

Sample and standard are represented by “sam” and “std” respectively.  represents fluorescence quantum yield, A is the absorbance and should be in the range 0.020.03. F is the integral area of fluorescence emission.  is the refractive index of experimental solvent. BDP-2 was used as standard (F = 4.4%. In toluene). 2.6. Nanosecond Transient Absorption Spectra. The nanosecond transient absorption spectra were measured on LP980 laser flash photolysis spectrometer (Edinburgh Instruments, Ltd UK) and the signal was digitized with a Tektronix TDS 3012B oscilloscope. The data (kinetic decay curve and spectrum) were obtained with the L900 software. All samples were deaerated with N2 for ca. 15 min before measurement. 2.7. Time Resolved Electron Paramagnetic Resonance (TREPR) Spectroscopy. Samples were dissolved in isopropanol/toluene (1: 1, v/v) at 0.3 mM. The time-resolved continuous-wave (TR CW) EPR measurements were performed on an X-band EPR Elexsys E-580 spectrometer (Bruker) at 80 K. Samples were photoexcited by pulse laser at 532 nm with energy 10 mJ per pulse. The spectra were simulated using the EasySpin package based on the Matlab.60 2.8. DFT Calculations. The geometries of the molecules, spin density surfaces and potential energy surfaces were optimized using density functional theory (DFT) with B3LYP and cam-

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B3LYP functional and 6-31G(d) basis set. The excited state energy were calculated by TDDFT at the B3LYP/6-31G(d) and cam-B3LYP/6-31G(d) level, based on the excited state geometry. All these calculations were performed with Gaussian 09W.61

3. RESULTS AND DISCUSSIONS 3.1. Molecular Structure Designing Rationales. Phenoxazine (PXZ) is a heterocyclic aromatic electron donor, and it has been intensively used in (singlet) photosensitizers for photovoltaics.62 Previously phenothiazine (PTZ) was used for preparation of SOCT-ISC electron donor/acceptor dyad.51 The HOMO energy level of PXZ is 5.02 eV, whereas it is 4.98 eV for PTZ. The different electron donating ability of these two chromophore may exert different effect on the ISC efficiency of the dyads. The selection of BDP as the visible light-harvesting chromophore is due to its lack of spin-orbit coupling, as such the ISC observed for the dyads can be easily assigned to SOCT-ISC. The PXZ moiety is connected to the BDP moiety at either the meso-position (BDP-PXZ-1 and BDP-PXZ2) or the 2-position (BDP-PXZ-3 and BDP-PXZ-4) (Scheme 1). The PXZ at meso position will experience significant steric hindrance, especially from the 1,7-methyl groups on the BDP ring, for instance BDP-PXZ-1 vs. BDP-PXZ-2. As such BDP-PXZ-1 will adopt orthogonal geometry as compared to BDP-PXZ-2 (Scheme 1). The steric hindrance of the rotation in BDP-PXZ-3 is smaller than BDP-PXZ-1. On the other hand, the alignment of the dipole moments of the BDP and the PXZ moieties in BDP-PXZ-1 and BDP-PXZ-3 is different, which may exert different effect on the SOCT-ISC efficiency. BDP-PXZ-4 is a reference of BDP-PXZ-3, as well as that of BDP-PXZ-2. BDP-PXZ-5 and BDP-PXZ-6 are two reference dyads with an extra intervening phenyl ring between the electron donor and acceptor, thus the electronic coupling between the

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electron donor and acceptor becomes weaker. Moreover, the geometry of the PXZ and the BDP may be not orthogonal, thus the ISC efficiency may be affected. Scheme 1. Synthesis of the Compounds.

(a) n-C4H9Br, DMF, KOH, N2, stirred at RT for 6 h, yield: 60%; (b) POCl3, DMF, N2, 90 °C, 2 h, yield: 65%; (c) 2,4-dimethylpyrrole, TFA, DDQ, TEA, BF3Et2O, DCM, RT, N2, yield: 6%; (d)TFA, anhydrous CH2Cl2, rt, 2 h; TCQ, TEA, BF3Et2O, 0.5 h, yield: 35%; (e) NBS, THF, ice bath for 0.5 h, yield: 30%; (f) n-C4H9Br, DMF, KOH, N2, stirred at RT for 12 h, yield: 65%; (g) Pd(PPh3)4, KOAc, toluene, reflux for 2 h, yield: 52%; (h) NIS, DCM, ice bath for 0.5 h, yield: 52%; (i) Pd(PPh3)4, K2CO3, toluene/EtOH/H2O, N2, 4 h, yield: 88%; (j) ICl, DCM/MeOH, ice bath for 1 h, N2, yield: 63%; (k) Pd(PPh3)4, K2CO3, toluene/EtOH/H2O, N2, refluxed for 1 h, yield: 12%;

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(l) Pd(OAc)2, P(t-Bu)3, Cs2CO3, toluene, N2, refluxed for 12 h, yield: 66%; (m) similar to step (c), yield: 14%; (n) similar to step (d), yield: 22%.

Formyl moiety of boronic acid ester moiety was introduced to the PXZ unit,51,52 then the BDP unit was introduced by appropriate Suzuki-Miyaura coupling reactions (Scheme 1). All the compounds were obtained with moderate to satisfactory yields. The molecular structures were fully characterized and verified. 3.2. UVVis Absorption and Fluorescence Emission Spectra. The UVVis absorption of the compounds was studied (Figure 1). BDP-PXZ-1 shows similar absorption in visible region as compared to that of BDP-1 (Scheme 1) at 502 nm (Figure 1a), indicating that the electron coupling between the PXZ and the BDP moiety at ground state is negligible. However, an extra broad band in the region of 500600 nm was observed for BDP-PXZ-2, besides the normal absorption band of BDP at 497 nm. The broad absorption band is attributed to a charge transfer (CT) absorption band, i.e. the S01CT transition.63 This result indicates that the electronic coupling of PXZ and BDP moieties in BDP-PXZ-2 is more significant as compared to that of BDP-PXZ-1. This different coupling between the electron donor and acceptor is due to the different molecular geometries (co-planarity) of the two units in the dyads. For BDP-PXZ-3, the absorption band in visible region is broader than BDP-1, and it is slightly red-shifted (Figure 1b). This result shows that the electronic coupling between the electron donor and acceptor in BDP-PXZ-3 is more significant than that in BDP-PXZ-1. Thus the 1,3-methyl groups exert less steric hindrance on the rotation of the PXZ moiety about the CC linker than that of the 1,7-methyl groups in BDP-PXZ-1, and a more coplanar geometry is resulted. For BDPPXZ-4, the absorption band at 629 nm is attributed to the CT absorption band, and the absorption band of the BDP moiety is blue-shifted to 473 nm, indicating the electronic coupling between the

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PXZ and BDP moiety is significant in BDP-PXZ-4. The significant electronic coupling in BDPPXZ-4 is due to the weak steric hindrance of the rotation about the linker between PXZ and BDP moieties.

PXZ BDP-1 BDP-2 BDP-PXZ-1 BDP-PXZ-2

0.6

0.6

Absorbance

0.8

0.4 0.2 0.0

300

400

0.8

a

500

Wavelength / nm

1.0

b

0.4 0.2 0.0

600

BDP-PXZ-3 BDP-PXZ-4

300 400 500 600 700 800

c

BDP-PXZ-5 BDP-PXZ-6

0.8

Absorbance

1.0

Absorbance

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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0.6 0.4 0.2 0.0

Wavelength / nm

300

400

500

Wavelength / nm

600

Figure 1. UVVis absorption spectra of (a) PXZ, BDP-1, BDP-2, BDP-PXZ-1 and BDP-PXZ2; (b) BDP-PXZ-3 and BDP-PXZ-4; (c) BDP-PXZ-5 and BDP-PXZ-6. c =1.0  105 M in nhexane. 20 C. The UVVis absorption of BDP-PXZ-5 and BDP-PXZ-6 were also studied (Figure 1c). Both dyads show similar absorption profile in visible spectral region as compared to BDP-1, indicating the negligible electronic coupling between the PXZ and the BDP moieties. We attribute the weak coupling to the presence of the intervening phenyl rings between the electron donor and acceptor, thus the larger distance between the donor and acceptor. The electronic coupling matrix elements (VDA) for the electronic excitation processes of the dyads were calculated based on the equation 3: 64



VDA cm

-1



 2.06  10 2  CT  CT  CT  ε max νmax Δ ν1/2   R  





1/ 2

(3)

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Table 1. The Parameters of S0  1LE and S0  1CT Bands of Compounds

BDP-PXZ-1

BDP-PXZ-2

BDP-PXZ-3

BDP-PXZ-4

BDP-PXZ-5

BDP-PXZ-6

solvent

λabs a ( b)

g c

Mabs d

VDA e

VDA* f

HEX

502 (7.73)

6.6

i

i

i

TOL

504 (7.39)

i

i

0.46

HEX

497 (5.72) g/548 (1.47) h

3.2

0.24

0.70

TOL

499 (5.56) g/573 (1.25) h

3.7

0.27

0.78

HEX

516 (5.66)

4.1

0.26

0.90

TOL

517 (4.88)

3.4

0.24

0.75

HEX

473 (2.81) g/629 (1.85) h

5.4

0.29

0.65

TOL

480 (2.66) g/644 (1.50) h

4.9

0.32

i

HEX

501 (7.19)

i

i

i

TOL

504 (6.85)

i

i

0.46

HEX

501 (4.91)

i

i

0.36

TOL

506 (4.45)

i

i

i

8.4

3.7

4.1

6.2

8.0

= 1.0 × 105 M, in nm. b Molar absorption coefficient.  = 104 M1 cm1. c The dipole moment of optimized ground state geometry at the B3LYP/6-31G(d)/level using Gaussian 09W, in Debye (D). d The transition dipole moment of S0  1CT transition, in Debye (D). e The electronic coupling matrix element for excitation, in eV. f The electronic coupling matrix element between 1CT state and 1LE state, in eV. g Absorption of S0  1LE band. h Absorption of S0  1CT. i Not applicable. ac

where R is the separation of the center of electron donor and acceptor, in Å; εCT max is the molar absorption coefficient at the maximum of the CT absorption band, in [M1 cm1]; CT max is the 1 maximum absorption of the CT band, in cm1; ∆CT 1/2 is the peak width at half height, in cm .

Note R value was estimated as the distance between center of BDP and PXZ moieties. Based on the CT absorption bands of the dyads, the VDA values of BDP-PXZ-2, BDP-PXZ-3 and BDPPXZ-4 were calculated as 0.24 eV ~ 0.32 eV (1936 cm1~2580 cm1) (Table 1). This is in the strong interaction regime, concerning the electron transfer process.21,45,46 Thus we propose

13



Page 14 of 61

efficient photo-induced CS for the dyads showing CT absorption bands. For those dyads do not show any distinct CT absorption bands, however, we assume the VDA magnitude is still in the strong interaction regime concerning CS, because only a very small coupling is required for efficient CS, ca. 1.7 cm1,33 and note these dyads are compact electron donor/acceptor dyads and the VDA values will be larger than this threshold. This postulation is confirmed to some extent by the fluorescence emission property of the dyads (see later section). The fluorescence of the dyads was studied (Figure 2 and Supporting Information Figure S39). For BDP-PXZ-1, the emission peak in n-hexane is centered at 515 nm, the same as that of BDP1. However, the fluorescence quantum yield (F) is 15.5%, much lower than BDP-1 (F = 61.9%) (Supporting Information Table S1). In other solvents, the fluorescence was quenched further (Supporting Information, Table S1). In toluene a minor emission band at 517 nm and a more intense emission band at 638 nm were observed for BDP-PXZ-1, respectively (Figure 2a). The emission band at higher energy is assigned as the locally excited state (LE), whereas the emission at lower energy is attributed the charge transfer state emission (CT). In other solvents, however, only LE emission band at ca. 517 nm was observed. For BDP-PXZ-2, more obvious CT emission and weaker LE emission were observed (Figure 2b and Supporting Information, Figure S39b). For BDP-PXZ-2, the CT emission in n-hexane is centered at 604 nm, whereas the CT emission in toluene is centered at 710 nm. The observation of the CT emission bands for both BDP-PXZ-1 and BDP-PXZ-2 indicate that the electronic coupling is significant for both dyads at the CT states. This is different from the scenario of the ground state, no CT absorption band was observed for BDP-PXZ-1. CT emission bands were also observed for other dyads (Supporting Information, Figure S40). For instance, emission band at 675 nm was observed for BDP-PXZ-3 in toluene. For BDP-PXZ-

14


Page 15 of 61

4, the LE emission bands were observed at around 509 nm, whereas the CT emission bands were observed centered at 588 nm and 727 nm in THF and n-hexane, respectively.

1.0

1.2

a

Hexane Toluene DCM MeCN

0.8 0.6 0.4 0.2 0.0

500

600

700

800

Wavelength / nm

900

Normalized Intensity

1.2

Normalized Intensity

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

Hexane Toluene

b

0.8 0.6 0.4 0.2 0.0

500

600

700

800

Wavelength / nm

900

Figure 2. Normalized fluorescence emission spectra of the compounds in different solvents (optically matched solution were used, λex = 470 nm, A = 0.186). (a) BDP-PXZ-1 and (b) BDPPXZ-2. 20 C. For BDP-PXZ-5 and BDP-PXZ-6, interestingly, the CT emission bands at 650 nm were observed in toluene and n-hexane, respectively, besides the LE emission bands at 515 nm (Supporting Information, Figure S40). These results indicate that the electronic coupling between the electron donor and acceptor is strong at the excited state, even there are intervening phenyl rings between the electron donor and acceptor.





64π 4 n 3   2 3 2 kr  VDA μ ν / ΔE1  ν  3h

(4)

According to eq. (4), 38 the electronic coupling matrix elements at excited state (VDA*) can be obtained, where the kr is the radiative rate constant (in sl) of CT fluorescence, h is the Planck constants, n is the solvent refractive index, VDA* is the electronic coupling matrix element between 1CT

state and 1LE state, * is the transition dipole moment (in Debye) of the S01LE transition,

15



Page 16 of 61

 is the energy (in cm1) of CT fluorescence, E1 is the energy gap between the locally excited state and the ground state. Electronic coupling can be evaluated by the value of electronic coupling matrix element. Therefore, the values of VDA* are larger than VDA (Table 1), which indicate that the electronic coupling between excited state (1LE and 1CT) is stronger than that between ground state and 1CT. Weaker electronic coupling (small VDA values) will lead to slower electron transfer, the relationship between the two is described by the equation 5:34



2 2π 3 2VDA exp  λ  ΔG  4 λk BT k et  12 hλk BT  2



(5)

where the h and kB represent the Planck and Boltzmann constants, respectively. The VDA is the electronic matrix element which couples the reactant and product states; The G is the Gibbs free energy change of the electron transfer and  is the total reorganization energy required to distort the product state to the equilibrium geometry of the reactant state. Note the kCS is depend on the separation distance of the electron donor/acceptor (r), by equation 6 (given the electron transfer is superexchange mechanism, hopping mechanism is unlikely because phenyl moiety is a poor electron donor):65,66

k CS  k 0 e  β ( r  r0 )

(6)

Where k0 is the rate constant at the contact distance r0,  is the attenuation coefficient. Hence, for the D-bridge-A system, the smaller distance of donor/acceptor is favorable for charge separation. The fluorescence decay kinetics of both the LE and CT emission bands of the dyads were studied (Supporting Information Figure S42). For BDP-PXZ-1, the emission at 515 nm decay with biexponential feature, the lifetime contains two components of 0.4 ns (species rate: 61%) and 4.7 ns

16


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(39%), respectively. The slow decay is similar as pristine BDP (3.9 ns) and the fast decay is attributed to charge separation and back charge transfer.47 For BDP-PXZ-5 (Supporting Information Figure S42), at 515 nm, a clear biexponential feature was observed. The decay contains two components: fast decay 0.5 ns (60%) and slow decay 17.5 ns (40%). On the other hand, the decay of the emission at 650 nm is single exponential with lifetime of 16.6 ns. It should be pointed out that the slow decay component of the emission at 515 nm (LE emission) is with longer lifetime than the fluorescence lifetime of BDP-1 (3.9 ns). There must be an energy reservoir interacting with the LE state. The fast decay component at 515 nm is attributed to the fast 1LE1CT and

back charge transfer, whereas the slow decay component at 515 nm can be assigned

as the result of state mixing because the slow decay component is with the same time constant to that of the CT emission band (16.1 ns). Such state mixing or back electron transfer was rarely reported.47 For dyads BDP-PXZ-3 and BDP-PXZ-4, only CT emission was observed (Supporting Information Figure S42). For BDP-PXZ-3, the CT emission lifetime is 3.5 ns and it is a mono exponential decay. For BDP-PXZ-4, the lifetime of CT emission is short as 0.7 ns at 727 nm, indicating that the charge recombination is fast. The lifetime of pristine BDP-2 is 0.4 ns, due to the intramolecular rotation (IMR) and butterfly distortion of the excited state.67 The LE emission lifetime of BDP-PXZ-2 and BDP-PXZ-6 are 2.3 ns and 2.5 ns, respectively, which are longer than the lifetime of BDP-2 (0.4 ns). For BDP-PXZ-2 at 515 nm, biexponential decay curve was observed, which contains the two components of 0.4 ns (77%) and 4.0 ns (23%), respectively. The fast decay should be attributed to the charge transfer process and back charge transfer, the slow decay can be assigned to state mixing as discussed above. For the CT emission band centered at 604 nm, a mono exponential decay with 5.1 ns lifetime was observed. The similar phenomenon

17



Page 18 of 61

can be observed for dyad BDP-PXZ-6 (Supporting Information Figure S42). For the biexponential decay curve, one fast decay of 0.41 ns (68%) and a slow one of 3.9 ns (32%) were observed.

Figure 3. Temperature-dependent fluorescence emission spectra of BDP-PXZ-1 (a) fluorescence spectra, (b) decay kinetics at 517 nm (LE emission) and (c) decay kinetics at 630 nm (CT emission) in deaerated toluene. c = 1.0 × 10−5 M. The instrument response function (IRF. Time resolution) of the spectrometer is 100 ps; Repetition rates are 10 MHz for 100 ns time range. The temperature-dependent fluorescence emission spectra of compounds were studied (Figure 3). For BDP-PXZ-1, the CT emission band is red-shift and the intensity decreases at lower temperature, but LE emission intensity did not change (Figure 3a). The decreased fluorescence emission at lower temperature is anomalous, usually enhanced fluorescence emission should be observed at lower temperature. The emissive CT state is with twisted intramolecular charge transfer (TICT) feature,68 at lower temperature, the geometry is more orthogonal, the TICT state energy level became lower. The energy level of CT state is lower at lower temperature, which will enhance the non-radiative transition rate (energy gap law). Therefore, the lifetime of CT emissive state decrease from 5.3 ns (295 K) to 3.7 ns (220 K) along with decreasing temperature (Figure 3c). The fast decay component of the LE emission increased from 42% (295 K) to 79% (220 K), (Figure 3b). It may be attribute to the charge separation (CS) process is more efficient at lower

18


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temperature (accordingly the back charge transfer is more inefficient).47 For BDP-PXZ-2, similar results were observed (Supporting Information Figure S44). For the other two dyads (BDP-PXZ-5 and BDP-PXZ-6), the temperature-dependency of the fluorescence is different (Supporting Information, Figure S45). The general trend is that the LE emission band became stronger at lower temperature, while the CT emission bands became weaker and red-shifting of the CT band was observed. For dyad BDP-PXZ-5, the fast decay component also contributes more to total decay of LE emission, similarly as BDP-PXZ-1. The ratio of the fast decay component increased from 85% (295 K) to 98% (210 K). The charge spearation process is more efficient but back electron transfer will be inhibited at lower temperature, as a result the fast decay of the 1LE state is more significant at lower temperature.47 As for BDP-PXZ-6, similar results were obtained as that of BDP-PXZ-2 (Supporting Information, Figure S45f). But the CT emission band shows biexponential decay along with decreasing temperature, which is different from BDP-PXZ-2. One posssible reason is that the rate of back charge transfer process is slowed down under low temperature.47 Similar phenomenon was also obeserved for CT emission of BDP-PXZ-5 (Supporting Information, Figure S45c). Therefore for BDP-PXZ-5, when the temperature is 210 K, the fast component 0.6 ns (60%) and slow component 4.5 ns (40%) of decay can be attributed to back electron transfer and CT emission, respectively. The fluorescence excitation spectra of the dyads were studied (Supporting Information Figure S42). The CT emission was set as the monitored emission wavelength, as such the 1LE1CT efficiency can be evaluated by the matching of the excitation spectrum with the UVVis absorption spectrum. For BDP-PXZ-2, the fluorescence excitation spectrum is superimposable to the UVVis absorption spectrum, as such we propose the internal conversion 1LE1CT is efficient, in other

19



words, the CS is efficient upon excitation of the BDP moiety. Similar results were observed for BDP-PXZ-3 and BDP-PXZ-4. Based on the separation of the LE and the CT emission band, the energy gap, or the driving force for the CS, was estimated in the range of 0.35 eV ~ 0.65 eV. Previously for the PBI-PTZ dyads, it was estimated that a driving force of 0.12 eV is sufficient for an efficient photo-induced CS.68 Therefore, we propose that the CS in the dyads is efficient. The postulation is supported by the low fluorescence quantum yields of the dyads (Supporting Information Table S1). 3.3. Electrochemical Characterization, Gibbs Free Energy Changes of Charge Separation and the CSS Energy Levels. The electrochemical properties were studied with cyclic voltammetry (Figure 4 and Supporting Information Figure S60). For BDP-1, oxidation wave at +0.79 V was observed, as well as a reduction wave at 1.65 V. For BDP-2, only reversible reduction wave at 1.22 V was obtained. For the other moieties phenoxazine and N-

a

b

Fc+ / Fc

Fc+ / Fc

Current

Current

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

Page 20 of 61

N-butyl-PXZ BDP-1 BDP-PXZ-1

1

0

-1

Potential / V

-2

N-butyl-PXZ BDP-2 BDP-PXZ-2

1

0

-1

Potential / V

Figure 4. Cyclic voltammogram of the compounds. (a) N-butyl-PXZ, BDP-1 and BDP-PXZ-1, (b) N-butyl-PXZ, BDP-2 and BDP-PXZ-2. Ferrocene (Fc) was used as internal reference. In deaerated CH2Cl2 containing 0.10 M Bu4N[PF6] as supporting electrolyte. Scan rates: 50 mV/s. c = 1.0 × 10−3 M, 20 °C.

20


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butylphenoxazine, only one reversible oxidation wave was observed at around +0.22 V, indicating PXZ is electron donor. In comparison the oxidation potentials of PTZ is +0.18 V (vs. Fc/Fc+).51 For BDP-PXZ-1, we observed two oxidation waves at +0.90 V and +0.36 V and one reduction wave at 1.65 V, indicating that the BDP unit act as the electron acceptor and the phenoxazine as the electron donor. Similar results were also obtained for other compounds. The redox potentials of the compounds were listed in Table 2 (see also Supporting Information, Table S4). The intramolecular electron transfer process and the charge separation state energy were calculated with Weller equation (eqs 710). GCS is the Gibbs free-energy change of charge separation process, GCR is the Gibbs free-energy change of charge recombination process and GS is the static Coulombic energy. e is the electronic charge, EOX is the half-wave potential for one-electron oxidation of the electron-donor unit, ERED is the half-wave potential for one-electron reduction of the electron-acceptor unit, E00 is the energy level approximated with normalized UV−Vis absorption spectra and fluorescence emission spectra, S is the static dielectric constant of the solvent, RCC is the center-to-center separation distance between the electron donor and oneelectron oxidation of the electron-donor unit, ERED is the half-wave potential for one-electron reduction of the electron-acceptor unit, E00 is the energy level approximated with normalized GCS  eEOX  E RED   E00  GS

GS  

e2 4 S  0 RCC



1  1 1  e2  1      8 0  RD RA   REF  S 

ECS  eEOX  E RED   GS GCR  G CS  E00 

(7) (8) (9) (10)

21



Page 22 of 61

Table 2. Redox Potentials, Driving Forces of Charge Separation (GCS), Charge Recombination (GCR) and the Energy Level of the CSS of the Compounds in Different Solvents a E(ox) d E(red) d /V BDP-PXZ-1 b

/V

+0.90

ΔGCS (eV) HEX

TOL

ECSS (eV)

DCM

ACN

HEX TOL DCM ACN

1.65

0.30 0.40

0.67

0.74

2.14

2.04

1.77

1.70

+0.36 BDP-PXZ-2 c

+0.42

1.25

0.47 0.63

0.98

1.08

1.93

1.81

1.46

1.36

BDP-PXZ-3 b

+0.84

1.63

0.41 0.43

0.78

0.88

2.14

2.06

1.66

1.56

+0.22 BDP-PXZ-4 c

+0.21

1.20

0.54 0.71

1.16

1.29

1.86

1.65

1.22

1.11

BDP-PXZ-5 b

+0.88

1.60

0.10 0.24

0.64

0.74

2.34

2.18

1.80

1.69

1.18

0.37 0.54

1.00

1.12

2.02

1.86

1.39

1.27

+0.39 BDP-PXZ-6 c

+0.40

a Cyclic voltammetry in

N2 saturated DCM containing a 0.10 M Bu4NPF6 supporting electrolyte; Pt electrode was used as counter electrode; working electrode is glassy carbon electrode; Ag/AgCl couple as the reference electrode. E00 is the energy level approximated with the crossing point of UV−Vis absorption and fluorescence emission spectra after normalization at the singlet excited state. b E00 = 2.44 eV. c E00 = 2.40 eV. d The value was obtained by setting the oxidation potential of Fc+/Fc as 0.

UV−Vis absorption spectra and fluorescence emission spectra, S is the static dielectric constant of the solvent, RCC is the center-to-center separation distance between the electron donor and electron acceptor determined by optimized conformation by DFT calculation, RD is the radius of electron donor, RA is the radius of electron acceptor, REF is the static dielectric constant of the solvent used for the electrochemical studies, and 0 is the permittivity of free space.35 The results are reported in Table 2.

22


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The calculations about Gibbs free-energy change of electron transfer show that the charge separation process is thermodynamically allowed for all the target compounds in both non-polar and polar solvents, which is agreement with the fluorescence (fluorescence quenching of BDP unit). We can determine the charge separation state energy level decrease along with increasing the polarity of solvent, which is agreement with the shift of CT emission. The CSS state energy levels of BDP-PXZ dyads are lower than the reported BDP-PTZ dyads in non-polar solvents (for instance, lower by 0.28 eV in toluene), which infer the CS process is more efficient.51 The efficient CS process is one important factor for enhancing SOCT-ISC. 3.4. Nanosecond Transient Absorption Spectroscopy: Triplet State Properties. In order to study the triplet state properties of the dyads, the nanosecond transient absorption spectra of the dyads were investigated (Figure 5). For BDP-PXZ-1, upon pulsed laser excitation into the BDP moiety (495 nm), an intense ground state bleaching band (GSB) at 504 nm was observed (Figure 5a). Excited state absorption bands (ESA) in the region of 350 nm700 nm were observed (note it superimposes with the GSB), which are the typical feature of BDP T1 state.69 Thus we concluded that triplet state is populated for BDP-PXZ-1 upon photoexcitation. The triplet state quantum yield (T) was determined as 54%, and the apparent triplet state lifetime was determined as 130.2 s under the experimental conditions (from a simple mono-exponential fitting of the experimental curves). It should be pointed out that the apparent triplet state lifetime is underestimated given the intrinsic lifetime is long and the triplet state quantum yield is high, because of the triplet-triplet annihilation (TTA) effect. We obtained the intrinsic triplet state lifetime of 539 s by fitting the T at two different concentration with the kinetic model taking account of the TTA self-queching effect (see Supporting Information for the detal model),67 which is much longer than the apparent

23


0.1

0.0

0.0 309.3 s ... 10.0 s 0.0 s

-0.2

O.D.

0.1

-0.1

a

-0.3

300

-0.2 400

500

600

Wavelength / nm

700

Page 24 of 61

169.1 s ... 5.8 s 0.0 s

-0.1

b

300 400 500 600 700 800

Wavelength / nm

0.0

0.0

-0.1

-0.2

T = 539.0 s

OD

OD

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

O.D.


T = 164.5 s

-0.2

-0.4

-0.3

c -0.6

0

200 400 600 800 1000

Time / s

d 0

200 400 600 800 1000

Time / s

Figure 5. Nanosecond transient absorption spectra of BDP-PXZ-1 and BDP-PXZ-2. Transient absorption spectra (a) BDP-PXZ-1 in toluene, ex = 495 nm and (b) BDP-PXZ-2 in n-hexane, ex = 493 nm. Decay trace (c) BDP-PXZ-1 at 504 nm in toluene and (d) BDP-PXZ-2 at 497 nm in nhexane. The T values in (c) and (d) are the intrinsic lifetimes obtained by fitting the decay curves with the kinetic model with taking into account of the TTA quenching effect (see Supporting Information for the details).67 c = 1.0  105 M, 20 C. experimental value (130.2 s). In literatures, the fitting of the decay kinetics of the triplet photosensitizers was rarely with the TTA quenching effect considered. Similar results were observed for BDP-PXZ-2 (Figure 5b and 5d). The triplet state quantum yield was determined as 28%, which is half of BDP-PXZ-1. The apparent experimental triplet state lifetime of BDP-PXZ2 was determined as 51.0 s. Fitting with the TTA kinetic model gives the intrinsic triplet state

24


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lifetime of 164.5 s for BDP-PXZ-2. Long-lived triplet state lifetime is beneficial for the application of the photosensitizers in intermolecular electron transfer or energy transfer, for instance, TTA upconversion, photo-catalysis and photodynamic therapy (PDT). In comparison, the traditional heavy atom effect photosensitizers such as diiodo-BDP-1 and diiodo-BDP-2 show triplet quantum yield of 85% and 57% in toluene, respectively. For diiodo-BDP-1 (Scheme 1), the triplet state lifetime is 276 s (in toluene. Fitting results with the TTA model).67,70 For diiodoBDP-2 (Scheme 1), the triplet state lifetime is 126 s (in toluene. Fitting results with the TTA model).67 Our results show that much longer triplet state lifetimes were observed for heavy atomfree triplet photosensitizers, which is one important advantage for applying the photosensitizers in PDT studies. Triplet state quantum yield of BDP-PXZ-1 (54%) is lower than the previous reported BDPPTZ dyads (ca. 96%).51,52 We propose that SOCT-ISC efficiency is not only dependent on the light-harvesting chromophores, but also related to the electron donor such as PTZ, anthracene or PXZ. The matching of the excited state energy levels with CT state may be changed, thus the ISC efficiency may be varied. We also selectively excited into the CT absorption band of BDP-PXZ-2 at 560 nm (Supporting Information, Figure S48), the transient absorption spectral features are literally the same to that with excitation into the BDP absorption band at 493 nm. The triplet state quantum yield with excitation into the CT absorption band is 24% (approximated with the singlet oxygen quantum yield), similar to that by photoexcitation into the BDP absorption band (28%). These result indicated that photoexcitation into the CT band is also efficient for ISC. Triplet state formation was also observed for other dyads (Supporting Information, Figure S46S52).

25



Page 26 of 61

Interestingly, the singlet oxygen quantum yields of BDP-PXZ-3 and BDP-PXZ-4 are much lower (Table 3). Moreover, moderate triplet quantum yields were observed for BDP-PXZ-5 and BDP-PXZ-6 (23%  29%. Table 3), although the geometry of the two dyads is not orthogonal (see later section). Table 3. Triplet State Properties of the Compounds in Different Solvents  a (%)

T b (%)

T (s)

BDP-PXZ-1

11 c/42 d

24 c/54 d

424.0 c, e/539.0 d, e

BDP-PXZ-2

28 c/8 d

28 c/5.9 d

164.5 c, e/142.0 d, e

BDP-PXZ-3

2.0 c/4 d

3.0 c/1.0 d

310.0 c, e/391.7 d

BDP-PXZ-4

 c, f/2 d

 c, f/ d, f

c, f/ d, f

BDP-PXZ-5

1.0 c/39 d

0.7 c/27 d

495.0 c/597.7 d, e

BDP-PXZ-6

23 c/ d, f

25 c/ d, f

131.1 c, e/ d, f

a Single

oxygen quantum yield (∆), diiodo-BDP-1 as standard (∆ = 0.85 in toluene). b Triplet quantum yield, diiodo-BDP-1 as standard (T = 0.88 in toluene). c In n-hexane. d In toluene. e Intrinsic lifetime. f Not observed. It is noteworthy to mention that moderate triplet state quantum yields were observed for BDPPXZ-5 and BDP-PXZ-6 (Table 3), although there is an intervening phenyl moiety between the BDP and the PXZ moieties. The intervening phenyl moiety may induce a non-orthogonal geometry, thus poor ISC was expected for the dyads (see the later section). Indeed, anthryl-BDP dyad and 9phenyl-anthryl-BDP dyads with an intervening phenyl moiety was reported to show weak ISC ability.71,72 These results indicate that the structure of the electron donor also plays a significant role in determining the triplet state quantum of the compact electron donor-acceptor dyads. Such a phenomenon was not reported previously. The energy levels of CT state of BDP-PXZ-2 (1.45 and 1.36 eV) are lower than BDP 3LE state (ca. 1.65 eV) in polar solvents, such as DCM and acetonitrile (Table 2). Therefore, we expect that

26


Page 27 of 61

the long lived 3CT state can be accessed by triplet-triplet energy transfer (TTET) process (direct ISC of BDP-PXZ-2 in polar solvent is non-efficient). We carried out the study with Pt(II) octaethylporphine (PtOEP) as the triplet sensitizer (ET1 = 1.92 eV) and BDP-PXZ-2 act as triplet state energy acceptor (Figure 6).

0.010

0.1 0.05

0.0

-0.05 270.6 s ... 9.3 s 0.0 s

-0.10 -0.15 400

500

-0.1

600

700

c

b 0

100

200

300

Time / s

-0.010

400

0.010

OD

0.00 -0.05

400

500

600

T = 14.4 s

-0.2 -0.3

d

Wavelength / nm

-0.1

700

OD

0.0

135.6 s ... 4.3 s 0.0 s

0

100

200

300

400

Time / s

0.1

0.05

-0.15

0.000 -0.005

-0.4

Wavelength / nm

-0.10

T = 63.5 s

-0.2 -0.3

a

T = 53.8 s

0.005

OD

OD

O.D.

0.00

O.D.

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


-0.4

e 0

100

200

300

Time / s

400

1 = 10.6 s 2 = 58.1 s

0.005

0.000

-0.005

f 0

100

200

300

Time / s

400

Figure 6. Intermolecular triplet−triplet energy transfer (TTET) from PtOEP to BDP-PXZ-2. (a) Transient absorption spectra; (b) Decay at 380 nm; (c) Decay at 550 nm; c (PtOEP) =2.5  106 M. And (d) Transient absorption spectra; (e) Decay at 380 nm; (f) Decay at 550 nm; c (PtOEP) =2.5  106 M, c (BDP-PXZ-2) =1.0  105 M, ex = 530 nm. In deaerated DCM. 20 C. In DCM, in the absence of BDP-PXZ-2, the featured triplet state signal of PtOEP was observed (Figure 6a). However, in the presence of BDP-PXZ-2, a new long lived species was generated after excitation into the PtOEP absorption band at 550 nm (Figure 6d. Note excitation of BDP-

27



Page 28 of 61

PXZ-2 alone at this wavelength does not produce any long-lived signal). The triplet state lifetime of PtOEP was reduced significantly from 63.5 s to 14.4 s, which indicates efficient TTET process. Furthermore, new GSB and ESA bands were observed at 495 nm and 550 nm, respectively. The new ESA band at 550 nm show increasing phase with 10.6 s, which coincides with the decay of the triplet state of PtOEP (14.4 s), indicating the TTET process. A decay phase of the transient at 550 nm was observed with lifetime of 58.1 s (Figure 6f). Similar results were obtained in acetonitrile (Supporting Information, Figure S53). Interestingly, compound BDP-2 can also be sensitized by PtOEP, but no obvious new ESA band at 550 nm can be observed in both DCM and acetonitrile (Supporting Information, Figure S55). Therefore, the ESA peak at 550 nm for the PtOEP/ BDP-PXZ-2 is attributed to the BDP radical anion (BDP), which is in agreement with spectroelectrochemical studies (Supporting Information, Figure S62c) and supported by the species associated decay spectra (SADS) analysis (Supporting Information, Figure S54). Normally, the lifetime of CSS show stronger solvent polarity dependence than the LE triplet state. The decay at 550 nm is faster in acetonitrile than in that DCM (12.6 s vs. 58.1 s), which is due to the lower 3CSS energy levels in polar solvents. We also used 5,10,15,20-tetraphenylporphyrin (TPP) as triplet photosensitizer to probe the CSS. The triplet state energy levels of TPP and BDP-2 are ca. 1.45 eV and 1.69 eV, respectively. The triplet state of TPP was not quenched by BDP-2 (Supporting Information, Figure S56c and S57c). However, the energy levels of 3CSS of BDP-PXZ-2 is 1.45 eV (in DCM) and 1.36 eV (in ACN), respectively. The TPP triplet state signal were significantly quenched by BDP-PXZ-2 in both DCM and ACN, which demonstrates that the BDP-PXZ-2 will quench the TPP triplet state (Supporting Information, Figure S56 and S57), and the 3CSS of BDP-PXZ-2 will be formed.

28


Page 29 of 61

However, we didn’t observe ESA band at 550 nm, which may be due to the relative low TTET efficiency. 3.5. Femtosecond Transient Absorption Spectroscopy. Femtosecond transient absorption spectroscopy was used to study the excited state dynamics of BDP-PXZ dyads (Figure 7). We hope to observe ISC in toluene due to the efficient triplet state quantum yield (T = 54% in toluene).

a

0.1

0.1

BDP

BDP

-0.2 -0.3

-0.1 0.4 ps 36.5 ps 3.8 ns Long

-0.2 -0.3

500

600

700

d

BDP

0.1

0.0

0.02

500

600

700

800

0

Wavelength / nm

e

0.2

BDP

0.1

0.12 ps 0.16 ps 1.00 ps 6.16 ps 57.4 ps

-0.2 -0.3 -0.4

 O.D.

0.0

-0.1

400

0.04

0.00

400

800

Wavelength / nm

0.06

0.2 ps 7.6 ps

-0.2 -0.3

600

700

800

Wavelength / nm

400

2000

3000

Time / ps

f

-0.1

500 nm 565 nm Fit result

-0.2 -0.3

-0.4 500

1000

0.0

-0.1

 O.D.

400

575 nm 650 nm Fit result

0.08

-0.4

-0.4

c

0.10

 O.D.

0.17 ps 0.40 ps 2.00 ps 118 ps 2.64 ns

 O.D.

 O.D.

-0.1

0.1

b

0.0

0.0

 O.D.

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


500

600

700

800

Wavelength / nm

-0.4

0

5

10

15

Time / ps

20

Figure 7. Femtosecond transient absorption spectra of BDP-PXZ-1. (a) Transient absorption spectra in toluene. ex = 505 nm, c = 1.0  105 M, (b) Species-associated difference spectra (SADS) and (c) Decay traces at selected wavelengths. (d) Transient absorption spectra of BDP-PXZ-1 in ACN (ex = 496 nm, c = 1  105 M), (e) Species-associated difference spectra (SADS) and (f) Decay traces at selected wavelengths. SADS were obtained by global fitting in sequential model. 20 C.

29



Page 30 of 61

As the control experiment, the ultrafast spectra were also measured in acetonitrile (ACN), which only simple charge separation (CS) and charge recombination (CR) process were expected, because no triplet state was observed in acetonitrile with ns TA spectroscopy. In toluene, a bleaching band at 505 nm was observed upon fs laser excitation (Figure 7a), which is due to the depletion of the ground state. The ESA band at 575 nm is attributed to BDP radical anion (BDP), which was reported to absorb at ca. 580 nm,73,74 this assignment is also in agreement with the spectroelectrochemical results (Supporting Information, Figure S62a). A weak shoulder ESA band at 538 nm was observed, which is in agreement with the reported PXZ radical cation absorption (535 nm) and the spectroelectrochemical data (Supporting Information, Figure S62b).75 The instant appearance of this band indicated the ultrafast CS in the dyad BDP-PXZ-1. Species-associated difference spectra (SADS) were obtained by global fitting in the sequential model to analyze the CS, CR and ISC process (Figure 7b). The species with 0.4 ps lifetime is attributed to the singlet excited state of BDP. The species with lifetime of 36.5 ps may be the vibrational unrelaxed CSS. The species with lifetime of 3.8 ns is the vibronically relaxed CSS. In other words, the slow CR process takes about 3.8 ns and the final state with infinite lifetime is the 3BDP*

state formed by CR. The weak ESA band at 650 nm  680 nm is attributed to T1Tn ESA

signal, this peak increase slowly along with the decay of ESA band of CSS at 575 nm. Therefore, we can conclude that ISC from 1CSS to BDP triplet state is resulted from the CR process, i.e. by SOCT ISC. The final long lived species can be attributed to the 3BDP* triplet state. Note the characteristic absorption of triplet state of BDP-PXZ-1 is not very clear in the long lived species due to the low triplet state concentration in the monitored time scale, as a result of the slow CR/ISC process.

30


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The fs TA spectra of BDP-PXZ-1 were also measured in ACN (Figure 7d, e, f), which show faster CS and CR process than that in toluene. The ESA band was observed at 580 nm is due to the CSS state (Figure 7d). In ACN, the ESA band blue shifted from 580 nm (in toluene) to 553 nm, as a result of better solvation in ACN. According to SADS data we found that the CS process takes 0.2 ps and the CR process takes 7.6 ps (Figure 7e). The rapid decay of the BDP indicates the SOCT ISC is inhibited in polar solvent, which is in agreement with the negligible triplet state quantum yield of BDP-PXZ-1 in ACN. For BDP-PXZ-2, similar measurement was applied to study the ultrafast photophyscial process (Supporting Information Figure S58). In n-hexane, the GSB band was observed at 497 nm and 590 nm immediately upon excitation, which is in accord with UVVis spectrum (Supporting Information, Figure S58a). The BDP anion and PXZ cation absorption are both located at the range 550 nm  600 nm based on the spectroelectrochemical results (Supporting Information, Figure S62c and S62d). In the fs TA spectra, we observed only one ESA band centered at 610 nm, which is due to the overlap of GSB and CSS absorption bands. The ESA band firstly increases on ps time scale, then decrease on ns time scale, which represents CS and the CR process, respectively. According to the SADS, the first species can be assigned to S1 state and a 0.4 ps CS process was obtained (Supporting Information, Figure S58b). Similar to BDP-PXZ-1, the species with 39.2 ps lifetime can be attributed to the hot CSS. Subsequently, a slow process with a time constant of 1.2 ns occurs along with the ESA band of 610 nm shifting to 650 nm, which is due to CR / ISC. Long lived T1Tn ESA signal of BDP-PXZ-2 will be generated by CR/ISC process, which show similar pattern of ns TA data (Supporting Information, Figure S58b and Figure S48a). In acetonitrile, both GSB at 495 nm and 610 nm can be observed and the ESA at 550 nm became stronger in 0.3 ps. The SADS data show two species (Supporting Information, Figure

31



Page 32 of 61

S58e). The species with 0.1 ps lifetime can be attributed to the characteristic signal of S1 state. Therefore, the CS takes less than 0.1 ps, which is faster than the CS in n-hexane. The CR is with a time constant of 2.1 ps. No triplet state formation was observed for BDP-PXZ-2 in ACN, which is in agreement with the ns TA spectral study. For compounds BDP-PXZ-3 and BDP-PXZ-4 which show weak ISC, only CS and CR process were observed (Supporting Information Figure S59). For BDP-PXZ-3, the 0.8 ps CS process was obtained in n-hexane. Consequently, the CR process of 2.9 ns was determined, no triplet state signal was observed (Supporting Information Figure S59b). For BDP-PXZ-4, CS and CR are with time constants of 0.4 ps and 62 ps in toluene, respectively (Supporting Information, Figure S59e). No ISC was observed. For BDP-PXZ-5 and BDP-PXZ-6 with intervening phenyl moiety between the electron donor and acceptor, different properties were observed (Supporting Information, Figure S60). For BDPPXZ-5 (in toluene), the ESA peak at 550 nm were observed, which is due to the overlapped BDP anion and PXZ cation absorption (Supporting Information, Figure S60a, S63a and S63b). The CS takes ca.14 ps, which is much slower than the dyads without the intervening phenyl group between the donor and acceptor (0.4 ps). The reason may be that the intervening phenyl moiety will reduce the electron coupling magnitude between the electron donor and acceptor. The CR is also very slow and it takes dozens of ns based on SADS, which indicate the ISC rate is slow. To be noted, the CR rate obtained from fs TA is not very accurate due to the detection time window (3.5 ns). Consequently, we estimate the CR/ISC dynamics as 10 ns  20 ns combining the CT emission lifetime (Supporting Information, Figure S42e). Due to the slow CR, the ISC can be considered inefficient in the time range, therefore no triplet state signal can be observed in the time range.

32


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For BDP-PXZ-6, the ESA band at 530 nm and 600 nm can be observed, which can be assigned to the absorption of PXZ cation and BDP anion, respectively. The CS takes 0.5 ps, the possible reason may be the stronger electron coupling than BDP-PXZ-5. Subsequently, slow CR process takes around 7.3 ns, but no ISC was observed (Supporting Information, Figure S60e). 3.6. Time-Resolved Electron Paramagnetic Resonance Spectroscopy (TREPR): Confirmation of the SOCT-ISC Mechanism. TREPR is a powerful tool to study the ISC mechanisms, for instance by the electron spin polarization (ESP) of the triplet excited states. Different ISC mechanisms, such as SO-ISC, SOCT-ISC or radical pair-intersystem crossing (RPISC), usually give different ESP patterns.46,48,76 Therefore, the ISC mechanism of dyads can be verified by TREPR.

Z' X A

Y Y'

X' Z

Ms = 2

E

diiodo-BDP-1

A E

100

diiodo-BDP-2

200

300

400

B / mT

500

Figure 8. Experimental TREPR spectra (black lines) and simulations (red lines) of the compounds diiodo-BDP-1 and diiodo-BDP-2. Samples were excited by pulsed laser at 532 nm with energy of 10 mJ per pulse. Solvent: isopropanol/toluene (1:1, v/v). c = 3.0 × 104 M. 80 K. The canonical orientations of each transition are presented.

33



Page 34 of 61

The experimental TREPR spectra of diiodo-BDP-1 are presented in Figure 8 along with the simulations. Triplet state signals are typical of those of randomy orientated triplet state and were obtained with ESP pattern as (e, e, e, a, a, a) (a and e represents for ehanced absorptive and emissive polarizations, respectively). The half-field signal (ΔMS = 2) at 160 mT for diiodo-BDP1 is the typical feature of the triplet state TREPR spectrum. The half-field signal was also observed for other systems studied herein, but the intensity is low and it is not shown in Figure 9. The zero field splitting (ZFS) parameters of D and E of diiodo-BDP-1 were determined as 103.5 mT and 22.5 mT, respectively. Note the sign of D is negeative, which is differen from most of the organic chromophores with oblate shape of spin density distributions of the triplet state.77,78 The ZFS E parameter can not be determined with TREPR at the temperature applied. Simulation indicate that the sublevel population rates of the sublevels of the triplet state of diiodo-BDP-1 is Px : Py : Pz = 0.0 : 0.22 : 0.78 (Table 4). For diiodo-BDP-2 (Figure 8), similar ESP was observed, i.e. (e, e, e, a, a, a). A half-field peak was observed at 145 mT. These features are same to that of diiodo-BDP-1. Interestingly, the simulation indicates that the D parameter of diiodo-BDP-2 is 156.3 mT, which is 1.5-fold of diiodo-BDP-1. The E parameter (36.4 mT) is also drasically different from that of diiodo-BDP1 (22.5 mT, Table 4). These parameters indicate that the triplet state wavefunction confinement of diiodo-BDP-2 is drastically different from that of diiodo-BDP-1. The increase in the D parameter indicates a decrease in the distance between the spin unpaired electrons, that is, the more confinement of the triplet state wavefunction. The spectra of BDP-PXZ-1 (Figure 9a) is also a typical feature of randomly oriented triplet state and show an ESP pattern as (e, e, e, a, a, a). We assume the SOCT-ISC is dominant for the dyad due to the absence of heavy atom of BDP-PXZ-1. This postulation was supported by the

34


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solvent polarity dependency of the singlet oxygen quantum yields (Table 2). It’s unlikely dominated by RP-ISC mechanism because of the very close proximity of the electron donor and acceptor. Furthermore, the ESP phase feature of triplet state formed via RP-ISC (a, e, e, a, a, e) or (e, a, a, e, e, a) was not observed.77 Therefore, the RP-ISC can be excluded. This is an interesting example that the ESP of the triplet state formed by SOCT-ISC mechanism is the same to that of the SO-ISC. Previously it was proposed that different ESP should be observed for the SOCT-ISC and the SO-ISC mechanisms.48

Z' X

Y

Y'

X' Z

Z' X

Y

Y'

X' Z

A

A

E A

E

E A

a 250

300

350

B / mT

400

450

b

E

250

300

350

B / mT

400

450

Figure 9. Experimental TREPR spectra (black lines) and simulations (red lines) of the dyads (a) BDP-PXZ-1 and (b) BDP-PXZ-2. The blue line and magenta line in (b) represent BDP-like triplet state and 3CT state, respectively. Samples were excited by pulsed laser at 532 nm with energy 10 mJ per pulse. Solvent: isopropanol and toluene (1:1, v/v). c = 3.0 × 103 M. 80 K. The TREPR spectra of BDP-PXZ-2 are presented in Figure 9b. The spectrum is not a typical triplet state. The ESP pattern is (e, e, a, e, a, e, a, e, a, a), which is much different from the TREPR spectra of BDP-PXZ-1 and the two diiodo-BDP compounds. Satisfactory simulation requires two components (Figure 9b, the blue and magenta lines). The fitting parameters are presented in Table

35



Page 36 of 61

4. The species with D value of 82.1 mT and E value of 16.9 mT can be attributed to BDP LE triplet state. The ESP patterns are the same as BDP-PXZ-1 (e, e, e, a, a, a). Another species with D value as 37.5 mT and E value as 8.2 mT can be attributed to a 3CT state. In this case, two electrons in the frontier molecular orbits are located at electron donor PXZ and electron acceptor BDP moiety, respectively, which results in larger distence of the two electrons than that in LE state, thus the magnetic dipolar interaction between the two eletrons are weaker, as a result, the D value of the 3CT state is much smaller than the 3LE state. The distance between the unparied electrons was estimated as 4.2 Å based on D value with point-dipole approximation, which is close Table 4. ZFS Parameters (D and |E|) and Relative Population Rates Px,y,z of the Zero-Field Spin States of Compounds a compounds

D (mT)

|E| (mT)

Px

Py

Pz

diiodo-BDP-1

103.5

22.5

0.00

0.22

0.78

diiodo-BDP-2

156.3

36.4

0.00

0.22

0.78

BDP-PXZ-1

85.6

16.9

0.00

0.09

0.91

Spec-1 82.1

19.1

0.00

0.10

0.90

Spec-2 37.5

8.2

0.00

1.00

0.00

Spec-1 153.4

35.7

0.00

0.37

0.63

Spec-2 53.5

10.8

0.17

0.83

0.00

83.9

19.6

0.17

0.00

0.83

BDP-PXZ-2

BDP-PXZ-5

BDP-PXZ-6 a All

zero-field populations are normalized populations as Px + Py + Pz = 1. b Sign of D < 0 was

used by analogy to diiodo-BDP-1 and BDP-AN dyads.78,79

36


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to the DFT optimazation results (3.5 Å for the centroid-to-centroid distance between the donor and acceptor). Furthermore, the zero-field populations of the LE triplet state is with population rate of Px : Py : Pz = 0.00 : 0.10 : 0.90, indicating the overpopulation of the Ty and Tz state (Table 4). However, for 3CT

state, assuming the D sign has not changed the overpopulation is on Ty state (Px : Py : Pz =

0.00 : 1.00 : 0.00), which is drastically different from the 3LE state (Table 4). It should be noted that species 2 is not spin correalted radical pair (SCRP), otherwise the width of the spectrum should be much smaller.80 Previously long-lived CT states were reported, but it was rarely confirmed as a 3CT state.34 In some cases SCRP was proposed with TREPR characterization.80 We attribute the observation of 3CT state, not SCRP, to the short bridge between the donor and acceptor in BDPPXZ-2, which results in strong spin-spin exchange for the two electrons, thus 3CT becomes an eigenstate. Weak spin spin exchange interaction for the dyads with long bridge will give SCRP.80 The electron coupling is strong for BDP-PXZ-2, which is unlikely to generate SCRP. For the two dyads with an extra intervening phenyl ring (BDP-PXZ-5 and BDP-PXZ-6 Supporting Information, Figure S64), the ESP pattens is similar to that of BDP-PXZ-1 and the LE state of BDP-PXZ-2. For BDP-PXZ-6, one heavy atom-free dyad, ESP pattern (e, e, e, a, a, a) indicates the SOCT-ISC mechanism is dominant. The population rate is (Px : Py : Pz = 0.17 : 0.00 : 0.83), which is different from diiodo-BDP-2, also prove that the ISC mechanism is unlikely SOISC (Table 4). For BDP-PXZ-5, very weak signals were obtained due to the weak ISC ability in non-ploar solvent. 3.7. DFT Computations: Ground State Geometry, Potential Energy Surfaces and Spin Density Surfaces. The ground state geometries of the dyads were optimized (Figure 10 and Supporting Information, Figure S74). For BDP-PXZ-1, the dihedral angle between the BDP

37



Page 38 of 61

moiety and the PXZ moiety is 85.6, very close to orthogonal geometry. This is due to the steric hindrance exerted by the methyl groups on the BDP moiety. For BDP-PXZ-2, which is without the methyl groups, however, the dihedral angle is 49.6, which is deviated from orthogonal geometry to large extent. As such the electronic coupling between the electron donor/acceptor moieties in BDP-PXZ-2 is expected to be larger than that of BDP-PXZ-1. This postulation is supported by the experimentally observed larger electronic coupling matrix elements of BDPPXZ-2 (0.27 eV). Orthogonal geometry is beneficial for SOCT-ISC,48 the triplet state quantum yield of BDP-PXZ-1 is 54% (in toluene), which is 2-fold higher than BDP-PXZ-2 (28% in toluene). Moreover, the geometry of the PXZ moiety is not a coplanar one, it takes a puckered geometry (dihedral angle 13), but the deviation from planarity is not significant as compared to that phenothiazine (dihedral angle 32).

85.6

BDP-PXZ-1

49.6

BDP-PXZ-2

19.7

BDP-PXZ-4

0.03

BDP-PXZ-5

Figure 10. Optimized conformations and the dihedral angles between the donor and acceptor of selected atoms of BDP-PXZ-1, BDP-PXZ-2, BDP-PXZ-4 and BDP-PXZ-5. Calculated by DFT at the B3LYP/6-31G(d) level with Gaussian 09W, based on the optimized ground state geometry.

38


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BDP-PXZ-3 adopts a dihedral angle of 52.9 between the BDP and the PXZ moieties, which is much smaller than the 85.6 of BDP-PXZ-1. This result indicates that the methyl groups at 1,7position in BDP-PXZ-1 exert more steric hindrance than the methyl groups at 1,3-position on BDP moiety in BDP-PXZ-3.52 The reason may be due to the different protruding angle of the methyl groups against the CC linker between the BDP and the PXZ moieties. Interestingly, although the two dihedral angle is similar to that of BDP-PXZ-2, the triplet state quantum yield of BDP-PXZ3 is much lower (Table 3). This result indicates that the geometry conformation is not exclusively sufficient for efficient SOCT-ISC, the orientation of the dipole moment of the electron donor/acceptor may also play a role. BDP-PXZ-4 shows a dihedral angle of 19.7 between the BDP and the PXZ moieties, indicating a nearly coplanar geometry, as such large electronic coupling can be expected for BDP-PXZ-4 as compared to that of BDP-PXZ-3. The UVVis absorption features of the dyads agree well with these calculation results (Figure 1). The triplet state quantum yields of BDP-PXZ-3 and BDP-PXZ-4 are much lower than that of BDP-PXZ-1 and BDP-PXZ-2. For BDP-PXZ-5, with an intervening phenyl linker between the BDP and the PXZ moieties, the PXZ and BDP planes take a 0.03 dihedral angle, indicating a coplanar geometry for the two chromophores. In principle, this is not beneficial for the SOCT-ISC. However, significant triplet state production was observed (T = 27%, Table 3). This result shows that not a conventional SOCT-ISC mechanism is responsible for the ISC of BDP-PXZ-5. Similar results were observed for BDP-PXZ-6 (T = 25%, Table 3). The potential energy curves of the dyads against the torsion angle between the electron donor and acceptor are present in Figure 11. For BDP-PXZ-1, the thermally accessible conformations are with torsion angles in the range of 70112 (Figure 11a), i.e. the molecules adopt an

39



orthogonal geometry to large extent. For BDP-PXZ-2, interestingly, the energy-minimized geometries are with torsion angles of 55 and 130 (Figure 11a), and the thermally accessible geometries are mostly restricted to these ranges (around 55 and 130 respectively). These conformations are deviated from the orthogonal to large extent, which is different from BDP-PXZ1. These different geometries may contribute to the higher triplet state quantum yield of BDPPXZ-1 (Table 3).

0.3

a

1.5

b

BDP-PXZ-1 BDP-PXZ-2

0.5

0.2

0.1 RT 0.0

RT 0.0 0

50

100

150

Dihedral angle / °

c

BDP-PXZ-3 BDP-PXZ-4

Energy / eV

Energy / eV

1.0

Energy / eV

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

Page 40 of 61

BDP-PXZ-5 BDP-PXZ-6

1.0

0.5

RT 0.0 0

50

100

150

Dihedral angle / °

0

50

100

150

Dihedral angle / °

Figure 11. Potential energy surfaces of the ground states established by the rotation about the CC linkers in (a) BDP-PXZ-1 and BDP-PXZ-2, (b) BDP-PXZ-3 and BDP-PXZ-4, (c) BDP-PXZ-5 and BDP-PXZ-6. The dotted lines show the thermal energy at room temperature (kBT = 0.026 eV). Calculated with DFT theory at the B3LYP/6-31G(d) level with Gaussian 09W. The PES of BDP-PXZ-3 is similar to that of BDP-PXZ-2, the energy-minimized geometries are with torsion angles in the ranges of 3780 and 101142, respectively. The triplet state quantum yield of BDP-PXZ-3 (3% in n-hexane) is lower than BDP-PXZ-2 (28% in n-hexane). These results indicate that the SOCT-ISC quantum yields are depended on the geometry, as well as the

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topological feature (the alignment of the electron donor/acceptor dipole moments, either parallel or perpendicular).

Figure 12. Selected frontier molecular orbitals and the energy levels (eV) of the dyads calculated by TDDFT at the B3LYP/6-31G(d) level with Gaussian 09W, based on the optimized ground state. The PES of BDP-PXZ-5 and BDP-PXZ-6 were also studied (Figure 11c). The PES of these two dyads were constructed against the rotation about the linkers, showed in Figure 10, at the mesoposition of the BDP moiety. The BDP-PXZ-5 conformations are thermally accessible at the

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torsion angles of 73107. For BDP-PXZ-6, the PES are similar as BDP-PXZ-2. The geometries with minimal potential energy are those with torsion angle of 4271 and 108136. The frontier molecular orbitals of the dyads are presented in Figure 12. For BDP-PXZ-1, the molecular orbitals are highly confined on the BDP or the PXZ moieties, no delocalization was observed. For instance, the HOMO orbit is exclusively localized on the PXZ moiety, the LUMO orbital is exclusively confined on BDP moiety. For BDP-PXZ-2, however, the geometry is no longer orthogonal, delocalization of the LUMO to PXZ was observed, indicating large electronic coupling between the two moieties. The frontier molecular orbitals of BDP-PXZ-3 and BDP-PXZ-4 are delocalized to some extent. For BDP-PXZ-5 and BDP-PXZ-6, the HOMO and LUMO are highly confined to BDP or the PXZ moieties, thus the coupling between the two moieties is weak.57 This is in agreement with the experimental results that no CT absorption bands were observed for the two dyads.

(a)

(b)

Figure 13. Isosurfaces of spin density of compounds at the optimized triplet state geometries. Calculation was performed at B3LYP/6-31G(d) level with Gaussian 09W. The numbers in the figure indicate the contribution of each atom to the total spin density. (a) BDP-PXZ-1 and (b) BDP-PXZ-2.

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The triplet state spin density surfaces are presented in Figure 13 (for other dyads, see Supporting Information Figure S76). The results show that the spin density surface distribution is dependent on the conformation, i.e. the electronic coupling between the electron donor and acceptor. For the weak electronic coupling dyads (BDP-PXZ-1 and BDP-PXZ-5), the unpaired electron is almost localized in BDP moiety. It agrees with the nanosecond transient absorption spectra, which shows typical features of BDP. As for dyad BDP-PXZ-6, minor delocalized spin density was obtained (Supporting Information Figure S74), therefore typical BDP ns TA spectra were observed. For dyads with stronger electronic coupling (BDP-PXZ-2 and BDP-PXZ-3), the spin density is delocalized, which is in agreement with TREPR analysis. To be mentioned, the magnitude of spin density in BDP moiety is almost the same as BDP-2 (Supporting Information, Figure S75) and BDP-PXZ-1 (Figure 13a), respectively. The result indicated that the unpaired electron should be still mainly localized on BDP moiety. For the dyads with efficient ISC, such as BDP-PXZ-1 and BDP-PXZ-2 (Table 3), the spin density of the connecting carbon is larger compared with the reference compounds (BDP-PXZ-3 and BDP-PXZ-4). The spin density value of the connecting carbon atom is 0.33 BÅ3 for BDPPXZ-1, but the value is only 0.09 BÅ3 for BDP-PXZ-3. Similar result can be obtained for BDPPXZ-2 (0.39 BÅ3) and BDP-PXZ-4 (0.10 BÅ3). Therefore, we propose that the SOCT-ISC efficiency is dependent on the spin density of the atoms at the connecting position of electron donor and BDP moiety. The spin density surfaces of radical anion and radical cation were also calculated (Supporting Information Figure S77). For dyad BDP-PXZ-1, the spin density of radical anion and radical cation is localized on BDP and PXZ moiety, respectively. The results indicate the BDP moiety act

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as the electron acceptor and PXZ as the electron donor. Similar conclusion can be obtained for dyad BDP-PXZ-2. 3.8. Application of the Dyads in TTA Upconversion: Larger Anti-Stokes Shift by Excitation into the CT Absorption Band. TTA upconversion system with heavy atom-free sensitizer is of particular interest.13,81 The dyads based on charge recombination induced ISC are used for TTA upconversion with perylene as the triplet energy acceptor (Figure 14). Especially for dyad BDPPXZ-1, it shows strong absorption of visible light ( = 7.4  104 M−1 cm−1 at 504 nm), efficient triplet state producing ability (54% in toluene) and long intrinsic triplet lifetime (539 s in toluene). Therefore the dyad was used for TTA UC with a 510 nm continuous wave (cw) laser as the excitation source. Perylene was used as the acceptor (Figure 14). For the BDP-PXZ-1 alone, emission band centered at 638 nm can be observed upon 510 nm cw laser excitation. Blue emission was observed at the range 400 nm490 nm upon addition of only 0.8 eq. acceptor, which can be attributed to the upconverted fluorescence of perylene. Significant upconversion was realized (UC = 12.3% in toluene). The upconverted fluorescence is visible to naked eye, which proves the efficient TTA upconversion (Figure 14c). The CIE coordinates of the emissions of the photosensitizer in absence or prescene of accopter is agreement with the photograph (Figure 14b). For other dyads BDPPXZ-5 and BDP-PXZ-6, TTA UC was also observed (Supporting Information, Figure. S69 and S70). Due to the low triplet quantum yield of the two ptotosensitizers (T = 27% in toluene for BDP-PXZ-5 and T = 25% in n-hexane for BDP-PXZ-6), the UC is 3.7% and 1.3% for the two dyads, respectively.

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Figure 14. TTA upconversion with BDP-PXZ-1 as the photosensitizer and perylene (Py) as the acceptor, ex = 510 nm. (a) Upconversion emission spectra. The asterisks indicate the scattered laser. (b) CIE diagram. (c) The photographs of BDP-PXZ-1 alone and the upconversion. (d) The photographs of upconversion solutions observed with band-pass filter (transparent in the range 380520 nm). Upconversion was performed upon excitation of the solution with 510 nm cw laser (power density: 53.2 mW/cm2). c (photosensitizer) = 1.0 × 10−5 M, c (acceptor) = 8.0 × 10−6 M, in toluene, 20 °C. BDP-PXZ-2 shows two absorption bands: LE absorption at 497 nm and CT absorption band at 543 nm (Figure 1). Interestingly, the TTA UC can be observed upon excitation at both the LE and the CT absorption bands. With exciting LE absorption band at 510 nm, obvious upconversion can be observed (Supporting Information, Figure S66) and the UC is 2.7% in n-hexane, even with low

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concentration of acceptor (0.4 eq. perylene). With exciting BDP-PXZ-2 at 589 nm (CT absorption band), it produces strong orange emission alone. In presence of perylene, upconverted fluorescence can be observed (Supporting Information, Figure S67). The UC is 3.2% in n-hexane. Furthermore, the anti-Stokes shift (5905 cm1) is larger by exciting the CT absorption band than that by excitation into the LE absorption band (3275 cm1), which is important for TTA UC. Finally, lifetime-based Stern−Volmer quenching curves and the kinetics of the delayed fluorescence of the TTA upconversion were studied, which can explain the TTET and TTA process of the efficient TTA upconversion (Table 5 and Supporting Information Figure S70S73). According to the delayed fluorescence studies (Supporting Information, Figure S73), we can observe that there was a growth phase before the decaying of the upconversion emission, which was attributed to the generation and accumulation of T1 state of perylene (12 s  23 s).51 The decay phase of the upconversion is the consumption of the acceptor’s triplet state through TTA (50 s  75 s). TTET process was studied by lifetime-based Stern−Volmer quenching curves (Supporting Information, Figure S68). Stern−Volmer quenching constants (KSV) were obtained, which demonstrated that the intermolecular TTET is efficient. Bimolecular quenching constants (kq) and quenching efficiency (fQ) were also calculated. Especially for dyad BDP-PXZ-1, the fQ was determined as 56.5% (Table 5). The photophysical processes of the dyads are summarized in Scheme 2. For both BDP-PXZ-1 and BDP-PXZ-2, photoexcitation leads to the population of the S1 state, then the CS gives the CSS. CSS is weakly emissive, thus the fluorescence of the dyads is quenched to large extent as compared to that of BDP. The CSS energy level is highly dependent on the solvent polarity. For some solvents, the CSS is with similar energy level to the Tn state and the ISC is enhanced, which

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may contribute to the solvent polarity-dependency of the ISC of the dyads. In highly polar solvents, the CSS is non-emissive. This result may be due to the fast CR, which gives the ground state. At the same time, the ISC yield becomes lower in polar solvents. For BDP-PXZ-1, the T is 24% in n-hexane and 54% in toluene. But no triplet state signal was observed in highly polar solvents such as DCM or acetonitrile. For BDP-PXZ-2, the highest T value is 28% in n-hexane. The energy levels of 1CSS are lower than the 3LE in polar solvents (DCM and acetonitrile), which indicate that the CR process from 1CSS to 3LE is inhibited. Therefore, the T value decease with increasing solvent polarity (T = 5.9% in toluene, T = 0.0% in DCM and acetonitrile). Table 5. Upconversion Related Parameters of the Dyads BDP-PXZ-1 a

BDP-PXZ-2 b

BDP-PXZ-5 a

BDP-PXZ-6 b

KSV c (105 M1)

7.3

15.9

10.6

13.1

kq d (109 M−1 s−1)

6.8

23.7

6.4

22.2

Rq e (10−10 m)

3.74

3.74

3.74

3.74

Dq f (10−6 cm2 s−1)

9.74

19.1

9.74

19.1

k0 g (1010 M−1 s−1)

1.24

23.7

1.23

23.8

fQ h (%)

56.5

99.0

52.3

93.1

UC i (%)

12.3

2.7

3.7

1.3

a

In deaerated toluene, 20 ºC. b In deaerated n-hexane, 20 ºC. c Stern−Volmer quenching constants, perylene was used as the triplet state quencher. d Bimolecular quenching constants. KSV = kqp. e The molecular radius of the quencher. f Diffusion coefficients of the energy acceptor. g Diffusion-controlled bimolecular quenching rating constants. h Quenching efficiency. i Upconversion quantum yields measured in n-hexane or toluene. Fluorescence quantum yield of diiodo-BDP-1 in acetonitrile (F = 2.7%) as standard (ex = 510 nm).

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For BDP-PXZ-3 and BDP-PXZ-4, we propose that ISC may occur between CSS and Tn state with similar energy levels. But due to the more planar structure, the ISC efficient is poor (3%). However, dyads BDP-PXZ-5 and BDP-PXZ-6 are without orthogonal configuration between donor and acceptor, but efficient ISC were observed. TREPR data provide clear evidence that the SOCT-ISC is dominant in the ISC process (no ESP pattern typical for RP-ISC was observed). Scheme 2. Photophysical Processes of (a) BDP-PXZ-1 and (b) BDP-PXZ-2 a [BDP-1PXZ*] 3.96 eV

[1BDP*-PXZ] 2.49 eV

CSS CR / ISC 3.8 ns

T3 BDP 3

2.04 eV

T2 3CSS

[1BDP*-PXZ]

2.49 eV 2.26 eV 1

CT

1.79 eV

T1 3BDP 1.65 eV

497 nm

CR

 = 4.9 ns

b

3.96 eV 2.63 eV

1

497 nm

[BDP-1PXZ*]

[BDP-3PXZ*]

CS 0.4 ps 2.14 eV 2.04 eV 1.77 eV 1.70 eV

a

DCM ACN

HEX TOL

T = 539 s

548 nm

[BDP-3PXZ*]

CS 0.4 ps

2.63 eV

1

2.14 eV 1.94 eV

CSS

2.34 eV

CR / ISC 1.2 ns

ISC

2.24 eV

3 T1 BDP

3

3

T =

164 s

CSS

TTET 530 nm

T = 58 s

Ground state

a

Energy level of 1CSS of BDP-PXZ-1 was obtained by electrochemical calculation; Energy level of 1CSS of BDP-PXZ-2 was approximated with the crossing point of UV−Vis absorption of CT band and fluorescence emission of CT state after normalization. Triplet state lifetime is the intrinsic lifetime, obtained by fitting with kinetic model with TTA effect considered.67

4. CONCLUSIONS In summary, we prepared a series of Bodipy-Phenoxazine (BDP-PXZ) compact electron donor/acceptor dyads. The aim is to study the relationship between the molecular geometry and the spin-orbit charge transfer induced intersystem crossing (SOCT-ISC) efficiency of the dyads. PXZ is the electron donor and BDP is the electron acceptor. The molecular geometry was restricted

48


[PtOEP]

1.92 eV

1.65 eV

 = 1.4 ns Ground state

[PtOEP]

T2 3BDP

1.45 eV 1.36 eV

CR  = 3.4 ns

1

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to be orthogonal, or more coplanar, by steric hindrance of the rotation about the linker between the electron donor and acceptor. Charge transfer (CT) absorption and fluorescence bands were observed for dyads with large electronic coupling between the electron donor and acceptor (electronic coupling matrix element, VDA, is in the range of 1936 ~ 2580 cm1). In general, orthogonal geometry results in smaller VDA values. We found the SOCT-ISC efficiency is dependent on solvent polarity, as well as molecular geometries (the mutual orientation of the electron donor/acceptor). With an intervening phenyl moiety between the PXZ and BDP moiety, moderate ISC was observed, which is different from the previously reported electron donor/acceptor dyads based on phenothiazine and Bodipy, which shows no SOCT-ISC. Femtosecond transient spectra show ultrafast charge separation (0.4 ps) and slow CR/ISC processes (1.2 ns  3.8 ns). With intervening phenyl group between the electron donor and acceptor, thus weaker electronic coupling between the two moieties, the CS and CR kinetics were both reduced (0.5 ps  14 ps for CS, 7.3 ns  ca. 20 ns for CR). The dyad with orthogonal orientation show highest ISC efficiency (T = 54%). Different dipole moment orientations result in different ISC ability (T = 54% and 3%, for dyads BDP-PXZ-1 and BDP-PXZ-3, respectively). Interestingly, the dyads with longer distance between donor and acceptor also show satisfactory ISC efficiency, which was rarely reported. Notably long triplet state lifetime (T = 539 s) were observed for the dyads, based on the fitting of the decay kinetics with a model with triplet-tripletannihilation (TTA) quenching effect considered. The triplet state lifetime is 2-fold of that accessed with heavy atom effect, indicating the advantage of using a heavy atom-free photosensitizer. Timeresolved electron paramagnetic resonance (TREPR) spectroscopy show that the electron spin polarization (ESP) of the triplet state formed by the SOCT-ISC is the same to that of SO-ISC. Charge transfer triplet state (3CT) and localized excited triplet states (3LE) were simultaneously

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observed for one dyad with TREPR, which is rare, previously the CT state was usually confirmed as spin correlated radical pair (SCRP), not a 3CT state. The dyads were used as triplet photosensitizer for triplet-triplet annihilation upconversion, upconversion quantum yields up to 12.3% was observed. For dyad BDP-PXZ-2 with large VDA values, the excitation into the CT band increase the anti-stokes shift of the upconversion to 5905 cm1 as compared to that with excitation into the LE band (3275 cm1). Our results are useful for designing efficient heavy atom-free triplet photosensitizers, and for study of the fundamental photochemistry of SOCT-ISC.

ACKNOWLEDGEMENT J. Zhao thanks the NSFC (21673031, 21761142005, 21911530095 and 21421005), the State Key Laboratory of Fine Chemicals (ZYTS201901) and Department of Education of the Xinjiang Uyghur Autonomous Region (TianShan Scholar Chair Professor), Violeta K. Voronkova thanks Zavoisky Physical-Technical Institute and FRC Kazan Scientific Center of RAS X. Li thanks NSFC (21576043), for financial supports. B. Dick thanks Dalian University of Technology for the Haitian Professorship support.

ASSOCIATED CONTENT

Supporting Information General experimental methods, molecular structure characterization and additional spectra. This material is available free of charge via the internet at http://pubs.acs.org.

 AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J. Z.)

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*E-mail: [email protected] (X. L.) *E-mail: [email protected] (B. D.) * E-mail: [email protected] (A. K.) * E-mail: [email protected] (V. K. V.) Notes The authors declare no competing financial interest.

 REFERENCES (1) Awuah, S. G.; You, Y. Boron Dipyrromethene (BODIPY)-Based Photosensitizers for Photodynamic Therapy. RSC Adv. 2012, 2, 1116911183. (2) Kamkaew, A.; Lim, S. H.; Lee, H. B.; Kiew, L. V.; Chung, L. Y.; Burgess, K. BODIPY Dyes in Photodynamic Therapy. Chem. Soc. Rev. 2013, 42, 7788. (3) Stacey, O. J.; Pope, S. J. A. New Avenues in the Design and Potential Application of Metal Complexes for Photodynamic Therapy. RSC Adv. 2013, 3, 2555025564. (4) Zhao, J.; Wu, W.; Sun, J.; Guo, S. Triplet Photosensitizers: from Molecular Design to Applications. Chem. Soc. Rev. 2013, 42, 53235351. (5) Xingshu, L.; Safacan, K.; Juyoung, Y.; U., A. E. Activatable Photosensitizers: Agents for Selective Photodynamic Therapy. Adv. Funct. Mater. 2017, 27, 1604053. (6) Shi, L.; Xia, W. Photoredox Functionalization of CH Bonds Adjacent to a Nitrogen Atom. Chem. Soc. Rev. 2012, 41, 76877697. (7) Ravelli, D.; Fagnoni, M.; Albini, A. Photoorganocatalysis. What for? Chem. Soc. Rev. 2013, 42, 97113. (8) Fukuzumi, S.; Ohkubo, K. Selective Photocatalytic Reactions with Organic Photocatalysts. Chem. Sci. 2013, 4, 561574.

51



Page 52 of 61

(9) Prasad, H. D.; Burkhard, K. The Photocatalyzed Meerwein Arylation: Classic Reaction of Aryl Diazonium Salts in a New Light. Angew. Chem. Int. Ed. 2013, 52, 47344743. (10) Hari, D. P.; Konig, B. Synthetic Applications of Eosin Y in Photoredox Catalysis. Chem. Commun. 2014, 50, 66886699. (11) Dai, F.; Zhan, H.; Liu, Q.; Fu, Y.; Li, J.; Wang, Q.; Xie, Z.; Wang, L.; Yang, F.; Wong, W. Platinum(II)-Bis(aryleneethynylene) Complexes for Solution-Processible Molecular Bulk Heterojunction Solar Cells. Chem.Eur. J. 2012, 18, 15021511. (12) Suzuki, S.; Matsumoto, Y.; Tsubamoto, M.; Sugimura, R.; Kozaki, M.; Kimoto, K.; Iwamura, M.; Nozaki, K.; Senju, N.; Uragami, C.; et al. Photoinduced Electron Transfer of Platinum(II) Bipyridine Diacetylides Linked by Triphenylamine- and NaphthaleneimideDerivatives and Their Application to Photoelectric Conversion Systems. Phys. Chem. Chem. Phys. 2013, 15, 80888094. (13) Zhao, J.; Ji, S.; Guo, H. Triplet-Triplet Annihilation Based Upconversion: from Triplet Sensitizers and Triplet Acceptors to Upconversion Quantum Yields. RSC Adv. 2011, 1, 937950. (14) Paola, C. Energy Up-Conversion by Low-Power Excitation: New Applications of an Old Concept. Chem.Eur. J. 2011, 17, 95609564. (15) Zhou, J.; Liu, Z.; Li, F. Upconversion Nanophosphors for Small-Animal Imaging. Chem. Soc. Rev. 2012, 41, 13231349. (16) Simon, Y. C.; Weder, C. Low-power Photon Upconversion Through Triplet-Triplet Annihilation in Polymers. J. Mater. Chem. 2012, 22, 2081720830. (17) Ye, C.; Zhou, L.; Wang, X.; Liang, Z. Photon Upconversion: from Two-Photon Absorption (TPA) to Triplet-Triplet Annihilation (TTA). Phys. Chem. Chem. Phys. 2016, 18, 1081810835.

52


Page 53 of 61 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


(18) Yanai, N.; Kimizuka, N. New Triplet Sensitization Routes for Photon Upconversion: Thermally Activated Delayed Fluorescence Molecules, Inorganic Nanocrystals, and Singlet-toTriplet Absorption. Acc. Chem. Res. 2017, 50, 24872495. (19) Zhao, J.; Xu, K.; Yang, W.; Wang, Z.; Zhong, F. The Triplet Excited State of Bodipy: Formation, Modulation and Application. Chem. Soc. Rev. 2015, 44, 89048939. (20) Turro, N. J.; Scaiano, J. C. Principles of Molecular Photochemistry: An Introduction; University Science Books: Sausalito, CA, 2009. (21) Gorman, A.; Killoran, J.; O'Shea, C.; Kenna, T.; Gallagher, W. M.; O'Shea, D. F. In Vitro Demonstration of the Heavy-Atom Effect for Photodynamic Therapy. J. Am. Chem. Soc. 2004, 126, 1061910631. (22) Yogo, T.; Urano, Y.; Ishitsuka, Y.; Maniwa, F.; Nagano, T. Highly Efficient and Photostable Photosensitizer Based on BODIPY Chromophore. J. Am. Chem. Soc. 2005, 127, 1216212163. (23) Sabatini, R. P.; McCormick, T. M.; Lazarides, T.; Wilson, K. C.; Eisenberg, R.; McCamant, D. W. Intersystem Crossing in Halogenated Bodipy Chromophores Used for Solar Hydrogen Production. J. Phys. Chem. Lett. 2011, 2, 223227. (24) Zhang, X.-F.; Yang, X. Singlet Oxygen Generation and Triplet Excited-State Spectra of Brominated BODIPY. J. Phys. Chem. B 2013, 117, 55335539. (25) Wong, W.-Y.; Ho, C.-L. Di-, Oligo- and Polymetallaynes: Syntheses, Photophysics, Structures and Applications. Coord. Chem. Rev. 2006, 250, 26272690. (26) Chou, P.-T.; Chi, Y.; Chung, M.-W.; Lin, C.-C. Harvesting Luminescence via Harnessing the Photophysical Properties of Transition Metal Complexes. Coord. Chem. Rev. 2011, 255, 26532665.

53



Page 54 of 61

(27) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti, F. Photochemistry and Photophysics of Coordination Compounds II. 2007, p 143203. (28) Zhao, Q.; Li, F.; Huang, C. Phosphorescent Chemosensors Based on Heavy-Metal Complexes. Chem. Soc. Rev. 2010, 39, 30073030. (29) You, Y.; Nam, W. Photofunctional Triplet Excited States of Cyclometalated Ir(III) Complexes: Beyond Electroluminescence. Chem. Soc. Rev. 2012, 41, 70617084. (30) Feng, Y.; Cheng, J.; Zhou, L.; Zhou, X.; Xiang, H. Ratiometric Optical Oxygen Sensing: a Review in Respect of Material Design. Analyst 2012, 137, 48854901. (31) Liu, Z.; He, W.; Guo, Z. Metal Coordination in Photoluminescent Sensing. Chem. Soc. Rev. 2013, 42, 15681600. (32) Ventura, B.; Marconi, G.; Broring, M.; Kruger, R.; Flamigni, L. Bis(BF2)-2,2’-Bidipyrrins, a Class of BODIPY Dyes with New Spectroscopic and Photophysical Properties. New J. Chem. 2009, 33, 428438. (33) Verhoeven J. W.; van Ramesdonk H. J.; Groeneveld M. M.; Benniston A. C.; Harriman A. Long-Lived Charge-Transfer States in Compact Donor-Acceptor Dyads. ChemPhysChem 2005, 6, 22512260. (34) Verhoeven, J. W. On the Role of Spin Correlation in the Formation, Decay, and Detection of Long-Lived, Intramolecular Charge-Transfer States. J. Photochem. Photobiol. C 2006, 7, 4060. (35) Raymond Z.; Ben D. A.; Dorota B. R.; Anthony H. Selective Triplet-State Formation during Charge Recombination in a Fullerene/Bodipy Molecular Dyad (Bodipy=Borondipyrromethene). Chem. - Eur. J. 2009, 15, 73827393. (36) Eric, V. Photoinduced Symmetry-Breaking Charge Separation. ChemPhyschem 2012, 13, 20012011.

54


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(37) KC Chandra B.; Lim Gary N.; Nesterov Vladimir N.; Karr Paul A.; D'Souza F. Phenothiazine-BODIPY-Fullerene Triads as Photosynthetic Reaction Center Models: Substitution and Solvent Polarity Effects on Photoinduced Charge Separation and Recombination. Chem.Eur. J. 2014, 20, 1710017112. (38) Verhoeven J. W.; Scherer T.; Wegewijs B.; Hermant R. M.; Jortner J.; Bixon M.; Depaemelaere S.; de Schryver F. C. Electronic Coupling in Inter- and Intramolecular DonorAcceptor Systems as Revealed by Their Solvent-Dependent Charge-Transfer Fluorescence. Recl. Trav. Chim. Pays-Bas. 1995, 114, 443448. (39) Benniston, A. C.; Harriman, A.; Li, P.; Rostron, J. P.; van Ramesdonk, H. J.; Groeneveld, M. M.; Zhang, H.; Verhoeven, J. W. Charge Shift and Triplet State Formation in the 9-Mesityl10-Methylacridinium Cation. J. Am. Chem. Soc. 2005, 127, 1605416064. (40) Benniston, A. C.; Harriman, A.; Verhoeven, J. W. Comment: Electron-Transfer Reactions in the 9-Mesityl-10-Methylacridinium Ion: Impurities, Triplet States and Infinitely Long-Lived Charge-Shift States? Phys. Chem. Chem. Phys. 2008, 10, 51565158. (41) Okada, T.; Karaki, I.; Matsuzawa, E.; Mataga, N.; Sakata, Y.; Misumi, S. Ultrafast Intersystem Crossing in Some Intramolecular Heteroexcimers. J. Phys. Chem. 1981, 85, 39573960. (42) Miyasaki, H.; Kiri, M.; Morita, K.; Mataga, N.; Tanimoto, Y. Picosecond Laser Photolysis Studies on the Intramolecular Photoreduction Process of Benzophenone by Diphenylamine Linked by Spacers in Acetonitrile Solution. Chem. Phys. Lett. 1992, 199, 2128. (43) Hiroshi, M.; Manabu, K.; Kazuhiro, M.; Noboru, M.; Yoshifumi, T. Picosecond Laser Photolysis Studies on Chain-Length, Solvent and Temperature Dependences of the Intramolecular

55



Page 56 of 61

Photoreduction Process of Benzophenone by Diphenylamine. Bull. Chem. Soc. Jpn. 1995, 68, 15691582. (44) Anthony, H.; J., M. L.; Gilles, U.; Raymond, Z. Rapid Intersystem Crossing in CloselySpaced but Orthogonal Molecular Dyads. ChemPhysChem 2007, 8, 12071214. (45) Weiss, E. A.; Ratner, M. A.; Wasielewski, M. R. Direct Measurement of Singlet-Triplet Splitting within Rodlike Photogenerated Radical Ion Pairs Using Magnetic Field Effects: Estimation of the Electronic Coupling for Charge Recombination. J. Phys. Chem. A 2003, 107, 36393647. (46) Dance, Z. E. X.; Mi, Q.; McCamant, D. W.; Ahrens, M. J.; Ratner, M. A.; Wasielewski, M. R. Time-Resolved EPR Studies of Photogenerated Radical Ion Pairs Separated by p-Phenylene Oligomers and of Triplet States Resulting from Charge Recombination. J. Phys. Chem. B 2006, 110, 2516325173. (47) Veldman, D.; Chopin, S. M. A.; Meskers, S. C. J.; Janssen, R. A. J. Enhanced Intersystem Crossing via a High Energy Charge Transfer State in a Perylenediimide-Perylenemonoimide Dyad. J. Phys. Chem. A 2008, 112, 86178632. (48) Dance, Z. E. X.; Mickley, S. M.; Wilson, T. M.; Ricks, A. B.; Scott, A. M.; Ratner, M. A.; Wasielewski, M. R. Intersystem Crossing Mediated by Photoinduced Intramolecular Charge Transfer: Julolidine-Anthracene Molecules with Perpendicular π Systems. J. Phys. Chem. A 2008, 112, 41944201. (49) Suneesh, C. V.; Gopidas, K. R. Long-Lived Photoinduced Charge Separation Due to the Inverted Region Effect in 1,6-Bis(phenylethynyl)pyrene-Phenothiazine Dyad. J. Phys. Chem. C 2010, 114, 1872518734.

56


Page 57 of 61 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


(50) Filatov, M. A.; Karuthedath, S.; Polestshuk, P. M.; Savoie, H.; Flanagan, K. J.; Sy, C.; Sitte, E.; Telitchko, M.; Laquai, F.; Boyle, R. W.; et al. Generation of Triplet Excited States via Photoinduced Electron Transfer in meso-Anthra-BODIPY: Fluorogenic Response toward Singlet Oxygen in Solution and in Vitro. J. Am. Chem. Soc. 2017, 139, 62826285. (51) Chen, K.; Yang, W.; Wang, Z.; Iagatti, A.; Bussotti, L.; Foggi, P.; Ji, W.; Zhao, J.; Di Donato, M. Triplet Excited State of BODIPY Accessed by Charge Recombination and Its Application in Triplet-Triplet Annihilation Upconversion. J. Phys. Chem. A 2017, 121, 75507564. (52) Wang, Z.; Zhao, J. Bodipy-Anthracene Dyads as Triplet Photosensitizers: Effect of Chromophore Orientation on Triplet-State Formation Efficiency and Application in Triplet-Triplet Annihilation Upconversion. Org. Lett. 2017, 19, 44924495. (53) Herbich, J.; Kapturkiewicz, A. Radiative and Radiationless Depopulation of the Excited Intramolecular Charge Transfer States: Aryl Derivatives of Aromatic Amines. Chem. Phys. 1991, 158, 143153. (54) Grabowski, Z. R.; Rotkiewicz, K.; Rettig, W. Structural Changes Accompanying Intramolecular Electron Transfer: Focus on Twisted Intramolecular Charge-Transfer States and Structures. Chem. Rev. 2003, 103, 38994032. (55) Jones, G.; Farahat, M. S.; Greenfield, S. R.; Gosztola, D. J.; Wasielewski, M. R. Ultrafast Photoinduced Charge-Shift Reactions in Electron Donor-Acceptor 9-Arylacridinium Ions. Chem. Phys. Lett. 1994, 229, 4046. (56) Zhu, Y.; Kulkarni, A. P.; Wu, P.-T.; Jenekhe, S. A. New Ambipolar Organic Semiconductors. 1. Synthesis, Single-Crystal Structures, Redox Properties, and Photophysics of Phenoxazine-Based Donor-Acceptor Molecules. Chem. Mater. 2008, 20, 42004211.

57



Page 58 of 61

(57) Pearson, R. M.; Lim, C.-H.; McCarthy, B. G.; Musgrave, C. B.; Miyake, G. M. Organocatalyzed Atom Transfer Radical Polymerization Using N-Aryl Phenoxazines as Photoredox Catalysts. J. Am. Chem. Soc. 2016, 138, 1139911407. (58) Tanaka, H.; Shizu, K.; Nakanotani, H.; Adachi, C. Twisted Intramolecular Charge Transfer State for Long-Wavelength Thermally Activated Delayed Fluorescence. Chem. Mater. 2013, 25, 37663771. (59) Liu, Y.; Xie, G.; Wu, K.; Luo, Z.; Zhou, T.; Zeng, X.; Yu, J.; Gong, S.; Yang, C. Boosting Reverse Intersystem Crossing by Increasing Donors in Triarylboron/phenoxazine Hybrids: TADF Emitters for High-Performance Solution-Processed OLEDs. J. Mater. Chem. C 2016, 4, 44024407. (60) Stoll, S.; Schweiger, A. Easyspin, a Comprehensive Software Package for Spectral Simulation and Analysis in EPR. J. Magn. Reson. 2006, 178, 4255. (61) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09W (Revision A.1), Gaussian Inc.: Wallingford, CT, 2009. (62) Wu, Y.; Zhu, W. Organic Sensitizers from D--A to D-A--A: Effect of the Internal Electron-Withdrawing Units on Molecular Absorption, Energy Levels and Photovoltaic Performances. Chem. Soc. Rev. 2013, 42, 20392058. (63) Sasaki, S.; Hattori, K.; Igawa, K.; Konishi, G.-i. Directional Control of π-Conjugation Enabled by Distortion of the Donor Plane in Diarylaminoanthracenes: A Photophysical Study. J. Phys. Chem. A 2015, 119, 48984906.

58


Page 59 of 61 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


(64) Pal, S. K.; Sahu, T.; Misra, T.; Ganguly, T.; Pradhan, T. K.; De, A. Synthesis, Characterization and Laser Flash Photolysis Studies of Some Naphthothiophenes Bearing Electron Donor and Acceptor Functional Groups. J. Photochem. Photobiol. A 2005, 174, 138148. (65) Weiss, E. A.; Ahrens, M. J.; Sinks, L. E.; Gusev, A. V.; Ratner, M. A.; Wasielewski, M. R. Solvent Control of Spin-Dependent Charge Recombination Mechanisms within DonorConjugated Bridge-Acceptor Molecules. J. Am. Chem. Soc. 2004, 126, 55775584. (66) Lewis, F. D.; Liu, J.; Weigel, W.; Rettig, W.; Kurnikov, I. V.; Beratan, D. N. Donor-BridgeAcceptor Energetics Determine the Distance Dependence of Electron Tunneling in DNA. P. Natl. Acad. Sci. 2002, 99, 1253612541. (67) Lou, Z.; Hou, Y.; Chen, K.; Zhao, J.; Ji, S.; Zhong, F.; Dede, Y.; Dick, B. Different Quenching Effect of Intramolecular Rotation on the Singlet and Triplet Excited States of Bodipy. J. Phys. Chem. C 2018, 122, 185193. (68) Zhao, Y.; Li, X.; Wang, Z.; Yang, W.; Chen, K.; Zhao, J.; Gurzadyan, G. G. Precise Control of the Electronic Coupling Magnitude between the Electron Donor and Acceptor in Perylenebisimide Derivatives via Conformation Restriction and Its Effect on Photophysical Properties. J. Phys. Chem. C 2018, 122, 37563772. (69) Sabatini, R. P.; McCormick, T. M.; Lazarides, T.; Wilson, K. C.; Eisenberg, R.; McCamant, D. W. Intersystem Crossing in Halogenated Bodipy Chromophores Used for Solar Hydrogen Production. J Phys. Chem. Lett. 2011, 2, 223227. (70) Wang, Z.; Xie, Y.; Xu, K.; Zhao, J.; Glusac, K. D. Diiodobodipy-Styrylbodipy Dyads: Preparation and Study of the Intersystem Crossing and Fluorescence Resonance Energy Transfer. J. Phys. Chem. A 2015, 119, 67916806.

59



Page 60 of 61

(71) Sooyeon, K.; Yang, Z.; Norimitsu, T.; Hirotaka, N.; Michiya, M.; Mamoru, F.; Mikiji, M.; Tetsuro, M. Aggregation-Induced Singlet Oxygen Generation: Functional Fluorophore and Anthrylphenylene Dyad Self-Assemblies. Chem.Eur. J. 2018, 24, 636645. (72) Filatov, M. A.; Karuthedath, S.; Polestshuk, P. M.; Callaghan, S.; Flanagan, K. J.; Telitchko, M.; Wiesner, T.; Laquai, F.; Senge, M. O. Control of Triplet State Generation in Heavy AtomFree BODIPY–Anthracene Dyads by Media Polarity and Structural Factors. Phys. Chem. Chem. Phys. 2018, 20, 80168031. (73) Hattori, S.; Ohkubo, K.; Urano, Y.; Sunahara, H.; Nagano, T.; Wada, Y.; Tkachenko, N. V.; Lemmetyinen, H.; Fukuzumi, S. Charge Separation in a Nonfluorescent DonorAcceptor Dyad Derived from Boron Dipyrromethene Dye, Leading to Photocurrent Generation. J. Phys. Chem. B 2005, 109, 1536815375. (74) Lifschitz, A. M.; Young, R. M.; Mendez-Arroyo, J.; Roznyatovskiy, V. V.; McGuirk, C. M.; Wasielewski, M. R.; Mirkin, C. A. Chemically regulating Rh(I)-Bodipy photoredox switches. Chem. Commun. 2014, 50, 68506852. (75) Akasaka, T.; Nakata, A.; Rudolf, M.; Wang, W.-W.; Yamada, M.; Suzuki, M.; Maeda, Y.; Aoyama, R.; Tsuchiya, T.; Nagase, S.; et al. Synthesis and Photoinduced Electron-Transfer Reactions in a La2@Ih-C80Phenoxazine Conjugate. ChemPlusChem 2017, 82, 10671072. (76) Van Willigen, H.; Jones, G.; Farahat, M. S. Time-Resolved EPR Study of Photoexcited Triplet-State Formation in Electron-Donor-Substituted Acridinium Ions. J. Phys. Chem. 1996, 100, 33123316. (77) Richert, S.; Tait, C. E.; Timmel, C. R. Delocalisation of Photoexcited Triplet States Probed by Transient EPR and Hyperfine Spectroscopy. J. Magn. Reson., 2017, 280, 103116.

60


Page 61 of 61 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


(78) Toffoletti, A.; Wang, Z.; Zhao, J.; Tommasini, M.; Barbon, A. Precise Determination of the Orientation of the Transition Dipole Moment in a Bodipy Derivative by Analysis of the Magnetophotoselection Effect. Phys. Chem. Chem. Phys. 2018, 20, 2049720503. (79) Wang, Z.; Sukhanov, A. A.; Toffoletti, A.; Sadiq, F.; Zhao, J.; Barbon, A.; Voronkova, V. K.; Dick, B. Insights into the Efficient Intersystem Crossing of Bodipy-Anthracene Compact Dyads with Steady-State and Time-Resolved Optical/Magnetic Spectroscopies and Observation of the Delayed Fluorescence. J. Phys. Chem. C 2019, 123, 265−274. (80) Suzuki, S.; Sugimura, R.; Kozaki, M.; Keyaki, K.; Nozaki, K.; Ikeda, N.; Akiyama, K.; Okada, K. Highly Efficient Photoproduction of Charge-Separated States in DonorAcceptorLinked Bis(Acetylide) Platinum Complexes. J. Am. Chem. Soc. 2009, 131, 1037410375. (81) Zhao, J.; Ji, S.; Wu, W.; Wu, W.; Guo, H.; Sun, J.; Sun, H.; Liu, Y.; Li, Q.; Huang, L. Transition Metal Complexes with Strong Absorption of Visible Light and Long-Lived Triplet Excited States: from Molecular Design to Applications. RSC Adv. 2012, 2, 17121728.

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Spin Orbit Charge Transfer Intersystem Crossing (SOCT-ISC) in

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