Can BODIPY-Electron Acceptor Conjugates Act as Heavy Atom-Free

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C: Energy Conversion and Storage; Energy and Charge Transport

Can BODIPY-Electron Acceptor Conjugates Act as Heavy AtomFree Excited Triplet State and Singlet Oxygen Photosensitizers via Photoinduced Charge Separation-Charge Reombination Mechanism? Wenbin Hu, Mingyu Liu, Xian-Fu Zhang, Yaling Wang, Yun Wang, Haikuo Lan, and Huaqing Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02961 • Publication Date (Web): 10 Jun 2019 Downloaded from http://pubs.acs.org on June 10, 2019

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The Journal of Physical Chemistry

Can BODIPY-Electron Acceptor Conjugates Act as Heavy Atom-Free Excited Triplet State and Singlet Oxygen Photosensitizers via Photoinduced

Charge

Separation-Charge

Recombination

Mechanism? Wenbin Hu†, Mingyu Liu†, Xian-Fu Zhang‡,*, Yaling Wang, Yun Wang†, Haikuo Lan†, Huaqing Zhao†

†

Department of Chemical Engineering, Hebei Normal University of Science and

Technology, Qinhuangdao, Hebei Province, China, 066004 ‡

MPC Technologies, Hamilton, Ontario, Canada L8S 3H4

*Corresponding author E-mail addresses: [email protected], [email protected]

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

ABSTRACT: To examine if BODIPY-electron acceptor conjugates can generate excited triplet state and singlet oxygen efficiently via photoinduced charge separation-charge recombination (CS-CR) mechanism, seven compounds of BODIPY-electron acceptor (A) type are synthesized, in which an x-nitro-phenyl (x = ortho, meta, and para) or a pyridyl subunit on the BODIPY meso-position acts as the acceptor. The photophysical properties and singlet oxygen formation efficiency are measured in seven solvents of different polarity. The excited triplet state and singlet oxygen formation efficiency decrease with the increase in the electron withdrawing ability of the phenyl moiety. In the mean time, the presence of a nitrophenyl or a pyridyl causes the substantial quenching of the excited singlet state of the linked BODIPY, and this quenching is enhanced by the solvent with higher polarity, indicating a strong photoinduced electron transfer (PET) occurs from BODIPY to the attached nitro-phenyl unit. The results imply that attaching the electron acceptors to BODIPY can not mediate excited triplet state formation via CS-CR mechanism, which is in sharp contrast to the case of BODIPY-electron donor type conjugates. Based on the electron movements in HOMO/LUMO for PET and CR, we obtain the general requirements for PET/CR mediated triplet formation: Eg(D)>Eg(A) when donor D is photo excited, in which Eg(D) and Eg(A) is the energy gap between HOMO and LUMO for D and A, respectively. The conclusion and the frontier orbital analysis are helpful in the design and synthesis of BODIPY based halogen-free photosensitizers.

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1. INTRODUCTION Photoinduced charge separation-charge recombination (CS-CR) mediated excited triplet state (T1) and singlet oxygen [O2(1g)] formation in a donor-acceptor (D-A) system have attracted high attentions recently,1-14 since this method provides halogenand heavy metal-free photosensitizers which are favored for health and environment. For a -conjugated compound (C), such as pristine BODIPY and perylene which exhibit low ability to generate T1 upon photoexcitation, usually heavy atom effect is used to tackle the problem, i.e. an atom or ion X (such as X=I, Br, Cu2+, Zn2+) is substituted into its molecules to significantly enhance T1 formation efficiency by increasing the ISC (intersystem crossing) rate constant.15 In the CS-CR method, however, another molecular unit, such as a donor (D) or an acceptor (A) containing no heavy atoms, is attached to C forming a D-C (or A-C) conjugate and promote T1 formation efficiency. The CS-CR process can occur with two possible mechanisms: spin-orbit charge-transfer intersystem crossing (SOCT-ISC) and radical pair intersystem crossing (RP-ISC), both of them do not need the presence of heavy atoms or ions.16-17 Pristine BODIPY (boron-dipyrromethene) dyes show negligible T1 formation capability, but a BODIPY linked to an electron donor unit can produce T1 with good efficiency,5,

7-12

since CS-CR can highly increase the rate constant of intersystem

crossing (ISC) and lead to highly efficient T1 formation. BODIPY photosensitizers based on this strategy have been shown to be potential candidates for photodynamic therapy of tumor and TTA (triplet-triplet annihilation) photon upconversion.18-19

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Fig. 1. Chemical structures (top) and geometries (bottom) of BODIPY compounds in this study. The computed dihedral angle is 90.0o, 87.2o, 89.6o, 90.0o, 83.2o, 88.9o, and 90.0o for compound 1 to 7 respectively. Compound geometries were calculated by DFT at B3LYP/6-311g level using Gaussian09.

Nevertheless some questions on CS-CR method remain open. Over the last years the focus has been on the donor-BODIPY type while the acceptor-BODIPY type D-A conjugates have not been reported. One interesting question arises: does a BODIPY linked with an electron acceptor unit (acceptor-BODIPY) also lead to the efficient T1 formation? To the best our knowledge, there has been no specific study to address the issues. To answer the questions, we have synthesized compounds 2 to 7 (Fig. 1) and the experimental results show that it in fact leads to a decreased T1 and singlet oxygen formation efficiency. We have measured their photophysics in detail and explain the results based on HOMO/LUMO analysis in this report.

2. EXPERIMENTAL SECTION 2.1. Reagents and Apparatus. Chemicals and reagents from Sigma-Aldrich, Acros Organics, Merck or Fluka at the highest commercial grade were used without further purification. All solvents were purified and distilled before use. Dry solvents were prepared with standard methods.20 Analytical thin layer chromatography (TLC) was performed on silica gel plates (Merck, Kieselgel 60, 0.25 mm thickness) with F254 indicator. Flash chromatography was performed using silica gel 60 (200-400 mesh) from Merck. Proton nuclear magnetic resonance spectra were recorded on a Bruker dmx NMR spectrometer at 600 MHz, and carbon nuclear magnetic resonance spectra were recorded on the Bruker dmx NMR spectrometer at 150 MHz. The spectra were recorded in either CDCl3 or DMSO-d6. Chemical shifts (d) are reported in ppm with CDCl3 (1H: 7.26 ppm; 13C: 77.16) used as reference. Coupling constants were measured in Herz (Hz). NMR data are given as follows: s (singlet), d (doublet), dd (doublet of doublets), t (triplet), q (quartet), m (multiplet) and br (broad). 4


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High-resolution mass spectra were recorded on a LTQ Orbitrap XL TOF spectrometer equipped with an atmospheric pressure chemical injection (APCI) source. 2.2. Synthesis of BODIPYs 2 – 7. The general procedure is described below. The BODIPY derivatives were synthesized according to our previously reported procedures. To a 50 mL solution of an appropriate aldehyde (1.0 mmol) and 2,4-dimethylpyrrole (0.200g, 2.1 mmol) in absolute dichloromethane, kept under nitrogen atmosphere, was added one drop of trifluoroacetic acid. The solution was then stirred at room temperature for 12 hours

and stopped when TLC analysis

showed the complete consumption of the aldehyde. In the same flask, the oxidation of the dipyrrolylmethane to dipyrrolylmethene was carried out by the addition of 2,3-dicyano-5,6-dichlorobenzoquinone (0.227g, 1.0 mmol) and the mixture was stirred for 120 min. The last step of the BODIPY synthesis requires the addition of N,N-Diisopropylethylamine (5ml) and boron fluoride ethyl ether (10 mL). After stirring for 12 hours, the reaction mixture was washed three times with water. The organic solution was dried over MgSO4, filtered and evaporated to dryness. The raw material was purified by column chromatography (SiO2 200 g, 20 % dichloromethane in n-hexane). Fraction containing the product was collected when the solvent was removed under reduced pressure. 5,5-difluoro-1,3,7,9-tetramethyl-10-(2-nitrophenyl)-5H-dipyrrolo[1,2-c:2',1'-f][1, 3,2]diazaborinin-4-ium-5-uide (2). Yield: 21%. Orange crystals. mp 248-250 °C. Rf = 0.54 (EtOAc–hexane, 3:7).

1

H NMR (CDCl3, 600 MHz): δ 8.12 (d, 1H, J=8.19),

7.73 (t, 1H, J=7.48), 7.64 (t, 1H, J=7.41), 7.40 (d, 1H, J=7.57), 5.92 (s, 2H), 2.49 (s, 3H), 1.29 (s, 6H).

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C NMR (CDCl3, 150 MHz): δ 155.14, 147.30, 140.70, 135.18,

133.07, 130.01, 129.63, 129.49, 129.16, 123.94, 120.58, 13.65, 12.81. HRMS (APCI) m/z: 350.1467 [M-F]+ ([M-F]+ calcd. 350.1476). 5,5-difluoro-1,3,7,9-tetramethyl-10-(3-nitrophenyl)-5H-dipyrrolo[1,2-c:2',1'-f][1, 3,2]diazaborinin-4-ium-5-uide (3). Yield: 17%. Orange crystals. mp 228-230°C. Rf = 0.57 (EtOAc–hexane, 3:7). 1H NMR (CDCl3, 600 MHz): δ 8.38 (d, 1H, J=7.04), 8.24 (s, 1H), 7.73 (t, 1H, J=7.97), 7.68 (d, 1H, J1=7.59), 6.02 (s, 2H), 2.57 (s, 6H), 1.36 (s, 6H). 13C NMR (CDCl3, 150 MHz): δ 156.74, 148.69, 142.49, 137.80, 136.79, 134.56, 5



131.02, 130.42, 124.06, 123.65, 121.94, 14.89, 14.71. HRMS (APCI) m/z: 350.1463 [M-F]+ ([M-F]+ calcd. 350.1476), m/z: 370.1521 [M+H]+ ([M+H]+ calcd. 370.1538) 5,5-difluoro-1,3,7,9-tetramethyl-10-(4-nitrophenyl)-5H-dipyrrolo[1,2-c:2',1'-f][1, 3,2]diazaborinin-4-ium-5-uide (4). Yield: 16%. Orange crystals. mp 273-274°C. Rf = 0.47 (EtOAc–hexane, 4:6). 1H NMR (CDCl3, 600 MHz): δ 8.39 (d, 2H, J=8.69), 7.55 (d, 2H, J=8.69), 6.02 (s, 2H), 2.57 (s, 6H), 1.36 (s, 6H). 13C NMR (CDCl3, 150 MHz): δ 156.71, 148.36, 142.53, 141.99, 138.33, 130.65, 129.67, 124.37, 121.89, 14.70, 14.67. HRMS (APCI) m/z: 350.1464 [M-F]+ ([M-F]+ calcd. 350.1476). 10-(2,4-dinitrophenyl)-5,5-difluoro-1,3,7,9-tetramethyl-5H-dipyrrolo[1,2-c:2',1'-f ][1,3,2]diazaborinin-4-ium-5-uide (5). Yield: 23%. Orange crystals. mp 213-214°C. Rf = 0.44 (EtOAc–hexane, 3:7). 1H NMR (DMSO-d6, 600 MHz): δ 9.00 (d, 1H, J=2.27), 8.72 (dd, 1H, J1=8.42, J2=2.20), 8.06 (d, 1H, J=8.42), 6.25 (s, 2H), 2.48 (s, 6H), 1.39 (s, 6H). 13C NMR (DMSO-d6, 150 MHz): δ 156.75, 149.04, 148.60, 142.31, 134.46, 134.25, 133.38, 130.10, 129.47, 122.56, 121.33, 14.80, 14.24. HRMS (APCI) m/z: 395.1288 [M-F]+ ([M-F]+ calcd. 395.1327) 5,5-difluoro-1,3,7,9-tetramethyl-10-(pyridin-2-yl)-5H-dipyrrolo[1,2-c:2',1'-f][1,3, 2]diazaborinin-4-ium-5-uide (6). Yield: 16%. Orange crystals. mp 220-221°C. Rf = 0.47 (EtOAc–hexane, 7:3). 1H NMR (CDCl3, 600 MHz): δ 8.78 (d, 1H, J = 4.68), 7.84 (t, 1H, J = 7.69), 7.44 (t, 2H, J = 7.63), 5.99 (s, 2H), 2.56 (s, 6H), 1.31 (s, 6H). 13

C NMR (CDCl3, 150 MHz): δ 156.35, 153.96, 150.23, 142.57, 138.54, 137.01,

131.39, 124.39, 123.89, 121.27, 14.70, 13.77. HRMS (APCI) m/z: 306.1560 [M-F]+ ([M-F]+ calcd. 306.1578), m/z: 326.1619 [M+H]+ ([M+H]+ calcd. 326.1640) 5,5-difluoro-1,3,7,9-tetramethyl-10-(pyridin-4-yl)-5H-dipyrrolo[1,2-c:2',1'-f][1,3,2]di azaborinin-4-ium-5-uide (7). Yield: 10 %. Orange crystals. mp 237-239°C. Rf = 0.54 (EtOAc–hexane, 7:3). 1H NMR (CDCl3, 600 MHz): δ 8.78 (d, 2H, J=5.89), 7.30 (d, 2H, J=5.93), 6.01 (s, 2H), 2.56 (s, 6H), 1.40 (s, 6H).

13

C NMR (CDCl3, 150 MHz):

δ156.49, 150.62, 143.65, 142.67, 137.62, 130.34, 123.33, 121.82, 14.64. HRMS (APCI) m/z: 306.1556 [M-F]+ ([M-F]+ calcd. 306.1578), m/z: 326.1616 [M+H]+ ([M+H]+ calcd. 326.1640). 2.3. photophysics measurement. This is similar to our previous report and details 6


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are given in the supporting information.5, 10, 12

3. RESULTS AND DISCUSSION 3.1. The design and synthesis for the BODIPY-acceptor compounds. The chemical structures of the BODIPY-acceptor compounds (Fig. 1) are designed based on PET requirements and geometry considerations. The two methyls at position 1 and 3 of the BODIPY unit are used to restrict the rotation of the meso-phenyl or meso-pyridyl, so that the phenyl (pyridyl) is almost perpendicular to the linked BODIPY, since the orthogonal orientation best favors T1 formation via PET.21 This perpendicular geometry is confirmed by the calculated structure (Fig. 1, the computed dihedral angle is 90.0o, 87.2o, 89.6o, 90.0o, 83.2o, 88.9o, and 90.0o for compound 1 to 7 respectively). As shown in Fig. 1, the BODIPY core and the linked phenyl or pyridyl in these compounds still maintain the planar structure. The position of a nitro group exhibits different influence on the geometry. Compared to compound 1 (in which the symmetric axis of the phenyl is coincident with that of the BODIPY unit), NO2 at 3’ and 4’ position of the phenyl shows little effect on the phenyl geometry (Fig. 1), but NO2 at 2’ position (for compound 2 and 5) makes the phenyl deviated from the symmetric axis of BODIPY plane (Fig. 1). For pyridyl, no stretched out substituents exist, pyridyl basically maintains the same orientation relative to the BODIPY as the phenyl in compound 1. For all these geometries, however, the electron cloud overlap (ground state interaction) between the phenyl (pyridyl) and the linked BODIPY is very small, so that the BODIPY unit and the phenyl moiety can be each viewed as an independent unit and their mutual interaction does not occur at their ground states but at their excited states. An attached nitro group makes the phenyl moiety a strong electron acceptor so that PET can occur easily from the BODIPY core to the phenyl. The position and number of nitro group are used to adjust the electron withdrawing ability of the acceptor unit. The occurrence of PET in a compound 2, 3, 4 and 5 is supported by the calculated HOMO/LUMO (Fig. 2). The HOMO and LUMO of the BODIPYs are 7



calculated by DFT method using Gaussian09 on the B3LYP/6-311g level, and the results are shown in Fig. 2. For compound 1, both HOMO and LUMO are localized on the BODIPY unit. For nitrophenyl modified BODIPYs, however, the HOMO is localized only on the BODIPY unit while the LUMO is located only on the nitrophenyl moiety, indicating that the photo excitation of a compound (any of 2-5) moves one electron from BODIPY to the linked phenyl unit (PET). For compound 6 and 7, comparing the HOMO and LUMO of each compound, it clearly shows that the HOMO is still solely located on the BODIPY unit, but the corresponding LUMO is partially located on the pyridyl moiety (although LUMO is mainly on the BODIPY unit), this suggests that PCT (photoinduced negative charge transfer) will occur from BODIPY to pyridyl upon photoexcitation. Therefore the pyridyl in the compounds acts as a negative charge acceptor, or electron acceptor in a broad sense. The computation is for the vacuum condition, but PCT/PET is usually stronger in solvent and increased with the increase in solvent polarity. Based on the calculation, compounds 2 to 5 are taken as the PET model while compound 6 and 7 are used as the PCT examples in this study on T1 generation.

Fig. 2. HOMO and LUMO of BODIPY compounds 1-7 in vacuum calculated by Gaussian09 with DFT method on 6-311g basis set level.

The compounds were then prepared according to the reaction shown in Fig. 3. 1H NMR,

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C NMR and HRMS are consistent with the structures. Pyridyl substituted

BODIPYs show lower yields than that of nitrophenyl BODIPYs. All the compounds show good solubility in both non polar and polar solvents.

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Fig. 3. Synthesis procedure for the BODIPY compounds.

Table 1a-Table 1g. Photophysical data in different solvents ( : dielectric constant) Table 1a. Fluorescence quantum yield f in different solvents. * solvent

1

2

3

4

5

6

7



CH3CN CH3OH Acetone Pinacolone THF EtOAc n-hexane

0.52 0.58 0.46 0.55 0.56 0.58 0.56

0.0032 0.027 0.0038 0.0064 0.050 0.016 0.023

0.019 0.14 0.070 0.074 0.25 0.23 0.42

0.015 0.030 0.027 0.047 0.11 0.14 0.19

0.0033 0.045 0.0091 0.012 0.0087 0.025 0.033

0.038 0.071 0.049 0.04 0.061 0.062 0.059

0.13 0.12 0.20 0.17 0.33 0.38 0.25

36.6 33.0 20.7 12.8 7.52 6.02 2.02

*: with excitation at 475 nm, emission range is 480-700nm. Fluorescein in 0.1 M NaOH was used as the reference.

Table 1b. The singlet oxygen formation quantum yield  in different solvents.** solvent

1

2

3

4

5

6

7




0.068 0.031 0.050 0.11 0.033 0.052 0.038

0.020 0.0083 0.051 0.079 0.026 0.027 0.018

0.0044 0.0062 0.029 0.07 0.028 0.031 0.01

0.0043 0.0036 0.0093 0.047 0.019 0.021 0.0067

0.0049 0.0055 0.012 0.073 0.026 0.026 0.021

0.024 0.013 0.012 0.012 0.012 0.0039 0.0052

0.037 0.012 0.031 0.020 0.024 0.032 0.0091

36.6 33.0 20.7 12.8 7.52 6.02 2.02

**: measured by DPBF chemical trapping method, with LED light excitation at 500 nm.

Table 1c. The absorption maximum abs (nm) in different solvents solvent

1

2

3

4

5

6

7




497 498 498 499 500 498 501

505 506 507 508 508 507 510

500 502 502 503 505 502 505

501 501 502 503 504 501 505

510 510 511 512 512 511 514

500 503 501 502 504 502 505

500 501 501 502 503 501 503

36.6 33.0 20.7 12.8 7.52 6.02 2.02

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Table 1d. The fluorescence emission maximum em (nm) in different solvents solvent

1

2

3

4

5

6

7




508 510 509 511 512 510 511

512 508 513 518 514 514 527

510 515 514 515 519 516 518

510 512 512 514 523 519 521

508 508 511 513 512 509 510

515 513 513 516 518 515 515

515 517 517 518 518 -517

36.6 33.0 20.7 12.8 7.52 6.02 2.02

Table 1e. Stokes shift (nm) values in different solvents. *** solvent

1

2

3

4

5

6

7




11 12 11 12 12 12 10

7 2 6 10 6 7 17

10 13 12 12 14 14 13

9 11 10 11 19 18 16

-2 -2 0 1 0 -2 -4

15 -12 14 14 13 10

15 16 16 16 15 -14

36.6 33.0 20.7 12.8 7.52 6.02 2.02

shift is calculated by em - abs. Table 1f. Absorption coefficients (M-1 cm-1) at absorption maximum in different solvents ***: Stokes

2 CH3CN MeOH Acetone Pinacolone THF EtOAc n-hexane

3

32000 65000 160000 40000 64000 81000 89000

89000 33000 42000 85000 76000 75000 98000

4 61000 71000 71000 44000 64000 85000 16000

5

6

40000 65000 45000 32000 48000 83000 26000

88000 55000 40000 66000 46000 64000 19000

7 76000 85000 120000 59000 140000 120000 82000

Table 1g. The fluorescence lifetime (ns) of each compound in different solvents.**** solvent

1

2

3

4

5

6

7

MeCN

3.81

0.42(35), 4.01

0.18(40), 2.95(25), 7.19

0.26(71), 1.81(20), 6.31

0.14(40), 2.30(29), 6.19

0.092(68), 0.52(29), 5.53

1.75(96), 8.93

MeOH

3.9

1.73(47),4.20

0.16(9.8),0.29(65),5.53

0.57(27),2.86(60),7.21

0.29(9),2.94(6),6.15

1.15(96.4),7.45

0.91(35),1.28(63),7.38

acetone

3.65

1.01(31),4.84

0.16(33),0.28(48),5.04

0.24(81),4.83

0.58(10),1.71(86),6.41

0.16(17),0.32(81),6.00

0.18(6),0.38(35),1.71

pinacolone

3.39

0.42(33),5.06

0.25(31),0.43(59),5.23

0.34(83),1.78(5),5.56

0.18(79),4.35

0.21(63),1.47(33),5.99

1.42(96),44.68

THF

3.73

0.66(83),5.12

0.18(10),1.86(86),6.69

0.27(26),1.19(62),5.65

0.31(65),1.76(30).5.59

0.21(69),4.64

1.06(33),1.98

EtOAc

3.98

0.68(93),4.76

0.42(42),1.38(54),7.83

0.33(62),1.11(32),5.78

0.74(32),1.64(33),5.42

0.29(89),3.13(6.3),13.63

0.50(28),1.87

n-hexane

3.37

1.83(11), 4.75

1.59(86),4.79

1.04(56),5.35

0.99(83),4.69

0.20(86),5.55(13)

1.22(97),7.45

****: excitation at 509 nm with a diode laser (50 ps pulse), emission monitored at 520 nm. The chi squared values for exponential fitting are within 1.00 to 1.24. The value in the parentheses is the percentage contribution of the emitting component to the total emission intensity.

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3.2. Singlet oxygen and excited triplet state formation. Nanosecond laser flash photolysis was used to detect the excited triplet state T1 of these compounds by recording the transient absorption spectra (TAS). For compound 1, weak transient signal was obtained in nitrogen saturated solvent (Fig. 4). The positive absorption at 405 nm is assigned to excited triplet state T1-Tn absorption, while the negative signal at 500 nm is due to ground state bleaching (Fig. 4). The decay of T1 state (405 nm) is concomitant with the recovery of ground state at 500 nm (T1S0). The triplet lifetime is 21 s in N2-saturated solvent based on the monoexponential fitting of T1 decay at 400 nm (Fig. 4 Right). In the air saturated solution, however, T1 lifetime is shortened to 0.22 s due to efficient oxygen quenching. This very efficient oxygen quenching to the 400 nm positive signal also strongly indicates that the 405 nm band is due to T1 state.

Fig. 4. Left: time resolved T1-Tn transient absorption spectra of compound 1 (20 M) in nitrogen saturated MeCN with OPO laser excitation (4 ns, 5 mJ) at 500 nm. Right: The decay of T1 state at 400 nm and the concomitant rise of ground state at 460 nm for compound 1 (20 M) in nitrogen saturated MeCN with OPO laser excitation (4 ns, 5 mJ) at 500 nm.

For all nitro and pyridyl substituted compounds, however, the T1-Tn TAS are too weak to be recorded by the instrument. In comparison to compound 1, apparently the nitrophenyl or pyridyl presence does not cause the increase but lead to the decrease in T1 formation. This is in sharp contrast to the Donor-BODIPY system in which PET or PCT can significantly mediate T1 formation.1-10 To be certain about this conclusion, we have measured the singlet oxygen formation efficiency () of these compounds, since  is proportional to T1 formation quantum yield. Fig. 5 shows the UV-Vis absorption change of DPBF (1,3-diphenylisobenzofuran) 11



in the presence of a BODIPY (as the singlet oxygen photosensitizer) in air saturated solvent with excitation of the BODIPY unit at 510 nm. Fig. 6 compares the value of these compounds in different solvents. With a few exceptions, the value follows the order in each solvent: 1>2>3>4~5, and 1>7>6 (Fig. 6). Compared to compound 1, it is clear that the electron acceptors (both nitro-phenyl and pyridyl) on BODIPY lead to the remarkable decrease of value, supporting the previous conclusion that the T1 formation quantum yield is decreased.

1.0 0.8 0.6 0.4 0.2 0.0

1.4 1.2

Abs

1.0 0.8 0.6 0.4 0.2 0.0

1.2 1.0

Abs

0.8 0.6 0.4 0.2 0.0

1.0 0.8 0.6 0.4 0.2 0.0

1.4 Absorbance (410nm)

Abs

1.2

1.5

0s 60 s 120 s 180 s 240 s 300 s 360 s 420 s 480 s 540 s 600 s

1.1 1.0 0.9 0

30 60 90 120 150 180 210 240 270 300 Time (s)

0

60 120 180 240 300 360 420 480 540 600 Time (s)

0

30 60 90 120 150 180 210 240 270 300 Time (s)

0

30 60 90 120 150 180 210 240 270 300 Time (s)

1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6

1.10 1.05 1.00 0.95 0.90 0.85 0.80

400 420 440 460 480 500 520 540 560 580 600 Wavelength (nm) 0s 30 s 60 s 90 s 120 s 150 s 180 s 210 s 240 s 270 s 300 s

1.2

1.4

400 420 440 460 480 500 520 540 560 580 600 Wavelength (nm)

0s 30 s 60 s 90 s 120 s 150 s 180 s 210 s 240 s 270 s 300 s

1.3

0.8

400 420 440 460 480 500 520 540 560 580 600 Wavelength (nm)

Absorbance (410nm)

1.4

0s 30 s 60 s 90 s 120 s 150 s 180 s 210 s 240 s 270 s 300 s

Absorbance (410nm)

1.6

1.00 0.98 Absorbance (410nm)

1.8

Abs

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

0.96 0.94 0.92 0.90 0.88 0.86

400 420 440 460 480 500 520 540 560 580 600 Wavelength (nm)

Fig. 5. Left: UV-Vis absorption change of DPBF decomposition by the acceptor-BODIPYs as photosensitizer in air saturated solution with excitation at 510 nm. Right: linear change of DPBF absorbance at 410 nm for the decomposition. From top to bottom: compound 1 in ethyl acetate (EE), compound 2 in EE, compound 6 in MeCN, compound 5 in MeCN.

12


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The values for these BODIPY-acceptor conjugates are generally smaller than 0.050 with a few exceptions, the value for these compounds is expected to be close to the corresponding  value. To understand why the BODIPY-acceptor conjugates exhibit lower  andvalue than that of compound 1, we made detailed studies on the photophysics of these compounds. These data are included in Table 1.

Fig. 6. Plot of singlet oxygen formation quantum yield against dielectric constant  (solvent polarity).

3.3. Electronic absorption spectra. The synthesized compounds 1 to 5 provide a good chance to investigate the spectroscopic and photophysical characteristics of these dyes as a function of the position and number of substituent NO2. These compounds were dissolved in the selected (non polar to highly polar) seven solvents to measure the UV-Vis electronic absorption and fluorescence emission spectra. The typical spectra are shown in Fig. 7. The compounds all display the characteristic absorption features of classic BODIPY dyes in all solvents examined (Fig. 7A and supporting information). A narrow main absorption band occurs with the peak maximum ( max abs ) in a very narrow range (495-515 nm) in different solvents (Fig. 7B). This visible absorption band is assigned to the S1←S0 transition. Compared to compound 1, the presence of NO2 substitutions does not change the spectral shape but causes a slight red shift of max abs within 55 nm, and max abs value ranks in the order: 2,4-NO2 > 2-NO2 > 3-NO2  4-NO2 indicating that the NO2 closer to BODIPY core causes a larger effect. 13



With the increase in solvent polarity, the absorption maximum of theses compounds all show a slight blue-shift (Fig. 7B). Over all, the insignificant change of absorption spectra upon NO2 substitution indicates that the ground state interaction between the nitrophenyl and the attached BODIPY core is indeed very weak.

Fig. 7. A: Normalized UV-Vis absorption spectra of the compound 1, 2, 3, 4 and 5 in EtOAc. B: Plotting of absorption maxima of each compound 1, 2, 3, 4 and 5 against dielectric constant (solvent polarity). C: Normalized fluorescence emission spectra of the compound (~10 M) 1, 2, 3, 4 and 5 in EtOAc with excitation at 475 nm. D: Plotting of emission maxima of each compound 1, 2, 3, 4 and 5 against dielectric constant (solvent polarity). E: Normalized absorption and emission spectra of compound 5 in different solvents. F: Plotting of stokes shift of each compound 1, 2, 3, 4 and 5 against dielectric constant (solvent polarity).

3.4. Fluorescence emission spectra. Except for compound 5 which contains two NO2 on the phenyl, the other compounds show the typical fluorescence features of BODIPY dyes (Fig. 7C): i.e., a narrow, slightly Stokes-shifted band at 5168 nm 14


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which is the mirror image of absorption band. The emission maximum is blue shifted with increasing solvent polarity for compound 2, 3, and 4. Compared to compound 1, the emission maximum of compound 2, 3, and 4 is red shifted in each solvent (Fig. 7D). In contrast to the case of absorption maximum ( max abs : 2,4-NO2 > 2-NO2 > 3-NO2  4-NO2), no clear correlation exists between emission maximum ( max em ) of these compounds, indicating that the excited state S1 structure is affected differently by the NO2 position. Compound 5 shows different emission spectra from others (Fig. 7E). In THF and hexane, broad and red-shifted new peaks are observed in addition to the usual emission band. The new band shows the features of charge-transfer (CT) emission (hCT) of a donor-acceptor (D-A) system: 1(D-A  D-A + hCT. This also implies that photoinduced electron transfer occurs and forms charge transfer state: D-A + h D(S1)-A

PET  

1

(D-A), in which superscript 1 represents singlet state.

Nanosecond laser flash photolysis provides no evidence for transient absorption spectra from triplet CT state: 3(D-A). No phosphorescence for 3(D-A) was found. It is therefore not likely that 3(D-A) is involved in the photophysical processes of the D-A type compounds. 2,4-dinitro phenyl is a much stronger electron acceptor than any of 2-, 3-, and 4-nitrophenyl, so that the electron transfer in compound 5 is much easier, the charge separated state is likely formed with much higher efficiency and more stable. The Stokes shift of compound 5 is also distinct, it is the smallest and ≤ 0 except in one solvent. 3.5. PET based on fluorescence quantum yield and fluorescence lifetime. For compound 1 that contains no PET (Fig. 8), fluorescence emission of BODIPY fluorophores comes from its excited singlet state S1 with rate constant kf: S1  S0 + h, the emission process usually competes with internal conversion IC (S1  S0 + heat, rate constant kic) and intersystem crossing ISC that forms excited triplet state T1 (S1  T1 with rate constant kisc). The fluorescence quantum yield f, lifetime f and T is then given by: f = kf/(kf+kic+kisc),

Eq. (1)

f = 1/(kf+kic+kisc).

Eq. (2) 15



T = kisc/(kf+kic+kisc),

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Eq. (3)

When a BODIPY is linked to an electron acceptor or donor, PET process also occurs from S1 with rate constant kpet (Fig. 8). In this case, the fluorescence quantum yield f’, lifetime f’ and triplet quantum yield T’ is calculated by: f’ = kf/(kf+kic+kisc+kpet),

Eq. (4)

f’ = 1/(kf+kic+kisc+kpet),

Eq. (5)

T’ = kisc/(kf+kic+kisc+kpet).

Eq. (6)

Fig. 8. The photophysical process for BODIPY-acceptor (D-A) and related energy level for the associated excited state. IC: internal conversion (S1S0 + heat, rate constant kic); FL: Fluorescence emission from S1 with rate constant kf; ISC: intersystem crossing (S1T1 with rate constant kisc); PET: photoinduced electron transfer from S1 with rate constant kpet; CT: charge transfer; phos: phosphorescence. S0: ground state; S1: lowest excited singlet state; T1: lowest excited triplet state.

Compared to that of compound 1, f’, T’ and f’ all become smaller due to the occurrence of kPET in the denominator. Therefore PET quenches S1 and leads to smaller T’ and also smaller . This result is consistent with what we observed for the nitro-phenyl BODIPYs. Fig. 8 shows the overall photophysical processes, in which D(S1) energy is obtained by 1240/mid, mid is the mid point of normalized absorption and emission spectra of BODIPY. D(T1) energy is based on BODIPY phosphorescence in our previous study.22 A(T1) energy is taken from literature,23 and the energy of 1(D-A) is calculated by D(S1) energy minus GPET (GPET will be computed in section 3.6). Fig. 9 plots the fluorescence quantum yield f’ as a function of solvent polarity (expressed by dielectric constant . Compared to compound 1, all nitrophenyl substituted BODIPYs show significantly smaller f values in each solvent, 16


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confirming that the nitrophenyl substitution indeed causes substantial fluorescence quenching. Since the ground state S0 of the BODIPY core is little affected by the nitro substitution (from the previous UV-Vis absorption study and calculated geometry), the fluorescence quenching must be due to the occurrence of a new process from excited state S1 (other than IC, radiation and ISC). Since NO2 is strongly electron withdrawing, this new process is ascribed to PET from the BODIPY core to the linked meso-nitrophenyl. The f value of each compound is also strongly affected by solvent polarity (Fig. 9 Left). With the increase in solvent polarity (represented by dielectric constant ),f value decreases for D-A type compounds (except for methanol =33, a protic alcohol solvent usually significantly increases f value due to H-bonding). This solvent effect also supports the presence of PET for nitrophenyl BODIPYs, since PET forms charged separated state: D-A + h 1(D-A) and the transient product 1(D-A) has larger dipole moment than that of the reactant D-A, which is stabilized by more polar solvent. f value is also determined by the position of NO2 group. Fig. 9 shows that f value is ranked by 1>3>4>25, and 1>7>6. In other words, PET efficiency is ranked by 5243, and 6>7. This is reasonable, because 2-NO2 substitution is more efficient for PET since it is closer to BODIPY core than 3- and 4-NO2. 4-NO2 is at the para-position and has a stronger electron withdrawing effect than the 3-NO2. 2-pyridyl is more efficient than 4-pyridyl for PCT since the electron-withdrawing N of 2 is closer to BODIPY core than that in 7, the f of 7 is then higher than that of 6. More percentage of S1 of 6 is quenched by PCT, so that less T1 is formed by ISC, T of 6 is hence smaller than that of 7, therefore  of 6 is smaller than that of 7.

17



Fig. 9. Plot of fluorescence quantum yield against dielectric constant  (solvent polarity).

To further support the presence of PET, we also studied the NO2 substitution effect on the fluorescence lifetime. Fig. 10 shows the fluorescence decay of compound 1 and 2 in different solvents (supporting information contains the fluorescence decays of other compounds). For compound 1, the emission decay is always monoexponential, and the lifetime is 3.37, 3.98, 3.73, and 3.81 ns in n-hexane, ethyl acetate, THF, and acetonitrile respectively, which is apparently not sensitive to the solvent polarity. For compound 2, however, the emission decay in polar solvents becomes faster and biexponential (Fig. 10), which are the typical features of PET in a D-A system. The long-lived component is 5.0 0.30 ns and assigned to the CT state emission. The short-lived component is from S1 emission of BODIPY and shortened by the PET process, the lifetime is 1.83, 0.68, 0.66 and 0.42 ns in n-hexane, ethyl acetate, THF, and acetonitrile respectively. Compared to that of compound 1, the lifetime of 2 in the same solvent is remarkably shorter, clearly supporting that the presence of NO2 leads to PET. Furthermore, the lifetime value of 2 is decreased with the increase in solvent polarity, which is another evidence of PET.

18


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Fig. 10. The fluorescence decay of 10 M compound 1 (left) and 2 (right) in different solvents with excitation at 485 nm laser (70 ps) and emission at 510 nm.

3.6. PET thermodynamic driving force. Thermodynamic data also support the PET occurrence. The free energy change (Gpet) must be negative when PET occurs. The tetramethyl BODIPY has an oxidation potential 1.12 V,24 while nitrobenzene, and dinitro benzene have the reduction potential of -1.0825 and -0.6926 V, respectively. Gpet is calculated by Eq. (7) in which Eox(D) is the oxidation potential of an electron donor, ERed(A) is the reduction potential of the electron acceptor, ES is the excitation energy of BODIPY (2.48 eV), and C is a constant (0.06 eV). We then have Gpet = -1.42 - ERed(A) for the BODIPY-acceptor systems. Gpet value is -0.34 and -0.73 eV for mononitro phenyl BODIPY compounds and compound 5 respectively. In both cases, Gpet value is negative, indicating that PET has sufficient driving force. Gpet = Eox(D) - ERed(A) - ES - C,

Eq. (7)

3.7. Why PET in the acceptor-BODIPYs does not lead to the triplet formation? We have shown that PET occurs in the nitrophenyl BODIPYs but does not lead to T1 formation via charge recombination, instead PET decreases T by quenching S1 of the BODIPY. This is in sharp contrast to the donor-BODIPY system in which T1 can be formed efficiently through charge recombination. Fig. 11 is given to explain the reason for the nitrophenyl BODIPYs.

19



Fig. 11. The electron transfer, electron spin reversion, and charge recombination in HOMO/LUMO of donor and acceptor.

Hereby BODIPY is the donor and photo excited to S1. We denote LD as the LUMO of a donor, HD as the HOMO of the donor, as shown Fig. 11. We consider the case in vacuum, Gpet = Eox(D) – ERed(A) – Eg(D) = ERed(D) – ERed(A) = ELA–ELD, For an efficient PET, GPET must be negative, then ELA–ELDELA. ELD , ELA is the LUMO energy of the donor and acceptor respectively. Eg(D) is energy gap between the HOMO and LUMO of the donor D. For the current systems, based on the absorption maximum (the BODIPY absorption maximum is 50010 nm, while the nitro benzene absorption maximum is 20


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less than 350 nm), we have Eg(D)EHA, since EHD=ELD–Eg(D)> ELA–Eg(D)> ELA–Eg(A)= EHA. Based on ELD>ELA and EHD>EHA, we can then draw the HOMO and LUMO shown in Fig. 11. (1) CT(S1) state is formed by PET. As shown in Fig. 11, after the excitation of the BODIPY (donor D) unit in D(S0)-A(S0), the excited singlet state D(S1)-A(S0) is formed by elevating one electron from HD to LD. Then PET occurs and results in the charge separated singlet state CT(S1): 1(D-A). PET process competes with IC, ISC and emission process of BODIPY. ISC of BODIPY is known being not efficient. (2) From CT(S1) state, there are three possible photophysical processes (Fig. 11 Bottom). The 1st is the charge recombination leading to ground state D(S0)-A(S0) via heat or light releasing, which is not beneficial for T1 formation. The 2nd is that one of the two unpaired electrons in LD or HA may reverse its spin and lead to the charge separated triplet state CT(T1): 3(D-A). This is usually a spin-forbidden process for a directly bound D-A system (the case in this study), but may become spin-allowed in a D-spacer-A system (spacer is a long distance bridge), since PET leads to a pair of free radicals (D-spacer-A). As shown in Fig. 11, other than back to CT(S1), CT(T1) have two possible fates by charge recombination: becoming D(T1)-A(S0) or D(S0)-A(T1). To become D(T1)-A(S0), the electron in LA needs to go back to LD, this can not be successful since the energy of LA is lower than that of LD (precondition of PET). To become D(S0)-A(T1) by charge recombination, the electron in HA must move to HD after PET (for PET, the electron in LD goes to LA), the following energy requirement must be met: the lost energy in PET can compensate the energy for charge recombination, i.e. ELD-ELA > EHD-EHA, or ELD-EHD > ELA-EHA, i.e. Eg(D) > Eg(A). This Eg(D) > Eg(A) is the general requirement for CT(T1) becoming D(S0)-A(T1) when donor D is photo excited for a D-A system. For the current systems, however, we know that Eg(BODIPY) < Eg(nitro benzene). This fact suggests that CT(T1) can not become D(S0)-A(T1) for the systems in this study. The 3rd process from CS(S1) state is that the direct transition from CS(S1) to D(S0)-A(T1) via CR process, in which the electron in HA moves into HD and reverse 21



its spin direction. This is called SOCT-ISC. Similar to the analysis in last paragraph, Eg(D)>Eg(A) must be met for SOCT-ISC possible, but Eg(BODIPY) < Eg(nitro benzene), SOCT-ISC can not occur for current systems. In summary, using the electron movements in HOMO/LUMO for PET and CR in Fig. 11, we obtain the general requirements for PET/CR mediated triplet formation: Eg(D)>Eg(A) and ELDELA when D is photo excited. Based on the obtained requirements, we can fully explain the results for the current systems: PET/CT mediated excited triplet formation can not occur.

4. CONCLUSIONS Seven BODIPY-acceptor type compounds are synthesized and characterized, in which nitro-phenyl and pyridyl act as the acceptor. Photoinduced electron transfer process from BODIPY to the acceptors is confirmed by fluorescence quenching, Gpet value calculation, and HOMO/LUMO orbital localization study. However, the PET does not enhance but decrease the T1 and singlet oxygen formation of the BODIPYs. This is in sharp contrast to the case of BODIPY-electron donor conjugates in which PET can enhance the T1 formation of BODIPY. The results can be explained by electron movements between HOMO/LUMO of D-A in Fig. 11. Using the electron movements in HOMO/LUMO for PET and CR in Fig. 11, we obtain the general requirements for PET/CR mediated triplet formation: Eg(D)>Eg(A) and ELDELA when D is photo excited. Due to Eg(D)

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