Triplet Excited State of BODIPY Accessed by Charge Recombination

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Triplet Excited State of Bodipy Accessed by Charge Recombination and Its Application in Triplet-Triplet Annihilation Upconversion Kepeng Chen, Wenbo Yang, Zhijia Wang, Alessandro Iagatti, Laura Bussotti, Paolo Foggi, Wei Ji, Jianzhang Zhao, and Mariangela Di Donato J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b07623 • Publication Date (Web): 04 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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The Journal of Physical Chemistry

Triplet Excited State of Bodipy Accessed by Charge Recombination and Its Application in Triplet-Triplet Annihilation Upconversion Kepeng Chen,a Wenbo Yang,a Zhijia Wang,a Alessandro Iagatti,b,c Laura Bussotti,b Paolo Foggi,b,d,c Wei Ji,e Jianzhang Zhao,*a and Mariangela Di Donato b, c* a

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] b

LENS (European Laboratory for Non-Linear Spectroscopy) via N. Carrara 1, 50019 Sesto Fiorentino, Italy. E-mail: [email protected] c

d

e

INO, Istituto Nazionale di Ottica Largo Enrico Fermi 6, I-50125 Florence, Italy

Dipartimento di Chimica Università di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy

School of Chemistry, Dalian University of Technology, E-208 West Campus, 2 Ling-Gong Road, Dalian 116024, P. R. China

RECEIVED DATE (automatically inserted by publisher)

ACS Paragon Plus Environment

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Abstract: The triplet excited state property of two Bodipy phenothiazine dyads (BDP-1 and BDP-2) with different length of linker and orientation of the components were studied. The triplet state formation of Bodipy chromophore was achieved via photo-induced electron transfer (PET) and charge recombination (CR). BDP-1 has a longer linker between the phenothiazine and the Bodipy chromophore than BDP-2. Moreover, the two chromophores in BDP-2 assume a more orthogonal geometry both at the ground and in the first excited state (87°) than that of BDP-1 (34°∼40°). The fluorescence of the BDP moiety was significantly quenched in the dyads. The charge separation (CS) and CR dynamics of the dyads were studied with femtosecond transient absorption spectroscopy (kCS = 2.2 × 1011 s−1 and 2 × 1012 s−1 for BDP-1 and BDP-2, respectively; kCR = 4.5 × 1010 s−1 and 1.5 × 1011 s−1 for BDP-1 and BDP-2, respectively. In acetonitrile). Formation of the triplet excited state of the Bodipy moiety was observed for both dyads upon photoexcitation, and the triplet state quantum yield depends both on the linker length and the orientation of the chromophores. Triplet state quantum yields are 13.4%, 97.5% and lifetimes are 13 μs, 116 μs, for BDP-1 and BDP-2, respectively. Spin-orbit charge transfer (SOCT) mechanism is proposed to be responsible for the efficient triplet state formation. The dyads were used for triplet-triplet-annihilation (TTA) upconversion, showing an upconversion quantum yield up to 3.2%.

1. INTRODUCTION Triplet state of organic chromophores is important for photosensitizing,1,2 photocatalysis,3−6 and organic spintronics.7−14 The population of the triplet state of chromophores is also critical for photodynamic therapy (PDT),1,2 and triplet-triplet-annihilation (TTA) upconversion.15−20 TTA upconversion is of particular interest because it only requires the use of non-coherent, low power


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density light sources. Moreover, the upconversion wavelength is tunable and the upconversion quantum yield are usually high, if compared to other photon upconversion methods, such as twophoton absorption or methods based on rare earth materials. In TTA upconversion two components are required, i.e. the triplet photosensitizer and the triplet energy annihilator/emitter. The triplet photosensitizer is responsible for excitation energy-harvesting and triplet state production via intersystem crossing (ISC). Triplet energy acceptor/annihilators play the role of upconverting the triplet energy and re-emit light at lower wavelength. One of the critical aspects in this process is that of developing novel triplet photosensitizers for TTA upconversion. Efficient ISC is prerequisite for high triplet state quantum yield. The ISC mechanisms of most of triplet photosensitizers are based on the heavy atom effect, i.e. the ISC is facilitated by the presence of heavy atoms such as iodine,1,21 bromine, and transition metal atoms such as Ir, Pt, Ru, and Re, etc. Some heavy atom-free organic triplet photosensitizers are known, such as the compounds with the n-π* → π-π* transition, which obeys El-Sayed rule,22,23 or with closelylying Sn/Tm states.24 Other approaches to access the triplet states of organic chromophores is by charge separation (CS) and charge recombination (CR).25−29 In this contest, a Bodipyphenothiazine dyad was prepared, and its CS process was studied, however, the photophysical properties of this compound were not analyzed in detail, especially the ISC property.27 Furthermore, application of similar kinds of photosensitizers for efficient triplet state generation is not common. To date, most of the triplet photosensitizers used for TTA upconversion are based on the heavy atom effect,16−18,30,31 while the employment of heavy atom-free photosensitizers is rare.23,32 Some of us reported the use of a Bodipy dimer with a C60 derivatives as heavy atom-free triplet photosensitizers.33 Besides the mentioned examples, triplet photosensitizers based on thermally activated delayed fluorescence were used for TTA


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upconversion.34 Although a few examples were reported, it is clear that much room is left to develop different heavy atom-free triplet photosensitizers for TTA upconversion. In order to address the above challenges, we studied the triplet excited state property of two Bodipy-phenothiazine dyads (BDP-1 and BDP-2. Scheme 1). Bodipy is the light absorbing chromophore in the dyads, due to its high stability, strong absorption of visible light, and versatile derivatization.35−40 Phenothiazine was selected as electron donor moiety due to its good redox property.41 As such the photo-induced electron transfer (PET), or the charge separation (CS), will occur upon photoexcitation. The charge recombination (CR) will probably induce ISC. Although similar dyads were reported previously, the triplet state properties were not studied.27,42 We studied the photophysical properties of the dyads using both steady-state and femtosecond/nanosecond transient absorption spectroscopies. Moreover we performed an electrochemical characterization and a computational analysis at Density functional theory (DFT) level to characterize the charge separation capabilities of the dyads. We found that both dyads undergo photo-induced CS and that upon CR the triplet excited state of Bodipy is formed. These new dyads were used as novel triplet photosensitizers for TTA upconversion and high upconversion quantum yield was observed.

2. EXPERIMENTAL SECTION 2.1. General Methods. All the chemicals used in synthesis are analytical pure and were used as received. Fluorescence quantum yields were measured in toluene with BDP (ΦF = 0.72 in tetrahydrofuran) as standards. The fluorescence lifetimes of the compounds were measured with OB920 luminescence lifetime spectrometer (with TCSPC mode. Equipped with EPL picosecond pulsed laser (445 nm, pulse width: 66.9 ps, maximum average power: 5 mW; Edinburgh


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Instrument Ltd., UK). Fluorescence spectra were recorded with a Shimatzu RF 5301PC spectrofluorometer. 2.2. Synthesis of Ptz-1. n-C4H9Br (3.84 g, 28 mmol) was added to a stirred mixture of phenothiazine (4.67 g, 23 mmol), Sodium hydride (NaH) (1.8 g, 75 mmol) in dry N,Ndimethylformamide (DMF) (30 mL). The mixture was heated at reflux for 2 h. Then the solvent was removed under reduced pressure. The mixture was washed with water and extracted with dichloromethane (DCM). The organic phase was dried over anhydrous Na2SO4. The solvent was removed under reduced pressure, and the residue was purified by column chromatography (silica gel; petroleum ether) to give the product as a light-yellow liquid. Yield: 3.6 g, 50.0%. 1H NMR (CDCl3, 400 MHz): δ = 7.13 (t, J = 7.6 Hz, 4H, Ar-CH), δ = 6.87 (d, J = 6.8 Hz, 4H, Ar-CH), 3.86 (s, 2H, CH2), 1.75–1.83 (m, 2H, CH2), 1.41–1.51 (m, 2H, CH2), 0.94 (t, J = 7.6 Hz, 3H, CH3) ppm. TOF−HRMS ([C16H17NS]+): calcd m/z = 255.1082, found m/z = 255.1085. 2.3. Synthesis of 1. A solution of NaOH (0.165 g, 1.61 mmol) in glacial acetic acid (10 mL) was added to a solution of Ptz-1 (0.35 g, 1.37 mmol) in chloroform (5 mL). Than a solution of bromine (0.07 mL, 1.37 mmol) in glacial acetic acid (3 mL) was added dropwise to the mixture at 0 °C. The mixture was stirred at 0 °C for 2 h. The reaction was quenched by a saturated aqueous solution of Na2S2O3. Organic layer was extracted by DCM and dried over Na2SO4. The solvents were removed under reduced pressure and the crude product was purified by column chromatography (silica gel; petroleum ether) to give the product as a light-yellow liquid. Yield: 0.27 g, 60.0 %. 1H NMR (DMSO-d6 , 400 MHz): δ = 7.36–7.32 (m, 2H, Ar-CH), δ = 7.23–7.19 (dt, J1 = 1.4 Hz, J2 = 7.3 Hz, 2H, Ar-CH), 7.16–7.13 (dd, J1 = 1.4 Hz, J2 = 8.6 Hz, 1H, Ar-CH), 7.02 (d, J = 7.3 Hz, 1H, Ar-CH), 6.98–6.92 (m, 1H, Ar-CH), 3.83 (t, J = 7.0 Hz, 2H, CH2), 1.67– 1.60 (m, 2H, CH2), 1.40–1.33 (m, 2H, CH2), 0.86 (t, J = 12.0 Hz, 3H, CH3) ppm. TOF−HRMS ([C16H16NSBr]+): calcd m/z = 333.0187, found m/z = 333.0182.


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2.4. Synthesis of 2. Phosphorous oxychloride (7.2 g, 4.3 mL, 47 mmol) was added dropwise to N, N-dimethyl formamide (4.58 g, 4.85 mL, 62.7 mmol) at 0 °C under N2. The white solid was formed after the mixture was stirred for further 15 min and compound Ptz-1 (3.97 g, 15.6 mmol) was added to the mixture. The reaction mixture was stirred at 0 °C for 3 h and then heated to 75 °C for 8 h. This solution was then cooled to room temperature, poured into ice/water mixture, and the mixture was neutralized slowly to pH 6-7 by adding saturated aqueous sodium bicarbonate (NaHCO3) solution. The mixture was extracted with DCM. Organic layer was dried over anhydrous sodium sulfate (Na2SO4). The solvents were removed under reduced temperature and the solid crude product was further purified by column chromatography (silica gel; DCM/petroleum ether, 1:2, v/v) to give the product as a light-yellow solid. Yield: 0.23 g, 51.0 %.1H NMR (CDCl3, 400 MHz): δ = 9.79 (s, 1H, CHO), 7.85–7.63 (dd, J1 = 1.4 Hz, J2 = 8.3 Hz, 1H, Ar-CH), 7.59 (s, 1H, Ar-CH), 7.17 (t, J = 8.3 Hz, 1H, Ar-CH), δ = 7.11 (d, J = 7.6 Hz, 1H, Ar-CH), 6.97 (t, J = 7.6 Hz, 1H, Ar-CH), 6.90 (t, J = 8.0 Hz, 1H, Ar-CH), 3.90 (t, J = 7.1 Hz, 2H, CH2), 1.84–1.76 (m, 2H, CH2), 1.52–1.42 (m, 2H, CH2), 0.96 (t, J = 7.4 Hz, 3H, CH3) ppm. TOF−HRMS ([C17H17NOS]+): calcd m/z = 283.1031, found m/z = 283.1038. 2.5. Synthesis of 3. Under an N2 atmosphere, the solution of 4-formylbenzeneboronic acid (1.50 g, 10.0 mmol) and pinacol (1.18 g, 10 mmol) in toluene (150 mL) was refluxed for 12 h at 120 °C. The solvents were removed under reduced pressure, white-yellow solid was obtained (2.3 g). This crude product was used for the next step of the synthesis without further purification. 2.6. Synthesis of 4. Under an N2 atmosphere, the above crude product of compound 3 (2.0 g, 8.7 mmol) was added to a solution of 2,4-dimethylpyrrole (2 mL, 20 mmol) in dry DCM (150 mL). A few drops of trifluoroacetic acid were added to the mixture, and the mixture was stirred overnight at room temperature. 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (2.0 g, 8.7


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mmol) was added to the reaction mixture. Then the mixture was stirred for another 2 h at room temperature, and then dry triethylamine (10 mL) was added. The solution was stirred for further 15 min. BF3·Et2O (10 mL) was added dropwise under ice bath cooling. After the reaction mixture was stirred for 2 h, water was added and the mixture was stirred overnight at room temperature. The mixture was then extracted with DCM. The organic phase was dried over anhydrous Na2SO4, and then the solvent was evaporated under reduced pressure. The residue was purified with column chromatography (silica gel, DCM) to give a red solid (0.75 g, yield, 10%). 1H NMR (CDCl3, 400 MHz): δ = 7.90 (d, J = 8.0 Hz, 2H, Ar-CH), δ = 7.29 (d, J = 8.0 Hz, 2H, Ar-CH), δ = 5.96 (s, 2H, Ar-CH), 2.54 (s, 6H, CH3), 1.38 (s, 12H, CH3), 1.36 (s, 6H, CH3) ppm. ESI−HRMS ([C35H35N3SBF2(M+1)]+): calcd m/z = 451.2461, found m/z = 451.2526. 2.7. Synthesis of BDP-1. Under N2 atmosphere, a solution of K2CO3 (35 mg, 0.25 mmol) in distilled water (1.4 mL) was added to a mixture of compound 1 (33.43 mg, 0.10 mmol) and 4 (44.23 mg, 0.10 mmol) in DMF (2.8 mL). After the mixture was purged with N2 for 30 min, then tetrakis(triphenylphosphane)palladium (3.8 mg, 0.0032 mmol) was added to the above mixture. the mixture was stirred at reflux for 14 h under N2. After cooling to room temperature, a solution of sodium sulfite and water was added to the reaction mixture. Organic layer was extracted by DCM and dried over Na2SO4. The solvents were removed under reduced temperature and the residue was purified by chromatography on silica gel (DCM/petroleum ether, 1:1) to give 16 mg (28%) of BDP-1 as a red solid. M.p. 217.4–218.2 °C. 1H NMR (CDCl3, 400 MHz): δ = 7.67 (d, J = 8.2 Hz, 2H, Ar-CH), δ = 7.46 (d, J = 7.6 Hz, 2H, Ar-CH), δ = 7.30 (d, J = 8.2 Hz, 2H, Ar-CH), 7.17 (t, J = 8.2 Hz, 2H, Ar-CH), 6.89–6.95 (m, 3H, Ar-CH), 5.99 (s, 2H, Ar-CH), 3.91(s, 2H, CH2). 2.58 (s, 6H, Ar-CH), 1.81–1.87 (m, 2H, CH2), 1.47–1.52 (m, 2H, CH2), 1.44 (s, 6H, ArCH), 0.97 (t, J = 7.4 Hz, 3H, CH3) ppm. 13C NMR (100 MHz, CDCl3, ppm): δ = 155.5, 145.0, 143.2, 143.1, 141.6, 140.5, 131.5, 128.5, 127.5, 127.4, 126.9, 125.8, 124.4, 122.6, 121.2, 115.6,


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115.5, 47.2, 29.7, 29.0, 20.2, 14.6, 13.9, OF−HRMS ([C35H34N3SBF2]+): calcd m/z = 577.2535, found m/z = 577.2511. 2.8. Synthesis of BDP-2. Under an N2 atmosphere, 2 (2.5 g, 8.7 mmol) and 2,4dimethylpyrrole (2 mL, 20 mmol) were dissolved in dry DCM (150 mL). A few drops of trifluoroacetic acid were added to the solution after degassed for 30 min, and the mixture was stirred overnight at room temperature. 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ) (2.0 g, 8.7 mmol) was added. The reaction mixture was stirred for another 2 h. Dry triethylamine (10 mL) was added. After this mixture was stirred for further 15 min, BF3·Et2O (10 mL) was added dropwise with ice bath cooling. After the reaction mixture was stirred for 2 h, water was added and the mixture was stirred overnight at room temperature. The mixture was then extracted with DCM. The organic phase was dried over anhydrous Na2SO4, and the solvent was evaporated under reduced pressure. The crude product was purified with column chromatography (silica gel, DCM/petroleum ether, 1:1, v/v) to give a red solid (0.994 g, yield, 23%). M.p. 189.0–189.4 °C. 1H NMR (CDCl3, 400 MHz): δ = 7.18 (t, J = 7.7 Hz, 1 H, Ar-CH), δ = 7.10 (d, J = 7.5 Hz, 1 H, Ar-CH), 7.01–6.99 (m, 2 H, Ar-CH), 6.98–6.88 (m, 3 H, Ar-CH), 5.98 (s, 2 H, Ar-CH), 3.89 (t, J = 6.9 Hz, 2H, CH2). 2.55 (s, 6H, Ar-CH), 1.86–1.79 (m, 2 H, CH2), 1.52 (s, 6H, Ar-CH), 1.48 (t, J = 7.5 Hz, 2 H, CH2), 0.89 (t, J = 7.3 Hz, 1 H, CH3) ppm. 13C NMR (100 MHz, CDCl3, ppm): δ = 155.4, 145.6, 144.8, 143.1, 140.9, 131.7, 128.5, 127.4, 127.0, 126.5, 126.0, 124.0, 122.7, 121.2, 115.7, 115.5, 47.4, 28.7, 20.0, 14.9, 14.6, 13.8. OF−HRMS ([C29H30BN3F2S]+): calcd m/z = 501.2222, found m/z = 501.2229. 2.9. Singlet Oxygen Quantum Yield (ΦΔ). 1,3-Diphenylisobenzofuran (DPBF) was used as 1

O2 scavenger. The 1O2 production was monitored by following the absorbance of DPBF at 414

nm. To determine the singlet oxygen quantum yield (ΦΔ), a relative method was used according to Eq. (1):


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Φ sam

⎛ 1 − 10− Astd ⎞ ⎛ msam ⎞ ⎛ ηsam ⎞ = Φ std ⎜ ⎟⎜ ⎟ − Asam ⎟ ⎜ ⎝ 1 − 10 ⎠ ⎝ mstd ⎠ ⎝ ηstd ⎠

2

(Eq. 1)

In the above equation, ‘sam’ and ‘std’ represent the sample and the standard. Φ, A, m and η represent the singlet oxygen quantum yield, the absorbance at excitation wavelength, the slope of the absorbance of DPBF changing over time, and the refractive index of the solvent used for measurement, respectively. Optically matched solutions were used (the solutions of the sample and the standard should give same absorbance at the excitation wavelength). Singlet oxygen quantum yields (ΦΔ) were measured in toluene with diiodo-BDP as the standard (ΦΔ = 0.85 in toluene). 2.10. Triplet State Quantum Yield (ΦT). To determine the triplet state quantum yield, a ground state bleaching method was used and the ΦT was calculated according to:

⎛ ε ⎞⎛ ΔA ⎞ Φ sam = Φ std ⎜ std ⎟⎜ sam ⎟ ⎝ ε sam ⎠⎝ ΔAstd ⎠

(Eq. 2)

In the above equation, ‘sam’ and ‘std’ represent the sample and the standard. Φ, ε and A respectively represent the triplet quantum yield, the molar absorption coefficient at steady state and the optical intensity of bleaching band in nanosecond transient absorption spectroscopy. Optically matched solutions were used (the solutions of the sample and the standard should give same absorbance at the excitation wavelength). Triplet quantum yield (ΦT) was measured with ground state bleaching method. Diiodo-BDP in toluene was used as standard (ΦT = 88%). The structures of BDP and diiodo-BDP are shown in Supporting Information (Scheme S1). 2.11. Femtosecond Transient Absorption Spectroscopy. Sub-picosecond transient absorption spectra were measured with a system based on a Ti:sapphire regenerative amplifier


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laser system (BMI Alpha 1000) and a fs-laser oscillator (Spectra Physics Tsunami). The details of the experimental system have been described in previous works.43 In all experiments samples were contained in a 2 mm quartz cuvette. All the samples were excited at 480 nm in order to minimize the scattering although in some cases it was still present. The excitation wavelength was generated using a home built Non-Linear Optical Parametric Amplifier (NOPA). In all cases excitation powers were on the order of 30 − 40 nJ. The light probe beam was obtained by focusing a portion of the 800 nm laser fundamental on a 3mm thick sapphire plate. A small portion of the white light continuum was splitted out and used as reference signal. Both pump and probe were focused on the sample with a 150 mm spherical mirror, the spot size of both beams on the focus was about 100 μm. Transient spectra have been recorded for a time interval spanning between −5 and 1500 ps. All data were analysed with a global analysis procedure using the software GLOTARAN.44 A sequential kinetic model was employed, from which both EADS (evolution associated decay spectra) and fit of kinetic traces are retrieved. In most cases three kinetic components were sufficient in order to correctly fit the data. 2.12. Nanosecond Transient Absorption Spectroscopy. The nanosecond time-resolved transient absorption spectra were measured with an LP920 laser flash photolysis spectrometer (Edinburgh Instruments, UK) and the signal was digitized with a Tektronix TDS 3012B oscilloscope. The samples were excited with a nanosecond pulsed laser (OPOLette 355II, tunable wavelength within the range 210−2400 nm). The lifetimes were obtained with the LP900 software. All samples for the flash photolysis experiments were deaerated with nitrogen (N2) for about 15 min before measurement, and the gas flow was maintained during the measurement. The delayed fluorescence of the upconversion mixtures was measured with the laser-induced-


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fluorescence (LIF) mode of a LP980 laser flash photolysis spectrometer (Edinburgh Instruments, UK). A focusing lens was placed between the cuvette and the detector to enhance the signal. 2.13. Theoretical Computations. The spin density surfaces of the compounds were calculated based on the DFT//B3LYP/6-31G(d)-optimized ground state geometries. The Gaussian 09 program package was used for the calculations.45

3. RESULTS AND DISCUSSIONS 3.1. Molecular Structure Designing Rationales. Brominated phenothiazine and 3-formyl phenothiazine are known compounds which can be prepared feasibly. As such we prepared dyad BDP-1 via the Pd(0)-catalyzed Suzuki coupling reaction with 3-bromophenothiazine and the meso-Bodipy boronic acid ester (Scheme 1). Dyad BDP-2 was prepared following a previously reported method, i.e. with the 3-formyl phenothiazine.42 The main structural difference between BDP-1 and BDP-2 is in the linker length, which may cause a decrease of the electronic coupling in BDP-1 compared with BDP-2. Moreover, based on the optimization of ground state geometry at DFT level (see later section), it appears that the orientation of the phenothiazine and Bodipy moieties is almost parallel in BDP-1 but orthogonal in BDP-2. Orthogonal orientation is beneficial for formation of triplet state following charge recombination (CR), due to an enhanced ISC through spin orbital charge transfer (SOCT).26,46,47 The photo-induced CS and the CR processes, as well as the production of the triplet excited state localized on the Bodipy unit, were studied for both dyads.


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Scheme 1. Synthesis of BDP Derivatives BDP-1, BDP-2. The Reference Compound BDP Is Also Presented.

C4H9

H N

a

b

N

S

C4H9

C4H9

N

c

N

S

S

S

Ptz-1

Br

1

O

B

O

N BDP-1

F

N B F

a N

N B

F C 4H 9

C4H9

N

d

S

N

e

S

CHO

N S

N

O HO

B

OH

F

N B

BDP-2

2

Ptz-1

F

C4H9

F

O B

f

g O

O B N

O

N

H

N F

B O

3

H

F

F

N B

4

F BDP

(a) n-C4H9Br, CTAB, NaOH, Ar, reflux for 6 h, yield: 50%; (b) Br2, KOH/AcOH, 0 °C, 2 h, yield: 60%; (c) Pd(PPh3)4, K2CO3, 1,2-dimethoxyethane/H2O, Ar, 14 h, yield: 28%. (d) POCl3, DMF, Ar, 90 °C, overnight, yield: 51%; (e) 2,4-dimethylpyrrole, TFA, DDQ, TEA, BF3·OEt2, DCM, RT, Ar, yield: 14%; (f) Pinacol, toluene, Ar, 120 °C, 4 h, yield: 96%; (g) Ar, CH2Cl2, TFA, DDQ, Et3N and BF3⋅Et2O, yield: 10%;


12

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3.2. UV−Vis Absorption and Fluorescence Spectra of the Dyads. The UV−Vis absorption spectra of the compounds are shown in Figure 1. The isolated phenothiazine molecule, Ptz-1, shows a strong absorption band at 257 nm. For BDP-1, an intense absorption band at 267 nm was observed, which is attributed to the phenothiazine moiety in the dyad. Moreover, an absorption band at 502 nm was observed, which is assigned to the Bodipy moiety.35 These absorption bands are similar to those of the respective components, indicating that no electronic interaction in dyad BDP-1 at ground state is established, and that the electronic coupling between the electron donor (phenothiazine) and the electron acceptor (Bodipy moiety) is weak, which may inhibit the direct photo-induced charge transfer. Accordingly, no charge transfer (CT) band was observed in the UV−Vis absorption spectra. Similar results were observed for BDP2.26,48

0.6

0.2

300

Ptz-1 BDP BDP-2

0.6

0.4

0.0

0.8

a

Ptz-1 BDP BDP-1

Absorbance

0.8

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


400 500 Wavelength / nm

600

b

0.4 0.2 0.0

300


600

Figure 1. UV−Vis absorption spectra of (a) Ptz-1, BDP and BDP-1; (b) Ptz-1, BDP and BDP-2. c = 1.0 × 10−5 M in DCM. 20 °C. The fluorescence spectra of dyads BDP-1 and BDP-2 are presented in Figure 2. The fluorescence intensity of BDP-1 is solvent polarity-dependent, being greatly reduced in polar solvents compared to that in less polar solvents (Figure 2a).42 It is well known that the


13


fluorescence of the pristine Bodipy shows no dependence on the solvent polarity, thus we tentatively propose that the fluorescence of the Bodipy moiety in BDP-1 is quenched by photo-

a BDP-1 Toluene DCM MeOH CH3CN

30

15

0

500

550

600

650

1.0 0.0

c Intensity / a.u

30 BDP-1 BDP-2

15

500

550 Wavelength / nm

600

BDP-2 Toluene DCM MeOH CH3CN

2.0

Wavelength / nm

0

b

3.0 Intensity / a.u

Intensity / a.u

45

Intensity / a.u

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

Page 14 of 45

500

550

600

650

Wavelength / nm

700

6

d

4

BDP-1 BDP-2

2 0

500

550

Wavelength / nm

600

Figure 2. Fluorescence emission spectra of (a) BDP-1; (b) BDP-2 in different solvents; (c) BDP-1 and BDP-2 in toluene; (d) BDP-1 and BDP-2 in DCM. Optically matched solutions were used (A = 0.037). λex = 475 nm. c = ca. 1.0 × 10−5 M. 20 °C. induced electron transfer (PET), or the charge separation (CS) processes.42 Interestingly, in case of BDP-2, the emission intensity is almost independent on the solvent polarity, but comparatively lower than that of BDP-1 in all solvents. This observation may indicate that PET is significant in BDP-2 since the fluorescence is already strongly quenched in non-polar solvents such as toluene. Interestingly, a charge transfer emission band at 658 nm was observed for BDP2, but not for BDP-1 (Figure 2a), in which the electronic coupling is weaker than that in BDP-2.


14

Page 15 of 45

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


To further support the hypothesis that PET is occurring in BDP-2, the fluorescence of the two dyads in toluene have been compared (Figure 2c). The emission of BDP-2 is much weaker than BDP-1. In more polar solvents, such as DCM, however, the two dyads show similar emission intensity, which is attributed to the more significantly quenched fluorescence of BDP-1. Table 1. Photophysical Properties of the Compounds

λabs/nm a

εb

λem/nm a

τF /ns c

ΦF (%) d

τT (μs)

ΦΔ(%) e

ΦT(%)

Ptz-1

257, 303

3.34, 0.53

−g

−g

−g

−g

−g

−g

BDP

231, 501

1.60, 7.54

510

3.67

72

−g

−g

−g

BDP-1

238, 267,

1.67, 2.21,

510

3.38

7.2 h,

13 j

0h

13.4 j

305, 502

0.75, 4.78

2.6 i

24.6 j 1.8 k

BDP-2

238, 267,

1.56, 2.05,

306, 502

0.73, 4.90

510

5.45

2.7 h, 2.4 i

116 j

1.3 h

97.5 j

67.3 j 34.9 k

DCM, c =1.0 × 10−5 M. b Molar absorption coefficient. ε : 104 M−1 cm−1. c Fluorescence lifetimes, double exponential fitting in DCM. This double exponential fitting means the majority of the singlet excited state was quenched by PET. d Fluorescence quantum yields. BDP in THF (ΦF = 72%) as reference. e Quantum yield of singlet oxygen (1O2). Rose Bengal (RB) was used as standard (ΦΔ = 0.8 in MeOH). f The triplet quantum yield, diiodo-BDP in toluene (ΦT = 88%) as standard. g Not applicable. h DCM. i Double exponential fitting in DCM. This double exponential fitting means the majority of the singlet excited state was quenched by PET. j In toluene. k In nhexane. a

The fluorescence emission of dyads BDP-1 and BDP-2 were also compared with that of the reference compound BDP (Supporting Information, Figure S16 and S17). Both BDP-1 and BDP-2 show weaker fluorescence emission than that of BDP, which is probably due to the


15

f


PET.42 BDP-2 is almost non-fluorescent as compared with BDP. The strong quenching effect may be due to the shorter distance between the phenothiazine and the Bodipy moieties.

a

b

n-hexane toluene p-xylene

24

Intensity / a.u

Intensity / a.u

48


n-hexane toluene p-xylene

6

48

c

..

480

840 mM Toluene

4

720

800

d BDP-2 in n-hexane 120 mM toluene 240 mM toluene

24 .. 12



36

BDP-1 in n-hexane 120 mM toluene 240 mM toluene

32

16

500

800

CPS/10

500

4

12

0

0

CPS/10

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 16 of 45

480

1200 mM Toluene BDP-3 in toluene

560 640 720 Wavelength / nm

800

Figure 3. Fluorescence emission spectra of different compounds; (a) BDP-1; (b) BDP-2. Optically matched solutions were used (A = 0.241). c = ca. 1.0 × 10−5 M. Changes of the emission spectrum of the compounds (c) BDP-1, c = 1.0 × 10−5 M; (d) BDP-2, c = 1.0 × 10−5 M in n-hexane in presence of different eq. toluene. λex = 475 nm, c = 1.0 × 10−5 M, 20 °C. We furthermore investigated the solvent dependency of the fluorescence by comparing the emission properties of the two dyads in three nonpolar solvents: n-hexane, toluene and p-xylene. For BDP-1, the fluorescence emission in three different solvents is similar (Figure 3a), while in the case of BDP-2 the emission in toluene is strongly quenched as compared to that in hexane (Figure 3b). Since the polarity of hexane and toluene is similar, the substantially quenched


16

Page 17 of 45

fluorescence of BDP-2 in toluene indicated strong interactions between the excited state of BDP-2 and solvent molecules. One possibility is the formation of exciplexes of Bodipy with toluene, similarly to what previously reported in the case of the naphthaleneimide fluorophore.49 In order to confirm the interaction between the BDP-2 and toluene molecules, we varied the concentration of toluene in hexane and recorded the fluorescence spectra of the system. The results show that the fluorescence of BDP-1 does not change with increased toluene concentration, but the fluorescence of BDP-2 is quenched and the intensity of the broadband emission observed at 658 nm becomes more significant at the increased toluene concentration (Figure 3d).

10000

10000

a BDP-2

BDP-2

1000

τ = 4.50 ns + 16.86 ns

Counts

τ = 3.60 ns + 6.94 ns

Counts

1000

100

c

b

BDP-1

1000

Counts

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


τ = 3.70 ns + 20.00 ns

100

100

10

10 0

20

40

60

Time / ns

80

100

10

0

10

20 30 Time / ns

40

50

0

20

40

60

80

100

Time / ns

Figure 4. Decay traces of the fluorescence of (a) BDP-1 at 510 nm; (b) BDP-2 at 510 nm; (c) BDP-2 at 658 nm. Excited with picoseconds pulsed laser (445 nm). c = 1.0 × 10−5 in toluene, 20 °C. Finally we measured the fluorescence lifetimes of the compounds (Figure 4). BDP-1 has a fluorescence lifetime of 4.6 ns, slightly longer that the pristine Bodipy (3.5 ns) and shows a biexponential decay, which is ascribed to the occurrence of photo-induced electron transfer.50−52 Similar results were observed for BDP-2. The fluorescence lifetime changes are in agreement


17


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 18 of 45

with the fluorescence intensity profile of the compounds. The photophysical parameters of the compounds are summarized in Table 1. 3.3. Electrochemical Characterization and the Gibbs Free Energy Changes of the PET of the Dyads. The electrochemical properties of the compounds were studied with cyclic voltammetry (Figure 5).25,53−55 For BDP, an oxidation wave at +0.75 V was observed, as well as a reversible reduction wave at −1.69 V. For BDP-1, an oxidation band at +0.32 V was observed, leading to propose that the phenothiazine moiety acts as electron donor and the BDP unit as the electron acceptor in the PET process. Similar results were observed for BDP-2. The redox potentials of the compounds are reported in Table 2. The free energy changes of the intramolecular electron transfer process can be calculated with the Weller equation (eqs 3 and 4). In the reported equations ΔGS is the static Coulombic energy, e = electronic charge, EOX = half-wave potential for one-electron oxidation of the electron-donor unit, ERED = half-wave potential for one-electron reduction of the electron-acceptor unit, E00 = energy level approximated with normalized UV−Vis absorption spectra and Fluorescence emission spectra, εS = static dielectric constant of the solvent, RCC = center-to-center separation distance between the electron donor (PTZ) and electron acceptor (BODIPY), determined by DFT optimization of the geometry, RD is the radius of the electron donor, RA is the radius of the electron acceptor, εREF is the static dielectric constant of the solvent used for the electrochemical studies, and ε0 is the permittivity of free space. The solvents used in the calculation of free energy of the electron transfer are toluene (εS = 2.38), CH2Cl2 (εS = 8.93), MeOH (εS = 33.7) and acetonitrile (εS = 37.5). The free energy differences for the charge-separation (ECS) were calculated with eqs 5 and 6. The data are collected in Table 3. Note that determination error exists for the free energy changes of the charge separation, but the trend of the free energy


18

Page 19 of 45

b

+

Fc / Fc

+

+

c

Fc / Fc

Fc / Fc

Current

Current

a 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


BDP-1

BDP

1.0

1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 Potential / V

0.5

0.0 -0.5 -1.0 -1.5 -2.0 Potential / V

BDP-2 1.0

0.5

0.0 -0.5 -1.0 -1.5 -2.0 Potential / V

Figure 5. Cyclic voltammogram of different compounds. Ferrocene (Fc) was used as the internal reference [E1/2 = +0.16 V (Fc+/Fc) vs standard hydrogen electrode]. (a) BDP; (b) BDP-1; (c) BDP-2; and Fc. In deaerated DCM solutions containing 0.10 M Bu4NPF6 as supporting electrode and with Ag/AgNO3 reference electrode. Scan rates: 100 mV/s. 25 °C. Table 2. Redox Potentials of Ptz-1, BDP, BDP-1, and BDP-2 a E(ox) (V)

E(red) (V)

Ptz-1

+0.30

−

BDP

+0.75

−1.69

BDP-1

+0.32, +0.78

−1.69

BDP-2

+0.36, +0.84

−1.69

a

Cyclic voltammetry in N2 saturated DCM containing a 0.10 M BuNPF6 supporting electrolyte; Counter electrode is Pt electrode; working electrode is glassy carbon electrode; Ag/AgNO3 couple as the reference electrode. changes in solvents with different polarity is clear, i.e. the CS is more favorable in polar solvents.

ΔGCS = e[ EOX − E RED ] − E00 + ΔGS


(3)

19


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

ΔGS = −

e2 4πε Sε 0 RCC

Page 20 of 45

1 ⎞⎛ 1 1 ⎞ e2 ⎛ 1 ⎟⎟⎜⎜ ⎜⎜ − ⎟⎟ − + 8πε 0 ⎝ RD RA ⎠⎝ ε REF ε S ⎠

(4)

(5)

ΔE CS = e[ EOX − ERED ] + ΔGS

(6)

ΔGCR = −(ΔGCR + E00 )

Calculation of the Gibbs free energy changes using the Weller equation shows that in both BDP1 and BDP-2 PET is thermodynamically allowed in polar solvents. PET is possible for BDP-2 even in the non-polar solvent toluene. This result is in agreement with the fluorescence emission study of the compounds, showing a fluorescence quenching even in toluene in comparison with the reference compound BDP. The PET driving force in polar solvents is however larger. The energy level of the charge transfer state (CTS) was also calculated (Table 3). It was found that the CTS energy levels decrease in polar solvents as compared with non-polar solvents. Table 3. Charge Separation Free Energy (ΔGCS) and Charge Separation Energy States (ECTS) for BDP-1 and BDP-2 in Different Solvents ΔGCS (eV)

ECTS (eV)

Toluene

CH2Cl2

Methanol CH3CN

Toluene

CH2Cl2

Methanol

CH3CN

BDP-1a

+0.29

−0.57

−0.80

−0.81

+2.73

+1.87

+1.64

+1.63

BDP-2 b

−0.03

−0.66

−0.83

−0.84

+2.42

+1.79

+1.62

+1.61

E00 = 2.44 eV. bE00 = 2.43 eV. E00 is the singlet state of Bodipy and λ is the wavelength of the crossing point of normalized UV−Vis absorption spectra and Fluorescence emission spectra. a


20

Page 21 of 45

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


3.4. Ultrafast Transient Absorption of the Compounds. In order to further verify the occurrence of PET in the dyads in different solvents and characterize the timescale of the process, and thereafter the possible charge transfer process, we recurred to ultrafast transient absorption spectroscopy. Compound BDP-1 was analyzed in two solvents, acetonitrile and toluene (Figure 6). In toluene excitation of the sample induces the immediate bleaching of the ground state absorption of the Bodipy moiety, and its stimulated emission. At the short time scale an excited state band peaking at ca 430 nm is visible, whose intensity is mostly decaying on a 6 ps timescale. The time evolution of the excited state signals is quite limited and closely reminds that of an isolated Bodipy. The negative band decays bi-exponentially on a 125 ps and 1.9 ns timescale. This ground state bleaching with bi-exponential decay means two processes occur after excitation which are the PET process and charge transfer process. However, bands indicating the involving of the phenothiazine moiety are not clearly identified in the spectra. The excited state spectra measured in acetonitrile appear to be qualitatively very similar to those observed in toluene, however the time evolution of the signal is quite different. The Bodipy bleaching recovers much faster in this solvent as indicated by the comparison between the kinetic traces registered in the two solvents in the bleaching region, reported in Figure 6c. In acetonitrile, a very broad weak band peaking at ca 650 nm is observed in the first and second evolution-associated difference spectrum (EADS) (see inset in Figure 6a), apparently decaying on a 20 ps timescale. The shortening lifetime of the Bodipy is most probably explained by the occurrence of electron transfer towards the phenothiazine moiety, occurring on a 4.5 ps timescale. The timescale of charge recombination is not easily retrieved, since no specific signals can be attributed to the charge separated state. Both the 590 nm band previously attributed to the Bodipy radical anion species and the 524 nm band assigned to phenothiazine radical cation were in fact not observed in the transient spectra,27 possibly because they are covered by the intense


21


negative bleaching/stimulated emission signal peaking at 505 nm. The only feature possibly attributable to the CS state could be the weak broad feature peaking at 650 nm, and according to this assignment the CS recombination would be 22 ps. In both solvents, no triplet signatures are visible, indicating that triplet formation occurs on a > 2 ns timescale. Compound BDP-2 has also been studied in acetonitrile and toluene. The results are similar to that previously reported.27 Further details on the influence of solvent polarity on PET process in both dyads, and the charge recombination process were studied. 0.1 0.0

0.0

BDP-1 in CH3CN 4.5 ps 22.7 ps 2.06 ns

-0.2

500

600

700

BDP-1 in Toluene 6 ps 125 ps 1.9 ns

-0.1

0.00 ΔA

-0.3

a -0.4

-0.02

-0.2

-0.2

b

-0.04


400

516 nm Toluene

-0.1 ΔA

0.02

BDP-1 502 nm CH3CN

0.0 ΔA

-0.1 ΔA

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 22 of 45


c 0

100 200 Time / ps

300

Figure 6. The evolution-associated difference spectrum (EADS) obtained by global analysis of the transient data recorded for compound BDP-1 in (a) acetonitrile and (b) toluene. Panel (c) reports the comparison of time traces registered in the two solvents on the Bodipy ground state (GS) bleaching band. Compound BDP-2 has also been studied in acetonitrile and toluene (Figure 7). In acetonitrile, the dynamic evolution of the excited state signals is quite rich. Upon excitation on the Bodipy moiety at 480 nm the immediate bleach of the ground state of the molecule is observed, peaking at 508 nm. The bleaching intensity significantly decreases on a very short 0.5 ps timescale, and on the same time a positive broad band peaking at about 550 nm develops, possibly indicating the formation of a CS state. The negative Bodipy signal further decays on a 5 ps timescale, while


22

Page 23 of 45

the positive band sharpens and blue shifts. The latter evolution could be indicative of vibrational relaxation within the CS state. Subsequently, on a 6.5 ps timescale, both the Bodipy negative band and the positive feature mostly decay, possibly indicating the occurrence of fast charge recombination. The small residual signal remaining on a longer timescale completely decays on a 790 ps timescale (the small positive feature at 480 nm observed in the blue and green EADS is most probably due to pump scattering). No significant triplet formation is expected for compound BDP-2 in acetonitrile.

0.1 0.0 -0.2

-0.1

BDP-2 in CH3CN

-0.2

0.5 ps 5.1 ps 6.5 ps 790 ps

-0.3

ΔA

ΔA

0.0

BDP-2 in Toluene 0.2 ps 4.7 ps 1.7 ns inf

-0.4 -0.6

a

b

-0.8



0.0

0.04 0.02 ΔA

-0.1 BDP-2 514 nm Toluene 506 nm CH3CN

ΔA

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


-0.2

-0.3

c 0

100 200 Time / ps

0.00 BDP-2 532 nm CH3CN

-0.02

534 nm Toluene -0.04

0

100

d

200

Time / ps

Figure 7. EADS obtained by global analysis of the transient data recorded for compound BDP-2 in (a) acetonitrile and (b) toluene. Panel (c), (d) report the comparison of time traces registered in the two solvents respectively on the Bodipy GS bleaching band, and in the 530 nm region.


23


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 24 of 45

In toluene the time evolution of the excited state spectra is quite different. The initial Bodipy bleaching signal recovers almost 50% on a fast 200 fs timescale possibly because of electron transfer towards the phenothiazine moiety. No positive bands are observed in the second EADS, possibly because they are covered by the intense negative band peaking at 505 nm. On a 4.7 ps broad positive feature extending between 530 and 630 nm develops while the bleaching intensity apparently increases (this could be an artifact due to scattering). Finally, the positive band recovers on a 1.7 ns timescale, while a negative signal, not decaying on the timescale of the experiment remains (green EADS, infinite lifetime). 3.5. Nanosecond Transient Absorption Spectroscopy: Triplet Excited State Properties. In order to verify the production of the triplet states upon photoexcitation of the dyads, nanosecond transient absorption (ns TA) studies were performed. It should be pointed out that previously similar dyads were reported, but the triplet state of those dyads were not studied.27,42 For BDP-1, a bleaching signal at 500 nm was observed upon pulsed excitation at 532 nm. The transient signal decays within 13 μs. The lifetime was greatly reduced to 391 ns in aerated solution, thus indicating that the transient observed is due to the triplet excited state (Supporting Information, Figure S18). Both the electrochemical study and the ultrafast transient absorption measurements presented above indicated the occurrence of PET for BDP-1. We propose that the triplet state is produced upon CR.27 The CS capabilities of phenothiazine-Bodipy dyads were previously studied, but the production of triplet excited states was not analyzed with ns TA spectroscopy.42 The signals measured for BDP-1 are very weak, preventing the registration of transient spectra with good signal-to-noise ratio. This indicates that the quantum yield of triplet production (ISC efficiency) of BDP-1 is low. Accordingly, the singlet oxygen quantum yield of BDP-1 in toluene was determined as 24.6% (in toluene. Table 1).


24

Page 25 of 45

Similar nanosecond transient absorption spectra study was carried out for BDP-2 (Figure 8a). Due to the smaller distance between the phenothiazine and the Bodipy moieties, the driving force for PET is larger in BDP-2. Accordingly, significant transient absorption signal was observed in toluene. The transient difference spectrum is very similar to that measured in case of triplet formation for an isolated Bodipy molecule. The triplet state lifetime was determined to be 116 μs by fitting the decay of the time trace at 505 nm. No signal was detected in acetonitrile.

0.00

0.00 600.00 μ s 570.00 μ s ... 30.00 μ s 0 μs

-0.08

-0.08

τ = 116 μ s λ = 505 nm

Δ O.D

ΔO.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


-0.16

a

-0.16

b -0.24

400


700

0

500

1000 1500 time / μ s

2000

Figure 8. Nanosecond transient absorption spectra of BDP-2. (a) Transient absorption with different delay times. (b) Decay trace at 505 nm. λex = 532 nm. c (BDP-2) = 1.0 × 10−5 M. In toluene. 20 °C. It should be noted that the singlet oxygen quantum yield of BDP-2 is high (ΦΔ = 67.3%).24 Although photo-induced charge separation was also observed in BDP-1, the triplet state yield of BDP-1 is much lower than BDP-2. We tentatively attribute the high triplet state quantum yield of BDP-2 to the shorter linker length and to the orthogonal orientation of the phenothiazine and the Bodipy chromophores. Such an orientation is beneficial for the SOCT enhanced ISC.26 The energy levels and photoinduced processes occurring in the two dyads are presented in the Jablonski Diagram (Scheme 2). Previously the energy diagram of similar dyads was proposed,


25


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 26 of 45

but the energy levels of the charge transfer states (CTS) were all lower than the Bodipy triplet state,42 which does not allow to rationalize the production of triplet state with the dyads. Scheme 2. Simplified Jablonski Diagram Illustrating the Photophysical Processes with (a) BDP-1; (b) BDP-2. BDP stands for ‘Bodipy’; PTZ stands for ‘Phenothiazine’ In CH3CN

a

[PTZ-1BDP*]

CSS

2.73 eV Toluene

b

[PTZ-1BDP*]

Toluene 1/k = 0.5 ps

CS

CS

1/k = 4.5 ps

2.49 eV 497 nm

[PTZ+•-BDP−•] 3 CR [PTZ- BDP] 1.63 eV CSS

CH3CN

τF = 3.2 ns

CSS 2.42 eV

1/k = 22 ps

2.49 eV 497 nm

[PTZ+•-BDP−•] 3 CR [PTZ- BDP] 1.61 eV CSS

CH3CN

1/k = 6.5 ps

1.52 eV

1.52 eV

τT = 13 μs in Toluene

τF = 5.7 ns

τT = 116 μs In Toluene

S0

S0

3.6. DFT Calculations. In order to complement the experimental study presented above and to further characterize the photophysical properties of these novel triplet photosensitizers, we carried out DFT calculations on the electronic excited states of the compounds.56−60

BDP-2

BDP-1

Figure 9. Spin density surfaces of the compounds at the optimized triplet state geometry; calculated at B3LYP/6-31G(d) level with Gaussian 09W.


26

Page 27 of 45

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


LUMO + 1

LUMO + 2

f = 0.009 400 nm

f = 0.683 326 nm

LUMO

LUMO

f = 0.378 436 nm

LUMO

LUMO

f = 0.000 694 nm

f = 0.000 591 nm

f = 0.000 454 nm

HOMO HOMO

HOMO− 1

S7 3.97 eV 2.90 eV S2

S2

CS S1

f = 0.699 427 nm

1.78 eV

LUMO

2.73 eV

T3

LUMO

T2 ISC CR

Absorption

f = 0.000 589 nm

HOMO

S3

S1 LUMO

HOMO − 4

3.10 eV

2.84 eV 2.10 eV

HOMO

2.10 eV

T1

f = 0.000 810 nm

1.53 eV HOMO − 1

Emission HOMO − 1

HOMO

S0

Figure 10. Selected frontier molecular orbitals involved in the excitation and triplet excited states of BDP-2. The butyl group was simplified as methylamine in the calculation. The UV−Vis absorption and the fluorescence emission were calculated by DFT and TDDFT// B3LYP/631G(d) level with Gaussian 09W, based on the optimized ground state and the excited state geometries, respectively. CS = charge separation, CR = charge recombination.

The energy-optimized geometry of BDP-2 at ground state indicated that the two parts (Bodipy and PTZ) takes an orthogonal geometry (87°. Figure 10). The phenothiazine moiety has non-


27


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coplanar geometry and the two phenyl rings are twisted by 32°. The calculated UV−Vis absorption bands for BDP-2 (S0 → S2 and S0 → S3) are located at 427 and 326 nm, respectively, for which H−1 → L and the H → L+2 are the respective major components of the transitions. On the other hand, the S1 state is a dark state and the S0 → S1 transition is a forbidden transition, due to the orthogonal geometry of the chromophores and the decoupled HOMO and LUMO orbitals.49,59,61,62 Similar results were observed for BDP-1 (Supporting Information, Figure S22 and Table S2). The dihedral angle between the phenothiazine and the BDP moieties is of 55°, which is much smaller than that of BDP-2 (87°). The CT feature of the S1 state is responsible for the observation of the red-shifted emission at 658 nm (Figure 2b). The S1 states of BDP-1 and BDP-2 were optimized, and both states have CT character (Figure S23 Supporting Information). At S1 state, the orientation of the phenothiazine and the BDP moiety in BDP-2 does not change (86°), which is due to the restricted rotation around the C−C bond at the meso- position of Bodipy (the linker between the Bodipy and the phenothiazine moiety). For BDP-1, however, the dihedral angle becomes smaller at the optimized S1 state geometry (40°). Moreover, in both the optimized S1 states of BDP-1 and BDP-2 the phenothiazine moiety becomes more coplanar (the two phenyl rings in the phenothiazine moiety are twisted by ca. 10°, much smaller than that of 32° at ground state). Note that the phenothiazine moiety has a radical cations form in the S1 state of the dyads. This conformational change is in agreement with previous reports.63,64 The virtual S0→Tn excitations (energy gap between the ground state and the triplet excited states) were calculated based on the optimized ground-state geometry with the TDDFT method, and the molecular orbitals involved in these transitions were analyzed (Figure 10 and Table 4). As shown from computed spin density surfaces of the compounds (Figure 9), the T1 state is


28

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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


localized on Bodipy chromophore and has not CT character. The localization of the triplet state on the Bodipy for both BDP-1 and BDP-2 is in agreement with the experimental results, because the nanosecond transient absorption spectra show the typical features of the triplet state of Bodipy.65,66 The geometry of the T1 states of both BDP-1 and BDP-2 were optimized (Figure S24 Supporting Information). At T1 state, the orientation of the phenothiazine and the BDP moiety in BDP-2 does not change (dihedral angle is 87°) compared with S1 state (86°). However, the phenothiazine moiety has non-coplanar geometry with the two phenyl rings twisted by 34° , while it has a coplanar geometry in the S1 state (the phenothiazine moiety ca. 10° and it is in the radical cations). For BDP-1, however, the dihedral angle of the two chromophores at the triplet state geometry (56°) of BDP-1 is larger than that of the optimized S1 state geometry (40°). Moreover, the phenothiazine moiety has non-coplanar geometry (34°) in the T1 state of BDP-1 compared with a coplanar geometry of S1 state (ca. 10°). Interestingly, the optimized S1 state energy (1.78 eV) is similar to that of the T1 state (1.53 eV). Note that the optimized S1 state has charge transfer character and the phenothiazine and the Bodipy moieties assume an orthogonal geometry (86° for BDP-2). As such the S1 → T1 ISC process may be enhanced because it has a SOCT nature.67 Similar results were observed for BDP-1 (Supporting Information, Figure S23). It should be noted that there is no heavy atom in BDP-1 and BDP-2, yet triplet state formation was observed for the dyads, especially for BDP-2 (Table 1 and Figure 8). As already mentioned above, we propose that CR may lead to the formation of the Bodipy T1 state. Normally direct 1

(CT)→3π−π* transition (a CR process) is forbidden, however, for BDP-2, the geometry

between the phenothiazine and the Bodipy moieties is almost orthogonal, thus the so called spin


29


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Table 4. Electronic Excitation Energies (eV) and Corresponding Oscillator Strengths (f), Main Configurations, and CI Coefficients of the Low-Lying Electronic Excited States of compound BDP-2. The Electronic Transition were Calculated by TDDFT//B3LYP/631G(d), Based on the DFT//B3LYP/6-31G(d)-optimized Ground State and Excited Geometries.

Excitation

Emission

Triplet

Electronic transition a

Energy b

S0 → S1

2.10 eV/589 nm

0.000

0.704

H→L

S0 → S2

2.90 eV/427 nm

0.575

0.699

H−1 → L

S0 → S7

3.97 eV/312 nm

0.101

0.683

H → L+2

S0 → S1

1.78 eV/694 nm e

0.000

0.705

H→L

S0 → S2

2.84 eV/436 nm e

0.378

0.671

H−1 → L

S0 → S3

3.10 eV/400 nm e

0.009

0.703

H → L+1

T0 → T1

1.53 eV/810 nm

0.000 f

0.709

H−1 → L

T0 → T2

2.10 eV/591 nm

0.000 f

0.703

H→L

T0 → T3

2.73 eV/454 nm

0.000 f

0.675

H−3 → L

f

c

CI d

Composition e

a

TDDFT//B3LYP/6-31G(d), based on the DFT//B3LYP/6-31G(d)-optimized ground state geometries. b Only the selected low lying excited states are presented. c Oscillator strengths. d CI coefficients are in absolute values. e TDDFT//B3LYP/6-31G(d)-optimized excited state geometries. f No spin−orbital coupling effect was considered; thus, the f values are zero. orbit charge transfer (in this case charge recombination. SO-CT ISC) may be responsible for the efficient ISC,67 since the electron spin angular momentum changes occurring during ISC can be compensated by the orbital angular moment change of CR. Another factor which contributes to the efficient ISC may be the closely-lying S1/T1 states. For BDP-1, however, the phenothiazine


30

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moiety and the Bodipy are in a more coplanar geometry (the dihedral angle is 40° for the optimized S1 state geometry), and as a result the ISC is less efficient. 3.7. Application of the Dyads for TTA Upconversion. Triplet photosensitizers used for TTA upconversion usually possess heavy atoms or are based on spin converters.16−18 To the best of our knowledge, triplet photosensitizers in which the triplet state is formed via charge recombination were not previously reported.16−18 For BDP-1, upconversion was observed using perylene as the triplet acceptor/emitter (ΦUC = 0.6%. See Supporting Information, Figure S20. The low efficiency of upconversion is in agreement with the lower triplet state quantum yield of BDP-1 (ΦT = 13.4%. Table 1). 0.9 300 Intensity / a.u

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


a

c

0.6

200

y

100

0

BDP-2 (0.46, 0.46)

BDP-2 4.0 eq. Perylene

400

600 Wavelength / nm

800

BDP-2 + Py (0.21, 0.22) 0.3

0.0 0.0

0.2

0.4

0.6

0.8

x Figure 11. TTA upconversion with BDP-2 as the triplet photosensitizer and perylene (Py) as the acceptor. (a) Upconversion spectra. (b) The photographs of triplet photosensitizers alone and the upconversion. (c) CIE diagram of BDP-2, the data are derived from (a). Upconversion was performed upon excitation of the solution with 510 nm continuous laser and the power density is 73.6 mW/cm2. c(Photosensitizer) = 1.0 × 10−5 M, c(Acceptor) = 4.0 × 10−5 M, in toluene, 20 °C. Interestingly, significant upconversion was observed for BDP-2 (ΦUC = 3.2%. Figure 11). For the BDP-2 alone, emission at 514 nm and 658 nm were observed upon 510 nm continuous laser


31


excitation. In the presence of triplet acceptor/annihilator perylene, the upconverted fluorescence of perylene in the range of 420 – 494 nm was observed. Excitation of the perylene alone with 510 nm laser does not produce this emission band, thus the upconversion of the mixture was confirmed. The TTA upconversion with the organic triplet sensitizers is visible even to unaided eyes (Figure 11). Compound BDP-2 alone gives a very weak emission upon 532 nm laser excitation. In the presence of perylene, however, the bright blue emission of perylene was observed, which is due to the significant TTA upconversion. The CIE coordinates of the emissions of the photosensitizer and the upconversion are presented in Figure 11c.

Power density 2 mW / cm

a

600

300

0 420

450

480

Wavelength / nm

900 integrated UCL Intensity

900 UCL intensity / (a.u)

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

Page 32 of 45

b

600

300 Slope = 0.91

10

30

100 -2

Power density (mW cm )

Figure 12. (a) Upconverted luminescence spectra of BDP-2 and Perylene upon excitation with a CW 510 nm laser at different incident power densities. (b) Integrated upconversion emission intensity data from part (a) plotted as a function of incident power density. All spectra are measured in deaerated toluene. c(BDP-2) = 1.0 × 10−5 M, c(perylene) = 4.0 × 10−5 M. 20 °C. The efficient TTA upconversion capability of BDP-2 was confirmed by the upconversion intensity−excitation power relationship (Figure 12). A linear relationship, instead of the typical quadratic relationship, was observed (Figure 12b). It was proposed that a linear relationship can


32

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be observed if the triplet-triplet-energy-transfer (TTET) and the TTA upconversion are efficient.68

0.024

a

BDP-2 0 ns 104 ns

0.08 ...

0.04

0.016

τ2 = 30.0 μs

0.008

4.888 μs

b

τ1 = 4.7 μs

AU

0.12

AU

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


0.000 0.00 400


800

0

60 120 Time / μs

180

Figure 13. Delayed fluorescence of the TTA upconversion observed upon OPO nanosecond pulsed laser excitation of the mixture of BDP-2 and perylene. (a) Nanosecond transient emission spectra of the mixture of BDP-2 (c = 1.0 × 10−5 M) with perylene (c = 4.0 × 10−5 M) after nanosecond pulsed laser excitation (λex = 510 nm). (b) Decay traces at 470 nm of the mixture upon nanosecond pulsed laser excitation (λex = 510 nm). In deaerated toluene, 20 °C. Finally, the kinetics of the delayed fluorescence of the TTA upconversion was studied with nanosecond transient absorption spectroscopy (Figure 13). Prior to the emission of the delayed fluorescence (upconverted emission), TTET and TTA exist. These processes are intermolecular and their kinetics is diffusion-controlled, thus it is slow and the processes may be recorded with nanosecond emission monitoring. The results show that the upconverted emission at 470 nm did not appear promptly, instead, it grows until 5 μs, then decayed within 30 μs. The growth is related to the accumulation of the acceptor (perylene) at the triplet state, and the second phase of the decay curve is related to the decay of the triplet state of the triplet acceptor. To the best of our knowledge, this is the first observation that the upconverted fluorescence emission follows a slow growth after the pulsed laser excitation.


33


The TTET process between the triplet photosensitizer and the triplet acceptor was studied by monitoring the decay of the triplet state signal of the dyads with different equivalent of perylene, and the lifetime-based Stern-Volmer quenching curves were constructed (Figure 14). The results show that the BDP-2 curve has a larger slope than the curve of BDP-1. The Stern-Volmer quenching constant of BDP-2 (6.6 × 104 M–1) is larger than that of BDP-1 (3.9 × 104 M–1) (Table 5). These quenching constants are in agreement with longer triplet state lifetime of BDP-2 (116 μs) than that of BDP-1 (13 μs. Table 1).

24

BDP-1 BDP-2

16 (I0/ It) - 1

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

Page 34 of 45

8

0 0

12 c / 10

24 M

36

-5

Figure 14. Stern–Volmer plots generated from phosphorescence intensity-quenching of BDP-1 and BDP-2 (λex = 502 nm), perylene was used as quencher. c (BDP-1 or BDP-2) = 1.0 × 10−5 M, in toluene, 20 ° C. We also calculated the bimolecular quenching constants (kQ. See Supporting Information for the details), in comparison to the diffusion controlled bimolecular rate constants (Table 5), we concluded that the TTET process is a diffusion controlled process, i.e. the intermolecular process is efficient. The quenching efficiency (fQ) were determined as 33% and 28% for BDP-1 and BDP-2, respectively.


34

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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


Table 5. Upconversion-related Parameters with BDP-1 and BDP-2 as the Photosensitizer a

BDP-1

BDP-2

KSV b/104 M–1

3.9

6.6

kq c/108 M–1 s–1

4.3

3.4

Rq d/10−10 m

3.74

3.74

Dq e/10−6 cm2 s−1

9.95

9.95

k0 f /109 M–1 s–1

1.30

1.22

fQ g/ %

33

28

ΦUC h / %

0.6

3.2

In deaerated toluene, c = 5 × 10−6, λex = 500 nm. b Stern–Volmer quenching constants. c Bimolecular quenching constants. KSV = kqτp. d The molecule radius of the quencher. e In toluene, η = 5.9× 10−4 Pa⋅s; T = 293.15 K; Rf (BDP-1) = 6.56 × 10−10 m, Rf (BDP-2) = 4.38 × 10−10 m; Df is the diffusion coefficients of the energy donor, Df (BDP-1) = 5.62 × 10−6 cm2s−1, Df (BDP-2) = 8.41 × 10−6 cm2s−1, Dq is the diffusion coefficients of the quencher. f Diffusion-controlled bimolecular quenching rating constants. g The quenching efficiency. h The upconversion quantum yields, diiodo-BDP (ΦF = 2.7%) as reference in CH3CN. a

3.8 Conclusions. We studied the triplet excited state property of two phenothiazine-Bodipy dyads which contain different length of linkers between the two chromophores. The geometry of the phenothiazine and the Bodipy moieties is orthogonal in the dyad with shorter linker (BDP-2) and it is more coplanar in the dyad with longer linker (BDP-1). The photophysical properties of the dyads were studied with steady-state absorption and emission spectroscopies, as well as theoretical computations. The fluorescence of the Bodipy unit in these dyads was significantly quenched as compared to that unsubstituted Bodipy. The occurrence of charge transfer and the production of the triplet excited state of Bodipy were confirmed by picosecond/nanosecond


35


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Page 36 of 45

transient absorption spectroscopy and the triplet state quantum yield was found to be higher for BDP-2 (ΦT = 97.5%, τT = 116 μs) than for BDP-1. ΦT = 13.4%, τT = 13 μs). Our results suggest that the orientation of the phenothiazine and the Bodipy moieties may play a role in the triplet state formation. We propose that intersystem crossing (ISC) of the Bodipy moieties in the dyads occurs via a spin-orbital charge transfer (SO-CT ISC) mechanism. The CR induced triplet state formation of the dyads were used for triplet-triplet-annihilation upconversion. The upconversion quantum yield was determined as 3.2%. These results are useful for study of fundamental chemistry of Bodipy chromophores, as well as for development of new heavy atom-free triplet photosensitizers.

ASSOCIATED CONTENT Supporting Information NMR and HR-MS spectra, steady state UV–vis absorption and luminescence spectra, nanosecond transient absorption spectra, TTA upconversion of BDP-1, and DFT/TDDFT calculation details about BDP-1 and BDP-2. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxx.

ACKNOWLEDGMENTS We thank the NSFC (21473020, 21673031, 21273028, 21421005 and 21603021), Program for Changjiang Scholars and Innovative Research Team in University [IRT_13R06], the Fundamental Research Funds for the Central Universities (DUT16TD25, DUT15ZD224, DUT2016TB12) for financial support. REFERENCES


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Triplet Excited State of BODIPY Accessed by Charge Recombination

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