Intramolecular Energy and Electron Transfers in Bodipy

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry

Intramolecular Energy and Electron Transfers in Bodipy-Naphthalenediimide Triads Mushraf Hussain, Ahmed M. El-Zohry, Habtom B. Gobeze, Jianzhang Zhao, Francis D'Souza, and Omar F. Mohammed J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b03884 • Publication Date (Web): 01 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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The Journal of Physical Chemistry

Intramolecular Energy and Electron Transfers in Bodipy-Naphthalenediimide Triads Mushraf Hussain,a¶ Ahmed M. El-Zohry,b¶ Habtom B. Gobeze,c Jianzhang Zhao,a* Francis D’Souza,c,* and Omar F. Mohammed b,* 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

KAUST Solar Center, Division of Physical Sciences and Engineering, King Abdullah

University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia. E-mail: [email protected] c

Department of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX 76203-5017, USA. E-mail:[email protected]

¶The authors contributed equally to this work.

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ABSTRACT: Borondipyrromethene (BDP) naphthalenediimide (NDI) triads (BDP-NDI) and Diiodo-BDP derivative (DiiodoBDP-NDI)) were synthesized to study the Förster Resonance Energy Transfer (FRET) and its impact on the triplet state formation and dynamics. In these triads, Diiodo-BDP and BDP are the energy donors and NDI is the energy accepter. Nanosecond transient absorption spectra of triads indicated that triplet state is localized on NDI moiety, either by selective photoexcitation of the Diiodo-BDP or NDI unit. The intersystem crossing (ISC) is attributed to intramolecular heavy atom effect. The triplet state quantum yield was found to be 54 % with a lifetime of 38 µs. On the other hand, no triplet state is observed for BDP-NDI system either by exciting BDP or NDI unit. Thus we confirmed that charge recombination does not produce triplet state. Interestingly, DiiodoBDP-NDI can be used as broadband excitable (500 ~ 620 nm) triplet photosensitizer and high triplet–triplet annihilation (TTA) upconversion quantum yield of ΦUC = 2.8 % was observed with 9,10-bis(phenylethynyl)-anthracene (BPEA) as a triplet accepter/emitter.

1. INTRODUCTION Triplet photosensitizers have a potential use in a variety of applications including photodynamic therapy,1–3 photocatalytic hydrogen production,4–7 photoinduced charge separation, photovoltaics,8–11 and photocatalytic synthetic organic reactions.12–16 Recently, these compounds are investigated for triplet–triplet annihilation (TTA) assisted upconversion.16–18 All these applications share a common feature: a long-lived triplet excited state of the photosensitizer which makes intermolecular energy transfer or/and electron transfer more efficient.19 Conventional triplet photosensitizers are usually based on single chromophore molecular structures, thus the absorption band in the visible spectral range is narrow (normally there is only one major absorption band in visible range).1,2 In order to harvest photo-energy from broadband


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light source from the singlet excited state, dyads and triads have been constructed which contain two or more chromophores with complimentary absorption and emission properties, also undergoing efficient Förster Resonance Energy Transfer (FRET).20,21 On the other hand, a few broadband absorbing triplet photosensitizers are recently reported. For instance, Bodipyperylenebisimide (BDP-PBI) dyad/triad and NDI triad show very efficient resonance energy transfer (RET).22–30 However, much room is still left for this area to study the intramolecular energy transfer in triplet photosensitizers. For instance, in BDP-PBI, intersystem crossing (ISC) is not completely inhibited by RET and charge separated state (CS) does not perturb the triplet state of PBI.23,25 In diiodobodipy-styrylbodipy dyad, on the other hand , ISC is inhibited by FRET to a large extent.25 A NDI triad was reported, which was based on three NDI units in which symmetry breaking CS is resulted.31 In Zinc porphyrin NDI (ZnP-NDI) dyad connected by 1,2,3-triazole linker, triplet state of ZnP is not suppressed by CS.32 However, in peptide linked ZnP-NDI triad, inhibition of triplet state is observed upon CS.33 To gain more insights into the photophysical processes in the broadband light−absorbing triplet photosensitizers, we have carefully designed two BDP-NDI triads (Scheme 1) to study the intramolecular energy transfer and its significant impact on the triplet state properties.34 With DiiodoBDP-NDI triad, we are able to explore and decipher the competition between FRET and ISC using steady−state and time−resolved spectroscopies. In this system, DiiodoBDP unit is singlet energy donor while NDI act as energy accepter. With the non−iodinated triad, BDP-NDI as a reference compound, we are able to clarify the effect of CS/CR on the triplet state population.

2. EXPERIMENTAL SECTION


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2.1. General Methods. Agilent 8453 UV−Vis spectrophotometer was employed for recording UV−Vis absorption spectra. Shimadzu RF-5301PC spectrofluorometer was used to collect the fluorescence spectra. Fluorescence lifetimes were measured by OB920 luminescence lifetime instrument (Edinburgh, U.K.) Scheme 1. Molecular Structures of BDP-NDI, DiiodoBDP-NDI and Reference Compounds


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2.2 Synthesis of Compounds. The detail of the synthesis and characterization

of

compound 1, compound 2, T-NDI, Diiodo-BDP, compound 3, compound 4，compound 5, compound 6, compound 7 and compound 8 are given in the supporting information. 2.3. Synthesis of the Compound BDP-NDI. Under Ar atmosphere, compound 3 (60 mg, 0.1 mmol) and compound 7 (90 mg, 0.2 mmol) were dissolved in mixture of CHCl3/EtOH/H2O (10:0.8:0.8, v/v). Et3N (0.1 mL, 0.71 mmol) was added and mixture was allowed to stir for 5 minutes. Sodium ascorbate (20 mg, 0.1 mmol) and CuSO4.5H2O (10 mg. 0.04 mmol) was added and the mixture was stirred for 48 h, at RT. Mixture was concentrated under reduced pressure and purified by column chromatography (Silica gel, CH2Cl2/CH3OH, 100:0.5, v/v). The product was obtained as blue solid (Yield: 115 mg, 81 %). mp > 250 °C. 1H NMR (CDCl3, 400 MHz); δ 9.75 (s, 2H), 8.24 (s, 2H), 7.87 (s, 2H), 7.16 (d, 4H, J = 8.0 Hz), 6.97 (d, 4H, J = 8.9 Hz), 5.95 (s, 4H), 4.83 (s, 8H), 4.46–4.43 (s, 4H), 4.12–4.03 (m, 4H), 2.53 (s, 12H), 1.95–1.88 (m, 2H), 1.38 (s, 12H), 1.36–1.28 (m, 16H), 0.94–0.87 (m, 12H). 13C NMR (100 MHz, CDCl3); δ166.4, 163.2, 158.4, 155.5, 148.6, 144.4, 143.0, 141.2, 131.7, 129.5, 128.2, 125.8, 123.3, 121.2, 118.3, 115.1, 102.5, 66.3, 49.9, 44.2, 38.8, 37.9, 30.8, 29.7, 28.7, 24.1, 23.1, 14.6, 14.1, 10.7. TOF MALDI−HRMS ([C78H88B2F4N14O6+Na]+) Calcd: m/z = 1437.7031. Found: m/z = 1437.7036. 2.4. Synthesis of the Compound DiiodoBDP-NDI. DiiodoBDP-NDI was synthesized by the same method as BDP-NDI. The product was obtained as dark blue solid. Yield: 78 %. mp > 250 °C. 1H NMR (CDCl3, 400 MHz); δ 9.66 (s, 2H), 8.11 (s, 2H), 7.87 (s, 2H), 7.10 (d, 4H, J = 8.5 Hz), 6.96 (d, 4H, J = 8.6 Hz), 4.80 (s, 4H), 4.75 (d, 4H, J = 3.9 Hz), 4.45 (s, 4H), 4.04–3.93 (m, 4H), 2.55 (s, 12H), 1.86–1.79 (m, 2H), 1.32 (s, 12H), 1.29–1.20 (m, 16H), 0.86–0.78 (m, 12H).

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C NMR (100 MHz, CDCl3); δ 166.2, 162.9, 158.8, 156.7, 148.4, 145.2, 144.3, 141.0,


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131.6, 129.4, 127.8, 125.6, 123.4, 121.2, 118.1, 115.4, 102.3, 85.7, 66.4, 49.9, 44.2, 38.8, 37.8, 30.8, 29.7, 28.7, 24.1, 23.1, 17.2, 16.0, 14.1, 10.7. TOF MALDI−HRMS ([C78H84B2F4I4N14O6+Na]+) Calcd: m/z = 1941.2897. Found: m/z = 1941.2877. 2.5. Nanosecond Transient Absorption Spectroscopy. LP920 laser flash photolysis spectrometer (Edinburg Instruments, UK) equipped with Tektronix TDS 3012B oscilloscope was used to study the nano- and microsecond dynamics. The samples were deaerated with argon for 15 min prior to investigations. The data was processed by LP920 software. 2.6. Femtosecond Transient Absorption Spectroscopy. The fs−TA experimental setup is detailed elsewhere.35,36 Briefly, the white-light continuum probe pulses were generated in a 200 µm CaF2 nonlinear crystal using intense 800 nm, and spectrally tunable pump femtosecond pulses (370−550 nm; a few µJ pulse energy) were generated in an optical parametric amplifier (Light conversion). The pump and probe pulses were spatially and temporally overlapped on the sample, and the transmitted probe pulse from the samples was collected on the broad-band UV−visible−near-IR detectors to record the transient absorption spectra.

3. RESULTS AND DISCUSSIONS 3.1. Design and Synthesis of the Triads. BDP is selected as one of the visible light absorbing chromophores, owing to its strong absorption of visible light, negligible non−radiative decay, and versatile derivatization.37–39 It should be noted that NDI is also one of the most investigated organic chromophores.40–44 DiiodoBDP-NDI triad (Scheme 1) is designed with the aim to study the FRET between Diiodo-BDP and NDI. As mentioned previously, Diiodo-BDP is the singlet energy donor and the NDI is the singlet energy acceptor.45,26 BDP-NDI, T-NDI, Diiodo-BDP and BDP (Scheme 1) are used as reference compounds. To synthesize triads,


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Diiodo-BDP/BDP and NDI moieties are connected together by click reaction i.e. cyclo addition of alkyne and azide in the presence of copper as catalyst. All reactions gave products with moderate to good yield.

3.2. UV− −Vis Absorption and Fluorescence Spectra. The UV−Vis absorption spectra of the compounds are shown in Figure 1. The Diiodo-BDP and BDP units give strong absorption spectra at 537 nm and 503 nm, respectively. T-NDI gives a weak absorption at 616 nm. Accordingly DiiodoBDP-NDI gives strong absorption peak at 537 nm and a weaker peak at 599 nm, which are assigned to the Diiodo-BDP and NDI moieties, respectively.42,46 Similar results were observed for BDP-NDI, which gives a strong characteristic absorption band of BDP moiety at 503 nm and a relatively weaker absorption band of NDI at 599 nm. Comparison of absorption at 537 nm and 503 nm, associated with Diiodo-BDP and BDP moieties in triads, indicates approximately two times absorbance as compared to the reference compounds. This absorption is due to the two units of BDP or Diiodo-BDP attached in the triads.

2.0

Absorbance

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


1.5

DiiodoBDP-NDI Diiodo-BDP T-NDI BDP-NDI BDP

1.0 0.5 0.0 300

400 500 600 Wavelength / nm

700

Figure 1. UV−Vis absorption spectra of the compounds. c = 1.0 × 10−5 M in toluene. 20 °C.


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The fluorescence of the DiiodoBDP-NDI triad was studied in various solvents (Supporting Information, Figure S23). We found that the fluorescence intensity of the NDI moiety in the triad decreased in polar solvents such as acetonitrile and methanol as compared to that in nonpolar solvents. But the fluorescence spectra of the T-NDI alone in different solvents does not show significant change (Supporting Information, Figure S23c). Thus, we propose that the Photoinduced electron transfer (PET) exist for the triad, and the fluorescence of NDI is quenched by PET.47–49 It is also proved by electrochemical study and Gibbs free energy changes (∆G°CS) of the electron transfer, which are negative in polar solvents, indicating that electron transfer is thermodynamically allowed only in high polar solvents (For detail see the electrochemical study section, Supporting Information). 50

350

a

210 140 70 0 600 700 wavelength / nm

800

500

DiiodoBDP-NDI Diiodo-BDP

30 20 10 0 500

600 700 Wavelength/ nm

800

500

c

DiiodoBDP-NDI T-NDI

400 Intensity / a.u.

400

b

40

DiiodoBDP-NDI Diiodo-BDP

Intensity / a.u.

Intensity / a.u.

280

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

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300 200 100

d

DiiodoBDP-NDI T-NDI

300 200 100

0 600

700 Wavelength / nm

800

0 600

700 Wavelength / nm

800

Figure 2. Fluorescence spectra of DiiodoBDP-NDI and Diiodo-BDP (λex = 500 nm) (a) in toluene and (b) in acetonitrile, c = ca. 1.0 × 10−5 M (A = 0.47). Fluorescence spectra of


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DiiodoBDP-NDI and T-NDI (λex = 575 nm) (c) in toluene and (d) in acetonitrile. c = ca. 1.0 × 10−5 M (A = 0.13 at 575 nm). Slight variation of concentration is necessary to prepare optically matched solutions, 20 °C. The fluorescence of the energy donor and accepter units in the DiiodoBDP-NDI were compared with the reference compounds. We found that the fluorescence of the energy donor was quenched in the triad as compared to Diiodo-BDP (Figure 2a, 2b). Since the PET process is normally inhibited in low polar solvent such as toluene,50 we assumed FRET exists in DiiodoBDP-NDI. Inhibition of PET in toluene is also supported by calculation of the ∆G°CS values, in toluene ∆G°CS is positive (see Supporting Information). Similarly with optically matched solutions, the fluorescence of T-NDI and the triad DiiodoBDP-NDI were also compared (Figure 2c, 2d). The fluorescence of the reference compound T-NDI and the triads DiiodoBDP-NDI showed that the fluorescence intensity of triads is drastically quenched in acetonitrile.

Table 1. Photophysical Parameters of the Compounds

λabs a

ε

b

λem a ΦF c / % τFd / ns τTe / µs ΦT f /%

DiiodoBDP-NDI 537, 599 1.71, 0.18 623

14.1 h

6.9

38

54

BDP-NDI

503, 599 1.65, 0.18 623

67.2 h

8.5

−

−

Diiodo-BDP

537

0.96

557

2.7 g

0.2

120

87 g

BDP

503

0.93

514

72.0 g

3.8

−

−

T-NDI

616

0.25

640

57.7 g

9.8

51

27 g

a

In toluene (1.0 × 10–5M), in nm. b Molar absorption coefficient. ε: 105 M–1cm–1. c Fluorescence quantum yield, d Fluorescence lifetimes. e Triplet state lifetime. f Triplet quantum yield. g Literature value.25,52 h Diiodo-BDP is used as reference.


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Similarly, the fluorescence of singlet energy donor and accepter parts in BDP-NDI was also investigated. In this triad, the emission of singlet energy donor part is also quenched (Supporting Information, Figure S24). The possibility of FRET in this triad is also evident when toluene is used. This is because PET is usually inhibited in low polar solvents such as toluene.50 Similarly emission of BDP-NDI is also compared with T-NDI and it was found that emission of triad is quenched in acetonitrile (Supporting Information, Figure S24d). These results indicate that PET may exist for the triad DiiodoBDP-NDI and BDP-NDI, especially in polar solvents. In order to study FRET, the UV–Vis absorption spectra and the fluorescence excitation spectra of DiiodoBDP-NDI were compared (Supporting Information, Figure S25).51 Difference of the peak area of normalized absorbance and excitation indicates that the singlet energy transfer in the compounds is not quantitative. Thus, we can consider the following facts, (1) the Diiodo-BDP is known to be with efficient ISC (86 %),25 (2) the fluorescence study suggests PET in the triad. Based on the above results, we propose the following photophysical processes, (1) the FRET should be faster than the ISC process of the Diiodo-BDP moiety, otherwise no such significant FRET will be observed; (2) CS process, if any, occurs after the FRET. In other words, the CS should be mainly driven by the singlet excited state of the NDI part, not the Diiodo-BDP part. Table 2. Singlet Oxygen Quantum Yields of the Compounds Diiodo-BDP

DiiodoBDP-NDI

BDP-NDI

T-NDI

λexc = 530

λexc = 600

λexc = 540

λexc = 600

λexc = 510

λexc = 600

λexc = 600

nm

nm

nm

nm

nm

nm

nm

Toluene

61 %

−a

33 %

39 %

− a,30 % b

− a, 32 % b

7%

CH3CN

86 %

−a

35 %

36 %

− a, 31 % b

− a, 32 % b

−a

a

Not observed. b Singlet oxygen observed in the presence of methyl iodide (50 % in solvent, v/v), Diiodostyryl bodipy is used as standard (Φ∆ = 62 % in DCM).


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3.3. Femtosecond Transient Absorption Spectroscopy. DiiodoBDP-NDI was excited by fs-laser pulses at 620 nm (preferentially exciting the NDI moiety). As can been seen in Figure 3a, ground-state bleach was observed at 609 nm at extracted spectra at 200 fs, which shifts within few picoseconds to 614 nm. The shifted spectrum starts to decay within hundreds of picosecond with the appearance of new positive peak at ca. 709 nm. The kinetic trace at 650 nm was selected to monitor the evolution of the excited NDI unit. As can be seen, the extracted kinetic trace shows a fast rise, followed by slow decay. The fast rise, ca. 1.3 ps, is assigned to proton transfer process between the (NH) and (CO) groups, in which the GSB is slightly redshifted (to 614 nm).53 The slow decay and the appearance of new peak at 709 nm are assigned to the formation of triplet state via ISC process, which takes about 1.1 ns (Figure 3b).

b

a

0.00 -0.01 -0.02 500

200 fs 10 ps 5 ns S. S. Abs. S. S. Em.

600 700 Wavelength (nm)

∆A (mO.D)

0.01 ∆A / O.D.

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


@ 650 nm Fitting @ 709 nm Fitting

0 1

10 100 Time (ps)

1000

Figure 3. Femtosecond transient absorption spectra of DiiodoBDP-NDI after 620 nm excitation in toluene (a) Extracted spectra at different time delays overlaid with steady-state measurements (b) The kinetic-decay trace of DiiodoBDP-NDI at 650 and 709 nm. Exciting the DiiodoBDP-NDI at 520 nm would preferentially excite the Diiodo-BDP unit. The extracted spectra at early times, ca, 280 fs, show strong GSB as a result of overlapped


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spectra of the absorption and emission of Diiodo-BDP unit (Figure 4a). The extracted spectrum is rapidly vanished within 4 ps, with appearance of new bleach at ca. 609 nm, which is assigned to the population of the singlet state of the NDI unit through FRET process. Again, the new NDI bleach starts to recover with the appearance of the previously observed peak at ca. 709 nm, which is attributed to the triplet-state formation of the NDI unit. The ISC herein takes about ca. 850 ps as shown from the extracted kinetic traces at Figure 4.

0

(ISC)

(FRET)

-10

@ 280 fs @ 20 ps @ 200 ps @ 5,5 ns S.S. Abs. S.S. Em.

(FRET)

-20

a 500

600 700 Wavelength (nm)

∆A (mO.D)

(FRET)

∆A (mO.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

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b 0 5

@ 520 nm Fitting @ 709 nm Fitting

100 1000 Time (ps)

Figure 4. (a) Femtosecond transient absorption spectra of DiiodoBDP-NDI after 520-nm optical excitation, in toluene with assigned processes, FRET and ISC (b) The kinetic decay trace of DiiodoBDP-NDI at 520 and 709 nm. 3.4. Nanosecond Transient Absorption Spectroscopy: Triplet State Properties. Triplet excited states of the compounds were studied by nanosecond transient absorption spectroscopy (Figure 5). Upon photoexcitation at 532 nm pulsed laser, the reference compound Diiodo-BDP gives a strong bleaching band at 535 nm (Figure 5b). Based on the excited state lifetimes, the transient absorption spectra above 400 nm and 550–700 nm spectral range can be attributed to triplet state absorption of Diiodo-BDP.25,45 The triplet state lifetime is determined to


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be 120 µs. For T-NDI, upon excitation at 532 nm, a bleaching band is observed at 610 nm (Figure 5c). This bleaching band is located at the characteristic absorption band of NDI and excited state absorption (ESA) bands are observed in the range of 400–500 nm and 650–750 nm that are associated with triplet state of NDI.26 The triplet state lifetime is determined as 51 µs. The NDI moiety alone is known to show low ISC efficiency.54

0.02

0.10

0.010

a 0.00

∆ O.D.

-0.02

-0.05 300

.... 33 µs 0 µs

-0.04

0.000

b

0.00

0.00

-0.03

τ = 38 µs

0

0.005

τ = 120 µs

-0.06

d

0.09

200 400 600 800 1000 Time / µs

0

400 500 600 700 Wavelength / nm

0.000

∆ O.D.

0.03 0.06

-0.005

-0.06 300 400 500 600 700 800 400 500 600 700 Wavelength / nm Wavelength / nm

96 µs .... 16 µs 0 µs

∆ O.D.

0.00

96 µs .... 16 µs 0 µs

c

0.005

299 µs

∆ O.D.

∆ O.D.

0.05

∆ O.D.

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


e 200 400 600 800 Time / µs

τ = 51 µs

f

0.010 0

100

200 300 Time / µs

Figure 5. Nanosecond transient absorption spectra of (a) DiiodoBDP-NDI (b) Diiodo-BDP (c) T-NDI at different delay times upon pulsed laser excitation (λex = 532 nm) and decay traces of (d) DiiodoBDP-NDI (e) Diiodo-BDP (f) T-NDI recorded at 600 nm, 537 nm and 600 nm respectively. c =1.0 × 10−5 M in toluene. 20 °C. When DiiodoBDP-NDI is selectively excited by pulsed laser at 532 nm, the profile of transient spectra obtained is exactly same as observed for T-NDI (Figure 5a). No ground state bleach due


13


to the Diiodo-BDP moiety was observed for DiiodoBDP-NDI, although it is mainly the DiiodoBDP part that was excited at 532 nm, which can be attributed to the fast FRET from Diiodo-BDP part to NDI as shown previously. The triplet state lifetime of DiiodoBDP-NDI was determined to be 38 µs, close to that of T-NDI. DiiodoBDP-NDI is also excited at 600 nm laser and profile of the transient spectra is similar to T-NDI (Figure 6a). The triplet quantum yield of DiiodoBDP-NDI is measured by TTET method and found to be 54 % (see Supporting Information for detail). Singlet oxygen quantum yield of DiiodoBDP-NDI was found to be 33 % which is almost half of the singlet oxygen quantum yield of Diiodo-BDP. But this value is assigned to NDI, because upon exciting Diiodo-BDP, there is 98 % FRET from Diiodo-BDP to NDI.

0.012

a

0 µs 4.3 µs ... 117.0 µs

0.004

b

0.06

0 µs 5.62 µs ... 112.4 µs

0.03

∆ O. D.

∆ O. D.

0.008

0.000

0.00

-0.004

-0.03

400


400

0.00

0.00

-0.02

-0.02


∆ O. D.

∆ O. D.

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

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τ = 77 µs -0.04

τ = 30 µs

-0.04

c -0.06

0

100

200 300 Time / µs

800

d 400

-0.06

0

50

100 150 Time / µs

200

Figure 6. Nanosecond transient absorption spectra of (a) DiiodoBDP-NDI (λex = 600 nm), c = 4.0 × 10–6 M and (b) BDP-NDI (λex = 515 nm) at different delay times upon pulsed laser


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excitation (methyl iodide was added). Decay traces of (c) DiiodoBDP-NDI (d) BDP-NDI collected at 590 nm. c = 1.0 × 10–5 M (methyl iodide was added). DiiodoBDP-NDI is analyzed in deaerated toluene and BDP-NDI is in mixture of deaerated toluene and methyl iodide (50 % v/v in toluene). 20 °C. We also studied the nanosecond dynamics of BDP-NDI (in which there is no iodine atom on BDP unit) and no triplet signals are observed either by exciting BDP or NDI unit. The lack of triplet state formation in BDP-NDI shows that CS in the dyads will not lead to triplet formation. The ns-TA signals are observed for T-NDI but lack of any TA signal in BDP-NDI upon excitation of NDI part may be due to some energy level changes that may cease ISC. However in the presence of methyl iodide very strong signals were observed for BDP-NDI which is due to external heavy atom effect. This effect of external heavy atom is also verified by measurement of singlet oxygen quantum yield by adding methyl iodide (50 % in solvent, v/v) in the analyzing solvents (Table 2). By exciting BDP-NDI at 500 nm and 600 nm laser, the transient absorption spectra are almost similar (Figure 6b. see Supporting Information, Figure S26a). Triplet lifetime observed for BDP-NDI at 500 nm excitation is found to be 30 µs, which is similar to 38 µs obtained by DiiodoBDP-NDI at 532 nm photoexcitation 3.5. Application of DiiodoBDP-NDI in TTA Upconversion. Here we used DiiodoBDP-NDI (three chromophores based compound) as triplet photosensitizer for TTA upconversion by exciting Diiodo-BDP unit with 532 nm laser and NDI unit with 589 nm laser (Figure 7). 9,10bis(phenylethynyl)anthracene (BPEA) is used as triplet accepter (For detail of TTA upconversion experiment see supporting information). The maximum upconversion quantum yield (ΦUC = 2.8 %) was observed at DiiodoBDP-NDI:BPEA (1:6, molar ratio) at 3.2 mW. Upon excitation at 589 nm laser, a relatively low upconversion quantum yield (1.8 %) was


15


observed as compared to 532 nm laser but it is still four times higher than the upconversion quantum yield reported by NDI-C60 dyads. 55 TTA upconversion of Diiodo-BDP is determined with BPEA as accepter, by exciting at 532 nm, and 3.7 % upconversion is obtained (see Supporting Information, Figure S27b). Although the upconversion quantum yields are similar for both the Diiodo-BDP and DiiodoBDP-NDI when excited at 532 nm, however the triad has superiority over Diiodo-BDP because it can be excited in a broad wavelength range. Our TTA upconversion experiments are done in toluene, as we already described that PET is inhibited in toluene, hence electron transfer has no effect on TTA upconversion. However based on FRET efficiency (which is 98 % based on fs TA), we concluded that upon excitation on 532 nm laser, we are actually indirectly exciting NDI. The difference of TTA upconversion at 532 nm and 589 nm excitation is due to different absorbance of chromophore at these wavelength.

1000

Integrated UCL Intensity

Emission Intensity / a.u.

1200 DiiodoBDP-NDI DiiodoBDP-NDI+BPEA

800 600

400 Upconverted 200

Fluorescence

a

*

100

Slope = 0.78

b

0 500

600 700 Wavelength / nm

10

800

500

100-2 Power Density / mW cm

100 DiiodoBDP-NDI DiiodoBDP-NDI+BPEA

400 300 200 100 Upconverted

Fluorescence

*

c

0 500

600 700 Wavelength / nm

800

Integrated UCL Intensity

Emission 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 16 of 28

10

Slope = 0.74

d 100 -2 Power density / mW cm


16

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Figure 7. TTA upconversion of BPEA with DiiodoBDP-NDI as triplet photosensitizer (a) λex = 532 nm (c) λex = 589 nm. Power density dependent integrated upconversion luminescence (UPL) intensity of BPEA (b) λex = 532 nm and (d) λex = 589 nm, with DiiodoBDP-NDI as photosensitizer. The asterisks indicate the scattered laser. c[DiiodoBDP-NDI] = 1.0 × 10–5 M, c[BPEA] = 6.0 × 10–5 M in deaerated toluene. 20 °C. DiiodoBDP-NDI was used as triplet energy donor to study TTET from DiiodoBDP-NDI to PBI, rubrene and perylene (Figure 7b and 7d, Table 3). The quenching of triplet lifetime of DiiodoBDP-NDI was monitored upon increasing the triplet accepter with 532 nm pulsed laser excitation. The maximum value of KSV = 3.05 × 105 M–1 is observed by PBI following by rubrene (KSV = 2.3 × 104 M–1 and perylene (KSV = 6.28 × 103 M–1).

Table 3. Stern-Volmer Quenching Rate Constant (Ksv) and Bimolecular Quenching Rate Constant (kq) of PBI, Rubrene and Perylene as Triplet Accepters

PBI

Rubrene

Perylene

KSV/M [10 ]

3.05

0.23

0.06

kq/M–1 s–1[109] b

3.23

0.24

0.09

Rq/m [10–10] c

7.11

4.96

4.33

Dq/cm2 s–1 [10–6] d

5.12

7.34

8.41

k0/M–1 s–1 [10–10] e

1.30

1.53

1.65

fQ (%) f

24.8

1.6

0.6

–1

5 a

a

Stern-Volmer Quenching Rate Constant (Ksv) calculated by ns TA with DiiodoBDP-NDI as photosensitizer (λex = 532 nm, c = 1.0 × 10–5 M), decay traces at 600 nm. b Bimolecular quenching rate constant (kq). c Radius of quencher (Rq). d Diffusion co–efficient of quencher (Dq).


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e

Diffusion controlled bimolecular quenching rate constant (k0). efficiency (fQ); In toluene, 20 °C.

f

Bimolecular quenching

3.6. Jablonski Diagram. Scheme 2 shows Jablonski diagram for the photophysical processes involved in DiiodoBDP-NDI. Upon selective excitation at Diiodo-BDP part (energy donor moiety), there is competition between ISC and FRET. fs-TA spectra shows that the ISC of the Diiodo-BDP (kISC = 3.3 × 109 s−1) is much slower than the FRET (kFRET = 2.5 × 1011 s−1), thus production of triplet excited state by Diiodo-BDP part in DiiodoBDP-NDI is inhibited. However, we get 33 % singlet oxygen quantum yield by exciting Diiodo-BDP part of DiiodoBDP-NDI (Table 2).

Scheme 2. Simplified Jablonski Diagram of DiiodoBDP-NDI Illustrating the Photophysical Processes Involved a

1

*

(DiiodoBDP -NDI)

2.31 eV

FRET ISC

τ = 300 ps

τ = 4 ps

1

*

(DiiodoBDP-NDI )

τ = 850 ps

3

537

(DiiodoBDP*-NDI) 1.68 eV 3 * (DiiodoBDP-NDI ) 1.46 eV

2.07 eV

ISC

599

τ = 38 µs

a

The red color of the compound names indicate the localization of the excited states of the triad. Note the direct ISC of the Diiodo-BDP moiety is inhibited by the fast FRET.


18

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We concluded that this singlet oxygen is produced due to excitation of NDI unit by singlet energy transfer from Diiodo-BDP moiety. Interestingly, 39 % singlet oxygen production was observed upon excitation of DiiodoBDP-NDI at 600 nm where the reference compound T-NDI produces only 7 % singlet oxygen. Thus we purpose that the production of triplet excited state with DiiodoBDP-NDI is due to intramolecular heavy atom effect, the iodine atoms on the BDP. It is confirmed by adding methyl iodide in the BDP-NDI solution and singlet oxygen quantum yield (Φ∆ = 32 %) was observed by photoexcitation of the NDI part (Table 2).

4.0. CONCLUSION In summary, BDP-NDI and DiiodoBDP-NDI triads were synthesized, in order to study the effect of FRET and intramolecular photo-induced electron transfer on the triplet state property of these compounds. The molecular structures of the two triads were designed in such a way that the BDP is a singlet energy donor and the NDI unit is the singlet energy acceptor. The photophysical

properties

of

the

triad

were

studied

with

steady–state

and

femtosecond/nanosecond transient absorption spectroscopies. We found that the ISC of the Diiodo-BDP (kISC = 3.3 × 109 s−1) is slower than the FRET (kFRET = 2.5 × 1011 s−1) between Diiodo-BDP and NDI units, thus the formation of the triplet excited state with selective photoexcitation of the Diiodo-BDP unit is inhibited by the FRET. Based on the fluorescence and excitation spectra, as well as the femtosecond transient absorption data, we propose that FRET is faster than the ISC of Diiodo-BDP. In DiiodoBDP-NDI triad upon selective excitation of NDI part, the triplet state is observed due to intramolecular heavy atom effect. DiiodoBDP-NDI is used as broadband excitable (500 ~ 620 nm) triplet photosensitizer and TTA–upconverted emission of BPEA is observed (quantum yield ΦUC = 2.8 %). These new findings could be very


19


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beneficial for future studies of the photophysical properties of multi–chromophores triplet photosensitizer molecular design and for the applications of these compounds. Supporting information Molecular structure characterization and additional spectra are available free of charge via the Internet at http://pubs.acs.org.

■ ACKNOWLEDGMENT We thank the NSFC (21473020, 21673031, 21761142005, 21603021, 21273028 and 21421005), the Fundamental Research Funds for the Central Universities (DUT16TD25, DUT15ZD224, DUT2016TB12), the State Key Laboratory of Fine Chemicals (ZYTS201801) for financial support.

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Intramolecular Energy and Electron Transfers in Bodipy

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