Covalently Bonded PeryleneâDiiodoBodipy Dyads for Thiol

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Covalently Bonded Perylene-DiiodoBodipy Dyads for ThiolActivatable Triplet-Triplet Annihilation Upconversion Kejing Xu, Jianzhang Zhao, and Evan G. Moore J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06922 • Publication Date (Web): 21 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Covalently Bonded Perylene-DiiodoBodipy Dyads for Thiol-Activatable Triplet-Triplet Annihilation Upconversion Kejing Xu,a Jianzhang Zhao a,* and Evan G. Moore b,* a

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

School of Chemistry & Molecular Biosciences, University of Queensland, Brisbane, QLD, 4072, Australia. E-mail: [email protected] (E. M.)

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

ACS Paragon Plus Environment

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Abstract: To achieve activatable triplet-triplet-annihilation (TTA) upconversion, we linked a diiodoBodipy

triplet

photosensitizing

unit

and

perylene

triplet

energy

acceptor/annihilation/emitter using a disulfide bond (dyad BP-1), which can be selectively cleaved by thiols. For comparison, a reference dyad featuring a shorter and more chemically robust 1,2,3-triazole linker between the two components was also prepared (dyad BP-2). The photophysical properties of these compounds have been studied using steady-state and timeresolved transient spectroscopies, forward singlet energy transfer and backward triplet energy transfer (ping-pong energy transfer) were observed. For BP-1, the rate for forward intramolecular Förster Resonance Energy Transfer from perylene to diiodoBodipy is kFRET = 1.9 × 108 s−1, while the backward triplet-triplet-energy-transfer (TTET) process from diiodoBodipy to perylene was slightly slower, with kTTET = 3.7 × 107 s−1. For BP-2, faster energy transfer kinetics were determined (kFRET = 3.1 × 108 s−1 and kTTET = 8.4 × 107 s−1, respectively). Interestingly, we found FRET rate constant is more critically dependent on the length of the linker than the corresponding TTET process, which may have important implications for the design of supramolecular TTA architectures. Lastly, upon cleavage of the disulfide bond in BP-1, intramolecular FRET was effectively shut down and instead intermolecular TTET was observed, allowing for thiol-activatable TTA upconversion with an improvement in upconversion quantum yield from 0.03% to 0.5% in the presence of thiols.

1. INTRODUCTION Triplet-triplet-annihilation (TTA) upconversion has attracted considerable attention due to its advantages for efficient harvesting of the photoexcitation energy, requiring only weak noncoherent light (solar irradiance is sufficient), and offering high upconversion quantum yields.1−8 Moreover, molecular structures capable of undergoing TTA can be readily derivatized, allowing


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easily tunable absorption and emission wavelengths for a given triplet photosensitizer/triplet acceptor pair.1−8 These advantages have resulted in the study of TTA upconversion for applications

in

luminescence

bioimaging,9

photovoltaics,10−12

photocatalysis,13−15

and

luminescence materials.9,16−19 For TTA upconversion, a triplet photosensitizer and triplet acceptor/emitter are the two essential components,2 with the former being responsible for efficient harvesting of photoexcitation energy, to produce a long-lived triplet excited state, and the latter acting as an triplet energy acceptor via intermolecular triplet-triplet-energy-transfer (TTET). Subsequent annihilation of two triplet acceptors can produce a singlet excited state, from which upconverted fluorescence emission can be observed. The sometimes disadvantageous aspect of these types of TTA systems is their bimolecular nature (as a mixture of triplet photosensitizer and acceptor), which can be inconvenient for applications such as luminescence bioimaging. Therefore, a matrix often must be utilized in which the two components can be confined.8,16−21 Alternative methods such as linking the triplet photosensitizer and triplet acceptor/emitter via covalent bonds have been investigated, thus producing a single molecular entity which may be more convenient for applications.22−25 Indeed, attempts to link the triplet photosensitizer and the triplet acceptor have been undertaken previously, for example using anthracene as an acceptor connected to Ru(II) complexes as the photosensitizer.22−24 Using a supramolecular approach, perylene was used as an organic acceptor connected to various organic photosensitizers, allowing host-guest interactions to facilitate the TTA process.25 However, in these systems, singlet energy transfer via Förster resonance energy transfer (FRET) from the triplet acceptor to the triplet photosensitizer can be detrimental to the TTA upconversion. In order to further elucidate this scenario, herein we have prepared two diiodoBodipy-perylene dyads BP-1 and BP-2 (Scheme 1).


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Scheme 1. Synthesis of BP-1, BP-2 and the Structure of Reference Compound IBDP a N3

O

O

N3 OH

CHO

a

N

N

I

N

B F F B-1

I

N

O B-2

Py-1

Py

SS O

d

F

B-2

O

c

b

B

F

O

F F B N N

SS O

O

O

O

O

O

I

O

O

O

O

I

O

O

O

I

N N O N

O O N N N

O

O

O

BP-1

I

O

N

I

B F

IBDP

g

F F B N N

O

O

N F

F F B-3

f

SS

I

I

N

N B

S-2

S-1

Py-3

Py-2

e

+

N3

N N O N

I I

N N F B F N

N

N

O

I BP-2

a

Key: (a) NIS, CH2Cl2, r.t., 6 h. Yield: 93%; (b) DMF/POCl3, o-dichlorobenzene, 100 °C, 2.5 h. Yield: 83%; (c) NaBH4, MeOH/CH2Cl2, r.t., 3 h. Yield: 60%; (d) PPh3/CBr4/NaN3, DMF, r.t., 24 h. Yield: 52%; (e) CuSO4·5H2O, and sodium ascorbate, 24 h, r.t, in Ar. Yield: 41%; (f) similar with step e, Yield: 86%. (g) Similar with step e, Yield: 79%. For BP-1, we have incorporated a reactive disulfide linker which can be readily cleaved in the presence of other thiols,26 thus allowing the upconversion efficiency prior to and after cleavage to be compared, and the effects of the covalent linkage between triplet photosensitizer and triplet acceptor to be examined directly. As a control, BP-2 contains a chemically stable linkage between the perylene and the diiodoBodipy units. The photophysical properties of these systems


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have been studied using steady-state and time-resolved transient spectroscopies, and we have determined that FRET is more sensitive to the length of the chemical linker than the TTET process, which is an important result for future molecular structure design.

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 with perylene (ΦF = 98% in n-hexane) as standard. The fluorescence lifetimes of the compounds were measured with EPL picosecond pulsed laser and TCSPC techniques (405 nm, pulse width: 66.9 ps, maximum average power: 5 mW; Edinburgh Instrument Ltd., UK). The fluorescence (fluorescence emission) was recorded with a FS5 spectrofluorometer (Edinburgh Instrument Ltd., UK). 2.2. Compound B-2. To a solution of B-1 (200 mg, 0.49 mmol) in anhydrous CH2Cl2 (25 mL) was added excess N-iodosuccinimide (NIS. 440 mg, 1.96 mmol). The mixture was stirred at room temperature for about 1 hour (monitored by TLC until complete consumption of the starting material). The reaction mixture was then concentrated under reduced pressure, and the crude product was purified by column chromatography (silica gel, PE/CH2Cl2, 2:1, v/v). The red band was collected, and the solvent was removed under reduced pressure to give the product as red solid. Yield: 300.0 mg, 93.0%. 1H NMR (400 MHz, CDCl3): δ 7.18 (d, 2H, J = 8.0 Hz), 7.08 (d, 2H, J = 8.0 Hz), 4.23 (t, 2H, J = 4.0 Hz), 3.69 (t, 2H, J = 4.0 Hz), 2.65 (s, 6H), 1.45 (s, 6H). MALDI-HRMS: calcd ([C21H20N5OBF2I2]+), m/z = 660.9819, found m/z = 660.9810. 2.3. Compound Py-1. Perylene (2.52g, 10 mmol) was added to a stirred mixture of anhydrous o-dichlorobenzene (5 mL) and anhydrous DMF (4.75 g, 65 mmol), the reaction mixture was heated to 100 °C, POCl3 (3.07 g, 20 mmol) were added dropwise through a dropping funnel over a period of 30 min, and then the mixture was stirred for an additional 4 h at same temperature.


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The reaction mixture was cooled and put into H2O (500 mL) and neutralized by dilute aqueous sodium acetate and the mixture was standed at 0 °C for 3 h. The precipitate was filtered off, washed with H2O (3 × 30 mL), and purified by column chromatography (silica gel, DCM as eluent). Compound 1 was obtained as orange crystal (2.32g, 83%). mp 233−235 °C. 1H NMR (400 MHz, CDCl3): δ 10.32 (s, 1H), 9.18−9.15 (m, 1H), 8.31−8.26 (m, 4H), 7.94 (d, 1H, J = 8.0 Hz), 7.82 (d, 1H, J = 8.0 Hz), 7.76 (d, 1H, J = 8.0 Hz), 7.71− 7.67 (m, 1H), 7.57− 7.54 (m, 2H). TOF HRMS EI+: Calcd ([C21H12O]+), m/z = 280.0888, found, m/z = 280.0890. 2.4. Compound Py-2. 1-Formylperylene (400mg, 1.4 mmol) were dissolved in dry THF: MeOH (50 mL, 2:1, v/v) mixture, and then NaBH4 (162 mg, 4.2 mmol) was added in small portions into the solution. The solution was stirred at RT overnight. Then reaction mixture was diluted with CH2Cl2 (100 mL) and the solution was washed with brine (100 mL). Then organic phase was dried over Na2SO4 and solvent was removed under reduced pressure. The residue was purified by column chromatography (silica gel, CH2Cl2). Compound 2 was obtained as dark yellow solid (237 mg, 60%); mp 208−210 °C; 1H NMR (400 MHz, CDCl3): δ 8.23−8.14 (m, 4H), 7.94 (d, 1H, J = 8.0 Hz), 7.70 (d, 2H, J = 8.0 Hz), 7.57−7.48 (m, 4H), 5.09 (s, 2H). TOF HRMS EI+: Calcd ([C21H14O]+), m/z = 282.1045, found, m/z = 282.1037. 2.5. Compound Py-3. In a 25 mL flask, hydroxymethylperylene (141 mg, 0.5 mmol) and triphenylphosphine (197 mg, 0.8 mmol) were dissolved in dry DMF (3 mL) and then CBr4 (249 mg, 0.8 mmol) was added at 0 °C. The solution was stirred for 10 min at 0 °C. Afterwards NaN3 (194 mg, 3 mmol) was added and slurry was stirred at RT for 24 h. Then the slurry was diluted with CH2Cl2 (100 ml) and the solution was washed with water (2×80 mL), the organic phase was dried over Na2SO4 and solvent was removed under reduced pressure. The crude product was purified by column chromatography (silica gel, hexane/CH2Cl2, 2:1, v/v). Compound 3 was


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obtained as a dark yellow solid (80 mg, 52%.); mp 158−160 °C; 1H NMR (400 MHz, CDCl3): δ 8.28−8.16 (m, 4H), 7.87 (d, 1H, J = 8.0 Hz), 7.73−7.70 (m, 2H), 7.59 (t, 1H, J = 8.0 Hz), 7.53−7.49 (m, 3H), 4.73 (s, 2H). TOF HRMS EI+: Calcd ([C21H13N3]+), m/z = 307.1109, found, m/z = 307.1101. 2.6. Compound S-2. A mixture of compound B-2 (50 mg, 0.08 mmol) and S-1 (120 mg, 0.24 mmol) were dissolved in deaerated CHCl3/ethanol/water (12/1/1, v/v, 14 mL). Triethylamine (3 drops) was added. Then CuSO4⋅5H2O (10.0 mg, 0.01 mmol) and sodium ascorbate (20.0 mg, 0.010 mmol) were added. The reaction mixture was stirred at RT for 24 h. After completion of the reaction, water (20 mL) was added, and mixture was extracted with dichloromethane (3×30 mL) and the solution is dried over anhydrous Na2SO4. After removal of the solvent under reduced pressure, the mixture was purified by column chromatography (silica gel, CH2Cl2/ ethyl acetate, 4/1) to give red solid. Yield: 36.9 mg (41%.). mp > 250 °C. 1H NMR (400 MHz, CDCl3): δ 7.83 (s, 1H), 7.34 (d, 4H, J = 8.0 Hz), 7.16 (d, 4H, J = 8.0 Hz), 7.00−6.95 (m, 6H), 5.25 (s, 2H), 5.10 (s, 4H), 4.83 (t, 2H, J = 4.0 Hz), 4.69 (d, 2H, J = 4.0 Hz), 4.46 (t, 2H, J = 4.0 Hz), 4.38−4.34 (m, 4H), 2.95−2.91 (m, 4H), 2.64 (s, 6H), 2.52 (d, 1H, J = 4.0 Hz), 1.41 (s, 6H). MALDI-HRMS: calcd ([C47H46N5O9BF2S2I2]+), m/z = 1191.0888, found m/z = 1191.0876. 2.7. Compound BP-1. Similar procedure with the synthesis of compound S-2 was used. Yield 86%. Mp > 250 °C. 1H NMR (400 MHz, CDCl3): δ 8.25−8.16 (m, 4H), 7.81−7.71 (m, 4H), 7.54−7.47 (m, 4H), 7.44 (d, 4H, J = 8.0 Hz), 7.32−7.30 (m, 2H), 7.27 (s, 1H), 7.25 (s, 1H), 7.13 (d, 2H, J = 8.0 Hz), 7.00−6.94 (m, 4H), 6.91 (d, 2H, J = 8.0 Hz), 5.92 (s, 2H), 5.22 (s, 2H), 5.12 (s, 2H), 5.07 (d, 4H, J = 8.0 Hz), 4.80 (t, 2H, J = 4.0 Hz), 4.43 (t, 2H, J = 4.0 Hz), 4.37−4.31 (m, 4H), 2.93−2.89 (m, 4H), 2.63 (s, 6H), 1.38 (s, 6H).

13

C NMR (100 MHz, CDCl3): δ = 158.7,


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158.5, 156.8, 154.8, 145.2, 144.2, 140.9, 134.5, 132.1, 130.4, 129.4, 128.7, 127.9, 127.8, 126.8, 126.7, 124.0, 122.5, 120.7, 119.6, 115.4, 114.9, 69.6, 66.4, 65.6, 62.1, 62.0, 52.6, 49.8, 37.1, 29.7, 17.2, 16.0, 14.1. MALDI-HRMS: calcd ([C68H59BN8O9F2S2I2]+), m/z = 1498.1997, found m/z = 1498.1989. 2.8. Compound BP-2. Similar procedure with the synthesis of compound S-2 was used. Yield 79%. Mp > 250 °C. 1H NMR (400 MHz, CDCl3): δ 8.25−8.16 (m, 4H), 7.78−7.71 (m, 3H), 7.53−7.44 (m, 5H), 7.09−7.02 (m, 4H), 5.95 (s, 2H), 5.29 (s, 2H), 2.62 (s, 6H), 1.35 (s, 6H). 13C NMR (100 MHz, CDCl3): δ = 159.1, 156.6, 145.3, 143.6, 141.2, 134.5, 133.3, 132.6, 132.2, 131.6, 130.6, 130.3, 129.3, 128.1, 128.8, 128.7, 128.4, 128.3, 127.9, 127.3, 126.8, 126.7, 122.8, 121.0, 120.9, 120.8, 119.5, 115.8, 62.0, 52.8, 17.1, 16.0. MALDI-HRMS: calcd ([C43H32BN5F2I2O]+), m/z = 937.0758, found m/z = 937.0776. 2.9. Sub-nanosecond Transient Absorption Spectroscopy. The excitation source utilised for transient absorption (TA) measurements was an amplified Ti:Sapphire laser system (Spitfire ACE, Spectra Physics) delivering ca. 120 fs 800 nm laser pulses at a 1 kHz repetition rate, which were coupled to an OPA system (Topas Prime, Light Conversion)delivering tuneable excitation pulses at 443 and 534 nm. Ground and excited state difference spectra (ΔOD) at various delay times were measured using a sub-nanosecond TA spectrometer (EOS, Ultrafast Systems) incorporating two 512 pixel CCD sensors as the sample and reference channel. A white light probe pulse between ca. 380-760 nm was generated using a supercontinuum source (Leukos, STM-2-UV), which was externally triggered using the Ti:Sapphire TTL output. The pump pulse polarisation was set to magic angle with respect to the probe using a Glan-Taylor polariser (GT10, Thorlabs), and the instrument response function (IRF) of this experimental setup has an estimated full width at half maximum (FWHM) of ca. 250 ps. Resulting time traces were


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analyzed globally using commercially available software (Igor, Version 6.1.2.1, Wavemetrics) and were fit to multi-exponential decay models of the form;

I(t) = A1 exp-1/τ1(t) + A2 exp-1/τ2 (t) + ...+ An exp -1/τn (t) where I(t) is the intensity of the ΔOD data at time (t), τn represent individual decay times, and An are pre-exponential scaling factors. Stock solutions of perylene or BP-1, BP-2 and IBDP were prepared in aerated DMF, and diluted to have an OD of 0.5 at ca. 440 nm or 0.6 at ca. 530 nm respectively over the 2 mm path length cell used. Samples were continuously stirred mechanically, and no detectable changes were observed in the UV−vis spectra of samples during data acquisition, indicating the lack of any significant decomposition. 2.10. DFT Calculations. The ground state structures of compounds were optimized using DFT with B3LYP functional and 6−31G(d) basis set. The excited-state related calculations (UV−vis absorptions) were carried out with the time-dependent DFT (TDDFT) with the optimized structure of the ground state (DFT/6−31G(d)). There are no imaginary frequencies in frequency analysis of all calculated structures. All of these calculations were performed with Gaussian 09W.69 2.11. TTA Upconversion. A continuous wave (cw) diode pumped solid-state laser (532 nm) was used as the excitation source for the upconversion. The diameter of the laser spot is 3 mm and power of laser beam was measured with VLP-2000 pyroelectric laser power meter. For the upconversion experiments, the mixed solution of the triplet photosensitizer, triplet acceptor was degassed with N2 for at least 15 min and gas flow is kept during the measurement. Then solution was excited with laser and upconverted fluorescence was recorded with a RF 5301PC spectrofluorometer. In order to repress the laser scattering, a small black box was put behind the fluorescent cuvette to dump the laser.


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3. RESULTS AND DISCUSSIONS 3.1. Design and Synthesis of Compounds. 2,6-diiodoBodipy and perylene were selected as a suitable TTA upconversion pair, since 2,6-diiodoBodipy is a known triplet photosensitizer,27−31 while perylene is a popular triplet acceptor in TTA upconversion.30,32−35 Importantly, however, studies of these two chromophores connected by covalent bonds for TTA upconversion have not been previously undertaken.1,2,36 Hence, two dyads (BP-1 and BP-2) were prepared in moderate to satisfactory yields, in which the covalent linker between the diiodoBodipy and the perylene unit are chemically different. For dyad BP-1, the linker utilized contains an oxidized S-S disulfide bond, which can be readily cleaved via thiol–disulfide exchange in the presence of other thiols (Scheme 1),26,37 and has previously been used for thiolactivatable photodynamic therapeutic reagents.26,37 A shorter dyad BP-2 (Scheme 1) featuring a chemical stable linker was also prepared and characterized. Lastly, since it was assumed that the likely FRET and the TTET kinetics would be strongly influenced by the different linkers, diiodoBodipy (IBDP) and perylene (Py) were also used as standard reference compounds. 3.2. UV−Vis Absorption and Fluorescence Spectroscopy. The UV−Vis absorption spectra of all compounds were studied, and are shown in Figure 1. Most notably, the results show that the UV−Vis absorption spectrum of BP-1 and BP-2 are the sum of the perylene and diiodoBodipy parts, indicating there are no significant electronic coupling between the chromophores in the ground state.38−44 Corresponding fluorescence studies of the dyads were undertaken, and compared with results using the reference compounds. Upon direct photoexcitation of the perylene unit, it is notable that fluorescence from the perylene unit for the BP-1 and BP-2 dyads is much weaker than that of the perylene unit alone (Py) using optically matched solutions, indicating the presence of


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efficient FRET occurring in the dyads.38,43,45,46 Importantly, the observed perylene emission from BP-2 is also considerably weaker than that of BP-1, which implies that the shorter linker favors more efficient FRET between the energy donor and energy acceptor for BP-2, as expected based on the Förster mechanism. The fluorescence lifetimes determined for the perylene unit of BP-2 (1.93 ns) is also shorter than that of BP-1 (2.02 ns), compared to 4.61 ns for the Py model compound, which is also an indication of more efficient FRET occurring in BP-2 compared to BP-1 (Table 1).

1.2

a

BP-1 IBDP Py BP-2

Normalized Intensity

0.8

Absorption

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


0.4

0.0 300

400

500

Wavelength / nm

600

2

b

1

BP-1 BP-2 IBDP Py

0

450

500

550

600

Wavelength / nm

650

700

Figure 1. (a) The UV−Vis absorption spectrum of BP-1, BP-2, IBDP and Py. (b) the normalized emission spectrum of BP-1(λex = 415 nm), BP-2 (λex = 415 nm), IBDP (λex = 520 nm) and Py (λex = 410 nm). c = 1.0 × 10−5 M in DMF, 20 °C.

The solvent polarity dependent fluorescence emission from each of the compounds was also studied, since photo-induced electron transfer (PET) reactions can sometimes be observed for multi-chromophore dyads or triads (Figure 2).47−53


11


BP-1

a

6

b

BP-2

Toluene DCM CH3OH CH3CN

3

0

500

600

4


Counts / 10

Counts / 10

4

6

5 4

2

450

500

550

600

Wavelength / nm

4

c

IBDP

4

0

700

Wavelength / nm

650

700

d

Py

2 1 0

550

600

650

Wavelength / nm

700

4


3

Counts / 10

4

3

Counts / 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

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2 1 0

450

500

550

Wavelength/ nm

600

Figure 2. The emission spectra of BP-1, BP-2, IBDP and Py in solvents with different polarity. (a) λex = 415 nm, A = 0.403; (b) λex = 415 nm, A = 0.293; (c) λex = 520 nm, A = 0.403; (d) λex = 410 nm, A = 0.305. Optically matched solutions were used. 20 °C. Dyad BP-1 was designed with a cleavable linker between the diiodoBodipy part and perylene. In the previous section, we have shown that the fluorescence of the perylene unity was quenched to a large extent by FRET occurring in the dyad. As a result, this dyad is an ideal compound for studying the effects of FRET in covalently linked photosensitizer/triplet acceptors on the resulting TTA upconversion,22−25 and may also function as an TTA upconversion system which can be activated (i.e. enhanced or switched on) by external stimulus such as light or biologically significant small organic molecules.54−60 Hence, we first studied the change in FRET (i.e. the


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Absorption

1.2

a

BP-1 + RSH 0 eq. 1.7 eq. 0.8 ... 10 eq. 15 eq. 0.4

0.0 300

400

500

Wavelength / nm

600 4 3 4

2

1

0

d

c

30 min ... 0.5 min 0 min

Counts / 10

4

3

Counts / 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


2

RSH 1 0

450

500

550

600

Wavelength / nm

650

0

5

10

15

20

Time / min

25

30

Figure 3. The UV−vis absorption and emission changes of BP-1 in the presence of RSH (mercaptoethanol). (a) UV−vis absorption changes; (b) the emission changes upon the addition of different amount of RSH, λex = 415 nm; (c) the emission changes of BP-1 in the presence of RSH at different time (100 eq. of RSH); (d) kinetic plot by monitoring the emission wavelength at 450 nm. c (BP-1) = 1.0 × 10−5 M, In DMF, λex = 415 nm, 20 °C. fluorescence changes of the perylene unit in dyad BP-1) upon cleavage of the disulfide bond via thiol exchange in the presence of other thiols, Upon the addition of differing amounts of thiols, the UV−Vis absorption spectra did not change (Figure 3a), in agreement with the lack of any significant interaction between the diiodoBodipy and the perylene units in the ground state. However, a substantial enhancement in the fluorescence from the perylene unit was observed in


13


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Page 14 of 45

the presence of added thiol (Figure 3b). This result is in agreement with the proposed decrease in intramolecular FRET upon cleavage of the disulfide bond (the cleavage was formed by Mass spectral analysis, Figure S36−S38). In addition, from this result we expect that the TTA upconversion from intact BP-1 will likely be weak, due to the low fluorescence quantum yield of the perylene unit, and since the fluorescence of perylene is enhanced upon disulfide bond cleavage, we anticipate (vide infra) that the TTA upconversion may also be enhanced. Lastly, the time-course for the disulfide bond cleavage reaction was also monitored by following the enhancement in perylene fluorescence upon addition of 100 equivalents of thiol, revealing the reaction to be relatively fast (Figure 3c and 3d), and was complete after ca. 5 minutes. It should be pointed out that the linker between the diiodoBodipy and the perylene moiety is flexible and the molecular can take folded conformation, so that the distance between the two chromophores is reduced, and the distance requirement for FRET can be satisfied. Even in the extended conformation, the distance between the two chromophores is 5 nm. The fluorescence lifetimes for dyad BP-1 were also studied in the absence and presence of thiol (Figure 4). For intact BP-1, the lifetime of perylene unit was determined as 2.02 ns, which is shorter than Py (4.6 ns) The observed slower decay kinetics supports the occurrence of rapid FRET in BP-1, with an evaluated rate constant of kFRET = 2.7 × 108 s−1 calculated by (1/2.02 ns − 1/4.4 ns = 2.7 × 108 s−1 ), which competes with the radiative decay of perylene unit.57 Upon addition of thiol, and hence cleavage the disulfide bond, the FRET is essentially switched off due to the much larger distance between the two cleaved units.61 Accordingly, we observe that the fluorescence decay from the perylene with a fluorescence lifetime of 4.4 ns (Figure 4a), which is very similar to the perylene reference compound (τPy = 4.6 ns, Figure 4b). Similar absorption,


14

Page 15 of 45

emission, and fluorescence lifetime features were also observed for BP-2 (Figure S19, S20, Supporting Information). 4

10

BP-1 + RSH

a

10

b

Py

BP-1 τ1 = 2.02 ns 8.7% τ2 = 0.31 ns 91.3%

2

10

τ450 nm = 4.6 ns

3

10

Py + RSH τ450 nm = 4.6 ns

Counts

τ450 nm = 4.4 ns

3

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


2

10 1

10

10

20

30

Time / ns

40

50

0

10

20

30

Time / ns

40

50

Figure 4. The fluorescence emission lifetime of BP-1 and perylene at 450 nm in the absence and presence of thiols (RSH, 2-mercaptoethanol). c (BP-1) = 1.0 × 10−5 M in DMF, λex = 405 nm, 20 °C. Table 1. Photophysical Parameters of the Compounds

a

λabs/nm

εb

λem/nm

τF /ns c

ΦF (%) d

ΦΔ (%) e

534

8.8

553

0.43, f 3.83 g

7.9, f 89.6 g

73.7 f, 82.0 g 1.4%

443

4.3

450

2.02, f 4.4 g

6.8i

534

8.6

553

0.22 f

2.7

443

4.2

450

1.93 f

1.1i

IBDP

534

8.8

552

0.18

Py

437

4.1

440

4.61

BP-1 BP-2

ΦUC

h f

1.8% g 75.2

2.1%

3.1

81.0

2.9%

98.0

−j

−j

In DMF, c =1.0 × 10−5 M. b Molar absorption coefficient. ε : 104 M−1 cm−1. c Fluorescence lifetimes. d Fluorescence quantum yields, perylene (ΦF = 98% in n-hexane) as reference. e The singlet oxygen quantum yield, IBDP (ΦΔ = 81% in DMF) as reference. f In the absence of RSH (2-mercaptoethanol). g In the presence of RSH. h Upconversion quantum yield, the total concentration of acceptor is 4 × 10−5 M. i The fluorescence quantum yields of the perylene part in BP-1 and BP-2. j Not applicable.

a


15


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Page 16 of 45

The photophysical properties of the compounds are summarized in Table 1. Clearly, both BP-1 and BP-2 show weak perylene fluorescence, due to the efficient intramolecular FRET, compare to Py which otherwise shows very strong fluorescence with a quantum yield of 98%. 3.3. Electrochemical Studies: The Gibbs Free Energy Changes of the Photo-induced Electron Transfer. In order to study the thermodynamics of any potential photo-induced electron transfer (PET) reaction, the electrochemical properties of the compounds were studied (Figure 5), allowing the corresponding Gibbs free energy changes to be calculated.49, 62−65 For perylene, only a single reversible oxidation wave at + 0.61 V was observed, with no reduction wave observable in the electrochemical window of the CH2Cl2 solvent used in the measurement. Table 2. Redox Potentials of BP-1, IBDP and Py. a E(ox) (V)

E(red) (V)

BP-1

+ 0.90, + 0.64

−1.44

IBDP

+ 0.89

−1.44

Py

+ 0.61

−

a

Cyclic voltammetry in N2 saturated CH2Cl2 containing a 0.10 M Bu4NPF6 supporting electrolyte; Counter electrode is Pt electrode; working electrode is glassy carbon electrode; Ag/AgNO3 couple as the reference electrode. Ferrocene (Fc) was used as internal reference, 20 °C. For diiodoBodipy, both a reversible oxidation and reduction wave were observed at + 0.89 V and −1.44 V, respectively. For the covalently linked BP-1 dyad, the observed pseudo reversible oxidation waves corresponding to the Py entity in BP-1 is located at 0.64 V versus Fc/Fc+, whereas the one-electron oxidation potential of Bodipy is located at 0.90 V.66,67 The corresponding reduction wave for the Bodipy component remains essentially intact, appearing at −1.44 V comparable to IBDP.


16

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20

Current / μA

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


+

10

Fc / Fc

0 -10

Py BP-1 IBDP

-20 1

0

-1

-2

Potential / V

Figure 5. Cyclic voltammogram of BP-1, IBDP and Py. In deaerated CH2Cl2 containing 0.10 M Bu4NPF6 as supporting electrode, Ag/AgNO3 reference electrode. Scan rates: 50 mV/s. Ferrocene (Fc) was used as internal reference, 20 °C. Table 3. Charge Separation (ΔGCS) and Charge Separation Energy States (ECTS) for BP-1 in Different Solution ΔG(CS) (eV)

Py+•-IBDP−•

a

ECS (eV)

Toluene

CH2Cl2

CH3CN

Toluene

CH2Cl2

CH3CN

+ 0.47 a

− 0.25 a

− 0.45 a

2.75 a

2.03 a

1.83 a

− 0.03 b

− 0.75 b

− 0.95 b

2.75 b

2.03 b

1.83 b

With IBDP at its excited state, E00 = 2.28 eV. b With Py at its excited state, E00 = 2.78 eV. Using the Weller equation,66,67 the Gibbs free energy change for PET was calculated to be +

0.47 eV in toluene, meaning that PET is inhibited in non-polar solvents. However, in more polar solvents such as CH2Cl2 and CH3CN, the free energy changes were calculated to be − 0.75 eV and − 0.95 eV, which indicates that PET may be more probable in these solvents. Importantly, the energy levels of the charge transfer state (CTS) were also calculated (Table 3) with these results showing that the CTS is higher in energy than the T1 state of both the perylene and


17


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Page 18 of 45

diiodoBodipy units. As such, we propose that while the fluorescence of the components may potentially be quenched by PET, the triplet state of the dyads will not be susceptible to PET quenching, which is similar to results we have found previously with other molecular systems.57, 68

3.4. Sub-Nanosecond Transient Absorption Spectroscopy. In order to reveal more details for the photophysical properties of the BP-1 and BP-2 dyads, and the IBDP and Py model compounds, these were studied using sub-nanosecond transient absorption (ns-TA) spectroscopy. The observed ns-TA spectra (λex = 443 nm) for a solution of the perylene (Py) model compound in DMF are shown in Figure 6a. Immediately upon excitation, a positive S1→Sn excited state absorption (ESA) feature centered at ca. 700 nm is evident, together with a combination of negative ground state bleach (GSB) and stimulated emission (SE) peaks at ca. 410, 440, 472 and 505 nm, which rapidly decay on the nanosecond timescale, with an evaluated lifetime of 4.50 ns. This result is in good agreement with TCSPC measurements for the fluorescence lifetime of perylene (Figure 4b, 4.6 ns). Despite the relatively small triplet excited state population expected, a second slowly decaying component with a lifetime of 191.5 ns can be clearly observed from a biexponential fit of the ΔOD data, and the corresponding DADS spectrum (Figure 6d), which reveals characteristic perylene T1→Tn ESA absorption peaks at ca. 420, 460, and 490 nm and an overlapping negative GSB feature at ca. 440 nm. For the IBDP model compound, the corresponding ns-TA data upon 534 nm excitation are shown in Figure 7. In this case, the spectra are dominated by a strong negative GSB feature at ca. 530 nm, a strong positive ESA feature centered at ca. 455 nm, and a weaker ESA band from ca. 580−730 nm.


18

Δ OD

b

a Time delay / μs

Wavelength / nm

Amplitude

Amplitude

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


Δ OD

Page 19 of 45

c Wavelength / nm

d Wavelength / nm

Figure 6. (a) Sub-nanoscend TA data for perylene in DMF (λex = 443 nm). (b) The decay kinetics of this intensity data integrated in 10 nm wavelength increments from 390 to 760 nm. (c) and (d) deconvolution of the globally fit kinetic data into their decay associated difference spectra (DADS). The corresponding decay kinetics of these features are satisfactorily reproduced in this case using a monoexponential function, with a lifetime of 579.9 ns, which also reveals the origin of these features to be from the T1 excited state,30 It should be pointed out that the lifetime reported herein is much shorter than previously reported (57.1 μs),30 due to aerated conditions being used in the present case. Similarly, the S1 excited state lifetime of a chemically similar diiodoBodipy has been previously reported31 to be ca. 130 ps, and was independently determined to be 0.18 ns


19


for IBDP herein using TCSPC measurements, which is faster than the available timing resolution of our ns-TA setup (IRF ≈ 250 ps). As such, the signals shown in Figure 7a can be exclusively

Δ OD

Amplitude

attributed to the triplet excited state of the IBDP chromophore.

Δ OD

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

Page 20 of 45

a Wavelength / nm

b Time delay / μs

c Wavelength / nm

Figure 7. (a) Sub-nanoscend TA data for IBDP in DMF (λex = 534 nm). (b) The decay kinetics of this intensity data integrated in 10 nm wavelength increments from 390 to 760 nm. (c) deconvolution of the globally fit kinetic data into their decay associated difference spectra (DADS). For BP-1, two sets of sub-nanosecond TA data have been collected using either 443 nm or 534 nm excitation to selectively excite the perylene or diiodoBodipy moieties respectively, with the resulting spectra and kinetic analyses shown in Figure 8 and Figure 9. Using λex = 534 nm (Figure 8), the decay data at earlier time delays are clearly dominated by spectral features which we attribute to the T1 excited state of the diiodoBodipy unit, including a strong negative GSB feature at 530 nm and positive ESA features at ca. 455 nm and from ca. 580−730 nm (Figure 8a).30,31,69 The lack of observable S1→Sn excited state features can again be understood due to


20

Page 21 of 45

the 250 ps IRF of our ns-TA setup, and the relatively short S1 lifetime of 0.43 ns evaluated independently by TCSPC. Unlike the IBDP model compound (Figure 7), the spectra for BP-1 at

Δ OD

Amplitude

c Δ OD

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


b

a Time delay / μs

Wavelength / nm

Wavelength / nm

Figure 8. (a) Sub-nanoscend TA data for BP-1 in DMF (λex = 534 nm). (b) The decay kinetics of this intensity data integrated in 10 nm wavelength increments from 390 to 760 nm. (c) deconvolution of the globally fit kinetic data into their decay associated difference spectra (DADS). later delay times clearly evolve, yielding excited state features including ESA peaks at 430, 460, and 490 nm and an overlapping negative GSB feature at ca. 445 nm, which are characteristic features for the triplet state of perylene.70 Our resulting analysis of the kinetic data yields two decay lifetimes of 25.8 ns and ca. 310 ns respectively for the initial T1 (diiodoBodipy) and subsequent T1 (perylene) excited states. The corresponding data upon 443 nm excitation of BP-1 are shown in Figure 9. In addition to the features described upon 534 nm excitation, an initial spectral feature characterized by an ESA peak at ca. 700 nm is observed, corresponding to S1→Sn absorptions of the initially populated perylene moiety. This peak decays with a 2.4 ns lifetime, which agrees well with the corresponding TCSPC data (cf. 2.02 ns), and is significantly shorter than the value of 4.5 ns


21


obtained for the perylene Py model compound (Figure 6), due to the presence of efficient intramolecular singlet energy transfer to populate the lower energy S1 excited state of the diiodoBodipy fragment. The difference in the perylene S1 excited state lifetimes values for BP-1 compared to the Py model compound can be used to estimate the rate constant for energy

Δ OD

Δ OD

transfer, yielding a value of kFRET = 1.9 × 108 s−1, which is similar with the TCSPC result (kFRET

b

a Time delay / μs

Wavelength / nm

c

d Amplitude

Amplitude

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

Wavelength / nm

Wavelength / nm

Figure 9. (a) Sub-nanoscend TA data for BP-1 in DMF (λex = 443 nm). (b) The decay kinetics of this intensity data integrated in 10 nm wavelength increments from 390 to 760 nm. (c) and (d) deconvolution of the globally fit kinetic data into their decay associated difference spectra (DADS).


22

Page 23 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


= 2.7 × 108 s−1). Interestingly, no transient features were observed which could be attributed to S1→Sn absorptions of the diiodoBodipy acceptor, with the ns-TA spectra instead rapidly evolving to form the T1 excited state of the diiodoBodipy moiety. This result is not unsurprising, however, given that the lifetime of the diiodoBodipy centered S1 excited state for BP-1 is very short (0.4 ns) compared to the S1 (perylene) excited state donor (2.4 ns), which will result in only a very small intermediate excited state population, prior to efficient ISC to form the lower energy diiodoBodipy centered T1 level. As was the case upon direct excitation of the diiodoBodipy moiety at λex = 534 nm (Figure 7), kinetic analysis reveals the T1 level of the diiodoBodipy fragment decays with a lifetime of 25.9 ns to form the lower energy T1 perylene excited state. Hence, using the T1 lifetime evaluated for the IBDP model compound (579.9 ns), the rate constant for this triplet-triplet energy transfer (TTET) step can be estimated to be kTTET = 3.7 × 107 s−1, followed by decay of the lowest energy perylene centered T1 state with a lifetime of ca. 310 ns. The intramolecular TTET from the perylene to diiodoBodipy units in BP-1 was also studied in deaerated solution (Supporting Information, Figure S22). In this case, the much longer triplet lifetime of the perylene chromophore due to the absence of quenching by 3O2 allows us to estimate the rate constant for the reverse TTET step, yielding a value of kTTET = 9.0 × 103 s−1. Corresponding date for the BP-2 dyad were also collected (Supporting Information, Figure S29 and S30) using λex = 443 and λex = 534 nm, respectively, and a similar analysis of these spectra has been undertaken, revealing values of kFRET = 3.1 × 108 s−1 and kTTET = 8.4 × 107 s−1. These rate constants are approximately twice as fast as those obtained for BP-1, which can be rationalized based on the shorter intramolecular separation between the diiodoBodipy and perylene chromophores for BP-2 compared to BP-1.


23


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Page 24 of 45

Lastly, reaction of BP-1 with 100 molar equivalents of 2-mercaptoethanol results in rapid cleavage of the disulfide bond linking the perylene and diiodoBodipy chromophores, and ns-TA spectra for the subsequent mixtures have also been collected (Figure 10 and Figure 11). As shown in Figure 10, after cleavage of the linking disulfide bond, excitation of the perylene chromophore at λex = 443 nm results in population of the perylene S1 excited state, which displays the characteristic positive ESA feature at ca. 700 nm, and overlapping negative GSB and stimulated emission peaks at ca. 420, 450, 480, and 510 nm, which decays with a considerably longer S1 excited state lifetime of 4.44 ns, which is comparable to the model perylene Py compound (4.50 ns). This can be rationalized by the loss of intramolecular singlet energy transfer quenching upon cleavage of the disulfide bond. In this case, the lower energy T1 state of perylene is formed directly, and subsequently decays with a ca. 4.2 μs lifetime under aerated conditions. Interestingly, this value is considerably longer than the value of 191.5 ns obtained for the model perylene compound, which may be due to the presence of the substituted 1,2,3-triazole linker providing some level of protection from diffusional quenching by 3O2, since these measurements were undertaken in aerated solution. Corresponding ns-TA spectra and kinetic analysis employing direct excitation (λex = 534 nm) of the cleaved diiodoBodipy fragment are shown in Figure 11. As noted earlier, spectral features due to S1 → Sn excited state transitions of the diiodoBodipy are not observed due to the short S1 excited state lifetime, with ns-TA spectra at early time delays instead displaying a strong negative GSB feature at 530 nm, and positive ESA features at ca. 455 nm and a. 580-730 nm, which are attributed to the T1 excited state of the cleaved diiodoBodipy fragment, with an apparent lifetime of 11.2 μs. This value is again longer than the IBDP model compound under


24

Page 25 of 45

identical conditions, (τ = 579 ns), which we suggest may be due to the steric bulk of the

Δ OD

Δ OD

substituted 1,2,3-triazole linker reducing quenching by 3O2.

a

b Time delay / μs

Amplitude

Wavelength / nm

Amplitude

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


c Wavelength / nm

d Wavelength / nm

Figure 10. (a) Sub-nanosecond TA data for BP-1 + 100 equivs RSH in DMF (λex = 443 nm). (b) The decay kinetics of this intensity data integrated in 10 nm wavelength increments from 390 to 760 nm. (c) and (d) deconvolution of the globally fit kinetic data into their decay associated difference spectra (DADS). Lastly, given the relatively long-lived T1 excited state of the cleaved iodoBodipy fragment, and the presence of an equimolar amount of perylene in the solution mixture, the opportunity for


25


intermolecular triplet-triplet energy is possible, which is indeed observed at longer time delays by a rise in spectral features corresponding to the T1 excited state of perylene, with ESA peaks at 430, 460, and 490 nm, which subsequently decay with a 107.5 μs lifetime. These observations are consistent with intermolecular triplet state energy transfer observed in other molecular

Δ OD

Amplitude

systems using nanosecond transient absorption spectroscopy.71−74

Δ OD

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

a Wavelength / nm

c

b Time delay / μs

Wavelength / nm

Figure 11. (a) Sub-nanosecond TA data for BP-1 + 100 equivs RSH in DMF (λex = 534 nm). (b) The decay kinetics of this intensity data integrated in 10 nm wavelength increments from 390 to 760 nm. (c) deconvolution of the globally fit kinetic data into their decay associated difference spectra (DADS). To the best of our knowledge, the results presented herein represent the first detailed report on various photophysical processes including FRET and the TTET occurring in covalently linked perylene triplet photosensitizer and Bodipy triplet energy acceptor/emitter moieties. A summary of the photophysical processes involved for the BP-1 dyad are shown in Scheme 2. Upon direct excitation of the diiodoBodipy unit (λex = 534 nm), ultrafast intersystem crossing occurs (< 250 ps), followed by rapid intramolecular TTET to populate the perylene triplet state, with an evaluated rate constant of kTTET = 3.7 × 107 s−1. Subsequent backward TTET to the diiodoBodipy


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


moiety was observed in deaerated solution, where the T1 excited state lifetime is sufficiently long-lived to allow this process to compete, with an experimentally determined rate constant of kb = 9.0 × 103 s−1, which agrees well the value of 7.3 × 104 s−1 calculated using the energy separation of Δ = 0.16 eV between these two states and the appropriate Boltzmann factor (exp(−Δ/kBT)) at room temperature. Since they decay with distinguishably different lifetimes, we consider the lowest energy triplet states of the diiodoBodipy and perylene moieties to not be in close equilibrium. Table 4. Summary of Photophysical Data Obtained by Sub-nanosecond Transient Absorption Measurement in Aerated DMF.

λex/nm

τ1 /ns

τ1 /ns

τ1 /ns

τ1 /ns

(Perylene S1)

(IBDP S1)

(IBDP T1)

(Perylene T1)

Perylene

443

4.50

N/A

N/A

191.5

IBDP

534

N/A

Covalently Bonded PeryleneâDiiodoBodipy Dyads for Thiol

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