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Different Quenching Effect of Intramolecular Rotation on the Singlet and Triplet Excited States of Bodipy Zhangrong Lou, Yuqi Hou, Kepeng Chen, Jianzhang Zhao, Shaomin Ji, Fangfang Zhong, Yavuz Dede, and Bernhard Dick J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10466 • Publication Date (Web): 30 Nov 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Different Quenching Effect of Intramolecular Rotation on the Singlet and Triplet Excited States of Bodipy Zhangrong Lou,a,b¶ Yuqi Hou,a¶ Kepeng Chen,a Jianzhang Zhao,*a Shaomin Ji,*c Fangfang Zhong,a Yavuz Deded* and Bernhard Dick*e

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]

b

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences (CAS), Dalian 116023, P. R. China

c

School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, P. R. China. E-mail: [email protected]

d

Department of Chemistry, Gazi University, Ankara, 06500, Turkey. E-mail: [email protected] e

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

Received Date (automatically inserted by publisher)

ACS Paragon Plus Environment

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Abstract: It is well known that the fluorescence of a chromophore can be efficiently quenched by the free rotor effect, sometimes called intramolecular rotation (IMR), i.e. by a largeamplitude torsional motion. Using this effect, aggregation induced enhanced emission (AIE) and fluorescent molecular probes for viscosity measurements have been devised. However, the rotor effect on triplet excited states was rarely studied. Herein, with molecular rotors of Bodipy and diiodoBodipy, and by using steady state and timeresolved transient absorption/emission spectroscopies, we confirmed that the triplet excited state of the Bodipy chromophore is not quenched by IMR. This is in stark contrast to the fluorescence (singlet excited state), which is significantly quenched by IMR. This result is rather interesting since a longlived excited state (triplet, 276 s) is not quenched by the IMR, but the shortlived excited state (singlet, 3.8 ns) is quenched by the same IMR. The unquenched triplet excited state of the Bodipy was used for triplet-triplet annihilation upconversion and the upconversion quantum yield is 6.3%.

1. INTRODUCTION The free rotor effect is a well-known non-radiative decay channel for the electronic singlet excited state which, in some cases, efficiently quenches fluorescence,13 and has been widely employed to develop fluorescent viscosity molecular probes,49 and aggregation induced emission (AIE) materials.1013 The principle is that the fast intramolecular rotation (IMR) of the rotor moiety may dissipate the energy of the electronic excited state, presumably via a conical


2

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intersection (CI) between the potential energy surfaces of the excited state and a lower lying state, usually the electronic ground state.14,15 In biological matrix or viscous solvents such as glycerol, the IMR or the free rotor effect is restricted to large extend due to the friction with the microenvironment, and the rotational nonradiative decay channel is blocked. Consequently the S1 state may decay radiatively, giving rise to fluorescence enhancement.16 Following this rationale, it is expected that a longlived excited state, such as the triplet excited state, for which the lifetime may be 105fold longer than that of a singlet excited state,17,18 would also be efficiently quenched by the free rotor effect, i.e. IMR. Therefore, one may design a viscosityactivated triplet photosensitizer for selective generation of cytotoxic singlet oxygen (1O2) as a potent photodynamic therapeutic agent.1924 Previously a binuclear Ir(III) complex was reported to show the AIE phosphorescence.25 The AIE activity is attributed to the restriction on the distortion of the molecule at the T1 state in viscous matrix. However, examples concerning the effect of IMR on the electronic triplet excited state are rarely studied in detail,13 and the most representative molecular rotor (e.g. BDP-1 in Scheme 1), has not been studied for this effect. Motivated by the above postulations, we chose compound BDP-1-Iodo (Scheme 1). It is known that BDP-1 is an efficient fluorescent viscosity molecular probe, i.e. the fluorescence of the Bodipy moiety is quenched efficiently by the IMR of the phenyl moiety at the meso position.5 In viscous solvents, the IMR is restricted and significant fluorescence enhancement was observed. We envisioned that the T1 state of BDP-1-Iodo might be quenched by the same free rotor effect of the mesophenyl ring in solvents with low viscosity. In viscous solvents,


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phenyl rotation would be restricted, thus the triplet excited state will not be quenched and 1O2 photosensitizing may be activated. Interestingly, the experimental results are in stark contrast with these expectations. We found that, unlike the singlet excited state, the triplet excited state cannot be quenched by the free rotor effect. This result is interesting and of fundamental importance in photochemistry, and it will inspire more in-depth investigation of the free rotor effect on the excited state of organic chromophores.10

2. EXPERIMENTAL AND COMPUTATIONAL SECTION. 2.1. General Methods. Chemicals used in synthesis are analytical reagent grade without further purification except as otherwise noted. 1H and 13C NMR spectra were obtained on a 400 MHz Varian Unity Inova spectrometer (TMS as the standard of the chemical shifts). The mass spectra were measured with a MALDI and ESI Micro MS spectrometer. UVvis absorption spectra were measured at room temperature on a HP8453 UVvisible spectrophotometer. Fluorescence spectra were recorded on a Shimadzu RF 5301PC spectrofluorometer. Fluorescence lifetimes were measured with a OB 920 fluorescence/phosphorescence lifetime spectrometer (Edinburgh Instruments U.K.). Fluorescence quantum yields were measured with BDP-2 as the standard (F = 0.72 in THF). 2.2. Compound BDP-1-Iodo. Compound BDP-1-Iodo was prepared following a modified literature method.26 Under Ar atmosphere, a solution of iodine chloride (ICl. 244.0 mg, 1.5 mmol) in dry ethanol (5.0 mL) was added to the solution of BDP-1 (134.0 mg, 0.5 mmol) in dry N,N dimethyl formamide (DMF. 25 mL) and 25.0 ml of dried ethanol (25 mL). The reaction mixture


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was stirred overnight at room temperature. The generated red mixture was washed with brine and extracted with dichloromethane. Then, the combined organic layers were dried over anhydrous Na2SO4 overnight and the solvent was evaporated under reduced pressure. The residue was purified on a shortcolumn chromatography (silica gel, petroleum ether : dichloromethane = 3:1, v/v), a red solid was obtained, 8.2 mg, yield: 3.2 %. 1H NMR (CDCl3, 400 MHz):  7.97 (s, 2H), 7.647.52 (m, 5H), 7.11 (s, 2H).

C NMR (CDCl3, 100 MHz):  148.5, 137.7, 136.1, 136.0,

13

133.0, 131.5, 130.4, 128.8. HRMS: Calcd (C15H9BF2I2N2) m/z = 519.8916, found m/z = 519.8906. 2.3. Nanosecond Time-resolved Transient Absorption Spectra. The nanosecond transient absorption spectra were detected by Edinburgh LP920 laser flash photolysis spectrometer (Edinburgh Instruments, UK). The signal was digitized on a Tektronix TDS 3012B oscilloscope and was analyzed by the LP900 software. 2.4. TTA Upconversion: A laser diode (continuous laser, 532 nm) was used as the excitation source for the upconversion. The mixed solution of the compound (triplet photosensitizers) and the triplet acceptors was degassed with N2 for at least 15 min before the upconversion experiments. The steady state 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 as beam dump to trap the laser. The upconversion quantum yields (UC) of BDP-1-Iodo and BDP-2-Iodo in toluene were determined with the fluorescence quantum yield of the BDP-2-Iodo as the external standard (F


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= 2.7 % in CH3CN). The upconversion quantum yields were calculated with the following equation (Eq. 5), where , A, I and  represents the quantum yield, absorbance, integrated photoluminescence intensity and the refractive index, respectively. Symbols with ‘std’ and 'sam' stand the corresponding parameter for the standard and sample (Eq. 5).

ΦUC

 1  10 Astd   Isam  sam   2Φstd     Asam    1  10  Istd  std 

2

(Eq. 5)

2.5. DFT and TD-DFT calculations. The computations were performed with the Firefly 8.2.0 program on personal computers.27 The B3LYP functional and the cc-pVDZ basis were used. Geometry optimizations were performed with no symmetry constraints imposed for S0 (RHFDFT), S1 (ROHF-DFT) and S1 states (RHF-TDDFT), resulting in structures 1, 2 and 3, respectively of Figure 8. The optimized Cartesian coordinates and pictures of the structures are given in the Supporting Information. Two further structures were calculated, namely structure 4 on the S1 state potential curve with CS symmetry imposed, where the mirror plane is perpendicular to the phenyl and BODIPY units. This structure is a transition state on the S1 surface. Structure 5 is the result of an attempt of optimization of S1 starting from the transition state of structure 4. This optimization did not converge but oscillated around a small region of configuration space, for which structure 5 is an example. At each of these 5 structures, the energies of the two other states were calculated, yielding vertical excitation and emission energies, respectively. These energies (in Hartree units) are


6

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summarized in the Table S1 and in Figure 8 of the main text of the paper. In order to obtain the barriers for rotation of the phenyl ring, stationary states on the S0 and T1 were calculated with the same symmetry restriction to CS point group as for the S1 state. These are included in Table S1 as structures 6 and 7. According to the Hessian matrix, structure 6 is a transition state, but structure 7 is a local minimum. I.e., the true transition state on the T1 potential deviates slightly from CS symmetry.

3. RESULTS AND DISCUSSION 3.1. Molecular Structure Design and Synthesis of the Compounds. BDP-1 and BDP-2 are iodine free reference compounds, prepared for comparing the free rotor effect on their emissive singlet excited state with that on the triplet excited state of BDP-1-Iodo and BDP-2-Iodo. BDP1 is a fluorescent viscosity molecular probe, i.e. it is weakly fluorescent in low viscosity solvents, but it is strongly fluorescent in viscous solvents, for instance glycerol.5 BDP-2, however, shows strong fluorescence and its emission intensity is independent on the solvent viscosity, because the IMR of the phenyl moiety is restricted by the methyl groups at the 1,7positions. Note that triplet state formation upon photoexcitation for BDP-1 and BDP-2 is much less efficient due to lack of the heavy atom effect.21


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Scheme 1. Synthesis of the Bodipy Derivatives. Perylene Was Used as the Triplet State Energy Acceptor in TTA Upconversion. BDP-2, BDP-2-Iodo and BDP-3-Iodo Are Reference Compounds a

CHO +

H N

1)

2)

3) N

NHHN

I

N

N

B F F BDP-1

N

N

I

F

F

BDP-2

F

B

F

BDP-1-Iodo

N

B Perylene

F

I

N

N B

I

I

F

BDP-2-Iodo

N F

N

I

B F

BDP-3-Iodo

Keys: Reagents and conditions: 1) HCl, H2O, RT, 6 h; 2) DDQ, Et3N, BF3Et2O, CH2Cl2, RT, 2 h; yield: 6.4%; 3) ICl, DMF/EtOH, RT, 14 h, yield: 3.2 %. a

The synthesis of BDP-1-Iodo is presented in Scheme 1. The iodination was accomplished with iodine chloride (ICl).26 BDP-2-Iodo was prepared as a reference compound for which the meso phenyl moiety is unable to rotate freely due to the steric hindrance exerted by the methyl groups at 1,7positions.28,29 Thus the triplet excited state of BDP-2-Iodo is not expected to be quenched. 3.2. UV−Vis Absorption Spectra and Fluorescence Emission Spectra. The UVvis absorption of the compounds were studied (Figure 1). The iodinated Bodipys (BDP-1-Iodo and BDP-2-Iodo) show red-shifted absorption as compared to the unsubstituted Bodipy derivatives


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BDP-1 and BDP-2 (Figure 1a). The absorption band of BDP-1-Iodo (546 nm) is slightly

1.0 0.8 0.6

a BDP-1 BDP-2 BDP-1-Iodo BDP-2-Iodo

0.4 0.2 0.0 350

400 450 500 550 Wavelength (nm)

600

Normalized Flurescence Intensity

redshifted when compared to BDP-2-Iodo (531 nm).

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


1.2 1.0

BDP-1 BDP-2 BDP-1-Iodo BDP-2-Iodo

b

0.8 0.6 0.4 0.2 0.0 500

550 600 650 Wavelength (nm)

700

Figure 1. (a) The UV–vis absorption spectra and (b) the normalized fluorescence spectra of BDP1, BDP2, BDP1Iodo and BDP2Iodo. c = 1.0  105 M in methanol (cuvettes with 1.0 cm light path were used). 20 C. The absorption band width of BDP-1-Iodo (FWHM = 1660 cm1) is larger than BDP-2-Iodo (FWHM = 1060 cm1), which may indicate more accessible FranckCondon geometries than BDP-2-Iodo. The same features hold for fluorescence emission (Figure 1b). The iodinated Bodipy compounds show redshifted emission as compared to the uniodinated parent compounds. Next, we studied the viscositydependency of the fluorescence emission of the compounds BDP-1 and BDP-2 (Figure 2). The fluorescence quantum yield (F) of BDP-1 is highly dependent on the viscosity of the solvents. For BDP-1, the F value of BDP-1 in methanol is 1.7%, the value increased to 49.1% in glycerol (in glycol the fluorescence quantum yield is


9


6.6%).5 The fluorescence in glycol (viscosity  = 22.10 cp) is 5-fold stronger than that in methanol ( = 0.59 cp), which indicates the effect of IMR on the fluorescence of this rotor.5 For BDP-2, however, no free rotor effect was observed, i.e. the F value is independent on the solvent viscosity (F = 69.7 % and 79.3 % in methanol and glycol, respectively. Table 1). The fluorescence emission intensity was hardly changed in different solvents, thus no significant free rotor effect was observed. Apparently the methyl groups at 1 and 7 positions restrict the rotation of the phenyl group (actually the bending of the molecular at the S1 excited state).526, 28-30

200 150

400

a



b Glycol Glycol/MeOH = 1:1 (V/V) MeOH

Intensity (a.u.)

250

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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100 50 0 480


640

Glycol Glycol/MeOH = 1:1 (V/V) MeOH

300

200  100

0 480


640

Figure 2. Change of fluorescence emission intensity for (a) BDP1 and (b) BDP2 with increasing solvent viscosity. Optically matched solutions were used for comparison of the emission (the absorbance of the solution at the excitation wavelength of 470 nm was kept constant for the solvents with different viscosities), c = ca. 1.0  105 M (slight variation of the concentration is necessary to prepare the optically matched solutions), 20 C. The solvent viscosity dependent fluorescence of BDP-1 indicated that the IMR of the phenyl moiety at the meso position of BDP chromophore is an efficient quencher of the fluorescence


10

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(singlet excited state). This situation is similar to the previous observations that the fluorescence of BDP can be efficiently quenched by the IMR of the rotor moiety directly attached to the πconjugation framework of Bodipy.3133 For rotor substituents without direct conjugated connection to the fluorophore, the quenching effect on the fluorescence is much less efficient.3436 Table 1. Photophysical Parameters of BDP-1 and BDP-2 in Solvents with Different Viscosity a

abs (nm) b

em (nm) c



Toluene

502

522

MeOH

497

M/G=1/1 g

F (%) e

F (ns) f

6.4

6.0

0.46

514

5.7

1.7

0.29

499

516

5.4

2.8

0.40

Glycol

501

518

5.4

6.6

0.59

Glycerol

502

5119

4.0

49.1

h

Toluene

503

515

8.1

86.2

3.8

MeOH

498

510

8.5

69.7

3.9

M/G=1/1

499

511

9.1

73.7

4.2

Glycol

500

513

7.9

79.3

4.7

Glycerol

502

514

6.8

70.6

h

Compds Solvents BDP-1

BDP-2

d

The excitation wavelength for BDP-1 and BDP-2 is 470 nm. c = 1.0 × 10−5 M, 20 °C. b Absorption wavelength. c Fluorescence emission wavelength. d Molar extinction coefficient. In 104 M−1cm−1. e Fluorescence quantum yields were determined with BDP-2 ( = 72 % in THF) as a reference. f Fluorescence lifetimes. g M stands for MeOH, G stands for glycol, v/v = 1:1. h Not determined. a

3.3. Nanosecond Transient Absorption Spectroscopy: The Unquenched Triplet Excited State of the Molecular Rotor. The triplet excited states of BDP-1-Iodo and BDP-2-Iodo were


11


studied with nanosecond transient absorption spectroscopy (Figure 3 and Figure 4). The quantum yields for triplet state formation of BDP-1 and BDP-2 are extremely low, due to the lack of the heavy atom effect.21 The bleaching band of BDP-1-Iodo at 548 nm is due to the depletion of the ground state upon photoexcitation (Figure 3a). The excited state absorption (ESA) at 433 nm and the bands in the region of 600  700 nm are typical for the T1Tn transitions of the Bodipy chromophore.37,38 It should be pointed out that the ESA band may overlap with the ground state bleaching band, thus the TA spectra are distorted.

0.02

0.04

a

b

0.00

0.00 -0.04 -0.02 -0.04

MeOH

 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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MeOH: Glycol = 1:1

-0.08

224.0 s ...... 13.3 s 0 s

Glycol

-0.12 -0.16

-0.06 -0.20 400


400

600 800 1000 1200 Time (s)

Figure 3. (a) Nanosecond time-resolved transient absorption spectra of BDP-1-Iodo in deaerated toluene. (b) Decay curves monitored at 546 nm of BDP-1-Iodo in solvents with different viscosity. The pulsed laser excitation wavelength is 552 nm, c = 1.0  105 M, 20 °C. The triplet state lifetime of BDP-1-Iodo in MeOH was determined as 126 s (this is the inverse of the unimolecular decay rate constant. see Table 2 and equations 1 and 2 for details). It should be noted that the self-quenching effect of the long-lived triplet state is not negligible, thus the inverse of the bimolecular self-quenching rate constant was calculated as 9.2 s (Table 2). In


12

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a MeOH/glycol (1:1, v/v) viscous medium, the inverse of the bimolecular self-quenching rate constant was calculated as 56.3 s (Table 2), indicated the suppressed diffusion in this viscous solvent. In glycol, the inverse of the bimolecular self-quenching rate constant increased to > 1000 s (Figure 3b and Table 2). The most interesting result is that the unimolecular decay (or the intrinsic triplet state lifetimes) of the BDP-1-Iodo in three representative solvents are the same. This is a clear indication that the triplet state is not quenched by any rotor effect, i.e. the rotation of the phenyl moiety, or the bending of the molecule at the excited state.39,40 It should be pointed out that the ∆ O. D. values of the ns TA spectra of BDP-1-Iodo in solvents with different viscosity are similar, indicating the triplet state quantum yields are similar (Figure 3). With singlet oxygen (1O2) quantum yields as approximation, the triplet state quantum yield of BDP-1-Iodo is close to that of BDP-2-Iodo (Table 2). Note the fluorescence of the uniodinated analogue, BDP-1, varies drastically in solvents with different viscosity (Figure 2a), which is completely different from the performance of BDP-2 (Figure 2b). Similarly, the triplet excited state of BDP-2-Iodo was studied in solvents with different viscosity (Figure 4 and Table 2). The inverse of the unimolecular decay rate constant is 276 s (see Table 2 and equations 1 and 2 for details). Similar to that of BDP-1-Iodo, the inverse of the bimolecular decay rate constants are increased from 19.0 s, to 66.7 s and finally >1000 s, respectively, indicating more suppressed diffusion in MeOH/glycol (1:1, v/v) and glycol than that in methanol.


13


0.05

0.05

a

0.00

0.00

b

-0.10 -0.15

386.6 s ...... 13.2 s 0 s

Glycol

-0.10

MeOH: Glycol = 1:1

MeOH

-0.15 -0.20 -0.25

-0.20 400

 O.D.

-0.05

-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

Page 14 of 34


800 1000 1200 1400 1600 Time (s)

Figure 4. (a) Nanosecond transient absorption spectra of BDP-2-Iodo in deaerated toluene; (b) Decay of the transient at 526 nm in deaerated solvents with different viscosities. The pulsed laser excitation wavelength is 523 nm, c = 1.0  105 M, 20 °C. Although the unimolecular decay of BDP-1-Iodo is faster than that of BDP-2-Iodo, for which the triplet state lifetimes are 126 s and 276 s, respectively, the scenario is clearly different from the fluorescence of BDP-1 and BDP-2. Thus it is clear that the triplet state in BDP-1-Iodo is not quenched by the rotation of the phenyl moiety. Our photophysical measurements with BDP-2-Iodo suggested that the extension of the apparent triplet excited state lifetime of the BDP-1-Iodo is due to the restricted bimolecular selfquenching effect, i.e. diffusion in viscous solvents, not by inhibition of the free rotor effect, i.e. IMR, in viscous solvents. This is clearly demonstrated by the persistent inverse of the unimolecular decay rate constants (Table 2). In order to support this conclusion, we studied the triplet excited state lifetimes of BDP-3-Iodo, in different solvents. There is no phenyl moiety at


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mesoposition of the Bodipy framework, thus IMR is completely ruled out. Similar results were observed (Figure 5 and Table 2).

0.05

0.05

a

0.00

0.00

-0.05

-0.05

-0.10

-0.10

386.9 s ...... 13.3 s 0 s

-0.15

 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


b Glycol MeOH: Glycol = 1:1

MeOH

-0.15 -0.20 -0.25 -0.30

-0.20 400

500

600

Wavelength (nm)

700

400 600 800 1000 1200 1400 Time (s)

Figure 5. (a) Nanosecond transient absorption spectra of BDP-3-Iodo in deaerated toluene; b) Decay traces were monitored at 526 nm in solvents with deaerated solvents with different viscosity. The pulsed laser excitation wavelength is 523 nm, c = 1.0  105 M, 20 °C. Now we discuss the fitting of the triplet state decay traces in detail. When both unimolecular and bimolecular reactions contribute to the decay of the triplet state, the appropriate differential k0  4 RND / 1000 

4 N 1000

R

f

 Rq



equation for the time dependence of the triplet concentration c(t) is:

dc(t )  k1c(t )  k2c(t )2 dt

(Eq. 1)

Which has the solution:

c(t ) 

c0 k1 exp(k1t ){c0 k2  k1}  c0 k2

(Eq. 2)


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A fit of this equation to the transient absorption, given by A(t) = c(t)(T  G), yields three parameters, namely the initial amplitude A0 and two decay times. The first is the inverse of the unimolecular rate constant 1 = 1/k1, the second is 2 = 1/(c0k2). The initial concentration is c0 = A0/∆. We included a further parameter to account for loss of sample by photodecomposition. When the three decay curves for the same compound in three different solvents were fitted simultaneously, with a common value for 1 and the loss factor, the parameters listed in Table 2 were obtained (Table 2). Table 2. Photophysical parameters of BDP-1-Iodo, BDP-2-Iodo and BDP-3-Iodo a Comp.

Solvent

abs (nm)

[b]

em (nm)

[c]



[d]

∆ (%) [e]

1 (µs) [f]

[f]

k2

[h]

 30.0 [g] 5.1 < Glycol 549 579 5.6 81 126 > 1000 0.6 Toluene 537 554 7.7 85 BDP-2-Iodo    MeOH 531 550 7.5 35 276 19.0 11 M/G=1/1 532 552 6.4 54 276 66.7 8.6 < Glycol 534 554 6.8 78 276 > 1000 0.6 Toluene 530 553 11.1 94 BDP-3-Iodo    MeOH 523 539 10.5 34 241 12.4 22 M/G=1/1 524 541 11.7 49 241 39.5 9.7 Glycol 525 542 11.0 75 241 > 1000 < 2 [a] The excitation wavelengths for BDP-1-Iodo and BDP-2-Iodo are 520, and 490 nm, respectively. c = 1.0 × 10−5 M, 20°C. [b] Absorption wavelength. [c] Fluorescence emission wavelength. [d] Molar extinction coefficient. In 104 M−1 cm−1. [e] Quantum yield of singlet oxygen (1O2). BDP-2-Iodo was used as standard (T = 85%, in toluene). [f] Triplet state lifetimes, measured by transient absorption in deaerated atmosphere. Parameters obtained from a global fit of a mixed unimolecular and bimolecular decay model to the transient absorption data in three solvents of different viscosity. [g] M stands for MeOH, G stands for glycol, the volume ratio is 1:1 (v/v). [h] In 109 M1 s1. k2 is the bimolecular rate constant as defined in eq. 1. BDP-1-Iodo

Toluene MeOH M/G=1/1

551 546 547

579 575 578

5.8 5.5 5.8

57 35 38

 126 126

2 (µs)  9.2 56.3


16

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In calculating the initial triplet concentration from the initial amplitude of transient absorption we assumed that only the ground state contributes to the bleaching at the peak wavelength. A comparison of the data with the fitted curves is shown in the SI. Note that for each compound this fit uses only 8 parameters for the three curves, whereas independent biexponential fits uses 15 parameters. Nevertheless, both types of fit result in the same sum of squares within a few percent. The photophysical parameters of the compounds BDP-1-Iodo, BDP-2-Iodo and BDP-3-Iodo are presented in Table 2. We find that the triplet state formation quantum yields of BDP-1-Iodo and BDP-2-Iodo are hardly dependent on the viscosity of the solvents, which is different from the singlet excited state properties of BDP-1 and BDP-2 (Table 1). This observation is interesting, since the shortlived singlet excited state of BDP is efficiently quenched by the free rotor effect, but the longlived triplet state of BDP was not quenched. This is clearly against the ordinary intuition that a long-lived electronic excited state (e.g. triplet state) should be more likely to be quenched by a fast nonradiative decay channel (e.g. IMR), than the short-lived excited state (e.g. singlet state). Previously it has been found that if a free rotor moiety is directly connected to the chromophore -conjugation framework, then the triplet state can be quenched. For instance, an Au(I) complex with a carbazole derived monoisocyano ligand was found to show the AIE phosphorescence.41 Moreover, an Ir(III) complex was reported to show the aggregationinduced phosphorescence enhancement.42 A binuclear Ir(III) complex was reported to show the AIE effect (enhanced phosphorescence),25 and the restriction of the rotation


17


Page 18 of 34

about the phenyl linker between the two Ir(III) coordination centers is believed to be responsible for the AIE on the phosphorescence. But the triplet state property was not studied in detail. In order to further confirm that the triplet state lifetime extension of iodo Bodipy compounds (BDP-1-Iodo, BDP-2-Iodo and BDP-3-Iodo) are due to the inhibited self-quenching, the diffusioncontrolled bimolecular quenching rate constants k0 of the compounds in different solvents were calculated with the Smoluchowski equation (Eq. 3.),2

k0  4 RND /1000 

4 N  R f  Rq  D f  Dq  1000

(Eq. 3)

where D is the sum of the diffusion coefficients of the energy donor (Df) and quencher (Dq), N is Avogadro’s number. R is the collision radius, the sum of the molecule radii of the energy donor (Rf) and the quencher (Rq). Diffusion coefficients can be obtained from StokesEinstein equation (Eq. 4):

D  kT 6 R

(Eq. 4)

where k is Boltzmann’s constant,  is the solvent viscosity, R is the molecular radius. The calculated k0 values are presented in Table 3 (these k0 values are the quenching constants with perylene as triplet energy acceptor). It is clear that upon changing the solvents from methanol, to methanol/glycol (1:1, v/v) to glycol, i.e. with increasing viscosity, the k0 values decreased from 1.20  1010 M1 s1 (in methanol) to 2.25  109 M1 s1 (in methanol/glycol, 1:1, v/v), and then


18

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further decreased to 2.60  108 M1 s1 (in glycol). Our experimental values for the bimolecular quenching constants of the BDP-1-Iodo and BDP-2-Iodo alone are in a similar range, i.e. (1.1 – 3.0)  1010 M1 s1 (in methanol), and (5.1 – 9.7)  109 M1 s1 (in methanol/glycol, 1:1, v/v). For pure glycol our fits find that the contribution of bimolecular reactions is negligible compared to the unimolecular decay. Therefore, the diffusion of BDP-1-Iodo in viscous medium was substantially reduced compared to that in low viscous environments such as methanol. As a result, the diffusion-controlled self-quenching process, e.g. triplet-triplet annihilation (TTA), will be inhibited and the triplet state lifetime will be prolonged. 3.4. Application of the Unquenched Triplet State in BDP-1-Iodo for Triplet−Triplet Annihilation (TTA) Upconversion. We further confirm that the triplet state of BDP-1-Iodo can be used for efficient intermolecular triplet energy transfer, herein we demonstrated with TTA upconversion. Application of BDP-2-Iodo in TTA upconversion was reported by our group previously.29 BDP-1-Iodo was used as triplet photosensitizer and perylene was used as triplet acceptor.4345 A 532 nm continuous laser was used as excitation source. Upconverted fluorescence emission in the range of 425 – 520 nm was observed (Figure 6a). The efficient upconversion (quantum yield: UC = 6.3%) with BDP-1-Iodo indicates the triplet state quantum yield in BDP-1-Iodo is high (singlet oxygen yield was determined as 57%), and the triplet excited state was not quenched by any IMR.


19


150

0.9

a

b BDP-1-Iodo BDP-1-Iodo + Per BDP-2-Iodo BDP-2-Iodo + Per

100

c

BDP-2-Iodo (0.40, 0.59)

1 1+ Per

2+ Per

2

BDP-1-Iodo (0.46, 0.46)

0.6

y

BDP-2-Iodo + Per (0.21, 0.22)

0.3

50

BDP-1-Iodo + Per (0.18, 0.14)

Excited with 532 nm Laser

0 450


0.0 0.0

0.2

0.4

0.6

0.8

x

650

Figure 6. (a) The upconversion fluorescence spectra with BDP-1-Iodo and BDP-2-Iodo as photosensitizer perylene (Per) was the acceptor. (b) Photographs of the upconversion. The compound names are simplified as 1, 2 and Per, respectively. (c) The CIE coordinate changes of the upconversion. Excited with a continuous 532 nm laser at power of 4.8 mW. c (Sensitizers) = 1.0  105 M, c (Perylene) = 3.0  105 M. In deaerated toluene. 20 C. 24

a

18 MeOHGlycol = 1:1 Toluene MeOH Glycol

12

b

12

(0/ t) - 1

18

(0/t) - 1

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 20 of 34

6

MeOHGlycol = 1:1 Toluene MeOH Glycol

6

0

0 0.0

1.2 2.4 3.6 -5 c (Perylene) / 10 M

4.8

0.0

1.2 2.4 -5 c (Perylene) / 10 M

3.6

Figure 7. Stern–Volmer plots generated from the quenched triplet state lifetimes of a) BDP-1Iodo and b) BDP-2-Iodo (ex = 525 nm) in different solvents and perylene as quencher. c (BDP1 or BDP-2) = 1.0  105 M, 25 C.


20

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The quenching of the triplet state of the BDP-1-Iodo and BDP-2-Iodo by perylene in different solvents was studied by the ns TA spectroscopy. SternVolmer plots were constructed based on the quenched triplet state lifetimes (Figure 7), and the quenching constants (KSV) were determined by linear fitting of the curves (Table 3). Interestingly, we found that the quenching of the triplet state is much less efficient in viscous solvents (glycol or MeOH/glycol mixture), than that in low viscous solvents such as methanol. Note that the apparent triplet state lifetimes of the photosensitizers are much longer in glycol, such that much larger rate constants should be observed. The apparent discrepancy between the triplet state lifetimes and the quenching Table 3. Photophysical parameters of BDP-1-Iodo and BDP-2-Iodo Compound

Solvent

b UC (%) a KSV

k0 c

kq d

BDP-1-Iodo

Toluene

6.3

4.87

8.56

8.25

MeOH

0.1

2.70

12.0

9.65

M/G=1/1  e

0.79

2.25

1.71

Glycol

e

0.56

0.26

0.54

Toluene

15.0

5.22

8.56

6.00

MeOH

9.2

3.20

12.0

10.7

M/G=1/1 7.2

1.79

2.25

1.28

Glycol

1.15

0.26

0.48

BDP-2-Iodo

1.8

a

The upconversion quantum yields, BDP-2-Iodo was used as reference (F = 2.7%, in CH3CN). b Stern–Volmer quenching constants. In 105 M–1. With perylene as quencher. c Bimolecular quenching constants calculated with the Smoluchowski equation. In 109 M1 s1. d Diffusion-controlled bimolecular quenching rating constants. In 109 M1 s1. The values were calculated based on the triplet state lifetimes of BDP-1-Iodo and BDP-2-Iodo and the SternVolmer constants, with KSV = kqp. e Not observed.


21


Page 22 of 34

constants in different solvent is attributed to the restricted diffusion of the photosensitizer and the quencher (triplet energy acceptor) molecules in viscous solvents such as glycol. Reduced diffusion will lead to smaller quenching constants, regardless of the magnitude of the triplet state lifetime. 3.5. Theoretical Computations: Rationalization of the Different Quenching Effect on Singlet and Triplet Excited States. We made a preliminary study to explain why the singlet excited state (fluorescence) of BDP-1 is quenched by IMR, but the triplet state is not. Initially we studied the potential energy curves of the ground state and the singlet/triplet excited states against the torsion angle of the CC connecting the phenyl moiety and the meso carbon atom of the Bodipy framework. DFT calculations with the B3LYP functional and CAS(12|12) calculations were performed using the cc-pVDZ basis set. When the symmetry was restricted to the C2 point group, a scan along the torsional coordinate resulted in potential energy curves that had almost constant energy difference and showed a large barrier of ca. 60 kcal/mole (see Supporting Information, Figure S12). The S1 state is in close vicinity to the second and third triplet state, with a crossing near a torsional angle of ca. 30°. These curves would indicate fast ISC from S1 to T2 followed by relaxation to T1. This could explain the low fluorescence quantum yield of BDP-1, but is at variance with the non-observation of any triplet population in this compound.


22

Page 23 of 34

80

S1

E (kcal)

60

40

T1 20

S0

0 1

2

3

4

5

1

2

3

4

5

60

torsion

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


40 20 0

structure

Figure 8. Top left: energies of the three lowest electronic states (upper panel) and torsional angle (lower panel) for the five structures of BDP-1. Ball-and-stick pictures of these 5 structures are also shown, both viewed perpendicular and along the rotor axis, including structure 1: global minimum of S0 state; 2: global minimum of T1 state; 3: local minimum of S1state; 4: CSsymmetric structure of S1, transition state; 5: lowest point found on S1 surface (optimization not converged). The dihedral angles (degree) at the CC bond at the meso-position of Bodipy (shown in structure 1) are supplied for every structure. For more geometry details, refer to the Supporting Information.


23


Page 24 of 34

However, when we lifted the symmetry restriction, we observed that torsion is accompanied by bending of the BODIPY moiety along the coordinate of the butterfly motion. Relaxed scans of the potential energy surfaces resulted in a much lower torsional barriers of ca. 11.5 kcal/mole in S0 and 5.5 kcal/mole in T1, and a very flat potential curve for S1. Figure 8 shows results obtained with (TD)DFT-B3LYP/cc-pVDZ calculations performed with the Firefly program.45 The horizontal axis in Figure 8 labels five structures. Ball-and-stick pictures of these 5 structures are also shown in Figure 8. For each case we show two views, one perpendicular to the rotor axis, and one along the rotor axis. The first three structures are the optimized structures for the lowest three electronic states, in the following order: 1 = geometry of the ground state S0, optimized with RHF-B3LYP, 2 = geometry of the lowest triplet state T1, optimized with ROHFB3LYP, 3 = geometry of the first excited singlet state S1, optimized with TD-DFT-B3LYP. The upper panel shows the energies of these states, relative to the global minimum of the S0 state, in kcal/mole units. The lower panel shows the torsional angle between the BODIPY unit and the phenyl group for each structure. Whereas structures 1 and 2 are the global minima of S0 and T1, respectively; structure 3 is a local minimum of S1. These three structures have almost C2 symmetry, with planar BODIPY and phenyl units. Hence these structures are well characterized by a single value for the torsional angle. When the torsional angle is further reduced, the potential energy curves for S0 and T1 rise until they reach a local maximum (transition state). Due to sterical hindrance the BODIPY unit bends,


24

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and symmetry is lowered to C1. In contrast to S0 and T1, the energy of the S1 state drops along this path. Optimization of the global minimum by DFT methods failed, however: the geometry oscillates around structure 5, which is strongly distorted. At this geometry the S0 and S1 states are very close in energy, and the T1 state is above the S1 state. This cannot occur in regular closed shell molecules, i.e., structure 5 corresponds to a ground state wavefunction that cannot be described with a single determinant. Obviously, the standard DFT technique is inadequate in such a situation. However, the S1 state could be optimized with the symmetry constrained to CS, with the mirror plane perpendicular to the phenyl ring. This geometry is structure 4 in Figure 8. Figure 8 offers a consistent explanation of all our experimental observations. BDP-1 is excited to the S1 state (red arrow). If ISC to the T1 state is fast (green arrow), the system is trapped in the T1 minimum, which has a geometry very similar to the S0 ground state. Hence the torsional movement is not excited. This is the situation in BDP-1-Iodo. In BDP-1, however, ISC is slower than relaxation of the S1 state towards structure 5 where it crosses to S0 via a conical intersection (blue arrow). In BDP-2 this geometrical path is blocked by the methyl groups, hence fluorescence is observed. Whereas distortion of the BODIPY unit towards a crossing between S1 and S0 states is downhill on the S1 potential surface, the path to the crossing between T1 and S0 is uphill on the T1 surface. Hence S1 is quenched when the molecule stays on the S1 surface long enough and the path is not blocked by methyl groups as in BDP-2-Iodo, but the T1 state is not quenched in any of the compounds considered in this paper.


25


Page 26 of 34

These conclusions are confirmed by calculations with the CAS(12|12) method. In contrast to DFT methods, CAS calculations are based on wavefunctions, and they obey the variational theorem (in contrast to methods based on perturbation theory). A detailed discussion of these calculations is outside the scope of this paper and will we presented elsewhere.

4. CONCLUSIONS In summary, although it has been well known that the free rotor effect, or intramolecular rotation (IMR), induces quenching of the fluorescence (singlet excited state) of chromophores, such as Bodipy, herein we demonstrated that the effect of the free rotor on the triplet excited state of Bodipy is drastically different, that is, the triplet state formation quantum yield, as well as the triplet excited state lifetime of Bodipy was not reduced by the free rotor effect (the free rotation of the mesophenyl ring in the Bodipy). Theoretical studies show that the S1 state potential energy curve can cross the S0 potential energy curve, however the relevant coordinate is not the phenyl torsion alone but a combination with the bending of the BODIPY moiety along the butterfly motion. This leads to very low fluorescence and triplet yield of BDP-1. In BDP-1Iodo ISC is faster than relaxation of S1 towards the conical intersection, resulting in high triplet yield. The unquenched triplet state of the iodinated rotor was used for efficient triplettripletannihilation (TTA) upconversion, and upconversion quantum yields up to 6.3% were observed. This is a fundamentally interesting observation in photochemistry. It will inspire more in-depth investigation into the free rotor effect on the property of the triplet excited state. Novel functional organic materials may be developed following these studies.


26

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ACKNOWLEDGEMENTS Z. Lou thanks the NSFC (21503224) and J. Zhao thanks the NSFC (21673031, 21473020 and 21421005), the Fundamental Research Funds for the Central Universities (DUT16TD25, DUT15ZD224, DUT2016TB12) for financial support. S. Ji thanks Guangdong Province Universities and Colleges Young Pearl River Scholar Funded Scheme (2016) and Guangzhou Science and Technology Program (201707010243) for financial support. Y. Dede thanks Muhammed Büyüktemiz of Gazi University for his assistance with the quantum chemical calculations. Turkish Academy of Sciences (TÜBA-GEBİP) and The Scientific and Technological Research Council of Turkey (TUBITAK, 114Z790) are acknowledged for financial support to Y. Dede. B. Dick thanks Dalian University of Technology for the Haitian Professorship support. ASSOCIATED CONTENT

Supporting Information General experimental methods, 1H, 13C NMR data and HRMS spectra of the compounds and the theoretical computation details. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]


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*E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. REFERENCES (1) Turro, N. J.; Ramamurthy, V.; Scaiano, J. C. Principles of Molecular Photochemistry: an Introduction, University Science Books, 2009. (2) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed., Kluwer Academic, New York, 1999. (3) Valeur, B. Molecular Fluorescence: Principles and Applications, Wiley-VCH Verlag, GmbH, 2001. (4) Haidekker, M. A.; Brady, T.; Lichlyter, P. D.; Theodorakis, E. A. A Ratiometric Fluorescent Viscosity Sensor. J. Am. Chem. Soc. 2006, 128, 398399. (5) Kuimova, M. K.; Yahioglu, G.; Levitt, J. A.; Suhling, K. Molecular Rotor Measures Viscosity of Live Cells via Fluorescence Lifetime Imaging. J. Am. Chem. Soc. 2008, 130, 66726673. (6) Peng, X.; Yang, Z.; Wang, J.; Fan, J.; He, Y.; Song, F.; Wang, B.; Sun, S.; Qu, J.; Qi, J.; Yan, M. Fluorescence Ratiometry and Fluorescence Lifetime Imaging: Using a Single Molecular Sensor for Dual Mode Imaging of Cellular Viscosity. J. Am. Chem. Soc. 2011, 133, 66266635.


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