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Controlling Triplet-Triplet Annihilation Upconversion by Tuning the PET in Aminomethyleneanthracene Derivatives Kejing Xu, Zafar Mahmood, Daniel Escudero, Jianzhang Zhao, and Denis Jacquemin J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 25 Sep 2015 Downloaded from http://pubs.acs.org on September 29, 2015

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The Journal of Physical Chemistry

Controlling Triplet-Triplet Annihilation Upconversion by Tuning the PET in Aminomethyleneanthracene Derivatives Kejing Xu,a Zafar Mahmood,a Daniel Escudero,b Jianzhang Zhaoa,* and Denis Jacquemin b,c* 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

CEISAM UMR CNRS 6230, Université de Nantes, 2 rue de la Houssinière, BP 92208, 44322Nantes Cedex 3, France. E-mail : [email protected] (D. J.)

c

Institut Universitaire de France, 103 boulevard St Michel, 75005 Paris Cedex 5, France.

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:

Activatable

triplet-triplet

annihilation

upconversion

Page 2 of 39

was

achieved

using

aminomethyleneanthracene derivatives. The molecular structures of the anthracene derivatives were varied by changing the number of phenyl substituents on the anthracene core (A-1, A-2 and A-3 containing no phenyl, one and two phenyl substituents, respectively). The structural modifications tune the intersystem crossing (ISC), the fluorescence as well as the distance between the electron donor (amino group) and the fluorophore by using methylene (A-1 and A-2) or a benzyl moiety (A-3) as a linker. Triplet-triplet annihilation upconversion is mainly tuned by photoinduced electron transfer (PET). Hence, the fluorescence of A-1 and A-2 can be switched on by protonation or acetylation of the amino group, whereas A-3 gives persistent strong fluorescence. Determination of the Gibbs free energy changes indicated significantly different PET driving forces for the three compounds. The mechanism of the fluorescence switching was studied with steady state UV−vis absorption, fluorescence emission spectroscopy, nanosecond transient absorption spectroscopy and ab initio computations. We found that the PET exerts different quenching effects on the singlet and triplet excited states of the anthracene derivatives. The triplet-triplet annihilation upconversion using these compounds as triplet acceptors/emitter was studied as well, and it was found that upconversion can be switched on by inhibition of the PET through acetylation and protonation. Keywords: Anthracene; Photochemistry; Triplet State; Triplet-triplet annihilation; Upconversion

1. INTRODUCTION Triplet-triplet annihilation (TTA) assisted upconversion has recently attracted considerable attention,1−15 due to the advantages of simultaneously having strong absorption of the visible light, high upconversion quantum yields, tunable excitation/emission wavelength, as well as molecular structures of both the photosensitizer and the acceptor that can be improved by organic


2

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synthesis. Indeed, a large panel of triplet photosensitizers16−19 and acceptors have been developed for TTA upconversion.1−5,20 TTA upconversion has already been applied in various research field, e.g., luminescence bioimaging,21 photocatalysis22 and photovoltaics.23−27 However, activatable, or switchable TTA upconversion was rarely studied. Indeed, recently we developed photoswitchable TTA upconversion using a photochromic unit, e.g., dithienylethene (DTE).28−30 However, to the very best of our knowledge, chemically-activatable TTA upconversion was never demonstrated before. Clearly, external stimuli-addressable TTA upconversion adds additional flexibility for the application of TTA upconversion, such as in super resolution fluorescence microscopy,31 and its development is a major improvement. In this framework, we report herein a new chemical-activatable TTA upconversion with aminoanthracene derivatives as triplet acceptor/emitter (A-1, A-2 and A-3, see Scheme 1). The fluorescence of these anthracene derivatives was quenched by the photo-induced electron transfer (PET) from the amino group to the anthracene core (A-1 and A-2),32,33 and is also modulated via intersystem crossing (ISC). Our conclusions are supported by the negative Gibbs free energy changes (∆G0CS) of the PET processes34 and by computational investigations. Interruption of the PET by acetylation of the amino group of the triplet acceptor switches on the TTA upconversion. Interestingly, we found that the singlet excited state of the anthracene derivatives was substantially quenched by the PET, whereas the triplet excited state of the same compounds was barely affected by any PET process. These results not only pave the way to the development of controllable TTA upconversion, but also help understanding the fundamental photochemistry of the triplet states in these chromophores.

2. RESULTS AND DISCUSSIONS


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2.1. Design and Synthesis of the Compounds. 9,10-diphenylanthracene (DPA) was widely used as triplet acceptor/emitter for TTA upconversion.1,3,16 It should be pointed out that the fluorescence quantum yield of DPA is very large (ΦF = 97.3%), whereas the fluorescence quantum yield of anthracene is much smaller (ΦF = 30%),35 due to the efficient ISC in anthracene (ΦISC = 0.70).36 This is why the number of the phenyl substituents on the anthracene Scheme 1. Synthesis of A-1, A-2 and A-3

a

NH

CHO

Br

b

a

d

c Br 2

1

3

NH

CHO

CHO

e

f

A-3

HN

CHO

h

g Br

A-1

4 5

A-2

j

i

O N N

O

N

O

N Pt

O

N

N

A-1' DPA

PtOEP

A-2'

a

Key: (a) Phenylboronic acid, K2CO3, Pd(PPh3)4, toluene/ ethanol/ water, reflux, 6 h. Yield: 93%; (b) Br2, CHCl3, r.t., 75 min. Yield: 82%; (c) 4-Formylphenylboronic acid, Na2CO3, Pd(PPh3)4, toluene/ ethanol/ water, reflux, 6 h. Yield: 77%; (d) n-butylamine, ethanol/ THF, reflux, 10 h; NaBH4, r.t., 3 h. Yield: 69%; (e) similar with step d. Yield: 87%; (f) Br2, CHCl3, r.t., 8 h. Yield: 62%; (g) similar with step a, Yield: 90%; (h) similar with step d, Yield: 48%; (i) (Boc)2O, Pyridine, methanol, r.t., 10 h. Yield: 97%; (j) similar with step i, Yield: 89%.


4

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core was varied to tune the ISC ability of the derivatives (A-1, A-2, A-3. Scheme 1). Indeed, high fluorescence quantum yield for the triplet acceptor (and thus low ISC efficiency) is desired for applications in TTA upconversion.1,5,16 PET is a widely used mechanism for switching of fluorescence,32,37 and we foresaw that attaching an amino group onto anthracene may result in a switchable triplet acceptor/emitter. On the other hand, the different fluorescence properties of both anthracene and DPA, prompted us to prepare three amino-containing anthracene derivatives varying by their substituent number on the anthracene core, A-1, A-2 and A-3 so to vary the ISC/fluorescence quantum yields.36 We also varied the distance between the amino donor and the anthracene core to modulate the impact of PET on the fluorescence.34 In this context, the fluorescence of A-2 and A-3 can be compared, and the inhibition of the PET can be induced by protonation (with acid), or by other methods, e.g., by the acetylation of the amino group. The preparation of the anthracene derivatives was based on the bromination of the anthracene core, and the stepwise Suzuki cross coupling reaction to introduce the phenyl moieties. The amine groups were introduced by condensation of the formyl moiety with butylamine, and then the reduction with NaBH4. All the products were obtained with moderate to satisfactory yields. The molecular structures of the products were fully verified with 1H NMR,

13

C NMR and HR

MS (see the Supporting Information, SI). 2.2. UV− −vis Absorption and Fluorescence Spectroscopy. A-1 absorbs at 347 nm, 365 nm, and 385 nm, which is the vibronic signature of anthracene (Figure 1b). Attaching a phenyl moiety to the anthracene moiety results in a 9 nm redshift of the absorption bands (Figure 1c and 1d). The three derivatives (A-1, A-2 and A-3) show similar vibronic progressions indicating that there is no significant charge transfer (CT) at play in the absorption. This statement was confirmed by a study of the solvent polarity-dependency of the absorption spectra


5


that found only trifling variations of the absorption maxima (Figure 1). These results indicate that the different substitutions of the anthracene in A-1, A-2 and A-3 do not significantly perturb the Franck-Condon (FC) state of the compounds. However, the singlet excited state may undergo CT or solvation prior to decay to the ground state, thus the fluorescence spectra are supposed to be solvent-dependent.

4.0 CH3CN Toluene CH2Cl2 Ethanol

2.4 1.6

PtOEP

0.8

0.05

A-1 0.00

0.0 300

0.15

CH3CN Toluene CH2Cl2 Ethanol

b 0.10

Absorbance

a

400

500

Wavelength / nm

300

600


c

0.10

350

400

450

0.05

0.15


d

0.10

0.05

A-3

A-2 0.00 300

500

Wavelength / nm

Absorbance

Absorbance

3.2

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

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0.00

350

400

450

Wavelength / nm

500

300

350

400

450

Wavelength / nm

500

Figure 1. UV−vis absorption spectra of (a) PtOEP, (b) A-1, (c) A-2 and (d) A-3 in different solvents. c = 1.0 × 10−5 M, 20 °C. The fluorescence study showed that the emission of A-1 is extremely weak in polar solvents such as ethanol and acetonitrile, whereas it becomes much stronger in less polar solvents such as dichloromethane and toluene (Figure 2a). This evolution of the fluorescence emission profile when changing the polarity of the medium is a clear evidence for PET.34 Quenching of the fluorescence in polar solvents was also observed for A-2 (Figure 2b). By contrast, for A-3, the fluorescence intensity remains constant in solvents with different polarity (Figure 2c), indicating


6

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that the PET is not significant in A-3, which is consistent with the large separation between the electron donor (the amino group) and the electron acceptor (the anthracene moiety).32,34 For the sake of comparison, the fluorescence of DPA in different solvents was studied as well and no change of the fluorescence was observed (see Figure S29 in the SI).

8

4

2

0

5

4 2

12

420

450

Wavelength / nm

480

400

450

A-3 Toluene CH2Cl2 Ethanol CH3CN

9

0 390

c

5

Toluene CH2Cl2 Ethanol CH3CN

6

Counts / 10

Toluene CH2Cl2 Ethanol CH3CN

A-2

b

Counts / 10

A-1

a

5

6

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


500

Wavelength / nm

6 3 0 400

450

500

Wavelength / nm

550

Figure 2. Fluorescence emission spectra of (a) A-1, λex = 360 nm, A = 0.05; (b) A-2, λex = 370 nm, A = 0.079; (c) A-3, λex = 370 nm, A = 0.096 in solvents with different polarity. c = 1.0 × 10−5 M, 20 °C. Optically matched solutions were used. In order to further ascertain the impact of the putative PET mechanism on the variation of the fluorescence of the compounds in different solvents, the effect of protonation of the amino group on the fluorescence spectrum was studied (Figure 3). Upon addition of acid such as the trifluoroacetic acid (TFA), the fluorescence of A-1 and A-2 was strongly intensified (Figure 3a and 3b), whereas for A-3 the fluorescence intensity did not change (Figure 3c). These results support the PET mechanism for the anthracene derivatives,32,34 as the PET becomes impossible upon protonation of the amino group. In short, we clearly demonstrated that the distance between the electron donor and the electron acceptor strongly modulates the efficiency of the PET, and subsequently the fluorescence intensity of A-1, A-2 and A-3.


7


A-1 + TFA

TFA

2

5

70 µL 50 µL ... 7 µL 5 µL 2 µL 0 µL

4

A-2 + TFA

8

Counts / 10

Counts / 10

5

6

70 µL 50 µL .. 4 µL 2 µL 0 µL

6 4

TFA

2

a

b 0

0 400

450

400

500

c

15

550

DPA + TFA 0 µL 5 µL 10 µL 20 µL 30 µL 50 µL 70 µL

Counts / 10

5

0 µL 2 µL 4 µL 10 µL 15 µL

5

500

d

15

A-3 + TFA 10

450

Wavelength / nm

Wavelength / nm

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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10

5

0

5

0

390

420

450

Wavelength / nm

480

510

400

450

500

550

Wavelength / nm

Figure 3. Changes of the emission spectrum of (a) A-1, (b) A-2, (c) A-3 and (d) DPA upon the addition of different eq. TFA (0.01 M) in ethanol. λex (A-1) = 360 nm, λex (A-2) = 370 nm, λex (A-3) = 370 nm, λex (DPA) = 370 nm, c = 1.0 × 10−5 M in ethanol, 20 °C. Inhibition of the PET in A-1 and A-2 is not limited to protonation (upon addition of acid in the solution),38−45 as a mild chemical reaction of acetylation of the amino group should also inhibit the PET from the amino moiety to the anthracene core.32,46 To this end, the fluorescence emission intensity changes of the compounds in the presence of acetylation reagent of (Boc)2O was studied (Figure 4). It turned out that the intensities of the fluorescence A-1 and A-2 were significantly enhanced in the presence of (Boc)2O (Figures 4a and 4b), whereas no increase of fluorescence was observed for A-3 (Figure 4c). These results parallels the one obtained with the acid (Figure 3).


8

2

5

36 min 26 min ... 3 min 1 min 0 min

3

1

A-2 + (Boc)2O

12

Counts / 10

5

a

b


9 6 3

0 390

420

450

Wavelength / nm

480

0

390

420

450

480

A-3 + (Boc)2O

15

510

Wavelength / nm

c

0 min 1 min 3 min 10 min 20 min

5

A-1 + (Boc)2O

4

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


Counts / 10

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10

5

0 390

420

450

480

510

Wavelength / nm

Figure 4. Changes of emission spectrum of (a) A-1, (b) A-2 and (c) A-3 upon the addition of (Boc)2O (5 µL) in ethanol. λex (A-1) = 360 nm, λex (A-2) = 370 nm, λex (A-3) = 370 nm, c = 1.0 × 10−5 M, 20 °C. The fluorescence lifetimes of the compounds were studied by Time-Correlated Single Photon Counting (TCSPC). For A-1, a fast component was observed in the decay curve (τ = 0.15 ns), as well as a slow decay kinetics (τ = 5.02 ns). The fast decay trace was attributed to PET (Figure 5a).47 The presence of (Boc)2O increases the lifetime and yields the disappearance of the fast decay component, an effect that is even clearer with addition of acid (τ = 7.05 ns). Similar results were observed for A-2 in which both a fast (τ = 0.15 ns) and a slow (τ = 4.45 ns) decay components were observed (Figure 5b). In the presence of acid, only the slow decay process pertains (τ = 6.25 ns). By contrast the luminescence lifetime of A-3 are unchanged in the presence of acid (Figure 5c). Thus, the fluorescence lifetime studies confirmed the presence (absence) of PET in A-1 and A-2 (A-3). The photophysical properties of the compounds are summarized in Tables 1 and 2. The fluorescence quantum yields of A-1 and A-2 are 4.7% and 7.2%, respectively. For A-3 the fluorescence yield attains 95.1%, which is similar to that of DPA


9


(ΦF = 97.3%). In the presence of (Boc)2O, the fluorescence quantum yield of A-1 (A-2) was increased to 26.3% (67.9%).

10

4

10

3

10

2

10

1

4

a

τ F = 7.05 ns

4

10

A-2 τ1 = 0.15 ns 30% τ2 = 4.45 ns 70% A'-2 τF = 6.77 ns

3

A-1 + (Boc)2O τ F = 5.21 ns A-1 τ1 = 0.15 ns 26.1% τ 2 = 5.02 ns 73.9%

10

2

b

c A-3 τF = 5.79 ns

3

10

A-2 + TFA τF = 6.25 ns

10

10

Counts

5

Counts

10

A-1 + TFA

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

Page 10 of 39

A-3 + TFA

τF = 5.88 ns

A-3 + (Boc)2O τF = 5.88 ns

2

10

1

10

1

10

0

20

40

60

80

Time / ns

100

0

20

40

60

80

100

Time / ns

0

20

40

60

80

100

Time / ns

Figure 5. The fluorescence decay curves of (a) A-1, (b) A-2 and (c) A-3 by monitoring the emission at 410 nm. λex = 405 nm, c = 1.0 × 10−5 M in ethanol with TFA (40 µL, 0.1 M) and (Boc)2O (10 µL) added. 20 °C.

The triplet state formation quantum yields (ΦT) of all compounds were determined. A clear trend emerged: the ISC decreases when adding more phenyl substituents on the anthracene core (A-1, A-2 to A-3). These different ISC efficiencies for A-1, A-2 and A-3 are attributed to the S1/T2 state energy gap. Hence, smaller S1/T2 gaps increase the efficiency of ISC (see Section 2.5). Interestingly, as seen in Table 1, we found that the prohibition of PET does not alter the quantum yield of singlet oxygen formation (Φ∆). For example, A-1’ and A-2’ (see Scheme 1) show similar Φ∆ values as A-1 and A-2. This indicates that, in contrast with the singlet state (see fluorescence study above), the triplet state of A-1 and A-2 were not quenched by PET (ΦF = 4.7 and 7.2% for A-1 and A-2 respectively, ΦF = 27.8 and 68.2% for A-1’ and A-2’ respectively). The analysis of the PET induced quenching of the fluorescence lifetime was presented in Table 2. Fast PET was observed for A-1 and A-2.


10

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Table 1. Photophysical Parameters for all Compounds

a

λabs/nm

εb

λem/nm

τF /ns c

ΦF (%) d

Φ∆(%) e

ΦT(%) f

A-1

365

1.08

392, 415

5.17 g, 7.05 h, 5.21i

4.7 g, 26.3 i

34.9 g

51.7

A-2

372

1.12

408, 429

4.60 g, 6.25 h, 6.82 i

7.2 g, 67.9 i

31.4 g

37.8

A-3

372

1.24

408, 427

5.79 g, 5.88 h, 5.88 i

95.1 g, 94.8 i

−m

6.3

DPA

373

1.40

406, 427

5.30

97.3 g

−m

−l

A-1’

365

1.07

392, 415

5.21

27.8

49.4

46.7

A-2’

372

1.12

408, 429

6.82

68.2

41.2

36.0

6.10

j

−l

−l

−l

PtOEP 534

644

52.0

g

k

In ethanol, c =1.0 × 10−5 M. b Molar absorption coefficient. ε : 10 4 M−1 cm−1. c Fluorescence lifetimes, average lifetime. d Fluorescence quantum yields, anthracene in ethanol (ΦF = 30%) as reference. e Quantum yield of singlet oxygen (1O2). Ru(bpy)3 was used as standard (Φ∆ = 0.57 in DCM). f The triplet quantum yield, anthracene in ethanol (ΦT = 70%) as reference. g In ethanol without TFA or (Boc)2O. h In ethanol with TFA (0.01 M). i In ethanol with (Boc)2O (10 µL) added. j Phosphorescence emission wavelength. k Phosphorescence lifetime, µs. l Not applicable. m Not observed. a

Table 2. Fluorescence Kinetic Parameters of the Compounds a τ1/ ns b

kPET/109 s−1 c

τ2/ ns d

kF/108 s−1 e

A-1

0.15 (26.1%)

6.67

5.02 (73.9%)

1.99

A-2

0.15 (30.0%)

6.67

4.45 (70.0%)

2.25

A-3

−f

−f

5.79

1.73

a

In ethanol, c =1.0 × 10−5 M. b The lifetime fitted from the fast decay component. c The lifetime fitted from the slow decay component. d The PET rate constant, kPET = 1/ τ1. e The fluorescence rate constant, kF = 1/ τ2. f Not applicable.

2.2. Electrochemical Studies: Cyclic Voltammetry. The electrochemical properties of the compounds have been studied by cyclic voltammetry (Figure 6). DPA shows a reversible oxidation wave at +0.87 V and a reversible reduction wave at −2.26 V. For both A-1 and A-2, the


11


oxidation waves are anodically shifted (Table 3), yet the reduction waves are similar to that of DPA. This difference can be attributed to the electron donating ability of the phenyl moieties on the anthracene core. Irreversible oxidation waves were observed for A-1, A-2 and A-3 at +0.59 V, which is attributed to the oxidation of the amino group.

b

a

+

Fc / Fc

+

Current

Current

Fc / Fc

A-1 1

c

0 -1 Potential / V

A-2

-2

1

0 -1 Potential / V

-2

+

+

Fc / Fc

d

Fc / Fc

Current

Current

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

Page 12 of 39

DPA

A-3 1

0

-1 -2 Potential / V

1

0 -1 Potential / V

-2

Figure 6. Cyclic voltammogram of (a) A-1, (b) A-2, (c) A-3 and (d) DPA in deaerated CH3CN containing 0.10 M Bu4NPF6 as supporting electrode and with Ag/AgNO3 reference electrode. Scan rates: 50 mV/s. Ferrocene (Fc) was used as internal reference; 20 °C. In order to study the photo-induced electron transfer (PET) in A-1, A-2 and A-3, the Gibbs free energy changes (∆GCS) of the electron transfer processes were calculated with the Rehm-Weller equation (Eqs. 1 and 2. See the SI for details),48−50

∆GCS = e[ EOX − E RED ] − E00 + ∆GS


(1)

12

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∆GS = −

e2 4πε Sε 0 RCC

e2  1 1  1 1    − + −  8πε 0  RD RA  ε REF ε S 

Table 3. Redox Potentials of A-1, A-2, A-3 and DPA

(2)

a

E(ox) (V)

E(red) (V)

A-1

+1.11, +0.75, +0.59

−2.32

A-2

+1.10, +0.74, +0.59

−2.28

A-3

+0.91, +0.68

−2.26

DPA

+0.87

−2.26

a

Cyclic voltammetry in N2 saturated CH3CN containing a 0.10 M BuNPF6 supporting electrolyte; Counter electrode is Pt electrode; working electrode is glassy carbon electrode; Ag/AgNO3 couple as the reference electrode.

where ∆GS is the static Coulombic energy, which is described by Eq. 2, e is the electronic charge, EOX is the half-wave potential for mono-electron oxidation of the electron-donor unit and ERED is the half-wave potential for one-electron reduction of the electron-acceptor unit. Here, the anodic and cathodic peak potentials were used because in some cases the oxidation is irreversible therefore the formal potential E1/2 cannot be derived. E00 is the energy level approximated as the fluorescence emission wavelength (for the singlet excited state). In Eq. 2, εS is the static dielectric constant of the solvent, RCC is the center-to-center separation distance determined using a DFT optimization of the geometry (see below), RD (RA) is the radius of the electron donor (acceptor), εREF is the static dielectric constant of the solvent used for the electrochemical studies and ε0 the permittivity of free space. The solvents used in these calculations are CH3CN (εREF = 37.5), CH2Cl2 (εREF = 8.93) and toluene (εREF = 2.4, 20 °C). The redox potentials and the free energy changes of the electron transfer are compiled in Table 4.


13


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Page 14 of 39

The Gibbs free energy changes indicated that the PET is thermodynamically allowed for A-1 and A-2, but the driving force for the PET of A-3 is much smaller (Table 4). The Gibbs free energies also indicate that the driving force for the PET is larger in polar solvents (such as acetonitrile) than that in apolar solvents (such as toluene), which fits the fluorescence emission changes (Figure 2). In order to study the triplet state-related photophysical properties of A-1, A-2 and A-3 ADC(2)/def2-TZVP calculations were carried out (see details in the Experimental Section). For A-1, the triplet energy state is located vertically and adiabatically at 2.33 and 1.83 eV, respectively, which are lower than the charge separation energy state (2.80 eV in CH3CN). In contrast, the singlet energy level is theoretically located at 3.56 eV. Similar results were observed for the singlet and triplet energy levels of A-2, A-3 and other aminomethyleneanthracene derivatives. These results indicate that while the triplet excited state of the energy acceptors cannot be quenched by charge transfer (see below), PET can influence the singlet energy states of the acceptors(see Figures 2-4 above). Table 4. Charge Separation Free Energy (∆GCS) and Charge Separation Energy States (ECTS) for A-1, A-2 and A-3 in Different Solvents ∆GCS (eV)

ECTS (eV)

Toluene

CH2Cl2

Ethanol

CH3CN

Toluene

CH2Cl2

Ethanol

CH3CN

A-1a

−0.14

−0.30

−0.34

−0.35

+3.02

+2.85

+2.81

+2.80

A-2b

−0.09

−0.24

−0.27

−0.28

+2.96

+2.81

+2.77

+2.76

A-3c

+0.30

−0.06

−0.13

−0.15

+3.34

+2.99

+2.91

+2.89

a

E00 = 3.16 eV. b E00 = 3.05 eV. c E00 = 3.04 eV.


14

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2.3. Acetylation-Activatable Triplet-Triplet Annihilation Upconversion. Both A-1 and A-2 show switchable fluorescence, i.e., the fluorescence emission intensity can be greatly enhanced in the presence of acid or (Boc)2O (Figure 4), suggesting switchable TTA upconversion. As stated in the Introduction, switchable TTA upconversion was rarely reported,28,29 although this approach can be very useful in areas such as luminescence bioimagaing,21,51,52 and supraresolution fluorescence microscopy.31

500

300

A-1 + (Boc)2O

a


100

*

500

200

*

0

600

400

700

b


300

100

0 400

A-2 + (Boc)2O

400

Intensity

Intensity

200

Wavelength / nm

500

600 Wavelength / nm

700

500

c

25 min ... 0 min

450

A-3 + (Boc)2O

400

Intensity

600

Intensity

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


300 150

A-2 + (Boc)2O 10 µL 30 µL 50 µL

300

d

200 100

*

0 400

500

0

600

700

*

400

Wavelength / nm

500

600 Wavelength / nm

700

Figure 7. The change of upconverted fluorescence intensity upon the addition of (Boc)2O (5 µL) at different reaction time, (a) A-1, (b) A-2, (c) A-3. (d) the upconversion intensity with different amount (Boc)2O. Upconversion was performed upon excitation with a 532 nm continuous laser (power density is 110.0 mW cm−2). c(PtOEP) = 1.0 × 10

−5

M, c(Acceptor) = 4.0 × 10−5 M, in

ethanol, 20 °C.


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Firstly we studied the effect of protonation on the upconversion. Although the fluorescence of both A-1 or A-2 can be unambiguously and reversibly enhanced, the upconversion emission intensity was firstly enhanced and then decreased. After a careful study, we found that DPA undergoes a similar enhancement/decrease response upon protonation, indicating that no reliable results can be obtained with addition of acid such as TFA (See Figure S31 in the SI). Therefore we studied the impact of inhibited PET on the TTA upconversion by acetylating the amino moiety with (Boc)2O (Figure 7). The acetylation of the amino moiety with (Boc)2O proceeds smoothly at room temperature. Upon addition of (Boc)2O, the upconversion emission intensity was enhanced with increased reaction time (Figure 7a). The enhancement is more significant for A-2 (Figure 7b) an effect related to the larger fluorescence quantum yield change of A-2 (Table 1). The different variations of the TTA upconversion intensity in A-1 and A-2 upon acetylation are attributed to the different fluorescence quantum yields of A-1 and A-2 after reaction with (Boc)2O. For A-3, no enhanced upconversion was observed in the presence of (Boc)2O (Figure 7c). For A-2, the upconversion emission intensity cannot be quenched by adding a large excess of (Boc)2O (Figure 7d). The upconversion quantum yields with A-1 (acetylated), A-2 (acetylated) and A-3 were determined as 1.03%, 10.4% and 22.4%, respectively (see Table 5 and the footnote h for the convention used). Therefore, the change in the upconversion efficiency of A-2 without and with (Boc)2O is substantial (×14). Since two mechanisms may be operative, i.e. the triplet-triplet-energy-transfer (TTET) and inhibited PET, we investigated the mechanism of the enhancement of the TTA upconversion in the presence of (Boc)2O (Figure 7). The study of the triplet state quenching of PtOEP with A-1, A-2 and A-3 as triplet energy acceptors showed that upon acetylation, the TTET efficiency was almost unchanged (see Figure 8 and Table 5). Thus, the enhanced TTA upconversion can only be


16

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attributed to the increased fluorescence quantum yields of A-1 and A-2 upon acetylation and not to enhanced TTET. The slightly decreased quenching constants of A-1, A-2 and A-3 upon acetylation are related to the increase of molecular size that reduces the diffusion controlled bimolecular collision constant (k0).53

2.8

A-1 A-1 + (Boc)2O A-2 A-2 + (Boc)2O

2.1

(I0/ It) - 1

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


1.4 A-3 A-3 + (Boc)2O DPA DPA + (Boc)2O

0.7

0.0 0.0

0.6

c / 10

1.2 -5

1.8

M

Figure 8. Stern–Volmer plots generated from phosphorescence intensity-quenching of complexes PtOEP (λex = 532 nm), A-1, A-2, A-2 or DPA as quenchers in ethanol with or without (Boc)2O (20 µL). c (PtOEP) = 1.0 × 10 −5 M, 20 °C. The bimolecular quenching efficiency (fQ) was also studied. Stern-Volmer quenching constants were calculated as KSV = 2.14 × 105 M−1 for A-1.53 The bimolecular quenching constant was calculated as kq = KSV/τ0 = 4.12 × 109 M−1 s−1, where τ0 is the triplet state lifetime of the triplet energy donor (52.0 µs). In order to study the quenching efficiency, which is given by fQ = kq/k0, where k0 is the diffusion-controlled bimolecular quenching rating constants, we used the Smoluchowski equation,53

k0 = 4π RND /1000 =

4π N ( Rf + Rq )( Df + Dq ) 1000


(3)

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Page 18 of 39

where D is the sum of the diffusion coefficients of the energy donor (Df) and quencher (Dq), N is Avogadro’s number and R is the collision radius obtained as the sum of the molecule radii of the energy donor (Rf) and the quencher (Rq). Diffusion coefficients can be obtained from StokesEinstein equation:53 (4)

D = kT 6πη R

where k is Boltzmann’s constant, η is the solvent viscosity and R is the molecule radius. This radii is 5.5 Å for the energy donor (PtOEP) and 6.3 Å for the quencher (A-1). According to Eq. (4), the diffusion coefficients of the energy donor (PtOEP) is 3.34 × 10−6 cm2 s−1 and quencher (A-1) is 2.91 × 10−6 cm2 s−1 (in ethanol at 20 °C). Consequently k0 attains 5.58 × 109 M−1 s−1 whereas the quenching efficiency is fQ = kq / k0 = 73.8 %.

Table 5. Upconversion-related Parameters with PtOEP as the Photosensitizer a A-1

A-2

A-3

DPA

KSV/M–1 [105] b

2.14 i, 1.92 j

1.35 i, 1.16 j

1.59 i, 1.50 j

2.11 i, 2.10 j

kq/M–1 s–1 [109] c

4.12 i, 3.69 j

2.60 i, 2.23 j

3.06 i, 2.88 j

4.06 i, 4.03 j

Rq/m [10−10] d

6.30 i, 6.50 j

8.26 i, 7.21 j

9.76 i, 9.70 j

6.30 i, 6.30 j

Dq/cm2s−1[10−6] e

2.91 i, 2.82 j

2.22 i, 2.55 j

1.88 i, 1.89 j

2.91 i, 2.91 j

k0/M–1 s–1 [109] f

5.58 i, 5.59 j

5.79 i, 5.66 j

6.02 i, 6.01 j

5.58 i, 5.58 j

fQ (%) g

73.8 i, 66.0 j

44.9 i, 39.4 j

50.8 i, 47.9 j

72.8 i, 72.2 j

ΦUC (%) h

0.13 i, 1.03 j

0.72 i, 10.4 j

21.9 i, 22.4 j

23.2 i, 23.1 j

a

In deaerated ethanol. b Stern–Volmer quenching constants. c Bimolecular quenching constants. KSV = kqτp, τP = 52.0 µs. d the molecule radius of the quencher. e In ethanol,η = 1.17 × 10−3 Pa⋅⋅s; T = 293.15 K; Rf = 5.5 × 10−10 m; Df is the diffusion coefficients of the energy donor, Df = 3.34 × 10−6 cm2s−1 , Dq is the diffusion coefficients of the quencher. f Diffusion-controlled bimolecular quenching rating constants. g The quenching efficiency. h The upconversion quantum yields, Rhodamine B (ΦF = 97 %) as reference in ethanol. We note that the reference values of Rhodamine B reported in Ref. 54 are largely scattered from 41% to 97%, herein the upper limit


18

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from the scatter is used. A value of ΦF = 49 % in ethanol is recommended in Ref. 55. (Boc)2O. j With (Boc)2O (20 µL).

i

Without

The intermolecular TTET between the photosensitizer PtOEP and the triplet acceptors was studied with nanosecond transient absorption spectroscopy (see Supporting Information, Figures S34-S39). The results demonstrated that intermolecular TTET did occur, which is essential for the TTA upconversion. Moreover, the triplet state features of the acceptors, such as the triplet state lifetimes and the transient absorption spectra were obtained (see Supporting Information). 2.4. Rationalization of the ISC mechanisms in A-1, A-2 and A-3. Compounds A-1, A-2 and A-3 were used triplet acceptors, and a clear evolution trend of the ISC of the acceptors was observed. It is essential to study the ISC processes since it may modulate the fluorescence efficiency. Though the ISC mechanism does not have a direct influence on the TTAUC triggering, it may indirectly harm the quantum yields of upconversion. In practice, if one aims to reach large quantum yields of upconversion, the ISC mechanisms in the acceptors should not be highly efficient, so that the upconverted singlet excitons are not fully quenched via ISC. Thus, in order to elucidation of the origin of the ISC in these compounds, we performed ADC(2)/def2TZVP calculations (see details in the Experimental Section) on compounds A-1, A-2 and A-3 to attain insights into their ISC mechanisms. For organic molecules in the weak coupling regime, the ISC rate between the n-singlet (Sn) and m-triplet (Tm) excited states (kISC) can be expressed by the Fermi Golden rule approximation (eq. 5),56

k ISC

2π = Sn Hˆ SO Tm h

2

× [ FCWD ]

(5)

where the bracket term accounts for their associated spin-orbit coupling (SOC) and FCWD stands for the Franck-Condon weighted density of states. The calculation of ISC rates first


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Page 20 of 39

requires the assignment of all energetically accessible ISC photodeactivation channels followed by accurate calculations of the magnitudes in Eq. (5). Based on the computation of kISC values relying on accurate ab initio electronic structure data it is found that two main factors, i.e., substantial electronic and/or vibronic SOC and small Tm−Sn energy gaps, govern the efficiency of the ISC process.56 Recently, it was shown in several organic dyes that vibronic SOC with higherlying singlet/triplet excited states may also effectively enhance the ISC rates.57 Computing all parameters on Eq. (5) rapidly becomes prohibitive for large molecules so that both semiquantitative58 and qualitative59 strategies are often used to rationalize the efficiency of ISC processes. Herein, we report theoretical estimates of relative Tm−Sn energy gaps and SOCs to rationalize the efficient ISC mechanisms occurring in both A-1 and A-2 as compared to those occurring in A-3. Obtaining accurate relative Tm−Sn energy gaps and SOCs is mandatory to attain a correct interpretation of the photodeactivation mechanisms. Towards this aim, highly accurate electronic structure methods are often required. In this regard, the accuracy of the ADC(2) method was tested with respect of the Thiel’s set for excitation energies and oscillator strengths of singlet and triplet excited states. The ADC(2) mean errors for the excitation energies of singlets and triplets are only of 0.22 and 0.12 eV, respectively.60 This is why ADC(2) was chosen to evaluate Tm−Sn energy gaps. Oppositely, experience shows that the electronic SOCs are often less sensitive to the level of theory.56 Table 6 lists both the ADC(2)/def2-TZVP vertical (at the S0 optimized geometry) and adiabatic (at the S1 optimized geometry) excitation energies. The agreement between the theoretical and experimental positions of the absorption and fluorescence emission bands is excellent (mean errors of less than 0.05 eV for all the compounds). For example, the calculated UV absorption of A-1 is located at 348 nm (3.56 eV), which is close to the experimental result of 365 nm (Table 1).


20

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The calculated fluorescence emission wavelength of A-1 is located at 397 nm (3.12 eV), which is in also good agreement with the experimental value of 392 nm (Table 1). As discussed above, upon photoexcitation to the spectroscopic state (S1) different competing

Table 6. Lowest Vertical (at S0 geometry) and Adiabatic (at S1 geometry) Singlet and Triplet Electronic Transition Energies (in eV) and Oscillator strengths (in Parentheses) of A-1, A-2 and A-3 at the ADC(2)/def2-TZVP Level of Theory. Vertical and Adiabatic (in Parentheses) Singlet-Triplet Splittings (in eV) and SOCs (in cm-1) between the Involved Tm and S1 States

A-1

A-2

A-3

a

b

States

ADC(2)/ def2-TZVP (S0 geom.)

ADC(2)/ def2-TZVP (S1 geom.)

∆E (S1-Tx)vert. ∆E(S1-Tx)adia a

S1

3.56 (0.125)

3.12 (0.131)

−

−

T1

2.33

1.83

1.22 (1.28)

(0.0; 0.1; 0.0)

T2

3.73

3.42

−0.17 (−0.30)

(0.1; −0.6; −0.7)

S1

3.47 (0.189)

3.01 (0.215)

−

−

T1

2.30

1.80

1.17 (1.22)

(0.0; 0.1; 0.1)

T2

3.69

3.40

−0.22 (−0.38)

(0.6; 0.8; −0.4)

S1

3.48 (0.238)

2.97 (0.303)

−

−

T1

2.31

1.80

1.16 (1.18)

(0.0; 0.0; 0.0)

T2

3.73

3.43

−0.25 (−0.46)

(−0.1; −0.2; 0.0)

Values in Parentheses correspond to the adiabatic energies

b

S1 Hˆ SO Tx

(x- ; y- ; zcomponents)

Values obtained at the QR-TD-

DFT/6-31G* level of theory at the S1 optimized geometry. photodeactivation mechanisms are opened up. As seen in Table 6, only the S1→T1 ISC mechanism is (vertically and adiabatically) energetically accessible for all compounds. In all


21


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Page 22 of 39

compounds, the S1→T1 ISC mechanism is characterized by a large energy gap (> 1 eV) and an almost negligible value of the corresponding electronic SOCs (values below 0.1 cm−1, see Table 6). Therefore, under these circumstances, a direct S1→T1 ISC channel seems quite unlikely. As stated above, higher-lying states have been proved to enhance the ISC rates57 via spin-vibronic coupling. It is known that the S1→T2 transition is responsible for the efficient ISC of anthracene.61 In Table 6 are also listed the vertical and adiabatic energies of the second triplet excited states, i.e. T2. The S1→T2 gaps decrease in the A-3 > A-2 > A-1 series. Additionally, the S1→T2 SOCs are substantially larger (by almost one order of magnitude) for both A-1 and A-2 than for A-3. In such a scenario, we propose enhanced S-T ISC channels in A1 and A-2. Thus, for these latter compounds, their ISC mechanisms gain intensity through spinvibronic coupling with the nearby T2 state. In contrast, for A-3, this mechanism is much less effective. These interpretations are in agreement with the ranking of the observed quantum yields of triplet formation.

3. CONCLUSIONS Three anthracene derivatives containing an amino group were prepared as triplet energy acceptors for switchable triplet-triplet annihilation upconversion. The fluorescence of the anthracene derivatives is tuned by controlling both the intersystem crossing (ISC) by attaching different number of the phenyl rings to the anthracene core (A-1, A-2 and A-3 are with no phenyl, one and two phenyl substituents, respectively), and the photo-induced electron transfer (PET) by variation of the distance between the electron donor (the nitrogen atom of the amino group) and the anthracene moiety (the linker is a methylene moiety for A-1 and A-2, but a benzyl group for A-3). We demonstrated that ISC is inhibited with increasing the phenyl substituents on the anthracene, whereas the PET is highly dependent on the linker: it is important in A-1 and A-2,


22

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but negligible in A-3. As a result, the fluorescence of A-1 and A-2 can be switched on by inhibition of the PET, either by protonation or by acetylation of the amino group, whereas the fluorescence intensity of A-3 is insensitive to these effects. The calculation of the Gibbs free energy changes (∆GCS) of the PET processes in these compounds validated the different PET efficiency. The triplet-triplet annihilation upconversion with these compounds as triplet energy acceptor and emitter was studied, and the TTA upconversion can be switched on with inhibition of the PET by acetylation of the acceptors. The switching mechanisms were studied with steady state UV−vis absorption and fluorescence emission spectroscopy, as well as nanosecond transient absorption spectroscopy. We found that although the fluorescence of the anthracene acceptors were quenched by PET, the triplet excited states of the anthracene derivatives were not quenched by PET. Furthermore, we found that the ISC efficiency of the anthracene derivatives can be substantially tuned by using different phenyl substituent on the anthracene core. Beyond constituting a fundamental investigation of the photochemistry of the triplet state of organic chromophores, these results constitute the first step to develop new activatable TTA upconversion systems.

4. EXPERIMENTAL SECTION 4.1. General Methods. All the chemicals used in synthesis are analytical pure and were used as received. Fluorescence quantum yields were measured with anthracene in ethanol as standard (ΦF = 0.30 in ethanol). The fluorescence lifetimes of the compounds were measured with EPL picosecond pulsed laser (473 nm, pulse width: 66.9 ps, maximum average power: 5 mW; Edinburgh Instrument Ltd., UK) which was synchronized to the FLS 920 spectrofluorometer. The fluorescence (fluorescence emission) was recorded with a FS5 spectrofluorometer (Edinburgh Instrument Ltd., UK).


23


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4.2. Compound 3. A solution of compound 2

29

Page 24 of 39

(400 mg, 1.2 mmol) in toluene (10 mL)

was mixed with a solution of 4-formylphenylboronic acid (270 mg, 1.8 mmol) in EtOH (5 mL). This mixture was mixed with an aqueous solution of Na2CO3 (745.2 mg in 5 mL water). The resulting suspension was purged for 30 min with a stream of Argon . Then Pd(PPh3)4 (10 mg) was added. The mixture was refluxed under Argon atmosphere for 6 h, at which time the heating was removed and the reaction mixture was allowed to cool to room temperature. Water (30 mL) was added and the organic layer was separated. The aqueous layer was extracted with CH2Cl2 (3 × 30 mL). The combined organic layer was dried over MgSO4, filtered and the solution was evaporated under reduced pressure. Purification by column chromatography (silica gel, DCM/ Petroleum ether = 1 : 1) afforded 332.1 mg (77.2%) of the product as a yellow solid. Mp: 259.5− 260.7°C. 1H NMR (400 MHz, CDCl3): δ 10.22 (s, 1H), 8.16 (d, 2H, J = 8.0 Hz), 7.74−7.68 (m, 4H), 7.64−7.55 (m, 5H), 7.50 (d, 2H, J = 8.0 Hz), 7.37−7.35 (m, 4H). MALDI-HRMS: calcd ([C27H18O]+), m/z = 358.1358, found m/z = 358.1351. 4.3. Compound 4. To a solution of 9-anthracenecarboxaldehyde (1.0 g, 4.9 mmol) in CHCl3 (20 mL) was added dropwise a solution of bromine (0.40 mL, 7.9 mmol) in CHCl3 (10 mL) over a period of 10 min at room temperature. The reaction mixture was stirred for 8 h, at which time it was quenched with a saturated solution of Na2S2O3 (100 mL). The organic layer was removed and washed with aqueous Na2CO3 (1 M, 100 mL) and water (2 × 100 mL). The organic layer dried over MgSO4, filtered and the solution was evaporated under reduced pressure. Purification by column chromatography (silica gel, DCM/ Petroleum ether = 1 : 1) afforded 0.86 g (62%) of the product as a yellow solid. mp 218.1− 219.5 °C. 1H NMR (400 MHz, CDCl3): δ 11.48 (s, 1H), 8.89 (d, 2H, J= 8.0 Hz), 8.67 (d, 2H, J = 8.0 Hz), 7.71−7.64 (m, 4H).

13

C NMR

(100 MHz, CDCl3) : δ = 193.2, 134.0, 131.8, 130.1, 128.88, 127.3, 125.5, 123.7. MALDI-HRMS:


24

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calcd ([C15H9OBr]+), m/z = 283.9837, found m/z = 283.9838. 4.4. Compound 5. A solution of compound 4 (300 mg, 1.05 mmol) in toluene (10 mL) was mixed with a solution of phenylboronic acid (256 mg, 2.1 mmol) in EtOH (5 mL). This mixture was mixed with an aqueous solution of Na2CO3 (667.8 mg in 5 ml water). The resulting suspension was purged for 30 min with a stream of Argon and treated with Pd(PPh3)4 (10 mg). The mixture was heated to reflux under Argon atmosphere for 6 h, at which time the heating source was removed and the reaction was allowed to cool to room temperature. Water (30 mL) was added and the layers were separated. The aqueous layer was extracted with CH2Cl2 (3 × 30 mL). The combined organic layers were dried over MgSO4, filtered and evaporated under reduced pressure. Purification by column chromatography (silica gel, DCM/ Petroleum ether = 1 : 1), afforded 267 mg (90%) of the product as a yellow solid. Mp: 167.7− 168.6°C. 1H NMR (400 MHz, CDCl3): δ 11.59 (s, 1H), 9.02 (d, 2H, J = 8.0 Hz), 7.71−7.57 (m, 7H), 7.43−7.39 (m, 4H). 13

C NMR (100 MHz, CDCl3): δ = 193.7, 145.4, 138.26, 131.6, 130.6, 129.9, 128.5, 128.4, 128.0,

125.5, 125.0, 123.4. MALDI-HRMS: calcd ([C21H14O]+), m/z = 282.1045, found m/z = 282.1042. 4.5. Compound A-1. 9-Anthracenecarboxaldehyde (500 mg, 2.42 mmol) and n-butylamine (2 ml) were dissolved in ethanol/THF (3:2, v/v). The mixture was refluxed with stirring for 10 h under N2. Then NaBH4 (448.5 mg, 12.1 mmol) was added in several portions and the mixture was stirred for 3 h at room temperature. The solvent was removed and the residue was taken up with dichloromethane (DCM), the organic phase was washed with brine and dried over Na2SO4, DCM was removed and the residue was purified with column chromatography (silica gel, DCM/PE, 2:1, V/V) to give light yellow solid (764 mg, 87%). Mp: 39.7−40.5°C. 1H NMR (400 MHz, CDCl3): δ 8.41 (s, 1H), 8.37 (d, 2H, J = 8.0 Hz), 8.03 (d, 2H, J = 12.0 Hz), 7.57 (t, 2H, J = 8.0 Hz), 4.73 (s, 2H), 2.90 (t, 2H, J = 6.0 Hz), 1.63−1.56 (m, 2H), 1.46−1.36 (m, 3H), 0.95 (t, 3H,


25


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J = 8.0 Hz). 13C NMR (100 MHz, CDCl3): δ = 132.1, 131.7, 130.4, 129.2, 127.2, 126.1, 125.0, 124.3, 50.5, 46.0, 32.4, 14.26. MALDI-HRMS: calcd ([C19H21N]+), m/z = 263.1674, found m/z = 263.1664. 4.6. Compound A-2. Compound 5 (100 mg, 0.354 mmol) and n-Butylamine (1 ml) were dissolved in mixed solvent ethanol/THF (3:2, v/v). The mixture was refluxed with stirring for 10 h under N2. Then NaBH4 (67 mg, 1.77 mmol) was added in several portions and the mixture was stirred for 3 h at room temperature. The solvent was removed and the residue was taken up with dichloromethane (DCM), the organic phase was washed with brine and dried over Na2SO4, DCM was removed under reduced pressure and the residue was purified with column chromatography (silica gel, DCM/PE, 2:1, v/v) to give white solid (55.2 mg, 48%). Mp: 80.5− 81.7°C. 1H NMR (400 MHz, CDCl3): δ 8.42 (d, 2H, J = 8.0 Hz), 7.67 (d, 2H, J = 8.0 Hz), 7.60−7.52 (m, 5H), 7.41−7.32 (m, 4H), 4.80 (s, 2H), 2.95 (t, 2H, J = 6.0 Hz), 1.67−1.59 (m, 2H), 1.47−1.38 (m, 2H), 0.96 (t, 3H, J = 8.0 Hz). 13C NMR (100 MHz, CDCl3): δ = 139.1, 137.5, 131.7, 131.2, 130.1, 129.9, 128.3, 127.8, 127.4, 125.8, 124.8, 124.1, 50.4, 46.0, 32.2, 20.6, 14.0. MALDI-HRMS: calcd ([C25H25N]+), m/z = 339.1987, found m/z = 339.1997. 4.7. Compound A-3. Compound 3 (50 mg, 0.14 mmol) and n-butylamine (1 mL) were dissolved in mixed solvent ethanol/THF (3:2, v/v). The mixture was refluxed with stirring for 10 h under N2. Then NaBH4 (26.4 mg, 0.7 mmol) was added in several portions and the mixture was stirred for 3 h at room temperature. The solvent was removed and the residue was taken up with dichloromethane (DCM), the organic phase was washed with brine and dried over Na2SO4, DCM was removed under reduced pressure and the residue was purified with column chromatography (silica gel, DCM/PE, 2:1, v/v) to give white solid (40 mg, 69%). Mp: 179.3 − 177.8 °C. 1H NMR (400 MHz, CDCl3): δ 7.90 (d, 2H, J = 8.0 Hz), 7.70 (d, 2H, J = 8.0 Hz), 7.62−7.53 (m, 7H), 7.47


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(d, 2H, J = 4.0 Hz), 7.34−7.26 (m, 4H), 4.35 (s, 2H), 3.02 (t, 2H, J = 8.0 Hz), 2.05−1.97 (m, 2H), 1.54−1.45 (m, 3H), 0.98 (t, 3H, J = 8.0 Hz). 13C NMR (100 MHz, CDCl3): δ = 140.5, 138.9, 137.6, 135.8, 132.3, 131.3, 130.4, 129.9, 129.9, 129.7, 128.4, 127.5, 127.1, 126.5, 125.3, 125.1, 50.7, 46.1, 28.3, 20.2, 13.6. MALDI-HRMS: calcd ([C31H29N]+), m/z = 415.2300, found m/z = 415.2292. 4.8. Compound A-1’. The compound A-1 (131.0 mg, 0.5 mmol) was dissolved in methanol (10 mL) at room temperature. Five drops of pyridine was added and the mixture was stirred for 5 min. A solution of di-tert-butyl dicarbonate (218 mg, 1.0 mmol) in methanol (2 mL) was added dropwise over 5 min. The mixture was stirred overnight at room temperature. Then the solution was evaporated to dryness under reduced pressure. The residue was purified by flash chromatography on silica gel (DCM/ PE, 1:1, v/v) to give white solid (177.0 mg, 97.4%). Mp: 95.5− 96.3 °C. 1H NMR (500 MHz, CDCl3): δ 8.45 (s, 1H), 8.43 (d, 2H, J = 10.0 Hz), 8.03 (d, 2H, J = 10.0 Hz), 7.56−7.46 (m, 4H), 5.54 (s, 2H), 2.78 (s, 2H), 1.55 (br, 9H), 1.31−1.25 (m, 2H), 1.01−0.97 (m, 2H), 0.63 (s, 3H). 13C NMR (100 MHz, CDCl3) : δ = 156.0, 131.5, 129.4, 129.2, 128.2, 126.5, 126.2, 125.3, 125.0, 124.4, 79.7, 44.3, 40.9, 30.5, 28.7, 20.0, 13.8. MALDI-HRMS: calcd ([C24H29NO2]+), m/z = 363.2198, found m/z = 363.2200. 4.9. Compound A-2’. The compound A-2 (34.0 mg, 0.1 mmol) was dissolved in methanol (10 mL) at room temperature. Five drops of pyridine was added and the mixture was stirred for 5 min. A solution of di-tert-butyl dicarbonate (106.0 mg, 2.0 mmol) in methanol (2 mL) was added dropwise over 5 min. The mixture was stirred overnight at room temperature. Then the solution was evaporated to dryness under reduced pressure. The residue was purified by flash chromatography on silica gel (DCM/ PE, 1:1, v/v) to give white solid (38.9 mg, 88.6%). Mp: 35.7 − 37.2 °C. 1H NMR (500 MHz, CDCl3): δ 8.49 (d, 2H, J = 10.0 Hz), 7.68 (d, 2H, J = 10.0


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Hz), 7.60−7.51 (m, 5H), 7.41−7.33 (m, 4H), 5.60 (s, 2H), 2.85 (s, 2H), 1.56 (br, 9H), 1.38−1.35 (m, 2H), 1.04 (s, 2H), 0.67 (s, 3H). 13C NMR (100 MHz, CDCl3): δ = 156.0, 139.2, 138.6, 131.3, 131.2, 130.1, 128.5, 128.1, 128.0, 127.7, 126.1, 125.0, 124.5, 79.7, 44.4, 41.0, 30.6, 28.7, 20.1, 13.8. MALDI-HRMS: calcd ([C30H33NO2]+), m/z = 439.2511, found m/z = 439.2503. 4.10. Nanosecond Transient Absorption. The nanosecond time-resolved transient absorption spectra were recorded on LP920 laser flash photolysis spectrometer (Edinburgh Instruments, UK). The signal was digitized on a Tektronix TDS 3012B oscilloscope. All samples in laser flash photolysis experiments were deaerated with argon for ca. 15 min before measurement. 4.11. Theoretical Calculations. The geometries of the singlet ground state (S0) of A-1, A-2 and A-3 were optimized at the B3LYP/6-311G* level of theory. The geometries of the lowest singlet (S1) excited states were also optimized at the TD- B3LYP/6-311G* level of theory. Gas phase ADC(2) vertical and adiabatic singlet and triplet excitation energies were obtained at these geometries using the def2-TZVP basis set. SOCs were computed using the quadratic-response TD-DFT approach,62,63 i.e. QR-TD-DFT, as implemented in the Dalton program64 at their S1 optimized geometries. The SOC operator makes use of a semi-empirical effective single-electron approximation.65 For the latter calculations the B3LYP functional in combination to the 6-31G(d) basis set was used. ADC(2) and DFT/TD-DFT calculations were performed with the TURBOMOLE66 and Gaussian 0967 programs. 4.12. Cyclic Voltammetry. The cyclic voltammetry curves were recorded by CH instruments (CH instruments, Inc. Shanghai, China). In N2 saturated CH3CN containing a 0.10 M Bu4NPF6 as supporting electrolyte; counter electrode is Pt electrode; working electrode is glassy carbon electrode; Ag/AgNO3 couple as the reference electrode. c [Ag+] = 0.1 M. 1.0 mM compounds in CH3CN, 20 °C. Scan rates: 50 mV/s. Ferrocene (Fc) was used as internal reference.


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Supporting information Experimental procedures, molecular structure characterization, additional spectra, details of the calculations for the electrochemical study. This material is available free of charge via the Internet at http://pubs.acs.org.

■ ACKNOWLEDGMENT We thank the NSFC (21073028, 21273028, 21473020 and 21421005), the Royal Society (UK) and NSFC (China-UK Cost-Share Science Networks, 21011130154), Ministry of Education (SRFDP-20120041130005), the Fundamental Research Funds for the Central Universities (DUT14ZD226), Program for Changjiang Scholars and Innovative Research Team in University [IRT_132206] and Dalian University of Technology for financial support (DUT2013TB07). D. E. acknowledges the European Research Council (ERC, Marches-278845) and the Région des Pays de la Loire for his post-doctoral grant. D.J. acknowledges the European Research Council (ERC) and the Région des Pays de la Loire for financial support in the framework of a Starting Grant (Marches-278845) and a recrutement sur poste stratégique, respectively. This research used resources of 1) the GENCI-CINES/IDRIS, 2) CCIPL (Centre de Calcul Intensif des Pays de Loire), and 3) a local Troy cluster.

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