Broadband Visible Light-Harvesting Naphthalenediimide (NDI) Triad

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Broadband Visible Light-Harvesting Naphthalenediimide (NDI) Triad: Study of the Intra-/Intermolecular Energy/Electron Transfer and the Triplet Excited State Shuang Wu, Fangfang Zhong, Jianzhang Zhao, Song Guo, Wenbo Yang, and Thomas Murray Fyles J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b01448 • Publication Date (Web): 28 Apr 2015 Downloaded from http://pubs.acs.org on April 29, 2015

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The Journal of Physical Chemistry

Broadband Visible Light-Harvesting Naphthalenediimide (NDI) Triad: Study of the Intra/Intermolecular Energy/Electron Transfer and the Triplet Excited State Shuang Wu,a Fangfang Zhong,a Jianzhang Zhao,*,a Song Guo,a Wenbo Yang a and Tom Fyles b

a

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024 (China). E-mail: [email protected]; Web: http://finechem2.dlut.edu.cn/photochem b

Department of Chemistry, University of Victoria, PO Box 3065, Victoria, Canada.

ACS Paragon Plus Environment

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Abstract: A triad based on naphthalenediimides (NDI) was prepared to study the intersystem crossing (ISC), the fluorescence-resonance-energy-transfer (FRET), as well as the photo-induced electron transfer (PET) processes. In the triad, the 2-bromo-6-alkylaminoNDI moiety was used as singlet energy donor and the spin converter, whereas 2,6-dialkylaminoNDI was used as the singlet/triplet energy acceptor. This unique structural protocol and thus alignment of the energy levels ensures the competing ISC and FRET in the triad. The photophysical properties of the triad and the reference compounds were studied with steady state UV−vis absorption spectra, fluorescence spectra, nanosecond transient absorption spectra, cyclic voltammetry and DFT/TDDFT calculations. FRET was confirmed with steady state UV−vis absorption and fluorescence spectroscopy. Intramolecular electron transfer was observed in polar solvents, demonstrated by the quenching of both the fluorescence and triplet state of the energy acceptor. Nanosecond transient absorption spectroscopy shows that the T1 state of the triad is exclusively localized on the 2,6-dialkylaminoNDI moiety in the triad upon selectively photoexcitation into the energy donor, which indicates the intramolecular triplet state energy transfer. The intermolecular triplet state energy transfer between the two reference compounds was investigated with nanosecond transient absorption spectroscopy. The photophysical properties were rationalized by TDDFT calculations. Keywords: Energy transfer; FRET; ISC; Naphthalenediimide; Triplet state;

INTRODUCTION Triplet photosensitizers are versatile compounds and have been widely used in photocatalysis, such as photocatalytic hydrogen (H2) production by photolysis of water,1−4 photoredox catalytic organic synthesis,5−8 photoinduced charge separation,9 molecular logic gates,10,11 non-linear


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optics,12,13 and more recently in the triplet-triplet annihilation upconversion.14−17 The conventional triplet photosensitizers usually contain only a single visible light-harvesting chromophore, as a result, there is only one major absorption band in visible spectral region for these conventional triplet photosensitizers.18−20 Multichromophore molecular assemblies have been studied, such as the Pt(II) porphyrin complex with BODIPY appendents.21−24 However, very few examples based on neat organic chromophores were reported. Recently we prepared BODIPY-iodoBODIPY dyads,25 rhodamine-iodoazaBODIPY dyad,26 and styryl-iodoazaBODIPY dyads,27 as broadband visible light-harvesting organic triplet photosensitizers. The designing of these dyad/triad triplet photosensitizers is based on intramolecular resonance energy transfer (RET) and the concept of the intramolecular spin converter,18 so that the photoexcitation energy-harvested by the two different chromophores can be funneled into one moiety in the dyad/triads.18 Interesting photophysical processes were observed for these dyads/triads, such as delocalization of the triplet state (triplet state equilibrium), as well as intermolecular resonance energy transfer from the singlet energy donor to the singlet energy acceptor (spin converter).18,26,27 These broadband visible light-absorbing multi-chromophore triplet photosensitizers have been used for photoredox catalytic organic reactions,28 and singlet oxygen (1O2) photosensitizing.25,26 The photosensitizing or the photocatalytic activity is more efficient as compared with the conventional triplet photosensitizers. However, much room is left to fully explore this kind of multi-chromophore broadband visible light-harvesting triplet photosensitizers. For example, the known broadband visible light-absorbing organic triplet photosensitizers are all based on the same molecular structural profile, that is, the singlet energy acceptor is the spin converter.21,25−27An alternative molecular structural profile, i.e. dyads with the singlet energy donor as the spin converter, have


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not been investigated. In this kind of multichromophore dyads, the intersystem crossing (ISC) of the singlet energy donor will compete with the Förster-resonance-energy-transfer (FRET). Previously it was reported that the ISC was completely inhibited by the FRET.11 However, more multi-chromophore photosensitizers are needed to fully unveil the photophysics involved in these new organic triplet photosensitizers. In order to address the above challenges, herein we prepared naphthalenediimide (NDI) triad (N-1, Scheme 1) to study the effect of competing ISC and FRET processes, as well as the photoinduced electron transfer, on the photophysical properties of the triad. In triad N-1, the 2-bromo6-aminoNDI part was used as the spin converter and the singlet energy donor,15 the 2,6diaminoNDI moiety in N-1 is the singlet energy acceptor. The photophysical processes of N-1 were studied with steady state UV−vis absorption and fluorescence spectroscopies, nanosecond transient absorption spectroscopy, electrochemical measurement (cyclic voltammetry, Gibbs free energy changes of the electron transfer processes), as well as DFT calculations. Recently we reported diiodoBodipy-rhodamine dyad,29 and diiodoBodipy-perylenebisimide (PBI) triad,30 which show competing ISC and FRET, however, the molecular structure and the photophysical properties are different and the comparison will be made in later sections.

EXPERIMENTAL SECTION Analytical Measurements. NMR spectra were recorded by a Bruker 400 MHz spectrometer with CDCl3 as solvent and TMS as standard at 0.00 ppm. High resolution mass spectra (HRMS) were determined in a TOF MALDI-HR MS system (UK). Fluorescence spectra were measured on a RF5301 PC spectrofluorometer (Shimadzu, Japan) while fluorescence lifetimes were measured with a OB920 luminescence lifetime spectrometer (Edinburgh Instruments, UK). The


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absorption spectra were recorded on UV2550 UV−vis spectrophotometer (Shimadzu, Japan) and Agilent 8453 UV/vis near-IR spectrophotometer. The chemicals used were analytically pure. Solvents were dried or distilled before use. Compounds 1, 2, 3, N-1 and N-2 were prepared according to the reported methods (see Supporting Information for synthesis of 2 and 3).15 4,31 7 and N-3,32 were synthesized according to the literature procedures. Synthesis and Characterization. Synthesis of the Compound 4. Under a N2 atmosphere, compound 3 (60 mg, 0.077 mmol) was dissolved in dichloromethane (30 mL), and trifluoroacetic acid (0.3 mL) was added. The mixture was stirred at 25 °C for 12 h. The precipitate was washed with a saturated sodium hydroxide solution (10 mL) and then with water (60 mL) twice. The organic layer was drying over Na2SO4. After removing the solvent under reduced pressure, the obtained blue powder was used in the next reaction directly without further purification (42.3 mg, Yield: 90% ).31 Synthesis of the Compound N-1. 4 was mixed with compound 2 (194 mg, 0.30 mmol) and 2methoxyethanol (5 mL), and the mixture was stirred at 120 °C for 7 h under a N2 atmosphere. After removing the solvent under reduced pressure, the crude product was purified by column chromatography (silica gel; CH2Cl2/methanol, 100:1, v/v). A purple solid was obtained (19.1 mg, yield: 11 %). 1H NMR (400 MHz, CDCl3): δ 10.40 (s, 2H), 9.68 (s, 2H), 8.68 (s, 2H), 8.31 (s, 2H), 8.08 (s, 2H), 4.09−4.02 (m, 20H), 1.86−1.78 (m, 4H), 1.58 (s, 2H), 1.25 (s, 48H), 0.92−0.83 (m, 36H). 13C NMR (100 MHz, CDCl3): δ 166.1, 162.7, 162.2, 161.8, 161.3, 151.9, 149.2, 138.3, 127.2, 125.4, 121.2, 120.9, 120.0, 118.0, 100.8, 45.2, 44.2, 43.2, 37.9, 32.1, 32.0, 30.8, 29.8, 29.4, 28.8, 24.1, 23.3, 23.2, 22.8, 14.3, 10.7. TOF MALDI-HRMS: calcd ([C94H119Br2N10O12]−), m/z = 1740.7389, found, m/z = 1740.7540.


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Synthesis of the Compound N-2. Under a N2 atmosphere, a mixture of compound 2 (100 mg, 0.15 mmol), 2-ethylhexylamine (5.2 mg, 0.082 mmol) and 2-methoxyethanol (3 mL) was stirred at 120 °C for 6 h. After removing the solvent under reduced pressure, the crude product was purified by column chromatography (silica gel; CH2Cl2/petroleum ether, 1:1 v/v). A red crystalline solid was obtained (77.1 mg, yield: 72%). 1H NMR (400 MHz, CDCl3): δ 10.19 (s, 1H), 8.87 (s, 1H), 8.30 (s, 1H), 4.18−4.08 (m, 4H), 3.50 (t, 2H), 1.92−1.91 (m, 2H), 1.82−1.76 (m,

1H),

1.56−1.29

(m,

24H),

1.01−0.88

(m,

8H).

TOF

MALDI-HRMS:

calcd

([C38H54BrN3O4]−), m/z = 695.3298, found, m/z = 695.3300. Synthesis of the Compound N-3. Under a N2 atmosphere, a mixture of compound 2 (100 mg, 0.15 mmol), 2-ethylhexylamine (0.46 mL) and 2-methoxyethanol (3 mL) was stirred at 140 °C for 2 h. After removing of the solvent under reduced pressure, the crude product was purified by column chromatography (silica gel; CH2Cl2/petroleum ether, 1:1 v/v). A blue crystalline solid was obtained (106.2 mg, yield: 92%). 1H NMR (400 MHz, CDCl3): δ 9.34 (t, 2H), 8.17 (s, 2H), 4.18−4.08 (m, 4H), 3.43−3.40 (m, 4H), 1.95−1.93 (m, 2H), 1.79−1.75 (m, 2H), 1.54−1.26 (m, 33H), 0.95−0.86 (m, 23H). TOF MALDI-HRMS: calcd ([C46H72N4O4]−), m/z = 744.5554, found, m/z = 744.5572. Nanosecond Transient Absorption Spectroscopy. Nanosecond transient absorption spectra were recorded on a LP920 laser flash photolysis spectrometer (Edinburgh Instruments, Livingston, UK). The samples were purged with N2 for 15 min before measurements excited with a nanosecond pulse laser and the data was digitized by a Tektronix TDS 3012B oscilloscope. Cyclic Voltammetry. Cyclic voltammetry was performed under a 100 mV/s scan rate, in CHI610D Electrochemical workstation (Shanghai, China). The measurements were performed at room temperature with tetrabutylammonium hexafluorophosphate (Bu4N[PF6], 0.1 M) as the


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supporting electrolyte, glassy carbon electrode as the working electrode and platinum electrode as the counter electrode. Ag/AgNO3 was used as reference electrode. Dichloromethane was used as the solvent and ferrocene (c = 1.0 × 10−3 M) was added as the internal references. The solutions were purged with N2 before measurement. DFT Calculations. Geometry optimization was calculated by using the B3LYP functional, while the vertical excitation energy was calculated with the time-dependent DFT (TD−DFT) method based on the singlet ground-state geometry. All the calculations were performed with Gaussian 09W software (Gaussian, Inc.).33

RESULTS AND DISCUSSIONS Design and Synthesis of the Compounds. NDI was widely used in fluorescent molecular probes,34−36 and as a scaffold in molecular arrays.37−50 However, application of NDI in study of triplet excited state is rare.15,32,51,52 Triad N-1 is composed of two 2-bromo-6-alkylaminoNDI moieties as singlet energy donors, as well as the spin converter for production of the triplet excited states (Scheme 1). The heavy atom effect of the bromine atom will facilitate ISC.20,53−55 The central 2,6-dialkylaminoNDI moiety is the singlet energy acceptor, which is devoid of ISC capability due to the lack of heavy atoms.15 2-Bromo-6-alkylaminoNDI (N-2) and 2,6dialkylaminoNDI (N-3) were prepared as reference compounds for study of the photophysical properties. The preparation of the compounds is with compound 2 as the starting material.56 Reaction of 2 with the mono-protected 1,2-diaminoethane gave compound 3. Deprotection of the Boc moiety with trifluoride acetic acid (TFA) and then reaction with compound 2 lead to compound B-1. The reference compounds N-2 and N-3 were prepared using similar methods (Scheme 1). All the compounds were obtained in moderate to satisfactory yields. The molecular structures of the compounds were fully verified with 1H NMR, 13C NMR and HRMS


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Scheme 1. Preparation of the Triad N-1, and the Reference Compounds N-2 and N-3.a The Molecular Structures of B-1 and B-2 Used in Fluorescence Quantum Yield Measurement Were Also Shown R O

O

O

O Br

N

O Br

(a)

Br

Br O

O

O

O

N R

1

R

O

2

(b)

NH2

H2 N

6

O

(c) Boc

H2N

N

O H N

N H

O

R Boc

3

7

R=

R

R O

(d)

3

H2N

N H

O

N

N

O

R R

H N

NH2

O

(e)

O

O

O O

H N N

O

Br

O

N

O

R

O

H N N H

O

N

O

R

N-1

R O

Br N

N

N

R

R

O

O

N H

4

N

N

O

Br

R

O

Boc

N H

O

N

H N

H N

O

H N

O R

(f)

2

(g)

R

N H

N

O

H N

I

R I

O

N

R

R

N-2

N-3

O

N

B

N

F

B-1

I

N F

B

N

I

F

F

B-2

a

Key: (a) 2-ethylhexylamine,120 °C, under N2, 2 h, yield: 37%; (b) (Boc)2O, 0 °C, 24 h; yield: 85%; (c) 140 °C, under N2, yield: 62%; (d) THF, 25 °C, 12 h, yield: 90%; (e) compound 2, 120°C, under N2, 7 h, yield: 11%; (f) 2-ethylhexylamine, 120 °C, under N2, 6 h, yield: 72%; (g) 2-ethylhexylamine, 140 °C, under N2, 2 h, yield: 92%.


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spectroscopies. Previously we reported the study of a diiodoBodipy-rhodamine dyad which show competing ISC and FRET,29 However, the linker between the two chromophores in the diiodoBodipy-rhodamine dyad is rigid cyanuric chloride.29 For the triad N-1 reported in this paper, the chromophores are naphthalenediimide (NDI), and the linkers are flexible. Moreover, different photophysical properties were observed for the dyad and the triad (see later section). We also studied a diiodoBodipy-PBI dyad/triad, but the linker between the different chromophores is rigid C−C bond.30 UV−vis Absorption and Fluorescence Spectroscopy. The UV−vis absorption spectra of the compounds were studied (Figure 1). The reference compounds N-2 and N-3 give absorption in the visible spectral region at 534 nm and 617 nm, respectively. Thus the singlet excited state (S1) of N-2 has higher energy level than that of N-3. For N-1, two major absorption bands at 526 nm and 606 nm were observed. Interestingly, the UV−vis absorption spectrum of N-1 is not the sum of N-2 and N-3. For example, the ε value of N-1 at 524 nm is close to that of N-2, although 0.4 0.3

Absorption

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


Mixture N-2/N-3 (2 : 1) N-1 N-2 N-3

0.2 0.1 0.0 300

400

500 600 700 Wavelength / nm

800

Figure 1. UV−vis absorption of compounds N-1, N-2, N-3 and the mixture of N-2/N-3 (2:1). c = 1.0 × 10−5 M in toluene, 20 °C.


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there are two 2-bromo-6-alkylaminoNDI moieties (analogue of N-2) in N-1. We noted the spectrum of N-1 is different from that of the N-2/N-3 mixture (2:1. Figure 1). Moreover, the absorbance of N-1 at 605 nm is only half of that of N-3. However, only slight broadening of the absorption bands was observed, thus we envisage the interactions between the different moieties in N-1 is not significant,21,57−62 π-π interaction, or excitonic interactions may be responsible for the observed UV−vis absorption profile of B-1. The fluorescence of the compounds was studied (Figure 2). The fluorescence emission of N-1 is highly sensitive to the solvent polarity. For example, the emission of N-1 in toluene is strong, but the emission was almost completely quenched in polar solvents such as dichloromethane, methanol and acetonitrile (Figure 2a). The emission of N-1 is due to the dialkylaminoNDI moiety in N-1, i.e. the N-3 unit. However, the fluorescence emission of N-3 is much less sensitive to the solvent polarity (Figure 2b). For example, the fluorescence emission intensity of N-3 is quenched only by half in polar solvents (methanol and acetonitrile). Thus we propose photoinduced electron transfer exists in N-1, especially in polar solvents, and, as a result, the fluorescence of the energy acceptor was quenched in polar solvents.28,63 This postulation was confirmed by the electrochemical studies on N-1 (given below), which indicates that photoinduced electron transfer is thermodynamically allowed in polar solvents such as dichloromethane, for which negative Gibbs free energy changes (ΔGCS) was obtained.64 All the compounds are stable upon white light photoirradiation (power density: 3 mW cm−2) (Supporting Information, Figure S11−S14). The UV−vis absorption of N-1 was compared with a previously reported diiodoBodipyrhodamine dyad.29 The separation between the steady state absorption bands of the diiodoBodipy-rhodamine dyad is only 23 nm (thus the serious overlap of the absorption bands of


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the chromophores in the dyad was resulted). For the previously reported diiodoBodipy-PBI dyad and triad, the absorption bands of the components overlap with each other.30 For the triad N-1 studied herein, the spectral separation is 81 nm. Large spectral separation is beneficial for assignment of the nanosecond transient absorption spectra of the compounds (see later section). Moreover, the study of the previous diiodoBodipy-rhodamine dyad was focused on switching of the triplet state by acid/base. The subject of study on N-1 is different. 300

600

a Intensity / a.u.

Toluene

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


200 MeCN DCM MeOH

100

500

b

400

Toluene DCM MeOH MeCN

300 200 100

0 600

650

700

750

Wavelength / nm

800

0 600


800

Figure 2. Emission spectra of compounds in different solvents. (a) N-1 (λex = 565 nm); (b) N-3 (λex = 580 nm). c = 1.0 × 10−5 M, 20 °C. In order to study the PET effect, the fluorescence emission of N-1 and the energy donor reference, i.e. compound N-2, were compared (Figure 3). With optically matched solution (the two sample solutions show the same absorbance at the excitation wavelength), the fluorescence emission of the energy donor (with N-2 as the analogue) is quenched in N-1, either in toluene or acetonitrile. The quenching of the N-2 emission in N-1 indicates the presence of FRET for N-1, especially in toluene.63−65 In toluene the PET process is less efficient, thus the FRET rate constant can be calculated based on the quenched fluorescence of the energy donor (Eq. 1), as kFRET = 4.4 × 109 s−1.66


11


⎡[Φ ( N -2) / Φ ( N -1) ] − 1⎤⎦ kFRET = ⎣

(Eq. 1)

τ ( N-2)

where kFRET is the rate constant of FRET process, Φ(N-2) is the fluorescence quantum yield of N-2, Φ(N-1) is the fluorescence quantum yield of N-1, τ(N-2) is the fluorescence lifetime of N-2.

100

a

N-2

200

Intensity / a.u.

Intensity / a.u.

250

150 100 50 N-1

0 550

600

650

Wavelength / nm

b N-2 50

N-1

0

700

550

600 650 Wavelength / nm

700

Figure 3. Emission spectra of compounds N-1 and N-2 in (a) acetonitrile, λex = 504 nm; A = 0.15; (b) toluene, λex = 491 nm, A = 0.11; c = ca. 1.0 × 10−5 M. Optically matched solutions were used, 20 °C. 250

40

a 20

b

200

30

Intensity / a.u.

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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N-3

10

150 N-3 100 50

N-1

0

N-1 0

600

650

700

750

Wavelength / nm

800

600


800

Figure 4. Emission spectra of compounds N-1 and N-3. (a) In acetonitrile, λex = 579 nm, A = 0.10. (b) In toluene, λex = 568 nm, A = 0.11. Optically matched solutions were used. c = ca. 1.0 × 10−5 M, 20 °C.


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Similar quenching results were observed for comparison of the fluorescence emission of N-3 and N-1 (Figure 4). However, the information derived from Figure 4 is different from that of Figure 3. The quenching of the singlet energy acceptor in N-1 indicates either photoinduced electron transfer,67 or ground state interaction between the chromophores in N-1. It was found that the emission wavelength of N-1 is different from that of N-3, indicating the emissive moiety in N-1 is resided in a different environment as compared with the reference compound N-3.63 The photoinduced electron transfer rate constant in N-1 can be calculated based in the quenched emission of the energy acceptor part according to eq. 2.66 Based on the fluorescence quantum yields of N-3 and N-2 (Table 1), as well as the fluorescence lifetime of N-3, the rate constant of the PET was calculated as 8.4 × 108 s−1 based on Eq. 2. Note this is a relatively slow electron transfer, but it is still efficient to quench the fluorescence of the N-3 part, which is with radiative rate constant of kr = 1.1 × 108 s−1 (derived from the fluorescence lifetime of N-3, Table 1).

⎡Φ ⎤ Intra kCS = ⎢ ( N-3) ⎥ τ ( N-3) ⎣ Φ ( N-1) ⎦

(Eq. 2)

where kIntraCS is the rate constant of charge separation process, Φ(N-3) is the fluorescence quantum yield of N-3, Φ(N-1) is the fluorescence quantum yield of N-1, τ(N-3) is the fluorescence lifetime of N-3. The fluorescence lifetime of N-1 (5.87 ns) is shorter than that of N-3 (8.9±0.1 ns. Table 1), thus the fluorescence quenching of N-3 in N-1 is not exclusively due to static quenching, otherwise the fluorescence lifetime of N-1 should be similar to that of N-3.68 In other words, the PET process in N-1 upon photoexcitation is highly likely. Naphthalenediimide is known as strong electron acceptor.69


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We noted the singlet oxygen (1O2) quantum yield (ΦΔ) of N-2 is only half of that of a watersoluble analogue.51 This discrepancy is probably due to the solvent D2O used in the literature, it is known that 1O2 is more stable in deuterated solvent.10 N-1 is soluble only in organic solvents thus no comparison was made. Table 1. Photophysical Parameter of the Compounds

N-1

λabsa

εb

λema

ΦF(%)c

τFd/ ns

ΦΔ(%)g

τT j / μs

526/606

2.0/1.1

622

6.6±0.4

(6.6±0.1)e

(10.6±0.1)h

276±5

(5.9±0.1)f

(1.0±0.1)i

N-2

534

1.6

560

22.3±0.4

4.9±0.1

17.2±0.1

N-3

617

2.3

640

49.7±0.4

8.9±0.1

−k

60±3 −k

In toluene. c = 1.0 × 10−5 M. In nm. b Molar absorption coefficient. In 104 M−1 cm−1. c Fluorescence quantum yields. In toluene. Errors were determined by repetition of the measurement. d Fluorescence lifetime in dichloromethane. Errors were determined by repetition of the measurement. e λex = 405 nm, λem = 555 nm. f λex = 405 nm, λem = 610 nm. g Singlet oxygen quantum yield in toluene using 2,6-diiodoBODIPY B-1 (Scheme 1) (ΦΔ = 79%) and 2,6-diiodobisstyrylBODIPY B-2 (Scheme 1) as references (ΦΔ = 69 %). Errors were determined by repetition of the measurement. h λex = 530 nm. i λex = 610 nm. j Triplet state lifetime, measured with transient absorption spectroscopy. Errors were determined by repetition of the measurement. k Not observed. a

Electrochemical Study: Cyclic Voltammetry and Gibbs Free Energy Changes of the Electron Transfer. In order to study the photoinduced electron transfer in N-1 from a thermodynamic point of view, the electrochemical properties of the compounds were studied with cyclic voltommetry (Figure 5).63,66,70,71 N-2 shows a reversible reduction wave at −1.25 V (vs. Ag/AgNO3), and a pseudo-reversible reduction wave at −1.68 V (Figure 5b). However, no reversible oxidation waves were observed for anodic scanning up to +1.50 V. These results indicate that the 2-bromo-6-alkylaminoNDI is a good electron acceptor. For N-3, two reversible reduction waves at −1.47 V and −1.85 V were observed (Figure 5c). Compared to N-2, N-3 is difficult to reduce because of the two electron-donating amino group on the NDI chromophore.


14

Page 15 of 41

In the anoidic scanning region, two reversible oxidation waves at +0.57 V and +1.04 V were observed for N-3. Compared to N-2, N-3 is more likely to be oxidized, due to the presence of two electron-donating moieties in N-3. For N-1, the CV curve is not the sum of that of N-2 and N-3 (Figure 5a). For example, there is only one reversible oxidation wave at +0.74 V. Several reduction waves were overlapped in the cathodic scanning region. Based on these results, we propose that there is interaction between the chromophores in N-1 in the ground state.70

c

b Fc/Fc

+

Fc/Fc

Current

+

1.5 1.0 0.5 0.0 -0.5 -1.0 -1.5

Potential / V

+

Fc/Fc

Current

a Current

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


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

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

Figure 5. Cyclic voltammogram of compounds (a) N-1, c = 5.0 × 10−4 M; (b) N-2; (c) N-3, 1.0 M Bu4NPF6 as supporting electrode, Ag/AgNO3 as reference electrode. The working electrode is glassy carbon electrode. Ferrocene (Fc) was added as internal reference. c = 1.0 × 10−5 M in dichloromethane. 20 °C.

In order to study the photo-induced electron transfer in N-1, the Gibbs free energy changes (ΔG°CS) of the photo-induced electron transfer processes were calculated with the Rehm-Weller equation (Eq. 3 and Eq.4).63,72,73

ΔG 0 CS = e[ EOX − ERED ] − E00 + ΔGS


(Eq. 3)

15


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

ΔGS = −

e2 4πε Sε 0 RCC

Page 16 of 41

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

(Eq. 4)

Where ΔGS is the static Coulombic energy, which is described by eq. 4. e = electronic charge, EOX = half-wave potential for mono-electron oxidation of the electron-donor unit, ERED = halfwave potential for one-electron reduction of the electron-acceptor unit; note herein 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 = energy level approximated with the fluorescence emission wavelength (for the singlet excited state). εS = static dielectric constant of the solvent, RCC = (11.9 Å) center-to-center separation distance determined by DFT optimization of the geometry, RD is the radius of the 2,6-dialkylaminoNDI donor, RA is the radius of the electron acceptor, εREF is the static dielectric constant of the solvent used for the Table 2. Electrochemical Redox Potential, Driving Forces Charge Recombination (ΔGCR), Charge Separation(ΔGCs) and Charge Transfer State Energy (ECS) for N-1, N-2 and N-3 a

ΔGCS (eV)c

ΔGCR (eV) ECS

−1.16, −1.29, −0.46d −1.42, −1.62, −0.16e −1.87

+0.01 d

−1.75b

1.75 b

+0.29 e

−2.22c

2.22 c

−1.25, −1.68

−0.39 f

+0.51 f

−h

−h

−1.47, −1.85

−0.10 g

+0.79 g

−h

−h

Eox (V)

Ered (V)

+0.74, +1.07

N-2

+1.11

N-3

+0.57, +1.04

N-1

ΔGCS (eV)b

Cyclic voltammogram in N2 saturated DCM of compounds: N-1, c = 5.0 × 10−4 M; N-2 and N-3, c = 1.0 × 10−5 M. 1.0 M Bu4NPF6 as supporting electrolyte, Ag/AgNO3 reference electrode. b In DCM. c In toluene. d Via 1[2-Bromo-6-alkylaminoNDI]*. e Via 1[2,6-dialkylaminoNDI]*. f Intermolecular electron transfer, via 1[N-2]*. g Intermolecular electron transfer, via 1[N-3]*. h Not applicable. a


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electrochemical studies, ε0 permittivity of free space. The solvents used in the calculation of free energy of the electron transfer is DCM (ε = 8.9, 20 °C) and toluene (ε = 2.4, 20 °C). The ΔGCS value in toluene was calculated as +0.10 eV (PET is assumed to be via the singlet excited state of 2-bromo-6-alkylaminoNDI) or +0.38 eV (PET is assumed to be via the singlet excited state of 2,6-dialkylaminoNDI) (Table 2). For N-1 with ET occurs in DCM, with the 2Bromo-6-alkylaminoNDI part as electron acceptor, and with the singlet excited state of the same unit the E00 is derived, the ΔGCS was calculated as ΔGCS = 0.74 − (−1.16) − 1.91 − 0.15 = −0.16 eV. Similarly the values under other conditions were calculated and the data were listed in Table 2. Therefore, the photoinduced electron transfer is thermodynamically more favorable in polar solvents, such as dichloromethane and acetonitrile, as compared with that in toluene. This result is in agreement with the more significant quenching of the fluorescence of N-1 in polar solvents as compared with that in toluene. The intermolecular electron transfer between N-2 and N-3 was also studied by calculation of the free energy changes.66 The results show that the free energy changes of the intermolecular electron transfer are positive values in toluene (Table 2). However, intermolecular PET is possible for N-2/N-3 in polar solvents such as dichloromethane and acetonitrile. Nanosecond Transient Absorption Spectroscopy: Localization of the Triplet Excited State of the Triad. The triplet excited states of the compounds were studied with the nanosecond transient absorption spectroscopy (Figure 6).13,74−76 For N-1 (in toluene), bleaching band at 607 nm was observed upon nanosecond pulsed laser excitation at 530 nm (Figure 6a). The bleaching band is due to the ground state bleaching of the 2,6-dialkylaminoNDI moiety in N-1, based on the steady state UV−vis absorption spectrum of N-1. Moreover, excited state absorption (ESA) of N-1 in the range of 400 − 500 nm and 650 nm


17


− 750 nm were observed. These features are attributed to the triplet state absorption of 2,6diaklyaminoNDI moiety.15,51,52 The triplet state lifetime was determined as 276±5 μs, which is much longer than the previously observed triplet state lifetime of NDI chromophore (90 μs).52 0.004

0.006

a

b 0.004 Δ O.D.

Δ O.D.

0.002 0.000

-0.002 -0.004

1265.3 μs … 23.7 μs 0 μs

400


0.03

312.2 μs … 7.6 μs 0 μs

0.02 0.01

τT = 276 μs

0.002 0.000

800

0

c

0.00

500

1000 1500 2000

Time / μs

d

0.03 0.02 Δ O.D.

Δ 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 18 of 41

0.01

τT = 60 μs

0.00

-0.01

400

500 600 700 800 Wavelength / nm

0

500

1000 1500 2000 Time / μs

Figure 6. Nanosecond transient absorption spectra of compounds N-1 and N-2. Transient absorption with different delay times: compound N-1 (a) λex = 530 nm, c = 1.0 × 10−5 M; (b) decay trace of N-1 at 610 nm (λex = 530 nm), (c) transient absorption spectra of compound N-2 (λex = 530 nm), c = 5.0 × 10−5 M; (d) decay trace of N-2 at 535 nm (λex = 530 nm), c = 5.0 × 10−5 M in toluene after pulsed laser excitation under N2 atmosphere. The laser pulse was trigged at 400 μs after start of the xenon lamp flash to attain a satisfactory baseline. This setting has no negative effect on data fitting. 20 °C. This triplet state lifetime is also much longer than that observed with a diiodoBodipy-PBI dyad/triad (150 μs).30 The T1 state of N-1 is localized on the 2,6-dialkylaminoNDI moiety


18

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(singlet energy acceptor). This postulation was supported by the nanosecond transient absorption of N-2 (Figure 6c). For N-2, the bleaching band is at 533 nm, and the ESA profile is very different from that of N-1. The triplet state lifetime of N-2 was determined as 60±3 μs (Figure 6d). No transient signal was observed for N−1 in polar solvents such as dichloromethane and acetonitrile, indicating that the triplet state is quenched by PET. This property is different from a previously reported diiodoBodipy-rhodamine dyad, for which the triplet state can be observed in polar solvents (such as in mixed CH2Cl2/MeOH solvent).29 We found that N-3 does not produce triplet excited state upon photoexcitation. Thus, we propose intramolecular triplet-triplet-energy-transfer (TTET) occurs in N-1. The evidence is as follows. First, we have shown that upon photoexcitation at 530 nm, the triplet state was produced with N-2. In N-1, however, the triplet state is exclusively localized on the singlet energy acceptor. We also show that upon excitation into the singlet energy acceptor in N-1 alone, the production of triplet state is much less efficient. Thus the production of triplet state in N-1 is mainly via the intramolecular TTET process. For a previously reported diiodoBodipy-rhodamine dyad, however, not such intramolecular TTET was observed, and the triplet state is confined on the diiodoBodipy moiety.29 For the previously reported diiodoBodipy-PBI dyad and triad, the ISC was not reduced by FRET.30 But for N-1, however, the singlet oxygen quantum yield ΦΔ = (10.6±0.1)% (can be approximated as the triplet state quantum yield, ΦT) was only 60% of that of N-2, for which ΦΔ = (17.2±0.1)%. That is, the ISC was reduced by the FRET, and probably also by the PET. Intermolecular Triplet Energy Transfer between N-2 and N-3. In order to support the putative intramolecular triplet state energy transfer in N-1, the intermolecular triplet state energy transfer between N-2 and N-3 was studied with the nanosecond transient absorption spectra of


19


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Page 20 of 41

the mixture of the two compounds (Figure 7).76,77 N-2 is the triplet state energy donor and N-3 is the triplet state acceptor. In the presence of N-3, we found that the decay of N-2 was accelerated, and the transient absorption spectra evolved into the feature of that of N-3, that is, a bleaching band at 607 nm, and ESA at 506 nm (Figure 7a, 7c, 7e). For example, with increasing the N-3/N2 molar ratio, the ground state bleaching of N-2 at 534 nm decreased sharply, as well as the ESA of N-2 in the range of 400 − 500 nm. At the same time, the ESA at 436 nm decreased sharply, and the transient feature of N-3 was observed, which shows a ground state bleaching band at 610 nm, and the relatively weak ESA in the range of 400 − 550 nm and 600 − 800 nm. Excitation of N-3 alone at the same excitation wavelength gave much weaker transient signals. Thus we propose that population of the N-3 triplet state is via intermolecular triplet-triplet-energy-transfer (TTET). In order to study the kinetics of the intermolecular triplet energy transfer, the decay traces at 535 nm and 610 nm were monitored (Figure 7b, d, f). Sequential decrease/increase feature was observed for the decay traces. For example, with molar ration of N-2/N-3 = 1:1, the decay trace at 535 nm recovers during the beginning 70 μs, the transient increased to positive region, then it decays to zero. The first stage of recovery toward the baseline is due to the triplet state energy transfer process, as a result the triplet state of N-3 is produced, which show weak ESA at 535 nm. The final phase is the decay of the N-3 state. It should be pointed out that these processes take place simultaneously, although different process dominate in the respective periods. This postulation was supported by the decay trace at 610 nm (Figure 7b, d, f), where the N-3 moiety shows steady state bleaching. Firstly the decay trace at 610 nm gave a sharp decrease from positive to negative values, then the bleaching decays to zero. The instant appearance of the positive transient is due to the population of triplet state of N-2, which show ESA in the region


20

Page 21 of 41

of 550 nm − 750 nm. The fast decrease of the transient at 610 nm to negative value corresponds to the production of the triplet excited state of N-3 via intermolecular TTET. This process takes about 60 ± 3 μs, very close to the decay trace of N-2 monitored at 535 nm. The final phase of the decay curve at 610 nm is the decay of the triplet state of N-3 to the ground state, the process

0.01

0.00

0.00

-0.01

-0.01

400

0.03


0.02

400

0.00

-0.01

0

200

400 600 Time / μs

400

0.03

0.01 0.00


f

0.02

800

535 nm 610 nm

0.01 0.00

-0.01

Donor/Acceptor =5:3

-0.02

800 1000

0.01

800

535 nm 610 nm

-0.01

Donor/Acceptor =5:1

-0.02


d

0.02 Δ O.D.

0.01

0.02

-0.01

800

535 nm 610 nm

0 μs 33.3 μs 66.6 μs ... 266.4 μs

e

0.00

0.03

b

0.03

Δ O.D.

0.01

0.02 Δ O.D.

Δ O.D.

0.02

0 μs 33.3 μs 66.6 μs … 266.4 μs

c

0.03

ΔO.D.

0 μs 33.3 μs 66.6 μs … 266.4 μs

a

0.03

Δ 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


0

200

400

600

800 1000

Time / μs

Donor/Acceptor =5:5

-0.02 0

200

400 600 Time / μs

800 1000

Figure 7. Nanosecond transient absorption spectra and decay curves of the mixture of compounds N-2 and N-3. Transient absorption spectra of the mixture with different molar ratio of the energy donor and acceptor, with concentration of N-2 fixed at 5.0 × 10−5 M, and concentration of N-3 at (a) 1.0 × 10−5 M; (c) 3.0 × 10−5 M; (e) 5.0 × 10−5 M. Decay traces at 535 nm and 610 nm of the mixture of (a), (c) and (e) are presented in (b), (d) and (f), respectively. Excited with 535 nm nanosecond pulsed laser. The laser pulse was trigged at 200 μs after start of the xenon lamp flash to attain a satisfactory baseline. This setting has no negative effect on data acquirements. 20 °C.


21


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Page 22 of 41

takes about 250 μs. Excitation of N-3 alone in separate solution did not produce any triplet excited state, thus the intermolecular TTET was confirmed. It should be pointed out that for the previously reported diiodoBodipy-PBI dyad and triad,30 the spectral overlap is serious and the kinetics of the intermolecular TTET are less unambiguous. DFT calculations: Singlet and Triplet Excited States. The photophysical properties of the triad N-1 were rationalized with DFT calculations, such as the confinement of the excited states, and the UV−vis absorption (Figure 8 and Table 3).79−81 The ground state geometry of N-1 was optimized with the DFT method. The molecule takes an configuration with the two 2-bromo-6aminoNDI units far away from each other. The three moieties are nearly co-planar (Figure 8, and see the geometry data in Supporting Information). The electronic excited states and the frontier molecular orbitals were calculated with the TDDFT methods (Figure 8 and Table 3).81 HOMO is localized on the 2,6-dialkyldiaminoNDI moiety, whereas the LUMO is localized on the 2-bromo-6-alkylaminoNDI moiety. Other occupied or unoccupied molecular orbitals are confined on either the diaminoNDI moiety or the bromoaminoNDI moiety, there are no molecular orbitals which are delocalized across over the two chromophores. Thus there is no π-conjugation between the chromophores. This result is in agreement with the UV−vis absorption spectra in which the N-1 shows absorption wavelengths which are very close to that of N-2 and N-3. Two low-lying dark excited states (charge transfer state) were observed for N-1. The oscillator strength is f = 0.064 for S1 state. The S1 state is a charge transfer state with 2,6-dialkylaminoNDI as electron donor and the two 2-bromo-6-alkylaminoNDI moieties as electron acceptors. Charge transfer transition is forbidden or weakly allowed transition due to the poor molecular orbital overlap. On the other hand, S3 state is due to the transition HOMO→LUMO+2, thus this


22

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


transition is confined on the 2,6-dialkylaminoNDI moiety. The calculated vertical excitation energy is 569 nm, which is close to the experimental result of 606 nm in the UV−vis absorption spectrum (Figure 1). S4 state is confined on the two 2-bromo-6-alkylaminoNDI moieties, respectively. The calculated excitation is at 488 nm, which is close to the experimental results of absorption band at 526 nm in the UV−vis absorption spectrum (Figure 1). We noted the discrepancy between the calculated absorption wavelength and the experimental results, but the trend of the calculated absorption wavelengths are in agreement with the experimental results.

Figure 8. Selected frontier molecular orbitals involved in the excitation and emission of N-1. Toluene was used as solvent in the calculations (PCM model). The calculations are at the B3LYP/Genecp/level using Gaussian 09W.


23


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Page 24 of 41

The fluorescence of the triad was calculated with TDDFT methods, based on the optimized singlet excited states. Since preliminary calculation shows that S3 state is the emissive state, thus the S3 state geometry was optimized, based on which the emission wavelength of N-1 was calculated as 617 nm (Figure 8 and Table 3), this value is in good agreement with the experimental value of 622 nm (Figure 2 and Table 1). S2 state is confined on the 2,6dialkylaminoNDI moiety. Moreover, two dark states of S1 and S2 were predicted by TDDFT calculation, which is in good agreement with the experimental results that N-1 is much weakly fluorescent (ΦF = 6.6±0.4 %, in toluene) as compared with that of the reference compound N-3 (ΦF = 49.7±0.4%, in toluene. Table 1). The triplet excited states were studied with the TDDFT methods, approximated with the ground state geometry. The T1 state of N-1 is localized on the 2,6-dialkylaminoNDI moiety with energy level of 1.37 eV (Figure 8 and Table 3), whereas the T2 and T3 states are localized on the two 2-bromo-6-alkylaminoNDI moieties, respectively. Note the T2 and T3 state are fully degenerate with the same energy level of 1.75 eV. These two states are localized on the two 2bromo-6-alkylaminoNDI moieties. The energy gap of these two high-lying triplet state with the T1 state is 0.38 eV, thus no triplet state equilibrium is likely to be observed.25,82 This postulation is in full agreement with the nanosecond transient absorption spectroscopy, which indicated that the T1 state of N-1 is exclusively localized on the 2,6-dialkylaminoNDI moiety (Figure 6). The spin density surface of N-1 was also calculated to study the distribution of the triplet excited state of the triad (Figure 9).73 The spin density surface is exclusively confined on the 2,6dialkylaminoNDI moiety, the 2-bromo-6-alkylaminoNDI moieties do not contribute to the T1 excited state of triad N-1. This conclusion is in full agreement with the nanosecond transient absorption of N-1 (Figure 6).


24

Page 25 of 41

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


Table 3. Selected Electronic Excitation Energies (eV) and Corresponding Oscillator Strengths (f), Main Configurations and CI Coefficients of the Low-lying Electronically Excited States of N-1.a Calculated by TDDFT//B3LYP/ Genecp, Based on the Optimized Ground State Geometries (Toluene was Used as Solvent in All the Calculations) Singlet

(UV−vis)

(Fluorescence)

(Triplet)

Electronic transition

TDDFT//B3LYP/Genecp Energy b

fc

Composition d

CI e

H→L

0.7051

S0→S1

2.01 eV / 616 nm 0.064

S0→S2

2.02 eV / 614 nm 0.0001 H→L+1

0.7058

S0→S3

2.18 eV / 569 nm 0.2790 H→L+2

0.7062

S0→S4

2.54 eV / 488 nm 0.4341 H−1→ L+1

0.5173

H−2→ L

0.4802

S0→S5

2.54 eV / 488 nm 0.0021 H−1→ L

0.5173

S0→S1

1.93 eV / 643 nm 0.0933 H→L

0.7053

S0→S2

1.94 eV / 640 nm 0.0000 H→L+1

0.7060

S0→S3 f

2.01 eV / 617 nm 0.2338 H→L+2

0.7081

S0→T1

1.33 eV / 933 nm 0.0000 H→ L+2

0.6985

S0→T2

1.75 eV / 707 nm 0.0000 H−2→ L+1

0.3718

S0→T3

H−1→L

0.3827

1.75 eV / 707 nm 0.0000 H−2→ L

0.3718

a

Calculated by TDDFT//B3LYP/ Genecp in Toluene. b Only selected low-lying excited states are presented. c Oscillator strength. d H stands for HOMO and L stands for LUMO. Only the main configurations are presented. e CI coefficients are in absolute values. f The geometry of this singlet excited state was optimized (note the experimental results of the emission is 622 nm).


25


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Page 26 of 41

Figure 9. Spin density surface of N-1 at T1 state. The calculations are at the B3LYP/Genecp/level using Gaussian 09W. Toluene was used as solvent in the calculation (PCM model).

Jablonski Diagram: Photophysical Processes Involved in the Triad. The photophysical processes of N-1 are summarized in Scheme 2 (see also Supporting Information, Scheme S1). Photoexcitation into the energy donor (2-bromo-6-alkylaminoNDI moiety) will establish the competing ISC and FRET processes. Since the triplet state energy level of the 2,6dialkylaminoNDI moiety is much lower than that of the 2-bromo-6-alkylaminoNDI part, the triplet state of the triad is finally localized on the energy acceptor part, via intramolecular TTET. In non-polar solvent such as toluene, the charge separated state (CTS) has an energy level of 2.40 eV, thus the triplet state of N-1 will not by quenched by charge separation (electron transfer), but the fluorescence of the energy acceptor will be significantly quenched. In polar solvents, such as dichloromethane and acetonitrile, the energy level of the CST is much lower (1.76 eV). As a result, the production of the triplet state localized on the 2,6-dialkylaminoNDI part may be inhibited by the electron transfer process. Experimentally no triplet excited state was observed for N-1 in dichloromethane and acetonitrile.


26

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


Scheme 2. Photophysical Processes Involved in N-1 Upon Photoexcitation in Toluene.

CONCLUSIONS In summary, naphthalenediimide (NDI)-triad was prepared to study the effect of intersystem crossing (ISC) and the competing fluorescence-resonance-energy-transfer (TTET), as well as the intramolecular electron transfer process on the triplet state property of the triad. The triad contains two 2-bromo-6-alkylaminoNDI moiety as the singlet energy donor and spin converter (for triplet state production, and as triplet energy donor), and one 2,6-dialkylaminoNDI as the singlet/triplet energy acceptor. The molecular structural motif is unique in that competing ISC and FRET is established in an organic multi-chromophore assembly. The photophysical properties of the triad and the reference compounds were studied with steady state UV−vis absorption and fluorescence emission spectroscopies, nanosecond transient absorption spectroscopy, electrochemical characterization (cyclic voltammetry) and DFT/TDDFT calculations. We confirmed the fluorescence-resonance-energy-transfer (FRET) for the triad with fluorescence spectroscopy. Furthermore, intramolecular electron transfer was observed in polar


27


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Page 28 of 41

solvents, demonstrated by the fluorescence quenching and the calculation of the Gibbs free energy changes (ΔG°CS) of the electron transfer. The fluorescence and the triplet state of the triad were quenched in polar solvents by the electron transfer process. The T1 state of the triad is exclusively localized on the 2,6-dialkylaminoNDI moiety (in toluene), indicating the intramolecular triplet state energy transfer. As a support of this postulation, the intermolecular long range triplet state energy transfer between the two reference compounds was investigated with the nanosecond transient absorption spectroscopy. The photophysical properties of the triad were rationalized by DFT calculations. These results are useful for study of the photophysical processes in multi-chromophore organic photosensitizers and for designing of organic triplet photosensitizers showing broadband visible light absorption.

AKNOWLEDGEMENT We thank the NSFC (20972024, 21273028, 21421005 and 21473020), the Royal Society (UK) and NSFC (China-UK Cost-Share Science Networks, 21011130154), and Ministry of Education (SRFDP-20120041130005), Program for Changjiang Scholars and Innovative Research Team in University [IRT_13R06], the Fundamental Research Funds for the Central Universities (DUT14ZD226) and Dalian University of Technology (DUT2013TB07) for financial support.

ASSOCIATED CONTENT Supporting Information Synthesis procedures, NMR and HR-MS spectra, nanosecond transient absorption spectra, and coordinates of the optimized geometries of N-1. This material is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES


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Broadband Visible Light-Harvesting Naphthalenediimide (NDI) Triad

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