Broadband Visible Light Harvesting N^N Pt(II) Bisacetylide Complex

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Broadband Visible Light Harvesting N^N Pt(II) Bisacetylide Complex with BODIPY and Naphthalene Diimide Ligands: Förster Resonance Energy Transfer and Intersystem Crossing Peili Wang, Yun Hee Koo, Woojae Kim, Wenbo Yang, Xiaoneng Cui, Wei Ji, Jianzhang Zhao, and Dongho Kim J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 02 May 2017 Downloaded from http://pubs.acs.org on May 6, 2017

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The Journal of Physical Chemistry

Broadband Visible Light Harvesting N^N Pt(II) Bisacetylide Complex with BODIPY and Naphthalene Diimide Ligands: Förster Resonance Energy Transfer and Intersystem Crossing Peili Wanga¶ Yun Hee Koo,b¶ Woojae Kim,b Wenbo Yang,a Xiaoneng Cui,a Wei Ji,a Jianzhang Zhao*a and Dongho Kimb* a

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2 Ling-Gong Road, Dalian 116024, P. R. China E-mail: [email protected] Web: http://finechem2.dlut.edu.cn/photochem

b

Spectroscopy Laboratory for Functional π-electronic systems and Department of Chemistry, Yonsei University, Seoul 03722 (Korea). E-mail: [email protected]

RECEIVED DATE (automatically inserted by publisher)

¶

These authors contribute equally to this paper.

ACS Paragon Plus Environment

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Abstract: A N^N Pt(II) complex, Pt-1, with two heteroleptic ligands was prepared, which is a rarely reported molecular structure. The two different acetylide ligands, i.e. borondipyyromethane (BDP) and naphthalenediimide (NDI) chromophores, show strong absorption in visible region. The photophysical properties of the complex were investigated by using steadystate and femtosecond/nanosecond time-resolved optical spectroscopies, as well as electrochemical characterization. Upon selective photoexcitation of the coordinated BDP acetylide ligand at 503 nm, Förster-resonance energy transfer (FRET, kFRET = 1.2 × 1011 s−1) process from BDP to NDI ligand was observed, which leads to the population of the singlet excited state of the latter. After that, intersystem crossing (ISC) process occurs (kISC = 3.3 × 109 s−1), which generates the triplet excited state of NDI ligand (τ = 28.1 μs). The overall excitedstate dynamics are fairly similar in both nonpolar toluene and polar benzonitrile, indicating that photo-induced charge separation dynamics between BDP and NDI is insignificant. This is presumably due to the strong interaction between NDI ligand and central Pt atom which can give rise to strong spin-orbit coupling. This hypothesis can be further supported by the excited-state dynamics obtained after photoexcitation at the S2 state of NDI ligand. The ultrafast ISC from the S2 state of NDI ligand to higher triplet state, which corresponds to the breakdown of KashaVavilov’s rule, was observed, suggesting that the NDI core strongly interacts with heavy Pt atom. Finally, we identified that this broadband visible light-excitable Pt(II) complex can be used as triplet photosensitizer for two-color excitable triplet-triplet annihilation upconversion, and the upconversion quantum yield was determined as 4.1 %.

1. INTRODUCTION Transition metal complexes showing visible light absorption and long-lived triplet excited state exhibit promising applications in several areas,1−5 such as photovoltaics,6 photocatalytic H2


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production,7,8 photoredox catalytic organic reaction,9−12 photo-initiated polymerization,13 photodynamic therapy,14−18 luminescent molecular sensor,19−23 and more recently the triplettriplet annihilation (TTA) upconversion.24−26 The fundamental photophysical processes involved in these applications are intramolecular energy and electron transfer.14 Along efficiency of energy and electron transfer, strong absorption of visible light, efficient intersystem crossing (ISC) from singlet to triplet state, and long-lived triplet excited state are required for these applications.27 Previously we prepared a series of Pt(II), Ru(II), Ir(II) and Re(I) complexes with strong absorption of visible light and long-lived triplet excited state.14,28 The common feature of these complexes is an usage of visible light-harvesting organic chromophores as coordination ligands. On the bases of direct metalation or π-conjugated linkage of the chromophores to the metal centre, these complexes show efficient ISC processes. One of the challenges left for transition metal complexes is their limited range of absorption in visible light. To harvest most of the energy from broadband light source, complex should have broadband absorption.29,30 However, most transition metal complexes only have single lightharvesting chromophore, thus, these complexes cannot harvest most of the energy from broadband light source, such as solar light. 31−36 To efficiently harvest energy from broadband light source, employment of more than one chromophore to expand the absorbable range of light is required. However, only few transition metal complexes containing two different chromophores were reported. N^N Pt(II) biacetylide complexes can be good candidates for application in photocatalysis and TTA upconversion owing to the easiness of preparation and high chemical stability.1,37−40


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Introduction of two different acetylide ligands to N^N Pt(II) biacetylide complexes is feasible, however, since the reaction is done in single step, mixture of products were produced.34,41,42 As a result, only few N^N Pt(II) bisacetylide complexes containing two different acetylide ligands were reported. 34,41,42 In 2009, Shuichi et al. reported a NDI containing N^N Pt(II) bisacetylide complex, containing two different acetylide ligands and photoinduced charge separation of the complex was studied. 41 We also reported a N^N Pt(II) bisacetylide complex which contains two different acetylide ligands based on boron-dipyrromethene (BDP),34 which is a well-known chromophore showing strong absorption of visible light. Photophysical properties of this complex were studied with steady state and time-resolved transient absorption spectroscopies. However, much room is left to explore more diverse visible light-harvesting organic chromophores to construct such interesting complexes, and to study the photophysical processes of these complexes. Herein we prepared a N^N Pt(II) bisacetylide complex Pt-1 (Scheme 1), which contains a Bodipy acetylide ligand and a naphthalenediimide (NDI) ligand. BDP ligand is assumed to be a singlet energy donor and NDI ligand is an energy acceptor and spin converter. FRET and ISC processes were studied with steady state and time-resolved transient absorption and emission spectroscopies. The complex was examined as a multi-wavelength-excitable triplet photosensitizer for TTA upconversion and efficient upconversion was observed. 2. RESULTS AND DISCUSSION 2.1. Molecular Structure Design and Synthesis. N^N Pt(II) bisacetylide complexes usually contain two acetylide ligands, which is feasible in preparation because the two ligands were


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Scheme 1. Synthesis of the complex Pt-1 and the reference compounds.

Br

a

OHC

c

b

Si

N

CHO

1

O

O

O

O

O

O

d

Br Br

O

O

O

O

O

N

O

e

N

2

H

N B F

F

BDP

O

O

N

O

f

H N

O

N

O

O

N

O

g

Br

H N

Br

Br

O

CHO

O

N

O

O

O

5

4

3

N

NDI F F

N B N Cl

N N

Pt

h

NDI

BDP Cl

N

Pt

O

N

6

N O

O N O

O

N

O

P(nBu)3 O

N

O

P(nBu)3 F F

B

N

H N

N H

Pt-1

Pt

N

Pt-2

Condition: (a) Trimethylsilylacetylene (TMSA), Et3N, (PPh3)2PdCl2, CuI, PPh3; 89 °C, reflux, 8 h, yield: 68.2%. (b) CH3OH, CH2Cl2, K2CO3; 25 °C, 2 h, yield: 86%. (c) CH2Cl2, 2,4Dimethyl-pyrrole, trifluoroacetic acid; DDQ; Et3N, BF3OEt2; 25 °C, 48 h, yield: 30%. (d) Bromine, 50% SO3, iodine; 50 °C, 48 h. (e) Acetic acid, 2-ehylhexylamine; 120 °C, reflux, 2 h, yield: 28%. (f) Methoxyetnanol, 2-ehylhexylamine; 120 °C, reflux, 2 h, yield: 70.9%. (g) TMSA, Et3N, (PPh3)2PdCl2, CuI, PPh3; 75 °C, 8 h. CH3OH, CH2Cl2, K2CO3; 25 °C, 30 min, yield: 72.2%. (h) CH2Cl2, CuI, diisopropylamine; 25 °C, 16 h, yield: 15.6 %.


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always introduced in one step reaction.1,39,43 Using two different acetylide ligands for preparation of N^N Pt(II) bisacetylide complexes will produce a mixture which contains at least three products.34,41,42 However, the advantage of using different acetylide ligands is to attain complexes in which photo-induced electron transfer or energy transfer is possible.34,41 Herein we prepared the N^N Pt(II) complex with heteroleptic acetylide ligands, i.e. BDP and NDI. Both Bodipy44−46 and NDI are popular chromophores showing strong visible light absorption.47−52 Previously we showed that the absorption of the NDI acetylide coordinated to Pt(II) center has absorption band at 583 nm,53 while the coordinated BDP acetylide ligand has fluorescence emission band at ca. 503 nm.34,54 Fluorescence emission band of BDP and absorption band of NDI overlaps, thus, FRET process from BDP to NDI is anticipated for Pt-1. NDI-containing Pt(II) complex with similar NDI moiety was reported to give room temperature phosphorescence at 784 nm (1.58 eV).53 Thus after FRET process from BDP to NDI, ISC at NDI ligand is anticipated. Triplet-triplet energy transfer (TTET) from T1 state of NDI to T1 state of BDP is not likely to occur, since the energy level of T1 state of BDP is expected to be higher than the T1 state of NDI. The T1 state of BDP at Pt-1 can be approximated from phosphorescence of Ir(III) complex with bodipy ligand, which was reported to be at ca. 733 nm (1.69 eV).55 Thus, we anticipate the T1 state of the Pt-1 complex would be localized on the NDI moiety. Both acetylides were prepared with routine methods (Scheme 1). The target complex Pt-1 was obtained with a three-component one-step reaction. A mixture was obtained with this reaction, and Pt-1 was isolated with a yield of 15.6%.


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2.2. Steady-state Absorption and Emission Properties. The UV-vis absorption spectra of Pt-1 complex and its reference compounds are shown in Figure 1(a). The absorption maximum of BDP ligand in Pt-1 (λabs = 503 nm) is almost the same as that of the pristine ligand (λabs = 504 nm), indicating that the electronic structure of BDP ligand is not perturbed upon Pt-coordination. For NDI ligand, however, its absorption maximum (λabs = 580 nm) is red-shifted by 45 nm as compared to the free NDI ligand (λabs = 535 nm). This means that the electronic structure of NDI is considerably affected by the coordinated Pt atom. Especially, it is important to note that the overall spectral shape of Pt-1 is similar to a linear summation of Bodipy and NDI spectra. Furthermore, the solvent polarity dependence was not observed in the absorption spectra (see Figure S14 in the Supporting Information). These results suggest that the electronic interaction between two coordinated chromophores is absent in the ground state.

800

a

NDI BDP Pt-1

0.8

0.4

0.0

400

500 600 Wavelength / nm

b Intensity / a.u.

1.2

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


700

600 400 200 0

Air

500

N2

600 700 800 Wavelength / nm

Figure 1. (a) UV−vis absorption spectra of NDI (black, solid), BDP (red, solid) and Pt-1 (blue, dash dot) in toluene. (b) Emission spectra of Pt-1, in air (red, solid) and N2 (black, dashed). λex = 470 nm. c = 1.0 × 10−5 M. 25 °C.


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In the ambient condition, Pt-1 exhibits two fluorescence bands as shown in Figure 1(b); one is originated from BDP at 517 nm and the other can be attributed to NDI at 600 nm. But in N2purged condition, we could observe an additional band at 785 nm, which can be assigned to the phosphorescence from NDI ligand. This is similar to the previous report of the N^N Pt(II) complex with homoleptic acetylide NDI ligands.53 Fluorescence/phosphorescence dual emission has also been reported for several Pt(II) complexes.53,56 Normally this kind of emission behavior can be attributed to the bulkiness of coordinated ligand, which makes the ISC process less efficient.20 Previously reported N^N PtCl2 complex with the NDI moiety attached to the phenanthracene ligand did not exhibit photoluminescence, either from the Pt(II) coordination center or the NDI moiety, due to the bulkiness of the ligand.57 In order to prove that the fluorescence bands of Pt-1 are not due to the free ligand impurities, we have measured fluorescence lifetimes at each emission band and compared those with lifetimes of free ligands. We found that upon coordination to Pt(II) metal center the fluorescence lifetimes of BDP and NDI ligands (1.1 ns and 0.6 ns, respectively) are considerably shortened as compared to those of free ligands (11.3 ns for BDP and 2.5 ns for NDI), indicating that additional nonradiative deactivation pathway exists in both S1 states of each ligand. In particular, the emission lifetime at 790 nm was estimated to be 34 μs which is sensitive to the presence of O2, again identifying that the emission band around 790 nm represents the phosphorescence from the triplet state of NDI ligand. The emission spectra of the free BDP and Pt-1 were compared by using optically matched solution (Figure 2a). It was found that the fluorescence of BDP ligand in Pt-1 complex is substantially quenched as compared with that of free BDP. Similar results were observed for comparison of the NDI and the Pt-1 emission (Figure 2b). There are three possible relaxation


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pathways for the S1 state of BDP: 1) Intersystem crossing (ISC) to the triplet state of BDP due to Pt atom, 2) Förster-resonance energy transfer (FRET) from BDP (energy donor) to NDI (energy

a

600 BDP

300 Pt-1 0


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Normalized intensity

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Intensity / a.u.

900 Intensity / a.u.

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NDI

300 150 0

Pt-1 500

600 Wavelength / nm

700

Ex Abs

c

2

0

300


600

Figure 2. (a) Emission spectra of Pt-1 (red, dashed) and BDP (black, solid), (b) Emission spectra of Pt-1 (red, dashed) and NDI (black, solid) measured by using optically matched solutions (aerated toluene, A = 0.169) in room temperature. The excitation wavelength is 460 nm for both cases (c) Comparison of the normalized UV−vis absorption and the fluorescence excitation spectra of the Pt-1 complex in toluene (λem = 640 nm, A = 0.197 at 502 nm).

acceptor), and 3) Photo-induced electron transfer (PET) from BDP (electron donor) to NDI (electron acceptor). Among these processes, we could find an evidence of FRET by comparing the fluorescence excitation spectrum and UV-vis absorption spectrum of Pt-1 complex (Figure 2c). The excitation spectrum monitored at 640 nm exhibits similar spectral features to the absorption spectrum. Especially, the appearance of the sharp band at 502 nm in the excitation spectrum infers that the emission of NDI arises from the photoexcitation of BDP, i.e. FRET process occurs from BDP to NDI. Note that the relative intensity from BDP in the excitation spectrum is smaller than that in the normalized absorption spectrum, indicating that not all excited-state energy of BDP can migrate to NDI. By integrating the areas of the absorption and


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excitation bands, the FRET efficiency could be estimated as ca. 88%.58,59 Other processes will be discussed in the following section in detail. Table 1. Photophysical Properties of the Ligands and the Pt-1 Complex.

Pt-1

λabs,a nm (ε b)

λem,a nm (τF, c ns)

τT,g μs

Φ F, h %

ΦΔ,n %

502(0.97) /

517 (1.11) / 600 (0.60) / 790 (34.19 μs)d

27.26

1.8i

90.8

580(0.31)

514 (1.37) / 605 (1.76) / 805 (50.13 μs) e

/ 28.13

2.6j

63.5

0.14k NDI

535(0.18)

BDP 504.5(0.87)

563 (11.30)d 561 (7.09)f

−o

78.5l

−o

520 (2.46)d 515 (5.32)f

−o

47.2m

−o

In toluene (c = 1.0 × 10−5 M). b Molar absorption coefficient. ε : in 105 M−1 cm−1. c Luminescence lifetimes of each emission peak (λex = 445 nm). d In deaerated toluene at RT and e 77 K. f In deaerated Methylcyclohexane : Me-THF : Ethyl iodide (2 : 1 : 1, v / v / v) at 77 K. g Triplet states lifetimes, in deaerated toluene at RT. h Luminescence quantum yields. i BDP (ΦF = 0.72 in THF) was used as standard for Pt-1 (λex = 460 nm). j Rho-B (ΦF = 0.65 in Ethanol) was used as standard for Pt-1 (λex = 526 nm). k Phosphorescence quantum yields. [Ru(dmb)3][PF6]2 was used as standard for Pt-1 (λex = 458 nm). l Rho-B (ΦF = 0.65 in Ethanol) was used as standard for NDI (λex = 507nm). m BDP (ΦF = 0.72 in THF) was used as standard for BDP (λex = 451 nm). n Singlet oxygen quantum yields. Diiodobodipy (ΦΔ = 0.85 in toluene) was used as standard (λex = 507 nm and 555 nm). o Not applicable. a

In the same manner, the emission spectra of Pt-1 complex and free NDI were comparatively analyzed by using optically matched solution (Figure 2b). In this case, we also observed that a large portion of fluorescence from Pt-coordinated NDI is quenched. This implies that the S1 state population of NDI ligand is greatly reduced via the possible nonradiative deactivation pathways as follows: 1) ISC to the triplet state of NDI, and 2) PET process from BDP to NDI. The photophysical properties of the compounds were summarized in Table 1. 2.3. Femtosecond and Nanosecond Transient Absorption. In order to investigate the excited-state dynamics of the Pt-1 complex, we have performed femtosecond transient


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0.02 Δ O.D.

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-0.02 -0.04

1 ps 5 ps 10 ps 17 ps 50 ps

0.01 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


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Intensity / a.u.

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100 ps 200 ps 500 ps 1 ns 3 ns

0.00

-0.03 -0.06

A 8.6 ps B X 2 308 ps C X 2 Long

500 600 700 Wavelenrth /nm

Figure 3. Femtosecond transient absorption spectra measured (a) from 1 to 50 ps and (b) from 100 ps to 3 ns at 490 nm excitation of Pt-1 in toluene (c) species-associated difference spectra obtained from global target analysis. absorption measurements (Figure 3). First, the excitation wavelength was tuned to 490 nm, corresponding to the S0→S1 transition of BDP ligand. From 1 to 50 ps, we could observe a decay of ground-state bleach (GSB) band of BDP at 510 nm and a simultaneous rise of GSB of NDI at 595 nm. This spectral change can be ascribed to excited-state population transfer, FRET, from BDP to NDI ligand. After 100 ps, while the excited state absorption (ESA) bands in the range of 650−730 nm decreased, the ESA band at ca. 770 nm was intensified. Especially the TA spectrum at 3 ns is very similar to the spectral signature of the triplet state of Pt-coordinated NDI as shown in nanosecond TA spectra (Figure 6).35 In this regard, the spectral evolution occurring in the


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hundreds of picosecond time scale can be assigned to the ISC process from the S1 state of NDI to its T1 state. By using a global target analysis assuming that three states, the S1 state of BDP and the S1 and T1 states of NDI, are involved in the excited-state dynamics and the generation of these states are in a consecutive order, we could extract species-associated difference spectra (SADS) of each state and the rate constants of FRET and ISC processes (Figure 3c). We also assumed that the initial population is distributed between the S1 state of BDP and S1 state of NDI at the ratio of 8:2, since NDI ligand possesses a small extinction coefficient at the pump wavelength as well. The species A shows a strong GSB band at 510 nm with negligible ESA, which corresponds to the S1 state of BDP.60 This spectrum changes into the species that exhibits negative GSB around 520 nm, corresponding to the S1 state of NDI. Therefore, it can be regarded as that A→B represents FRET process from BDP to NDI, with the time constant of 8.6 ps (kFRET = 1.2 × 1011 s−1). The species C corresponds to the T1 state of NDI, and therefore B→C represents the ISC process in NDI ligand with the time constant of 308 ps (kISC = 3.2 × 109 s−1). Previously a NDIcontaining N^N PtCl2 complex was reported,57 and the S1 state lifetime of NDI was found as 0.9 ps. This short singlet excited state lifetime of the NDI moiety was attributed to the result of internal conversion and intersystem crossing. Our results on Pt-1 show that amino-substitution may significantly alter the ISC kinetics of the NDI moiety. Much slow ISC was observed for the NDI moiety in Pt-1 (308 ps), even with a direct linkage of the NDI moiety to the Pt(II) centre. Recently we reported a N^N Pt(II) bisacetylide complex which contains two different acetylide ligands, but the complex is with two Bodipy derived acetylide ligands, and the photophysical process was not studied in detail with femtosecond transient absorption spectrosocpy.34


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The TA spectra of reference compound, Pt-2 was also recorded (see Figures S21 and S22 in the Supporting Information), the spectral evolution of Pt-2 is very similar to that of Pt-1, which means that Pt-2 also undergoes FRET from BDP to NDI and ISC in NDI in sequential order. Especially, the time constant of FRET processes for Pt-2 was determined as 14.3 ps (kFRET = 7.0 × 1010 s−1), which is almost twice of that for Pt-1. The longer time constant of Pt-2 is due to a dependence of FRET rate on the distance between donor and acceptor,61 because the distances between BDP and NDI ligands are 11.7 Å and 21.0 Å for Pt-1 and Pt-2, respectively. We also calculated the theoretical FRET rate based on the optimized structure of Pt-1, and compared it with the experimental results. However, in the optimized geometry, the transition dipole moments of BDP and NDI ligands are aligned almost perpendicular so that an inaccurate value reaching nanoseconds was obtained (see the Supporting Information). However, it is commonly known that the rotation energy barrier along the C≡C triple bond is very small in both the ground and excited states.62 From this point of view, it can be considered that the quasi-free rotation of BDP and NDI ligands along the C≡C triple bond is possible. Thus, we could calculate the FRET rates at all possible orientations of these two ligands. The averaged time constant was calculated to be 1.4 ps (7.0 × 1011 s−1), which is shorter than the experimental value (8.6 ps). The difference between the theoretical and experimental time constants can be explained by steric interactions between BDP and NDI, which can inhibit some orientations, so that the rate is deviated from quasi-free rotation approximation. Next, the excitation wavelength was selected at 600 nm, which matches the S0 →S1 transition of NDI ligand (Figure 4). In the resulting spectra, we observed the spectral evolution corresponding to the ISC process in NDI ligand as previously assigned. Through a global target analysis assuming there are two species, S1 and T1 states of NDI ligand, sequentially


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participating in the excited state dynamics, we could obtain SADS of each state which resembles the species B and C obtained at 490 nm excitation (Figure 4b). Therefore, B→C represents the ISC process in NDI ligand with the time constant of 266 ps (kISC = 3.7 × 109 s−1), which is similar to the time constant of ISC assigned at 490 nm excitation (308 ps).

Δ O.D.

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0.01

-0.01

1 ps 10 ps 100 ps 300 ps 1.1 ns 3 ns

0.00

-0.01

B 260 ps C Long


Figure 4. (a) Femtosecond transient absorption spectra of Pt-1 measured at 600 nm excitation in toluene. (b) Species-associated spectra obtained from global target analysis. In order to observe the photoinduced charge transfer (PET) occurring in polar solvents, the TA measurements of Pt-1 in benzonitrile have been performed by photoexcitation at 490 and 600 nm (See the Supporting Information, Figures S23 and S24). In the next section, we calculated the free energy of charge separation and found that the charge separation should be spontaneous in polar solvents. However, the TA spectra measured in benzonitrile did not exhibit any significant differences in the spectral evolution compared to the measurements in toluene. Inhibition of charge separation from the S1 state of BDP ligand is a result of faster rate of FRET than that of charge separation, so that charge separation is outcompeted by faster FRET. On the other hand,


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the absence of charge separation from the S1 state of NDI ligand is rather unclear. However, we propose that strong spin-orbit coupling between central Pt(II) atom and NDI ligand inhibits charge separation process between BDP and NDI ligands.

0.01 Intensity / a.u.

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0.00 -0.01

A 8.1 ps B x 2 307 ps C x 2 Long

-0.02 500


Figure 5. Species-associated difference spectra of Pt-1 obtained from global target analysis at 380 nm excitation in toluene. The strong spin-orbit coupling between the central platinum and NDI ligand could be confirmed by the TA measurements of Pt-1 in toluene with 380 nm excitation, which corresponds to the S0→S2 transition of NDI ligand. Recently it has been reported that in rNDI with bromine as a core substituent, ultrafast ISC process from the S2 to T4 state was observed as a result of strong spin-orbit coupling between NDI core and bromine.47 From the transient absorption spectra of Pt-1, SADS extracted by the global target analysis with three species (Figure 5), the S1 state BDP ligand and the S1 and T1 states of NDI ligand, with an initial population ratio of 1:2:2 using a consecutive model revealed similar SADS obtained at 490 nm. The presence of initial population of T1 state implies that there is a portion of triplet excited-state species which decay to the T1 state faster than our instrumental response time (200 fs), generated through ultrafast ISC from the S2 state of NDI. Through ultrafast ISC from the S2 state of NDI


15


ligand, we can conclude that there is a strong interaction between the central platinum atom and NDI ligand. The nanosecond transient absorption spectra of Pt-1 were measured (Figure 6) to study the triplet state property of Pt-1. The complex was excited at 532 nm, at which mainly the NDI moiety was excited. Bleaching band at 580 nm was observed, which is assigned to the ground state depletion of the coordinated NDI moiety. Moreover, through excited state absorption (ESA) bands in the ranges of 335−495 nm and 600−800 nm are attributed to the triplet state absorption of the NDI moiety.50,53

0.10

0.00

0.00

0.05

-0.05 -0.10

τ = 490 ns

-0.04

-0.05

120.00 μs 114.00 μs ... 6.00 μs 0.00 μs


a 800

Δ O.D.

0.00

τT = 28.1 μs 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 16 of 42

-0.08

-0.10

b 0

50

100 150 Times / μs

c 200

-0.12

0

1

2 3 Times / μs

4

Figure 6. Nanosecond time−resolved transient absorption spectra of (a) Pt-1 (λex = 532 nm) and (b) decay traces at 583 nm in deaerated toluene, (c) in aerated toluene. c = 1.0 × 10−5 M, at 25 °C. Interestingly, a minor bleaching band was observed at 500 nm, which is assigned to the ground state bleaching of the coordinated BDP ligand. Thus, a minor triplet-triplet energy transfer from the T1 state of NDI to the T1 state of BDP occurs at Pt-1, even the energy of T1 state of BDP is located higher than the T1 state of NDI. This postulation was confirmed by the nanosecond TA spectra of the Bodipy complexes.34,54


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In order to confirm that the transient state is the triplet excited state, lifetimes in deaerated solution and aerated solutions were compared. The lifetime of the triplet state of Pt-1 in deaerated solution was determined as 28 μs (Figure 6b).50 However, the lifetime of Pt-1 measured in aerated toluene solution was reduced to 0.49 μs (Figure 4c). This result clarifies that this transient state is the triplet excited state. DFT calculations were done to characterize the excited triplet state of Pt-1. DFT calculations were done using B3LYP functional and LANL2DZ basis sets were employed for Pt, while 631G(d,p) basis sets were imployed for atoms other than Pt. All of the calculations were done with Gaussian 09W.63 The spin density surface of the complex Pt-1 was calculated at optimized triplet-state geometry (Figure 7). It shows that the spin density is mainly localized on the NDI moiety as well as the Pt(II) atom. This result is in agreement with the ns-TA results.

Figure 7. Isosurfaces of spin density of Pt−1. The photophysical processes of Pt-1 upon photoexcitation were summarized in Scheme 2. Based on the ns TA spectra, the triplet state of the BDP ligand was also populated, but to much less extent as compared with the triplet state of the NDI ligand. In both toluene and benzonitrile, charge separation was insignificant. It should be noted that the formation of triplet state of BDP ligand is completely inhibited due to the fast FRET to the NDI moiety. Previously it was


17


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Page 18 of 42

reported that the ISC of the coordinated Bodipy ligand may take up to 100 ps.34 The T1 states of the coordinated NDI and the BDP moieties are close, thus a poor equilibrium between the excited states was observed (Figure 4a). The energy diagram also show that the T1 states of both the coordinated NDI and BDP ligands are not quenched by CS. Scheme 2. Photophysical processes of Pt-1 upon photoexcitation. See the main text for the detail photophysical processes.

CTS (2.48 eV) In toluene

NDI

BDP 2.43 eV

CTS (1.88 eV)

2.10 eV ISC 308 ps

600 nm

FRET 8.2 ps

1

λex =

1

3

NDI (1.58 eV)

In CH2Cl2 CTS (1.71 eV) In CH3CN

TTET

3

BDP (1.70 eV)

λex = 490 nm

τT = 26.3 μs τT = 28.1 μs

[NDI]

Ground state

[BDP]

2.4. Electrochemical Properties and Gibbs Free Energy Changes of the Putative PET. The electrochemical properties of the complex and the ligands were studied (Figure 8).64 These data, together with the optical spectral data, will be used for evaluation of the thermodynamics of the PET process. For NDI ligand, two reversible reduction waves were observed at −1.23 V and −1.64 V, respectively (Table 2). These waves correspond to the sequential one electron reduction process of the NDI ligand. No oxidation waves were observed within the electrochemical window of the solvent we used. Thus we postulate that the NDI ligand is more likely an electron


18

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acceptor, instead of an electron donor. The cyclic voltammogram of BDP ligand shows an irreversible oxidation wave at +0.82 V. One quasi-reversible reduction wave was observed at −1.60 V. For the complex Pt-1, an irreversible oxidation wave was observed at +0.70 V, which can be assigned to the BDP unit. In the reduction region, one reversible reduction band at −1.33 V was observed, which is cathodically shifted as compared with that of the un-coordinated NDI ligand. One quasi-reversible reduction wave at −1.65 V was observed, which is tentatively assigned to the BDP ligand as well as the coordinated NDI ligand, and the reversible reduction band at −1.33 V can be assigned to the coordinated NDI ligand.

a

b +

Fc /Fc

+

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


NDI Pt-1

1

0 -1 Potential / V

-2

BDP Pt-1

1

0 -1 Potential / V

-2

Figure 8. Cyclic voltammogram of (a) Pt−1 and NDI, (b) Pt-1 and BDP. Ferrocene(Fc) was used as internal reference (E1/2 = +0.18 V, Fc+/Fc). In deaerated DCM solution containing 0.5 mM photosensitizers with the ferrocene, 0.10 M Bu4N[PF6] as supporting electrolyte, Ag/AgNO3 reference electrode, scan rates: 100 mV/s. 25 °C. The Gibbs free energy change (ΔGCS) of the photoinduced electron transfer, ΔGcs, was calculated by the Rehm−Weller equation (eq. 1).


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Page 20 of 42

ΔGCS = e [EOX − ERED ] − E00 + ΔGS

(eq 1)

e2 ⎛ 1 1 ⎞⎛ 1 1⎞ ΔGS = − − + − ⎟ ⎜ ⎟⎜ 4πε sε 0RCC 8πε 0 ⎝ RD RA ⎠⎝ ε REF ε S ⎠

(eq 2)

e2

Table 2. Electrochemical Potentials of the Compounds Versus Fc(+/0)a Compound

Oxidation, V

Reduction, V

Pt-1

+0.70

−1.33

BDP

+0.82

−1.60

NDI

−

−1.23

a

Cyclic voltammetry was studied in deaerated dichloromethane containing a 0.10 M Bu4NPF6 supporting electrolyte; counter electrode is Pt electrode; working electrode is glassy carbon electrode; Ag/AgNO3 couple as the reference electrode. [Ag+] = 0.1 M, 0.5 mM photosensitizers in deaerated DCM. 25 °C.

where ΔGS = the static Coulombic energy, e = electronic charge, EOX = half-wave potential for one-electron oxidation of the electron-donor unit, ERED = half-wave potential for one-electron reduction of the electron-acceptor unit, E00 = energy level for the singlet excited state approximated with the fluorescence emission wavelength or the triplet excited state approximated with the phosphorescence emission wavelength, εS = static dielectric constant of the solvent, RCC = the center-to-center separation distance determined by DFT optimization of the geometry, RD = the radius of the electron donor, RA = the radius of the electron acceptor. εREF = the static dielectric constant of the solvent used for the electrochemical studies, and ε0 = the permittivity of free space. The solvents used in the calculation of free energy of the electron transfer are toluene (εS = 2.4), dichloromethane (εS = 8.9), and acetonitrile (εS = 37.5).


20

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Table 3. Free Energy Changes of Charge Recombination (ΔGCR), Charge Separation (ΔGCS), and Charge Separated States Energy Level (ECS) of Pt−1 with the BDP as the Electron Donor and the NDI as the Electron Acceptor .a Electro transfer

ΔGCS, eV

ΔGCR, eV

ECS, eV

BDP →1NDI*b

+0.38 c

−2.48c

+2.48c

−0.22d

−1.88d

+1.88d

−0.39e

−1.71e

+1.71e

+0.90c

−2.48c

+2.48c

+0.30d

−1.88d

+1.88d

+0.13e

−1.71e

+1.71e

+0.05c

−2.48c

+2.48c

−0.55d

−1.88d

+1.88d

−0.72e

−1.71e

+1.71e

+0.78c

−2.48c

+2.48c

+0.18d

−1.88d

+1.88d

+0.01e

−1.71e

+1.71e

BDP→3NDI*b

1

3

BDP*→NDIb

BDP*→NDIb

a

The arrow means the direction of charge transfer. b Electron transfer process in Pt−1. c In toluene. d In dichloromethane. e In MeCN.

We estimated the free energy changes with the singlet and triplet excited states of both chromophores as a driving force for the PET process using Rehm-Weller equation (Table 3).65−67 Clearly the free energy changes (ΔGCS) are dependent on the energy levels of the excited states. The energy levels of the charge transfer states were also calculated. It should be noted that the CTS energy levels are also higher than the triplet excited state of NDI moiety. The triplet excited state of NDI moiety is unable to drive the charge separation because positive values were


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Page 22 of 42

obtained for the Gibbs free energy changes. On the other hand, the singlet excited state of the BDP moiety is able to drive the CS, especially in polar solvents (ΔGCS = −0.55 eV and −0.72 eV in dichloromethane and acetonitrile, respectively). Based on all these results and the femtosecond transient absorption spectral studies, we propose that the lack of formation of CTS in even polar solvents is due to the fast FRET in Pt-1, and the CS is inhibited kinetically, for which an exact reason is unknown at the moment. 2.5.

Application

of

the

Complexes

as

Multiwavelength-Excitable

Triplet

Photosensitizer for TTA Upconversion. Most triplet photosensitizers used for TTA upconversion contain only a single light-harvesting chromophore.24−26,68−71 Thus, these photosensitizers cannot efficiently harvest broadband light sources for TTA upconversion. On the other hand, since Pt-1 contains two different light-harvesting chromophores that can efficiently funnel energy through FRET and ISC. As a consequence, Pt-1 is expected to be an efficient broadband-excitable triplet photosensitizer for TTA upconversion. To examine the upconversion with excitation at two different light harvesting chromophores of Pt-1, two different CW-lasers were employed for upconversion experiments (510 and 589 nm, Figure 9). Perylene was selected as the triplet acceptor. Firstly, the upconversion experiment was carried out with 510 nm laser, which selectively excites BDP. Strong upconversion at 448 nm was observed, and its upconversion quantum yield was determined as 4.1%. Next, the upconversion was carried out with laser excitation at 589 nm. In this case the coordinated NDI part is selectively excited. Strong upconversion was also observed, and the upconversion quantum yield was determined as 1.4% under the same condition.


22

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400

b

*

200 100 0 400

a

Pt-1 + Perylene

300

400

Intensity / a.u.

Intensity / a.u.

Pt-1

300

Pt-1 + Perylene *

200 100

Pt-1

0 400

500 600 700 800 Wavelength / nm

500 600 700 800 Wavelength / nm

Figure 9. Upconversions with Pt-1 as triplet photosensitizer and perylene as triplet acceptor. (a) Excited by 510 nm laser (4.8 mW, 68.6 mW cm−2), (b) Excited by 589 nm laser (4.8 mW, 24.5 mW cm−2). The asterisks indicated the scattered laser. c = 1.0 × 10−5 M in deaerated toluene, 25 °C.

.9

y

b

a

Pt-1 Pt-1+Py

.6

Pt-1 (0.52, 0.45)

.3 Pt-1+Py (0.26, 0.21)

0.0 0.0 .2

.4

x

.6

.9

.8

d

c

Pt-1 Pt-1+Py

a

.6 Pt-1 (0.59, 0.41)

y

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


.3 Pt-1+Py (0.22, 0.16)

0.0 0.0 .2

.4

x

.6

d

.8

Figure 10. CIE diagram and the photographs of the emission of the sensitizer alone and the upconversion. (a) and (b): λex = 510 nm (4.8 mW); (c) and (d): λex = 589 nm (4.8 mW). c = 1.0 × 10−5 M , in deaerated toluene, 25 °C.


23


The upconversion photo and the CIE coordinate changes were presented in Figure 10. The upconverted blue emission is visible to unaided eyes with green (510 nm) and yellow laser excitation (589 nm). The CIE coordinates were changed from (0.52, 0.45) to (0.26, 0.21) for the upconversion with 510 nm laser excitation. Similar results were observed for the upconversion with 589 nm laser excitation.

500 Intensity / a.u.

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

Page 24 of 42

*

400 300 200

*

L1 + L2 L1 L2 sum of L1, L2

100 400


800

Figure 11. Upconversions with Pt-1 as triplet photosensitizer and perylene as triplet acceptor. L1 stands for excitation at 510 nm, L2 stands for excitation at 589 nm. L1+L2 stands for simultaneous excitation with 510 nm and 589 nm CW-laser (the two laser beams are in pseudocollinear geometry within the cuvette). The asterisks indicate the scattered laser. c = 1.0 × 10−5 M. In deaerated toluene. 25 °C. We also carried out a preliminary two-wavelength co-excitation experiment for the TTA upconversion (Figure 11). The two cw-laser beams (510 and 589 nm) are pseudo-collinear, i.e. they superimpose with each other. The result was compared to single laser excitation under the same setup, blocking one of the two laser beams. Interestingly, we found that the upconversion intensity with simultaneous excitation with two laser beams is stronger than the sum of upconversion with a single laser beam. We tentatively assign the intensified upconversion with


24

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the simultaneous excitation of the solution with two laser beams is due to dual-excitation of the photosensitizer Pt-1. In this case, both of the coordinated BDP and NDI moieties were excited at the same time. As a result, local concentration of excited photosensitizer was increased, leading to more efficient TTET process which intensifies the TTA upconversion. Broadband wavelength excitable TTA upconversion was rarely reported. Previously Pd(II) benzoporphyrin complex was used for TTA upconversion, and focused solar light was used as excitation source, but the complex shows a narrow range of absorption in visible region (not broadband absorption).72 Recently we prepared Pt(II) complex showing broadband absorption in visible spectral region, but those complexes were not used for TTA upconversion due to inappropriate triplet state energy levels.33,34,54 Our results will be useful to improve the efficiencies in photovoltaics and photocatalysis.69,73−76 2.6. Conclusions. In summary, we prepared a N^N Pt(II) bisacetylide complex with heteroleptic acetylide ligands (BDP and NDI ligands). The photophysical properties of the complexes were studied with steady state and femtosecond/nanosecond time-resolved transient absorption spectroscopies, as well as electrochemical characterizations. Based on the steady state fluorescence emission spectra and the ultrafast transient absorption spectra, we confirmed the Föster-resonance-energy-transfer (FRET) from the BDP ligand to the NDI ligand (8.2 ps), which was followed by the slow intersystem crossing (ISC) of the NDI moiety (305 ps). In the ultrafast transient absorption spectra, no charge transfer state (CTS) was observed, although charge separation is thermodynamically allowed based on the Gibbs free energy changes of the putative CS process. The T1 triplet state of the complex is exclusively localized on the NDI moiety with the triplet state lifetime of 28.1 μs. The complex was examined as broadband light-excitable triplet photosensitizer for TTA upconversion, and the upconversion quantum yield of 4.1% was determined.


25


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Page 26 of 42

3. Experimental Section. 3.1. Synthesis BDP. Under Ar atmosphere, to a solution of 4-ethynylbenzaldehyde (0.13 g, 1.0 mmol, compound 2) and 2,4-dimethylpyrrole (0.22 g, 2.3 mmol) in CH2Cl2 (250 mL) was added trifluoroacetic acid (7.6 mL, 0.1 mmol). The mixture was stirred at room temperature overnight. DDQ (0.246 g, 1.0 mmol) was added before the mixture was stirred for another 12 h. Then, Et3N (1.7 g, 17.0 mmol) was added, 10 min later, BF3·OEt2 (2.60 g, 18.6 mmol) was added and stirred overnight. The reaction mixture was washed with water and dried over anhydrous Na2SO4. The solvent was removed under reduced pressure and the residue was purified with chromatography (petroleum ether/CH2Cl2 = 1/1, v/v) to give an orange solid (104.8 mg, yield 30%). 1H NMR (400 MHz, CDCl3): δ 7.62 (d, J = 4.0 Hz, 2H); 7.28 (s, 2H); 5.99 (s, 2H); 3.18 (s, 1H); 2.55 (s, 6H); 1.40 (s, 6H). LTQ−Orbitrap HRMS: calcd for ([C21H19BF2N2 + H]+) m/z = 349.1609, found m/z = 349.1679. Compound 3. Naphthalenetetracarboxylic dianhydrides (NDA, 10 g, 37.3 mmol) was dissolved in oleum (50% SO3, 150 mL) after iodine (60 mg, 0.24 mmol) were placed in reaction bottle. Then bromine (11.92 g, 74.56 mmol) was injected to the mixture before the temperature was increased to 50 °C and stirred for 48 h. After reaction was finished, the mixture carefully poured into ice. The precipitated solid was filtered off, and washed with water and methanol. The crude yellow product was the mixture of monobromonaphthalene tetracarboxylic diimides (NDIs) and dibromo NDI, which were used as the starting materials of NDI acetylide ligand without further purification. Compound 4. Under Ar atmosphere, the crude compound 3 (1.06 g, 2.5 mmol), 2ethylhexylamine (0.97 g, 7.5 mmol) was refluxed in acetic acid (25 mL) for 2 h. After cooling to


26

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room temperature, the mixture was poured into cold water (100 mL). The solid was collected by filtration, and was washed with water and methanol. After drying under vacuum, the residue was purified with column chromatography (CH2Cl2/petroleum ether = 1:1, v/v) to give an yellow solid (0.785 g; yield 48.5%). 1H NMR (400 MHz, CDCl3): δ 9.00 (s, 2H), 4.12−4.19 (m, 4H), 1.94 (m, 2H), 1.26−1.39 (m, 16H), 0.94 (t, J = 7.2 Hz, 6H), 0.88 (t, J = 7.2 Hz, 6H). MALDI−TOF HRMS: calcd for ([C30H36Br2N2O4]+) m/z = 648.1021, found m/z = 648.0999. Compound 5. Under Ar atmosphere, the mixture of compound 4 (130.0 mg, 0.20 mmol), 2ethylhexylamine (28.6 mg, 0.22 mmol) and 2-methoxyethanol (5 mL) was stirred at 120°C for 8 h. After cooling to room temperature, the solvent was evaporated under reduced pressure. Then the crude product was purified with column chromatography (CH2Cl2/petroleum ether = 1:1,v/v) to give a red solid (98.8 mg; yield 70.6%). 1HNMR (400 MHz, CDCl3): δ 10.19 (s, 1H), 8.87 (s, 1H), 8.30 (s, 1H), 4.18–4.08 (m, 4H), 3.48 (t, J = 8 Hz, 2H), 1.96–1.93 (m, 2H), 1.80–1.77 (m, 1H), 1.39–1.26 (m, 24H), 1.01–0.88 (m, 18H). MALDI−TOF HRMS: calcd for ([C38H54BrN3O4]+) m/z = 695.3298, found m/z = 695.3289. NDI. under Ar atmosphere, Compound 5 (78.8 mg, 0.113 mmol) was dissolved in 10 mL of Et3N. PPh3(2.6 mg, 0.01 mmol) and Pd(PPh3)2Cl2(3.5 mg, 0.005 mmol) were added, followed by CuI (1.9 mg, 0.01 mmol). Trimethylsilylacetylene (0.8 mL, 0.005 mmol) was injected before the mixture was stirred under Ar for 8 h at 75 °C. After the reaction was finished, the solvent was removed under reduced pressure. The residue was purified with column chromatography (CH2Cl2/petroleum ether = 1:1, v/v) to give a red solid (65.5 mg, yield 81.1%). The red solid (65.5 mg, 0.092 mmol) was dissolved in the mixed solvent of CH2Cl2 (5mL) and methanol (10mL), then K2CO3 (80 mg, 0.58 mmol) was added. The mixture was stirred at room temperature for 1 h. Water was added and the mixture was extracted with CH2Cl2 (3 × 15 mL).


27


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Page 28 of 42

The organic layer was dried over anhydrous Na2SO4. The solvent was removed under reduced pressure and the residue was purified with column chromatography (CH2Cl2 /petroleum ether = 1:1, v/v) to give an red solid 42.5 mg; yield: 72.2 %. 1H NMR (400 MHz, CDCl3): δ 10.24 (s, 1H), 8.77 (s, 1H), 8.28 (s, 1H), 4.18 – 4.08 (m, 4H), 3.74 (s, 1H), 3.38 (t, J = 8.0 Hz, 2H), 1.99 – 1.91(m, 2H), 1.81 − 1.77 (m, 1H), 1.39 − 1.26 (m, 24H), 1.01 – 0.87 (m, 18H). MALDI−TOF HRMS: calcd for ([C49H55N3O4 + H]+) m/z = 642.4193, found m/z =642.4285. Pt-1. Under Ar atmosphere, Pt(dbbpy)Cl2 (dbbpy = 4,4’-di(tert-butyl)-2,2’-bipyridine) (26.7 mg, 0.05 mmol), BDP (17.4 mg, 0.05 mmol), and NDI (32.1 mg, 0.05 mmol) were dissolved in CH2Cl2 (10 mL). After addition of diisopropylamine (1 mL) to the mixture, CuI (2.9 mg, 0.10 equiv) was added. Then the mixture was stirred at room temperature for 16 h. Deionized water was used to quench the reaction. The mixture was extracted with CH2Cl2 (3 × 15 mL). The organic layer was dried over anhydrous Na2SO4. After removal of the solvents, the solid was purified with column chromatography (Silica gel, CH2Cl2 was used as eluent). Pt-1 was collected as a red solid (11.3 mg, yield 15.6%). 1H NMR (500 MHz, CDCl3): δ 10.51 (s, 1H), 9.98 (d, 2H), 8.97 (s, 1H), 8.24 (s, 1H), 8.00 (d, J = 10 Hz, 2H), 7.74−7.66 (m, 4H), 7.20 (d, J = 10 Hz, 2H), 5.99 (s, 2H), 4.25−4.14 (m, 4H), 3.48−3.51 (m, 2H), 2.09−1.95 (m, 2H), 1.81−1.77 (m, 1H), 1.52−1.26 (m, 48H), 0.99−0.85 (m, 18H). 13C NMR (125 MHz. CDCl3): 166.9, 163.8, 163.7, 163.3, 163.3, 163.2, 156.6, 156.0, 155.1, 153.6, 151.6, 151.3, 143.6, 142.6, 141.1, 133.1, 131.7, 131.6, 129.3, 127.0, 124.9, 124.6, 124.4, 122.5, 121.1, 120.3, 119.0, 118.8, 118.6, 103.1, 101.8, 101.0, 88.7, 77.3, 77.0, 76.8, 53.4, 46.4, 44.6, 43.9, 39.4,37.9, 37.8, 35.9, 35.8, 31.9, 31.3, 30.7, 30.3, 29.7, 29.4, 28.9, 28.7, 28.6, 24.6, 24.2, 24.1, 23.2, 23.1, 23.0, 22.7, 14.8, 14.6, 14.1, 14.1, 14.0, 11.0, 10.9, 10.7. MALDI−TOF HRMS: calcd for ([C79H96BF2N7O4Pt + H]+) m/z = 1451.7311, found m/z = 1451.7338.


28

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3.2. 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 supporting electrolyte, glassy carbon electrode as the working electrode, and platinum electrode as the counter electrode. Dichloromethane was used as the solvent, and ferrocene (Fc) was added as the internal reference. The solution was purged with N2 before measurement, and the N2 gas flow was kept constant during the measurement. 3.3. Nanosecond Transient Absorption Spectroscopy. Nanosecond transient absorption spectroscopy was studied with a LP920 laser flash photolysis spectrometer (Edinburgh Instruments, Livingston, U.K.). The samples were purged with N2 for 15 min before measurements, and the N2 gas flow was kept constant during the measurement. The signal was digitized with a Tektronix TDS 3012B oscilloscope. 3.4 Femtosecond Transient Absorption. The femtosecond time-resolved transient absorption (fs-TA) spectrometer pumped by a Ti:sapphire regenerative amplifier system (Integra-C, Quantronix) operating at 1 kHz repetition rate and an optical detection system was used. Pump pulse was generated by Optical Parametric Amplifiers (Palitra, Quantronix), and had width of ~ 100 fs and average power of 100mW. White light continuum was generated by a sapphire window with 3 mm of thickness by focusing a small portion of fundamental 800 pulse. Variable optical delay (ILS250, Newport) was used to control the time delay between pump and probe beams making pump beam travel along the delay. To obtain polarization-independent signals, the polarization angle between pump and probe beam were set to magic angle (54.7°) using a half-wave Glan-laser polarizer. The pump pulses were chopped at 500 Hz and intensities of the WLC probe pulses were recorded alternately with or without pump pulse by High Speed spectrometer (Ultrafast Systems) to obtain the time-resolved transient absorption difference


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signal (ΔA) at a specific time. Cross-correlation FWHM in pump-probe experiments was less than 200 fs and chirp of WLC probe pulses was measured to be 800 fs in the 400 − 800 nm region. Reflection optics were used at the probe beam path and the 2 mm path length of quartz cell were used in order to minimize chirp. HPLC grade solvents were used in all fs-TA measurements. Global target analyses were done using Glotaran.77 Supporting Information Experimental procedures, molecular structure characterization, additional spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENT We thank the NSFC (21273028, 21421005, 21673031 and 21473020), Program for Changjiang Scholars and Innovative Research Team in University [IRT_13R06], the Fundamental Research Funds for the Central Universities (DUT16TD25, DUT15ZD224, DUT2016TB12) for financial support. The work at Yonsei University was financially supported by the Global Research Laboratory (2013K1A1A2A02050183).

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TOC Graphic


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Broadband Visible Light Harvesting N^N Pt(II) Bisacetylide Complex

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