Communication Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Efficient Electrogenerated Chemiluminescence of Pyrrolopyrrole Aza-BODIPYs in the Near-Infrared Region with Tripropylamine: Involving Formation of S2 and T2 States Ryoichi Ishimatsu,*,† Hirosato Shintaku,† Yuto Kage,† Misaki Kamioka,† Soji Shimizu,*,†,‡ Koji Nakano,† Hiroyuki Furuta,†,‡ and Toshihiko Imato*,†
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†
Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡ Center for Molecular Systems (CMS), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *
3) and TPA•, R* is produced (eq 4). TPA′ represents a product of the electron transfer reaction. R* includes singlet and triplet excited states. Normally, for organic molecules, the S1 state is the emissive state,
ABSTRACT: Efficient electrogenerated chemiluminescences (ECLs) of three pyrrolopyrrole aza-BODIPYs in the near-infrared region by using tripropylamine as a coreactant are reported. Kinetic analysis based on Marcus theory indicates the direct formation of S2 and T2 states through the electron transfer reaction, which affects the ECL efficiencies.
1
za-boron-dipyrromethene (aza-BODIPY) and its analogues, which show unique photophysical properties such as small Stokes shifts (SSs) and small full widths at halfmaximum as BODIPY shows,1,2 are attractive to realize high molar extinction coefficients and high photoluminescent (PL) quantum yields (ϕPL) in the near-infrared (NIR) region.3−11 BODIPYs and aza-BODIPYs have been employed for NIR bioimaging12−14 and photovoltaic cells.15−17 For organic lightemitting diodes (OLEDs)18 and electrogenerated chemiluminescence (ECL), they are important to produce efficient NIR light-emitting systems. ECL involves homogeneous and heterogeneous electron transfers and radiative transition processes.19 Several research studies were conducted to reveal the electrochemical properties including ECL of BODIPY and aza-BODIPY derivatives.20−24 It was reported that using tripropylamine (TPA) as a coreactant was effective to enhance the ECL intensity of the derivatives, where, energetically, ion annihilation between the radical anion and cation of the BODIPY derivatives is inefficient to produce their S1 states, but favorable to yield their T1 states. In the presence of TPA, the excited states of a fluorescent molecule (R*) can be formed through the following reactions,25 (1)
TPA•+ → TPA• + H+
(2)
R − e− → R•+
(3)
R•+ + TPA• → R* + TPA′
(4)
(5)
Equation 5 represents a radiative transition process from the S1 state of R (1R*). The population of 1R*, which is a result of competition with the kinetics of electron transfer reactions to form other states such as the T1 state, is related to the ECL efficiency.26,27 Therefore, kinetic analysis gives a deep understanding for the enhancement of the ECL with TPA. Here, we report efficient NIR ECLs emitted by three pyrrolopyrrole aza-BODIPYs (PPABs: PPAB-Ph, PPAB-Th, and PPAB-Th-Fl, Figure 1) with TPA. There are several papers reporting NIR ECLs from nanoclusters with coreac-
A
TPA − e− → TPA•+
R * → R + hν
Figure 1. Molecular structures of PPABs employed in this research.
•+
The electrochemically generated TPA (eq 1) undergoes deprotonation to yield TPA• (eq 2), and then through the electron transfer between electrochemically generated R•+ (eq © XXXX American Chemical Society
Received: May 16, 2019
A
DOI: 10.1021/jacs.9b05245 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society tant;28−33 however, examples of efficient NIR ECL from organic molecules are relatively rare. This is also related to the fact that the guideline for designing organic molecules for NIR ECL is not sufficiently clear. We discuss the kinetics of homogeneous electron transfer processes to form the excited states based on Marcus theory that involves the Franck− Condon factor to consider the nuclear tunneling in the “inverted region”. Importantly, we elucidate that the formation of not only the S1 but also T2 (for all PPABs) and S2 (for PPAB-Th-Fl) states is possible through the electron transfer reaction (eq 4), and the formation of the S2 state may enhance the ECL intensity. Recently, it is inferred that the formation of the T2 state affects the efficiency of OLEDs containing thermally activated delayed fluorescence molecules.34,35 Therefore, direct formation of higher excited states by charge transfer is very important. Figure 2 shows visible (Vis) absorption and PL spectra of the PPABs in dichloromethane (DCM). The Vis and their
Figure 3. CVs of PPABs in DCM. Concentration: 0.5 mM. Scan rate: 100 mV s−1. Arrows indicate scan direction.
and Em of the second reduction waves (Em(Red2)). The Em values are listed in Table 2. The DFT calculations suggest that, energetically, a dianion and dication are formed at the respective second redox waves. For the second reduction wave of PPAB-Ph, the current in the backward scan was much smaller than that in the forward scan, indicating that the dianion of PPAB-Ph is rather unstable. We used TPA as a coreactant, and the potential-dependent ECLs and currents of the PPABs with TPA are presented in Figures S2, S3, and S4. The ECL spectra of the PPABs are displayed in Figure 4. The maximum wavelength of ECL (λECL) appeared at 659, 709, and 777 nm for PPAB-Ph, PPAB-Th, and PPAB-Th-Fl, respectively, and they are slightly red-shifted compared with their corresponding λPL because of inner filter effects (Figure S5). The ECL efficiencies (ϕECL) of the PPABs relative to Ru(bpy)32+, which were obtained through eq S1, are tabulated in Table 2 with λECL. Their ϕECL values are high, and particularly, ϕECL of PPAB-Th-Fl is about twice that of a Ru(bpy)32+/TPA system. Moreover, interestingly, the order of ϕECL is inconsistent with that of ϕPL. We discuss the kinetics to form the excited states to understand the ϕECL values of the PPABs. According to Marcus theory considering the Franck−Condon factor, the rate constants of the electron transfer (ket) can be written as36 2π 2 1 ket = Hif exp(−S) ℏ 4πλokBT
Figure 2. Vis absorption (broken lines) and PL spectra (solid lines) of PPABs. Solvent: DCM. Concentration: 5 μM.
corresponding PL spectra are “mirror image”, and vibrational structures can be seen. The maximum wavelength of absorption and PL spectra (λabs and λPL, respectively) were red-shifted in the order of PPAB-Ph, PPAB-Th, and PPABTh-Fl. We performed density functional theory (DFT) calculations to estimate HOMOs and LUMOs (Figure S1) and time-dependent (TD) DFT calculations to obtain the energy levels of the Sn and Tn states of PPABs, E(Sn) and E(Tn), respectively, where n = 1 and 2. These values together with λabs, λPL, SS, and ϕPL are summarized in Table 1. Figure 3 presents the cyclic voltammograms (CVs) of the PPABs. All PPABs gave two one-electron reduction and oxidation waves. The midpoint potentials (Em) for the first oxidation waves (Em(Ox1)) were shifted to a negative potential direction in the order of PPAB-Ph, PPAB-Th, and PPAB-ThFl, whereas for the Em of the first reduction waves (Em(Red1)), a slight shift to a positive direction was observed. The CVs indicate that the gaps of Em(Ox1) and Em of the second oxidation wave (Em(Ox2)) are smaller than those of Em(Red1)
×
ij (λ + ΔG 0 + jhνi)2 yz zz expjjj− o zz j! jk 4λokBT {
∑S j=0
j
(6)
where ℏ = h/2π. h, kB, and T denote the Plank constant, Boltzmann constant, and absolute temperature, respectively. ΔG0 and Hif are the Gibbs energy difference and mixing energy
Table 1. Photophysical Properties and Energy Levels of the Excited States in DCM λabs/nm PPAB-Ph PPAB-Th PPAB-Th-Fl
589, 636 633, 694 683, 752
λPL/nma 657, 719 703, 778 772, 860
sh
SS/nm
S
νi/cm−1
ϕPL
E(S1)/eV
E(T1)/eV
E(S2)/eV
E(T2)/eV
21 9 20
0.42 0.33 0.34
1250 1400 1340
0.87b 0.42c 0.24c
1.88d (1.93e) 1.76d (1.76e) 1.61d (1.59e)
1.02e 0.85e 0.79e
2.80e 2.55e 2.20e
2.34e 2.03e 1.74e
5 μM. sh: shoulder. bFrom ref 8. cFrom ref 11. dFrom λPL. eFrom TD-DFT calculations with a B3LYP/6-31+G(d) level.
a
B
DOI: 10.1021/jacs.9b05245 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Journal of the American Chemical Society Table 2. Electrochemical and ECL Properties of PPABs in DCMa PPAB-Ph PPAB-Th PPAB-Th-Fl
Em(Red1)/V
Em(Red2)/V
−0.93 −0.85 −0.82
−1.46 −1.39 −1.31
Em(Ox1)/V
Em(Ox2)/V
0.80 0.75 0.58
1.08 1.13 0.90
ΔEm/Vb 1.73 1.60 1.40
ΔEOx1/TPA•/Vc 2.50 2.45 2.28
λECL/nmd 659, 718 709, 780 777, 861
sh
ϕECLe 1.0 1.7 2.1
a Potentials are vs SCE. sh: shoulder. bΔEm = Em(Ox1) − Em(Red1), cΔEOx1/TPA• = Em(Ox1) − E(TPA•), where E(TPA•) = −1.7 V vs SCE. d10 μM PPABs in the presence of 20 mM TPA. eCompared with that of Ru(bpy)32+/TPA.
Figure 5. kd/et against ΔG0 for PPAB-Th-Fl. The vertical bands indicate the energy gap between ΔEOx1/TPA• and E(Sn) and E(Tn) of PPAB-Th-Fl. Hif = 0.016 eV.
Figure 4. ECL spectra of 10 μM PPABs in DCM in the presence of 20 mM TPA.
of the initial and final states, respectively. S is the Huang−Rhys factor, related to the electron-vibrational coupling strength, and νi is an averaged vibrational frequency. λo represents the reorganization energy of the solvent molecules. λo for oneelectron transfer reactions was calculated with eq S2, and the values are listed in Table S1. To obtain S and νi, the Vis absorption spectra of the PPABs were fitted with eq S3 (Figure S6 and Table 1). Hif for the electron transfer reaction between the radical cation of the PPABs and TPA• was calculated with eq S4.37 By considering the diffusion process for eq 4, the rate constant of the electron transfer to form the Sn and Tn states, kd/et(Sn) and kd/et(Tn), respectively, may be written as38 kd/et(Sn , Tn) =
kd/et(T1) is in the diffusion-limited region, and for PPAB-Th and PPAB-Th-Fl, kd/et(T1) is on the order of 109 M−1 s−1. Importantly, for PPAB-Th-Fl, its ΔG0(S2) is ∼ − 0.1 eV, and kd/et(S2) is limited by the diffusion, whereas, for PPAB-Ph and PPAB-Th, the formation of their S2 states is endothermic reactions, and their kd/et(S2) values are on the order of 107−109 M−1 s−1. Normally, the rate constants of the internal conversion (kIC) are large (1011−13 s−1),39 and for BODIPY derivatives, 1012−13 s−1 was found for the S2 → S1 conversion.40,41 In addition, the rate constants of the intersystem crossing (kISC) for organic molecules without heavy atoms are much lower than kIC.39,42 Hence, it is reasonable to assume kIC ≫ kISC for the PPABs. Under the condition, the S2 and T2 states are immediately relaxed to the S1 and T1 states, respectively. From the spin statistics, the formation of Tn states is favorable compared with that of Sn states; i.e., the ratio of Sn and Tn states formed by the electron transfer is 1:3.43,44 Thus, by considering the spin statics with the kinetics, ϕECL may be written as
kd k −d ket
+1
(7)
where kd and k−d are the rate constants of diffusion-limited ionpair formation and dissociation, respectively. Here, kd = k−d is assumed, and kd was set to be 2 × 1010 M−1 s−1, which is a typical value for the PPABs determined from eq S5 by considering their diffusion coefficients and radii (Table S1). In Figures 5, S7, and S8, we plotted kd/et (where j = 0 to 5 in eq 6) against ΔG0. The curves at S = 0, meaning that the contribution of the nuclear tunneling effect is neglected in the kinetics, are also drawn for comparison. The energy levels of a pair that includes the radical cation of the PPABs and TPA• (where the redox potential of TPA• is ∼ − 1.7 V vs SCE25); ΔEOx1/TPA• (= Em(Ox1) − (−1.7)) were compared with its E(Sn) or E(Tn). The vertical bands represent the energy gaps between ΔEOx1/TPA• and E(Sn) or E(Tn), ΔG0(Sn), and ΔG0(Tn), respectively. The width of the bands (0.1 eV) covers the contribution from the reorganization energy of the inner sphere (Table S1). For the PPABs, the formation of the S1 and T2 states is in the diffusion-limited region. Although their ΔG0(T1) values are negatively large, because of the nuclear tunneling effect, kd/et(T1) values are not suppressed much. For PPAB-Ph,
ϕECL ≈
ϕPL × (kd/et(S1) + kd/et(S2 )) kd/et(S1) + kd/et(S2 ) + 3kd/et(T) 1 + 3kd/et(T2) (8)
By neglecting kd/et(S2) for PPAB-Ph and PPAB-Th, and kd/et(T1) for PPAB-Th and PPAB-Th-Fl, ϕECL of PPAB-Ph, PPAB-Th, and PPAB-Th-Fl may be described as their ϕPL with a coefficient of 1/7, 1/4, and 2/5, respectively. Furthermore, with ϕPL, the ratios of ϕECL for PPAB-Ph and PPAB-Th relative to PPAB-Th-Fl became 1.3 and 1.1, respectively. Noticeably, due to the different kd/et values to form the excited states, the theoretical ϕECL ratios are smaller than the ϕPL ratios. The theoretical ratio of ϕECL for PPAB-Th moderately agrees with the experimentally obtained one (0.81, and this experimental value suggests kd/et(S1)/3kd/et(T1) ≈ 1.4 in eq 8). For PPAB-Ph, the difference between the theoretical and experimental ratio of ϕECL (0.48) is large. This is probably because of the generation of the unstable dianion of PPAB-Ph C
DOI: 10.1021/jacs.9b05245 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society by TPA•, which may facilitate the depletion of PPAB-Ph near the electrode. In conclusion, efficient NIR ECLs from the PPABs generated with TPA were described. ϕECL was influenced by the kinetics of the electron transfer reaction to form the excited states. It is emphasized that the direct formation of higher excited states such as S2 and T2 states, which affects the ECL efficiency, should be considered for NIR fluorescent molecules because their energy levels are relatively low. Our results indicate that the light-emitting efficiency may be manipulated by changing ΔG0 with molecules having a different redox potential.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b05245.
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Experimental section, HOMOs and LUMOs of PPABs, potential-ECL curves, comparison of ECL and PL spectra, fitting absorption spectra, kinetic analysis, and Table S1 (PDF)
AUTHOR INFORMATION
Corresponding Authors
*
[email protected] (R.I.) *
[email protected] (S.S.) *
[email protected] (T.I.) ORCID
Ryoichi Ishimatsu: 0000-0003-2697-5167 Soji Shimizu: 0000-0002-2184-7468 Koji Nakano: 0000-0002-4860-5389 Hiroyuki Furuta: 0000-0002-3881-8807 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science (JSPS) through its JSPS KAKENHI Grant Number JP 18K05176.
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