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Efficient Intersystem Crossing in Heavy-Atom-Free Perylenebisimide Derivatives Wenbo Yang,† Jianzhang Zhao,*,† Christoph Sonn,‡ Daniel Escudero,§ Ahmet Karatay,∥ H. Gul Yaglioglu,∥ Betül Kücu̧ ̈köz,∥ Mustafa Hayvali,*,⊥ Chen Li,*,‡ and Denis Jacquemin*,§,# †

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 ‡ Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany § CEISAM UMR CNRS 6230, Université de Nantes, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France ∥ Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Beşevler, Ankara, Turkey ⊥ Department of Chemistry, Faculty of Science, Ankara University, 06100 Beşevler, Ankara, Turkey # Institut Universitaire de France, 1, rue Descartes, 75005 Paris Cedex 5, France S Supporting Information *

ABSTRACT: Efficient intersystem crossing (ISC) in heavyatom-free organic chromophores remains rare because of the lack of strong spin−orbit coupling effects in such compounds. Finding organic chromophores with ISC ability is important for applications in several areas, e.g., photocatalysis and photodynamic therapy. Herein, we report new perylenebisimide (PBI) chromophores with tetraphenylethynyl substituents at the 2,5,8,11-positions of the PBI core (ortho-positions, not the usually reported bay-positions of PBI), which show efficient ISC without the presence of any heavy atoms. Steadystate and picosecond−nanosecond transient absorption spectroscopies as well as time-dependent density functional theory computations were used to reveal the photophysical properties. For one of the PBI derivatives, excitation wavelength-dependent ISC was observed. The efficient ISC was attributed to the S1/S2 → Tn (n > 1) processes. Photochemical reduction of the PBI derivatives in the presence of a sacrificial electron donor (triethanolamine) produced a stable PBI radical anion. molecules (monoaza[5]helicenes),52 1-nitropyrene,53 4-thiothymidine,54 6-thioguanosine,55 and 1-cyclohexyluracil.56 Previously, the ISC of asymmetrically substituted peryleneimide derivatives was reported to yield phosphorescence at 77 K (in solid matrix).50 In fluid solution at room temperature, the formation of a triplet state upon photoexcitation of the asymmetrically substituted peryleneimide derivatives was also confirmed with nanosecond transient absorption spectroscopy. The triplet state lifetimes were determined to be in the 130− 140 μs range. The singlet oxygen (1O2) quantum yields (ΦΔ) of these derivatives lie in the 0.4−0.6 range. However, the origin of the ISC in these heavy-atom-free PBI derivatives was not investigated.50,57 Triplet state formation was also observed in well-defined intramolecular perylenediimide dimeric aggregate.5 In this case, the triplet state lifetime was determined to be 200 μs, and the singlet oxygen quantum yield (ΦΔ) is 0.5.5 It was proposed that the triplet state was formed in this case via a polar transition

1. INTRODUCTION Perylenebisimide (PBI) is a versatile chromophore that has been widely used in molecular assembly,1−6 artificial photosynthesis,7 two-photon absorption dyes,8,9 and photoinduced charge separation (CS).5,10−14 However, most of these studies focused on fluorescence or the singlet excited state,4,15−20 and the investigations devoted to the triplet excited state of PBI remains rather limited. This is surprising, because triplet excited states are crucial in photocatalysis,21−26 photodynamic therapy,27−35 molecular logic gates,36 photovoltaics,37 and in triplet−triplet annihilation upconversion processes.38−42 Previously, the triplet excited state of PBI was accessed with the well-known heavy atom effect present in transition-metal complexes.43−47 Interesting photophysical properties were unraveled, e.g., the triplet state lifetime is significantly compound-dependent.48,49 However, the triplet formation is rare in metal-free PBI derivatives5,50 because of the lack of significant spin−orbit coupling (SOC) effects in the vast majority of organic compounds, although ISC has been reported in some heavy-atom-free organic chromphores, such as 9,10-dioxa-antibimanes,51 nonplanar aromatic heterocyclic © XXXX American Chemical Society

Received: February 16, 2016 Revised: April 21, 2016

A

DOI: 10.1021/acs.jpcc.6b01584 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Structures of PBI Derivatives Used in This Studya

a

PBI-2 and PBI-6 show intersystem crossing upon photoexcitation.

Scheme 2. Synthesis of PBI Derivativesa,b

a

B-1 was used as triplet energy acceptor. bMolecular structures of the PBI derivatives studied here. (a) Trimethylsilylacetylene, (PPh3)4Pd, CuI, dried (iPr)2NH/toluene, Ar, RT, 12 h, yield: 85%. (b) K2CO3, CH2Cl2, CH3OH, RT, 4 h, yield: 76%. (c) Iodine, dioxane, pyridine, 0 °C, 4.5 h, yield: 52%. (d) Trimethylsilylacetylene, Pd(PPh3)4, PPh3, CuI, triethylamine, 89 °C, 10 h, yield: 80%. (e) K2CO3, CH2Cl2, CH3OH, RT, 12 h, yield: 68%. (f) 3-Aminopentane, imidazole, Ar, 160 °C, 7 h, yield: 90%. (g) Bromine, CH3Cl, 60 °C, 36 h, yield: 95%. (h) Phenylacetylene, (PPh3)4Pd, CuI, dried (iPr)2NH, Ar, 40 °C, 9 h, yield: 87%. (i) 2, (PPh3)4Pd, CuI, dried (iPr)2NH, Ar, 40 °C, 10 h, yield: 78%. (j) 5, (PPh3)4Pd, CuI, dried (iPr)2NH, Ar, 40 °C, 12 h, yield: 56%. (k) Phenylacetylene, (PPh3)4Pd, CuI, dried (iPr)2NH, Ar, 40 °C, 10.5 h, yield: 92%. (l) 2, (PPh3)4Pd, CuI, dried (iPr)2NH, Ar, 40 °C, 12 h, yield: 74%.

B


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The Journal of Physical Chemistry C

Figure 1. UV−vis absorption spectra of the compounds. c = 1.0 × 10−5 M in toluene, 20 °C.

compounds of PBI-6 and PBI-7. The compounds PBI-3 and PBI-4 used as reference also are new derivatives (Scheme 1). Herein, with Sonogashira cross coupling, we prepared the tetraphenylethynyl derivatives PBI-6 and PBI-7 (Scheme 2). Compounds with similar substituents at the bay-positions, i.e. 1,7-positions (PBI-2 and PBI-3), were used as the reference to study the photophysical properties of PBI-6 and PBI-7 (see Scheme 1). Moreover, the photophysical properties of the 1,7dibromoPBI (PBI-1) and the 2,5,8,11-tetrabromoPBI (PBI-5) were also investigated. 2.2. UV−Vis Absorption and Fluorescence Spectroscopy. The UV−vis absorption spectra of the compounds are reported in Figure 1. PBI-1 shows slightly red-shifted absorption compared to its tetrabromo counterpart (PBI-5; Figure 1a). This red-shifted absorption of PBI-1 can be attributed to the twisted geometry of the molecule (see section 2.5).60 For PBI-5, interestingly, the molecule presents a planar geometry (see section 2.5). Both compounds show similar vibronic progression of their main absorption band. The comparison of the UV−vis absorption of PBI-1−PBI-4 indicates that the absorption wavelength is red-shifted when the π-conjugation length is increased (Figure 1b). However, it is the electron-donating substituents that induce the most significant red-shifting of the absorption spectra. A similar trend was observed for the 2,5,8,11-substituted derivatives PBI5− PBI-7 (Figure 1c). Unfortunately, we failed to prepare the counterpart of PBI-4, i.e., the 2,5,8,11-substituted PBI derivative with electron−donating substituents, which were found to be unstable. The fluorescence emission spectra of the compounds were studied (Figure 2a−f). For PBI-1−PBI-3, the fluorescence quantum yield (ΦF) is up to 80%. In contrast, PBI-4 shows a ΦF value of 5.2%, which is attributed to the near-IR absorption and thus the small S1/S0 energy gap, which makes the nonradiative decay prominent. We underline that PBI derivatives showing near-IR absorption and room temperature near-IR emission were rarely reported, as the previous examples were related, on the one hand, to PBI J-aggregates that absorb in near IR spectral region,69,70 and on the other hand, to triply linked bay-region fused di(perylenebisimide)s (DiPBIs) that also display near-IR absorption but no room temperature nearIR fluorescence.61 Much lower ΦF were observed for PBI-5−PBI-7. Note that PBI-5−PBI-7 bear a planar geometry, instead of the twisted geometry of PBI-1−PBI-4. It is a general belief that the planar geometry is more beneficial for efficient fluorescence than the twisted geometry, and the present molecules therefore stand as counterexamples of this empirical rule. Thus, likely other nonradiative decay channels specific to the singlet excited states of PBI-5−PBI-7 are responsible for the quenching of

state (charge-separated state). Indeed, the formation of a triplet state by charge recombination (CR) is known for other chromophores.58 In addition, bay-substituted PBI derivatives (with amino substituents at the bay position) demonstrated triplet state formation ability, and 1O2 photosensitizing.59 No nanosecond transient absorption spectroscopic study of these compounds was reported to date, but theoretical calculation showed that the SOC between the S1 state and T4 state, ⟨S1|HSO|T4⟩, is as large as 11.5 cm−1 for the y component. This large SOC constant was proposed to justify the ISC ability.60 Triply bay-region fused di(perylenebisimide)s (DiPBIs) were prepared and were found to show efficient ISC (90%) and a triplet state lifetime ranging from 35 to 56 μs.61 Singlet oxygen quantum yield varied from 0.59 to 0.67. It was proposed that the twisted X-shaped configuration enhanced the SOC and hence allowed ISC, such as tetraphenoxyPBIs, but no detailed theoretical study was reported.61 Thionated PBI derivatives were also found to show efficient ISC;62 similar results were observed for sulfur-containing squaraines,63 2,4-dithiothymine,64 2,4-dithiouracil,65 and thioxanthone.66 Indeed, the ISC process may take only 0.88 ps to occur, the S1 → T1 transition being probably facilitated by the sulfur atoms, though no detailed mechanistic study was performed.62 In short, to the best of our knowledge, no heavy-atom-free PBI derivatives with a planar geometry have been reported to show efficient ISC. Herein, we prepared new PBI derivatives, with tetraphenylethynyl substituents at the 2,5,8,11-positions of the PBI core (ortho-positions, not the bay-positions of PBI), and efficient ISC was observed. The photophysical properties were studied in detail with steady-state and nanosecond− picosecond time-resolved transient absorption spectroscopy, and we found for some derivatives photoexcitation into the S2 state can enhance the ISC. Theoretical computations indicated large SOC for S1/S2 → Tn (n > 1) transitions, which is responsible for the efficient ISC of the new PBI derivatives. Photochemical reduction of the compound to produce persistent radical anion was investigated as well.

2. RESULTS AND DISCUSSION 2.1. Design and Synthesis of the Compounds. PBI derivatives with substituents at the bay-positions were extensively studied, but it is only more recently that PBI derivatives with substituents at the 2,5,8,11-positions of the PBI core (ortho-positions, not the bay-positions of PBI) were reported, such as the tetrabromo and the tetracyano derivatives.67,68 However, no PBI derivatives with substituents at the ortho-positions allowing the extension of the πconjugation framework of the PBI core were reported previously, and we first fill this gap; thus, we prepared new C


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Table 1. Luminescence Lifetime (τL) and Oscillator Strength ( f) of the Compounds λema [nm] (τLa [ns]) PBI-1

547 (5.34), 585 (5.16), 636 (5.20)

PBI-2

597 (7.91), 644 (7.75), 700 (12.66) 582 (6.64), 630 (6.70), 680 (12.21) 523 (3.95), 561 (4.02), 608 (4.14), 666 (4.80) 542 (1.80), 594 (39.09), 643 (43.04)

PBI-3 PBI-5 PBI-6

λabsb [nm] ( fc) 525 (0.1464), 490 462 (0.0633) 568 (0.1471), 527 494 (0.0422) 565 (0.1093), 524 491 (0.0338) 512 (0.1630), 477 447 (0.0593) 532 (0.1480), 492 465 (0.1638)

(0.1069), (0.1061), (0.0879), (0.1667), (0.2053),

a

In 2-methyltetrahydrofuran: methylcyclohexane = 1:1 (v:v), at 77 K (−196 °C). bMaximum absorption wavelength in toluene, at 20 °C. c = 1.0 × 10−5 M. cOscillator strength in toluene, at 20 °C.

other than the simple relaxation to the S1 state followed by radiative decay. Inspired by a recent report that S2 → T4 ISC is more efficient than the S1 → T1 ISC in a core substituted naphthalenediimide dye,71 we envisioned that the higher-lying ISCs are likely more efficient in both PBI-2 and PBI-6 than S1 → T1. As a proof of this postulate, the excitation wavelength dependencies of the singlet oxygen quantum yields of the compounds (ΦΔ) were determined (Table 2). For PBI-2, the ΦΔ value was determined to be 0.078 by excitation at 561 nm. Interestingly, the ΦΔ value increased to 0.355 with photoexcitation at 336 nm. Similar results were obtained for PBI-6 (Table 2). These results indicate that higher-lying ISCs are indeed more efficient than the S1 → T1 ISC for both PBI-2 and PBI-6 (see section 2.5). 2.3. Nanosecond Transient Absorption Spectroscopy: Triplet Excited State of the PBI Derivatives. The triplet excited states of the PBI derivatives were studied with the nanosecond transient absorption (TA) spectroscopy. Because PBI-5 and PBI-6 show the most significant 1O2 photosensitizing ability (see Table 2), the transient absorption (TA) spectra of these compounds were studied (Figure 5a−d). For PBI-5, ground-state bleaching bands at 475 and 510 nm were observed upon pulsed laser excitation; excited-state absorption (ESA) in the range of 520−650 nm was observed as well (Figure 5a). These features are typical of the triplet excited state of the PBI moiety.48,49,72 Similar evolutions of the main bands were obtained for PBI-6 (Figure 5c). The triplet state lifetime of PBI-5 was determined to be 10.4 μs (Figure 5b), whereas for PBI-6 the lifetime is 504.9 μs (Figure 5d). In comparison, the triplet state signal is much weaker for PBI-1, and only the decay curve can be determined but no satisfactory TA spectra can be recorded (see Figure S25). Recently PBI derivatives with aryl appends at the orthopositions were reported to show ISC, which was attributed to intramolecular charge transfer (ICT).73 We note that the aryl appends in those PBI derivatives are twisted against the πconjugation plane of the PBI chromophore, which is drastically different from the geometry of PBI-6. Moreover, for these compounds, the triplet state lifetime (30.6 μs) is shorter than that of PBI-6 (504.9 μs), although the triplet state quantum yield (ΦISC = 86%) is higher than that of PBI-6 (ΦISC = 51%, see Table 2). The triplet state quantum yield (ΦT) of compound PBI-6 was determined as 51.3%, using the ground-state bleaching method (see Experimental Section for details). This ΦT value of PBI-6 is similar to those reported for other heavy-atom-free

Figure 2. Photoluminescence spectra of the compounds in different solvents: (a) PBI-1 (λex = 480 nm), (b) PBI-2 (λex = 520 nm), (c) PBI-5 (λex = 470 nm), (d) PBI-6 (λex = 445 nm), (e) PBI-3 (λex = 510 nm), and (f) PBI-4 (λex = 355 nm). Optically matched solutions were used in each panel (each of the solutions gives the same absorbance at the excitation wavelength); 20 °C.

fluorescence. ISC may of course be one of such nonradiative decay channels, as proven by the nanosecond transient absorption spectra (see following section). The E00 values (the energy gap between the 0−0 vibrational energy levels of S0 and S1 states) of the PBI derivatives were estimated to be PBI-1 (2.31 eV), PBI-2 (2.13 eV), PBI-3 (2.15 eV), PBI-4 (1.72 eV), PBI-5 (2.39 eV), PBI-6 (2.30 eV), and PBI-7 (2.30 eV) by using the crossing point of the normalized UV−vis absorption and the respective fluorescence emission spectra. Interestingly, we observed different fluorescence lifetimes for the vibronic progression bands, especially at low temperature (77 K) (Table 1). For example, the fluorescence lifetime was determined to be 1.8 ns for PBI-6 at 542 nm (Figure 3a), but becomes much longer at both 594 and 643 nm (39−43 ns, Figure 3b,c). These different fluorescence lifetimes can be attributed to the different oscillator strengths of the emission bands (Table 1), which are dictated by the Franck−Condon principle, i.e., by selection rules for the transitions between the vibrational states accompanying the electronic transition. The fluorescence excitation spectra of compounds PBI-2 (Figure 4a) and PBI-6 (Figure 4b) were compared with their UV−vis absorption spectra. Normalizing the intensities in the long wavelength range, we found that the intensity in the short wavelength region of the excitation spectra is lower than its UV−vis counterpart. This result indicates that upon photoexcitation into higher-lying singlet excited state than S1, the energy of the excited state is dissipated through channel(s) D


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Figure 3. Fluorescence lifetime decay curve of PBI-6 at different wavelengths: (a) 542 nm, (b) 594 nm, and (c) at 643 nm (λex = 445 nm) in 2methyltetrahydrofuran: methylcyclohexane (1:1, v:v); c = 1.0 × 10−5 M, 77 K.

Figure 4. Comparison of the normalized UV−vis absorption and the fluorescence excitation spectra of the compounds: (a) PBI-2 (λem = 638 nm, A = 0.188 at 568 nm) and (b) PBI-6 (λem = 586 nm, A = 0.190 at 328 nm). In toluene; 20 °C.

PBI derivatives.61,73 It should be pointed out that this value is not always well approximated by the singlet oxygen quantum yields (ΦΔ).74,75 PBI-6 was used as a triplet photosensitizer to initiate intermolecular triplet−triplet energy transfer (TTET, Figure 6a−i and Figure S26a−i and Figure S27a−f).76−79 To this end, we selected B-1 as triplet acceptor, a choice justified by, on the one hand, its low T1 state energy level (1.06 eV, which is smaller than that of PBI-6, 1.34 eV), and on the other hand, its very red-shifted absorption band compared to PBI-6. These properties ensure an efficient intermolecular TTET and the feasible spectral discrimination for monitoring the evolution of the TA. Upon excitation of PBI-6 at 470 nm, the initial TA spectrum is equivalent to the previously described spectrum of PBI-6. As time goes, the features of the PBI-6 TA spectrum disappear and those of the B-1 TA spectrum start to develop

Figure 5. Nanosecond transient absorption spectra of compounds (a) PBI-5 and (c) PBI-6. Decay curves of (b) PBI-5 at 512 nm (λex = 470 nm) and (d) PBI-6 at 532 nm (λex = 530 nm) are presented. c = 1.0 × 10−5 M in deaerated toluene; 20 °C.

(Figure 6a,d), as indicated by the bleaching of the band at 628 nm and in the 650−800 nm range. Photoexcitation of B-1 alone in a separate experiment shows that no triplet state can be produced. The intermolecular TTET was monitored with the transient change at two specific wavelengths (532 and 628 nm). The

Table 2. Photophysical Properties of the Compounds λabsa [nm] (εb [×104 M−1 cm−1]) PBI-1 PBI-2 PBI-3 PBI-4 PBI-5 PBI-6 PBI-7

490 310 337 353 477 328 335

(3.73), 525 (5.57) (4.55), 527 (2.99), 568 (4.70) (4.59), 524 (2.45), 565 (3.85) (6.42), 492 (2.86), 661 (2.68) (4.59), 512 (6.67) (11.41), 465 (4.80), 492 (6.01), 532 (7.33) (9.43), 463 (3.28), 494 (3.50), 532 (4.55)

λema [nm] (τFc [ns]) 547 595 588 791 527 544 545

(4.89), (7.10), (6.76), (1.45) (2.64), (0.38), (0.77),

585 (4.90) 638 (7.15) 632 (6.81) 563 (2.62), 608 (2.63) 586 (0.54), 637 (0.55) 588, 638

ΦF [%] d

ΦΔ [%]/ΦTk [%] f

84.5 , 75.9 89.6e, 88.7f 80.8e, 80.2f 5.2f 56.1d, 42.2f 7.0d, 3.5f 11.6d, 8.0f

g

22.6 7.8g, 35.5h j j 84.6g 56.9g, 66.6h/51.3k j

τTi 410 ns j j j 1.04 μs 504.86 μs j

In toluene (c = 1.0 × 10−5 M). bMolar absorption coefficient. cFluorescence lifetimes of each emission peak (λex = 445 nm). c = 1.0 × 10−5 M, in toluene. dRelative fluorescence quantum yields in toluene. Bodipy (BDP, see Scheme S1 for molecular structure. ΦF = 72% in THF) was used as standard for PBI-1, PBI-5, PBI-7 (λex = 470 nm), and PBI-6 (λex = 458 nm). eRelative fluorescence quantum yields in toluene. Rhodamine B (ΦF = 65% in EtOH) was used as standard for PBI-2 and PBI-3 (λex = 515 nm). fAbsolute fluorescence quantum yields in toluene (c = 5.0 × 10−6 M). g Singlet oxygen (1O2) quantum yields in toluene. For PBI-1 and PBI-5, λex = 518 nm; for PBI-2, λex = 561 nm; for PBI-6, λex = 538 nm. Rose bengal (RB; ΦΔ = 80% in MeOH) was used as standard. hSinglet oxygen (1O2) quantum yields in toluene. λex = 336 nm. Anthracene (ΦΔ = 70% in MeOH) was used as standard. iTriplet state lifetimes. c = 1.0 × 10−5 M, in deaerated toluene. jNot determined. kTriplet states quantum yield in toluene, with 2,6-diiodoBodipy (B-2, see Scheme S1 for molecular structure. ΦT = 88% in toluene) as the standard, λex = 532 nm. a

E


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Figure 6. Study of the intermolecular triplet−triplet energy transfer by using nanosecond transient absorption spectra of compounds PBI-6 and B-1. (a) Transient absorption spectra of the mixture with molar ratio (PBI-6/B-1) of 1:0.125, decay curves at (b) 532 nm and (c) 628 nm (λex = 470 nm); (d) transient absorption spectra of the mixture with molar ratio of 1:0.375 (PBI-6/B-1), and the decay curves at (e) 532 nm and (f) 628 nm (λex = 470 nm); (g) transient absorption spectra of the mixture with molar ratio of 1:0.625 (PBI-6/B-1), decay curves at (h) 532 nm and (i) 628 nm (λex = 470 nm). [PBI-6] = 1.0 × 10−5 M; in deaerated toluene at 20 °C.

decay of the triplet excited state of PBI-6 was monitored at 532 nm, and we found that its lifetime was shortened in the presence of B-1. Moreover, increasing the B-1 concentration reduces the triplet state lifetime of PBI-6, clearly indicating more efficient TTET. The evolution of the triplet state of B-1 was monitored at 628 nm. Interestingly, we observed complicated kinetics. First, there is a sharp decreasing stage, indicating the formation of the triplet state of B-1, which gives a ground-state bleaching band at 628 nm. We underline that both the decay of the PBI-6 triplet excited state (either intrinsically or by intermolecular TTET) and the formation of the triplet state of B-1 will cause a decrease of the transient absorption at 628 nm, so that the first sharp decrease of the transient absorption at 628 nm shows a kinetics which is roughly twice that monitored at 532 nm (where the decay of the PBI-6 triplet state only is followed) (Figure 7). For example, with the PBI6/B-1 molar ratio of 1:0.125, the decay time of the transient bands at 532 and 628 nm are 240.69 and 125.63 μs, respectively. Similar results were observed for the PBI-6/B-1 molar ratio of 1:0.375, for which the two lifetimes are 135.35 and 64.0 μs, respectively. See Table 3 for more details. The triplet state intermolecular energy-transfer rate constant, kET, and the triplet state intermolecular energy transfer efficiency ΦET can be calculated by eqs 1 and 2, respectively.80 kET = (1/τ ) − (1/τ0)

Figure 7. Nanosecond transient absorption spectra of the mixtures of PBI-6 and B-1 at different delay times (0 and 266.4 μs). Molar ratio of PBI-6/B-1 is 1:0.375 (λex = 470 nm). [PBI-6] = 1.0 × 10−5 M; in deaerated toluene at 20 °C.

ΦET = 1 − τ /τ0

(2)

where τ0 is the triplet state lifetime of the triplet energy donor in the absence of an energy acceptor and τ is the triplet state lifetime of the triplet energy donor in the presence of an energy acceptor. The slope of a Stern−Volmer plot is the Stern−Volmer quenching constant KSV (Figure 8). KSV was calculated as 7.40 × 105 M−1.

kq = KSV /τ0

(1) F

(3) DOI: 10.1021/acs.jpcc.6b01584 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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2.4. Femtosecond Transient Absorption Spectroscopy: Intersystem Crossing and Excitation WavelengthDependent Photophysics. Ultrafast pump−probe experiments were performed for PBI-1, PBI-2, PBI-5, and PBI-6. Pump wavelengths were chosen based on their maximum absorption bands, and white light continuum was used as probe beam. To study the excitation wavelength-dependent photophysical process of compounds PBI-2 and PBI-6, we pumped both compounds at 350 nm, which is the highest-energy absorption band for both compounds. Ultrafast pump probe experiments of PBI-5 were performed at 512 nm (see Figure S28a). Bleaching bands at ca. 450, 480, and 515 nm were observed, that is, at wavelengths where PBI-5 gives steady-state absorption bands. An extra bleaching signal was observed at 565 nm, which cannot be seen on absorption spectra. Because fluorescence spectra of this compound give a signal at ca. 565 nm, we attributed this additional signal to stimulated emission (see Figure S28a). Furthermore, the positive signals seen above 590 nm and below 413 nm can be ascribed to excited-state absorption. Another positive signal appears at 544 nm after a ca. 700 ps time delay. This signal can be assigned to T1 → Tn transition with an ISC rate constant of 1.4 × 109 s−1 (see Figure S28b). We also performed ultrafast pump probe experiment with a 532 nm excitation for PBI-6 (Figure 9a). In the line of PBI-5,

Table 3. Kinetic Parameters of the Intermolecular Triplet State Energy Transfer between Compounds PBI-6 and B-1a molar ratio PBI6:B-1b

τ532 nmc (μs)

τ628 nmd (μs)

τ628 nme (μs)

kETf (s−1)

ΦETg (%)

PBI-6 only 1:0.125 1:0.25 1:0.375 1:0.5 1:0.625 1:0.75 1:0.875

555.14 240.69 177.34 135.35 111.71 102.73 87.12 75.42

−h 125.63 82.40 64.00 53.79 46.36 41.59 36.67

580.05 818.81 717.19 700.62 572.06 527.66 457.49 453.30

−h 2.35 3.84 5.59 7.15 7.93 9.68 1.15

−h 56.6 68.1 75.6 79.9 81.5 84.3 86.4

× × × × × × ×

3

10 103 103 103 103 103 105

a

Determined with nanosecond time-resolved transient difference absorption spectroscopy (λex = 470 nm). b[PBI-6] = 1.0 × 10−5 M in deaerated toluene. cTriplet state lifetime at 532 nm. dTriplet state lifetime of the rising stage of decay curves at 628 nm. eTriplet state lifetime of the decline stage of decay curves at 628 nm. fTriplet state intermolecular energy-transfer rate constant. gTriplet state intermolecular energy-transfer efficiency. hNot applicable.

Figure 8. Stern−Volmer plot for the quenching of the triplet state lifetime of PBI-6 with increasing concentration of B-1. λex = 470 nm; [PBI-6] = 1.0 × 10−5 M; in deaerated toluene at 20 °C.

where kq is the bimolecular quenching constant. According to eq 3, kq was calculated as 1.33 × 109 M−1 s−1. The quenching efficiency, f Q, was determined as fQ = kq /k 0

Figure 9. Femtosecond transient absorption spectra of PBI-6. (a) Spectra at different delay times (λex = 532 nm). (b) Decay curves at 587 and 750 nm (λex = 532 nm). c = 1.0 × 10−5 M in deaerated toluene at 20 °C.

(4)

PBI-6 showed bleaching signals around 465, 495, and 535 nm wavelengths, which were attributed to ground-state bleaching signals. The extra bleach signal at 585 nm was attributed to stimulated emission consistently with PBI-5 (Figure 9a). Additionally, ESA signals (above 600 nm and below 445 nm) and T1 → Tn transition at 587 nm were also observed in the transient absorption spectra. By analyzing the decay curve at 750 nm, we determined an ISC rate of kISC = 3.6 × 109 s−1 for PBI-6 (Figure 9b), which is 2.5 times faster than that for PBI-5. We repeated the experiments for compound PBI-6 with pumping at 350 nm to assess the influence of the excitation wavelength on the photophysical processes (see Figure S29a). Behaviors similar to that obtained with the 532 nm pumping are obtained, but for a slightly slower ISC rate (kISC = 3.2 × 109 s−1). Nevertheless, the ISC kinetics is much slower than for the previously reported thionated PBI derivatives (ISC process is with rate constant of kISC = 1.1 × 1012 s−1).62 The decay curves of T1 → Tn transitions at 587 nm wavelengths of these two experiments are given in Figure S29b. Unfortunately, we could not obtain ISC rate for compounds PBI-1 and PBI-2, because of limited range of time delay of our experimental setup to observe triplet−triplet transitions (see Figures S30 and S31a,b).

where k0 is the diffusion−controlled bimolecular quenching rating constant that can be calculated with the Smoluchowski equation:81 k 0 = 4πRND/1000 =

4πN (R f + R q)(Df + Dq ) 1000

(5)

where D is the sum of the diffusion coefficients of the energy donor (Df) and quencher (Dq), N Avogadro’s number, and R the collision radius, the sum of the molecule radii of the energy donor (Rf) and the quencher (Rq) that were determined by DFT optimization of the geometry to be 10.8 Å for the PBI-6 donor and 8.7 Å for the B-1 quencher. The diffusion coefficients can be obtained from Stokes−Einstein equation:81 D = kT /6πηR

(6)

with k the Boltzmann’s constant, η the solvent viscosity, and R the molecule radius. According to eq 6, the diffusion coefficients of the energy donor (PBI-6) is 3.40 × 10−6 cm2 s−1 and that of the quencher (B-1) is 4.19 × 10−6 cm2 s−1 (in deaerated toluene at 20 °C). Thus, k0 was calculated to be 1.12 × 1010 M−1 s−1, which led to a quenching efficiency of 11.9% by applying eq 4. G


Article

The Journal of Physical Chemistry C 2.5. Theoretical Computations: Rationalization of the Efficient ISC and the Excitation Wavelength-Dependent ISC. We have performed B3LYP/6-31G(d) calculations (see details in the Experimental Section) on compounds PBI-1, PBI-2, PBI-5, and PBI-6 to rationalize their ISC mechanisms. In the weak coupling regime, the ISC rate (kISC) between the nsinglet (Sn) and m-triplet (Tm) excited states of an organic molecule can be expressed by the Fermi golden rule approximation82 2π kISC = ⟨Sn|Ĥ SO|Tm⟩2 × [FCWD] (7) ℏ where FCWD stands for the Franck−Condon weighted density of states and the bracket term accounts for their associated SOCs matrix elements (SOCMEs). The calculations of ISC rate require (i) the assignment of all energetically accessible ISC photodeactivation channels upon photon absorption and (ii) the calculation of the magnitudes in eq 7. Usually, it is found that two main actors, i.e., substantial electronic SOC and small Tm−Sn energy gaps, govern the efficiency of the ISC process.82 Recently, it was shown for several organic dyes that vibronic SOC involving higher-lying singlet/triplet excited states may also effectively enhance the ISC rates.83 Computing all parameters of eq 7 becomes rapidly prohibitive for large molecules so that semiquantitative strategies are often used to rationalize the efficiency of ISC processes.84−86 Herein, we report theoretical estimates of relative Tm−Sn energy gaps and SOCMEs to rationalize the efficient ISC mechanisms occurring in these PBI derivatives. Particular focus is put on (i) disentangling the excitation wavelength-dependent ISC occurring on PBI-2 and (ii) rationalizing the efficient ISC mechanisms in PBI-6. Table 4 lists the TD-B3LYP/6-31G(d) vertical excitation energies for the bromine-substituted PBI compounds (PBI-1

previously proven successful for the excited-state and spin− orbit properties of other perylenebisimide derivatives.60 PBI-1, in contrast to PBI-5, presents a nonplanar ground-state optimized geometry. Due to the heavy-atom effect, these derivatives are in principle prone to efficient ISC processes. As seen in Table 4, for PBI-1, only the S1−T1 ISC mechanism is energetically accessible upon excitation to the S1 band. The S1− T1 ISC mechanism is characterized by a large energy gap (>1 eV) and non-negligible SOCMEs (values up to 2 cm−1, see Table 4). Therefore, under these circumstances, the large S1− T1 energy gap in PBI-1 prevents a very efficient ISC process (in agreement with the experimental evidence, see its moderate quantum yield of triplet formation in Table 2). In contrast, upon excitation of the S1 band in PBI-5, further S1−Tm ISC mechanisms are energetically accessible, i.e., S1 → T2 and S1 → T3 (see Table 4). These processes are characterized by very low energy gaps and very large SOCMEs (up to 79 cm−1) and thus very efficient ISCs. Indeed, the largest quantum yield of triplet formation among all the PBI derivatives is obtained for PBI-5 (again, see Table 2). As stated above, the bromine-free derivatives, i.e., PBI-2 and PBI-6, also exhibit ISC processes. This deserves further investigation. In Table 5 we collect the main vertical singlet Table 5. Main Lowest Vertical (at S0 Geometry) Singlet and Triplet Electronic Transition Energies (in Electronvolts) and Oscillator Strengths (in Parentheses) of PBI-2 and PBI6 at the TD-B3LYP/6-31G(d) Level of Theorya

PBI-2

Table 4. Lowest Vertical (at S0 Geometry) Singlet and Triplet Electronic Transition Energies (in Electronvolts) and Oscillator Strengths (in Parentheses) of PBI-1 and PBI5 at the TD-B3LYP/6-31G(d) Level of Theorya state/ character PBI-1

S1/ππ*

PBI-5

T1/ππ* T2/ (nπ)π* S1/ππ* T1/ππ* T2/nπ* T3/nπ*

TD-B3LYP/631G(d)b 2.37 (0.517)/ 2.10 (0.492) 1.30/0.92 2.59/2.41 2.54 (0.616)/ 2.32 (0.616) 1.43/1.14 2.42/2.22 2.43/2.30

ΔE (S1− Tm)vert

c

SOCMEs (S1/Tm) (x-; y-; z-components)

PBI-6

− 1.07 −0.22

(−2.1; 0.0; 0.0) (−0.3; 25.3; 4.3)

−

−

1.11 0.12 0.11

(0.0; 0.0; 0.0) (−1.4; −0.5; 0.0) (0.0; −79.0; 0.0)

state/ character

TD-B3LYP/631G(d)

SOCMEs Sn/Tmb (x-; y-; zcomponents)

S1/ππ* S3/ππ* T1/ππ* T2/ππ*

2.02 (0.505) 2.82 (0.268) 1.12 2.09

T3/ππ* T4/ππ*

2.32 2.63

T5/ππ*

2.73

S2/ππ* S3/ππ* T1/ππ* T2/ππ* T3/ππ* T4/ππ* T5/ππ*

2.33 (0.575) 2.35 (0.384) 1.34 1.86 1.95 1.95 2.39

− − S1−3/T1: (0.0; 0.0; 0.0) S1T2: (0.0; −0.1; 0.0)/S3T2: (−0.2; 0.1; 0.0) S1−3/T3: (0.0; 0.0; 0.0) S1/T4: (0.0; 0.0; 0.0)/S3/T4: (−0.1; −0.1; 0.0) S1/T5: (−0.3; −0.1; 0.0)/S3/T5: (−0.1; −0.1; 0.0) − − S2−3/T1: (0.0; 0.0; 0.0) S2−3/T2: (0.0; 0.0; 0.0) S2−3/T3: (0.0; 0.0; 0.0) S2−3/T4: (0.0; 0.0; 0.0) S2−3/T5: (0.0; 0.0; 0.0)

a

The character of the states is indicated. The main SOCMEs (in reciprocal centimeters) between the involved Tm and Sn states are also given. bValues obtained at the QR-TD-DFT/6-31G(d) level of theory at the S0 optimized geometry

a

The character of the states is indicated. Vertical singlet−triplet splittings (in electronvolts) and SOCMEs (in reciprocal centimeters) between the involved Tm and S1 states are also given. bData in italics correspond to values of the same excited states obtained at the S1 (rather than S0) optimized geometries. cValues obtained at the QRTD-DFT/6-31G(d) level of theory at the S0 optimized geometry.

and triplet excited states of PBI-2 and PBI-6 along with the main SOCMEs for the main ISC mechanisms. Overall, as expected, SOCMEs are very small as compared to the brominesubstituted derivatives (only up to 0.3 cm−1, see Table 5). It is noteworthy that SOCMEs values between 0.2 and 5.0 cm−1 are considered large enough to induce ISC on a nanosecond time scale.87 In the case of PB1−2, upon excitation of the S1 band, the most likely ISC mechanism in view of the vanishing energy gap and the non-negligible SOCME values is the S1 → T2 ISC. The observed low singlet oxygen quantum yield (7.8% in Table 2) is in accordance with the very small SOCMEs figures. This

and PBI-5). Time-dependent density functional theory (TDDFT) locates the S1 absorption band of PBI-1 and PBI-5 at 2.37 and 2.54 eV, respectively, in reasonable agreement with the experimental data (2.53 and 2.60 eV, respectively). In this regard, TD-DFT using the B3LYP functional has been H


Article


Figure 10. Out-of-plane low-frequency vibrational modes of PBI-6 (calculated at B3LYP/6-31G(d) levels).

relationship between the ISC property and the molecular structures are known to date. The presence of n-π* transition (small electron exchange, J, values) and heavy atom effect (strong spin−orbital coupling) are normally responsible for ISC, but as shown above one can have ISC between S1 and Tn state (n > 1). There are no easy principles to predict an energy matching between the S1 and Tn states. 2.6. Photochemical Studies: Observation of the Radical Anion of PBI-6 in the Presence of Sacrificial Electron Donor. Radical ions are interesting because they can be used as powerful redox agents once photoexcited.91 PBI is electron deficient, and its one electron reduction product, that is, its radical anion, was previously obtained by an electrochemical method91,92 or in the presence of reductant, such as dithionite Na2S2O493,94 or n-tetrabutylammonium cyanide.93−95 We note that, in the photochemical reactions by intramolecular photoinduced electron transfer, the PBI radical anion is shortlived, and it decays rapidly by charge recombination (within ca. 250 ns),96 though even much faster processes have been reported (within 50 ps).91 Herein, we studied the formation of persistent radical anion of PBI-6 by a photochemical method, i.e., by photoirradiation of the mixture of PBI-6 and a sacrificial electron donor triethanolamine (TEOA). PBI-6 shows UV−vis absorption below 550 nm. The structured absorption and the relative intensity of the vibronic progression indicated that no aggregation takes place in the solution.97 Upon photoirradiation (532 nm) of a mixture of PBI-6 and TEOA, near-IR absorption bands at 783, 877, and 1078 nm developed (Figure 11a). On the basis of the previous studies, these near-IR absorption bands can be attributed to the absorption of the radical anion of PBI (2[PBI-6]•−), or more precisely, to the D0 → Dn transitions. These near-IR absorption bands disappeared in the presence of air, and the formation of these near IR bands was reproduced again in the presence of TEOA and photoirradiation (Figure 11b). No dianions, [PBI6]2−, were formed, which would be indicated by a decrease of the radical anion absorption.97 To the best of our knowledge, this is the first report of persistent PBI radical anions formed with a photochemical reduction method. Similar changes were observed with reductant dithionite Na2S2O4 (see Figure S32a,b). For other PBI derivatives, without significant ISC ability, such photochemical reduction was not observed, i.e., no stable radical anion was observed. The nanosecond transient absorption spectra of PBI-6 upon formation of radical anion demonstrated that the triplet state lifetime of PBI-6 was greatly reduced as compared with that of PBI-6 alone. Moreover, the transient absorption spectra acquired upon photoexcitation at 780 nm did not produce any signal. These results indicate that the triplet state of PBI-6 is probably quenched by the radical anion (doublet state). Moreover, the excited state of the radical anion of PBI-6

evidence is in agreement with other heavy-atom-free PBI derivatives bearing a nonplanar ground-state geometry (as is the case of PBI-2). Furthermore, PBI-2 exhibits excitation wavelength-dependent ISC. Upon irradiation with a higher energy photon (λex = 336 nm), the S3 state can be populated (because the S2 is dark in nature) and new ISC processes are opened up. As shown in Table 5, the S3 is energetically aligned with T4 and T5. Additionally, slightly larger SOCME values between these states are obtained (as compared to those between S1 and T2). Therefore, the availability of the S3 → T4 and S3 → T5 paths is the reason for the ca. 4-fold increase (up to 35.5%, see Table 2) of triplet formation with PBI-2 upon excitation with a higher-energy photon. Finally, PBI-6, which is characterized by a moderate triplet formation quantum yield (51.3%) was also investigated. Importantly, PBI-6 presents a planar ground-state geometry. As can be seen in Table 5, the electronic SOCMEs between all possible energetically aligned singlet (we remark that the first bright excited state is S2, see Table 5) and triplet excited states are all close to 0 cm−1; therefore, the direct SOC mechanism must be disregarded for PBI-6. Recently, it was shown for several organic dyes that the main mechanism responsible for their enhanced ISC processes is not the direct electronic one, but through vibronic SOCs. To confirm this hypothesis for PBI-6, we have analyzed several out-of-plane low-frequency modes leading to nonplanar structures. These modes are represented in Figure 10. We note that their associated frequencies are very small (e.g., 3.3 and 12.8 cm−1), so that their upper vibrational states will be highly populated at room temperature (at room temperature, kT ≈ 200 cm−1). We have computed the SOCMEs at geometries slightly distorted along these modes. The SOCME values at these geometries are considerably larger than at the ground-state geometry (e.g., SOCME(S2/T5) = 0.2 cm−1, and up to 14 cm−1 for other higher-lying Sn and Tm states). Thus, considering this evidence, we can confirm that for PBI-6, their ISC mechanisms gain intensity through spin-vibronic couplings. In short, PBI-6 stands as the first proof that planar, heavy-atom-free PBI derivatives may lead to efficient ISC processes. Recently, PBI derivatives with aryl appends at the ortho-positions were reported to show ISC, which was attributed to intramolecular charge transfer (ICT).73 The ISC mechanism is different from those previously reported PBI derivatives. There are also other examples in which Tn states can induce ISC, such as nitronaphthalene88,89 and naphthalenediimide derivatives.71 The S2 state was also reported to be responsible for ISC, such as for m-xylene90 and naphthalenediimide derivatives.71 Generally, a simple relationship relates the f luorescence and the molecular structures, such as the rigidity, planarity, and size of the π-conjugation of the chromophores known to influence directly the emission yields. However, no straightforward I


Article


It is noteworthy that the PBI-6 derivative does not contain any heavy atoms, such as bromine or iodine atoms. The photophysical properties of all compounds were studied in detail with steady-state and nanosecond−picosecond timeresolved transient absorption spectroscopies. We found the PBI derivative (PBI-2) with bisphenylethynyl substituents at the 1,7-positions of the PBI core (the bay positions of PBI) showed less efficient ISC. Interestingly, excitation wavelength-dependent ISC was observed in PBI-2 (ΦΔ = 7.8% with excitation at 561 nm, and ΦΔ = 35.5% with excitation at 336 nm). The ISC rate constant of PBI-6 was determined to be 3.6 × 109 s−1. In view of the TD-DFT results, the efficient ISC of PBI-6 occurs through vibronic spin−orbit couplings. The excitation wavelength-dependent ISC of PBI-2 was attributed to the presence of S3 state (2.82 eV), which lies slightly above the T4 (2.63 eV) and T5 (2.73 eV) states, along with their non-negligible SOCME values.

4. EXPERIMENTAL SECTION 4.1. General Methods. All the chemicals used in syntheses are analytical pure and were used as received. Fluorescence quantum yields were measured in toluene with BDP (ΦF = 0.72 in tetrahydrofuran) and rhodamine B (ΦF = 0.65 in ethanol) as standards. Singlet oxygen quantum yields were measured in toluene with rose bengal (ΦΔ = 0.80 in methanol) and anthracene (ΦΔ = 0.70 in methanol) as standards. Triplet quantum yield was measured in toluene with B-2 (ΦT = 0.88 in toluene) as standard. The structures of BDP and B-2 are shown in Scheme S1. The fluorescence lifetimes of the compounds were measured with an EPL picosecond pulsed laser (445 nm; pulse width. 66.9 ps; maximum average power, 5 mW; Edinburgh Instrument Ltd., U.K.) which was synchronized to the OB920 spectrofluorometer. The fluorescence (fluorescence emission) was recorded with a FS5 spectrofluorometer (Edinburgh Instrument Ltd., U.K.). General method to synthesize PBI-2−PBI-4, PBI-6, and PBI-7. BromoPBI (0.05 mmol) and acetylide compound (0.4 mmol) was dissolved in 15 mL of dried (iPr)2NH under Ar. (PPh3)4Pd (5.7 mg, 0.005 mmol) and CuI (0.95 mg, 0.005 mmol) were added before the mixture was stirred for 8−12 h at 40 °C. After the reaction was finished, the solvent was removed under reduced pressure. The residue was purified by column chromatography (silica gel). PBI-2. Yield: 87%. 1H NMR (400 MHz, CDCl3): 10.13− 10.09 (m, 2H), 8.89−8.87 (m, 2H), 8.76−8.72 (m, 2H), 7.68− 7.65 (m, 4H), 7.50−7.46 (m, 6H), 5.51−5.04 (m, 2H), 2.37− 2.24 (m, 4H), 2.02−1.91 (m, 4H), 0.95 (t, 12H, J = 14.9 Hz). MALDI−TOF−HRMS ([C 50 H 38 N 2 O 4 ] −): calcd m/z = 730.2832; found m/z = 730.2845. PBI-3. Yield: 78%. 1H NMR (400 MHz, CDCl3): 9.98−9.95 (m, 2H), 8.92 (s, 2H), 8.77−8.75 (m, 2H), 7.79−7.73 (m, 8H), 5.13−5.06 (m, 2H), 2.35−2.23 (m, 4H), 2.02−1.91 (m, 4H), 0.94 (t, 12H, J = 7.4 Hz). MALDI−TOF−HRMS ([C52H36N4O4]−): calcd m/z = 780.2737; found m/z = 780.2730. PBI-4. Yield: 56%. 1H NMR (400 MHz, CDCl3): 10.22− 10.18 (m, 2H), 8.87−8.70 (m, 4H), 7.55 (d, 4H, J = 8.9 Hz), 6.76 (d, 4H, J = 8.7 Hz), 5.15−5.05 (m, 2H), 3.08 (s, 12H), 2.36−2.25 (m, 4H), 2.00−1.90 (m, 4H), 0.97−0.93 (m, 12H). MALDI−TOF−HRMS ([C 54 H 48 N 4 O 4 ] −): calcd m/z = 816.3676; found m/z = 816.3685. PBI-6. Yield: 92%. 1H NMR (400 MHz, CDCl3): 8.73−8.64 (m, 4H), 7.84−7.82 (m, 8H), 7.46−7.45 (m, 12H), 5.28−5.20

Figure 11. (a) UV−vis absorption spectra and (c) emission spectra of PBI-6 in the presence of TEOA in air and after irradiation by 532 nm Xe lamp for 15 min in N2. The reversibility of the formation of [PBI6]−• shown by (b) absorption at 783 nm and (d) emission at 540 nm. [PBI-6] = 1.0 × 10−5 M; [TEOA] = 1.0 × 10−2 M; in THF; 20 °C.

(*2[PBI-6]•−) is short-lived, and no intersystem crossing to the quartet state occurs.91 The short life of *2[PBI-6]•− may be due to the small energy gap of D1 and D0 states, making the nonradiative D1 → D0 internal conversion particularly efficient.91 To unambiguously confirm the formation of PBI radical anion, the electron spin resonance (ESR) spectrum of the solution of PBI-6 was recorded (Figure 12). The ESR signal

Figure 12. EPR spectra of the radical anions generated from PBI-6 in the presence of TEOA after irradiation by 532 nm laser for 15 min in deaerated solution. [PBI-6] = 1.0 × 10−5 M; [TEOA] = 1.0 × 10−2 M; in THF; 20 °C.

proves the existence of free radicals. The g factor is g = 2.003. These results are similar to the previously reported PBI radical formed by the chemical reduction by Na2S2O4.93

3. CONCLUSIONS In summary, we described a series of new PBI derivatives. Among the new perylenebisimide chromophores synthesized here, PBI-6, bearing tetraphenylethynyl substituents at the 2,5,8,11-positions of its core (ortho-positions, not the baypositions of PBI) was shown to provide a very efficient intersystem crossing (ΦT = 51.3% at an excitation of 532 nm). J


Article


4.6. Theoretical Computations. The geometries of the singlet ground state (S0) of PBI-1, PBI-2, PBI-5, and PBI-6 were optimized at the B3LYP/6-31G(d) level of theory. Gasphase TD-B3LYP vertical singlet and triplet excitation energies were obtained at this geometry using the same atomic basis set as in the optimizations. The Gaussian09 program package was used for these calculations.99 SOCs were computed using the quadratic-response TD-DFT approach,100,101 QR-TD-DFT, as implemented in the Dalton program102 at the S0 optimized geometry using the same exchange-correlation functional and basis set as in the geometry optimizations.

(m, 2H), 2.34−2.23 (m, 4H), 2.01−1.90 (m, 4H), 1.42−1.23 (m, 40H), 0.83−0.80 (m, 12H). MALDI−TOF−HRMS ([C86H86N2O4]−): calcd m/z = 1210.6588; found m/z = 1210.6580. PBI-7. Yield: 74%. 1H NMR (400 MHz, CDCl3): 8.78−8.69 (m, 4H), 7.89 (d, 8H, J = 8.3 Hz), 7.75 (d, 8H, J = 8.3 Hz), 5.26−5.19 (m, 2H), 2.29−2.22 (m, 4H), 2.01−1.91 (m, 4H), 1.30−1.22 (m, 40 H), 0.86−0.80 (m, 12H). MALDI−TOF− HRMS ([C90H82N6O4]−): calcd m/z = 1310.6398; found m/z = 1310.6388. 4.2. Singlet Oxygen Quantum Yield (ΦΔ ). 1,3Diphenylisobenzofuran (DPBF) was used as 1O2 scavenger for monitoring the 1O2 production by following the absorbance of DPBF at 414 nm. To determine the singlet oxygen quantum yield, a comparative method was used and was calculated according to eq 8: Φsam

2 ⎛ 1 − 10−Astd ⎞⎛ msam ⎞⎛ ηsam ⎞ ⎟⎟ = Φstd⎜ ⎟⎜⎜ ⎟⎜ ⎝ 1 − 10−Asam ⎠⎝ mstd ⎠⎝ ηstd ⎠

■

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01584. Experimental procedures, molecular structure characterization and additional spectra (PDF)

■

(8)

In the above equation, sam and std indicate the sample and standard, respectively. Φ, A, m, and η represent the singlet oxygen quantum yield, absorbance at excitation wavelength, slope of the absorbance of DPBF changing over time, and refractive index of the solvent used for measurement, respectively. Optically matched solutions were used (the solutions of the sample and the standard should give the same absorbance at the excitation wavelength). 2,6-DiiodoBodipy (B-2, see Scheme S1 for the molecular structure) was used as standard (ΦT = 88% in toluene, λex = 532 nm). 4.3. Triplet State Quantum Yield (ΦT). To determine the triplet state quantum yield, a ground-state bleaching method was used, and the yield was calculated according to98 ⎛ ε ⎞⎛ ΔA sam ⎞ Φsam = Φstd⎜ std ⎟⎜ ⎟ ⎝ εsam ⎠⎝ ΔA std ⎠

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +8641184986236. E-mail: [email protected] (J.Z.). *Tel.: +496131-379350. E-mail: [email protected] (C.L.). *Tel.: +903122126720. E-mail: [email protected] (M.H.). *Tel.: +33251125564. E-mail: [email protected] (D.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSFC (21273028, 21473020, and 21421005), Ministry of Education (SRFDP-20120041130005), the Fundamental Research Funds for the Central Universities (DUT14ZD226), Program for Changjiang Scholars and Innovative Research Team in University [IRT_132206], and Dalian University of Technology for financial support (DUT2013TB07). D.E. acknowledges the European Research Council (ERC, Marches-278845) and the Région des Pays de la Loire for his postdoctoral grant. D.J. acknowledges the European Research Council (ERC) and the Région des Pays de la Loire for financial support in the framework of a Starting Grant (Marches-278845) and the LUMOMAT project, respectively. This research used resources of (1) the GENCI-CINES/ IDRIS, (2) CCIPL (Centre de Calcul Intensif des Pays de Loire), and (3) a local Troy cluster.

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In the above equation, sam and std indicate the sample and standard, respectively. Φ, ε, and A represent the triplet quantum yield, molar absorption coefficient at steady state, and optical intensity of bleaching band in nanosecond transient absorption spectroscopy, respectively. Optically matched solutions were used (the solutions of the sample and the standard should give the same absorbance at the excitation wavelength). 4.4. Electron Spin Resonance Spectroscopy. ESR spectra were recorded at room temperature using a Bruker ESP-300E spectrometer at 9.4 GHz, X-band, with 100 kHz field modulation. Sample was irradiation by 532 nm laser for 15 min in N2 before being quantitatively injected into specially made quartz capillaries for ESR analysis. 4.5. Femtosecond Transient Difference Absorption Spectroscopy. The ultrafast wavelength-dependent pump− probe spectroscopy measurements were performed using a Ti:sapphire laser amplifier−optical parametric amplifier system (Spectra Physics, Spitfire Pro XP, TOPAS) and commercial experimental setup (Spectra Physics, Helios). Pulse duration was measured as 100 fs inside the pump−probe experimental setup via cross correlation. Wavelengths of the pump beam were chosen according to the steady-state absorption spectra of studied compounds. White light continuum was used as a probe beam.

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REFERENCES

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