Aggregation-Induced Enhancement of Molecular Phosphorescence

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C: Energy Conversion and Storage; Energy and Charge Transport

Aggregation-Induced Enhancement of Molecular Phosphorescence Lifetime: A First-Principle Study Jinxiao Zhang, Edward Sharman, Li Yang, Jun Jiang, and Guozhen Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07087 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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The Journal of Physical Chemistry

Aggregation-Induced Enhancement of Molecular Phosphorescence Lifetime: A First-Principle Study Jinxiao Zhang†, Edward Sharman‡, Li Yang†, Jun Jiang†, Guozhen Zhang*†

†Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials, CAS Center for Excellence in Nanoscience, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China

‡Department of Neurology, University of California, Irvine, California 92697, USA

ACS Paragon Plus Environment

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ABSTRACT: Pure organic phosphorescent molecules are promising compounds for applications of phosphorescence, yet their utilization are restricted due to inefficient intersystem crossing (ISC) between singlet and triplet states. Molecular aggregation has been deemed as a viable strategy to modulate molecular luminescence in solution, yet their impact on the phosphorescence is rarely investigated. In this work, we carried out first-principle studies to elucidate how aggregation of selected phosphorescent molecules will affect their phosphorescence behaviors. Our calculations show that the overall ISC rate is appreciably enhanced thanks to the decrease of energy gaps (ΔE) and the increase of ISC channels between singlet and triplet states as the degree of aggregation develops.

This

facilitates

the

singlet-to-triplet

conversion.

More

importantly,

phosphorescence lifetime increases with the increase of degree of molecular aggregation. The long-lived phosphorescence associated with aggregation benefits from multiple factors, including small singlet-triplet gaps, enhanced overall ISC rates and suppression of fluorescence. We believe this aggregation-induced intersystem crossing (AI-ISC) mechanism may be employed as an alternative approach for realizing persistent phosphorescence.


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1. Introduction Persistent phosphorescent materials have received considerable attention in the bioimaging,1,

chemical sensors,3,

2

applications.9,

10

4

photovoltaic devices,5,

6

security signs,7,

8

and other

The range of luminophores is essentially limited to inorganics or

organometallic complexes,11-13 which benefits from various allowed electronic transitions thanks to abundant d orbitals in transition metal atoms. In contrast to the expensive and toxic metal-contained phosphorescent materials, pure organic molecules are cheaper, environmentally safer and offer better molecular design versatility.14 However, the tight binding feature of valence electrons in pure organic molecules restrains the emission caused by the transition from triplet excited states to ground state. Thereby, pure organic molecules exhibit weak or non-phosphorescence.15, 16 Only recently have they been brought into focus.9, 14, 17-20

Despite of this, rational design of long-lived phosphorescence in pure organic

molecules still remains in a preliminary state. Tang et al proposed that sufficient long-lived phosphorescence can be achieved by rational design molecular systems with fast intersystem crossing (ISC) rate and tunable phosphorescent radiative decay.21 ISC between the singlet and triplet excited states of molecules is prerequisite for phosphorescence.22 Unfavourable ISC rate in pure organic molecules makes it challenging to attain sufficiently high levels of long-lived phosphorescence.23 A conceivable strategy favoring an increased ISC rate is to minimize the energy gap between the Sm and Tn states according

to

an

empirical

||2/(ΔES–T)2.24,

25

formula

based

on

perturbation

theory:

kISC

∝

Here HSO is the spin-orbit perturbation Hamiltonian and

is the spin-orbit matrix element (SOCME) between the mth singlet excited


4

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state and the adjacent nth triplet excited state. ΔES–T is a singlet-triplet energy gap. Clearly, even a small SOCME would be sufficient to trigger a nonradiative transition (i.e. ISC) between two multiplicities when the energy gap is small enough. Instead of subtle modulation of molecular properties aiming at narrowing ΔES–T in dye molecules, aggregation may serve as an viable strategy according to the Kasha exciton model.26 In past decades, organic luminescence generally consider to be suffered from aggregation-caused quenching (ACQ), whose luminescence is weakened or quenched at high concentrations due to inter/intra-molecular collisional interactions.27, 28 To address the issue, Tang et al. have been systematically developing the strategy of “aggregation-induced emission” (AIE) and paved a practical way of overcoming the challenge of ACQ since 2001.29 Noteworthy, AIE can be extended to make phosphorescence or thermally activated delayed fluorescence, such as our previous work in π-π stacking aggregates30 and B. Voit’s research in polymers31. In this work, we explore the impact of molecular aggregation on the phosphorescence lifetime. According to the Kasha molecular exciton model, the excitonic coupling during aggregation will cause splitting of the excited energy levels.26 For a long time, ISC was considered to be uncompetitive with internal conversion (IC). Recently researches revealed that ISC and IC can have comparable timescale and ISC can occur in high-lying excited states.22, 32 Therefore, we argue that besides S1 to the adjacent triplet excited state, highlying ISC channels in aggregated molecules can also appreciably contribute to the overall ISC rates. As illustrated in Fig. 1a,30 in the monomer, the energy levels are sparse and SOC between the two multiplicities is negligible. In the dimeric aggregate, the excited states undergo a splitting of their energy levels, resulting in smaller ΔES–T than in the monomer.


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As aggregation proceeds, both singlet and triplet excited states energy levels develop bandlike patterns. Thus, molecular aggregation brings about many more favourable ISC channels between a singlet and its adjacent triplet excited sates, along with a suppression of the lowest singlet exited states. This concept of “aggregation-induced intersystem crossing” (AI-ISC) has been established in our previous study on aggregates of fluorescent molecules.30 AI-ISC allows simultaneous suppression of fluorescence and activation of phosphorescence and can be considered as complementary to AIE. However, the influence of AI-ISC on phosphorescence lifetime remains unexplored. Herein we chose several recently reported organic phosphorescent molecules and studied the impact of aggregation on their phosphorescence lifetime. Benzophenone, a prototype short-lived luminophore with π and n orbitals that are perpendicular to each other, has been chosen as the model system for studying phosphorescence.33,

34

We adopted three

phosphorescent π-conjugated aromatic derivatives of benzophenone, namely 1(dibenzo[b,d]furan-2-yl)phenylmethanone

(BDBF,

1),

1,3-phenylene

bis(4-

fluorophenyl)methanone (mFDBP, 2) and dibenzo[b,d]thiophen-2-yl(4-fluorophenyl) methanone (FBDBT, 3), as shown in Fig. 1b. These are of interest because they exhibit phosphorescence at low temperature or in amorphous/crystalline states, while their molecular morphologies and intermolecular interactions in solution remain unknown.21 In particular, aggregation is an effective strategy for manipulating the phosphorescence properties. π-extended aromatic derivatives are prone to aggregate in concentrated solution which results in significant variation of their luminescence performance. Therefore, investigation of such systems can provide a platform to further elucidate the mechanisms of phosphorescence emission. Additionally, substituent effect and inter-/intra-molecular


6

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motions have significant influence on the performance of phosphorescence.

9, 35

The

incorporation of different functional groups into benzophenone can not only allow us to tune the emission wavelength, but also provide a more comprehensive understanding of phosphorescence lifetime of benzophenone derivatives.

Figure. 1 (a) Schematic diagram of aggregation-induced intersystem crossing (AI-ISC). The diagram displays the energy splitting during the process of aggregation. F: fluorescence, P: phosphorescence. The thickness of the straight arrows shows the emission strength of fluorescence or phosphorescence. The purple arrows denote intersystem crossing from singlet to triplet excited


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states. (b) The structures of the organic molecules and their corresponding optimized tetramers (with an average intermolecular distance of 3.3~3.5 Å) that were studied in this work. In this study, the π-π slipped stacking aggregates of BDBF, mFDBP and FBDBT were investigated from the point of view of the AI-ISC mechanism. Our calculation showed that the phosphorescence lifetime is lengthened in aggregates compared to monomers. We think several factors resulting from molecular aggregation are beneficial for long-lived phosphorescence. Firstly, the overall ISC rates from singlet to triplet excited state are greatly increased; secondly, the singlet-triplet energy gap (e.g. ΔET1-S0) is steadily reduced as the aggregation develops; thirdly, the inhibition of vibrations owing to aggregation with appropriate inter-molecular distance help suppress nonradiative decay; finally, the fluorescence decay is severely suppressed as molecular aggregation proceeds. 2. Computational methods All the density functional theory/time-dependent density functional (DFT/TD-DFT) calculations in this work were performed with the ADF 2017 program package.36-38 The geometry optimizations of ground state S0 are carried out using the B3LYP hybrid functional. For the lowest singlet and triplet excited states S1 and T1, we optimized the structures using the same functional in the TDDFT framework. It has been reported that the statistical average of orbital potentials (SAOP) method39 can yield much better energies for HOMOs and excitation energies in TDDFT calculations.40-42 A scalar relativistic effect (also known as a pseudo-relativistic correction) was taken into account under the Zero Order Regular Approximation (ZORA) scheme to deal with the SOC between singlet and triplet states.43, 44 Then SAOP was used in conjunction with a scalar ZORA Hamiltonian


8

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for phosphorescence spectra simulations. In this case, SOC was considered as a perturbation based on relativistic orbitals (pSOC-TDDFT).45,

46

Because the relativistic

effect is small in pure organic molecules, this approach can avoid time-demanding full SOC calculations. All calculations used a Slater-type double-zeta polarization (DZP)47 basis set with Grimme3 BJ damping dispersion effects (DFT/TDDFT-D3-BJ) that include van der Waals dispersion48. Cyclohexane was used as solvent and the solvent effects were treated with the COSMO continuum solvation model in all calculations. 3. Results and discussion We firstly built the (BDBF)3 trimer with a π-π stacking pattern and test the intermolecular distance from 2.4 ~ 4.0 Å, from which we found that the aggregate with an intermolecular distance of ~3.4 Å was most likely stable structures. So we chose 3.4 Å as the intermolecular distance for all initial aggregated structures. The structure of the optimized BDBF tetrameric aggregate was modeled as a slipped π-π stack with an average intermolecular distance of 3.3~3.5 Å owing to the intermolecular van der Waals attraction as shown in Fig. 1b. Besides, it has been reported that unsaturated aromatic systems in concentrated solution tend to form a slipped stacking packing instead of perfect facial stacking, which further validates our optimized parallel displaced packing structure.49, 50 Based on appropriate molecular distance and slipped stacking structure, we firstly evaluate the frontier molecular orbitals (FMOs) of the lowest singlet and triplet excited states. The highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of the tetrameric aggregate excited states show profound differences in distribution of electrons between the lowest singlet and the triplet excited states (Fig. 2a). The HOMO and LUMO for the lowest singlet excited state, which are closely associated with fluorescence, are delocalized with wavefunction spanning over almost the entire tetramer aggregate. Delocalization MOs make


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electrons more mobile and more fluctuations of electron densities in a molecule, which will increase inter-/intra-molecular motion such as vibrations and rotations. These motions further facilitate the consumption of part of the excitation energy via nonradiative decay channels. The delocalized nature of the HOMO may exert a significant influence over the suppression of the radiative transition from S1 to S0, leading to quenching of fluorescence.51, 52 In sharp contrast to the delocalization of S1, the HOMO and LUMO of T1 are much more localized. Both α-spin HOMO and β-spin LUMO are primarily centered at one BDBF units, which are beneficial for maintaining phosphorescence and leads to more persistent emission.30 Fig. 2b displays the calculated energy levels of the excited states of the monomer, dimer, trimer and tetramer of BDBF. It shows that the energy of the lowest singlet excited state steadily drops from 2.76 eV in monomer to 2.44 eV in tetramer. It is evident that the excited state energy levels split into an increasing number of levels as the degree of aggregation increases, ultimately resulting in a band-like structure. There are only two singlet excited states below 3.0 eV for the monomer, this number rises to 6 for the dimer, 10 for the trimer and 18 for the tetramer. Similar behavior can also be observed in the triplet excited states. The phenomenon of orbital splitting induced by intermolecular interaction leads to the emergence of numerous unoccupied and occupied orbitals enabling an enhanced probability of transition between different multiplicities. ΔES-T, the energy gap between the lowest singlet excited state and the nearest adjacent triplet excited state, decreases as n increases (Fig. S2a), indicating that aggregation leads to a narrowing of the singlet-triplet energy gap. As a consequence, we have smaller ΔES-T in molecular aggregates. It has been recognized that small singlet-triplet gaps appreciably boost ISC rates between them so that ISC process can compete with IC.53 Besides, vibrational intensity in the tetramer is


10

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restricted compared with that in the monomer of BDBF (Fig. S8a), which can reduce nonradiative decay. Therefore, it is reasonable to assume that ISC is competitive with IC.

Figure. 2 (a) HOMO and LUMO for S1 and T1 of (BDBF)4 tetramer. The calculation results suggest electrons are delocalized and localized in S1 and T1, respectively. (b) Calculated excited state electronic energy levels. (c) SOCMEs of aggregated (BDBF)4 of all the possible ISC channels below the red line in Fig. 2b. In isolated BDBF, the major ISC channels are S1-T1 and S1-T2 with SOCMEs of 3.40 and 3.20 cm-1, respectively. We assume that ISC can occur in high excited states in aggregated (BDBF)4 given its intense energy splitting. For comparison, we choose the S1 state of each monomer as the reference. Then we collect all excited states below that reference level (see the excited states below the red line in Fig. 2b, the exact excited


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energies can be found in Table S2). After that, we calculated all SOCMEs of the possible ISC channels formed by these selected excited states. (m: 0~6 , n: 0~8, a total of 48 channels as listed in Fig. 2c). Obviously, there are much more possible ISC channels in tetramer than the isolated molecule. Since kISC is proportional to ||2/(ΔES–T)2, only those channels with both non-trivial SOCMEs and sufficiently small ΔES-T will be significant. To simplify the comparison of ISC rates between monomer and tetramers, we list ISC channels within the limited range of Sm-Tn (m: 1~3, n: 1~4). Apparently, there are more non-trivial ISC channels in tetramer than monomer (Table 1). For example, ISC rates of channels such as S1-T2, S2-T3, S3-T3 and S3-T4 in tetramer are comparable to the S1-T2 in monomer. Consequently, the overall ISC rate of tetramer which benefits from summation of many more channels, surpasses that of monomer. In addition, the oscillator strength of S0-S1 which is directly associated with the emission of fluorescence in aggregates generally decreases compared with that of the isolated molecule (Fig. S5a), leading to a suppression of fluorescence. Combining factors of enhancement of ISC rates and suppression of fluorescence and nonradiative decay together, we can expect that AI-ISC will facilitate phosphorescence. Table 1. Comparison of ISC rate of BDBF and (BDBF)4 that contributed by selected ISC channels. kISC BDBF (BDBF) 4

S1 S1

T1 85.8 5.88

T2 2006 2058

T3

T4

S2

14.3

10.3

S3

10.6

0.49

81698 3 477

10911 5

Table 2. The phosphorescence lifetimes of BDBF. Aggregates

BDBF

(BDBF)2

(BDBF)3

(BDBF)4


12

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SAOP 1.00 3.30 12.2 31.7 CAM-B3LYP 1.00 3.85 8.85 18.7 *We set the phosphorescence lifetime of isolated molecule as 1 and phosphorescence lifetimes of aggregates are relative values compared to monomer. The equation of phosphorescence lifetime in a ZORA p-SOC TDDFT calculation can be expressed as: τ=c3/[2(ΔE)2∙f],46 where c=137.036, ΔE is the energy gaps between T1 and S0, and f is the oscillator strength of T1 (all in atomic units). Both (ΔE)2∙and f are inversely proportional to τ, either the decline of ΔE or f will cause the increase of phosphorescence lifetime. Note that the reverse of phosphorescence lifetime 1/τ is phosphorescence radiation rate. Phosphorescence lifetime was calculated with two different functionals including cost-effective SAOP/DZP and long-range corrected CAM-B3LYP/DZP, as seen in Table 2. For the clarity and convenience of comparison, we adopted scaled phosphorescence time. Phosphorescence lifetime increases as the degree of aggregation develops, which is independent of theoretical methods. The persistent phosphorescence can be ascribed to the following reasons: optimizing molecular distance allow us to achieve favourable energy splitting, suppression of fluorescent emission and fast overall ISC rates; inhibition of vibration reduce the nonradiative decay. The increase in phosphorescence lifetime is of significance for the improvement in quantum yield as well as in energy utilization. In addition to the increasing of phosphorescence lifetime, the emission peaks also experience a red shift from monomer to tetramer because that the energies of both S1 and T1 are lowered as the degree of aggregation progresses (Fig. S4a and Fig. S6a). Therefore, AI-ISC mechanism is a promising strategy for optimizing phosphorescent properties.


13


Page 14 of 27

Figure. 3 (a) HOMO and LUMO for S1 and T1 of the (mFDBP)4 tetramer. (b) The calculated excited state energy levels. (c) SOCMEs of aggregated (mFDBP)4 of all the possible ISC channels below the red line in Fig. 3b. Then we continued exploring the AI-ISC mechanism by studying mFDBP with the same computational setup. The structure of the mFDBP tetramer contains a stack of three benzene rings on adjacent molecules as displayed in Fig. 1b. All optimized mFDBP aggregates display similar offset stacking with intermolecular distances in the range of 3.3~3.5 Å. The aggregates of mFDBP are not perfectly stacked because of the rotational freedom of the benzophenone moiety. Likewise, the FMOs of S1 and T1 are delocalized and localized, respectively (Fig. 3a), contributing to the prolongation of the phosphorescence lifetime. In order to better depict the delocalized/localized features of FMOs of S1/T1, we calculated the mFDBP octamer at the level of M06-2X/DZP. As Fig.


14

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S1b shows, FMOs of S1 and T1 of (mFDBP)8 exhibit more remarkably delocalized S1 and localized features, respectively. Energy splitting is evident in both singlet and triplet manifolds (Fig. 3b); ultimately a band-like structure is realized for large aggregates. This scenario validates the presence of energy splitting and overlap arising from aggregation. Both energy gaps between the lowest singlet and the adjacent triplet excited states (Fig. S2b) and HOMO-LUMO energy gaps (Fig. S3b) are reduced with increased aggregation, manifesting in more facile formation of the excited state and of ISC from the singlet space to the triplet manifold. As we can see from Fig. 3b, it is obvious that the major ISC channel in monomer is S1T1, whose SOCME is 5.21 cm-1. In tetramer, there are ten singlet and eleven triplet excited states below the reference line in Fig. 3b. Similar to the case of BDBF’s aggregates, ISC channels with large SOCMEs or small ΔES-T will show large ISC rates. SOCMEs of S1-T1, S3-T2 and S3-T4 are 2.957 cm-1, 0.470 cm-1 and 0.117 cm-1, respectively, and their corresponding ΔES–T are 0.288 eV, 0.0304 eV and 0.0008 eV, respectively. Those three either have a relatively large SOCMEs or an extremely small ΔES–T, so their ISC rates are relatively large (Table 3). Therefore, it is speculated that the overall ISC rate of tetramer is substantially larger than that of monomer because it benefits from summation of many more non-trivial ISC channels. Therefore, we can easily conclude that aggregation of mFDBP leads to enhancement of the overall ISC rate. Meanwhile, both of the oscillator strength and transition dipole moment of S0-S1 approach zero for the tetramer (Fig. S5b). The inhibition of fluorescence and increase of ISC can directly improve the phosphorescent efficiency. As expected, the phosphorescence lifetime of mFDBP is increased due to aggregation (Table 4), demonstrating the persistence of aggregate luminescence induced


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by the AI-ISC mechanism. Additionally, the phosphorescence emissive wavelength undergoes a red shift due to the decrease of T1 with the increase of aggregation degree, (Fig. S6b). This property permits color adjustment by varying the degree of aggregation. It also allows us to or estimate degree of aggregation by measuring the emission peak wavelength. Table 3. Comparison of ISC rate of mFDBP and (mFDBP)4 that contributed by selected ISC channels. kISC mFDBP (mFDBP) 4

S1 S1 S2 S3 S4

T1 149 105 0.9 1.2 1.4

T2

T3

T4

11.1 238 33.9 22339 2.9 5.8 13.6

T5

234

Table 4. The phosphorescence lifetime of mFDBP. Aggregate mFDB (mFDBP) (mFDBP) (mFDBP) s P 2 3 4 Lifetime 1 3.17 13.8 17.3 *Here we set the phosphorescence lifetime of isolated molecule as 1 and phosphorescence lifetimes of aggregates are relative values compared to monomer. To further assess the reproducibility of the phenomena described above, we applied the same set of DFT calculations to the aromatic molecule FBDBT. Similar to BDBF and mFDBP, the HOMO and LUMO of FBDBT aggregates display a delocalized S1 and a localized T1 (Fig. 4a), which is beneficial for enhancing the emission of phosphorescence. The aggregation of FBDBT is also based on π-π stacking interactions and causes splitting of excited state energy levels. As a result, the excited state manifolds in the aggregates become denser than in the monomer (Fig. 4b) and the energy gaps are narrower (Fig. S2c).


16

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Fig. 4 (a) HOMO and LUMO for S1 and T1 of (FBDBT)4 tetramer. (b) The calculated excited electronic energy levels. (c) SOCMEs of aggregated (FBDBT)4 below of all the possible ISC channels below the red line in Fig. 4b. Similarly, there are more ISC channels in (FBDBT)4 than isolated FBDBT below the same reference line (red line in Fig. 4b). Some of the ISC channels in (FBDBT)4 show larger SOCMEs than S1-T1 in FBDBT (0.65 cm-1), in combination with narrow ΔES-T, their ISC rates become competitive to the major channel the monomer (Table 5). SOCMEs of S3-T3 and S4-T4 in (FBDBT)4 are 0.163 cm-1, 0.242 cm-1, respectively, and their corresponding ΔES–T are 0.0391 eV, 0.0482 eV, respectively. Because of their small ΔEST,

they show relatively large ISC rates. Taking all ISC channels into consideration, the

overall ISC in (FBDBT)4 is appreciably larger than FBDBT. Simultaneously, the oscillator strength of S0-S1 approaches zero and thus completely quenches fluorescence emission.


17


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Meanwhile, vibrational intensities in the tetramer are lower than in the monomer of FBDBT (Fig. S8c). Such restriction of vibrations normally suppresses the relaxation of the excited states and thereby help prolong phosphorescence lifetime. Finally one notes that the aggregation-induced increasing in phosphorescence lifetime calculated for the first two molecules is reproduced in FBDBT (Table 6). The triplet emission spectra again show a red shift with increasing degree of aggregation (Fig. S6c). Results of (FBDBT)n, together with (mFDBP)n and (BDBF)n, suggest that our AI-ISC mechanism may be common feature for many luminogenic aggregate systems. Table 5. Comparison of ISC rate of FBDBT and (FBDBT)4 that contributed by selected ISC channels. kISC FBDBT (FBDBT) 4

S1 S1 S2 S3 S4 S5

T1 4.00 1.04 2.60 1.79 41.4 0.28

T2

T3

T4

T5

3.68 4.79 17.4 0.04 0.60 25.2 1.51 6.99 6.86 2.36

Table 6. The phosphorescence lifetime of mFDBP. Aggregate mFDB (mFDBP) (mFDBP) (mFDBP) s P 2 3 4 Lifetime 1 1.42 54.9 57.1 *Here we set the phosphorescence lifetime of isolated molecule as 1 and phosphorescence lifetimes of aggregates are relative values compared to monomer. All three molecules studied in this work contain benzophenone moiety, which has traditionally served as a prototype for establishing triplet state chemistry. By varying the substituents and incorporated aromatic groups of BDBF, we attain mFDBP and FBDBT. All the dominating ISC channels in three systems follow EI-Sayed’s rules (1nπ*→3ππ*), which is helpful for enhancing the ISC rate. As seen in Table 7, tetramers show longer phosphorescence lifetime compared with


18

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counterpart monomers. Meantime, the phosphorescent quantum yield ФP and lifetime τ measured by experiments become increasingly unfavorable from BDBF to mFDBP to FBDBT. As Tang proposed, trends of efficiency of phosphorescence may be associated with trends of ISC rates.21 Since quantum yield cannot be calculated directly, we rationalize relative phosphorescence efficiencies based on kISC values of dominating ISC channel(s). The phosphorescent performances of calculated aggregates share similar trends with the experiment result. BDBF tetramer shows both large kISC and sharp enhancement of phosphorescent lifetime compared with the monomer, which can be attributed to its small ΔES-T in dominating ISC channel, large SOC as well as weak nonradiative decay. Both the performances of phosphorescence of mFDBP and FBDBT aggregates become less prominent compared with BDBF aggregates. For mFDBP aggregates, this can be ascribed to the strong nonradiative decay caused by more intensive molecular vibrations and rotations. FBDBT aggregates exhibit significant increase of phosphorescence lifetime along with smallest kISC among three species. The incorporation of F, S atoms results in delocalized T1, which will ultimately lead to the annihilation of excitons by molecular vibrations or rotations. Besides, for phosphorescence emission, persistent lifetime is accompanied with low efficiency.54 This is consistent with the equation of phosphorescence lifetime: τ=c3/[2(ΔE)2∙f].

Table 7. The phosphorescence performance of experiments and calculations.

Structure Exp.a

ФP/%

34.5

17.7

6.5


19


Calc.b

Page 20 of 27

τ/ms

232

210

110

kISC

2006→2058

149→105

4.00→1.04

τ

τ→31.7τ

τ→17.3τ

τ→57.1τ

a

Experimental data of quantum yield Фp and phosphorescence lifetime τ are obtained from Tang’s work (Ref 21). b Calculation data of kISC and τ represent changes from monomer to tetramer. kISC is the ISC rate of the dominating ISC channel in monomer and tetramer, which is S1-T2 in BDBF and S1-T1 for mFDBP and FBDBT. Here we set the calculated phosphorescence lifetime of isolated molecules as τ and phosphorescence lifetimes of tetramers are relative values compared to monomer. 4. Conclusions In summary, we have elucidated the effects of AI-ISC and exploring its application in achieving persistent phosphorescence lifetime by performing comprehensive electronic structure calculations on three types of benzophenone-based molecules. Appropriate intermolecular distances of stable aggregates of these molecules give rise to delocalization of orbitals in the singlet excited state and to localization of orbitals in the triplet excited state, resulting in the suppression of fluorescence. The suppression of fluorescence is a consequence of the substantial reductions in oscillator strength of the lowest singlet excited state. Importantly, the energy levels undergo significant splitting and become band-like, facilitating efficient energy overlap between singlet and triplet excited states. As the degree of aggregation increases, the energy levels of both singlet and triplet excited states become increasingly band-like, which gives rise to more ISC channels. This new feature brought about by aggregation is crucial for enhancing total ISC rates between the singlet and triplet manifolds. Because of suppression of fluorescence, increase of ISC rate as well as reduction of molecular vibration intensities, longer phosphorescence lifetimes are realized. Importantly, AI-ISC allows us to increase phosphorescence lifetime by increasing degree of aggregation. Additionally, the emission wavelength can be tailored by modulating


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degree of aggregation as well. Noticeably, Li et al. reported recently that purely organic long-lived phosphorescence can be achieved through molecular packing in experiment.55 This latest experimental discovery supports our theoretical prediction of enhancing phosphorescence lifetime via AI-ISC. Therefore, it is to be expected that AI-ISC can be used as a promising supplement to AIE mechanisms and help gain a better understanding of utilizing triplet excited states. Certainly, many more factors such as molecular polarity, aggregation pattern and solvent environment can have non-trivial impacts on aggregates. Exploration of these conditions on the phosphorescent properties of molecular aggregates are underway. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. FMOs of octamers, Energy gaps, HOMO-LUMO gaps, Photo-absorption peaks, Oscillator strengths and transition dipole intensities (S1→S0), Phosphorescence (T1 → S0) emission peaks, Aggregation patterns investigation, Vibrational intensities in monomers and tetramers, Energy levels of singlet and triplet excited states, SOCMEs of tetramers and all the optimized Cartesian coordinates are included in Figures S1−S8 and Table S1−S43. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ORCID Guozhen Zhang: 0000-0002-6116-5605 Funding Sources MOST (No. 2016YFA0400904) NSFC (No. 21703221, 21633006) Fundamental Research Funds for the Central Universities (WK2060030027) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by MOST (No. 2016YFA0400904), NSFC (No. 21703221, 21633006), the Fundamental Research Funds for the Central Universities (WK2060030027). University of Science and Technology of China High Performance Computing Center is acknowledged for computing time. REFERENCES (1) Zhao, Q.; Huang, C.; Li, F. Phosphorescent Heavy-Metal Complexes for Bioimaging. Chem. Soc. Rev. 2011, 40, 2508-2524.


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Aggregation-Induced Enhancement of Molecular Phosphorescence

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