Intramolecular-locked triphenylamine derivatives with adjustable room temperature phosphorescence properties by the substituent effect

Abstract

Organic room temperature phosphorescent materials have attracted a lot of research interest due to their wide applications. Herein, a series of intramolecular-locked triphenylamine derivatives have been synthesized with adjustable room temperature phosphorescence (RTP) properties, in which the RTP lifetime ranged from 30 to 288 ms and the emission color varied from sky blue to green. These results are mainly related to the substituent effect, which can tune the intramolecular charge transfer and intermolecular interactions, contributing to different intersystem crossing (ISC) processes, and the stability and energy levels of excited triplet states.

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Room-temperature phosphorescence (RTP) of pure organic materials has attracted considerable attention for their potential applications in biological imaging,^1–4 organic light-emitting diodes (OLEDs),^5,6 sensors,^7–9 optics and anti-counterfeiting technologies.^10–20 However, it is still a challenge for organic luminogens to achieve persistent RTP with a long lifetime and a high efficiency, which is mainly related to two critical points: (a) the spin-forbidden intersystem crossing (ISC) process, which is hard to be overcome, and (b) the unstable excited triplet state, which can be easily relaxed by molecular motions as nonradiative transitions and quenched by oxygen and moisture.^21–30

Recently, some efficient strategies have been proposed to achieve persistent RTP in pure organic materials.^31–36 The introduction of heteroatoms with lone-pair electrons can promote the ISC process with increased spin–orbit coupling (SOC).^37,38 Also, molecular rigidity is the key factor to the stability of excited triplet states, since the rigid conjugated system can suppress the possible nonradiative transitions by molecular motions.^39,40 Alternatively, the substituents as the periphery groups play an assistant role in optimizing the molecular packing and intermolecular interactions.^41–45 As one of the impressive examples, through the incorporation of substituents with different electronic properties into the 10-phenyl-10H-phenothiazine 5,5-dioxide core, the enhanced π–π electronic coupling of conjugated cores is obtained by the increased electron-withdrawing ability of substituents, contributing to the ultralong RTP effect.⁴⁵

In this work, the commonly used triphenylamine (TPA) unit is selected as the conjugated skeleton,⁴⁶ which exhibits nearly no RTP in the natural state. With the aim to enhance the molecular rigidity, an intramolecular locking strategy is employed to bind the two phenyl units together by carbon chains. The resultant compound C-TPA exhibits the observed afterglow with a RTP lifetime of 30 ms (Fig. 1), confirming the crucial role of molecular rigidity. Furthermore, different substituents are introduced to the unbinding phenyl in the TPA moiety to promote the intermolecular interactions. The RTP lifetimes of C-TPA-OMe and C-TPA-Me with methoxyl and methyl modification increase to 184 ms and 288 ms, respectively, while the emission color changes from sky blue to green. It is mainly related to the additional C–H⋯π interactions induced by alkyl chains and to the optimized molecular packing by the substituent effect. Herein, we would like to present the synthesis, characterization, and crystal structures to shed some light on the relationship among the molecular structure, molecular packing and RTP effect.

Fig. 1 Molecular design strategy, molecular structures, and RTP lifetimes of C-TPA, C-TPA-OMe, and C-TPA-Me (crystals), and photographs taken before and after irradiation (365 nm) under ambient conditions.

Synthesis and emissive properties

The target compounds C-TPA, C-TPA-OMe and C-TPA-Me were facilely prepared through C–N coupling reactions (Scheme S1, ESI†). Their molecular structures were fully characterized by ¹H NMR and ¹³C NMR spectroscopy, elemental analysis, HPLC and single-crystal X-ray diffraction (Fig. S1–S3 and S26–S31, ESI†). Their UV-visible absorption spectra in tetrahydrofuran (THF) solution (concentration: 10 μM) exhibit similar absorption peaks at about 270 nm (Fig. S4 and S5, ESI†), which become broader in the solid state. Compared to the photoluminescence (PL) spectrum of C-TPA without substituents in THF solution (Table 1), those of C-TPA-OMe and C-TPA-Me show blue-shifted emissions (Fig. S4, ESI†), mainly due to the electron-donating ability of methoxyl and methyl moieties. However, for their single crystals, after turning off the ultraviolet (UV) lamp (365 nm) (Fig. 1), sky-blue afterglow of C-TPA and green one of C-TPA-OMe could be visualized with a lifetime of up to 2.0 s. The persistent afterglow can be further confirmed by the time-dependent phosphorescence spectra (Fig. S6–S8, ESI†).

Table 1 Photophysical properties of the three C-TPA derivatives

Sample	λ abs(solution) (nm)	λ F(solution) (nm)	λ F(crystal) (nm)	λ P(crystal) (nm)	τ P(crystal) (ms)	Φ F(crystal) (%)	Φ p(crystal) (%)
C-TPA	275	369	351	475	30	13.47	2.26
C-TPA-OMe	270	362	386	508	184	83.55	—
C-TPA-Me	270	363	361	529	288	20.83	2.0

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Comparing the maximum PL wavelengths of these crystals carefully, there is a red-shifting tendency from C-TPA (351 nm) to C-TPA-Me (361 nm), then to C-TPA-OMe (386 nm). Similarly, the RTP emission peaks change from 475 nm for C-TPA with a lifetime of 30 ms (Fig. S9, ESI†), to 508 nm for C-TPA-OMe with a lifetime of 184 ms (Fig. 2a), then to 529 nm for C-TPA-Me with the increased RTP intensity and lifetime (288 ms), which is over 10 times of that of C-TPA (Fig. 2). When the temperature decreased to 77 K, the phosphorescence intensity increased largely, mainly due to the suppression of molecular motions as nonradiative transitions (Fig. S10–S14, ESI†). Once these crystals have been ground into powder, the XRD spectra exhibited the crystalline state to some extent (Fig. S15, ESI†), mainly related to the good crystallinity of these compounds. Accordingly, the RTP lifetimes of C-TPA, C-TPA-OMe and C-TPA-Me powders changed to 22 ms, 104 ms and 275 ms, respectively (Fig. S16 and S17, ESI†).

Fig. 2 (a) The steady-state PL and phosphorescence and (b) temperature-dependent phosphorescence decays for C-TPA, C-TPA-OMe and C-TPA-Me crystals.

The persistent RTP effect of these C-TPA derivatives in the crystalline state or single crystal form can be explained by the theoretical calculations and crystal analysis.

Theoretical calculations

With the aim to explore the possible mechanisms of the emissive properties of three C-TPA derivatives, the optimized geometry, electrostatic potential (ESP) (Fig. S18, ESI†), frontier molecular orbitals and energy levels of TPA, C-TPA, C-TPA-OMe and C-TPA-Me were investigated by time-dependent density functional theory (TD-DFT) (Tables S2–S14, ESI†). H and L represent the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively. The blue arrows refer to the possible ISC channels. The major ISC channels were mainly determined by two elements: T_m gave a small energy gap (<0.3 eV) with S₁ and contained the same transition configurations as the S₁ state.^47,48 When the energy of T_m was similar to or lower than that of S₁, the latter element was considered to be more important. For TPA, as indicated in Fig. 3a, ΔE_(S1–T1) is relatively large (0.75 eV), and the natural transition orbital (NTO) shows that S₁ and T₁ are locally excited (LE) states, and there are no channels for the ISC process (Tables S3 and S14, ESI†). Thus, TPA exhibits nearly no RTP properties. Once the two phenyl moieties are locked together by alkyl chains, the resultant C-TPA derivatives show twisted conformations with a torsion angle of about 90° between the free phenyl and the locked moiety. Furthermore, the DFT calculations of C-TPA derivatives exhibited a poor overlap between the HOMO and LUMO of intramolecular charge transfer (ICT) states, resulting in a reduced ΔE_(S1–T1) and thus enhanced ISC processes.^49–51 Accordingly, there are many ISC channels from S₁ to T_m for these C-TPA derivatives (Tables S4–S13, ESI†), contributing to the RTP effect.^52–54 Therefore, the NTOs of the excited states are shown in Table S14 (ESI†), and the S₁ state of locked structures exhibits charge transfer (CT) character, while high-energy triplets with small ΔE_(S1–Tm) exhibit more CT properties, and the T₁ state shows locally excited (LE) states, which also exhibit ³(π,π*) transitions. Thus, intramolecular charge-transfer states (¹CT and ³CT) are generated, and a stepwise energy transfer process of ¹CT → ³CT → ³π–π* could be achieved to serve as intermediate states for minimizing ΔE_ST. In theory, this could accelerate k_ISC and promote the strong emission of phosphorescence.^30,55–57

Fig. 3 Diagrams of the TD-DFT calculated energy levels and possible ISC channels of single molecular TPA (a), C-TPA (b), C-TPA-OMe (c) and C-TPA-Me (d) at the excited singlet (S₁) and triplet (T_m) states. H and L represent the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), respectively. LE and CT mean locally excited and charge transfer states, respectively. The insets show the Kohn–Sham frontier orbitals obtained from the isolated states of these compounds.

Compared to the single molecule, substantial electronic interactions among the dimers of C-TPA derivatives result in the overlap of the excitonic orbitals, giving rise to energy splitting.³⁰ Furthermore, the energy gaps between S₁ and T₁ states decrease (Fig. S19–S21 and Table S2, ESI†), which could lead to a reduced energy loss in the process of internal conversion at excited triplet states.⁵⁸ The increased ISC channels and decreased ΔE_(S1–T1) in the molecular dimer will jointly promote the generation of phosphorescence. The above calculation results theoretically explain the effective promotion of room temperature phosphorescence by molecular dimers and further prove the tremendous influence of molecular packing on their luminescence properties.

Crystal analysis

Fig. 4 presents the molecular conformations, dimers and packing of these three molecules in single crystals. All the C-TPA derivatives show twisted conformations, and the dihedral angles between the phenyl and the acridine moiety (as the locked structure) are in the range of 84–86°. The dihedral angle of the two phenyl groups in the acridine ring is about 30°, forming a butterfly-like spatial structure. Efficient intermolecular interactions, such as C–H⋯π and C–H⋯N, are observed, while no π–π interactions were found (Fig. S22–S24, ESI†). The intermolecular interactions would restrict the intermolecular motions effectively and lead to reduced non-radiative transitions, which could contribute greatly to the relatively high PL efficiencies of these materials.^58–60 To simplify the packing analyses (Fig. S23, ESI†), for C-TPA, the molecules are intertwined, making the order of the crystal arrangement relatively poor. Furthermore, the herringbone arrangement of C-TPA-OMe is composed of mostly anti-parallel dimers. Then, the layered and orderly herringbone stacking was found in methyl-substituted C-TPA-Me, and thus the introduction of substituents can optimize molecular stacking.

Fig. 4 Single-crystal structures of the three target compounds: (a) The monomer, (b) dimer A of intra-unit-cell and (c) dimer B of inter-unit-cell; (d) packing of the molecules of C-TPA, C-TPA-OMe and C-TPA-Me. d represents the distances of carbon chains to the locked moiety. I represents the centroid–centroid distance of unbinding phenyl.

To analyse crystals in detail, we choose dimer A (Fig. 4b), which accounts for the majority in one unit cell, to represent the interaction within the unit cell. The distances d of C (lock) in one molecule to a locked moiety of another molecule in dimer A are regarded as a typical element to monitor the distance of adjacent molecules. As for the inter-unit-cell dimer, the anti-parallel dimer B (Fig. 4c) as a representative to study the interaction between the unit cells is chosen, except C-TPA, because the two unbinding phenyls of the two molecules are interlaced at an angle of 52.11°. For the substituents of C-TPA derivatives varying from H to OCH₃ to CH₃, the resultant compounds exhibit a decrease in distance d from 6.240 Å to 5.986 Å to 5.239 Å, which is supposed to be associated with luminous properties. For the intra-unit-cell dimer A of C-TPA, there are four types of C–H⋯π interaction (2.983–3.770 Å) and two types of C–H⋯N interaction (3.450 and 3.632 Å) (Table S16, ESI†). Furthermore, the inter-unit-cell dimer B contained two types of C–H⋯π interaction (3.328 Å and 3.743 Å). Moreover, in comparison with the H substituted compound, the additional group of the OCH₃ substituted one provides more kinds of C–H⋯O and C–H⋯π interactions. For the intra-unit-cell dimer A of C-TPA-OMe, four types of C–H⋯π interaction (2.905–3.830 Å) and two types of C–H⋯N interaction (3.598 and 3.909 Å) are found. Additionally, there are two types of C–H⋯O interaction (3.875 Å) and two types of C–H⋯π interaction (3.554 Å) in the anti-parallel dimer B of inter-unit-cell (centroid–centroid distance of the unbinding phenyl is 6.35 Å). It can be found from the data that the intermolecular interaction of C-TPA-OMe is obviously stronger than that of C-TPA, and with more compact packing, which also contributed to the persistent RTP effect of C-TPA-OMe. As for C-TPA-Me, multiple hydrogen bonds of C–H⋯π (distances ranging from 2.800 to 3.694 Å) and C–H⋯N (distances of 3.556 and 3.898 Å) are found in dimer A. In the anti-parallel dimer B of inter-unit-cell (centroid–centroid distance of the unbinding phenyl is decreased to 5.08 Å), there are four kinds of C–H⋯π, whose distances range from 3.749 Å to 3.915 Å. It can be derived from the comparation that compact and orderly molecular packing leads to a low nonradiative transition of the conjugated system, resulting in the persistent RTP effect with a long RTP lifetime. These intense intermolecular interactions could largely reduce the possible energy loss through non-radiative relaxation, which is harmful for its phosphorescence. In summary, tight stacking with multiple C–H⋯π intermolecular interactions (listed in Table S16, ESI†) can also be further proved as a crucial factor to RTP lifetime.

The above experimental results and theoretical calculations illustrate that molecular stacking plays an important role in the luminescence process of the C-TPA compound derivatives. As shown in Fig. 5, from TPA to C-TPA-R, for the achievement of persistent RTP, two factors should be noted: two phenyl moieties locked by alkyl chains produce a CT orbital, generating a stepwise energy transfer process of ¹CT → ³CT → ³π–π* for enhancing the intersystem crossing, which promoted phosphorescence.^55–57 Furthermore, the results proved that the intramolecular locking strategy was highly necessary. In addition, the introduction of substituents not only promoted intramolecular interactions but also optimized molecular packing, which can facilitate the ISC process and suppress the molecular motions to achieve persistent RTP.^{41,42,58–62}

Fig. 5 The proposed mechanism for PL processes in isolated molecules of TPA and C-TPA derivatives’ transition from the S₀ to the S₁ state. The ISC transition is not favoured in TPA. Formation of a CT transition bridge within molecular C-TPA could facilitate the communication between the excited singlet and triplet states to produce the RTP effect. C-TPA derivatives with strong intermolecular interactions can stabilize triplet states to extend the phosphorescence lifetime.

Lastly, considering their application, due to their different photophysical properties the three compounds may have potential use as anti-counterfeiting materials. As shown in Fig. S25 (ESI†), a simple encryption pattern was fabricated and combined with the prominent RTP feature under UV excitation. Under 365 nm UV light irradiation, the shape of two birds emit deep blue. After ceasing the UV irradiation, the visible green RTP of bird (up) can be readily visualized and lasts for seconds because of the long RTP lifetime; at the same time, the bird (bottom) emits sky-blue light but disappears quickly. Thus, the feasible application of the three compounds in information encryption and decryption is successfully demonstrated.

Conclusions

In summary, a series of intramolecular-locked triphenylamine derivatives have been synthesized with the incorporation of different substituents to the unlocked phenyl moiety. Compared to C-TPA without a substituent, C-TPA-OMe and C-TPA-Me with methoxyl and methyl modification exhibited largely increased RTP lifetimes of 184 ms and 288 ms, respectively, and the emission color varied from sky blue to green. This is mainly related to the additional C–H⋯π interactions induced by alkyl chains and to the optimized molecular packing by the substituent effect, which can extend the RTP lifetime from 30 ms to 288 ms. Thus, the idea of how molecular structure design regulates molecular packing and interactions can give a better understanding of the molecular structure–packing–performance relationship. This phenomenon could be termed as a Molecular Uniting Set Identified Characteristic (MUSIC).²¹ It is envisaged that the results of the present study could promote further studies based on rational structure design for the development of efficient organic p-RTP materials.

Author contributions

Li Zhen and Li Qianqian conceived the project. Han Mengmeng synthesized all the materials and performed all photophysical measurements. Xu Zhichen and Lu Jie performed a part of the synthetic experiments. Xie Yujun discussed the data. Han Mengmeng performed the theoretical calculations and wrote the paper. Li Qianqian and Li Zhen discussed and revised the manuscript. All the authors approved the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (no. 51973162 and 21734007), Excellent Youth Foundation of Hubei Scientific Committee (2020CFA084), and the Fundamental Research Funds for the Central Universities (2042020kf0200) and Wuhan City (2019010701011412) for financial support. We also thank the national supercomputing center in Shenzhen.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2081904–2081906. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1qm01184g

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