The intensive pursuit of materials with efficient room temperature phosphorescence (RTP) have been continuous these years for their essential applications in organic light-emitting diodes, data recording, security protection, photodynamic therapy [1-8], etc. Although great progress has been achieved in inorganic and organometallic phosphors [9-14], taking advantage of heavy atom effect to boost intersystem crossing (ISC), less attention has been paid to pure organic luminogens with relatively poor spinorbital coupling (SOC).
Compared with inorganic and organometallic compounds, pure organic luminogens take advantages of facile preparation, easy functionalization, low toxicity, and low cost. In the past, the studies on pure organic phosphors, however, were normally carried out at cryogenic conditions owing to the susceptibility of triplet excitons to molecular motions and external quenching [15, 16]. To obtain efficient pure organic RTP, on one hand, people normally endeavored to increase the SOC and thus facilitate the ISC process through incorporation of aromatic carbonyls, heavy atoms, and heteroatoms [15-20]; on the other hand, they tried to depress the nonradiative deactivation pathways through absorption on solid matrix , crystallization [22-27], embedding into rigid matrix , supramolecular interactions , and metal-organic framework coordination [30, 31]. Among these approaches, the discovery of crystallization-induced phosphorescence (CIP) in pure organics such as benzophenone and 4, 4'-dibromobiphenyl suggested a possible crystal engineering way to efficient RTP .
Meanwhile, efficient luminogens with high contrast mechanochromism also attracted great attention because of their fundamental implications on the molecular packing and promising applications in optical recording, mechanical sensors, security papers [32-39], etc. CIP-active pure organic RTP luminogens are also promising mechanochromic candidates due to the susceptibility of triplets to molecular motions and external quenchers [22, 27]. With continuous efforts to construct pure organics with efficient RTP and remarkable mechanochromism, in this work, a series of benzophenone-aromatic amine conjugates were synthesized (Fig. 1). Such molecular design based on the following considerations: (1) Aromatic carbonyl groups can effectively promote SOC and share rapid ISC rates [15, 16, 40]; (2) Nitrogencontaining diphenylamine (DPA) and triphenylamine (TPA) may enhance n-π* transition to facilitate ISC process [18-20]; (3) Such electron donating-accepting (D-A) structure may narrow the energy gap between excited singlet and triplet states, thus facilitating the ISC process; (4) Twisted D-A structure and possible RTP emission may synergistically endow the compounds with remarkable mechanochromism . Herein, their detailed photophysical properties were investigated.
The target compounds were facilely prepared according to the synthetic routes shown in Scheme S1 (Supporting information) in moderate to high yields (62%–74%). They were characterized by spectroscopic methods, with satisfactory results obtained (Figs. S1–S7 in Supporting information). Particularly, single crystal structures definitely suggest the successfully preparation of these compounds [CCDC 1845357 (BPDPA), 1845358 (BPTPA), 1845359 (BP2DPA), and 1845360 (BP2TPA)]. When dissolved in n-hexane (Hex), toluene (Tol), trichloromethane (TCM), and tetrahydrofuran (THF), their absorption maximum are generally at about 356–382 nm (Table 1, Fig. S8 in Supporting information), with slightly varied peaks due to the poor tuning effect on ground states by solvent polarity. However, their emission spectra demonstrate typical bathochromic shift with increasing solvent polarity (Fig. S9 in Supporting information, Table 1), owing to the intramolecular charge transfer (ICT) effect.
Previously, the combination of BP with carbazole yielded luminogenic crystals with RTP and even persistent RTP emissions . Herein, these compounds consisting of BP and D(T)PA are thus also expected to generate RTP emissions in the crystalline state. Upon irradiation, the crystals of BPDPA, BP2DPA, BPTPA, and BP2TPA exhibit bright blue emissions with maxima at 437, 445, 463, and 463/475 nm (Fig. 2a), respectively. Through a time-gated technology, their delayed emissions could be recorded since nanosecond signals can be definitely excluded with a delay time (td) no less than 0.1 ms. As can be seen from Fig. 2b, Figs. S10 and S11 in Supporting information, with a td of 0.2 or 0.5 ms, emission maxima at 441/521/589, 540, 470/562, and 487/540 nm are recorded for BPDPA, BP2DPA, BPTPA, and BP2TPA, respectively. While the peaks at 437, 470, and 487 nm approaching those of prompt emissions are ascribable to the delayed fluorescence (DF), the others are assignable to the RTP emissions. Notably, with a td of 0.2 ms, BP2TPA shows a broad emission peaking at 487 nm with a full width at half maximum (FWHM) of 122 nm, from which no obvious RTP can be derived. However, with a longer td of 0.5 ms, the faded emission at 487 nm and an emerging peak at 540 nm are observed, which can be readily assigned to DF and RTP, respectively. The absence of RTP in solutions and the appearance of apparent RTP in crystals highly suggest the CIP nature of these compounds.
|Fig. 2. Emission spectra of different crystals with td of (a) 0 and (b) 0.2 or 0.5 ms|
To gain more insights into their emission properties, single crystal structures of the compounds were checked. Generally, these luminogens adopt highly twisted conformations (Fig. 3, Figs. S12 and S13 in Supporting information) with abundant intermolecular interactions in the crystals. As in BPDPA, each molecule is electronically related to six neighboring ones which give rise to numerous C－H…π (2.810, 2.849, 2.850 Å) short contacts Figs. 3a and b). These intermolecular interactions help stiffen the conformations, thus activating the radiative channels for triplets. Moreover, the dense packing and ordered crystalline lattice in crystals also help to protect the susceptible triplets from common quenchers like moisture and oxygen. Similarly, BPTPA holds a 3D network composed by intermolecular interactions among adjacent molecules Figs. 3c–e), precisely C－H…π (2.820, 2.887, 2.813, 2.814, 2.875 Å), C－H…HC (2.325, 2.696 Å), and π-π (3.372 Å) short contacts.
|Fig. 3. Single crystal structure and molecular packing of BPDPA (a and b) and BPTPA (c–e) with denoted intermolecular interactions|
The other two compounds, BP2DPA and BP2TPA, are also gifted with versatile interactions in crystals, namely C－H…π (2.740, 2.888, 2.819, 2.875 Å), C－H…H－C (2.363, 2.369 Å), π-π (3.390 Å) electronic communications in BP2DPA (Fig. S12) and C－H…π (2.863, 2.856, 2.897, 2.843, 2.870, 2.725, 2.802, 2.881, 2.821, 2.813, 2.867 Å), C－H…H－C (2.382 Å), C＝O…C＝O (3.155 Å), π-π (3.349 Å) short contacts in BP2TPA (Fig. S13). Despite the absence of heavy atoms, benzophenone and aromatic amines contribute to generate triplet excitons; meanwhile, such abundant and effective intermolecular interactions significantly restrict the vibrational stretching, thus generating remarkable RTP in crystals. Specifically, those suppress C－H stretching play a predominant role, owing to the highest energy vibration of C－H stretching (~0.37 eV, C＝C 0.20 eV) in aromatic molecules and the exponential dependence of the nonradiative rate constant (knr) on the phonon energy .
At 77 K, persistent phosphorescence was observed after the stop of UV irradiation (Fig. S14 in Supporting information), which highly suggests the significantly decreased nonradiative deactivations. Meanwhile, the absence of the afterglow even in vacuum at room temperature precludes oxygen quenching as the main cause. Namely, further conformation rigidification is responsible for the persistent phosphorescence at cryogenic temperatures.
To quantitatively evaluate the emission of these crystals, their quantum efficiencies (Φ) were determined, which are 7.5%, 8.2%, 50.0%, and 10.5% for BPDPA, BP2DPA, BPTPA, and BP2TPA, respectively, derived from which their RTP efficiencies are calculated as 1.5%, 1.9%, 3.2%, and 3.6%. These values are comparable to the reported data of other organic RTP materials [25, 30]. The twisted D-A structure, bright and triplet-involved emission, as well as abundant intermolecular interactions of the crystalline samples make them highly possible as mechanochromic materials [22, 32, 42]. To confirm it, mechanical responses for the emission of these luminogens were verified. As depicted in Fig. 4, upon manual grinding, the emission color of the compounds generally changes from blue to cyan or green, with emission maxima variations by 35 (437 nm to 472 nm), 33 (463 nm to 496 nm), 26 (445 nm to 471 nm), and 30 nm (463 nm to 493 nm) for BPDPA, BP2DPA, BPTPA, and BP2TPA Figs. 4c, e, and Fig. S15 in Supporting information), respectively, demonstrating obvious mechanochromism. Meanwhile, with a td of 0.1 ms, the ground solids also exhibit extremely weak delayed emissions with maxima close to those of prompt fluorescence (Fig. S16 in Supporting information). No apparent RTP, however, can be obtained, which might be ascribed to the destruction of intermolecular interactions and increased exposure to oxygen.
|Fig. 4. Photographs of the (a) recrystallized and (b) ground luminogens taken under 365 nm UV light. (c, e) Emission spectra and (d, f) XRD patterns of recrystallized and ground solids of (c, d) BPDPA and (e, f) BPTPA. λex = 370 nm except for recrystallized BPTPA (367 nm)|
Further XRD measurements were carried out to gain more information on the mechanism of mechanochromism. As depicted in Figs. 4d and f, the diffraction patterns for the recrystallized solids of BPDPA and BPTPA exhibit sharp and intense peaks, thus verifying their ordered structure and crystalline nature. Those of their ground powders, however, are basically flat lines, suggestive of their predominantly disordered amorphous characteristics. BP2DPA and BP2TPA demonstrate similar transition from crystalline to amorphous states upon grinding (Figs. S15b and d). Namely, the emission mechanochromism is highly related to the transition from crystalline to amorphous states before and after grinding. While the molecules adopt highly twisted conformations in the crystalline state, they may undergo conformation planarization and enhanced π-π interactions upon grinding, thus resulting in red-shifted emission.
Notably, unlike most CIP luminogens, which generally became less emissive upon grinding owing to the activated nonradiative pathways of phosphorescence, these luminogens, except BPTPA, however, demonstrate obviously boosted efficiency upon mechanical grinding, with Φ values of 34.0%, 11.6%, 12.0%, and 36.8% for the amorphous BPDPA, BP2DPA, BPTPA, and BP2TPA, respectively. Such opposite trends might be caused by multiple contradictory effects on the emission. In crystals, the luminogens could adopt a more twisted conformation to arrange themselves as a bulk lattice. The destruction of confined structures should have caused the quantum yield to decrease, which is common in many systems [22, 41, 42]. However, considering the confined packing modes are also twisted, the molecular structure could be tuned planar in response to external stress if the molecules are symmetric and with proper size. In that case, the destruction of their previous confined structures was counteracted by the formation of new kinds of dense packing modes, sothe effectiveintramolecular conjugation canbe expanded and emissions are therefore enhanced , which is the case of BPDPA, BP2DPA, and BP2TPA. As in BPTPA, its molecular structure is neither symmetric nor small enough, making it hard to form the above new packing modes, so its quantum yield suffers a decrease by the collapse of crystalline lattice.
It is also noted that the ground solids of BPDPA and BP2TPA own comparable efficiencies (~35%), whereas those of BP2DPA and BPTPA (~12%) are also approaching. The fluorescence lifetimes of the recrystallized solids before/after grinding are 1.50/5.04, 1.18/1.52, 2.73/4.35, and 2.48/4.15 ns for BPDPA, BP2DPA, BPTPA, and BP2TPA, respectively, which highly indicate the presence of unfavorable strong exciton interactions in the ground solids. The final efficiencies, however, should be determined by multiple factors, which may be associated with chemical and electronic structures, as well as the excimer-like interactions.
In summary, a group of D-A structured twisting compounds based on BP and D(T)PA were prepared and studied in view of their photophysical properties. These heavy-atom free pure organic luminogens exhibit typical CIP characteristics, generating fluorescence-phosphorescence dual emission at crystalline states. Such intrinsically dual emissive pure organics might be useful in revealing the underlying spin correlations of organic semiconductors within the lifetime of the excitons . Meanwhile, upon manual grinding, they undergo distinct phase transition from crystalline to amorphous states, accompanying conformation planarization and destruction of intermolecular interactions. Such changes induce obvious variations in emission color, wavelength, as well as efficiency, thus demonstrating remarkable mechanochromism.Acknowledgment
This work was financially supported by the National Natural Science Foundation of China (No. 51473092).Appendix A. Supplementary data
Supplementarymaterial related to this article can befound, in the online version, at doi: https://doi.org/10.1016/j.cclet.2018.08.001.
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