Positional Variation of Monopyridyl-N in Unsymmetrical Anthracenyl π-Conjugates: Difference between Solution- and Aggregate-State Emission Behavior.

Fluorescence enhancement on aggregation for π-conjugates linked with pyridyl ring has been established as a part of widely studied smart organic functional materials. Therefore, the photophysical features in the solution and aggregate states for such compounds remain impressive. In this work, we synthesized three series of photostable unsymmetrical aryl-substituted anthracenyl π-conjugates linked to pyridyl ring with a variation of the position of a pyridyl-N atom and examined the difference in the photophysical properties preferably in the aggregate state. The so-called "aggregation-induced emission (AIE)" behavior was discernible for the 2- and 4-pyridyl- but not 3-pyridyl-10-p-tolyl or mesityl-substituted π-conjugates. Curiously, a variation of the position of a pyridyl-N atom does not solely control the AIE phenomenon for 10-thiophenyl-substituted π-conjugates, where all of the isomers are found to be AIE-active. Hence, the dissimilarity in emission behavior in the aggregate state is governed by the position of N-atom for pyridine and also the substituent at the 10th position of the anthracyl ring. The mechanistic insight behind these observations is demonstrated by concentration-dependent fluorescence studies, time-resolved fluorescence, single-crystal X-ray diffraction studies (largely supportive to understand the molecular structure and packing in the aggregate), and average particle size measurement of the aggregates and partly by the density functional theory studies for a few representative molecules.

Introduction

The evolution in the discovery of organic small molecules as aggregation-induced emission active luminogens (n class="Chemical">AIEgens) has been continuing and being highlighted in many recent reviews.[1] The tremendous applications of such AIEgens in numerous fields, such as optoelectronic materials, optical security, and fluorescence sensing, have made a steady and common platform for researchers from interdisciplinary areas.[2] Among many such AIEgens, pyridyl-linked extensive π-conjugates could acquire its place as promising AIEgens in the literature.[3,4] There are typical and extensively reported AIEgens, such as tetraphenylethene (TPE), that are linked to pyridyl rings and established as potential smart materials and metal sensors.[3a,3b] The significant attractions of pyridine core are mainly due to the freely available lone pair on nitrogen residing at sp2 orbital and strong N···H bonding in the aggregated state resulting many changes in the fluorescence behavior under different external stimuli, such as acid/base, pressure, and temperature.[3,4] Apart from TPE, pyridine-linked anthracenyl π-conjugates are well established as useful AIEgens, as shown in Figure .[4,5]

Figure 1

Reported pyridyl π-conjugates A–C and their applications.

The pyridyl π-conjugate A was blended with tyrosine polymer to afford highly emissive polymeric material[5a] and was also used as a ligand to make complexes with solvatochromic behavior along with significant photophysical studies.[5b,5c] Further, the symmetrical n class="Chemical">di-styrylanthracene compound (B) is quite well recognized as a valuable material, where the pyridyl core played a crucial role[6] for the various applied field, as mentioned in Figure . Further, our literature search resulted in the molecule of type C that was reported as a patent related to the light-emitting device.[7] Notably, the electron-donating substitution (such as −methyl) at the 10th position of the anthracyl ring for A had the advantageous effect for the charge transfer.[5d] However, the AIE studies were not investigated for such aryl-substituted A. In spite of many disputes, the restricted intramolecular motion in the aggregate state is well established and documented as the main cause of the AIE effect by Prof. Tang and others.[8,1c] Thus, the influence of large twisting on the system would be beneficial to restrict the intramolecular motion to impose enhanced emission. With this clue, we were earlier successful to generate distinct anthracenyl π-conjugates[9] as new AIEgens, including these two pyridyl analogues ATh4P and AT4P(9b) (Figure ), where AT4P was established as multiple metal-ion sensors.[9c] Meanwhile, the literature reports on the substitution[10] and regioisomeric effect[11] on the AIE behavior due to different molecular packing in the aggregate state and electronic structure prompted us to synthesize a variety of regioisomeric pyridyl compounds (Figure ) and explore their photophysical behaviors in both solution and aggregate states.

Figure 2

Pyridyl π-conjugates linked to substituted anthracenyl skeleton.

Pyridyl π-conjugates linked to substituted n class="Chemical">anthracenyl skeleton.

More importantly, a precise change in the position of pyridyl nitrogen atom by keeping the other part intact was expected to exert a substantial effect on the molen class="Chemical">cular conformation, crystal packing, electronic structure, and subsequently the optical properties. Thus, such molecules with the same molecular formulae can display different photophysical behavior just because of the different conformational flexibility within the molecule.[10] At present, we focus on generating and studying the AIE properties for three series of regioisomeric pyridyl π-conjugates that are linked with (hetero)aryl-substituted anthracene (Figure ). The subtle change in the position of nitrogen for the pyridyl ring resulted in different absorption and emission behaviors on aggregation. In fact, the substitution effect at the 10th position of anthracenyl ring was also found to be exciting. The tolyl- and mesityl-substituted π-conjugates are AIE-active except 3-pyridyl isomers AT3P and AM3P that favor aggregation-caused quenching (ACQ) behavior. Surprisingly, all thiophenyl-substituted pyridyl isomers including ATh3P are recognized as relatively better AIEgen under similar conditions. Such interesting features need explanation. To understand the observed photophysical behaviors, the molecular structures were determined for selected molecules by single-crystal X-ray diffraction studies to find the supramolecular interactions in the aggregates. The observed facts are explained through the concentration-dependent Fl. studies, measurement of excited-state lifetime, and aggregate particle size along with the electronic structure of selected molecules.

Results and Discussion

Synthesis of the Molecules

These compounds were designed based on our earlier observations,[9b] and particularly, the n class="Chemical">thiophene substitution made the system unique as AIEgen. All of these compounds were synthesized in high yields via simple but efficient Horner–Wadsworth–Emmons reactions of phosphonates with the corresponding aldehydes in the presence of NaH or KOBu (Scheme ).

Scheme 1

Synthesis of Anthracene-Based π-Conjugates Linked to Regioisomeric Pyridine

All of the compounds were characterized using multinuclear NMR and mass spectroscopy. The trans-coupling was observed for some of these compounds; however, the molecular structures for few compounds were determined unequivocally by single-crystal X-ray diffraction studies. Initially, KOBu was preferred as a base compared to n class="Chemical">NaH (60% dispersed in mineral oil) due to the operational simplicity and easy purification process. All of these π-conjugates were synthesized using KOBu except ATh2P as it could not be synthesized using KOBu/tetrahydrofuran (THF) unexpectedly even after repetitive attempts. Instead, the reduced compound ATh2PR was obtained in 75% yield every time (Scheme ). Nevertheless, ATh2P was prepared in 72% yield using NaH. The compound with the presence of two CH2’s was identified by 1H NMR and finally characterized by single-crystal X-ray diffraction (Scheme ; right). Such reactions could be possible with KOBu/THF due to the unusual effect in organic synthesis.[12]

Scheme 2

Formation of Compound ATh2PR and Its Molecular Structure

However, such a compound is considered to be an analogue of C, as mentioned earlier (Figure ). Due to the loss of conjugation with pyridyl ring and being quite different from our fon class="Chemical">cus, we withhold ATh2PR for AIE studies. All of these π-conjugates are soluble in most of the water-miscible solvents and significantly stable under photoexcitation in both the solution and solid states, whereas the photostability was an issue for the previously reported system with similar properties, in which two 9-vinylanthracene parts are attached to only 1,2-positions of the benzene ring.[10]

AIE Studies

A solvent–nonsolvent system is typically preferred to study the difference in AIE behaviors. Among n class="Chemical">water-miscible solvents, including tetrahydrofuran, acetonitrile, and 1,4-dioxane, acetonitrile was preferred because of its relatively lower quantum yield compared to other solvents (Figure S1). Photophysical studies were carried out for all of the compounds in acetonitrile solution (10 μM) at room temperature. The AIE properties were examined by measuring the absorption and emission spectra for each compound in acetonitrile solution upon gradual addition of water fraction [a nonsolvent fw (v/v %)].

The absorption maxima peaked at ∼396 nm was profoundly observed in the UV–vis absorption spectra due to the familiar π–π* transitions of anthracene,[13] and the peak had a red shift by 5–15 nm upon addition of n class="Chemical">water (Figure a), indicating the possibility of J-aggregate formation.[14] However, red shift and enhanced fluorescence are not essentially limited to J-aggregates. The formation of nanoaggregate without J- or H-aggregation may also cause such effect called the Mie scattering effect.[14a] In fact, the crystal packing of AT2P also reveals no parallel alignment of the molecules (H-aggregation), rather it is more closer to the shape of J-aggregation (head-to-tail directional packing).[14b] The compound AT2P was fairly emissive at λmax = 490 nm with a quantum yield (Φf) of 10%, which quenched gradually with the increase of water fraction fw ∼ 70% (Figures b and 4a) due to the polarity effect that can stabilize the excited state.[11] Due to the presence of a pyridine ring, there could be a chance to generate the twisted intramolecular charge-transfer (TICT) state. However, the effect would be subtle, not like typical push–pull systems. To confirm the TICT effect, we have measured the emission spectra in solvents of different polarities, such as hexane, 1,4-dioxane, and acetonitrile. The red shift (25 nm) was somewhat significant for both AT3P and AT4P, whereas for AT2P, the shift is only 12 nm (Figure S2). The quenching of Fl. intensity was observed for all of these compounds upon increasing the polarity. Thus, the presence of the TICT state can be anticipated, although the effect is not much significant. There was a sudden enhancement in the Fl. intensity with 10 nm red shift when fw > 70% and that reached to a maximum between 80 and 90% of fw. The red shift can also be attributed to the molecular interactions in the aggregates. Although the measurements of Φf for the aggregates are known to be erroneous due to the light scattering effect of the nanoaggregates,[15] the quantum yield is measured to find the emission enhancement factor or AIE measurement numerical parameter αAIE [(Φf)a/(Φf)s; a: aggregate and s: solution].

Figure 3

(a) Absorption spectra and (b) emission spectra of compound AT2P (10 μM; λex = 405 nm) at different fw’s in acetonitrile.

Figure 4

I versus I0 (I0: Fl. intensity before addition of water; I: Fl. intensity after addition of water) for (a) AT2P, (b) AT3P, and (c) AT4P. [Concentration of the probe: 10 μM, λex = 405 nm.] The image is taken at fw = 0 and 90% for all of these three compounds under 365 nm UV light.

(a) Absorption spectra and (b) emission spectra of compound AT2P (10 μM; λex = 405 nm) at different fw’s in n class="Chemical">acetonitrile.

I versus I0 (I0: Fl. intensity before addition of water; I: Fl. intensity after addition of n class="Chemical">water) for (a) AT2P, (b) AT3P, and (c) AT4P. [Concentration of the probe: 10 μM, λex = 405 nm.] The image is taken at fw = 0 and 90% for all of these three compounds under 365 nm UV light.

However, considering the quantum yield and Fl. intensity, the emission enhancement factor is stated in Table . The compound AT2P showed an ∼2-fold enhancement in fluorescence intensity at fw = 90%. The formation of nanoaggregates for n class="Chemical">AT2P is validated through dynamic light scattering (DLS) studies that showed the average particle size of 116 nm. Thus, this compound was found to exhibit AIE behavior. The aggregate formation for few other aggregates was also measured by DLS where the average particle size was found to be within 200 nm (Table ). Moreover, the solution of aggregate without any precipitate indicated the particle size apparently within the nano range. Under similar conditions, the AIE properties were studied for other regioisomers AT3P and AT4P, where aggregation-caused quenching (ACQ) effect was observed only for AT3P. Interestingly, the compound AT3P was more emissive at λmax = 480 nm in the solution state (Φf = 22%) compared to the other isomers (Figure b; Figure S3 for spectra). The absorption and fluorescence data for all of the compounds studied herein are tabulated in Table . Although the compound AT4P is weakly fluorescent at λmax = 500 nm (Figure S4) (Φf = 2%), the AIE property was identified (Figure c) with a 5-fold intensity enhancement.[9b] Hence, the subtle change in the position of the nitrogen atom in pyridine could affect the emission behavior in both the solution and aggregate states.

Table 1

Photophysical Parameters Obtained from Absorption and Emission Studies

comp.	water fraction (fw %, average particle size in nm)	Abs λmax (nm)	Emi λmax (nm)	relative QYa (%)	αAIE (QY)	Fl. Int	αAIE (Fl. Int)
AT2P	00%	396	490	10	1	1785	1.92
	90%, 116	409	500	12		3429
AT3P	00%	395	480	22		5036
	99%, 73	403	500	5		2350
AT4P	00%	396	500	2	3.5	342	5.08
	90%	409	510	7		1740
AM2P	00%	398	485	14	0.92 (see text)	1105	1.05 (see text)
	80%, 187	406	490	13		1170
AM3P	00%	395	480	29		8773
	99%, 109	402	490	5		776
AM4P	00%	396	490	4	2	1353	1.61
	90%	407	500	8		2189
ATh2P	00%	398	475	<1	30	66	8.18
	90%, 126	408	504	6		540
ATh3P	00%	397	480	<1	25	119	11.73
	99%, 168	408	510	5		1396
ATh4P	00%	398	480	<1	90	29	42.6
	99%	413	510	9		1237

Relative quantum yield is calculated using coumarin-53B as a reference.

Relative quantum yield is calculated using n class="Chemical">coumarin-53B as a reference.

Besides, we replaced tolyl with n class="Chemical">mesityl to afford AM2P, AM3P, and AM4P that are simply reachable and expected to have better Fl. intensity. Apart from little higher quantum yield, the absorption and emission band appeared at almost similar λmax (∼480 nm) in the solution state compared to ATP series. The compound AM2P was fairly fluorescent in solution (Figure S5), and polarity-induced quenching was noted upon increasing the water fraction, yet it was suddenly enhanced at fw > 60% due to the AIE effect (Figure a). However, the Fl. intensity became almost equal in the solution and aggregate states and hence αAIE was trivial.

Figure 5

I versus I0 (I0: Fl. intensity before addition of water; I: Fl. intensity after addition of water) with fw for (a) AM2P, (b) AM3P, and (c) AM4P. [Concentration of the probe: 10 μM, λex = 405 nm.] The image is taken at fw = 0 and 90% for all of these three compounds under 365 nm UV light.

I versus I0 (I0: Fl. intensity before addition of water; I: Fl. intensity after addition of n class="Chemical">water) with fw for (a) AM2P, (b) AM3P, and (c) AM4P. [Concentration of the probe: 10 μM, λex = 405 nm.] The image is taken at fw = 0 and 90% for all of these three compounds under 365 nm UV light.

Alike to AT3P, n class="Chemical">AM3P also showed ACQ while it was significantly emissive in solution (Φf = 29%, higher than AT3P; Figures b and 6). Even though the compound AM4P was comparatively weak in emission behavior in solution (Φf = 4%), the AIE behavior was identified (Figures c and S6).

Figure 6

(a) Absorption spectra and (b) emission spectra of compound AM3P with various water fractions (10 μM; λex = 405 nm).

(a) Absorption spectra and (b) emission spectra of compound AM3P with various n class="Chemical">water fractions (10 μM; λex = 405 nm).

Based on our earlier observations on the role of heterocycles in originating the AIE effect,[9b] the comparative photophysical studies were fon class="Chemical">cused on the regioisomers of thiophene-attached π-conjugates ATh2P, ATh3P, and ATh4P (Scheme ). Interestingly, all of these regioisomeric compounds were almost nonemissive (Φf = <1%; 0.1–0.2%, Table ) in the solution state compared to other two series but considerably emissive in the aggregate state (Figure ).

Figure 7

I versus I0 (I0: Fl. intensity before addition of water; I: Fl. intensity after addition of water) with fw for (a) ATh2P, (b) ATh3P, and (c) ATh4P. [Concentration of the probe: 10 μM, λex = 405 nm.] The image is taken at fw = 0 and 90% for all of these three compounds under 365 nm UV light.

I versus I0 (I0: Fl. intensity before addition of water; I: Fl. intensity after addition of n class="Chemical">water) with fw for (a) ATh2P, (b) ATh3P, and (c) ATh4P. [Concentration of the probe: 10 μM, λex = 405 nm.] The image is taken at fw = 0 and 90% for all of these three compounds under 365 nm UV light.

For all of these molecules, the Fl. intensity remained almost unchanged when n class="Chemical">water was added up to 70–80%, and later, it was enhanced 8 times for ATh2P (Figure S7), 12 times for ATh3P (Figure ), and 45 times for ATh4P (Figure S8 and Table ). Thus, the AIE effect was very much prominent for this set of molecules. About 10 nm bathochromic shift was observed in the Fl. spectra for both aryl-substituted series in the aggregate state, whereas the thiophenyl compounds showed an ∼30 nm shift. A remarkable difference was noted for this series where all of the regioisomers inclusive of ATh3P were AIE-active as shown in Figures and 8.

Figure 8

(a) Absorption spectra and (b) emission spectra of compound ATh3P with different water fractions (10 μM; λex = 405 nm).

(a) Absorption spectra and (b) emission spectra of compound ATh3P with different n class="Chemical">water fractions (10 μM; λex = 405 nm).

Analysis

To give an insight into the observed differences in the emission behavior, we have initially made an effort to analyze the molecular structures and their possible conformational changes in the solution and aggregate states. The high Fl. intensity for n class="Chemical">AT3P and AM3P can be accounted to the involvement of different emitting species, i.e., few conformers that can contribute to decay in a radiative pathway.

Moreover, the concentration-dependent fluorescence was studied for all of these molecules to find out the molen class="Chemical">cular form present in the solution and that can be responsible for the variation of Fl. intensity in the solution state. When the low concentration is maintained at 10–8–10–10 M, the structured band at ∼450 nm was noted along with a signature at ∼500 nm, which became prominent with an increase in the concentration of probe AT2P (Figure b). The only emission peaked at ∼500 nm was observed at the concentration of 10–4–10–6 M, where the maximum intensity was observed at 10–5 M solution (Figure a). Based on this observation, the peak at ∼450 nm can be attributed to the monomer and the ∼500 nm peak can be accounted for the excimer.[16] Hence, the Fl. behavior is sensitive to the concentration of the solution. Such facts reveal the excimer as an origin of the emission at this concentration. The λmax values for monomer and excimer are very close to the reported anthracenyl compounds, indicating the major role of anthracene skeleton in emission behavior.[17]

Figure 9

Concentration-dependent emission spectra of AT2P: (a) 10–3–10–10 M in 1,4-dioxane and (b, c) closer look to observe monomer and excimer.

Concentration-dependent emission spectra of AT2P: (a) 10–3–10–10 M in n class="Chemical">1,4-dioxane and (b, c) closer look to observe monomer and excimer.

There were no such recognizable changes in the emission pattern by changing the concentration for AT3P (Figure a), and this can perhaps support the nonexistence of excimer and can be considered as an origin of comparatively higher Fl. intensity. For n class="Chemical">AT4P, the phenomenon was similar to AT2P, as observed in Figure b.

Figure 10

Concentration-dependent emission spectra of (a) AT3P and (b) AT4P in 1,4-dioxane; (c, d) closer look to observe monomer and excimer.

Concentration-dependent emission spectra of (a) AT3P and (b) n class="Chemical">AT4P in 1,4-dioxane; (c, d) closer look to observe monomer and excimer.

Further, the same studies were continued for thiophene series where all of the regioisomeric compounds exhibited the monomer and excimer emission at a lower concentration with extremely low intensity (Figure ). On gradual increment of concentration, the intensity became somewhat significant, although these compounds were almost nonemissive in the solution state. Such ocn class="Chemical">currence can also be depicted by the fast intramolecular rotation process for a small thiophene group, which can serve as a nonradiative relaxation decay path.[1c]

Figure 11

Concentration-dependent emission spectra of (a) ATh2P, (b) ATh3P, and (c) ATh4P in 1,4-dioxane.

Concentration-dependent emission spectra of (a) ATh2P, (b) ATh3P, and (c) n class="Chemical">ATh4P in 1,4-dioxane.

Further, fluorescence lifetime measurements were carried out to find the excited-state decay behavior for all of these regioisomers. The excited-state decay pattern was biexponential with major (>90%) and minor components (<10%) for most of these molecules (except n class="Chemical">AM3P that showed single exponential decay; see Table S1). The lifetime decay profiles for all of the compounds in solution and aggregate states are documented in the Supporting Information (Table S1 and Figures S9–S17). In the aggregate state, all of the molecules started relaxing through three pathways from the excited state. As expected, the solutions of AT3P and AM3P had relatively larger weighted mean lifetimes (1.98 and 2.04 ns, respectively) in comparison to their other regioisomers, and such relatively higher lifetimes supported the intense emission for these isomers. The nonfluorescent nature of the aggregate state for these isomers can be attributed to the relatively shorter excited-state lifetime (0.11 ns for AT3P and 0.25 ns for AM3P) compared to the other isomers where lifetime was found to be almost equal to or even higher than the solution state. The relatively larger lifetime for AM3P also indicated the higher emissive character. A completely different scenario was noted for thiophene series where the lifetime was significantly short for all of the isomers (within 0.02–0.04 ns) in solution state as evident from the poor emitting ability. Still, in comparison to solution state, the fluorescence lifetime was significantly boosted for the aggregates (0.34 ns for ATh2P and 0.68 ns for ATh3P). In particular, ATh4P showed a substantial rise in the lifetime (from 0.02 ns in the solution state to 0.97 ns in the aggregate state) to acquire the striking AIE properties.

Crystal Structure Analyses

As the Fl. behavior differs in the aggregate state, it is very much essential to understand the molecular structure with the possible conformational flexibility in the aggregate state. The supramolen class="Chemical">cular interactions in the aggregates can also play a significant role in emission behavior.[18] In this context, the determination of X-ray structures for the regioisomeric molecules and detailed scrutiny on inter/intramolecular interactions within the molecular packing are very much crucial. The crystals of some of the compounds (AT2P, AM2P, AM3P, ATh4P, ATh3P) were easily grown from dichloromethane/hexane (1:1) or EtOAc/hexane mixture by the solvent evaporation method, and the structures were solved successfully (see Table S2). Few cases (AM2P, AM3P, ATh4P), disorder issues were noted; however, it could be fixed and refined successfully. Of note, we could not crystallize all of the regioisomers from the same series. In fact, we were questioning the difference in molecular conformation and packing between AM3P and ATh3P, where the major difference in AIE behavior appeared. Compound AT2P crystal grows in the monoclinic space group P21/c with two symmetry-independent molecules per asymmetric unit (Figure b).[19,10a]

Figure 12

(a) Molecular packing through ac view for AT2P and (b) two symmetry-independent molecules with selected torsional angles (°) and distances (Å).

(a) Molecular packing through ac view for n class="Chemical">AT2P and (b) two symmetry-independent molecules with selected torsional angles (°) and distances (Å).

The torsion angle between anthracene and n class="Chemical">tolyl ring is 78.7° for one and 85.6° for another molecule, and it indicates the better conjugations in one molecule than the other. There are nine effective intermolecular interactions present in this crystal packing of AT2P. The strong π···π interaction with a distance of 3.320 Å was found to be coupled with multiple C–H···π interactions. However, AM2P crystallizes in a triclinic space group (P1̅) with one symmetry-independent molecule per asymmetric unit (Figure S18). The twisted molecular structure has a 95.4° torsional angle between anthracene and mesityl group. This compound has 10 supramolecular forces like π···π stacking and C–H···π (Table ), including strong H···H interactions (in the range of 2.2–2.4 Å). Therefore, a good number of intermolecular interactions can confine the molecule in the twisted conformation within the crystal lattice and restrict the rotation of molecules, which is proved to be the cause of the AIE effect.[1,2] The detrimental effect of these multiple interactions within the molecule might also be the cause for mild AIE effect. Both the compounds AM3P and AT3P exhibit the ACQ effect on aggregation, and the analysis of crystal structure and packing for AM3P, as obtained, are very important to explain the observation.

Table 2

List of Various Interactions within the Crystal Packing

comp.	C–H···π (Å)	C···C (π···π, (Å)	H···H (Å)	centroid···centroid (Å)	S···H (Å)	C···S (Å)	N···H (Å)
AT2P	2.852, 2.859, 2.808, 2.875, 2.776, 2.661, 2.694, 2.846	3.320		5.090	NA	NA
AM2P	2.883, 2.837, 2.771, 2.716, 2.867	3.347, 3.397	2.157, 2.362, 2.335	6.199	NA	NA
AM3P	2.884, 2.781, 2.809, 2.872, 2.892		2.329, 2.330	8.984	NA	NA
ATh3P	2.891, 2.892	3.315	2.307	5.037	2.985, 2.989	3.435
ATh4P	2.830, 2.835, 2.866, 2.827			5.090		3.294	2.717

Although the intermolecular interactions within n class="Chemical">AM3P are relatively less (only seven), the molecular structure allowed to pack with almost 100% overlap with one another from different planes. In spite of having a twisted molecular structure, it could align exactly in a parallel orientation to facilitate strong crystal packing where energy transfer becomes effective. Few strong C–H···π interactions (Table ) with distances of 2.781 Å (between two anthracenyl rings), 2.872 Å (anthracene···ethylene), and 2.892 Å (pyridine···pyridine) play an important role for such tight molecular packing (Figure ) with maximum overlap.

Figure 13

(a) Molecular structure with selected torsional angles (°) and bond distances (Å); (b) molecular packing one after another that leads to the formation of 100% overlap (with ∼0° interplanar angle between two layers); and (c) molecular packing (bc view) with maximum overlap.

(a) Molecular structure with selected torsional angles (°) and bond distances (Å); (b) molen class="Chemical">cular packing one after another that leads to the formation of 100% overlap (with ∼0° interplanar angle between two layers); and (c) molecular packing (bc view) with maximum overlap.

Next, the AIE characteristics for n class="Chemical">ATh3P made us curious to find the packing of the molecules in the condensed phase. A detailed scrutiny on thiophene-based molecular structure reveals comparatively much better conjugation as the thiophene ring is twisted about 81° for ATh4P and 74° for ATh3P. In fact, ATh3P had relatively more supramolecular forces where S gets involved in three types of intermolecular interactions [Figure ; two S···H (2.989 Å) and one C···S (3.435 Å), see Table ], including strong π···π stacking (3.315 Å), but the twisted molecular structure rigidifies the molecular conformations to undergo intramolecular rotation process. Thus, despite the presence of numerous interactions, twisted molecular conformation did not assist the formation of exciplex or excimer in the aggregate state (unfavorable to the fluorescence), rather locked the intramolecular rotation to result in the enhancement in emission intensity by blocking the nonradiative pathways.

Figure 14

(a) Molecular structure of ATh3P with selected torsional angles (°) and distances (Å) and (b) molecular packing (bc view).

(a) Molecular structure of n class="Chemical">ATh3P with selected torsional angles (°) and distances (Å) and (b) molecular packing (bc view).

The thermal stabilities of these molecules were determined, and the TGA (thermogravimetric analysis) studies revealed the decomposition of all of these molen class="Chemical">cules only after 300 °C (Figure S20).

Electronic Structure

Following usual expectations, the distribution of electron cloud plays a crucial role in dictating the optical properties of these molecules. Ground-state optimization using DFT/cam-b3lyp/6-311++g(d,p) indicates that the highest occupied molecular orbital (HOMO) is primarily located on anthracene, while the lowest unoccupied molecular orbital (LUMO) spills over toward pyridine (Figure ). However, the overall contribution from anthracene toward HOMO and LUMO indicates the major role of this skeleton to control the optical properties of these molecules, as observed experimentally. We find a reasonable contribution from pyridine to build the LUMO for these compounds. The relatively small ground-state dipole moment (ca. 2–3 Debye, Table S3) values in these compounds indicate a possible absence of typical electron donor–acceptor moieties.

Figure 15

HOMOs and LUMOs of AM3P and ATh3P.

HOMOs and LUMOs of AM3P and n class="Chemical">ATh3P.

Conclusions

In conclusion, three series of pyridinyl molen class="Chemical">cules have been synthesized by varying the position of pyridinyl N-atom where the quenching could favor for AM3P or AT3P (anticipated being similar packing motif), but the molecular packing of ATh3P did not favor the Fl. quenching in the aggregate state, rather rigidified the intramolecular rotation that relaxes the molecules from excited state in a radiative pathway. The multiple supramolecular interactions with the twisted conformational structure are responsible for such changes. Thus, the emissive behavior not only differs with the change in position of pyridyl N-atom, but it also changes with different substitution at the 10th position of the anthracyl ring. We could differentiate the emission behavior of the molecules through concentration-dependent Fl. studies, where only AT3P and AM3P showed intense fluorescence in solution, originated from monomers. The lifetime of the excited states could clearly identify the reason for variation of the Fl. behavior for these regioisomeric molecules with different substituents. Comparatively better AIE behaviors for thiophenyl-substituted regioisomers were supported by much longer lifetime in the aggregate state compared to the solution state. This study will be useful to explore different AIEgens with the pyridyl core in terms of their variances in photophysical behaviors by changing the position of N-atom.

Experimental Section

General Consideration: Reagents

All experiments were carried out in hot-air oven-dried glassware under nitrogen and n class="Chemical">argon atmosphere. Diethyl ((10-(aryl)anthracen-9-yl)methyl)phosphonates were prepared in our lab using the reported procedure.[9e] KOBu was purchased from Sigma-Aldrich and used as received. THF was redistilled from sodium metal and benzophenone mixture. All other reagents were purchased from common suppliers and used without further purification. Column chromatography was performed by using silica gel 100–200 mesh. Reactions were monitored by thin-layer chromatography on precoated silica gel 60 F254 plates (Merck & Co.) and were visualized by UV light (∼365 and ∼254 nm).

Analytical Methods

1H, n class="Chemical">13C NMR spectra were recorded in CDCl3 solution using Bruker Avance DRX (400 and 500 MHz). The signals were referenced to tetramethylsilane, and the solvent used is deuterated chloroform (7.26 ppm in 1H, 77.16 ppm 13C). Chemical shifts are reported in ppm, and multiplicities are indicated by singlet (s), doublet (d), triplet (t), and doublet of a doublet (dd). Elemental (elem.) analyses were carried out on a CHN analyzer. The fluorescence spectra were recorded on a Hitachi spectrofluorimeter. The electronic absorption spectra were recorded with a JASCO-650V UV–vis scanning spectrophotometer. ESI-LCMS was recorded in Shimadzu LCMS-2020. The X-ray quality crystals of the compounds were grown by slow diffusion of n-hexane over CH2Cl2 solution. The X-ray quality crystals of the salts of the compounds were grown by slow diffusion of acetonitrile and hydrochloric acid. Single-crystal X-ray data were collected on a Rigaku XtaLAB Pro 200 diffractometer using graphite-monochromated Mo or Cu radiation. Data were collected and processed using CrysAlisPro (Rigaku Oxford Diffraction). The structures were solved by direct methods and refined by full-matrix least-squares method using standard procedures. Absorption corrections were done using Lorentz and polarization effects, where applicable. In general, all non-hydrogen atoms were refined anisotropically; hydrogen atoms were fixed by geometry or located by a difference Fourier map and refined isotropically. All bond angles, bond or other distances, and dihedral angles are determined using Mercury 3.3 software. Time-resolved measurements were performed by using a time-correlated single-photon counting (TCSPC) spectrometer (Edinburgh, OB920) with a laser diode source (λexc. = 405 nm). A dilute ludox solution in water was used to measure lamp profile. F900 decay analysis software was used to analyze the decay curves by using the nonlinear least-squares iteration method. The quality of the fit was judged by the chi-square (χ2) values. Solid-state quantum yields were calculated using SC-30 integrating sphere module on FS5 spectrofluorimeter (Edinburgh Instruments). DLS particle size analysis was carried out using a Zetasizer Nano S from Malvern Instruments at 25 °C. The DFT studies were performed using the basis set CAM-B3LYP/6-311++g(d,p).

General Synthesis

((10-(Aryl)anthracen-9-yl)methyl)phosphonate was taken in a 25 mL round-bottom flask and dissolved in dry n class="Chemical">THF under inert atmosphere. An activated base was added to the above solution under nitrogen atmosphere. The solution was stirred for 2–3 min. Then, pyridine-n-carboxaldehyde (n = 2–4) was added and the reaction was stirred for 2–3 h. Completion of the reaction was monitored by thin layer chromatography. The reaction mixture was quenched with water, washed with brine, and extracted with ethyl acetate (20 mL × 2). The resulting organic layer was dried over anhydrous sodium sulfate and concentrated. And the resulting compound was purified by column chromatography using fractions of ethyl acetate in hexane.

AT2P: (E)-2-(2-(10-(p-Tolyl)anthracen-9-yl)vinyl)pyridine[19]

Diethyl ((10-(p-tolyl)anthracen-9-yl)methyl)phosphonate (0.3 g, 0.716 mmol), n class="Chemical">potassium tert-butoxide (0.321 g, 2.867 mmol), and pyridine-2-carboxaldehyde (0.076 g, 0.716 mmol) were used. The product was obtained as orange crystals in a yield of 78% (0.207 g); mp 192–196 °C; IR (ν cm–1, in KBr): 3564, 2923, 2851, 1731, 1583, 1508, 1464, 1431, 1386, 1262, 1021, 972, 812, 762; 1H NMR (400 MHz, CDCl3) δ 8.79 (d, J = 4.8 Hz, 1H), 8.63 (d, J = 16.3 Hz, 1H), 8.49 (d, J = 8.8 Hz, 2H), 7.77 (dd, J = 8.3, 2.6 Hz, 3H), 7.54–7.46 (m, 3H), 7.44 (d, J = 7.7 Hz, 2H), 7.41–7.35 (m, 4H), 7.29 (ddd, J = 7.5, 4.7, 1.0 Hz, 1H), 7.11 (d, J = 16.3 Hz, 1H), 2.57 (s, 3H); 13C NMR (101 MHz, CDCl3) δ 155.39, 149.92, 137.43, 137.10, 136.74, 136.67, 135.95, 132.21, 131.18, 130.19, 129.89, 129.39, 129.12, 127.44, 126.12, 125.27, 125.05, 122.50, 122.45, 77.31, 77.05, 76.80, 21.42; ESI-MS: 372 [MH]+; X-ray structure is determined for this sample.

AT3P: (E)-3-(2-(10-(p-Tolyl)anthracen-9-yl)vinyl)pyridine

Diethyl ((10-(p-tolyl)anthracen-9-yl)methyl)phosphonate (0.3 g, 0.716 mmol), n class="Chemical">potassium tert-butoxide (0.321 g, 2.867 mmol), and pyridine-3-carboxaldehyde (0.076 g, 0.716 mmol) were used. The product was obtained as a yellow-orange solid in a yield of 75% (0.199 g); mp 196–200 °C; IR (ν cm–1, in KBr): 3440, 3028, 1632, 1565, 1511, 1477, 1437, 1413, 1383, 1177, 1106, 1023, 968, 923, 812, 766, 747, 703; 1H NMR (400 MHz, CDCl3) δ 8.99–8.83 (m, 1H), 8.64 (d, J = 3.7 Hz, 1H), 8.39 (d, J = 8.8 Hz, 2H), 8.16–8.01 (m, 2H), 7.76 (d, J = 8.7 Hz, 2H), 7.54–7.31 (m, 9H), 7.00 (d, J = 16.6 Hz, 1H), 2.57 (s, 3H); 13C NMR (101 MHz, CDCl3) δ 149.02, 148.56, 137.63, 137.17, 135.81, 133.82, 132.95, 132.91, 131.81, 131.14, 130.17, 129.38, 129.14, 127.69, 127.55, 125.76, 125.42, 125.07, 123.70, 77; ESI-MS: 372 [MH]+; elem. analysis: found C, 90.42; H, 5.61; N, 3.85. C28H21N requires C, 90.53; H, 5.70; N, 3.77.

AT4P: (E)-4-(2-(10-(p-Tolyl)anthracen-9-yl)vinyl)pyridine: reported[9b,9c]

AM2P: (E)-2-(2-(10-Mesitylanthracen-9-yl)vinyl)pyridine

Diethyl ((10-(mesityl)anthracen-9-yl)methyl)phosphonate (0.3 g, 0.672 mmol), n class="Chemical">potassium tert-butoxide (0.328 g, 2.688 mmol), and pyridine-2-carboxaldehyde (0.072 g, 0.672 mmol) were used. The product was obtained as a greenish yellow solid in a yield of 80% (0.214 g); mp 254–258 °C; IR (ν cm–1, in KBr): 2913, 1609, 1582, 1562, 1516, 1465, 1430, 1376, 1191, 1145, 1025, 985, 906, 853, 762, 750; 1H NMR (400 MHz, CDCl3) δ 8.82–8.74 (m, 1H), 8.63 (d, J = 16.2 Hz, 1H), 8.49 (d, J = 8.8 Hz, 2H), 7.78 (td, J = 7.7, 1.8 Hz, 1H), 7.55–7.45 (m, 5H), 7.36 (ddd, J = 8.6, 6.5, 1.1 Hz, 2H), 7.31–7.26 (m, 1H), 7.13 (t, J = 8.1 Hz, 3H), 2.49 (s, 3H), 1.77 (s, 6H); 13C NMR (101 MHz, CDCl3) δ 155.41, 149.92, 137.62, 137.15, 136.75, 136.62, 136.03, 134.73, 131.88, 129.82, 129.60, 129.54, 128.27, 126.40, 126.39, 125.41, 125.36, 122.49, 122.47, 21.27, 20.07; ESI-MS: 400 [MH]+; elem. analysis: found C, 90.10; H, 6.23; N, 3.45. C30H25N requires C, 90.19; H, 6.31; N, 3.51. X-ray structure is determined for this sample.

AM3P: (E)-3-(2-(10-Mesitylanthracen-9-yl)vinyl)pyridine

Diethyl ((10-(mesityl)anthracen-9-yl)methyl)phosphonate (0.3 g, 0.672 mmol), n class="Chemical">potassium tert-butoxide (0.328 g, 2.688 mmol), and pyridine-3-carboxaldehyde (0.072 g, 0.672 mmol) were used. The product was obtained as a greenish yellow solid in a yield of 74% (0.198 g); mp 184–188 °C; IR (ν cm–1, in KBr): 2913, 1608, 1582, 1562, 1465, 1430, 1376, 1192, 1145, 1026, 985, 906, 853, 762, 750; 1H NMR (400 MHz, CDCl3) δ 8.92 (d, J = 1.7 Hz, 1H), 8.68–8.57 (m, 1H), 8.40 (d, J = 8.8 Hz, 2H), 8.15–8.02 (m, 2H), 7.58–7.46 (m, 4H), 7.43 (dd, J = 7.8, 4.8 Hz, 1H), 7.40–7.32 (m, 2H), 7.12 (s, 2H), 7.04 (d, J = 16.6 Hz, 1H), 2.48 (s, 3H), 1.75 (s, 6H); 13C NMR (101 MHz, CDCl3) δ 148.95, 148.52, 137.55, 137.22, 133.75, 133.04, 133.02, 132.91, 131.47, 129.59, 129.54, 128.33, 128.30, 127.67, 126.50, 126.05, 125.54, 125.44, 123.72, 21.25, 20.01; ESI-MS: 400 [MH]+; elem. analysis: found C, 90.08; H, 6.27; N, 3.58. C30H25N requires C, 90.19; H, 6.31; N, 3.51. X-ray structure is determined for this sample.

AM4P: (E)-4-(2-(10-Mesitylanthracen-9-yl)vinyl)pyridine

Diethyl ((10-(thiophen-2-yl)anthracen-9-yl)methyl)phosphonate (0.3 g, 0.672 mmol), n class="Chemical">potassium tert-butoxide (0.328 g, 2.688 mmol), and pyridine-4-carboxaldehyde (0.072 g, 0.672 mmol) were used. The product was obtained as a yellowish orange solid in a yield of 84% (0.225 g); mp 242–246 °C; IR (ν cm–1, in KBr): 3017, 1741, 1590, 1515, 1376, 1370, 1206, 1133, 962, 850, 763; 1H NMR (400 MHz, CDCl3) δ 8.72 (d, J = 6.1 Hz, 2H), 8.38 (d, J = 8.8 Hz, 2H), 8.25 (d, J = 16.5 Hz, 1H), 7.61–7.53 (m, 4H), 7.50 (ddd, J = 8.8, 6.5, 1.3 Hz, 2H), 7.37 (ddd, J = 8.6, 6.5, 1.1 Hz, 2H), 7.13 (s, 2H), 7.01 (d, J = 16.6 Hz, 1H), 2.49 (s, 3H), 1.76 (s, 6H); 13C NMR (101 MHz, CDCl3) δ 150.43, 144.51, 137.51, 137.28, 136.67, 134.95, 134.53, 130.91, 130.25, 129.55, 128.34, 126.56, 125.90, 125.71, 125.51, 120.97, 21.26, 20.01.; ESI-MS: 400 [MH]+; elem. analysis: found C, 90.06; H, 6.39; N, 3.48. C30H25N requires C, 90.19; H, 6.31; N, 3.51.

ATh2P (E)-2-(2-(10-(Thiophen-2-yl)anthracen-9-yl)vinyl)pyridine

Diethyl ((10-(mesityl)anthracen-9-yl)methyl)phosphonate (0.3 g, 0.730 mmol), n class="Chemical">sodium hydride (0.07 g, 2.923 mmol), and pyridine-2-carboxaldehyde (0.078 g, 0.730 mmol) were used. The product was obtained as a yellow-orange solid in a yield of 72% (0.191 g); mp 166–170 °C; IR (ν cm–1, in KBr): 3446, 2976, 1637, 1583, 1561, 1464, 1432, 1374, 1027, 971, 840, 762; 1H NMR (400 MHz, CDCl3) δ 8.74 (m, 1H), 8.59 (d, J = 16.3 Hz, 1H), 8.48–8.42 (m, 2H), 7.89 (dd, J = 8.1, 1.1 Hz, 2H), 7.79 (td, J = 7.7, 1.8 Hz, 1H), 7.64 (dd, J = 5.2, 1.1 Hz, 1H), 7.52–7.42 (m, 5H), 7.34 (dd, J = 5.2, 3.4 Hz, 1H), 7.32–7.28 (m, 1H), 7.22 (dd, J = 3.4, 1.1 Hz, 1H), 7.08 (d, J = 16.3 Hz, 1H); 13C NMR (101 MHz, CDCl3) δ 155.17, 149.92, 139.20, 136.88, 136.79, 133.84, 131.69, 129.60, 129.49, 129.23, 128.85, 127.16, 127.02, 126.72, 126.10, 125.66, 125.39, 122.61, 122.57; ESI-MS: 364 [MH]+; elem. analysis: found C, 82.59; H, 4.76; N, 3.78. C25H17NS requires C, 82.61; H, 4.71; N, 3.85; S, 8.82.

ATh3P: (E)-3-(2-(10-(Thiophen-2-yl)anthracen-9-yl)vinyl)pyridine

Diethyl ((10-(mesityl)anthracen-9-yl)methyl)phosphonate (0.3 g, 0.730 mmol), n class="Chemical">potassium tert-butoxide (0.328 g, 2.923 mmol), and pyridine-3-carboxaldehyde (0.078 g, 0.730 mmol) were used. The product was obtained as a greenish yellow solid in a yield of 75% (0.199 g); mp 208–212 °C; IR (ν cm–1, in KBr): 3433, 2923, 2853, 1634, 1565, 1517, 1477, 1436, 1415, 1373, 1336, 1262, 1221, 1121, 1025, 970, 886, 841, 823, 799, 768, 736; 1H NMR (400 MHz, CDCl3) δ 8.96–8.88 (m, 1H), 8.65–8.58 (m, 1H), 8.34 (d, J = 8.4 Hz, 2H), 8.14–8.00 (m, 2H), 7.89 (d, J = 9.1 Hz, 2H), 7.63 (dd, J = 5.2, 1.2 Hz, 1H), 7.52–7.42 (m, 5H), 7.32 (dd, J = 5.2, 3.4 Hz, 1H), 7.21 (dd, J = 3.4, 1.2 Hz, 1H), 6.97 (d, J = 16.6 Hz, 1H); 13C NMR (101 MHz, CDCl3) δ 149.11, 148.56, 139.04, 134.10, 133.44, 133.01, 132.80, 131.69, 129.55, 129.24, 129.09, 127.39, 127.22, 127.17, 126.81, 125.78, 125.73, 125.59, 123.76; ESI-MS: 364 [MH]+; elem. analysis: found C, 82.51; H, 4.76; N, 3.81. C25H17NS requires C, 82.61; H, 4.71; N, 3.85; S, 8.82.

ATh4P: (E)-4-(2-(10-(Thiophen-2-yl)anthracen-9-yl)vinyl)pyridine[9b]

ATh2PR: 2-(2-(10-(Thiophen-2-yl)anthracen-9-yl)ethyl)pyridine

Diethyl ((10-(mesityl)anthracen-9-yl)methyl)phosphonate (0.3 g, 0.730 mmol), n class="Chemical">potassium tert-butoxide (0.328 g, 2.923 mmol), and pyridine-2-carboxaldehyde (0.078 g, 0.730 mmol) were used. The product was obtained as orange crystals in a yield of 75% (0.199 g); mp 190–194 °C; IR (ν cm–1, in KBr): 3442, 3065, 2924, 1587, 1565, 1518, 1468, 1432, 1375, 1328, 1220, 1175, 1145, 1033, 986, 882, 836, 761, 747, 726; 1H NMR (400 MHz, CDCl3) δ 8.70 (d, J = 5.7 Hz, 1H), 8.41 (d, J = 8.9 Hz, 2H), 7.88 (d, J = 8.8 Hz, 2H), 7.66–7.58 (m, 2H), 7.52 (ddd, J = 8.9, 6.5, 1.3 Hz, 2H), 7.41 (ddd, J = 8.7, 6.5, 1.0 Hz, 2H), 7.32–7.29 (m, 1H), 7.23–7.16 (m, 3H), 4.17–4.09 (m, 2H), 3.39–3.32 (m, 2H); 13C NMR (101 MHz, CDCl3) δ 161.29, 149.37, 139.70, 137.00, 135.39, 131.98, 129.63, 129.36, 128.15, 127.72, 127.27, 126.78, 125.74, 125.48, 124.41, 123.36, 121.69, 39.44, 28; ESI-MS: 364 [MH]+; X-ray structure is determined for this sample.