Solid-State Emission Enhancement via Molecular Engineering of Benzofuran Derivatives.

A series of linear benzofuran derivatives consisting of either a vinylene or a cyanovinylene were prepared in order to investigate their emission properties. The X-ray crystallography of structurally similar derivatives was also evaluated. The crystalline structures of the vinylene derivatives showed only lateral contacts that involved the benzofurans and no π-stacking. In contrast, π-stacking was observed for the bisbenzofuran and benzofuran-phenyl cyanovinylene derivatives. No intermolecular π-π stacking was observed for the extended cyanovinylene structures. Intermolecular bonding between the nitrile and a furan atom was found. The fluorescence quantum yields (Φfl) of the vinylene derivatives were consistently high (>50%) in both solution and the crystal state. The exception was the benzofuran-furan-vinylene-phenyl, the Φfl of which was <10% when in the solid state. The cyanovinylene counterparts emitted weakly in solution (Φfl < 2%). Their luminogenic property was demonstrated with a ca. 15-fold increase in emission in the solid state. A 6-fold emission enhancement was also found when they were aggregated in a 90 vol% methanol/water mixture. The solid-state emission enhancement of the cyanovinylene benzofurans was in part attributable to intermolecular contacts that suppressed excited-state deactivation by molecular motion.

Introduction

Fluorophores having emission that is enhanced when they are either aggregated or in the solid state can be defined as luminogens. All-organic luminogens that are highly emissive in the solid state are of interest for using in various optoelectronic applications, such as organic light-emitting diodes and solid-state lasers.[1] Both the emission efficiency and the emission wavelength of π-conjugated materials in the solid state are contingent on molecular structures. Their photophysical properties are also highly influenced by intermolecular interactions that take place between neighboring molecules in the solid state.[2]

Recent advances in aggregation-induced emission (AIE) have established certain luminogen design criteria.[3] In general, the emission can be enhanced by restricting molecular motion (RMM).[4] Enhancement in the solid-state emission can also be achieved by limiting the intermolecular π-stacking of luminogens.[5] This occurs by engaging both intra- and intermolecular supramolecular interactions.

Furans have recently emerged as interesting fluorophores because of their strong intrinsic emission.[6] In fact, fluorescence quantum yields (Φfl) upward of 50% in solution have been reported for oligofurans of varying degrees of oligomerization, ranging from a trimer to a nonamer.[7] The high emission is in part due to the rigidity of the conjugated structure, which prevents nonradiative deactivation processes, such as out-of-plane vibration.[6,8] The high emission is in contrast to the observation with thiophenes, the singlet excited state of which is quenched by intersystem crossing (ISC) upon photoexcitation.[9] While benzofuran is structurally similar to furan, its fused phenyl-furan structural is more rigid, making it highly emissive.[10] For example, the Φfl of BF1, consisting of two benzofurans covalently connected to a central furan (Figure ), was 71% in solution,[11] and that of its anthryl counterpart, BF5, was 43%.[12]

Figure 1

Benzofuran luminogens previously reported.[13,14]

The crystalline structures adopted by benzofurans in the solid state are also factors that contribute to their luminogen behavior. For example, the linear BF2 derivative was found to form a herringbone arrangement in the solid state.[13] This supramolecular structure suppressed intermolecular π-stacking, which is an efficient mode of fluorescence deactivation, resulting in Φfl = 75%. This is in contrast to its star (BF3) and cruciform (BF4) counterparts that π–π stack and have solid-state emissions reduced by 30 and 80%, respectively, compared to that of BF2. Luminogens having solid-state emission are therefore made possible by engineering intermolecular contacts that suppress unwanted fluorescence quenching modes, such as π-stacking.[14]

While the molecular-level requirements for AIE are well-established,[3,4] the supramolecular luminogen design principles for solid-state emission enhancement remain relatively underexplored.[15] Within this context, the benzofuran series 1–7 (Figure ) was prepared as luminogens to contribute to unraveling the supramolecular/emission enhancement factors. The benzofuran was targeted as the common framework because of its intrinsic fluorescence. Meanwhile, the relative ease of synthesizing the series allows access to a range of structurally variable counterparts. Their relative small size, linearity, and overall degree of conjugation were expected to result in their high degree of crystallinity. This, in turn, would provide key information about the structural effects, such as the roles of (i) cyanovinylene, (ii) the type of terminal aromatic, and (iii) the furan spacer, on modulating the solid-state emission enhancement. As such, the crystallography and both the solution and solid-state emission of 1–7 are herein presented.

Figure 2

Benzofuran derivatives prepared and investigated for emission enhancement.

Results and Discussion

Luminogen Preparation

The luminogens 1–7 were prepared according to Scheme S1. 1 was prepared in 70% yield by McMurry coupling of 2-benzofuran carbaldehyde. The other compounds were prepared by either Knoevenagel condensations or Wittig–Horner reactions from the corresponding aldehydes. 3 and 5 were prepared by Wittig–Horner reaction by adding a slight excess of BuONa to a stoichiometric amount of the corresponding aldehyde and phosphonate. The resulting E isomers of 3 and 5 were isolated in 45 and 65% yield, respectively. The Knoevenagel condensations between the corresponding aldehyde and acetonitrile derivatives were done at room temperature in EtOH with a catalytic amount of BuONa. The resulting precipitate was washed with a small amount of EtOH to afford 2, 4, 6, and 7 as microcrystalline powders in 55–70% yields.

The identity of each compound and their corresponding reagents was confirmed by NMR spectroscopy (1H NMR spectra of 1–7 are presented in Figures S3, S5, S7, S9, S11, S13, and S15). More specifically, coupling constants of 16 and 15.5 Hz were respectively measured for the vinylic protons by 1H NMR for the asymmetric luminogens 3 and 5. The coupling constants are consistent with an E isomer of the unsaturated bond. In contrast, only one signal was observed for the vinylic protons for the symmetric 1 and the cyanovinyl derivatives 2, 4, 6, and 7. This is consistent with a single isomer. An exact number of carbons was observed for all the compounds in the 13C NMR spectra (13C NMR spectra of 1–7 are presented in Figures S4, S6, S8, S10, S12, S14, and S16). This, taken together with the single vinylene proton peak, provides sound evidence for only one configurational vinylene isomer being formed. However, the E and Z configurations of 1 and the cyanovinyl derivatives cannot unequivocally be confirmed exclusively with the NMR data.

All the luminogens prepared were analyzed by single-crystal X-ray diffraction, in part to assign the absolute configuration of the vinylene. The E configuration of the vinylene segment of 1, 3, and 5 was confirmed by the X-ray data. Similarly, the Z configuration of the cyanovinylene fragment of 2, 4, 6 and 7 was confirmed by the crystallographic data (Figure ).

Figure 3

Resolved X-ray crystallographic structures of 1–7 showing their configuration adopted in the solid state and the dihedral angle of the vinyl-phenyl planes.

The diffraction studies were most importantly undertaken to evaluate the solid-state intra- and inter-supramolecular interactions of the vinyl and cyanovinyl derivatives. These non-covalent contacts are of particular interest because the emission properties of conjugated materials in the solid state are known to strongly correlate with their packing modes.[16] Making such correlations was possible with the compounds studied, owing to their crystals being void of any included solvent. The crystals were therefore structurally robust, and they could be handled for photophysical measurements without compromising their structural integrity. Although the compounds examined were different and were expected to have different crystal structures, general trends from the X-ray data could nonetheless be drawn. For example, the aromatics and the vinylene moieties of all the compounds examined were found to be coplanar. The planes described by the benzofuran and furan of the more extended structures (5–7) were also coplanar. The furans adopted an anti orientation with respect to the benzofuran. The crystallographic data confirmed that the benzofurans of 7 were perfectly coplanar and also coplanar with respect to the unsaturated bond that links them. In contrast, the phenyl of 6 was twisted by 35° from the plane defined by the furan and the cyanovinylene. For 5, the similar dihedral angle was twisted by only 7°.

The crystal packing obtained from the single-crystal X-ray crystallographic data is of importance for confirming both intra- and intermolecular interactions. For example, both 1 and 3 have common stacking modes. They have intermolecular interactions that occur at distances less than 3.5 Å (green dotted lines Figure ). These correspond to lateral contacts between the phenyl and the benzofuran atoms. Although 1 and 3 had similar intermolecular contacts, they differed in their crystal packing. In the case of 1, the contacts occurred cofacially between adjacent molecules that were offset from each other. The contacts occurred between the longitudinal termini of adjacent molecules for 3. Additional intermolecular contacts occurred between adjacent molecules within the unit cell of 1 and 3. These are represented by the blue lines in Figure . Despite their packing modes and intermolecular contacts, no discernible π-stacking occurred in the solid state for 1 and 3. In contrast, cofacially overlapping interatomic distances less than 3.5 Å (green dotted lines in Figure ) were found for 2 and 4. These attributes are consistent with π-stacking modes. The nitrogen atom of each of the nitriles was also found to hydrogen bond with the vinylic hydrogen atom of an adjacent molecule (orange dotted lines Figure c,d). The hydrogen bond distances (dN–H) were calculated to be 2.68 and 2.57 Å for 2 and 4, respectively.

Figure 4

Stacking modes of 1 (a), 3 (b), 2 (c), and 4 (d) assigned from the X-ray crystallographic data.

The crystallographic data confirmed that subtle structural differences, such as the nitrile substituted vinylene of 3 compared to 1, have resounding effects on the supramolecular arrangement. This was also the case for 5–7. Similar to 1–4, 5–7 also exhibited intermolecular contacts. These involved multiple contacts between the benzofurans of one molecule and the furan of another molecule in the unit cell (green lines Figure ). These inter-heteroatomic contact distances were found to be less than 3.5 Å. 5–7 were further found to pack into columns. The molecules in each column were shifted and they did not perfectly overlap cofacially. The nitriles of the cyanovinylene were also found to hydrogen bond. In the case of 6, the nitrile hydrogen bonded with a furan atom of a different molecule in the unit cell. The dN–H bond was calculated to be 2.64 Å. The similar hydrogen bond (dN–H) of 7 was slightly longer at 2.72 Å (orange lines Figure c). The collective single-crystal X-ray data confirmed that intermolecular contacts indeed occurred in the solid state of 1–7. These were expected to influence the emission properties of the luminogens. Emission differences were therefore expected between the solid state, where the intramolecular contacts are prevalent, and in dilute solution, where such contacts are nonexistent.

Figure 5

Stacking modes and intramolecular contacts identified from the single-crystal X-ray data of 5 (a), 6 (b), and 7 (c).

Theoretical Calculations

The electronic properties of the molecules 1–7 were evaluated by gas-phase DFT theoretical calculations. These were done to validate both the X-ray crystallographic data and to bridge the effects of structure on the spectroscopic properties (vide infra). The HOMO and LUMO energy levels were calculated from the fully optimized geometries (see Table S3). The planes described by the aromatics and the vinylene of the luminogens were found to be coplanar in the optimized gas-phase geometry (see Figure S1). The calculated structures were therefore consistent with those observed by X-ray crystallography. The exceptions were 5 and 6, whose benzofuran and phenyl were calculated to be coplanar, whereas the crystal data showed them to be twisted from coplanarity with respect to each other (vide supra). This notwithstanding, the HOMO and LUMO frontier orbitals were calculated and they were found to be delocalized entirely across the conjugated framework of the luminogens. The frontier orbitals, along with the calculated energy gap of 1, 2, and 7, are illustrated in Figure . The effect of the nitrile on the HOMO and LUMO energy levels is also evident in the figure. Notably, the nitrile substitution lowers both the LUMO energy level by 0.64 eV and the HOMO energy level by 0.35 eV, compared to its unsubstituted vinyl counterpart. The overall effect of lowering the two energy energies reduces the HOMO–LUMO energy gap, being 0.28 eV smaller for 2 relative to 1. A similar trend in energy gap reduction was found when adding the furan. In this case, the additional heterocycle extends the degree of conjugation and lowers the energy gap by ca. 0.3 eV. The effect of extending the conjugated chain with furan mainly destabilizes the HOMO level by 0.24 eV for 7 compared to 2. The perturbation effect of the nitrile group on the HOMO and LUMO energy levels was experimentally confirmed from the oxidation and reduction potentials by cyclic voltammetry (see Table S3 and Figure S2 for the cyclic voltammograms of 1, 2 and 7).

Figure 6

HOMO and LUMO frontier orbitals along with their corresponding energy levels calculated gas-phase by DFT.

Solution Spectroscopic Properties

In light of the solid-state intermolecular contacts that were expected to affect the luminogen emission, the spectroscopic properties of 1–7 were evaluated. Their absorption and fluorescence were first evaluated in dilute (10–5 M) chloroform (Figures and 8). These conditions were chosen to benchmark the properties and to ensure the absence of any supramolecular contacts and aggregates. The dilute conditions further ensured that the intrinsic emission properties of the luminogens were evaluated without any bimolecular effects. The latter would otherwise lead to lower than expected emission yields. Chloroform proved the best solvent for solubilizing 1–7. Moreover, its aproticity ensured that no solute–solvent hydrogen bonds occurred. These would otherwise lead to spectral shifts and potentially inaccurate emission yields.

Figure 7

Normalized absorption (dashed line) and emission (solid line) spectra at 10–5 M in chloroform (black line) and normalized solid-state emission spectra (blue line) of 1 (a), 3(b), and 5(c).

Figure 8

Normalized absorption (dashed line) and emission (solid line) spectra at 10–5 M in chloroform (black line) and normalized solid-state emission spectra (blue line) of 2 (a), 4(b), 6 (c), and 7(d).

A well-defined vibronic structure was observed for the absorption spectrum of 1 (Figure a) along with two maxima (374 and 355 nm) and a shoulder at 338 nm. Similarly, the emission spectrum was also well structured with a maximum at 374 nm and a shoulder at 396 nm. In fact, 3 and 5 exhibited structured absorption and emission (Figure b,c). It is worthy to note that a moderate Stokes shift of ca. 3300 cm–1 was observed for all the luminogens (Table ). This is consistent with the excited state being moderately stabilized by the solvent. This is in part due to a rigid system having little rotation around the benzofuran–vinyl bond. This locked configuration limits polarization in the excited state.

Table 1

Spectroscopic Data Measured in Both Chloroform Solution and the Solid State

	solutiona					solid stateb
compd	λmax, nm	λem, nm	ΔStokes, cm–1c	Φ, %d	τ, nse	kf, ×10–8 s–1 f	λem, nm	Φ, %d	τ, nse	kf, ×10–7 s–1 f,g
1	342	387	3300	72	0.87	8.3	415sh	55	2.03	27.1
	358	410					427
	380	435					463sh
2	395	440sh	3100	2	n.d.	n.d.	546	24	7.81 (88%)	2.74
	415sh	451							13.0 (12%)	(8.78 ns)
3	324sh	365	3700	55	0.95	5.8	437	50	2.13	23.5
	336	383					463sh
	355	406
4	353	410sh	5100	2	n.d.	n.d.	490	35	9.68 (5%)	1.71 (2.04 ns)
	365sh	430							20.9 (95%)
5	377sh	415	2500	77	1.55	4.9	470	7	n.d.	n.d.
	396	440					497
	415	465					528
6	400	480	4200	<1	n.d.	n.d.	502	15	1.14 (27%)	2.97 (5.05 ns)
							530sh		3.16 (65%)
									10.9 (8%)
7	437	494sh	3800	<1	n.d.	n.d.	590	10	2.20 (89%)	1.22 (8.19 ns)
		523							15.2 (11%)

Measured in deaerated chloroform.

Crystals deposited in a 2 mm U-shape quartz slide, sealed with a quartz coverslip, and measured under ambient conditions.

Calculated from the difference between absorption and emission spectra maximum, converted to cm–1, and uncorrected for λ2.

Absolute emission quantum yield measured with an integrating sphere.

Excited with a 360 nm ps-laser diode. Values in parentheses are lifetime weights (α), calculated according to I(t) = ∑(a e–).

Radiative rate constant, calculated according to kf = Φfl/τavg.

Values in parentheses are intensity average lifetimes, ⟨τavg⟩, expressed in ns, according to tavg = ∑(aτ2)/∑(aτ). n.d. = not determined.

Similar to what was observed with the crystallographic data, modifying the luminogen structure affected both the absorption and emission spectra. For example, replacing the benzofuran with a phenyl, as in 3, blue-shifted the absorption and emission maxima. However, this substitution preserved both the fine vibronic structure and the small Stokes shift. In contrast, the absorption and emission maxima were red-shifted when adding a furan to the luminogens, such as for 5. This substitution did not increase the Stokes shift. This implies that 5 remained rigid, similar to the other investigated luminogens. This is consistent with previously investigated oligofurans where the overall rigidity was not perturbed with the increasing degree of conjugation.[6] This is apparent from the measured fluorescence quantum yields (Φfl) of the luminogens. The measured absolute emission yields for both 1 and 5 in chloroform and methanol were greater than 70% (Table ). Replacing a furan in the conjugated framework with a phenyl, as in 3, decreased the Φfl to 50%. The emission is nonetheless significantly more intense than its all phenyl counterpart, trans-stilbene (Φfl = 5% in hexane).[17]

From the emission yield, it is apparent that the benzofuran reduces the intrinsic fluorescence deactivating pathways by photoisomerization and ISC. These quenching manifolds are typically extremely efficient in stilbenes.[18] In fact, it is worthy to note that no photoisomers were spectroscopically detectable either in solution and the crystalline state. Modifying the luminogen structure was also found to impact the excited lifetime (τ) when measured in solution. Common to the three vinylene derivatives was their mono-exponential fluorescence kinetics. The fluorescence lifetime increased by ca. 20%, when replacing the benzofuran with a phenyl (1 vs 3). Similarly, the lifetime increased by nearly 2-fold with an additional furan (5 vs 1). The fluorescence rate constants (kf), calculated according to τ/Φfl, decreased when progressing along the series from 1 to 3 and to 5. The respective calculated kf values were 8.3-, 5.8-, and 4.9 × 108 s–1 (Table ). These are comparable to the known kf for stilbene (5.9 × 108 s–1).[17] Meanwhile, the nonradiative rate constants (knr) were ca. 2.5-, 1.2-, and 3.4-fold slower than the corresponding kf values for 1, 3, and 5, respectively. Non emissive deactivation modes nonetheless account for >20% of the excited-state deactivation pathways, according to 1 – Φfl. The excited-state kinetics demonstrate that slight structural modification affects the excited lifetime. They also confirm that nonradiative deactivation, although present to a lesser degree than emission, is a slower mode of quenching. Meanwhile, the mono-exponential decay suggests that the emission is from only one species. This precludes photoisomerization and rotamer formation, as these photoproducts would also emit. This is consistent with a structurally similar derivative of 3, 2,3-distyryl benzofuran, that exhibited very low photoisomerization efficiency (ca. 3%).[19]

The effect of adding the nitrile to the vinylene is apparent when comparing the structurally similar series 1 vs 2, 3 vs 4, and 5 vs 6. Appending the nitrile to the unsaturated bond resulted in the loss of the vibronic structure in both the absorption and emission spectra of 2, 4, and 6, relative to their unsubstituted vinylene counterparts. This was also accompanied by a broadening of the absorption and a red shift, relative to the corresponding unsubstituted vinylene counterparts. Moreover, the emission spectra of the cyanovinylene derivatives were consistently red-shifted by ca. 40 nm relative to their counterparts for the three sets of luminogens studied. This is a result of the greater dipole inducing effect of the electronic withdrawing nitrile that enhances an intramolecular electronic push–pull with the electron rich benzofuran in the excited state. The resulting polarized state is stabilized more in the polar solvent used for the measurements than its unsubstituted vinylene counterparts, giving rise to the red-shift. Substituting the ethylene with the nitrile also impacted the emission yields. The Φfl of 2, 4, and 6 was essentially quenched (<2%) compared to their unsubstituted vinylene counterparts. This was expected as the nitrile of cyanostilbene derivatives is well known to enhance the rotational disorder of its aromatic neighbors.[20] This nonradiative deactivation mode is efficient in solution, resulting in suppressed emission.[21]

Solid-State Emission

To evaluate the cyanovinylene mediated solid-state emission enhancement, the emission of the unsubstituted vinylene counterparts was first examined in the solid state. These were examined to benchmark the solid-state emission properties. Substantial fluorescence quenching in the solid state would be expected with classical aggregation-caused quenching (ACQ) emission if this mode of excited-state deactivation was prevalent. The solid-state emission 1, 3, and 5 was red-shifted and less structured than when measured in solution. The perceived color emitted from 1 and 3 was blue, occurring at 437 and 427 nm, respectively. This was also accompanied by a weakly detectable green emission that was observed at 497 nm. The solid-state Φfl of 3 was consistent with its emission yield in solution. In contrast, the solid-state emission of 1 was 25% lower, compared to when measured in solution. It nonetheless fluoresced appreciably (Φfl = 55%) in the solid state. The solid-state arrangements adopted by 1 and 3 are in part responsible for their consistently high Φfl both in the solid state and in solution. π-Stacking modes that are known to contribute to ACQ were not observed in the X-ray data of 1 and 3. Rather, only lateral interactions were observed. This crystal arrangement is well-known not to quench the emission.[2b] In fact, 3 packs in a criss-cross fashion with two molecules in the unit cell being orthogonal and edge-on with their phenyls. Correlating the emission yields with the X-ray data, it is apparent that structurally modifying the luminogen by incorporating a furan affects the solid-state packing, which in turn impacts that the quantum yields. It is noteworthy that the furan is not expected to promote ISC, as the ISC yields are known to be consistent, regardless of the degree of oligomerization for oligofurans.[9b]

The fluorescence lifetimes of both 1 and 3 were uniquely mono-exponential, similar to what was observed in solution. A single species therefore emits in the solid state. This suggests that only one crystalline supramolecular arrangement is adopted. If other forms occurred, they were not fluorescent. The fluorescence lifetimes also increased by more than 2-fold in the solid state. Meanwhile, the knr reduced by 1.5- and 2-fold for 1 and 3, respectively. The longer solid-state lifetime, and hence a reduced radiative rate, of 1 and 3 further suggests excited-state deactivation by molecular motion. Hindering this internal quenching pathway via supramolecular means increases the fluorescence lifetime. In contrast to 1 and 3, the Φfl of their molecularly extended counterpart 5 decreased significantly (7%) in the solid state. This fluorescence quenching is most likely due to partial intermolecular π-stacking that takes place between the benzofuran and furan. These contacts were found in the single-crystal X-ray structure.

The cyanovinylenes 2, 4, 6, and 7 emitted more intense in the solid state than in solution. This was expected owing to the well-known solid-state-induced planarization of cyanostyryls, which are reinforced by intermolecular contacts involving the nitrile nitrogen.[22] It should be noted that the solid-state emission yields were measured with an integrating sphere. Both the direct and indirect scattering and emission were factored in.[23] This approach takes into account anisotropy effects and both reabsorption and emission from indirect excitation. Therefore, the measurements should be independent of the solid-state layer thickness and alignment of the crystals with respect to the excitation beam. Taking this into account, the fluorescence enhancement of the cyanovinylenes was found to be contingent on the molecular structure. For example, the solid-state emission was more enhanced for the smaller molecules (2 and 4) compared to their more structurally extended counterparts, 6 and 7. The emission of these two cyanovinylenes increased 12- and 17-fold, respectively, in the solid state. Comparing the absolute emissions of 2 vs 7 and 4 vs 6, the emission of the structurally extended derivatives was reduced. This is a result of the additional aryl–aryl bond that increases the excited-state quenching modes by bond rotation and vibration. The emission of 6 and 7 was nonetheless enhanced 15- and 10-fold, respectively, when measured as a powder and compared to their emission in solution. The weak fluorescence of the cyanovinylene derivatives in solution precluded their lifetime measurement. This aside, the solid-state emission enhancement for the cyanovinylenes can be in part assigned to intermolecular hydrogen bonds involving the nitrile, when factoring in the crystallographic data. These interactions rigidify the aggregate and suppress intrinsic fluorescence deactivation modes that involve molecular organization. Given the solid-state emission of the cyanovinylenes was far less than unity, hydrogen bonding of the nitrile is insufficient to completely suppress the fluorescence deactivation modes. In fact, the crystallographic data show that 2, 4, 6, and 7 have a propensity to π-stack. This arrangement is in part responsible for offsetting the full effect of the hydrogen bond mediated emission enhancement.

The emission wavelength was contingent on the luminogen structure. For example, the perceived color emitted from 4 was green with a broad emission centered at 490 nm. The emissions of 2 and 6 were perceived both as yellow, with their maximum at 547 and 530 nm, respectively. Meanwhile, the observed color emitted from 7 was orange with a maximum at 597 nm. The emission of 2 was red-shifted by 119 nm compared to counterpart 1. In the crystal packing, 2 adopted parallel and planar sheets that were connected via hydrogen bonds between the nitrile nitrogen and the vinylic hydrogen. The heteroatoms were found to be parallel and aligned along a common axis. This parallel configuration is assumed to lead to a large dipole moment, which contributes to the observed extended red shift. In contrast, 1 adopted a herringbone-like crystal packing. In this configuration, the molecules are offset and any collective dipole moment is canceled. The solid-state emission of both 3 and 7 was similarly red-shifted by ca. 60 nm compared to when measured in solution. The exception was 6, whose solid-state emission was shifted by only 20 nm relative to solution. The latter aside, the shifts indicate that the excited state of the luminogens is more stabilized in the crystal state than in solution. This is a result of the various intermolecular interactions that occur in the crystal packing. The excited states of both 3 and 4 were similarly stabilized, based on the minimal emission difference between them (7 nm). In contrast, the solid-state emission of 6 was red-shifted by 35 nm compared to 5. The increased stabilization of 6 can in part be ascribed to the overlapping aromatics and π-stacking in the crystal packing. These arrangements are not prevalent for 5, where the aromatics arrange in a zig-zag arrangement. The data demonstrate that both solid-state intermolecular contacts and electronic effects therefore affect the emission.

Unlike their unsubstituted vinylene counterparts, the emission lifetimes of the cyanovinylenes were best fitted to multi-exponential decays. In fact, two lifetimes were measured for 2, 4, and 7 in the solid state. This is consistent with two emitting species. The faster decay component of 2, 4, and 7 was the major contributor to the fluorescence kinetics. The shorter lifetime of 2 and 4 was ca. four times longer than the solid-state lifetime of their vinylene counterparts 1 and 3. The longer lifetimes of 2, 4, and 7 were ca. 2-fold longer than those of their fast components. It is worthy to note that crystalline needles of the luminogens were used for the solid-state emission. Correlating their emission lifetime with the resolved crystal structures should therefore be possible for the luminogens. However, accurate correlations are possible only when the excitation beam irradiates a unique crystal. Given such an alignment was not possible with for the lifetime measurements, only loose correlations can be drawn between the measured lifetimes and the solid-state packing. Nonetheless, the calculated radiative decay constants of the cyanovinylenes were an order of magnitude smaller than their unsubstituted vinylene counterparts. This is a result of the longer excited lifetime of the cyanovinylene derivatives. With the slow kr of 2, 4, and 7, the non-radiative deactivation modes are efficient and they can compete with emission. However, the knr values calculated for 2 and 4 from the intensity average lifetimes and quantum yields for the cyanovinylenes were an order of magnitude smaller than their counterparts 1 and 3. In fact, the kf/knr ratio in the solid state for 2, 6, and 7 was calculated to be 3, 5.5, and 9 respectively, whereas the ratio was ca. 1 for the other luminogens. This further demonstrates that the cyano group suppresses nonradiative modes in the solid state.

Given that the emission of AIE active luminogens can be enhanced when they aggregate, this effect was examined for 2 in varying water/methanol mixtures. Although the emission measurements were done in chloroform, methanol was chosen because of its high degree of miscibility with water. This is important for ensuring a homogeneous solution even at high water/methanol ratios that are often required for aggregating luminogens. The emission of 2 was first examined in neat methanol to benchmark any emission enhancement. Similar to what was observed in chloroform, 2 only weakly emitted in methanol (Φfl = 2%). Its emission intensity increased with increasing water fractions in methanol/water mixtures (Figure ). In fact, the most intense emission was observed with 80 and 90 vol% water; Φfl = 10 and 13%, respectively. The AIE effect of 2 in methanol/water is both qualitatively and quantitatively evident in Figure . The emission of the aggregate of 2 in methanol/water was red-shifted compared to the emission in homogeneous solution. The perceived color of the red-shifted emission was green with a maximum at 540 nm. The ca. 90 nm emission bathochromic shift was consistent with what was observed in the solid state. The large red shift implies a high degree of stabilization of the excited state upon aggregation. This is in part due to a high degree of intermolecular stacking. While the emission can be enhanced with aggregation, it can be increased 2-fold further in the solid state. The additional emission enhancement in the solid demonstrates the advantage of suppressing excited-state deactivation processes by molecular motion. These are possible by engaging intermolecular contacts, especially those that prevent π-stacking modes. The latter are also efficient emission quenching pathways.

Figure 9

Emission spectra of 2 in 100% methanol (black), 80 vol% (blue), and 95 vol% (red) water/methanol mixtures. Inset: photograph of 2 in methanol (left) and 80 vol% water in methanol (right) irradiated with UV lamp (350 nm).

Conclusion

The emission and crystallographic studies of a series benzofuran cynano-vinylenes and unsubstituted vinylenes derivatives were investigated. These were to provide insight into the structural effects on the emission yields and kinetics. Based on these findings, key information about the structural requirements for fluorescence enhancement can be derived. (i) The benzofuran-vinylene framework is required for high emission in both solution and the solid state. (ii) The terminal aromatic (phenyl vs benzofuran) does not perturb the emission in either state. (iii) While incorporating a furan into the luminogen framework does not affect the quantum yield in solution, it quenches the solid-state emission by an order of magnitude. This is in part owing to the crystal packing modes. (iv) In contrast, intermolecular hydrogen bonding in the solid involving the nitrogen of the cyanovinylene derivatives enhances the solid-state emission by an order of magnitude relative to when measured in solution. The solid-state emission of the cyanovinylenes is nonetheless less than its unsubstituted vinylene counterparts. While it is possible to modulate the solid-state emission by supramolecular interactions, additional intermolecular contacts are required to fully enhance the solid-state emission of benzofuran cyanovinylenes. Extending the cyano mediated hydrogen bonding to include additional supramolecular contacts could potentially offset the solid-state emission deactivation of furans and result in highly emitting furan vinylenes.

Experimental Section

General Experimental Methods

Unless otherwise stated, all the reactions were carried out under argon atmosphere with commercially available reagents and solvents. Anhydrous THF was obtained by distilling over sodium and benzophenone. The other reagent grade solvents were used without further purification. Melting points were obtained on a Start SMP melting point apparatus and they are uncorrected. Mass spectrometric (MS) measurements were recorded on a MALDI-TOF Bruker Biflex III instrument using a positive-ion mode. NMR spectra were recorded at 298 K on a NMR Bruker Avance III 300 spectrometer. Chemical shifts for 13C NMR spectra were recorded in parts per million using either the central peak of deuterated chloroform (77.23 ppm) or deuterated DMSO (39.51 ppm) as the internal standards. Proton NMR spectra were recorded in parts per million and they were referenced to the residual solvent proton according to the literature. Characteristic splitting patterns due to spin–spin coupling are expressed as follows: br = broad, s = singlet, d = doublet, t = triplet, q = quartet, p = pentet, m = multiplet.

Benzofuran-2-carbaldehyde (8), diethyl benzylphosphonate (14), benzofuran-2-ylboronic acid (10), 5-bromofuran-2-carbaldehyde (11), benzofuran-2(3H)-one (15), and 2-phenylacetonitrile (13) were purchased from either Aldrich or Jansen and they were used as received.

Spectroscopy

The emission spectra were recorded on a combined steady-state/time-resolved spectrometer with a double excitation and emission monochromator. Samples for solution analyses were prepared with an absorbance <0.05 at the corresponding excitation. The solutions were purged with nitrogen for at least 20 min to remove the dissolved oxygen. They were then sealed with a screw top with a Teflon-coated septum. The absolute quantum yields were measured with an integrating sphere. The absorptions of the solution samples were referenced against the scattering peak of the solvent at the corresponding excitation wavelength. The measurements were done with a minimum dwell time/wavelength of 1 s, measured at 1 nm intervals, and averaged over three scans. For the solid-state absolute quantum yields, crystals of the given compound were deposited on a rectangular quartz slide and caution was taken to neither compress nor disrupt the crystalline structure. The excitation beam was focused on the sample and the sample was irradiated with the entire beam. The sample emission from indirect light absorption in the sphere was factored into the absolute quantum yield calculation according to known means.[23] The excitation wavelength used for solution and solid-state emissions of 1–7 were respectively 350/350, 380/420, 320/340, 330/350, 380/400, 350/380, and 440/440 nm.

Excited-state lifetimes were calculated by single-photon time-correlated spectroscopy using a multi-channel plate time-resolved detector and exciting with a ps-laser diode. The instrument response frequency was determined by scattering the ps-laser diode excitation beam with a solid Teflon prism. The solid-state lifetimes were acquired with a thin film of the sample spread over a quartz slide. The slide was positioned at 45° to the excitation beam and the emission detector. The lifetimes were obtained by fitting either for a mono- or bifunctional decay function, whichever gave the best correlation of χ2 < 1.4.

Synthesis

The synthetic pathway and the characterization of the compounds 1–7 are presented in the Supporting Information.

Structure Refinement and Crystal Data

Single crystals suitable for X-ray diffraction analyses were obtained for all the compounds by the slow evaporation of ethanol–chloroform solution. The data collection details are found in Tables S1 and S2.