Effects of nonaromatic through‐bond conjugation and through‐space conjugation on the photoluminescence of nontraditional luminogens

Photoluminescence (PL) mechanisms of nontraditional luminogens (NTLs) have attracted great interest, and they are generally explained with intra/intermolecular through‐space conjugation (TSC) of nonconventional chromophores. Here a new concept of nonaromatic through‐bond conjugation (TBC) is proposed and it is proved that it plays an important role in the PL of NTLs. The PL behaviors of the three respective isomers of cyclohexanedione and gemdimethyl‐1,3‐cyclohexanedione were studied and correlated with their chemical and aggregate structures. These compounds show different fluorescence emissions as well as different concentration, excitation and solvent‐dependent emissions. The compounds which undergo keto‐enol tautomerism and hence with a conjugated ketone‐enol structure (i.e., nonaromatic TBC) show more red‐shifted emissions. TBC effect reduces the energy gaps and facilitates the formation of stronger TSC in the aggregate state. The compounds in the ketone‐enol form are also prone to occur excited state intra/intermolecular proton transfer (ESIPT). The cooperative effect of nonaromatic TBC and TSC determines the PL behaviors of NTLs. This work provides a novel understanding of the PL mechanisms of NTLs and is of great importance for directing the design and synthesis of novel NTLs.

The photoluminescence (PL) mechanisms for NTLs have attracted great interest.Tang et al. and Yuan et al. proposed "clusteroluminescence (CL)" [24] and "clusterizationtriggered emission (CTE)" mechanisms, [25,26] respectively.The key point of these two mechanisms is the intra-and/or intermolecular through-space conjugation (TSC) [24,[27][28][29] through the overlap or sharing of electron clouds of NCCs, which leads to the formation of extended n-n, n-π and π-π conjugation and hence lowered energy levels, promoting electron transitions and photoluminescence emissions.32][33] The PL behaviors of NTLs can be preliminarily explained by TSC effect, and it has been proved by some indirect evidence and theoretical calculations.For example, oligo(maleic anhydride)s (OMAhs) [24] in both solution and solid powders emit bright fluorescence, while poly[(maleic anhydride)-alt-(2,4,4-trimethyl-1-pentene)] (PMP) is nearly nonemissive.Theoretical calculations show that the distances between two ─C═O groups of adjacent MAh units on OMAhs are in the range of 2.84-3.18Å, enabling the formation of strong TSC through π-π interactions.But, for PMP, due to the steric hindrance of 2,4,4-trimethyl-1-pentene groups, the distances between two ─C═O groups are greater than 5 Å, therefore, no or only weak TSC can be formed.In 2017, we also found that TSC effect is present in the fluorescent aggregates of aliphatic oximes (R─C═N─OH) [34] through intermolecular hydrogen bonds.
In fact, intra/intermolecular TSC effect also exists in aromatic compounds and it has an important impact on the emissions of traditional luminescent compounds.In 2017, Zhang et al. [35] reported the AIE behaviors of 1,1,2,2-tetraphenylethane (s-TPE) and 1,1,2,2-tetrakis(2,4,5trimethylphenyl)ethane (s-TPE-TM), both are with isolated phenyl rings.The fluorescence emissions of the two compounds in the aggregated state are red-shifted with comparison to those in the dispersed state, and s-TPE has a higher quantum yield (QY) and more red-shifted emission than s-TPE-TM.Theoretical calculations prove that the electron clouds of the benzene rings in s-TPE and s-TPE-TM overlap in the aggregated state, resulting in obvious intramolecular TSC effect and lowered HOMO-LUMO energy gaps.In contrast, due to the steric hindrance of the methyl groups on the benzene rings of s-TPE-TM, its intramolecular TSC effect is significantly weaker than that of s-TPE.Recently, Tang et al. [36] regulated the TSC effect of triphenylmethane and its derivatives by introducing different electron-donating (─OCH 3 , ─N(CH 3 ) 2 ) and -withdrawing groups (─NO 2 , ─CN) onto their phenyl rings, achieving the adjustment of their luminescent colors and QYs.
Strong TBC is always present in traditional organic luminescent compounds constructed by large π-conjugated systems and it plays a dominant role in the luminescence of traditional luminogens.The combinational effect of TBC and TSC determines the photoluminescence behaviors of the luminogens.For traditional luminogens showing ACQ behavior, due to their rigid and planar π-conjugated systems, the degree of intramolecular TBC is very large, so they show strong emissions in single-molecule state.However, in the aggregated states, the combined action of strong TBC and strong TSC will make excited molecules to form excimer/complex, causing fluorescence quenching.On the contrary, if the compounds possess weak even no TBC effect, intra/intermolecular TSC effect can promote their emissions.In addition, both TSC and TBC effects are beneficial to rigidify the molecular conformations and hence inhibit the nonradiative decay of excited molecules.Therefore, regulating TBC and/or TSC effect is the key to obtaining proper PL behaviors.
Up to now, most studies on the PL mechanisms of NTLs are limited to the discussion of the aggregated structure and the intra/intermolecular TSC effect of NCCs.In fact, only a few NTLs completely rely on TSC interactions to generate fluorescence.These NTLs contain only atoms with n electrons or isolated unsaturated bonds, such as various saccharides and pentaerythritol. [37]On the contrary, most reported natural and synthetic NTLs contain two or more continuous unsaturated bonds and/or atoms with n electrons, which can form π-π, p-π or multiple conjugations.For example, C═O and O (or N) atoms in carboxyl group [38] and amide group [39] can form π-p conjugation, and C═C, C═O and N atoms in acrylamide [40] can form π-π-p conjugation.The reported NTLs with high QYs and red-shifted emissions are generally with TBC effect. [32,38,41]44] For easy distinction and understanding, we proposed the concepts of aromatic and nonaromatic TBC to distinguish the TBC of π-conjugated benzene ring or aromatic heterocycle in traditional luminogens and the TBC of NCCs in NTLs, respectively.
Up to now, less attention has been paid to the contribution of nonaromatic TBC on the photoluminescence of NTLs.In this work, we studied the relationships between the photophysical properties and structures of three isomers of cyclohexanedione (CHD) and three isomers of dimethyl-1,3-cyclohexanedione (DMCHD).Owing to the different positions of keto-carbonyl and gem-dimethyl groups, some of these compounds can undergo keto-enol tautomerism, leading to the formation of nonaromatic TBC among adjacent C═O, C═C and ─OH groups in the isomerized products.Meanwhile, because of the different positions of functional groups and substituents, TSC in these compounds are also affected.These molecules have simple chemical structures and their single crystal structures can be easily obtained, so it is easy to clarify the relationship between their PL behaviors and the nonaromatic TBC and TSC effects, and then further clarify the PL mechanism of NTLs.

Photophysical properties
The chemical structures of the isomers of CHD, that is, 1,2-CHD, 1,3-CHD and 1,4-CHD, and the isomers of DMCHD, that is, 2,2-DMCHD, 4,4-DMCHD and 5,5-DMCHD, are shown in Figure 1A.The isomers of CHD and DMCHD emit bright fluorescence in both solid state and solutions under 365 nm UV light, but their emissions vary significantly from sample to sample.As shown in Figure 1B, in solid state, the three isomers 1,2-CHD, 1,3-CHD and 1,4-CHD emit light blue, yellow and blue-white fluorescence, while the three isomers 2,2-DMCHD, 4,4-DMCHD and 5,5-DMCHD emit dark blue, yellow-green and green fluorescence, respectively.Under 365 nm UV light, the solutions of 1,2-CHD, 1,3-CHD and 1,4-CHD exhibit blue, blue-green, and blue-white emissions, while the solutions of 2,2-DMCHD, 4,4-DMCHD and 5,5-DMCHD exhibit dark blue, yellow-green and blue emissions, respectively (Figure 1C).The fluorescence excitation spectra and emission spectra of the isomers of CHD and DMCHD in solid state are  F I G U R E 2 (A,C) Fluorescence excitation spectra and (B,D) emission spectra of the isomers of CHD and DMCHD in (A,B) solid state and in (C,D) methanol solutions (0.2 mol L −1 ).The excitation and emission spectra were scanned with the maximum emission and excitation wavelengths, respectively.
shown in Figure 2A,B, respectively.For the isomers of CHD, 1,2-CHD and 1,4-CHD show narrow excitation spectra with maximum excitation wavelengths ( max ex ) at around 380 nm, while 1,3-CHD shows a much wider excitation spectrum in a wide wavelength range of 300-550 nm and with a  max ex significantly red-shifted to 439 nm.The maximum emission wavelength ( max em ) of 1,2-CHD and 1,4-CHD are at around 460 nm, while that of 1,3-CHD is the longest, up to 555 nm, corresponding to a yellow emission.Compared with 1,2-CHD, 1,4-CHD shows a wider emission peak with a strong shoulder at 530 nm, corresponding to blue-white color.For the isomers of DMCHD, 2,2-DMCHD shows very narrow excitation and emission spectra, with two excitation peaks at 360 and 380 nm, and two emission peaks with similar intensities at about 408 and 434 nm, leading to dark blue fluorescence.4,4-DMCHD shows a very wide excitation spectrum with a  max ex of 459 nm and a shoulder peak at around 380 nm.The  max em of 4,4-DMCHD is 540 nm, which The fluorescence excitation and emission spectra of the solutions (0.2 mol L −1 ) of the compounds are shown in Figures 2C and 2D, respectively.The  max ex of 1,2-CHD, 1,4-CHD, 2,2-DMCHD and 5,5-DMCHD solutions are almost the same, at about 365-370 nm, and the 2,2-DMCHD solution shows a shoulder peak at 303 nm.The  max ex of the 1,3-CHD and 4,4-DMCHD solutions are at 414 and 439 nm, respectively, and the 4,4-DMCHD solution shows a shoulder peak at about 365 nm.PL intensities of the solutions are obviously different.The 1,4-CHD solution shows the strongest PL intensity, followed by the 4,4-DMCHD and 1,3-CHD solutions, and the 2,2-DMCHD solution shows the weakest PL intensity.The  max em of the 2,2-DMCHD solution is the shortest (409 nm), and those of the 1,2-CHD, 1,4-CHD and 5,5-DMCHD solutions are in the range of 465−480 nm.The  max em of the 1,3-CHD and 4,4-DMCHD solutions are red-shifted to 519 and 539 nm, respectively.Among all compounds, the  max ex and  max em of 2,2-DMCHD are the shortest in both solid state and solutions, while in solid state those of 1,3-CHD are the longest, and in the solutions those of 4,4-DMCHD are the longest.For most NTLs, the  max ex and  max em of solid state are longer than those of solutions, but for 1,2-CHD, the  max em of solid state (460 nm) is shorter than that of the solution (480 nm).
The photoluminescence lifetimes of the compounds are all in nanoseconds (Figure S1), indicating the fluorescence nature of the emissions.The QYs of the compounds in solid state and methanol solutions were measured and are shown in Table 1.The QYs in solid state are generally more than 1.4 %, and the QYs of 1,3-CHD and 4,4-DMCHD are 3.5% and 5.4 %, which are the highest for the isomers of CHD and DMCHD, respectively.Except for 1,4-CHD, the QYs of the compounds are generally higher in solid state than in solutions and they increase with increasing concentration, exhibiting aggregation-enhanced emission (AEE) behavior.

Concentration-dependent and excitation-dependent emissions
The isomers of CHD and DMCHD in solutions exhibit concentration-dependent emission (CDE).The fluorescence spectra of the compounds in solutions with different concentrations are shown in Figure 3 and Figure S2.For the excitation spectra, an excitation peak at about 309 nm appears in the very dilute solutions of 1,3-CHD, 1,4-CHD, 4,4-DMCHD and 5,5-DMCHD, and it disappears when the concentration is more than 2 × 10 −2 mol L −1 , while a new excitation peak appears at 365 nm or longer wavelengths.The  max ex of 1,2-CHD and 1,3-CHD, 1,4-CHD keeps nearly constant at 370 nm, 405 and 365 nm with the further increase of concentration, respectively (Figure 3A-C).Note that two peaks at 376 and 439 nm appear in the excitation spectra of the 4,4-DMCHD solutions with concentrations more than 2 × 10 −2 mol L −1 , and the peak at 439 nm becomes more significant with increasing concentration (Figure 3D).The PL intensities of the isomers of CHD mostly increase with increasing concentration.The  max em of 1,2-CHD and 1,4-CHD red-shifts slightly (Figure 3A,C), while the  max em of 1,3-CHD red-shifts significantly with increasing concentration (Figure 3B).For the isomers of DMCHD, the CDE of 2,2-DMCHD is similar to that of 1,2-CHD (Figure S2a), but the CDE of 4,4-DMCHD and 5,5-DMCHD is very different from the isomers of CHD.Their PL intensity firstly decreases and then increases with increasing concentration, accompanied by an abrupt red-shift of  max ex and  max em (Figure 3D and Figure S2b).For instance, when the concentration is less than 4 × 10 −3 mol L −1 , the  max ex and  max em of 4,4-DMCHD are 309 and 406 nm, respectively, but when the concentration is more than 2 × 10 −2 mol L −1 , its  max ex and  max em red-shift more than 100 nm, and when the concentration is more than 1 × 10 −1 mol L −1 , they are 439 and 537 nm, respectively.
In addition, these compounds in both solid state and solutions exhibit excitation-dependent emission (EDE) (Figure S3), a common feature of NTLs.With the increase of excitation wavelength (λ ex ), their PL intensity generally firstly increases and then decreases, and their  max em firstly keeps almost constant or changes very slightly and then gradually red-shift.An abrupt red-shift in  max em from 380 to 460 nm is observed for 1,4-CHD in solid state when the λ ex is increased from 300 to 320 nm (Figure S3b).And the first peak at 399 nm of the dual emission peaks of 4,4-DMCHD in a dilute solution vanishes while the second peak at 522 nm becomes dominant with the increase of λ ex (Figure S3f).The EDE behavior is explained as the presence of different clusters with different extents of TSC in the aggregates. [11,14,19]

Solvent, viscosity and temperature-dependent emissions
The fluorescence spectra of 1,4-CHD and 1,3-CHD in different solvents (2 × 10 −2 mol L −1 ) are shown in Figures 4A and 4B, respectively.The PL intensity of 1,4-CHD solutions decreases with increasing solvent polarity, while the  max ex and  max em keep almost constant at 375 and 447 nm, respectively.On the contrary, the PL intensity of 1,3-CHD in the nonpolar solvents, dichloromethane (DCM) and acetone (Ace), are very similar but low, it increases significantly in the polar solvents, especially in DMF.The  max em of 1,3-CHD in the nonpolar solvents is at 492 nm, while it blue-shifts to 482 and 477 nm in the polar solvents DMF and methanol, respectively.Two peaks at 372 and 390 nm are shown in the excitation spectra of 1,3-CHD in the nonpolar solvents DCM and Ace, while in the polar solvents, the      peak at 372 nm still presents, but in methanol the peak at 390 nm red-shifts to 401 nm, and in DMF the peak at 390 nm becomes a shoulder peak and a new peak appears at 352 nm, respectively.Viscosity-dependent fluorescence emissions of the compounds were studied by measuring the emission spectra of them in mixed solvents of glycerol and methanol with different glycerol fractions (Figure 4C and Figure S4).The PL intensity generally increases with the increasing glycerol fraction of the solutions, due to the increase of viscosity and hence the inhibition of nonradiative decay of excited state by retarding molecular motion.The increase of PL intensity with the glycerol fraction is more significant for 5,5-DMCHD and especially 1,2-CHD than other compounds.The emission wavelengths of the compounds keep constant or change slightly with the increase of glycerol fraction.The PL intensity of the 1,3-CHD methanol solution decreases with increasing temperature, and it increases when the temperature is decreased (Figure 4D), due to the lowered molecular motion at a low temperature.

Structural characterizations
To understand the electronic transitions of the isomers of CHD and DMCHD, the UV-Vis spectra of their methanol solutions were measured (Figure 5).For their dilute solutions (2 × 10 −5 mol L −1 ), UV absorption bands in the range of 200-310 nm, ascribed to π-π* and n-π* transitions, are observed.For the isomers of CHD, the 1,3-CHD, 1,4-CHD and 1,2-CHD solutions show strong, weak and nearly no absorptions at 208 nm, respectively, and the 1,2-CHD and 1,3-CHD solutions show strong absorption at 265 and 278 nm, respectively.For the isomers of DMCHD, 4,4-DMCHD and 5,5-DMCHD show strong absorptions at 208 and 278 nm, but the absorption of the latter is stronger, while the absorbance of 2,2-DMCHD at 208 nm is the highest, but its absorbance at 278 nm is the weakest, only stronger than that of 1,4-CHD.These results prove that the different positions of functional group (carbonyl) and gem-dimethyl group lead to the variation of electronic structures and energy levels of the compounds.1,3-CHD, 4,4-DMCHD and 5,5-DMCHD show stronger absorption in the long-wavelength range than other compounds, indicating the greater conjugation of these compounds.
The UV-vis spectra of their solutions with different concentrations are shown in Figure 5B-D and Figure S5.With increasing concentration, the absorbances at 208 and 278 nm of these compounds all increase.Moreover, a new absorption peak appears at 271 nm for 1,4-CHD (Figure 5C), and for the 1,4-CHD, 1,3-CHD and 4,4-DMCHD solutions, absorption bands appear in the range of 350-500 nm, indicating the formation of new chromophores. [25]t is well-known that some diketones with enolizable α-protons undergo keto-enol tautomerism. 1 H NMR characterization results of these compounds are shown in Figure 6 and Figure S6.In CDCl 3 and CD 3 OD solutions, 1,2-CHD undergoes full tautomerism, in which a carbonyl group is transformed into an enol group, resulting in a conjugated ketone-enol structure (Figure 6A).1,3-CHD undergoes partial tautomerism, in which both diketone and ketoneenol structures coexist (Figure 6B).While 1,4-CHD keeps the diketone structure (Figure 6C).Among the isomers of DMCHD, 2,2-DMCHD is completely in a diketone structure (Figure S6a), while diketone and ketone-enol structures coexist in 4,4-DMCHD and 5,5-DMCHD (Figure S6b,c). 13C NMR spectra also prove the occurrence of keto-enol tautomerism or not of the compounds (Figure S7).
The ratios of diketone to ketone-enol (n k :n e ) structures of 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD in CDCl 3 and CD 3 OD solutions with different concentrations are shown in Table 2. Solvent affects the keto-enol tautomerism of the compounds.In CDCl 3 , at the same concentration, the ratio of ketone-enol structure in the 1,3-CHD solutions is the largest, while it is the smallest in the 5,5-DMCHD solutions.In CD 3 OD, the ratio of ketone-enol structure in the 4,4-DMCHD solutions is the highest, while it is the lowest in the 1,3-CHD solutions.And the ratio of ketone-enol structure increases with increasing concentration in both CDCl 3 and CD 3 OD solutions.For example, the n k :n e of 4,4-DMCHD in CD 3 OD solutions decreases from 5.27:1 at 0.02 mol L −1 to 0.58:1 to 0.1 mol L −1 (Figure S6d and Table S1).
The single crystal structures of the compounds are obtained from the CCDC database (Table S2).Crystal structures further confirm that 1,4-CHD [45] is in a diketone structure and 1,3-CHD, [46] 4,4-DMCHD [47] and 5,5-DMCHD [48] are in ketone-enol structures in solid state.The crystal data of 1,2-CHD have not been reported, but theoretical calculations show that 1,2-CHD is in a ketone-enol structure. [49]hese characterizations confirm that, for the isomers of CHD, 1,2-CHD is in a ketone-enol structure and 1,4-CHD is in a diketone structure in both solutions and solid state, while, for 1,3-CHD, diketone and ketone-enol structures coexist in solutions and the proportion of the ketone-enol structure increase with increasing concentration, and it is in a ketoneenol structure in solid state.For the isomers of DMCHD, 2,2-DMCHD is in a diketone structure in both solutions and solid state, while 4,4-DMCHD and 5,5-DMCHD undergo tautomerism in solutions and solid state similar to that of 1,3-CHD.

PL mechanism based on TBC and TSC
In the ketone-enol structures, the C═O, C═C and ─OH groups form π-π-p conjugation, that is, a type of nonaromatic through-bond conjugation (TBC).The difference in the UVvis absorptions and fluorescence emissions of the isomers of CHD and DMCHD in the discrete state solutions) can be explained by the presence or absence of TBC effect.The TBC effect stabilizes the enol structures and extends conjugation, leading to the change of molecular electronic structures and the decrease of energy gaps of electronic transitions, and hence the red-shifted UV absorptions and fluorescence emissions.For 1,2-CHD, 1,3-CHD, 4,4-DMDCHD and 5,5-DMCHD, due to the partial or total transformation from the diketone to the ketone-enol structures, the TBC effect leads to the appearance of an absorption peak at about 265 nm (for 1,2-CHD) and 278 nm (for the other three compounds) (Figure 5), and the generally more red-shifted fluorescence emissions in both solutions and solid state (Figures 1 and 2).On the contrary, due to the lack of enolizable α-protons, 1,4-CHD and 2,2-DMCHD cannot undergo keto-enol tautomerism and hence no TBC is formed.Therefore, they show only the UV-vis absorption peaks of separated ketone groups at about 208 nm in dilute solutions and short-wavelength emissions.
However, the photoluminescence behaviors of the isomers of CHD and DMCHD cannot be fully explained with the presence or absence of TBC effect, especially in concentrated solutions and solid state.For example, 1,2-CHD and 1,3-CHD are both in ketone-enol structures with TBC effects, but their emission behaviors in solutions and solid state are very different.Moreover, the UV-vis absorptions and fluorescence emissions of some compounds change dramatically with increasing concentration.TSC effect is the other important factor affecting the luminescence behaviors of these compounds.
The single crystal structures provide precise conformations, molecular stacking mode and intermolecular interactions of the compounds, facilitating the study their TSC effects.As shown in Figure 8A, 1,4-CHD adopts a boat conformation, the two carbonyl groups O═C⋅⋅⋅C═O (d = 2.936 Å) in a molecule are in short distance and hence intramolecular TSC can be formed. [50]Moreover, short contacts of C⋅⋅⋅O (d 1 = 3.192 Å, d 2 = 3.207 Å) and O⋅⋅⋅O (d 5 = 3.691 Å) of the carbonyl groups amongst inter-layer 1,4-CHD molecules may also lead to the formation of intermolecular π-π and n-n TSC, respectively.1,3-CHD adopts a chair-like conformation (Figure 8B), in which the O═C─C═C─OH moiety forms ππ-p TBC in a plane.Moreover, adjacent 1,3-CHD molecules are connected with each other through strong hydrogen bonds (d 1 = 1.573Å), forming long chains of hydrogen-bonded O═C─C═C─OH groups in a plane, which is beneficial to electron delocalization and the formation of strong intralayer TSC.The parallel long chains are staggered in the c direction, in which the O⋅⋅⋅O ' distance between the adjacent two layers is d 2 = 3.615 Å, enabling the formation of interlayer TSC.Therefore, strong TBC and TSC are present in 1,3-CHD.1,2-CHD with a ketone-enol structure also adopts a chair conformation (Figure S8a), and TBC is formed in the O═C─C(OH)═C moiety.Unfortunately, intramolecular hydrogen bond (d = 2.217 Å) formed between the carbonyl (C═O) and hydroxyl (O─H) may inhibit the formation of effective intermolecular TSC.
To study the effects of TBC and TSC on molecular electron cloud surfaces and energy levels, theoretical calculations on these compounds were performed.We firstly calculated the electric potential distributions of the first excited state (S 1 ) of single molecular 1,4-CHD, 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD in the ketone-enol structure (Figure 9 and Figure S8b).The negative charges of 1,4-CHD are mainly located on the O of the carbonyl group, but the positive charges are distributed in the six-membered ring.For 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD, the negative charges are also mainly located on the O of the carbonyl group, but the positive charges are mainly on the H of the hydroxyl group.After introducing gem-dimethyl group into 4,4-DMCHD and 5,5-DMCHD, the positive and negative electron clouds are more aggregated on the conjugated O═C─C═C─OH, which is beneficial to the stabilization of TBC effect, resulting in stronger intermolecular electron transfer power and the smaller energy gap of 4,4-DMCHD and 5,5-DMCHD than 1,3-CHD. [51]he HOMO-LUMO energy levels of these compounds in single molecule are shown in Table 3.For comparison, the HOMO-LUMO energy levels of 1,3-CHD in diketone and ketone-enol structures were both calculated.Among the isomers of CHD, the HOMO-LUMO energy gap of 1,4-CHD is the highest (5.94 eV), while that of 1,2-CHD is the smallest (4.80 eV).The energy gap of 1,3-CHD is between those of 1,4-CHD and 1,2-CHD, and that in the diketone structure (5.71 eV) is higher than that in the ketone-enol structure (5.28 eV).And among the isomers of DMCHD, the energy gap of 2,2-DMCHD (5.58 eV) is higher than those of 4,4-DMCHD (5.21 eV) and 5,5-DMCHD (5.18 eV).These calculation results suggest that the compounds with TBC have lower HOMO-LUMO energy gaps than those without.Based on the molecular conformations from single crystal data, the energy gaps of different molecule numbers of 1,4-CHD, 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD were calculated (Figure 10 and Figure S9).Generally, the HOMO-LUMO energy gaps of all compounds decrease with increasing molecule number, due to the TSC effect induced by the overlap and/or sharing of intermolecular electron clouds when the molecules are in close contact in the aggregate state.Note that for the compound 1,4-CHD in a diketone structure, the energy gap decreases slightly with increasing molecule numbers (Figure 10A), but, for the compounds in the ketone-enol structures (e.g., 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD, Figure 10B,C and Figure S8b), their HOMO-LUMO energy gaps decrease dramatically from monomer to dimer and then gradually decrease with increasing molecule number, reaching very low HOMO-LUMO energy gaps of 2.39 eV and 3.26 eV for the tetramers of 1,3-CHD and 4,4-DMCHD, respectively.With comparison to 1,3-CHD, the energy gaps of 4,4-DMCHD and 5,5-DMCHD monomers are lower, but the decrease in energy gaps of them with increasing molecule number is less significant, therefore, the emission wavelengths of 4,4-DMCHD and 5,5-DMCHD in aggregate states are shorter than that of 1,3-CHD.These results indicate that TBC and TSC effects both contribute to the photoluminescence of NTLs, and the cooperative effect of TBC and TSC can effectively reduce the energy gaps of electron transitions and hence lead to red-shifted emissions of NTLs.
The crystal structures and theoretical calculations confirm that the isomers of CHD and DMCHD have different TBC and/or TSC effects.The combination of TBC and TSC effects determines the HOMO-LUMO energy levels and hence the fluorescence emissions of the compounds.
1,4-CHD and 2,2-DMCHD are always in the diketone structure in both solutions and solid state, and hence they cannot form TBC. This is the fundamental reason for their short-wavelength absorptions and emissions.The difference in their UV-vis absorptions and fluorescence emissions arises from their different TSC effects in the aggregate states.1,4-CHD can easily form strong intra-and intermolecular TSC, while 2,2-DMCHD fails to form strong TSC effect due to the steric hindrance of the gem-dimethyl group.Therefore, 1,4-CHD exhibits more red-shifted UV-vis absorptions and fluorescence emissions than 2,2-DMCHD in concentrated solutions and solid state (Figures 2, 5 and Figure S5).The  max ex of 2,2-DMCHD keeps almost constant in solutions with different concentrations and in solid state, while that of 1,4-CHD red-shifts slightly with increasing concentration (Figures 2, 3 and Figure S2) due to the formation of stronger TSC.Note that 1,4-CHD shows higher QY in solutions than in solid state, which is very uncommon in NTLs.A possible explanation is that 1,4-CHD molecules in solutions are easier to form aggregates in which the molecules can adopt proper conformations to form more clusters of nonconventional chromophores (ketone groups).
1,2-CHD is always in a ketone-enol structure with TBC, but the strong intramolecular hydrogen bonding between ─C═O and ─OH groups inhibits the formation of effective TSC effect, and hence it exhibits short-wavelength UVvis absorptions and blue fluorescence emissions.Notice that its UV-vis absorptions and fluorescence emissions are even blue-shifted with those of 1,4-CHD in the aggregate states.Its emission in solutions is slightly red-shifted with comparison to that in solid, the possible reason is that its intramolecular hydrogen bonding can be partially broken in solutions and hence intermolecular TSC can be formed.
1,3-CHD, 4,4-DMCHD and 5,5-DMCHD undergo partial tautomerism in solutions and the ratio of ketone-enol structure increases with increasing concentration.They are mainly in the diketone structure in dilute solutions, the lack of TBC and the low extents of TSC lead to their excitation and emission peaks in the low wavelength ranges.With the increase of concentration, the aggregation of molecules and the occurrence of keto-enol tautomerism lead to the formation of TBC (ketone-enol structure) and the enhanced TSC, leading to the red-shifted emissions.For 1,3-CHD, the keto-enol tautomerism occurs at a very low concentration, as evidenced by the significantly higher ratios (n k :n e ) of ketone-enol to diketone structures than 4,4-DMCHD and 5,5-DMCHD at the same concentrations (Table 2).Therefore, it shows a gradual red-shift rather than abrupt change in the emission wavelength as that for 4,4-DMCHD and 5,5-DMCHD with increasing concentration.The steric hindrance of the gem-dimethyl group in 4,4-DMCHD and 5,5-DMCHD leads to their weakened TSC and hence the blue-shifted fluorescence emissions of them with comparison to those of 1,3-CHD in solid state.Note that the 4,4-DMCHD exhibits more red-shifted  max ex and  max em and higher QY even than 1,3-CHD in solutions.The possible reason is that 4,4-DMCHD molecules are easier to adopt proper conformations to form more clusters with larger TSC than other compounds.
The appearance of very wide excitation and emission peaks, dual excitation and emission peaks as well as the EDE behavior of the compounds are all related to the formation or presence of clusters of NCCs with different extents of TSC in them.For example, the origin of the excitation peak at about 309 nm in the very dilute solutions is due to the isolated molecules in the diketone form.With the increase of concentration, the partial transformation of diketone to ketone-enol form leads to the red-shifted excitation wavelengths.The appearance of the excitation peak at 439 nm in the 4,4-DMCHD solutions, same as that in the solid state, is due to the TSC effect of 4,4-DMCHD in the ketone-enol form (Figures 2 and 3).
The different solvent-dependent fluorescence behaviors of 1,4-CHD and 1,3-CHD can also be explained with the difference in their TBC and TSC effects induced by the solvents.1,4-CHD in the diketo structure adopts a boat conformation, and hence it is a polar molecule.Therefore, it is easier to aggregate in a nonpolar solvent, leading to the enhanced PL intensity.While for 1,3-CHD, it undergoes keto-enol tautomerism and strong hydrogen bonds are formed between 1,3-CHD molecules in the aggregates, the polar solvents can compete to form hydrogen bonding with the hydroxyl group of the enol structure, leading to the lowered TSC effect and hence the blue-shifted emissions.On the other hand, the formation of hydrogen bonding between 1,3-CHD and the polar solvent is beneficial to the keto-enol tautomerism and the stabilization of the ketone-enol structure, leading to the enhanced PL intensity.

Excited state intra/intermolecular proton transfer mechanism
Excited state intra/intermolecular proton transfer (ESIPT/ESPT) is a fundamental photochemical process that plays a major role in both chemical and biological systems. [52,53]ESIPT process essentially involves the translocation of a proton between two hydrogenbonded groups (proton donor and proton acceptor) along the intra/intermolecular hydrogen bond within the same molecule or between two molecules in the excited state.Fluorophores that undergo ESIPT often give rise to dual emissions and large Stokes shifts.In recent years, organic photoluminescent materials exhibiting dual mechanisms of AIE and ESIPT have been reported and they show promising applications. [54,55]Intra/intermolecular hydrogen bonding is present in some NTLs, but no ESIPT has been reported in NTLs.
1,2-CHD, 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD undergo partial or total keto-enol tautomerism and form strong intra or inter-molecular hydrogen bonding in aggregate states.Therefore, these compounds are possible to occur ESIPT processes.We performed theoretical calculations on 1,2-CHD, 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD monomers to verify the presence of ESIPT.The conformations of the molecules in the diketone and ketone-enol forms and their transition state (TS) in methanol solutions in the ground state (S 0 ) and the first excited state (S 1 ) were firstly optimized (Figure S10).The Gibbs free energies of each conformation were calculated, and the potential barriers in the S 0 and S 1 states of each single molecule proton transfer process and its reverse process are shown in Figure 11.In methanol solutions, the potential barriers for the proton transfer of 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD in the ketone-enol structures in the S 1 state is only 38.59, 45.10 and 28.53 kJ mol −1 , respectively, which are much smaller than those for the proton transfer in the S 0 state, proving that ESIPT process is more prone to occur from the ketone-enol form to the diketone form.On the contrary, the proton transfer potential barriers for 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD in the diketone structures in the S 1 state are higher than those in the S 0 state, suggesting that the ESIPT process is unlikely to occur from the diketone form to the ketone-enol from.On the other side, 1,2-CHD has high energy barriers in both S 0 and S 1 states, indicating that intramolecular proton transfer is difficult to occur at normal temperature and pressure.
Furthermore, the intermolecular proton transfer processes of the compounds were investigated by scanning the S 0 and S 1 state potential energy curves using the dimer conformations with different O─H bond lengths.The original dimer structures in crystals and intermolecular proton transfer structures of 1,3-CHD, 4,4-DMDCHD, 5,5-DMCHD and 1,2-CHD are provided in Figure S11.As shown in Figure 12, as the O─H bond length increases, the S 1 energy of 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD gradually decreases, while the S 1 energy of 1,2-CHD first slightly increases and then decreases.No energy barrier or a low energy barrier for proton transfer in the S 1 state of the 1,3-CHD, 4,4-DMCHD, 5,5-DMCHD and 1,2-CHD dimers suggests a fast ESPT process.
These calculations prove that the compounds in the ketoneenol form can occur both intra and intermolecular ESPT processes.The difference in the ESIPT process of the compounds should be at least part of the reasons for their different PL behaviors.For 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD, ESIPT is easy to occur in both single molecule and aggregate states and no energy barrier is present for proton transfer in the S 1 state in aggregate state, while for 1,2-CHD, ESIPT is difficult to occur in single molecule state and a low energy barrier is present for proton transfer in dimers.Therefore, 1,2-CHD exhibits blue-shifted emissions than    solutions.On the other hand, 1,4-CHD and 2,2-DMCHD are incapable of occurring ESIPT process due to their chemical structures.This should be one reason for their blue-shifted emissions.
It is well-known that the ESIPT process is easily affected by solvent environment. [56,57]In protic solvents, the strong intermolecular interaction between solute and solvent molecules can promote the occurrence of ESIPT process.This is verified by the fact that the fluorescence emission of 1,3-CHD is enhanced in polar solvents.

CONCLUSION
In this work, the photophysical properties of the isomers of CHD and DMCHD in solid state and solutions were systematically investigated.Their fluorescence emissions vary with the position of carbonyl and the gemdimethyl groups.By comparing the nonaromatic TBC and TSC effects of the compounds, their luminescence mechanism was discussed.Structural characterizations prove that the compounds undergo keto-enol tautomerism or not in solid state and solutions, leading to different TBC in them.Moreover, single crystal structure and theoretical calculations show that intra-/intermolecular TSC effects of these compounds also vary with the chemical structures and conformations in the aggregate state.TBC effect reduces the HOMO-LUMO energy gap of single molecule, and it facilitates the formation of stronger TSC in the aggregate state.The cooperative effect of TBC and TSC leads to more significantly red-shifted emissions.Theoretical calculations also show that the compounds in the ketone-enol form can occur ESIPT process, which demonstrates an ESIPT mechanism in NTLs.This work reveals that nonaromatic TBC plays an important role in the photoluminescence of NTLs.The proposal of the concept of "nonaromatic TBC", together with nonaromatic TSC of NTLs and aromatic TBC and TSC of traditional luminogens, completes the puzzle of intra-and interactions on the photoluminescence of organic luminescent materials.The cooperative effects of nonaromatic and aromatic TBC and TSC, or more generally TBC+TSC, are very possibly the fundamental mechanism for the photoluminescence of both traditional and nontraditional organic luminogens.

Characterization
FT-IR spectra of the samples were recorded on a Nicolet Avatar E. S. P. 360 FTIR spectrometer (Thermo Electron Corporation, USA) at a resolution of 1 cm −1 for 32 scans.
1 H NMR spectra were recorded on an NMR spectrometer (400 MHz, JEOL, Japan) with an auto-sampler at ambient temperature, using CDCl 3 and (CD 3 ) 2 CO as the solvents.

Fluorescence and UV-vis spectroscopy
Fluorescence spectra and the luminescence quantum yields were measured on an FLS-980 fluorescence spectrometer (Edinburgh instruments, UK) with Xe-lamp (450 W) as an excitation source at room temperature.The excitation and the emission slit widths were 3 and 5 nm for the solids and solutions, respectively, unless otherwise stated.An integrating sphere was used to record fluorescence data.The PL quantum yields were measured with an FS980 fluorescence spectrometer tester (Quantaurus-QY, Hamamatsu, Japan).UV-vis spectra were recorded with a UV-vis spectrophotometer (UV-2450, Shimadzu, Japan).The photos were taken by Canon D600 camera.The ISO and exposure time were 400 and 1/60 s for solid, 800 and 1/8 s for solutions, respectively.

Theoretical calculation
All the electronic structures and energy levels of these compounds were calculated by Gaussian 09 (version D.01) [58] using B 3 LYP [59] density functional theory and 6-31G (d,p) [60][61][62] basis.The dispersion effect was corrected by Grimme's DFT-D 3 . [63]The intrinsic reaction coordinate (IRC) jobs were performed to get the energy barriers between the transition state structure and ketone-enol or diketone structure.All Gibbs free energies are calculated at normal temperature (25 • C) and pressure (1.0 atm).

A U T H O R C O N T R I B U T I O N S
Xiaomi Zhang and Yunhao Bai contributed equally to this article.The manuscript was written through contributions of all authors.All authors have given approval to the final version of the manuscript.

A C K N O W L E D G E M E N T S
This research was funded by National Natural Science Foundation of China (grant no.21574015) and the Program for Changjiang Scholars and Innovative Research Team (PCSIRT) in University.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflicts of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available in the supplementary material of this article.

F
I G U R E 1 (A) Chemical structures of the isomers of CHD and DMCHD.Fluorescence photos of the isomers of CHD and DMCHD in (B) solid state and (C) in methanol solutions (0.2 mol L −1 ) under 365 nm UV light irradiation.

TA B L E 2 7
Ratios of diketone to ketone-enol structures (n k :N e ) of 1,3-CHD, 4,4-DMCHD and 5,5-DMCHD in CDCl 3 and CD 3 OD solutions.shown in Figure7, for 1,4-CHD and 2,2-DMCHD, strong absorption bands at 1720 cm −1 assigned to the stretching vibration of carbonyl (C═O) and its overtone band at 3400 cm −1 appear.For 1,2-CHD, in addition to the absorption band of carbonyl group at 1730 cm −1 , strong absorption bands at 3425 cm −1 and 1665 cm −1 assigned to FT-IR spectra of the isomers of CHD and DMCHD.

TA B L E 3
HOMO-LUMO energy levels of single molecule of the isomers of CHD and DMCHD.

F I G U R E 1 2
Potential energy curves of S 1 and S 0 states for (A) 1,3-CHD, (B) 4,4-DMCHD, (C) 5,5-DMCHD and (D) 1,2-CHD dimers as a function of the O─H bond length.The energy represents the total energy of the system.

TA B L E 1
Quantum yields of the isomers of CHD and DMCHD in solid state and solutions.