Structural and Spectral Features of 4'-Substituted 2'-Hydroxychalcones

Article Cite This: J. Phys. Chem. A 2018, 122, 2030−2038

pubs.acs.org/JPCA

Structural and Spectral Features of 4′-Substituted 2′Hydroxychalcones in Solutions and Crystals: Spectroscopic and Theoretical Investigations Illia E. Serdiuk,†,‡ Michal Wera,‡ and Alexander D. Roshal*,§ †

Faculty of Mathematics, Physics and Informatics and ‡Faculty of Chemistry, University of Gdańsk, Wita Stwosza 57, 80-308 Gdańsk, Poland § Institute of Chemistry, V. N. Karazin Kharkiv National University, Svoboda sqr. 4, Kharkiv 61022 Ukraine S Supporting Information *

ABSTRACT: The article describes investigations of 2′-hydroxychalcone and its three derivatives bearing differently sized alkyloxy groups at position 4′. The compounds are investigated from the point of view of crystal structure, electronic absorption, fluorescence features in solutions and crystals using X-ray diffraction and electronic spectroscopy methods, and quantum chemistry calculations. In general, both in solutions and in the crystal phase, the influence of substituents on absorption spectra of chalcones was found to be insignificant. Exclusively in the case of 4′-(4-methoxybenzyloxy)-2′-hydroxychalcone, molecular packing influences the absorption features, which is because of the intermolecular interactions of substituent’s phenyl ring and chromophore fragment of the neighboring molecules. The lack of fluorescence of the excited enol form of chalcones in solutions and crystals is mainly due to intersystem crossing and excitedstate intramolecular proton transfer. Fluorescent properties of the phototautomer keto species formed by the proton transfer depend on molecular conformation. In solutions, the excited keto form is twisted and effectively deactivates nonradiatively due to conical intersection. In the crystal phase, the fixed planar geometry disables the conical intersection and the fluorescence of the keto form becomes detectable.

1. INTRODUCTION 2′-Hydroxychalcones are one of the most important biochemical precursors of a large family of plant metabolites (flavonoids).1,2 These compounds are used in synthesis of flavones, flavonols, isoflavones, anthocyanidins and other synthetic antioxidants.3,4 Unlike other chalcone derivatives demonstrating only cis/trans-isomerization5,6 and dimerization7−9 in the excited state, 2′-hydroxychalcones have more diverse reactivity. For example, the presence of the hydroxyl group at position 2′ causes dark acid−base4 and photoinduced3,10,11 cyclization of chalcones to flavanonones, which are synthetic precursors of some groups of flavonoids. The hydrogen bond between hydroxyl and carbonyl groups enables intramolecular proton transfer in the excited state (ESIPT)11,12 (or, in the opinion of some authors, hydrogen atom transfer13−16), which leads to the formation of phototautomer. Owing to their high reactivity in the excited state, 2hydroxychalcones exhibit complex photochemical behavior in liquid and frozen amorphous solutions,12,17 leading to completely nonradiative deactivation of their excited state which disables use of fluorescent spectroscopy methods for investigations of such compounds. In the condensed and ordered crystalline state, chalcones do not undergo reactions © 2018 American Chemical Society

caused by diffusion (photodimerization) or conformational changes (cis/trans photoisomerization, cyclization). For this reason, some of 4′-(dimethylamino)-2-hydroxychalcones exhibit intense aggregation induced emission (AIE) in the visible and near-infrared regions, which makes them attractive for application in organic lasers18−21 The 2′-hydroxychalcones described here (Figure 1) also exhibit green to orange fluorescence in a crystalline state, and the investigations presented below are aimed to shed light on the photophysics and photochemistry of these compounds. In the case of crystalline state, the type of crystal packing and the presence of intermolecular interactions and “contacts” can affect the spectral properties of 2-hydroxychalcones. Therefore, the dependence of spectral features on the packaging parameters should be considered. The aim of this work is to investigate the structure of a family of crystalline 2′-hydroxychalcones with electron-releasing substituents and to analyze the effect of crystal lattice structure Received: October 19, 2017 Revised: January 10, 2018 Published: February 5, 2018 2030

DOI: 10.1021/acs.jpca.7b10361 J. Phys. Chem. A 2018, 122, 2030−2038

Article

The Journal of Physical Chemistry A

Cary Eclipse spectrofluorometer, respectively. The spectral characteristics of the compounds investigated were extracted from experimental absorption and fluorescence spectra using the Spectral Data Lab program.23 Unconstrained and constrained geometry optimizations of isolated molecules of 2′-hydroxychalcone and its derivatives in the ground (S0) and excited singlet (S1) electronic states, as well as calculations of the energies of electronic transitions between the ground state and lowest excited singlet and triplet states were carried out at the DFT or TDDFT levels of theory,24 respectively, using the M062X functional25 and ccpVDZ basis sets26,27 implemented in the GAUSSIAN 09 program package.28 The Gibbs free energy contributions at 298.15 K and standard pressure were made by the built-in computational program of statistical thermodynamics routines. Mulliken charges on atoms in the ground state have been also calculalted using the RM1 method29 implemented in the MOPAC 2012 program.30 The calculations were carried out on the cluster of Ukrainian-American Laboratory of Computational Chemistry (UALCC, Kharkiv, Ukraine) and Wroclaw Centre for Networking and Supercomputing (WCSS, Poland).

on their spectral properties and the excited-state nonradiative deactivation processes.

2. MATERIALS AND METHODS The structures of the investigated 2′-hydroxychalcone derivatives are presented in Figure 1. The compounds were

Figure 1. Structures of 2′-hydroxychalcones under investigations.

synthesized according to the procedures described previously.22 Electronic spectra of 2′-hydroxyhalcones were measured in commercial methanol and dichloromethane of spectroscopic grade. The solvents were dried and distilled before use. X-ray data for 2′-hydroxycalcones were collected on an Oxford Diffraction Gemini R Ultra Ruby CCD diffractometer (Mo Ka radiation, graphite monochromator). Data collection and cell refinement were carried out with CrysAlis CCD and data reduction with CrysAlis RED. Structure solution and refinement was carried out with the SHELXS97 package. Absorption and fluorescence spectra were recorded on a PerkinElmer Lambda UV/vis 40 spectrophotometer and Varian

3. RESULTS AND DISCUSSION 3.1. X-ray Diffraction Analysis of 2′-Hydroxychalcones. Figure 2 shows the character of 2′-hydroxychalcones molecular packing. Cell parameters in crystal lattices are listed in Table 1. The presented data show that all the chalcones

Figure 2. Arrangement of 2′-hydroxychalcone molecules in the crystal lattice. 2031


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The Journal of Physical Chemistry A Table 1. Crystallographic Data for 2′-Hydroxychalconesa cell

unit cell dimensions, Å and deg

I

triclinic

II

monoclinic

III

triclinic

IV

triclinic

a = 6.8370(4) b = 10.4929(7) c = 17.1245(13) a = 15.3056(10) b = 5.4818(3) c = 16.5949(11) a = 8.2343(7) b = 10.0710(9) c = 10.2440(9) a = 5.409(4) b = 8.3510(5) c = 20.6710(13)

α = 73.971(6) β = 84.363(6) γ = 87.669(5) β = 108.976(7)

α = 90.993(7) β = 110.215(8) γ = 111.000(8) α = 97.201(5) β = 91.531(6) γ = 92.299(6)

Vcell, Å3

Z

Vmol, Å3

θ, deg

lHbond, Å

1174.9

4

293.7

10.47b 10.14

1.66b 1.58

1316.7

4

329.2

8.55

1.63

734.5

2

367.3

7.03

1.62

925.1

2

462.6

9.24

1.60

Vcell is the volume of a cell, Z is the quantity of molecules in a cell, Vmol is the molecular volume (Vcell/Z), θ is the torsion angle between planes of the benzene rings, lHbond is the length of intramolecular hydrogen bond. bValues θ and lHbond are presented for two conformers of I, which form a common crystal cell. a

Figure 3. Absorption spectra of 2′-hydroxychalcones: (a) in solutions (methanol, solid line; dichloromethane, dashed line) and (b) in crystals. Lines: (blue) I; (purple) II; (pink) III; (orange) IV.

Table 2. Spectral Parameters of I−IV in Dichloromethane and the Crystal Phase cmpd I II III IV a

medium DCMb solid DCM solid DCM solid DCM solid

long-wavelength bands ν, cm−1 (λ, nm) 27895 27500 28570 28200 28660 28215 28845 26315

(358) (364) (350) (355) (349) (354) (347) (380)

short-wavelength bands ν, cm−1 (λ, nm) 31995 32280 32660 32380 32605 32460 32230 32230

(313) (310) (306) (309) (307) (308) (310) (310)

f S0→S1/f S0→S3a 0.515 1.036 1.278 1.183 1.051 0.711 1.102 1.234

emission bands ν, cm−1 (λ, nm)

Stokes shift ΔνSt, cm−1

18900 (529)

8600

16380 (610)

11820

18860 (530)

9355

16740 (597)

9575

b

The ratio of experimental oscillator strengths of long-wavelength and short-wavelength bands. Dichloromethane.

under investigations have substantially different crystallographic parameters. Molecules in lattices of all the chalcones are weakly bonded to each other. Weak intermolecular hydrogen bonds of CH··· O type were found in compounds I (2.53−2.97 Å) and III (2.55−2.58 Å); π−π contacts are present in the case of III (3.55−3.62 Å). Some contacts were detected in the case of chalcone IV. Compound I has a special crystal structure, which distinguishes it from the others: cells of the lattice of I contain molecules with slightly different conformations. They have different flatness and different lengths of intramolecular hydrogen bonds. A very important common feature of all the chalcones is their almost planar geometry: torsion angles between planes of

benzene rings in chalcone moieties are in range from 7 to 10.5°. Theoretical values of these angles obtained by DFT method for the gaseous phase are from 0 to 4°, RM1 method gives higher values, in the range from 10° to 26°. Importantly, positions of bulky substituents relative to the molecular planes of III and IV substantially differ from those predicted by theoretical methods for gaseous phase. For this reason, the influence of substituent electronic effect on chalcone moiety can differ in gaseous and crystal phases. 3.2. Absorption Spectra and Nature of Electronic Transitions of 2′-Hydroxyhalcones. The main factors affecting the spectral behavior of 2′-hydroxychalcones are the electronic effects of substituents and the properties of environment. The influence of these factors was investigated 2032


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Table 3. Electronic Transition Parameters of 2′-Hydroxychalcones in Vacuum Predicted on the TD DFT Level of Theory S0 → S1 ππ* (B → CH) −1

ν, cm I II III IVc

26810 27175 27250 27475

S0 → S2 nπ* a

(λ, nm)

CI

(373) (368) (367) (364)

0.66χ1→1′ 0.55χ1→1′ 0.51χ1→1′ 0.56χ1→1′

−1

ν, cm

27100 27780 27780 27475

S0 → S3 ππ* (C → CH) a

(λ, nm)

CI

(369) (360) (360) (364)

0.67χ3→1′ 0.48χ3→1′ 0.48χ3→1′ 0.48χ5→1′

ν, cm−1 (λ, nm) 31445 31745 31845 28330

(318) (315) (314) (353)d

CIa

f S0→S1/f S0→S3b

0.57χ2→1′ 0.59χ2→1′ 0.59χ2→1′ 0.53χ2→1′

0.17 0.47 0.35 0.48

a CI is the predominant configuration matrix elements. bThe ratio of oscillator strenghts of S0 → S1 and S0 → S3 (S0 → S4 for IV) ππ* electronic transitions. cThe order of orbitals does not correspond to that of I−III. dππ* transition of the 4-methoxybenzyl fragment.

The next electronic transition of higher energy with theoretically predicted maximum at 360−370 nm occurs between the nonbinding molecular orbital and the vacant CH-type orbital φ1′. This transition has the nπ* nature and is forbidden, and thus its oscillator strength has zero value. In the experimental absorption spectra it is not observed. The short-wavelength absorption band centered in the experimental spectra at 314−316 nm is due to the transition between the occupied C-type orbital φ2 and the vacant orbital φ1′. This transition (calculated values of 314−318 nm) is due to the redistribution of the electron density mainly on the cinnamoyl fragment. In the case of IV, calculations predict an additional lowintensity band at 353 nm, probably due to the absorption of the para-methoxybenzyl moiety. The theoretical analysis of the nature of electronic transitions in studied chalcones allows one to explain to some extent the spectral behavior of corresponding experimental absorption bands. Because the low-energy electronic transition is accompanied by the interfragmental charge transfer, the longwavelength absorption band is most sensitive to electronreleasing properties of the substituents. It is supported by the fact that the computationally predicted f S0→S1/f S0→S3 values correlate with the experimentally obtained ratios f1/f 2. Spectral parameters of 2′-hydroxychalcones in crystal phase are presented in Table 2. The analysis of spectral data indicates, compared to the case for solutions, in the crystal phase, the short-wavelength absorption band is shifted negligibly by ±280 cm−1. Taking into account the localization of corresponding ππ* transition predominantly on the cinnamoyl fragment, it can be concluded that the electronic structure of this fragment changes negligibly in the condensed phase as compared to that in solutions. In the crystal phase, the long-wavelength absorption band of 2′-hydroxychalcones undergoes a slight 370−450 cm−1 bathochromic shift (Figure 3) evidencing the absence of a pronounced substituent effect. According to the crystallographic data this phenomenon can be explained by the formation of short intermolecular contacts (2.7−2.9 Å) between the carbonyl-group oxygen atom and hydrogen atoms of the phenyl rings of the neighboring molecules. Such contacts result in some decrease of the electron-withdrawing ability of the carbonyl groups that reduces the interfragmental charge transfer. In the case of IV, the long-wavelength absorption band exhibits a higher intensity than the short-wavelength one. Moreover, if compared to the case for the unsubstituted 2′hydroxychalcone I, the long-wavelength band of VI is shifted batochromically by 1185 cm−1. In our opinion, such spectral behavior of the long-wavelength band can be due to the different conformation of IV in solutions and crystals.

in liquid solvents of 2′-hydroxyhalcones at room temperature. The absorption spectra of compounds I−IV are shown in Figure 3a. Solid lines show the spectral curves obtained in dichloromethane, dashed lines for those in methanol. As can be seen in Figure 3a, the spectra of 2′-hydroxychalcones almost completely coincide. Taking into account that methanol has significantly higher polarity, electrophilicity and nucleophilicity than dichloromethane,31 it can be concluded that 2′hydroxychalcones show very low sensitivity both to nonspecific effects of the medium and to specific interactions such as formation of strong intermolecular hydrogen bonds or nucleophilic complexes with solvent molecules. The results of deconvolution of the experimental absorption bands shown in Table 2 indicate that the introduction of electron-releasing substituents has a negligible effect on the positions of these bands. However, comparison of the normalized absorption spectra in Figure 3a allows us to conclude that the introduction of electron-releasing substituents leads to an increase in the long-wavelength absorption band intensity with respect to the short-wavelength one. The ratio of the long-wavelength and short-wavelength band area, equal to the oscillator strengths ratio of the corresponding transitions (f1/f 2), does not correlate with the σ-para Hammett constants of the substituents. However, the f1/f 2 values show linear dependence on the total charges of the substituents in the ground state, calculated by the semiempirical method RM1 (r2 = 0.987). A worse correlation is obtained using the DFT method (r2 = 0.734), but it generally reflects the tendency of the f1/f 2 value increase with the increase of electron-releasing ability of substituent. Calculations of the theoretical absorption spectra performed by the DFT method showed that the long-wavelength transitions in the spectra of the chalcones are predominantly single-configuration ones. Therefore, the conclusions on the nature of these transitions can be made on the basis of analysis of the localization of corresponding molecular orbitals. As can be seen in Figure 1S, the molecular orbitals of 2′hydroxychalcones can be divided by localization to four types: localized (or predominantly localized) on benzaldehyde (B) and cinnamoyl (C) fragments, the nonbinding orbital of the carbonyl group (N), and delocalized on the whole molecule (CH). In the case of IV, there are also phenol type orbitals (P) localized on the para-methoxybenzyl moiety. The long-wavelength absorption bands of 2-hydroxyhalcones with the experimental maximum in the 355−360 nm range and corresponding computationally predicted maximum in the 365−375 nm range (Table 3) are due to the transition between the B-type φ1 orbital and the CH-type orbital φ1′, localized mainly on the cinamoyl fragment. Therefore, such a transition must be accompanied by the transfer of electronic density from the benzaldehyde to cinnamoyl fragments. 2033


Article

The Journal of Physical Chemistry A The optimized geometry of IV on the DFT level of theory is planar (Figure 4a): the angle between the planes of molecule

Figure 4. Conformations of IV: (a) optimized geometry in the S0 state; (b) conformation in the crystal lattice; (c) contacts between neighboring molecules in the crystal.

and benzene ring of benzyl substituent is 0.01°. Evidently, such geometry is expected to be optimal in gaseous and liquid phases. In crystals, this angle is equal to 76.81°; i.e., the benzene ring of benzyl substituent is almost orthogonal to another part of the molecule (Figure 4b). It can be assumed that the position of the benzene ring affects in some way the electronreleasing properties of the para-methoxybenzyl fragment and, as a consequence, the energy of the long-wavelength ππ* transition. However, as it was concluded above for I−III, the direct influence of substituents on the interfragmental charge transfer and, consequently, on the position of long-wavelength band is weak. Therefore, we suggest the presence of another factor affecting the electronic transition energy, which is also due to intermolecular interaction in crystals. Namely, there are close contacts between the benzaldehyde fragments and paramethoxybenzyl substituents of the neighboring molecules, as shown in Figure 4c. The interfragmentary charge transfer can thus depend not only on the conformation the paramethoxybenzyl group in a molecule but also on the polarizing effect of the para-methoxybenzyl groups of neighboring molecules. 3.3. Fluorescence Spectra. In solutions, I−IV do not fluoresce. Three reasons can be suggested for the complete nonradiative deactivation of the excited state of these compounds. The first reason can be the intensive excited-state structural relaxation. As shown in Figure 5a, all the 2′-hydroxychalcones

Figure 5. Optimized geometry of 2′-hydroxychalcone I: (a) unconstrained planar “normal” N form of chalcone in the S0 state (typical for crystalline and gaseous phase) and in the relaxed S1 state (for crystalline phase); (b) unconstrained “normal” N* form of chalcone in the relaxed S1 state and nonrelaxed S0 state (gaseous phase); (c) constrained planar T* form in the S1 state and the nonrelaxed S0 state (suggested for the crystalline phase); (d) unconstrained “phototautomer” T form in the relaxed S1 state (gaseous phase).

in the ground state are planar. Formation of the intramolecular hydrogen bond between the benzaldehyde and cinamoyl fragments favors such a planar geometry. After excitation, negative charges on the oxygen atoms of the hydroxyl and carbonyl groups decrease, which leads to a weakening of the hydrogen bond and twisting of the molecule (Figure 5b). The torsion angle in the S1 state of studied chalcones was estimated to be in the range from 51° (I) to 61° (II−IV). Because the S0 → S1/S1 → S0 electronic transitions are accompanied by the interfragmentary charge transfer, molecule twisting and decrease of conjugation between the fragments lead to a decrease of the oscillator strength of such transitions and, consequently, the decrease of radiative deactivation rate constant (kf). 2034


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The Journal of Physical Chemistry A Table 4. Relative Gibbs Free Energies of Tautomeric Forms in the Ground and Excited Statesa initial “enol” form (N) I II III IV

I II III IV

ΔG (S0)

ΔG (S1FC)

0.00 0.00 0.00 0.00

348.78 368.99 368.86 361.66

ΔG (S1)

ΔG (S1) planar

320.95 324.89 325.89 341.04 325.10 340.12 319.32 338.82 phototautomer “keto” form (T)

ΔG (S1)

ΔG (S0NR)

ΔG (S0NR) planar

246.73 260.71 258.91 267.57

72.13 122.26 118.20 107.40

25.48 36.57 35.82 35.27

ΔG (S1)

ΔG (S0NR)

ΔG (S1) planar

ΔG (S0NR) planar

246.73 260.71 258.91 267.57

243.59 258.78 255.06 258.74

299.45 314.51 314.47 302.08

68.83 73.76 72.97 82.76

ΔGN*→T* (ΔGN*→T* planar) −74.22 −65.19 −66.19 −51.76

(−25.44) (−26.53) (−25.65) (−36.74)

a All the values (kJ/mol) are calculated relatively to the ΔG values of N form in the S0 state. S1FC is the the Franck−Condon excited state, S1 is the the relaxed excited state, S1(planar) is the constrained planar molecule in the excited state, S0NR is the nonrelaxed ground state, and S0NR (planar) is the nonrelaxed ground state. ΔGN*→T* and ΔGN*→T* (planar) are the changes of Gibbs free energies of ESIPT for relaxed and constrained planar tautomeric forms, respectively.

Previous investigations of cinnamoyl-α-pyrones structurally close to 2′-hydroxychalcones35,36 showed that the former fluoresce only in the case of specific substituent or solvent effects, leading to a bathochromic shift of the long-wavelength absorption band above 380 nm. One can thus assume that the nπ*-transition band of the carbonyl group of the cinnamoyl fragment is centered at 370−380 nm. The experimental absorption ππ* band maxima of 2′-hydroxyhalcones are approximately in the same or shorter wavelength region, so the assumption on the nonradiative deactivation from the lowest nπ* state seems quite plausible. As follows from the data listed in Table 3 and depicted in Figure 7, the energies of long-wavelength transitions S0 → S1

The twisting of molecules results also in the decrease of an energy gap between the relaxed excited and nonrelaxed ground state from 299−304 to 249−204 kJ/mol (i.e., from 3 to 2 eV, approximately) (Table 4, Figure 6), which substantially

Figure 6. Diagram of the interplay of 2′-hydroxychalcone tautomeric forms in the ground and excited states. Red lines: twisted forms (liquid or gaseous phase). Blue lines: constrained planar forms (suggested for the crystalline state).

increases the probability of direct thermal radiationless deactivation of the S1 state and, consequently, leads to the increase of corresponding rate constant (kd). Here, it is important to note that the twisting of 2′-hydroxychalcones does not lead to conical intersection of the mentioned states. The increase of kd and decrease of kf values must result in a significant decrease of the emissive ability of studied chalcones. The second reason for the lack of fluorescence can be very efficient intersystem crossing (ISC) from the lowest singlet nπ* state typical for carbonyl and heterocyclic compounds.32,33 The well-known s-trans → s-cis photoisomerization of 2′-hydroxychalcones in liquid media34 with subsequent cyclization to flavanones serves as a proof for the S1(nπ*) → Tn(ππ*) ISC in such compounds. The s-trans → s-cis isomerization proceeds after the intramolecular proton transfer in the triplet state;17 therefore, this reaction would not be possible in the absence of ISC.

Figure 7. Diagram of excited singlet and triplet states of the chalcone III.

and S0 → S2 are very similar, which evidences energetic closeness of S1 (ππ*) and S2 (nπ*) states. The energy gap between these states is in the range from 0 (IV) to 605 cm−1 (II). Therefore, even though the lowest energy state is of the ππ* nature, the nπ* state can be easily populated due to the thermal ππ* → nπ* activation. According to the El Sayed rule, the efficient ISC can take place between energetically close states of different nature.32 As it is shown in Figure 7, the efficiency of ISC can be high between the energetically close S2 (nπ*) and T4 (ππ*) states (the energy gap between S2 and T4 is from 220 (I) to 1020 cm−1(IV)) and also, to a lesser extent, 2035


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in I−IV. However, some of hydroxychalcones exhibit the redshifted ESIPT fluorescence attributed to the emission of the phototautomer.17−19,38 In addition, fluorescence with a large Stokes shift was also observed in the case of I−IV in the crystalline state. According to the ΔGN*→T* values given in Table 4 and depicted on Figure 6, ESIPT is energetically spontaneous in 2′-hydroxychalcones. The calculated values for the N* → T* process are in the −50 to −75 kJ/mol range. In the case of 4-(dimethylamino)-2′-hydroxychalcone,38 ESIPT is very fast process, whose rate constant is about 1011 s−1. It can be due to the presence of the powerful electronreleasing dimethylamino group directly conjugated with the carbonyl fragment. That increases basisity of the last in the ground and especially excited states and promotes the phototautomerization reaction. In the case of I−IV, which do not have an electron-releasing fragment in position 4, a lower proton transfer rate can be expected. In solutions of most 2′-hydroxychalcones, both N* and T* forms do not fluoresce or have extremely weak emission with quantum yields below 10−6−10−5.15,16 The chalcones investigated do not fluoresce in liquid media either. Taking into account three possible ways of excited enol form deactivation mentioned above, the absence of the fluorescence can be explained either by considerably higher ISC and radiationless internal conversion rates in the initial enol form N* as compared to the ESIPT rate (kNISC* ≫ kN*→T*) or, in the opposite case, by complete dominance of nonradiative deactivation processes over the radiative ones in the phototautomer T* (kdT* ≫ kTf *). In the first case, the enol fluorescence is not observed due to fast N* → N deactivation, and the emission of the T* form is not detectable because of low phototautomer concentration. In the second case, vice versa, the lack of N* form emission is due to the low quantity of the excited N* form, and the absence of T* fluorescence intensity can be explained by fast radiationless deactivation of the keto tautomer. To get more information on the spectral behavior of the tautomeric T* form, its optimized geometry in the gaseous phase was analyzed. In the case of T*, the torsion angle between the benzaldehyde and cinnamoyl fragments was found to be 85−89° (Figure 5d). Therefore, there is no conjugation between these fragments in the excited state that leads to a significant increase of the energy of the long-wavelength π → π* transition, as well as to a dramatic decrease of its oscillator strength and, hence, emissive ability of the T* form. As in the case of the N* form, twisting of the fragments leads to the mixing πσ* and σπ* thus some contribution of ISC in the nonradiative deactivation of T* can be expected. Another possible and, in our opinion, most likely way of nonradiative deactivation of T* is conical intersection of the excited and nonrelaxed ground states. Decrease of the fluorescence intensity as a result of the conical intersection was suggested previously for 4-(dimethylamino)-2-hydroxychalcone38 and compounds related to chalcones.40,41 The calculated values of the free energies of T* in the relaxed excited state (ΔGS1) and nonrelaxed ground state (ΔGS0,NR) (Table 4, Figure 6) are very close. For this reason, the direct nonradiative S1 → S0 deactivation in T* can be highly efficient. In the crystalline state, the conformation changes in the molecules are hindered. For this reason, the absence of N* fluorescence can be explained only by two of the three reasons mentioned above: the presence of a long-wavelength nπ* transition or ESIPT. The presence of weak T* fluorescence of

between S1 (ππ*) and T3 (nπ*) states (the energy gap between S1 and T3 is from 1000 (II) to 1830 cm−1 (IV)) The efficiency of ISC can also increase due to the formation of mixed states. The latter can be formed by mixing of the energetically close ππ* and nπ* states mentioned above. Moreover, one should take into account that twisting leads to the formation of mixed πσ* and σπ* states, which also causes activation of intersystem crossing.37 From the point of view of possible application of the studied group of compounds in light-emitting devices it is important to note that the above-mentioned way of the excited-state ISC deactivation can be realized for the studied chalcones, but it is unlikely for 2′-hydroxy-4-aminochalcones.18,19,38 The energy of the lowest nπ* state of the latter compounds is much higher than that of the lowest ππ* state, and S1(ππ*) → S2(nπ*) → Tn(ππ*) deactivation is not possible. This can explain the more intense 4′-(dimethylamino)chalcone fluorescence38 in comparison with that of other chalcones. The third reason for the lack of fluorescence of 2′hydroxychalcones can be intramolecular proton (or hydrogen atom) transfer in the excited state (ESIPT, Figure 8). One of

Figure 8. ESIPT in 2′-hydroxychalcones.

the driving forces in ESIPT is the increase of acidity of proton donor and basicity of proton acceptor upon excitation. In some cases, such a change of acid−base properties can be predicted using calculated charges on atoms, i.e., the decrease of negative charge on the oxygen atom of hydroxyl group (proton donor) and the growth of negative charge on carbonyl group (proton acceptor). This approach works well in the case of structurally and electronically similar flavonols that undergo ESIPT.39 In the case of the aforementioned cinnamoyl-α-pyrones, the charge decreases on the oxygen atoms of both carbonyl and 4′hydroxyl groups after excitation. Thus, despite the increase in the acidity of the hydroxyl, the basicity of the carbonyl group decreases in the S 1 state. The analysis of predicted thermodynamic characteristics of tautomeric forms and experimental data indicated the absence of proton transfer in cinnamoyl-α-pyrones.36 Data in Table 5 show that the change of charges on the oxygen atoms of the 2′-hydroxychalcones is similar to that of cinnamoyl-α-pyrones, so one can expect the absence of ESIPT Table 5. Mulliken Charges on Oxygen Atoms of 2′-Hydroxyl and Carbonyl Groups of 2′-Hydroxychalcones in the Ground and Excited Relaxed Statesa S0 state I II III IV a

S1 state

qO (CO)

qO (O−H)

qO (CO)

qO (O−H)

−0.313 −0.323 −0.323 −0.319

−0.208 −0.212 −0.212 −0.211

−0.196 −0.260 −0.257 −0.237

−0.180 −0.172 −0.172 −0.178

qO is the Mulliken charge on oxygen atoms, e.̅ 2036


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The Journal of Physical Chemistry A crystals with Stokes shift values of 8600−11800 cm−1 (Table 1, Figure 9) evidences the occurrence of ESIPT. The absence of

rate in the crystal phase returns us to the question of the important contribution of nonradiative processes in the original enol form N*.

4. CONCLUSION The analysis of crystallographic data showed that 2′hydroxychalcones form crystal lattices of various types with different packing patterns, depending on the size of the substituent. A distinguishing general feature of all the investigated compounds except for IV is the absence of strong intermolecular interaction in the crystals. For this reason in general, small differences in the absorption spectra of 2′hydroxychalcones in solutions and crystals can be explained by different orientations of the substituents with respect to the plane of molecules in crystal and liquid media, and therefore different electronic influence of the substituents on the πsystem. In particular, this influence can be noticed on the energies and oscillator strengths of long-wavelength π → π* transition formed by the interfragmentary charge transfer. In the crystalline phase, all the 2′-hydroxychalcones are planar and unable to change their conformation upon excitation. This leads to the suppression of conformational isomerization in the S1 state, intersystem crossing, and conical intersection, which are supposed to be the main ways of the excited-state deactivation in liquid media. The compounds undergo ESIPT, which is reflected in the appearance of lowintensity red-shifted fluorescence in crystals.

Figure 9. Fluorescence spectra of 2′-hydroxychalcones in crystals: (blue line) I; (purple line) II; (pink line) III; (orange line) IV.

T* fluorescence in solutions and its presence in the crystalline state indicate the crucial influence of the conformational factor on the nonradiative deactivation of the ESIPT phototautomer. Analysis of thermodynamic and spectral parameters of the 2′hydroxychalcones in crystals was conducted with the help of quantum-chemical calculations. Geometry optimizations of the N and T forms were carried out with the constrained 0° torsion angle between the benzaldehyde and cinnamoyl fragments. Thus, optimized geometries are planar in the ground and excited states and close to those obtained from the X-ray diffraction experiments. The ΔG values of the excited planar N* forms are higher by 4 (I) to 20 (IV) kJ/mol as compared to values for the nonplanar ones. In the case of T* forms, planarization causes an energy increase by 15−50 kJ/mol (Table 4). As a result, the ΔGN→T values in planar molecules are 1.5−3 times lower than in twisted ones but remain significantly negative. The calculations thus confirm the abovementioned conclusion on the occurrence of ESIPT in the N* species. However, the energy differences between S1 and S0NR states of the planar T* species are in the range of −242 (III) to −219 (IV) kJ/mol, which excludes the possibility of conical intersection for these states. The predicted wavelength of the S1 → S0NR electronic transitions for “planar” photothatomers are in the 480 (II) to 524 (IV) nm range. Taking into account the accuracy of the TD DFT method used, one can consider this value in a satisfactory agreement with the experimental values (530−610 nm). As was mentioned before, the lowest excited singlet state of the 2′-hydroxychalcone keto-forms in the twisted conformation is of the nπ* nature. In a planar T* molecule, the predicted energy of the ππ* state is 0.55−0.90 eV lower than the nπ* state, and thus the lowest excited state of T* in the crystalline phase is of the ππ* nature. Consequently, one can assume much lower efficiency of nonradiative decay caused by ISC in the crystalline phase than in less condensed media. At the end of discussion of the fluorescent properties, it can be concluded that the ESIPT-induced fluorescence of I−IV appears only in the crystal phase, which is caused by the suppression of conformational isomerization in the S1 state, ISC, and conical intersection. Low fluorescence quantum yields of the T* species could therefore be caused by other nonradiative deactivation ways, e.g., energy transfer in the crystalline phase. At the same time, the expected lower ESIPT

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b10361. Figures demonstrating localization character of several occuped and unoccuped molecular orbitals of I and IV; crystallographic parameters of I−IV; RMS deviations from planarity, hydrogen bond lengths, and parameters of π−π contacts obtained by X-ray diffractometry; Cambridge Crystallographic Data Centre numbers of deposited crystallographic data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*A. D. Roshal. Phone: +38-057-707-53-35. Phone/fax: +38057-707-51-30. E-mail: [email protected], alexandre.d. [email protected]. ORCID

Illia E. Serdiuk: 0000-0002-4563-0773 Alexander D. Roshal: 0000-0003-1537-9044 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by grants of Ministry of Education and Science of Ukraine 0116U000835 and 0115U000484 (A.D.R.) and the BMN grant of University of Gdansk 5385200-B464-17 (I.E.S.).

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REFERENCES

(1) Bohm, B. A. Introduction to Flavonoids, v.2. Chemistry and Biochemistry of Organic Natural Products; Harwood Academic Publishers: Amsterdam, 1998. 2037


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