Excited-State Intramolecular Proton Transfer (ESIPT) Emission of

Article pubs.acs.org/JPCA

Excited-State Intramolecular Proton Transfer (ESIPT) Emission of Hydroxyphenylimidazopyridine: Computational Study on Enhanced and Polymorph-Dependent Luminescence in the Solid State Yasuhiro Shigemitsu,*,†,§ Toshiki Mutai,‡ Hirohiko Houjou,‡ and Koji Araki*,‡ †

Industrial Technology Center of Nagasaki, 2-1303-8 Ikeda, Omura, Nagasaki 856-0026, Japan Institute of Industrial Science, University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan § Graduate School of Engineering, Nagasaki University, 1-14, Bunkyo-machi, Nagasaki 852-8521, Japan ‡

S Supporting Information *

ABSTRACT: Although 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridine (HPIP) is only weakly fluorescent in solution, two of its crystal polymorphs in which molecules are packed as stacked pairs and in nearly coplanar conformation exhibit bright excited-state intramolecular proton transfer (ESIPT) luminescence of different colors (blue-green and yellow). In order to clarify the enhanced and polymorph-dependent luminescence of HPIP in the solid state, the potential energy surfaces (PESs) of HPIP in the ground (S0) and excited (S1) states were analyzed computationally by means of ab initio quantum chemical calculations. The calculations reproduced the experimental photophysical properties of HPIP in solution, indicating that the coplanar keto form in the first excited (S1) state smoothly approaches the S0/S1 conical intersection (CI) coupled with the twisting motion of the central C−C bond. The S1−S0 energy gap of the keto form became sufficiently small at the torsion angle of 60°, and the corresponding CI point was found at 90°. Since a minor role of the proximity effect was indicated experimentally and theoretically, the observed emission enhancement of the HPIP crystals was ascribed to the following two factors: (1) suppression of efficient radiationless decay via the CI by fixing the torsion angle at the nearly coplanar conformation of the molecules in the crystals and (2) inhibition of excimer formation resulting from the lower excited level of the S1-keto state compared to the S0−S1 excitation energy in the enol form. However, the fluorescence color difference between the two crystal polymorphs having slightly different torsion angles was not successfully reproduced, even at the MS-CASPT2 level of theory.

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INTRODUCTION Luminescent organic solids have been studied extensively because of their potential for use in various photofunctional applications such as nonlinear optics,1−3 organic light-emitting diodes,3−6 and fluorescent biosensors.7−9 In general, fluorescent organic compounds tend to lose their emissive properties in the condensed phase owing to the various pathways of radiationless energy dissipation through stacking and other intermolecular interactions. Therefore, it is essential to suppress radiationless deactivation of the excited state for compounds in the solid state and many efforts have been devoted to this topic. Recently, a novel class of luminescent compounds that do not (or only weakly) show luminescence in solution but exhibit greatly enhanced luminescence in the solid state or upon formation of aggregates have been reported and are attracting considerable interest.10−22 The process is referred to as aggregation-induced emission enhancement (AIEE), but examples of AIEE systems are still quite limited. Aromatic siloles by Tang et al.,11 phenylenevinylenes by Park et al.,12 terpyridine by our group,23 and several other compounds have been reported to be AIEE systems, and they all have similar © XXXX American Chemical Society

structural characteristics. In these compounds, two or more aromatic systems are connected by a single bond, and a critical role of the rotation around the single bond in the excited state has been indicated. Though several discussions on the origin of the AIEE effect have appeared, the mechanism of the packingto-luminescence transduction in these compounds is not sufficiently clear. Applications based on the tuning and switching of solid-state luminescence are attracting considerable interest, and increasing numbers of reports on the tuning and switching of solidstate luminescence by controlling molecular packing have recently appeared. We previously reported polymorph-dependent luminescence of 2-(2′-hydroxyphenyl)imidazo[1,2-a]pyridine (HPIP),24 which has the common structural features of other AIEE systems.25 HPIP shows very weak fluorescence with a large Stoke’s shift in apolar solvent, which is ascribed to excited-state intramolecular proton transfer (ESIPT) emission, Received: August 27, 2012 Revised: November 17, 2012

A

dx.doi.org/10.1021/jp308473j | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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orbital localized on N3 (enol form) or one lone pair occupied orbital localized on O1 (keto form), using atomic natural orbital large basis set (ANO-L)34 with the contractions C,N,O[4s3p2d]/H[3s2p]. A real level-shift parameter35 of 0.3 was employed for all MS-CASPT2(10,9)/ANO-L calculations with careful monitoring of the excited state reference weights. The S1 geometries and the energy profile as a function of the torsion angle (ϕ) were evaluated by CASSCF(6,6)/ANO-S. The fully relaxed S1 PESs were explored at CASSCF(6,6)/ANO-S-MB (minimal base) level. Solvent effects were not considered throughout these computations. The DFT, TD-DFT, and EOM-CCSD calculations were performed with Gaussian09.36 The CASSCF and MS-CASPT2 calculations were performed with MOLCAS 7.4.33

as illustrated in Figure 1. However, two crystal polymorphs of HPIP exhibit bright luminescence of different colors, blue-green

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RESULTS AND DISCUSSION Photophysical Properties of HPIP. In order to enhance the present discussion, previously reported24 and newly obtained photophysical properties of HPIP in solution, together with those in the solid state, are summarized in Table 1. HPIP shows almost identical absorption bands at

Figure 1. (a) Tautomeric structures of HPIP and (b) normal and ESIPT luminescence.

Table 1. Photophysical Properties of HPIP λabs (nm)

and yellow, both with high quantum yield, showing a clear AIEE effect. Furthermore, switching between the luminescence colors can be achieved by thermal interconversion of the two polymorphs, offering an excellent example of packingcontrolled luminescence color tuning. ESIPT luminescence has attracted much attention in various fields of photochemistry and photophysics.26,27 In the present study, we focused on the bright ESIPT luminescence of HPIP in the solid state. Although the excited states associated with the inter-ring twisting motion have been investigated from a theoretical viewpoint,28−31 the mechanistic details remain unclear. Herein, we report the potential energy surfaces (PESs) of HPIP in the ground (S0) and excited (S1) states explored by means of ab initio quantum chemical calculations. Attempts to elucidate the origin of emission enhancement in HPIP in the solid state and the possible mechanism for the packing-controlled luminescence color tuning based on both the experimental and theoretical studies are also discussed.

solution

solid

a

ethanol

327

cyclohexane

337

THF

332, 346

THF (77 K)a polymer matrixb amorphous powder BG-form crystal Y-form crystal

332, 349 332 337 334 335

λem (nm) (Φ, τ (ns)) 373 (0.11, 1.6)

377 (0.08, 2.6) 370 (−, 2.7)

578 (0.04, 0.73) 602 (0.02, 0.52) 521 (−, 5.3) 523 (0.37, 4.9) 537 (0.39, 6.4) 496 (0.50, 5.9) 529 (0.37, 5.8)

Frozen state at 77 K. bIn PMMA film.

around 330−340 nm in common organic solvents but displays two different solvent-dependent fluorescence bands, i.e, very weak ESIPT fluorescence that is yellow to orange in color in apolar and aprotic cyclohexane, normal blue fluorescence in protic ethanol, and both normal and ESIPT dual fluorescence in THF (Figure 2). According to the study by Douhal et al.,37−40 the enol form in the planar conformation is the most stable form in the ground state owing to stabilization of the intramolecular hydrogen bond between the imino and phenolic groups. The two crystal polymorphs of HPIP, the blue-greenemitting (BG) and yellow-emitting (Y) crystals, showed markedly enhanced and polymorph-dependent ESIPT luminescence with high quantum yields (Table 1). The crystal structures of BG (Pbca, Z = 8) and Y (P21/c, Z = 8) are shown24 in Figure 3 and Supplementary Table S-1. The Y crystal consists of two further conformers, Y1 and Y2 (Figure 3b). In the crystals, HPIP is in the enol form, and no apparent distortion of the structure is observed. The hydrogen bond between the phenol OH and the ring nitrogen is formed exclusively in the intramolecular mode, and absence of an intermolecular hydrogen bond is confirmed. In both crystals, two HPIP molecules form a stacked pair unit in an antiparallel mode (Figure 3c,d), and the pair units are aligned in a zigzag

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COMPUTATIONAL DETAILS The S0 geometries of HPIP were optimized using density functional theory (DFT) with the 6-31G(d,p) basis set in conjunction with the B3LYP functional. For the optimized S0 geometries, the vertical excitation energies were evaluated with time-dependent DFT (TD-DFT) at the TD(B3LYP)/631+G(d,p) level. As reference, equation-of-motion with coupled cluster singles and doubles (EOM-CCSD) were employed to estimate the transition energies using cc-pVDZ basis set. Alternatively, the S0 geometries were optimized with state averaged CASSCF(6e,6o) level of theory over the lowest four states with even weight 0.25, where 6π-electrons were distributed into 3π (HOMO, HOMO−1, HOMO−2) and 3π* orbitals (LUMO, LUMO+1, LUMO+2), using atomic natural orbital small basis set (ANO-S)32 with the contractions C,N,O[3s2p1d]/H[2s1p]. The quantitative vertical excitation energies including dynamical electron correlation were evaluated by multistate CASPT2 (MS-CASPT2).33 The single-point MS-CASPT2 calculations employed 4 to 8 stateaveraged CAS (10e,9o), which include one lone pair occupied B



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cence, showing marked luminescence enhancement. Since HPIP was surrounded by the polymer matrix or solvent molecules in these cases, the results strongly indicate that the major reason for the observed emission enhancement of HPIP in the solid state should be ascribed to factor (1). Ground State Geometry and Absorption Spectra. The relative keto−enol stability was computationally analyzed, and the results are shown in Table 2. The enol form (S0-enol) was Table 2. Total and Relative Energies of Coplanar HPIP in the Ground Statea species enol keto (ϕ = 0)

B3LYP/6-31G +(d,p)

CASSCF(6,6)/ ANO-S

MS-CASPT2(10,9)/ ANO-L

−686.1873 (not found)

−682.0288 −681.9744(+142)

−684.4210 −684.3838(+97.6)

a

Total energies are in a.u., and energies of the keto form relative to the enol form are in kJ/mol in parentheses.

Figure 2. (a) Solid-state absorption and luminescence spectra of the BG form (dotted line) and Y form (solid line). (b) Absorption and emission of HPIP at room temperature (broken line) and at 77 K (solid line) in THF solution. (Reproduced with permission from ref 24. Copyright 2008 Wiley.)

always more stable than the keto form (S0-keto), and the most stable conformations of S0-enol and S0-keto were both confirmed to be the coplanar conformation. High stability of the coplanar conformation is attributed to the formation of the intramolecular hydrogen bond. The optimized geometries of S0-enol and S0-keto forms in the coplanar conformation are shown in Supplementary Figure S-1. The energy difference between S0-keto and S0-enol states was computed to be 142 kJ·mol−1 by CASSCF(6,6)/ANO-S//B3LYP/6-31G(d,p) and 97.6 kJ·mol−1 by MS-CASPT2(10,9)/ANO-L//B3LYP/6-31G(d,p). Therefore, HPIP exists predominantly as the coplanar enol form in the ground state. Absorption spectra of HPIP in various solvents showed a single band at around 330 nm. The S0 → S1 vertical transition energies in the coplanar S0-enol form were estimated for several low-lying states by means of TD-DFT(B3LYP), CASSCF(6,6)/ ANO-S, and MS-CASPT2/ANO-L as shown in Table 3. Computation by the three methods predicted five ππ* excited states with no interleaf of the nπ* state. The energy level of the Franck−Condon (FC)-S2 was sufficiently separated from that of the FC-S1 by 0.61 eV at the MS-CASPT2(10,9)/ANO-L level. The S0 → S1 excitation energy predicted by CASSCF was 4.44 eV, a considerable overestimation of the experimental value of 3.68 eV. The lack of dynamic electron correlation in CASSCF tends to overestimate vertical excitation energies, especially for ionic or charge-transfer states.41 The overestimated energy was lowered by MS-CASPT2 to 3.95 eV. The S0 → S2 and higher excitation energies were also overestimated by CASSCF, which were also adequately lowered by MSCASPT2. EOM-CCSD/cc-pVDZ results were comparable to those obtained by CASSCF for S0 → S1 and to MS-CASPT2 for the higher energies. The agreements for S0 → S1 would be improved with more extensive basis such as aug-cc-pVTZ or higher, which go beyond our present computational capacity. As shown in Table 4, excitation to the FC-S1 can be dominantly described as HOMO−LUMO excitation of the entire π system, while excitation to the FC-S2 state is mainly described by (HOMO−1)−LUMO excitation with moderate charge transfer from the phenyl ring to the imidazopyridine ring, where (HOMO−1) resides on the phenyl ring and LUMO is delocalized on the whole molecular plane. The charge redistribution upon excitation to FC-S1 and FC-S2 did not show large charge transfer between the phenolic and

Figure 3. ORTEP structures of HPIP in the BG (a) and Y (b) crystals. Top and side views of the stacked pairs of the BG (c) and Y (d) crystals.

fashion (Supplementary Figure S-2). Therefore, the mode of molecular packing in the two crystals is not significantly different. However, closer examination of the structures shows that there is a slight difference between the N3−C10−C11−C16 torsion angles (ϕ) of BG (5.88°) and the two conformers of Y (Y1/Y2: 1.38°/−1.0°). In the crystals, the molecules are packed closely together, and their conformations are strictly confined. The effect of molecular packing on the luminescent properties can be generally understood from the following two factors: (1) packing-induced conformational fixation and/or alteration at a single molecular level and (2) intermolecular interactions with neighboring molecules. Since the amorphous powder of HPIP also showed bright yellow luminescence, it is unlikely that the specific mode of molecular packing is required for the observed emission enhancement in the solid state. It should also be noted that HPIP in a polymethylmethacrylate matrix (0.5 wt %) and in a frozen dilute THF solution (1.00 × 10−5 mol·dm−3) at 77 K displayed bright yellow ESIPT luminesC



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Table 3. Low-Lying Excited States of the Coplanar Enol Form of HPIP for the S0-Optimized Geometry (in eV) excited state

TD-B3LYP/6-31+G(d,p)

EOM-CCSD/cc-pVDZ

CASSCF(6,6)/ANO-S

MS-CASPT2(10,9)/ANO-L

experiment (in cyclohexane)

2A′(S1) 3A′(S2) 4A′(S3) 1A″(S4) 2A″(S5)

3.55 4.07 4.25 5.46 6.10

4.31 4.61 4.99 5.76 5.80

4.44 5.71 6.38 6.63 6.94

3.95 4.56 4.93 6.34 6.85

3.68

Table 4. Natural Orbital Occupations of the S0, S1, and S2 States of the Coplanar Enol Form of HPIPa

a

Occupations calculated using 8 state averaged CASSCF(10,9)/ANO-L with even weight 0.125.

Table 5. Natural Orbital Occupations of the S0, S1, and S2 States of the Coplanar Keto Form of HPIPa

a

Occupations calculated using 8 state averaged CASSCF(10,9)/ANO-L with even weight 0.125.

S1 PES Scan along the Central C−C Bond Torsion. Historically, the theoretical interpretation of radiationless transitions from the excited state in polyatomic molecules has been roughly categorized into two models: the proximity effect model and the conical intersection (CI) model. The proximity effect model44−51 has been applied to many luminescent/ nonluminescent heterocyclic compounds sensitive to temperature, chemical substitution, hydrogen bonding, and solvents. This sensitivity has been attributed to the vibronic couplings invoked by near-degeneracy of the nπ*/ππ* excited levels with out-of-plane deformation of the aromatic ring in excited states. However, the involvement of CI on PESs has been widely employed as an alternative model to explain radiationless decay.52−54 CIs can be accessed directly or indirectly,55−59 acting as efficient decay funnels from excited states to ground states within a single vibrational period. A critical role of CIs has recently been reported for many compounds including 2(2′-hydroxyphenyl)benzo[d][1,2,3]triazole (BZT), 2-hydroxyphenyl-1,3,5-triazine, and related compounds. BZT, known as an effective UV absorbent, forms a strong intramolecular hydrogen bond in the ground state. Paterson et al. reported a detailed systematic quantum chemical study of BZT, showing that fast excited energy dissipation through CI is essential for its high photostability.60 Additionally, ab initio computational analyses of the luminescence of HPIP-related compounds have been recently reported using the DFT, TD-DFT, and CIS levels of theory.61 However, the excited-state PESs have not been satisfactorily explored using a multiconfigurational level of theory for this series of compounds.

imidazopyridine moieties, according to Mulliken population analysis (Supplementary Table S-1). More specifically, the positive charge density is localized on the C7 and C10 imidazopyridine carbons, whereas the negative charge density remained on the N3, C5, and C8 imidazopyrizine atoms. As a result, no drastic change in the dipole moment was observed upon the S1 or the S2 excitations. This agrees well with the small solvent dependence observed for the absorption maxima in solutions (Table 1). The S2 character strongly depends on the computational level; the main configuration is expressed in HOMO−LUMO excitation by 4 state-averaged CASSCF(6,6)/ ANO-S and in (HOMO−1)−LUMO excitation by 4 stateaveraged CASSCF(10,9) level, respectively. The present largest scale calculation (8 state-averaged CASSCF(10,9)) may not give the definitive answer owing to insufficient inclusion of the active space. According to Platt’s nomenclature,42,43 the two low-lying excited states can be approximately described with the four-orbital model containing HOMO−1, HOMO, LUMO, and LUMO+1. That is, the S1 and S2 states are ascribed to La and Ba states, respectively. The S0-keto form possesses a larger dipole moment than the S0-enol form (Supplementary Table S-2), owing to the localized electron distribution on the hydroxyphenyl ring in the S0-keto form. As shown in Table 5, upon S0 → S1 (HOMO−LUMO) and S0 → S2 HOMO−(LUMO+1) excitations, a drastic decrease in the dipole moment was invoked by intramolecular electron transfer from the phenol ring to the imidazopyridine moiety, for instance, an increase in electron density on N3 and a decrease on O1. D



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Table 6. Vertical Transition Energies of Coplanar HPIP for the S1-Optimized Geometryc,d enol

keto (ϕ = 0)

keto (ϕ = 180)

states

ΔE

strength

ΔE

strength

ΔE

2A′(S1) 3A′(S2) 4A′(S3) 1A″(S4) 2A″(S5) 3A″(S6) 4A″(S7)

3.46 4.67 5.55 5.80 6.74 7.37 9.54

0.057 0.027 0.050 0.21 × 10−2 0.55 × 10−3 0.017 0.69 × 10−3

2.29 3.53 4.26 3.74 4.99 5.42 7.80

0.247 0.018 0.551 0.27 × 10−6 0.11 × 10−5 (forbidden) 0.23 × 10−6

2.17 3.74 4.14 3.33 4.69 5.07 5.85

experiment

strength 0.326 0.027 0.376 0.947 0.185 0.139 0.113

× × × ×

ΔEa

ΔEb

3.43

2.21

10−6 10−3 10−3 10−4

a

In ethanol. bIn cyclohexane. cGeometries optimized using 4 state averaged CASSCF(6,6)/ANO-S. dVertical excitation energies calculated by 4 state MS-CASPT2(10,9)/ANO-L.

The S1 → S0 vertical transition energies of the three coplanar forms for the S1-optimized geometries are shown in Table 6. The emission peak observed at 3.43 eV in ethanol can be assigned to the enol S1 → S0 emission 3.46 eV, and the peak at 2.21 eV in cyclohexane can be assigned to the keto S1 → S0 emission 2.29 eV, in quantitative accuracy within the MSCASPT2 level. In cyclohexane, the most stable closed-keto form was responsible for the red-shifted emission. The emission maximum of the keto form (ϕ = 180) was predicted at 2.17 eV, with a small red shift from that of the keto (ϕ = 0) being 2.29 eV. The enol form was stabilized in ethanol by intra- and intermolecular hydrogen bonds with solvents, and this was responsible for the blue-shifted emission. In THF at room temperature, dual fluorescence was observed because both the keto (ϕ = 0) and enol forms can coexist in equilibrium. For all three species, the lowest nπ* emission was more than 1 eV higher than the S1 state. Geometry optimization under the planar constraint for the S1 states of the enol and keto forms were performed using CASSCF(6,6)/ANO-S. The coplanar S1 states (2A′) of the enol form were found to be pure ππ* states with little involvement of the nπ* states because the lowest nπ* state (1A″) is 2.19 eV higher than the 2A′ state. The coplanar S1keto form was more stable (by 32.5 kJ·mol−1) than the S1-enol form using MS-CASPT2 as shown in Table 7. This result

Figure 4. S0 and S1 potential energy profiles for the keto and enol forms at S1-optimized geometries (CASSCF(6,6)/ANO-S).

In contrast, monotonic elevation of the S0-keto level was observed as the torsion angle increased across the whole range of torsion angles examined. The difference in the S0 → S1 energy gap of the keto form became sufficiently small at 60°, and the corresponding crossing point was found at 90°. The presence of the S0/S1 CI provided the efficient and fast decay process of the excited state, resulting in the low quantum yield of fluorescence in solution. Furthermore, in order to study the whole picture of HPIP in S1, we performed fully optimized PES explorations without constraint of the torsion angle (CASSCF(6,6)/ANO-S-MB), including the ESIPT process and subsequent regeneration of the S0-enol form. The results gave a PES profile similar to that obtained by fixing the torsion angles. The reaction path from the FC-S1 state in the enol form leads to a nonplanar keto-S1 minimum. Energetically, there is a shallow minimum of the S1enol level at ϕ = 6.6°, which deviated only slightly from the coplanar conformation. However, the energy difference from that of the planar conformation is only 4 kJ·mol−1, and as described earlier, ESIPT is the ultrafast process. Therefore, it is likely that the keto form is formed immediately after excitation to the S1 state. The twisting process is associated with the enolto-keto tautomerization. The local minimum of the keto-S1 state is found at the twisting angle of 37.7° and is approximately 3.1 kJ·mol−1 lower in energy than the FC-S1 level. During the process, there is no avoided crossing between the ππ* and rapidly descending nπ* excited states. The electronic state of the keto-S1 minimum is a valence ππ* state, which is contrasted with a diradical-like open-shell state of BZT.60 The overall process is a hydrogen-atom transfer (or coupled electron plus proton transfer), as indicated by the net charge on the

Table 7. Total and Relative Energies of Coplanar HPIP Species for the Geometries Optimized in the S1 Statea species

CASSCF(10,9)/ANO-Lb

MS-CASPT2(10,9)/ANO-Lc

enol keto (ϕ = 0) keto (ϕ = 180)

−682.0276 −682.0202 (+19.4) −682.0176 (+26.2)

−684.2709 −684.2833 (−32.5) −684.2772 (−16.5)

a

Total energies are in a.u., and energies of the keto form relative to the enol form are in kJ/mol in parentheses. bThe 8 state averaged CASSCF. cThe 4 state averaged MS-CASPT2.

indicates that HPIP can smoothly undergo ESIPT from the enol FC-S1 state. The observed lifetimes of the ESIPT excited states were 0.52 and 5.26 ns in the THF and frozen solutions, respectively (Table 1). Since the ESIPT process is extremely fast, occurring within a subpicosecond time scale,62−64 tautomerization to the keto form in the S1 state is completed immediately. We then studied the effect of the torsion angle, ϕ, in the S1keto state as shown in Figure 4. An increase of the torsion angle from 0 to 45° induced a relatively small change in the keto-S1 level with a small local minimum at around 30°, which then decreased gradually as the torsion angle increased beyond 45°. E



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process such as exciton migration or excimer formation in the case of the HPIP crystals. Thus, the BG and Y crystals showed efficient luminescence similar to that of the matrix-separated state, though the stacked molecular pair was formed in the crystals. Polymorph-Dependent Luminescence Color of the HPIP Crystals. From Figure 4, it is apparent that the energy gap between the S1-keto and S0-keto states depends greatly on the torsion angle. The energy gap gradually decreases from the coplanar conformation to the CI point. Unless the gap becomes too small to induce efficient radiationless decay or the torsion angle becomes too large to prevent the ESIPT process, an increase in the torsion angle is expected to induce red-shift of the ESIPT luminescence. Interestingly, however, the S1-keto level shows a small local minimum at around 30°, and an increase of the torsion angle does not induce monotonic decrease of the energy gap. The torsion angles ϕ in the crystals are small enough; however, there is a slight difference (Figure 3a), i.e., the BG crystal has the torsion angle of 5.8°, and in the Y crystal, the torsion angles of conformers Y1 and Y2 are 1.3° and −1.0°, respectively. Although the torsion angle of BG is larger than those of Y, the luminescence of BG appears at the higher energy side, as shown in Table 8. Since the difference between the emission

transferred hydrogen, which remains unchanged during the transfer. The intersection point between the S0 and S1 states was found at the torsion angle of 60°, approximately 19 kJ·mol−1 downhill from the keto-S1 minimum at 37.7°. The barrier-free landscape from the keto-S1 minimum to the S0/S1 CI allows an efficient twisting motion to proceed to the CI. The molecule at the CI switches to the keto-S0 PES, goes downhill to the local keto-S0 minimum at the torsion angle of 10.2°, and then surmounts the barrier between the keto and enol forms to finally reach the original coplanar S0-enol form. Enhanced Solid-State Luminescence of HPIP. Though HPIP showed very weak ESIPT luminescence in solution, it showed bright luminescence not only in crystals, but also in frozen dilute solutions and polymer matrixes. Therefore, the observed AIEE effect in the solid state should be explained primarily from the single molecular level. The CASSCF study clearly showed the presence of the S0/S1 CI, and the efficient radiationless decay process of the molecule proceeds via CI coupled with the twisting motion. This is the reason for the weak emission in solution. In the frozen solution or polymer matrix, HPIP is thought to be fixed in the most stable coplanar enol conformation. In the BG and Y crystals, the torsion angles are less than 6°, and the excited states at these angles are sufficiently separated from the CI. Therefore, fixation of the torsion angle ϕ at or close to the coplanar conformation in the solid state should be the origin of the observed emission enhancement in the crystalline state. Suppression of the efficient radiationless decay process via the S0/S1 CI allows emissive decay from the keto-S1 to the keto-S0 state on a nanosecond time scale. The fact that no phosphorescence was observed in the solid state or frozen solution at 77 K indicates that no spin−orbit coupling process was involved in the S1 state. The present quantum chemical studies indicate that the proximity effect (nπ*−ππ* coupling in the excited states) plays a minor role in the relaxation process. Though there are many discussions on the enhanced solid-state luminescence because of the importance of solid-state organic luminescent materials, the present study indicates that the suppression of efficient radiationless decay via CI by fixation of the torsion angle at or near the coplanar conformation is the primary reason for the efficient luminescence in the solid-state. However, in order to understand the observed AIEE effect of the HPIP crystals, further discussion is required. As shown in Figure 3b, in the HPIP crystals, two molecules are packed close enough to form a stacked pair in an antiparallel mode, and their inter-ring distances are the typical stacking distance of 3.4 Å. There is sufficient molecular overlap between the stacked pair (Figure 3), though the extent of the overlap is slightly different for the BG and Y crystals. In the case of aromatics having planar molecular structures, exciton migration or excimer formation between the stacked molecules provide efficient energy dissipation pathways.65 However, the S0−S1 excitation energy in the enol form is much higher than the Stokes-shifted ESIPT emission from the S1-keto state, indicating that the S1-keto state can be regarded as an energy trap to inhibit exciton migration. Further, there is a large difference between the electronic structures of the keto and the enol forms. Therefore, it is likely that there is no strong intermolecular orbital interaction to induce excimer formation between the stacked-pair molecules. Indeed, as shown in Table 1, the emission energies, quantum yields, and emission lifetimes of the HPIP crystals were not much different from those of the amorphous solid or the frozen dilute solution, indicating that there was no additional decay

Table 8. Vertical Transition Energies (in eV) of HPIP for the BG and Y1/Y2 Crystal Geometriesa experiment method

BG

Y1/Y2

BG

Y1/Y2

CASSCF(10,9)/ANO-Lb MS-CASPT2(10,9)/ANO-Lc EOM-CCSD/cc-pVDZ

1.84 2.19 2.42

1.94/1.95 2.12/2.19 2.41/2.42

2.58

2.34

a Geometries partially optimized with the twist angles ϕ fixed at 5.8 (BG), 1.3(Y1), and −1.0(Y2). bThe 4 state averaged CASSCF. cThe 4 state averaged MS-CASPT2.

energies and the torsion angles of the BG and Y crystals is quite small, 0.24 eV, we conducted higher-level CASSCF(10,9)/ ANO-L and MS-CASPT2(10,9)/ANO-L calculations in order to qualitatively estimate the effect of the torsion angle for the emission energies. The estimated emission energies of the conformers at each torsion angle are listed in Table 8. The CASSCF(10,9)/ANO-L computations apparently failed to reproduce the observed results, predicting a higher emission energy for Y1/Y2 owing to the lack in dynamical correlation. Though the MS-CASPT2 calculations predicted a slightly lower energy for Y1, there was no difference between BG and Y2. The disappointing results primarily indicate that the larger-scale MSCASPT2 computations are required to correctly reproduce the small gap between BG and Y1/Y2, including the more extended CAS, averaged states, and basis sets. Alternatively, semiquantitative EOM-CCSD/cc-pVDZ computations unexpectedly gave the best absolute agreements with the experiments, despite the compact basis set employed. Although the predicted gap between BG band Y1/Y2 was negligibly small, 0.01 eV, more extensive EOM-CC computations may be promising in the quantitative predictions. In addition, since the estimation is based only on the conformation of the single molecule, this failure would be derived from the oversimplified estimation model to reproduce the small energy difference of the polymorph-dependent blue-green and yellow luminescence. It may be necessary to consider the effect of the surrounding F



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molecules, degree of conformational freedom in the crystal, and other factors66−68 in order to explain the small emission energy difference.

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CONCLUSIONS In the present study, ab initio electronic structure calculations were conducted to investigate the distinctive ESIPT luminescence enhancement of HPIP in the solid state. The calculations reproduced the experimental photophysical properties of HPIP in solution, and the coplanar keto form in the S1 state was indicated to approach smoothly to the S0/S1 CI coupled with the twisting motion of the central C−C bond. The S1−S0 energy gap of the keto form became sufficiently small at the torsion angle of 60°, and the corresponding CI point was found at 90°. Since a minor role of the proximity effect was indicated experimentally and theoretically, the observed emission enhancement of HPIP in the crystals was ascribed to the following two factors: (1) suppression of the efficient radialtionless decay via CI by fixation of the torsion angle at the nearly coplanar conformation in the crystals and (2) inhibition of excimer formation resulting from the lower excited level of the S1-keto state compared to the S0 → S1 excitation energy in the enol form. However, the fluorescence color difference between the two crystal polymorphs having slightly different torsion angles was not successfully reproduced even at MS-CASPT2 levels of theory. The reproduction of the experiments will require the larger scale computations considering the extended CAS, the averaged states, and the basis sets for future tasks.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic details of BG and Y1/Y2 crystals of HPIP, Mulliken charges and dipole moments of S0-enol (planar) and S0-keto (planar), optimized geometries of S0-enol, S0-keto, S1enol, and S1-keto, and crystal packing drawings of HPIP in unit cell. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-957-52-1133. Fax: +81-957-52-1136. E-mail: [email protected] (Y.S.); [email protected] (K.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported in part by Grants-in-Aid for Scientific Research (B) (No. 21350109, to K. A.) and (C) (No. 20510094, to T. M.) from the Japan Society for the Promotion of Science (JSPS) and a grant from the Science and Technology Agency of Japan (to Y.S.). Financial assistance (to Y.S.) from Shiseido, Co., Ltd. is also acknowledged.

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REFERENCES

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Excited-State Intramolecular Proton Transfer (ESIPT) Emission of

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