Synthesis, X-ray Structure, Optical, and Electrochemical Properties of a White-Light-Emitting Molecule

A new white-light-emitting molecule (1) was synthesized and characterized by NMR spectroscopy, high resolution mass spectrometry, and single-crystal X-ray diffraction. Compound 1 crystallizes in the orthorhombic space group Pnma, with a = 12.6814(6), b = 7.0824(4), c = 17.4628(9) Å, α = 90°, β = 90°, γ = 90°. In the crystal, molecules are linked by weak intermolecular C-H···O hydrogen bonds, forming an infinite chain along [100], generating a C(10) motif. Compound 1 possesses an intramolecular six-membered-ring hydrogen bond, from which excited-state intramolecular proton transfer (ESIPT) takes place from the phenolic proton to the carbonyl oxygen, resulting in a tautomer that is in equilibrium with the normal species, exhibiting a dual emission that covers almost all of the visible spectrum and consequently generates white light. It exhibits one irreversible one-electron oxidation and two irreversible one-electron reductions in dichloromethane at modest potentials. Furthermore, the geometric structures, frontier molecular orbitals (MOs), and the potential energy curves (PECs) for 1 in the ground and the first singlet excited state were fully rationalized by density functional theory (DFT) and time-dependent DFT calculations. The results demonstrate that the forward and backward ESIPT may happen on a similar timescale, enabling the excited-state equilibrium to be established.


Crystal Structural Determination
A single crystal of 1 with dimensions of 0.56 mmˆ0.40 mmˆ0.25 mm was selected. The lattice constants and diffraction intensities were measured with a Bruker Smart 1000CCD area detector radiation (λ = 0.71073 Å) at 297(2) K (Bruker, Billerica, MA, USA). An ω-2θ scan mode was used for data collection in the range of 2.83˝ď θ ď 29.16˝. A total of 6225 reflections were collected and 2013 were independent (R int = 0.0961), of which 1340 were considered to be observed with I > 2σ(I) and used in the succeeding refinement. The structure was solved by direct methods with SHELXS-97 [43] and refined on F 2 by full-matrix least-squares procedure with Bruker SHELXL-97 packing (Bruker, Billerica, MA, USA) [44]. All non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms refined with riding model position parameters isotropically were located from difference Fourier map and added theoretically. At the final cycle of refinement,

Steady State Spectral Measurements
All the spectral measurements were done at 10´5 M concentration of solute in order to avoid aggregation and self-quenching. The fluorescence quantum yield of 1 and 2 in ethyl acetate was measured relative to quinine sulphate in 1 M sulphuric acid (Φ f = 0.57) as secondary standard [45] and calculated on the basis of the following equation: where n 0 and n are the refractive index of the solvents; A 0 and A are the absorbances; Φ f and Φ 0 f are the fluorescence quantum yields; and the integrals denote the area of the fluorescence band for the standard and the sample, respectively.

Computational Methods
The Gaussian 03 program (Gaussian, Pittsburgh, PA, USA) was used to perform the ab initio calculation on the molecular structure [46]. Full geometry optimizations of compound 1 were carried out with the 6-31G** basis set to the B3LYP functional. The hybrid DFT functional B3LYP has proven to be a suitable DFT functional to describe hydrogen bond [47]. Vibrational frequencies were also performed to check whether the optimized geometrical structures for 1 were at energy minima, transition states, or higher order saddle points. After obtaining the converged geometries, the TD-B3LYP/6-31G** was used to calculate the vertical excitation energies. Emission energies were obtained from TDDFT/B3LYP/6-31G** calculations performed on S 1 optimized geometries.
The phenomenon of photo-induced proton transfer (PT) reaction in 1 can be most critically addressed and assessed by evaluating the potential energy curve (PEC) for the PT reaction. For the S 0 state, all of the other degrees of freedom are relaxed without imposing any symmetry constraints. The excited-state (S 1 ) PEC for the ESIPT reaction in 1 has been constructed on the basis of TD-DFT optimization method. Figure 1 depicts the chemical structures and the synthetic routes of white-light-emitting small molecules 1 and 2. The synthesis of 1 started from a bromination of 7-methoxy-1-indanone (6), followed by the elimination of 5, giving a dienophile 4. The naphthalene ring can then be fused onto the C(2)-C(3) double bond by placing 4 through a reaction with α,α,α'α'-tetrabromo-o-xylene [41], yielding 3. Subsequently, deprotection of 3 with BBr 3 produced compound 2. Finally, the regioselective alkylation at the 4-position of 2 was executed by the reaction of 2 with tert-butyl alcohol and sulfuric acid, giving 1 with an overall product yield of 65%. The presence of a single tert-butyl group of 1 can be verified by the presence of a signal at δ 1.42 ppm (9H, singlet) and eight signals at δ 7.0-8.2 ppm (8H) in the 1 H-NMR spectrum. To confirm its structure, a single crystal of 1 was obtained from a dichloromethane solution, and the molecular structure was determined by X-ray diffraction analysis. Additionally, its X-ray structure is compared with that of 2.

Hydrogen Bond Studies
The dominance of an enol-form for 1 and 2, namely the intramolecular hydrogen-

X-ray Structures
Compound 1 crystallizes in the orthorhombic space group Pnma, whereas the closely related compound 2 crystallizes in the monoclinic space group P2 1 /c (Table 1). Figure 3 shows the ORTEP (Oak Ridge Thermal Ellipsoid Plot) diagram of 1. The molecule is completely planar (except for tert-butyl substituent), as indicated by the key torsion angles ( Table 2) angle is expected to be deviated from 120˝, a perfect six-membered-ring hydrogen-bonding formation. This viewpoint is confirmed by the =O(2)-H(2A)-O(1) angle of 145˝(143˝), according to the X-ray structure analysis. Note that compound 1 (2) has a weaker intramolecular hydrogen bond than most other ESIPT chromophores [49,50], which may account for its unique dual emission feature (vide infra).    and generating a C(10) motif. Careful examination of the crystal structure also depicts that there is no substantial π-π stacking between the tetracyclic plane and its adjacent one. As a result, we can ascertain that the bulky tert-butyl substituent not only increases the solubility of 1 compared with 2, but also reduces intermolecular contact and aggregation.  Figure 5 shows the steady state absorption and emission spectra of 1 in ethyl acetate. Compound 1 exhibits the lowest lying absorption band maximized at 423 nm, attributed to a π Ñ π* transition, which is also supported by the calculated frontier orbitals (vide infra). Additionally, the absorption spectrum of 1 is nearly identical with that of 2, which demonstrates that the introduction of the tert-butyl group does not substantially affect the bandgap energy of 1 compared with that of 2. As depicted in Figure 5, dual emission is well resolved in the steady-state measurement of 1, which is composed of a normal emission band (enol form), justified by its mirror image with respect to the lowest lying absorption, and a large Stokes shifted (6605 cm´1) emission band maximized at 477 and 587 nm, respectively. Accordingly, the assignment of a 587 nm emission for 1 in ethyl acetate to a proton-transfer tautomer emission is unambiguous, and ESIPT takes place from the phenolic proton (O-H) to the carbonyl oxygen, forming the keto-tautomer species shown in Figure 6. Incidentally, the dual emission achieves a nearly white light generation with Commission Internationale de l'Eclairage (CIE) (0.35, 0.36). The overall quantum yield of 1 is measured to be 0.15 and is about four times larger than that of 2 (0.04), which can be explained by the fact that the bulky tert-butyl substituent reduces the intermolecular π-π stacking of 1 so that the quantum yield can be substantially enhanced.

Quantum Chemistry Computation
To gain more insight into the molecular structures and electronic properties of 1, quantum mechanical calculations were performed using the density functional theory (DFT) at the B3LYP/6-31G** level. The values of bond lengths, bond angles, and torsion angles for 1 were compared with its crystal structure data. Table 2 compares the crystallographic and optimized geometric parameters of 1. There are no substantial differences between the experimental and DFT/B3LYP calculated geometric parameters. Consequently, we can conclude that basis set 6-31G** is suited in its approach to the experimental results.
The optimized geometric structures and the corresponding hydrogen bond lengths of enol and keto form for 1 in the ground and the first singlet excited state were calculated using DFT and TD-DFT with the B3LYP functional and the 6-31G** basis set (Figure 7). From E (K*) to E* (K), one can see that the intramolecular hydrogen bond length decreases from 1.89 (1.72) Å to 1.81 (1.65) Å, whereas the other bond lengths do not significantly change. The results clearly provide evidence for the strengthening of the intramolecular hydrogen bond from S 0 Ñ S 1 (S 1 Ñ S 0 ), which is consistent with previous studies [51][52][53]. Therefore, there is no question that the decreases of intramolecular hydrogen bond lengths from E (K*) to E* (K) is a very significant positive factor for the ESIPT (GSIPT: ground state intramolecular proton transfer) reaction.  Figure 8 depicts the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of the enol and keto form of 1, both of which are strongly delocalized over the entire π-conjugated system. It also shows that the electron density around the intramolecular hydrogen bonding system is mainly populated at hydroxyl oxygen and carbonyl oxygen at HOMO and LUMO, respectively. The results clearly show that upon electronic excitation of 1, the hydroxyl proton (O(2)-H(2A)) is expected to be more acidic, whereas the carbonyl oxygen O(1) is more basic with respect to their ground state, driving the proton transfer reaction (forward ESIPT). After the forward ESIPT (E* Ñ K*), the electron density located on O(2) increases while that on O(1) decreases, which shows the prominent intramolecular charge transfer from O(1) to O (2). This may supply the driving force for the proton transfer from O(1) to O(2) (backward ESIPT), so that the excited-state equilibrium can be established. In addition, the absorption and emission spectra of 1 were calculated by time-dependent DFT calculations (Franck-Condon principle, Figure 7). The calculated excitation, normal emission, and tautomer emission wavelengths for the S 0 Ñ S 1 (S 1 Ñ S 0 ) transitions are 411 nm, 467 nm, and 572 nm, respectively, which is very close to the experimental results ( Figure 4). In order to explain the ESIPT properties of compound 1, the potential energy curves of the intramolecular proton transfer as a function of the O(2)-H(2A) bond length (i.e., the transformation from the enol form to the keto form) at both the ground state and the excited state were studied ( Figure 9). On the one hand, the full geometry optimization based on the B3LYP/6-31G** theoretical level shows that the enol form (E) of 1 (2) in the ground state is more stable than the corresponding proton-transfer tautomer (K) by 12.8 (15.0) kcal/mol. As a result, proton transfer from K to E is populated in the ground states. It is also apparent that the increased phenolic (O(2)-H(2A)) acidity (hydrogen bonding strength, see 3.2) lowers the tautomerization energy by stabilizing the tautomers due to inductive effect of the bulky tert-butyl group. On the other hand (for the first singlet excited state), one can clearly see that the potential energy barriers of the forward (6.1 kcal/mol) and the backward (1.8 kcal/mol) ESIPT are in the same order of magnitude, which is in good agreement with previous theoretical studies of 2 [54]. Accordingly, the forward and the backward ESIPT may happen on a similar timescale, and hence leads to the rapidly established excited-state equilibrium.  Figure 10 shows the cyclic voltammogram of 1. When placed in dichloromethane and subjected to modest potentials, compound 1 shows one oxidation and two reduction waves, all of which are chemically irreversible. The first oxidation and reduction potentials of 1 are almost identical to those of 2 (Table 3), showing that the alkylation of 2 has no significant impact on both their electrochemical properties as well as their optical properties. The redox potentials and the HOMO and LUMO energy levels estimated from cyclic voltammetry (CV) for 1 are summarized in Table 3. The HOMO/LUMO energy levels of 1 are estimated to be´5.87/´2.94 eV, and are in good agreement with the theoretical calculations.

Conclusions
In conclusion, we have successfully synthesized and characterized a new ESIPT-based white-light-emitting small molecule (1) with a bulky tert-butyl group. Compound 1, as well as compound 2, undergoes an intramolecular proton transfer reaction in the excited state, resulting in a tautomer that is in equilibrium with the normal species, exhibiting a dual emission that generates white light. The introduction of the tert-butyl substituent not only increases the solubility of 1 compared with 2, but also improves the fluorescence intensity. Furthermore, analysis of the geometric structures clearly demonstrates that the intramolecular hydrogen bond length is shortened upon the photoexcitation, which is considered to be a very important factor for ESIPT. The potential energy curves demonstrate that the forward ESIPT and backward ESIPT may happen on a similar timescale and leads to the rapidly established excited-state equilibrium. Research on its application to single-molecule-based WOLEDs is currently in progress.