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Excited-State Intramolecular Proton Transfer (ESIPT) Fine Tuned by Quinoline-Pyrazole Isomerism: π-Conjugation Effect on ESIPT Min-Wen Chung,† Tsung-Yi Lin,† Cheng-Chih Hsieh,† Kuo-Chun Tang,† Hungshin Fu,† Pi-Tai Chou,*,† Shen-Han Yang,‡ and Yun Chi*,‡ Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan and Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 300, Taiwan ReceiVed: April 21, 2010; ReVised Manuscript ReceiVed: June 7, 2010
A series of quinoline/isoquinoline-pyrazole isomers (I-III), in which the pyrazole moiety is in a different substitution position, was strategically designed and synthesized, showing a system with five-membered intramolecular hydrogen bonding. Despite the similarity in molecular structure, however, only I undergoes excited-state intramolecular proton transfer, as evidenced by the distinct 560 nm proton-transfer emission and its associated relaxation dynamics. The experimental results support a recent theoretical approach regarding the conjugation effect on a proton (or hydrogen atom) transfer reaction (J. Phys. Chem. A 2009, 113, 4862-4867). The concept simply predicts that more extended π conjugation, i.e., resonance, for protontransfer tautomer species could allow for efficient delocalization of excess charge in the reaction center, resulting in a larger thermodynamic driving force for proton transfer. 1. Introduction Proton (or hydrogen atom) transfer represents one of the most fundamental processes involved in chemical reactions as well as in living systems.1 A vast amount of research has been focusing on various types of proton transfers in ground states as well as in the excited states to explore the associated reaction mechanism.2 Recently, a simple and an intriguing concept was reported by Dunietz and co-workers3 on the relationship between the degree of π conjugation and the occurrence of excited-state proton transfer. In this theoretical approach, they virtually constructed a set of derivatives with different numbers and positions of aromatic rings based on parent 7-azaindole dimers and then performed calculations to analyze the relative energy and structures between normal and tautomer species in the excited state. As a result, a general pattern appears that the degree of π conjugation for the proton-transfer tautomer species in the excited state has a significant influence on the protontransfer process. Extended π conjugation would result in better delocalization of excess charge at the reaction center and hence lower the relative energy of the tautomer species in the excited state. As a result, a larger thermodynamic driving force is formed for the proton transfer. Experimentally, the proof of concept may be attained straightforward by investigating various 7-azaindole derivatives proposed therein.4 Unfortunately, to our knowledge, synthesis of these derivatives is a nontrivial task. Alternatively, in this study, we strategically designed and synthesized a series of quinoline/isoquinoline-pyrazole isomers I-III for which their molecular structures are depicted in Scheme 1. Although the relative position between the proton-accepting site, i.e., the nitrogen atom in (iso)quinoline, and the proton-donating site, i.e., the N-H proton in the pyrazole moiety, is the same, the degree of π conjugation in the proton-transfer tautomer can be * To whom correspondence should be addressed. P.-T.C.: e-mail
[email protected]. Y.C.: e-mail
[email protected]. † National Taiwan University. ‡ National Tsing Hua University.
SCHEME 1: Molecular Structures of a Series of Quinoline/Isoquinoline-Pyrazole Isomers with Different Positions of π Conjugationa
a Note that syn and anti structures are exemplified using compound I for the convenience of discussion (see text).
systematically varied by changing the direction of extending π conjugation imposed on the pyridine moiety (vide infra). We then performed comprehensive spectroscopic and dynamic measurements for I-III. The results, in combination with theoretical approaches, may allow us to extract valuable information necessary for the proof of concept. 2. Experimental Section General Information. All reactions requiring anhydrous conditions were conducted in flame-dried apparatus under a nitrogen atmosphere. 1H and 13C NMR spectra in CDCl3 were recorded using a Varian Mercury 400 spectrometer. 3-Acetyl isoquinoline, 2-acetyl quinoline, and 1-acetyl isoquinoline were synthesized according to literature procedures,5 while 5-substituted, 3-isoquinolinyl, 2-quinolinyl, and 1-isoquinolinyl-3trifluoromethyl pyrazole were prepared employing basecatalyzed Claisen condensation of acetyl heterocycles with ethyl trifluoroacetate, followed by treatment with 98% hydrazine hydrate in refluxing ethanol solution.6 Details of the synthetic procedure and characterization of compounds I-III and Ia are described in the Supporting Information.
10.1021/jp1036102 2010 American Chemical Society Published on Web 07/14/2010
ESIPT Fine Tuned by Quinoline-Pyrazole Isomerism
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Figure 1. Absorption and normalized emission spectra of I-III and Ia recorded in cyclohexane at room temperature: (a) I (red), (b) Ia (black), (c) II (blue), (d) III (dashed line).
Cyclohexane used for photophysical measurement was spectroscopic grade (Merck Inc.) and refluxed several hours over sodium metal under a nitrogen atmosphere. It was then transferred, prior to use, through distillation to the sample cuvette. Steady-state absorption and emission spectra were recorded by a Hitachi (U-3310) spectrophotometer and an Edinburgh (FS920) fluorimeter, respectively. The excitation light source of the fluorimeter has been corrected by the Rodamine B spectrum. In addition, the wavelength-dependent characteristics of the monochromator and photomultiplier have been calibrated by recording the scattered light spectrum of the corrected excitation light from a diffused cell in the 220-700 nm range. In order to obtain the precise extinction coefficient, five different concentrations ranging from 5 × 10-5 to 5 × 10-7 M were performed. The deuterated compound I was synthesized by dissolving I in CH3OD, and then CH3OD was gradually evaporated in the vacuum line. This procedure was repeated three times, and the deuterated products were stored in the N2purged drybox where sample preparation was also performed. Formation of deuterated compound I was checked by 1H NMR, where ∼93% of the N-H proton disappeared. Detailed fluorescence lifetime measurement was described in a previous report.7 In brief, nanosecond lifetime studies were performed with an Edinburgh FL 900 photon-counting system with a hydrogen-filled or a nitrogen lamp as the excitation source with 40 kHz repetition rate. The emission decays were fitted by the sum of exponential functions with a temporal resolution of ∼200 ps by deconvolution of the instrument response function. The setup of picosecond dynamical measurements consists of a femtosecond Ti-Sapphire oscillator (82 MHz, Spectra Physics). The fundamental pulse train (760-800 nm) was pulse selected (Neos, model N17389) to reduce its repetition rate down to typically 0.8-8 MHz and then used to produce second harmonics (380-400 nm). Third harmonics (253-267 nm) was then generated by combining residual fundamental and second harmonics and used as an excitation light source. A polarizer was placed in the emission path to ensure that the polarization of the fluorescence was set at the magic angle (54.7°) with respect to that of the pump laser to eliminate the fluorescence anisotropy. An Edinburgh OB 900-L time-correlated single-photon counting system was used as a detecting system. The time-dependent fluorescence data were fitted by the sum of exponential functions with a temporal resolution of ∼50 ps after deconvolution. Computational Methodology. Ground-state and the lowest singlet excited-state (S1) geometries were optimized by density
functional theory (DFT) with B3LYP hybrid functional8 and CIS theory,9 respectively. The 6-31G(d,p) basis sets were employed for all atoms.10 Vibrational frequencies were then calculated based on their optimized geometries to verify that each of the calculated geometries is the global-optimized structure. All calculations were carried out using the Gaussian 03 program.11 3. Results and Discussion Figure 1 depicts the steady-state absorption and emission spectra of molecules I-III in cyclohexane at room temperature. For all I-III, the lowest-lying S0 f S1 absorption is located in the region of 310-350 nm, which is attributed to the pyrazole f quinoline/isoquinoline π electron transition (vide infra). However, despite the similarity in molecular structures, remarkably different emission spectra were resolved. All isomers I-III exhibit a major emission band maximized at ∼350-370 nm. The emissions of I and II possess significant vibronic progressions, while emission of III is relatively weaker and structureless. Nevertheless, these emissions all show spectral features that are a mirror image with respect to the S0-S1 absorption profile. Therefore, the assignments to the normal emission of I-III are straightforward. Despite this sameness, a major difference appears that I exhibits an additional emission band maximized at 570 nm, while no other emission bands can be resolved for both II and III. This 570 nm emission band of I, which is weak though, could not be ascribed to any spectroscopic artifact or aggregation effect based on the following observation. (i) To avoid second-order light leakage via the monochromator, a 400 nm long-pass filter was installed during data acquisition, the results of which showed no interference to the 570 nm emission. (ii) The excitation spectra monitored at either 350 or 570 nm bands are identical (see Figure S1 of the Supporting Information) and are the same as the absorption spectrum. (iii) The ratio of emission intensity for the 350 versus 570 nm band remains unchanged in a wide concentration range of 10-6-10-3 M. Moreover, in a control experiment, Ia (Scheme 1) was synthesized via methylation of I on the pyrazolic group. Due to the lack of pyrazolic proton, any proton-transfer event should be prohibited in Ia. As expected, only normal emission maximized at ∼338 nm was resolved for Ia (see Figure 1). Accordingly, for I, the 570 nm emission with a Stokes shift of ∼8000 cm-1 (peak-to-peak between absorption and emission) clearly originates from the
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Chung et al. SCHEME 2
Figure 2. Relaxation dynamics of I in cyclohexane monitored at (a) 350 and (b) 570 nm and (c) the instrument response function. (Inset) Logarithm plot of intensity versus time for I.
excited-state intramolecular proton-transfer (ESIPT) reaction between the pyrazolic proton and the nitrogen of the isoquinoline fragment. Further support of the above viewpoint is provided with dynamical approaches. For I, upon monitoring at the normal emission of 350 nm, a prompt rise component (system response limit of
