Photochemistry and Photobiology, 2010, 86: 1197–1201

Excited-state Intramolecular Proton–transfer-induced Charge Transfer of Polyquinoline Sun-Young Park1, Hyeok Jeong1, Hyunung Yu1§, Soo Young Park2 and Du-Jeon Jang*1 1 2

School of Chemistry, Seoul National University, Seoul, Korea Department of Materials Science and Engineering, Seoul National University, Seoul, Korea

Received 13 April 2010, accepted 3 August 2010, DOI: 10.1111/j.1751-1097.2010.00797.x

ABSTRACT The excited-state intramolecular proton–transfer-induced charge transfer of semirigid polyquinoline (PQH) is explored in 1,1,2,2tetrachloroethane (TCE) and N-methyl-2-pyrrolidinone (MP) using picosecond time-resolved fluorescence spectroscopy. Reaction mechanisms are found to depend on the rotational conformations of PQH at the moment of excitation; whereas the trans-enolic form does not undergo intramolecular proton transfer within its excited-state lifetime, the cis-enolic form does within 15 ps to form a tautomeric zwitterion species. While the subsequent intramolecular charge transfer of the zwitterionic species to yield a tautomeric keto species takes place on time scales of 25 ps in TCE (e = 8.50) and 62 ps in MP (e = 32.55), its reverse reaction is also followed on time scales of 28 ps in TCE and 20 ps in MP. The lack of a kinetic isotope effect in both forward and reverse charge-transfer reactions support our proposed mechanisms.

polar charge-transfer state (21–25). The efficiencies of excitedstate charge-transfer processes via p-electron delocalization are known to increase with solvent polarity because of the stabilization of a more polar charge-transfer state, which causes the intrinsic activation energy to decrease (26,27). Photoinduced proton-transfer and charge-transfer reactions have attracted considerable attention from many research groups for practical applications (21–25,28–31). Considering that the Franck–Condon excited state of an ESIPT molecule undergoes a huge dipolar change because of charge separation, we suggest that the equilibrium polarizations of the normal and the proton-transferred tautomeric species might be extensively different (26,27). Although many studies of this point have been reported, the detailed molecular coupling mechanism of proton transfer and charge transfer remains unclear (25,31). In this respect, the design of molecular structures whose photochemical behavior involves intramolecular proton–transfer-induced charge transfer is expected to provide new insight into the subject.

INTRODUCTION Proton- and charge-transfer reactions are most fundamental processes involved in chemical reactions as well as in living systems (1–8). Among the various studies of proton transfer, excited-state intramolecular proton transfer (ESIPT) via a cyclic intramolecular hydrogen (H) bond has received much attention because of their many applications (9–17). The unique properties of ESIPT, large Stokes’-shifted fluorescence without self-absorption and easy population inversion between the two lowest singlet states of the proton-transferred tautomer, make it possible to produce advanced polymer devices, such as electroluminescence devices (10–14), proton-transfer lasers (11–14) and UV absorbers (15,16). Fluorescence sensors and probes for macromolecular science and cellular biology are other promising application fields of ESIPT because of its extreme sensitivity (17–19). Electron rearrangement is basically intramolecular charge transfer (ICT) from the electron donor site of a molecule to the electron acceptor site without notable change in nuclear geometry to produce a tautomeric zwitterion species (20). In these ICT processes, characteristic solvent– polarity-dependent emission is often observed from a highly §Current address: Korea Research Institute of Standards and Science, Daejeon 305-600, Korea *Corresponding author email: [email protected] (Du-Jeon Jang)  2010 The Authors. Journal Compilation. The American Society of Photobiology 0031-8655/10

Scheme 1. Structure of polyquinoline (PQH).

Semirigid polyquinoline (PQH), which has been synthesized for light-emitting devices because of its unique emitting properties, is a good polymeric model system to study ESIPT-induced charge transfer as well as intramolecular resonance-assisted H-bonding (32) (Scheme 1). Thus, in this article, we report the dynamics and mechanisms of the ESIPT and the subsequent electron rearrangement of PQH tautomers (Scheme 2). Whereas the trans-enolic form (trans-E) of PQH does not undergo ESIPT within its excited-state lifetime, the cis-enolic form (cis-E) goes through ESIPT rapidly to form a tautomeric zwitterion species (Z). We have also found that the subsequent electron rearrangement of Z to form a tautomeric keto species (K) and the reverse electron rearrangement of K to form Z compete with each other in the first-excited singlet

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1198 Sun-Young Park et al. state. The ESIPT mechanism of PQH is discussed in details from the viewpoints of the molecular conformation and the torsional motion of PQH in solutions, and the subsequent charge-transfer dynamics of PQH is described with solvent– polarization-coupled dipolar changes in solutions.

Scheme 2. Moiety structures and symbols of polyquinoline (PQH) tautomers.

MATERIALS AND METHODS Materials. The preparation of PQH has been already described (10). PQH shows excellent thermal stability with glass transition and initial decomposition temperatures of 230 and 450C, respectively, and good solubility in 1,1,2,2-tetrachloroethane (TCE), N-methyl-2-pyrrolidinone (MP), m-cresol and formic acid, sufficient to prepare an optically clear film. The weight-averaged molecular weight of PQH was estimated as 8000 g mol)1 using gel permeation chromatography. The protic 1H atoms of PQH were exchanged with 2H atoms to produce deuterated polyquinoline (PQD) by refluxing a mixture of PQH and 2HCO22H (isotopic purity ‡99.5%, received from Merck) for 1 h and subsequently removing the solvent completely in a vacuum. Measurements. Absorption spectra were obtained using a UV ⁄ vis diode array spectrophotometer (Scinco; S-2040). Photoluminescence and excitation spectra were obtained with a home-built fluorometer, which consisted of a Xe lamp of 75 W (Acton Research; XS 432) with a monochromator of 0.15 m (Acton Research; Spectrapro 150) and a photomultiplier tube (Acton Research; PD 438) attached to a monochromator of 0.30 m (Acton Research; Spectrapro 300). Excitation and stray light were blocked using an appropriate combination of filters. Fluorescence spectra were not corrected for the wavelengthdependent sensitivity variation of the detector. To obtain time-resolved fluorescence kinetic profiles, a mode-locked Nd:YAG laser (Quantel; YG 501) with the pulse duration of 25 ps was employed for excitation, and a streak camera of 10 ps (Hamamatsu; C2830) attached to a CCD detector (Princeton Instruments; RTE128H) was used for detection. Samples were excited with 290 nm pulses generated from a Raman shifter, filled with CH4 gas of 15 atm and pumped by the fourthharmonic pulses of 266 nm from the laser. Emission was collected at a near magic angle for fluorescence kinetic measurements. Fluorescence kinetic constants were extracted by fitting measured kinetic profiles to computer-simulated kinetic curves convoluted with instrument temporal response functions. All the optical measurements were carried out at ambient temperature.

RESULTS AND DISCUSSION The absorption and the emission spectra of PQH in two organic solvents of TCE and MP at room temperature are given in Fig. 1. The lowest electronic absorption maximum of PQH at 360 nm in TCE (e = 8.50) shifts to the blue by 7 nm in MP (e = 32.55) as the solvent polarity increases. The drastic increase of PQH acidity with excitation (21–23) is

Figure 1. Absorption (solid) and emission (dashed, with excitation at 310 nm) spectra of polyquinoline (PQH) in (a) 1,1,2,2-tetrachloroethane (TCE) and (b) N-methyl-2-pyrrolidinone (MP).

believed to be the origin of the blue absorption shift in MP (33,34). PQH in TCE shows dual emission bands at 401 and 592 nm (Fig. 1a). The blue band at 401 nm is because of normal emission from the enolic form (E), whereas the red band at 592 nm arises from tautomeric emission from K. This indicates that upon absorption of a photon, E undergoes ESIPT-induced charge transfer to produce K in the firstexcited singlet state (see below). In contrast, PQH in MP shows a dominant blue emission band at 401 nm with a green shoulder emission band at 490 nm and a red tail emission band at 592 nm (Fig. 1b) (35). Although the origins of the blue and the red emission bands have already been described in Fig. 1a, the green band can be attributed to emission from tautomeric Z intermediate species. The electron rearrangement of Z following the ESIPT of cis-E produces K in the first-excited singlet state (see below). A close look at Fig. 1a reveals that the excitation of PQH in TCE also yields the green emission band. Green emission is stronger in MP than in TCE because charged intermediate species Z is more stable in the more polar solvent, MP (36). The dominant blue emission band of Fig. 1b suggests that externally H-bonded trans-E cannot undergo ESIPT within its excited-state lifetime (see below) (37,38). To gain more insight into the ground-state structural configurations of PQH tautomers, we have also recorded fluorescence excitation spectra (Fig. 2). The excitation spectrum of the red fluorescence band is almost superimposable with that of the blue fluorescence band. This indicates that the precursor of the tautomeric species K is the same as that of the normal E species. Thus, PQH exists predominantly as the E form in the ground state, and its Franck–Condon excited state is the precursor of K. However, a close look at Fig. 2 reveals that the excitation spectrum of the red fluorescence band is longer in wavelength and broader in spectral width than the excitation spectrum of the blue fluorescence band. This suggests that the precursor species of the tautomeric K species is H-bonded at the moment of excitation and that intramolecular H-bonding is a prerequisite to the ESIPT of PQH (10).

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Figure 2. Excitation spectra of polyquinoline (PQH) in 1,1,2,2-tetrachloroethane (TCE) monitored at 400 nm (solid) and at 600 nm (dashed).

Time-resolved fluorescence kinetic profiles of PQH in organic solvents were measured with excitation at 290 nm at three distinct wavelengths of 400, 490 and 600 nm to monitor the dynamic behavior of E, Z and K, respectively. Figure 3 and Table 1 provide a direct evidence that the red emission band of Fig. 1a at 592 nm is because of fluorescence from the K species following ESIPT, rather than phosphorescence from the E species. Figure 3a indicates that the fluorescence kinetic profile of PQH in TCE at 400 nm has biexponential decay times of 15 ps (98%) and 650 ps (2%) at room temperature, whereas that at 490 nm has two decay times of 25 ps (87%) and 190 ps (13%). The kinetic profile of the same sample at 600 nm has a rise component of 27 ps and two decay components of 28 ps (88%) and 290 ps (12%). Upon photoexcitation, the intramolecularly H-bonded cis-E form undergoes ESIPT from the enolic group to the nitrogen atom within 15 ps to produce the Z intermediate species whereas the transE form cannot undergo ESIPT within its relaxation time of 650 ps. The subsequent ICT of the Z intermediate species from the oxygen atom to the nitrogen atom, as the rate-determining step of excited-state PQH tautomerization, takes place on a time scale of 25 ps to form the K tautomeric species, which also undergoes the reverse ICT in 28 ps to reproduce the Z intermediate species. Whether hydrogen-atom transfer from the hydroxyl group to the pyridinic nitrogen atom occurs via one-step hydrogenatom transfer or rate-determining charge transfer following fast proton transfer can be clarified by examining the kinetic isotope effects of the protic hydrogen atom. One-step hydrogen-atom transfer would have a significant deuterium kinetic isotope effect, whereas rate-determining charge transfer following fast proton transfer would exhibit no deuterium kinetic isotope effect (39,40). Figure 3b shows that the rate constants of both the ESIPT-induced charge transfer and the reverse charge transfer remain invariant regardless of the isotope exchange of the protic hydrogen atom. The lack of a kinetic isotope effect suggests that the reversible chargetransfer reactions following the ESIPT of the intramolecularly H-bonded cis-E form are nuclear–mass-independent processes indeed. Thus, the proposed tautomerization reaction of an intramolecularly H-bonded cis-E conformer occurs sequentially involving two mechanistically distinct proton-transfer and charge-transfer steps (21–25).

Figure 3. Fluorescence kinetic profiles of (a) polyquinoline (PQH) in 1,1,2,2-tetrachloroethane (TCE), (b) PQD in TCE and (c) PQH in Nmethyl-2-pyrrolidinone (MP). Samples were excited at 290 nm and monitored at 400 nm (1), 490 nm (2), and 600 nm (3).

Table 1. Fluorescence kinetic constants extracted from Fig. 3.* kem (nm) 400 490 600

Sample PQH PQD PQH PQH PQD PQH PQH PQD PQH

in in in in in in in in in

TCE TCE MP TCE TCE MP TCE TCE MP

Rise time (ps)

Decay time (ps)

Instant Instant Instant Fast Fast Fast 27 27 Fast†

15 15 15 25 25 62 28 28 20

(98%) (99%) (73%) (87%) (87%) (60%) (88%) (89%) (94%)

+ + + + + + + + +

650 400 280 190 280 620 290 300 290

*kem is the collection wavelength of emission, and numbers in parentheses are initial amplitude percentages. †Unresolved because of the fast dominant decay component. PQH = polyquinoline; PQD = deuterated polyquinoline; TCE = 1,1,2,2-tetrachloroethane; MP = N-methyl-2-pyrrolidinone.

The fluorescence kinetic profiles of PQH in MP (Fig. 3c) well support the just described mechanisms. The fluorescence of PQH in MP at 400 nm has two decay components of 15 ps (73%) and 280 ps (27%). The percent amplitude of the slow component is much larger in MP than that in TCE because some fraction of E-forms are externally H-bonded to the oxygen atom of MP to have trans-E rotational conformers of the enolic group (37,38). The fluorescence of PQH in MP at

1200 Sun-Young Park et al.

Figure 4. Mechanistic description for the proton transfer (PT)-induced charge transfer (CT) and relaxation of polyquinoline (PQH) tautomers in organic solvents.

reverse ICT on a time scale of 28 ps in TCE to produce the Z species. The rate constants of both the ESIPT-induced charge transfers and the reverse charge transfer remain invariant regardless of the isotope exchange of the protic hydrogen atom. The lack of a kinetic isotope effect in both forward and reverse charge-transfer reactions implies that the charge transfers following the ESIPT of the intramolecularly H-bonded cis-E form are nuclear–mass-independent processes indeed (39,40). Thus, the plausible tautomerization reaction of an intramolecularly H-bonded cis-E conformer occurs sequentially involving two mechanistically distinct proton-transfer and charge-transfer steps (21–25). As the charged Z species is stabilized greatly in the strongly polar solvent of MP, the charge-transfer time (62 ps) of Z to form K is three times longer than the reverse charge-transfer time (20 ps) of K to form Z.

Table 2. Observed values for the rate constants of Fig. 4. Sample PQH in TCE PQH in MP PQD in TCE

kPT)1 (ps)

kCT)1 (ps)

k)CT)1 (ps)

kE)1 (ps)

kZ)1 (ps)

kK)1 (ps)

15 15 15

25 62 25

28 20 28

650 280 400

190 620 280

290 290 300

PQH = polyquinoline; PQD = deuterated polyquinoline; TCE = 1,1,2,2-tetrachloroethane; MP = N-methyl-2-pyrrolidinone.

490 nm has two decay times of 62 ps (60%) and 620 ps (40%), whereas that at 600 nm decays on time scales of 20 ps (94%) and 290 ps (6%). As the charged Z intermediate species is stabilized greatly in the strongly polar solvent of MP (36), the rate constant of the ESIPT-induced charge transfer of Z to generate K is three times smaller than that of the reverse charge transfer of K to produce Z. In a polar reaction such as proton or charge transfer, generally, the reaction rate is sensitive to solvent polarity if the reactant and the product have different polarities (41). In the Marcus (42) normal region, the rate constant decreases with increasing solvent polarity if the reactant is more polar than the product. Correspondingly, our results show that the electron rearrangement time is much longer in MP (62 ps) than in TCE (25 ps) because MP, which is much more polar than TCE, stabilizes the charged Z species better than TCE does. We can also note that owing to the stabilization of Z in MP, the relaxation time of Z is longer than that of either E or K. We consider that the apparent rise time of the K species, monitored at 600 nm, could not be resolved with our current temporal resolutions because the reverse charge transfer of K to form Z took place on the short time scale of 20 ps. Figure 4 schematically describes mechanisms for the ESIPTinduced charge transfer and relaxation of PQH tautomers in organic solvents of TCE and MP, and Table 2 displays the rate constants of Fig. 4. The reaction and relaxation pathways of excited-state PQH are found to depend primarily on the rotational conformations of the ground-state E species at the moment of excitation: whereas the trans-E form relaxes without going through ESIPT within its excited-state lifetime of 650 ps in TCE, the cis-E form undergoes ESIPT within 15 ps to form a tautomeric Z intermediate species. While the Z intermediate experiences ICT subsequently on a time scale of 25 ps in TCE to generate a tautomeric K species, the K species is also subject to

CONCLUSION The coupled reactions of proton transfer and charge transfer have been investigated for semirigid PQH in TCE and MP. Whereas the trans-E relaxes without undergoing proton transfer, the cis-E undergoes intramolecular proton transfer within 15 ps to produce a zwitterionic intermediate species. While the zwitterionic species experiences ICT subsequently to form a keto tautomer on time scales of 25 ps in TCE and 62 ps in MP, the keto species also goes through reverse ICT on time scales of 28 ps in TCE and 20 ps in MP. The rate constants of proton transfer and subsequent charge transfer remain invariant regardless of the isotope exchange of the protic hydrogen atom. The lack of a kinetic isotope effect in both forward and reverse charge-transfer reactions supports that the plausible tautomerization of an intramolecularly H-bonded PQH occurs sequentially involving two mechanistically distinct protontransfer and charge-transfer steps. Acknowledgements—This work was financially supported by research grants through the National Research Foundation of Korea by the Ministry of Education, Science and Technology (2010-0015806 and 2010-0001635). S.-Y.P. acknowledges the Seoul fellowship and the BK21 scholarship as well.

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