DOI: 10.1002/cphc.200800352 Cyano Analogues of 7-Azaindole: Probing Excited-State Charge-Coupled Proton Transfer Reactions in Protic Solvents Cheng-Chih Hsieh, Kew-Yu Chen, Wan-Ting Hsieh, Chin-Hung Lai, Jiun-Yi Shen, Chang- Ming Jiang, Hsin-Sheng Duan, and Pi-Tai Chou* [a] 1. Introduction Due to its fundamental importance in chemical and biochemi- cal reactions, the excited-state proton-transfer (ESPT) reaction has received much attention. [1–2] One relevant, important area of current interest which is based on ESPT, is the coupled excit- ed-state charge-transfer (ESCT) reaction. Theoretical ap- proaches undertaken by Hynes and co-workers [3] have de- scribed the key features of ESCT in terms of dynamics and elec- tronic structure of the intermolecular proton-transfer reactions in protic solution. One of the questions raised is the nature of the quantum character of the proton in a proton transfer reac- tion. It has been revealed that the motion is always faster than the rearrangement of protic solvent molecules, and thus the Born-Oppenheimer approximation can be analogously made for the correlation between proton and solvent molecules in this reaction. As a consequence, the equilibrium between the moving proton and the surrounded solvent molecules is estab- lished at each instant, and the reaction activation free energy is thus essentially dominated by the solvent reorganization since charge redistribution is involved rather than the height of the proton migration barrier. On this basis, various experimental approaches have been made and, according to the reaction pattern, can be catego- rized into two classes, namely the excited-state intermolecular proton transfer (i.e. deprotonation to the solvent pool) and in- tramolecular proton transfer. In view of excited-state intermo- lecular proton transfer, several elegant experiments have been performed and the results are well described by the aforemen- tioned theoretical models. [3, 4] On the other hand, cases regard- ing intramolecular proton-transfer may ease the reaction com- plexity simply due to its unimolecular type of reaction (ne- glecting the ion-pair forming process that involves the solvat- ed proton and its counterpart). Technically, however, studies of proton/charge coupled reactions for the latter case cannot be facilitated without certain ingenious design of molecules pos- sessing large dipolar changes between normal and proton transfer tautomers in the excited state. However due to synthetic design of molecules, studies re- garding solvent polarity influencing the excited-state intramo- lecular proton transfer have made significant progress. [5–9] Many potential ESCT/ESPT systems have been designed and in- tensively investigated during the past few years to gain de- tailed insights into this fundamental issue. Prototypical exam- ples include para-N,N-dialkylaminosalicylaldehyde [5, 7e] , N,N-di- ACHTUNGTRENNUNGalkylamino-3-hydroxy flavones, [6–7] 2-hydroxy-4-(di-p-tolyl-ami- no)benzaldehyde [8] and 2-(2’-hydroxy-4’-dietheylaminophenyl)- benzothiazole, [9] in which the N,N-dialkyl group is strategically designed to act as an electron donor (D), while the carbonyl oxygen or the nitrogen group within the parent ESPT moiety serves as an electron acceptor (A). Studies have concluded that most systems applied so far can be ascribed to an ultrafast ACHTUNGTRENNUNG( !150 fs) ESCT prior to ESPT. Using para-N,N-dialkylaminosali- cylaldehyde [5, 7e] as a model (see Scheme 1), the corresponding ESCT is an adiabatic type, that is, an optical electron-transfer [a] C.-C. Hsieh, K.-Y. Chen, W.-T. Hsieh, Dr. C.-H. Lai, J.-Y. Shen, C.-M. Jiang, H.-S. Duan, Prof. P.-T. Chou Department of Chemistry National Taiwan University No.1, Sec. 4, Roosevelt Rd., Da-an District, Taipei 106 (Taiwan) Fax: (+ 886) 2-23695208 E-mail : chop@ntu.edu.tw The interplay between excited-state charge and proton transfer reactions in protic solvents is investigated in a series of 7-azain- dole (7AI) derivatives : 3-cyano-7-azaindole (3CNAI), 5-cyano-7- azaindole (5CNAI), 3,5-dicyano-7-azaindole (3,5CNAI) and dicya- noethenyl-7-azaindole (DiCNAI). Similar to 7AI, 3CNAI and 3,5CNAI undergo methanol catalyzed excited-state double proton transfer (ESDPT), resulting in dual (normal and proton transfer) emission. Conversely, ESDPT is prohibited for 5CNAI and DiCNAI in methanol, as supported by a unique normal emission with high quantum efficiency. Instead, the normal emission undergoes prominent solvatochromism. Detailed relaxation dynamics and temperature dependent studies are carried out. The results con- clude that significant excited-state charge transfer (ESCT) takes place for both 5CNAI and DiCNAI. The charge-transfer specie pos- sesses a different dipole moment from that of the proton-transfer tautomer species. Upon reaching the equilibrium polarization, there exists a solvent-polarity induced barrier during the proton- transfer tautomerization, and ESDPT is prohibited for 5CNAI and DiCNAI during the excited-state lifespan. The result is remarkably different from 7AI, which is also unique among most excited- state charge/proton transfer coupled systems studied to date. ChemPhysChem 2008, 9, 2221 – 2229 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2221