Efficient enhancement of internal proton transfer of branched π-extended organic chromophore under one-photon and near-infrared two-photon irradiation
Graphical abstract
Introduction
Internal proton transfer in the excited state receives intensive attention as it is one of the significant phenomena in life and nature [1], [2]. Enol and keto four-level cycle phototautomerization (E → E* → K* → K → E, E, Enol, K, Keto) is considered as the most representative mechanism to realize ultra fast internal proton transfer in the excited state [3], [4]. It means that this process has to be permitted by the potential energy barrier of enol–keto phototautomerization [5]. Some small size molecules can undergo energy barrierless or nearly barrierless intramolecular proton transfer in the excited state [6], [7].
It was proposed to develop enlarged organic molecules bearing with proton transfer segments through chemical covalent bond [8], [9], [10]. However, Zhao and his colleagues pointed out that the insertion of π-conjugation units can inhibit excited state intramolecular proton transfer (ESIPT) of an organic chromophore [8]. It is considered that the potential energy barrier of the internal proton transfer in the excited state can increase with the increase in the molecular size. So far, it is not found which sort of chemical strategies can be used to increase internal proton transfer in the excited state of π-extended molecules.
For an organic molecule, ESIPT emission band possesses an exceptionally large Stokes shift without self-absorption, which is found great application potentials in various fields such as fluorescence sensors and optical materials [11], [12]. Enlarged organic chromophore with ESIPT can be more conveniently applied, and it possesses more application potentials. Particularly, π-extended organic chromophore with ESIPT can be utilized as a two-photon absorption (TPA) dye by near-infrared (near-IR) femtosecond laser which has various superior physical advantages such as low energy, deep penetration and negligible damage to biological tissues [13].
Salicylidene methylamine can undergo internal proton transfer in the excited singlet state via energy barrierless mechanism [14]. Triphenylamine is normally employed as representative parent core to develop two-photon organic fluorophores [15]. A new π-extended triphenylamine-based chromophore containing two salicylidene methylamine units was designed and synthesized in this letter. The target chromophore contains the armed proton transfer segments with the same chemical environments. Such molecular design is proposed to be favor of internal proton transfer in the excited state. New organic chromophores C1–C6 shown in Figure 1 were prepared through multi-step route (the detail of synthesis is shown in Supplementary materials).
Section snippets
Experimental
The organic solvents used in photophysical determination are spectral grade purchased from Aldrich Chemical Corporation. The new target chromophores were synthesized by our laboratory. 1H and 13C nuclear magnetic resonance (NMR) spectra were measured by a Bruker 500 MHz apparatus using tetramethylsilane (TMS) as an internal standard at room temperature. IR spectra were recorded by Fourier transform infrared spectrometer. Mass spectra were measured by Acquity SQD from WATERS using the electro
Single crystal analysis
Internal hydrogen bonding effect can exist between the adjacent CHN or NH and OH groups in C1–C4. X-ray single crystal diffraction analysis shows that the dihedral angle of C3–O1–H1–N1 in C1 is remarkably smaller than that in C2 (7.78°, 17.68°, Figure 2). The dihedral angle of C8–N1–C1–C2 in C1 is 0.37°, while it increases (14.87°) in C2. X-ray single crystal diffraction analysis suggests that there is better coplanarity between diphenylene part and salicylidene methylamine in C1 comparing with
Conclusions
To be summarized, this letter demonstrates internal H-bonding effect in enlarged π-extended organic chromophores by the analysis of single crystal diffraction, 1H NMR spectra and the UV–vis absorption spectra. Intramolecular proton transfer in the excited state of π-extended organic chromophores C1 and C3 is successfully confirmed by the observation of well-separated dual emission bands. As comparing with C1, C3 possesses larger ability to undergo intramolecular proton transfer in the excited
Supplementary material
The details of synthesis, the computation, Table S1 and Figures S1–S23, Scheme S1 association with this letter are shown in the Supplementary information.
The supplementary crystallographic data for this work were contained in CCDC 977222 and CCDC 977223. These data can be free available http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge crytstallographic data center, 12, Union Road, Cambridge CB2 1EZ, UK. Fax: +44 1223 336033.
Acknowledgements
This work was financially supported by Chongqing Natural Science Foundation CSTC2012jjB50007 and CSTC2010BB0216. H. L. is grateful to the Postdoctoral Science Foundation of China for financial supporting (Grant Nos. 22012T50762 and 2011M501388). L.Y. also thanks Graduate Student Innovation Foundation of Chongqing University (CDJX11131146).
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