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In this perspective we introduce the basic photophysics of the excited-state intramolecular proton transfer (ESIPT) chromophores, then the state-of-the-art development of the ESIPT chromophores and their applications in chemosensors, biological imaging and white-light emitting materials are summarized. Most of the applications of the ESIPT chromophores are based on the photophysics properties, such as design of fluorescent chemosensors by perturbation of the ESIPT process upon interaction with the analytes, their use as biological fluorescent tags to study DNA-protein interaction by probing the variation of the hydration, or design of white-light emitting materials by employing the large Stokes shift of the ESIPT chromophores (to inhibit the Föster energy transfer of the components). The photophysical mechanism of these applications is discussed. Furthermore, a new research topic concerning the ESIPT chromophores is proposed based on our group's results, that is, to develop organic triplet sensitizers with ESIPT chromophores.

Triplet-triplet annihilation (TTA) based upconversions are attractive as a result of their readily tunable excitation/emission wavelength, low excitation power density, and high upconversion quantum yield. For TTA upconversion, triplet sensitizers and acceptors are combined to harvest the irradiation energy and to acquire emission at higher energy through triplet-triplet energy transfer (TTET) and TTA processes. Currently the triplet sensitizers are limited to the phosphorescent transition metal complexes, for which the tuning of UV-vis absorption and T(1) excited state energy level is difficult. Herein for the first time we proposed a library of organic triplet sensitizers based on a single chromophore of boron-dipyrromethene (BODIPY). The organic sensitizers show intense UV-vis absorptions at 510-629 nm (ε up to 180,000 M(-1) cm(-1)). Long-lived triplet excited state (τ(T) up to 66.3 μs) is populated upon excitation of the sensitizers, proved by nanosecond time-resolved transient difference absorption spectra and DFT calculations. With perylene or 1-chloro-9,10-bis(phenylethynyl)anthracene (1CBPEA) as the triplet acceptors, significant upconversion (Φ(UC) up to 6.1%) was observed for solution samples and polymer films, and the anti-Stokes shift was up to 0.56 eV. Our results pave the way for the design of organic triplet sensitizers and their applications in photovoltaics and upconversions, etc.

Ru(Phen)(bpy) 2 (1) and its new derivatives (2-5) with pyrenyl or ethynylated pyrene and phenyl units appended to the 3-position of the phenanthroline (Phen) ligand were prepared and these complexes generate long-lived room temperature phosphorescence in the red and near IR range (600-800 nm). The photophysical properties of these complexes were investigated by UV-Vis absorption, luminescence emission, transient absorption spectra and DFT/TDDFT calculations. We found the luminescence lifetime (s)can be drastically extended by ligand modification (increased up to 140-fold), e.g. s ¼ 58.4 ms for complex 3 (with pyrenyl ethynylene appendents) was found, compared to s ¼ 0.4 ms for the reference complex 1. Ethynylated phenyl appendents alter the s also (complex 2, s ¼ 2.4 ms). With pyrenyl appendents (4 and 5), lifetimes of 2.5 ms and 9.2 ms were observed. We proposed three different mechanisms for the lifetime extension of 2, 3, 4 and 5. For 2, the stabilization of the 3 MLCT state by p-conjugation is responsible for the extension of the lifetime. For 3, the emissive state was assigned as an intra-ligand (IL) long-lived 3 p-p* state ( 3 IL/ 3 LLCT, intraligand or ligand-to-ligand charge transfer), whereas a C-C single bond linker results in a triplet state equilibrium between 3 MLCT state and the pyrene localized 3 p-p* triplet state ( 3 IL, e.g. 4 and 5). DFT/TDDFT calculations support the assignment of the emissive states. The effects of the lifetime extension on the oxygen sensing properties of these complexes were studied in both solution and polymer films. With tuning the emissive states, and thus extension of the luminescence lifetimes, the luminescent O 2 sensing sensitivity of the complexes can be improved by ca. 77-fold in solution (I 0 /I 100 ¼ 1438 for complex 3, vs. I 0 /I 100 ¼ 18.5 for complex 1). In IMPES-C polymer films, the apparent quenching constant K SV app is improved by 150-fold from 0.0023 Torr À1 (complex 1) to 0.35 Torr À1 (complex 3). The K SV app value of complex 3 is even higher than that of PtOEP under similar conditions (0.15 Torr À1 ).

Room temperature (RT) phosphorescence is observed from a naphthalimide species for the first time in the square-planar chromophore Pt(dbbpy)(C[triple bond]C-NI)(2), where NI = N-butyl-4-ethynylnaphthalimide and dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine. The combination of static and time-resolved absorption and photoluminescence data is uniformly consistent with triplet-state photophysics localized on an appended C[triple bond]C-NI unit following excitation into the low-energy absorption bands. This molecule features rather impressive long-lifetime, high-quantum-efficiency NI-based RT phosphorescence (tau = 124 micros; Phi = 0.215) centered at 621 nm, exemplifying how the platinum acetylide linkage strongly promotes intersystem crossing in the NI subunit, representative of a class of molecules whose excited states are typically dominated by singlet fluorescence.

Transition metal complexes of Ru(II), Pt(II) and Ir(III) with strong absorption of visible light and long-lived T 1 excited states were summarized. A design rationale of these complexes, i.e. direct metalation of organic chromophore, was proposed. Alternatively an organic chromophore can be dangled on the peripheral moiety of the coordination center. In both cases the long-lived intraligand triplet excited state ( 3 IL) can be accessed. However, the 3 IL excited state is usually emissive for the former case and it is very often non-emissive for the latter case. Two methods used for study of the long-lived triplet excited state, i.e. the time-resolved transient difference absorption spectroscopy and the spin density analysis, are briefly introduced. Preliminary applications of the complexes in luminescent O 2 sensing and triplet-triplet annihilation (TTA) upconversions were discussed.

2-Thienyl and 2,6-bisthienyl BODIPY derivatives (BS-SS and BS-DS) were prepared that show intense absorption (ε = 65000 M(-1) cm(-1) at 507 nm) and a large Stokes shift (96 nm) vs the small Stokes shift of typical BODIPY (<15 nm). Control compounds with a thienyl unit at the 8-position or phenyl substituents at the 2,6-positions were prepared (BS-1 and 9). BS-1 shows absorption/emission in the blue-shifted range and a small Stokes shift (12 nm). Compound 9 shows absorption in the red-shifted range, but the Stokes shift (<30 nm) is much smaller than that for BS-SS and BS-DS. DFT calculations propose the large Stokes shifts of BS-SS and BS-DS are due to the remarkable geometry relaxation upon photoexcitation and its substantial effect on the energy levels of molecular orbitals. For the dyes with small Stokes shifts, much smaller geometry relaxations were found. The fluorophores were used for fluorescent thiol probes, with 2,4-dinitrobenzenesulfonyl (DNBS) as the fluorescence switch. Both fluorescence OFF-ON and unprecedented ON-OFF transduction were observed, which are attributed to the different photoinduced intramolecular electron-transfer (PET) profile. All the photophysics were rationalized by DFT calculations based on the concept of "electronic states" instead of the very often used approximation of "molecular orbitals".

Room temperature near-IR phosphorescence of naphthalenediimide (NDI) was observed with N^N Pt(II) bisacetylide complex (Pt-NDI) in which the NDI was connected to Pt(II) center via acetylide. Pt-NDI shows intense absorption of visible light and long-lived NDI-localized excited state ((3)IL) (τ(T) = 22.3 μs). Pt-NDI was used as a triplet sensitizer for upconversion.