Peng Song

Rational Design of d-PeT Phenylethynylated-Carbazole Monoboronic Acid Fluorescent Sensors for the Selective Detection of α-Hydroxyl Carboxylic Acids and Monosaccharides

We have synthesized three new phenylethynylated carbazole boronic acid sensors, which were predicted to display novel d-PeT fluorescence transduction (PeT, photoinduced electron transfer; fluorophore as the electron donor of the electron transfer, ET) by DFT/TDDFT calculations. The d-PeT effect is characterized by a lower background fluorescence at acidic pH than at neutral pH, which is in stark contrast to the normal a-PeT effect (fluorophore as the electron acceptor of the ET) that shows a strong and undesired background fluorescence at acidic pH. Our experimental results confirmed the theoretical predictions and d-PeT was observed for two of the sensors (with p-dimethylaminophenylethynyl substitution at 6- position of the carbazole core). For the third sensor (with phenylethynyl substitution at 6- position of the carbazole core), however, not d-PeT but rather the normal a-PeT was observed. The discrepancy between the DFT/TDDFT calculations and the experimental observations can be rationalized using free energy changes (Rehm-Weller equations) and the rate constants for the ET (k(ET), Marcus equation). These new d-PeT boronic acid sensors show improved photophysical properties compared to the known d-PeT sensor reported previously by us. In particular, the fluorescence transduction efficiency of the new sensors was improved 8-fold when compared to the known d-PeT boronic acid sensors. Novel fluorescence enhancement/reduction was observed for one of the sensors upon binding with mandelic acid or tartaric acid at pH 5.6. The effect of pH as well as the bonding with analytes on the emission of the sensors were rationalized using DFT/TDDFT calculations. We believe that rational sensor design aided by DFT/TDDFT calculations as well as using free energy changes and electron transfer rate constants to study the emission properties of PeT sensors will become an essential tool in the design of new fluorophores or fluorescent sensors with predetermined photophysical properties.

Tuning the luminescence lifetimes of ruthenium(II) polypyridine complexes and its application in luminescent oxygen sensing

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 (τ)can be drastically extended by ligand modification (increased up to 140-fold), e.g. τ = 58.4 μs for complex 3 (with pyrenyl ethynylene appendents) was found, compared to τ = 0.4 μs for the reference complex 1. Ethynylated phenyl appendents alter the τ also (complex 2, τ = 2.4 μs). With pyrenyl appendents (4 and 5), lifetimes of 2.5 μs and 9.2 μs were observed. We proposed three different mechanisms for the lifetime extension of 2, 3, 4 and 5. For 2, the stabilization of the 3MLCT state by π-conjugation is responsible for the extension of the lifetime. For 3, the emissive state was assigned as an intra-ligand (IL) long-lived 3π–π* state (3IL/3LLCT, intraligand or ligand-to-ligand charge transfer), whereas a C–C single bond linker results in a triplet state equilibrium between 3MLCT state and the pyrene localized 3π–π* triplet state (3IL, 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 O2 sensing sensitivity of the complexes can be improved by ca. 77-fold in solution (I0/I100 = 1438 for complex 3, vs. I0/I100 = 18.5 for complex 1). In IMPES-C polymer films, the apparent quenching constant KSVapp is improved by 150-fold from 0.0023 Torr−1 (complex 1) to 0.35 Torr−1 (complex 3). The KSVapp value of complex 3 is even higher than that of PtOEP under similar conditions (0.15 Torr−1).

Tuning the emission properties of cyclometalated platinum(II) complexes by intramolecular electron-sink/arylethynylated ligands and its application for enhanced luminescent oxygen sensing

We have synthesized five novel cyclometalated Pt(II) complexes (aryl-ppy)Pt(acac) (ppy = 2-phenyl pyridine, aryl = N-butyl naphthalimide (NI) ethynylene for Pt-1, N-butyl naphthalimide (NI)–CH2 –CO– for Pt-2, 4-cyanophenyl – CH2 – CO– for Pt-3, naphthal ethynylene for Pt-4 and naphthal-diketo for Pt-5). For the first time, π-conjugation of the ppy ligands was extended via the CC bond. Deep red/near IR emission (638 nm–700 nm) was observed for the complex containing naphthalimide ethynylene subunit (Pt-1), whereas the close analogue Pt-2 (in which the linker between the NI and the ppy subunit is a –CH2CO– group) shows a relatively blue-shifted emission (540 nm–570 nm) but much longer luminescent lifetime (τ = 25.5 μs) than Pt-1 (τ = 6.6 μs). Simultaneous fluorescence/phosphorescence emissions were observed for Pt-1 and Pt-2, but other complexes show sole phosphorescent emission. The red-shifted phosphorescence of the complexes compared to the model complex ppyPt(acac) (486 nm) was attributed to either the significant electron-sink effect of the NI fragment (Pt-1) (for which the electron withdrawing effect is stronger than the previously reported fluoren-9-one), or the extended π-conjugation of the ppy ligand (via CC bond) (e.g.Pt-4). The substantial tuning of the emission color and the luminescent lifetimes (0.86 μs–25.5 μs) of the complexes were rationalized by theoretical calculations (DFT/TDDFT), i.e. the emissive triplet excited states were assigned as the normal 3MLCT state (give smaller τ values) or the novel ligand-localized 3IL emissive state (give larger τ values). With tuning the luminescent lifetimes, the luminescent O2 sensitivity of the complexes was improved by 117-fold (Stern–Volmer quenching constants KSV = 0.234 Torr−1 for Pt-2vs. KSV = 0.002 Torr−1 for Pt-5).

Effect of the Electron Donor/Acceptor Orientation on the Fluorescence Transduction Efficiency of the d-PET Effect of Carbazole-Based Fluorescent Boronic Acid Sensors

We have synthesized three new carbazole-based fluorescent boronic acid sensors to investigate the fluorescence transduction efficiency of the novel d-PET effect, in which the fluorophore acts as the electron donor and the protonated amine/boronic acid group as the electron acceptor of the photoinduced electron transfer process (PET). Aryl ethynyl groups are attached at the 3,6-position of carbazole (aryl = 4-dimethylaminophenyl for sensor 1 or phenyl for sensor 2). Sensor 3 is without 3,6-substitutions. The phenylboronic acid moiety is attached at the 9-position (N-atom) of the carbazole in these sensors. We found that 1 and 3 are d-PET sensors (fluorophore as the electron donor, supported by DFT/TDDFT calculations), which show diminished emission at acidic pH but intensified emission at neutral/basic pH, which is in stark contrast to the normal a-PET (fluorophore as the electron acceptor) sensors, e.g., 2, which shows intensified emission at acidic pH but diminished emission at neutral pH. The fluorescence modulation efficiency of the d-PET effect of the new sensors, i.e., the emission intensity enhancement upon switching from acidic pH to neutral pH, is up to 10-fold, which is greatly improved compared to our previous d-PET sensors (ca. 3-fold). The efficient d-PET effect of the new sensors is attributed to the proper orientation of the electron donor/acceptor; i.e., the dipole moment and the transition moment (the direction of PET) of the new sensors are oriented in the same direction, and the dipole moment values of the new sensors along the vector direction of the PET are larger than that of the reported d-PET sensors. Selective recognition of alpha-hydroxyl carboxylic acids, such as tartaric acid, was achieved with the d-PET sensors, and a novel fluorescence transduction profile of enhancement/diminishment for chemoselectivity was observed. Herein we propose that the orientation of the electron donor/acceptor may significantly affect the fluorescence modulation efficiency of the PET effect; this discovery will be important for the future design of PET sensors with improved fluorescence transduction efficiencies.

The Charge Transfer Phenomenon in Benzene–Pyrene–Sulfoxide/Methanol System: Role of the Intermolecular Hydrogen Bond in Excited States

AbstractnSulfoxide is an ideal functional group containing S=O for exploring excited-state hydrogen bond dynamics. Benzene–pyrene–sulfoxide (BPS) molecule, as one of the important fluorescent chemosensors, was selected to complete the hydrogen bond dynamics of sulfoxides functional group connecting to methanol (MeOH). The ground-state and excited-state geometric structures were investigated based on density functional theory and the time-dependent density functional theory methods, respectively. The calculated absorption and emission spectra of BPS chemosensor agreed well with the experimental results, demonstrating the theory we adopted is reasonable and effective. The phenomenon of variable-length S=O and H–O bands in the first excited state (S1) as well as variable-short hydrogen bond S=O···H–O demonstrated that the intermolecular hydrogen bond were strengthened. Bathochromic shift stretching vibrational modes of both S=O and H–O regions in the S1 state manifested hydrogen bond were strengthened authentically. In addition, the frontier molecular orbitals (MOs), depicting the nature of the electronically excited states, supported that the S1 state of BPS–MeOH was a local excited state with a π–π* transition, whereas the second excited state was the charge transfer state.

Rotational Reorientation Dynamics of Oxazine 750 in Polar Solvents

The rotational reorientation dynamics of oxazine 750 (OX750) in the first (with pump pulse at 660 nm) and a higher excited state (with pump pulse at 400 nm) in different polar solvents have been investigated using femtosecond time-resolved stimulated emission pumping fluorescence depletion (FS TR SEP FD) spectroscopy. In both excited states, three different anisotropy decay laws have been observed for OX750 in different solvents. Only in acetone and formamide could the anisotropy decays of OX750 be described by single-exponential functions, whereas the anisotropy decays have been found to exhibit biexponential behavior in other solvents. The slower anisotropy decay observed in all of the solvents has been assigned to the overall rotational relaxation of OX750 molecules, and a quantitative analysis of this time constant has been performed using the Stokes-Einstein-Debye hydrodynamic theory and the extended charge distribution model developed by Alavi and Waldeck. In both methanol and ethanol, a faster anisotropy decay on the order of picoseconds and a slower anisotropy decay on the hundreds of picoseconds time scale are observed. The most likely explanation for the faster anisotropy involves the rotation of the transition dipole moment in the excited state of OX750 resulting from the electron transfer (ET) reaction taking place from the alcoholic solvents to the OX750 chromophore. As a possible explanation, the wobbling-in-the-cone model has been used to analyze the biexponential anisotropy decays of OX750 in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The observed faster anisotropy decays on the hundreds of femtoseconds time scale in DMF and DMSO are ascribed to the wobbling-in-the-cone motion of the ethyl group of OX750, which is sensitive to the strength of the hydrogen bond formed between the solvent and the protonation site of OX750.

Theoretical and experimental study on the intramolecular charge transfer excited state of the new highly fluorescent terpyridine compound.

Experimental and theoretical methods have been used to investigate the relaxation dynamics and photophysical properties of the donor-acceptor compound 4-(4-N,N-diphenylaminophenyl)-2,2:6,2-terpyridine (DPAPT), a compound which is found to exhibit efficient intramolecular charge transfer emission in polar solvents with relatively large Stokes shifts and strong solvatochromism. The difference between the ground and excited state dipole moments (Deltamu) is estimated to be 13.7D on the basis of Lippert-Mataga models. To gain insight into the relaxation dynamics of DPAPT in the excited state, the potential energy curves for conformational relaxation are calculated. From the frontier molecular orbital (MO) pictures at the geometry of the twisted ICT excited state, the intramolecular charger transfer mainly takes place from HOMO (triphenylamine) to LUMO (terpyridine) in this donor-acceptor system.

Mechanism for the Excited-State Multiple Proton Transfer Process of Dihydroxyanthraquinone Chromophores

The single and dual cooperated proton transfer dynamic process in the excited state of 1,5-dihydroxyanthraquinone (1,5-DHAQ) was theoretically investigated, taking solvent effects (ethanol) into account. The absorption and fluorescence spectra were simulated, and dual fluorescence exhibited, which is consistent with previous experiments. Analysis of the calculated IR and Raman vibration spectra reveals that the intramolecular hydrogen bonding interactions (O20-H21···O24 and O22-H23···O25) are strengthened following the excited proton transfer process. Finally, by constructing the potential energy surfaces of the ground state, first excited singlet state, and triplet state, the mechanism of the intramolecular proton transfer of 1,5-DHAQ can be revealed.

Theoretical Investigation of the ESIPT Mechanism for the 1-Hydroxy-9H-fluoren-9-one and 1-Hydroxy-11H-benzo[b]fluoren-11-one Chromophores

The excited state intramolecular proton transfer (ESIPT) dynamics of the 1-hydroxy-9H-fluoren-9-one (HHF) and 1-hydroxy-11H-benzo[b]fluoren-11-one (HHBF) chromophores were investigated theoretically. The calculated bond lengths and angles, hydrogen bond energies and infrared vibrational spectra involved in the hydrogen bonding of O–H···O indicated that the intramolecular hydrogen bond was strengthened in the S1 state. Our calculated results accurately reproduced the experimental absorbance and fluorescence emission spectra, demonstrating that the adopted time-dependent density functional theory (TDDFT) method is reasonable and effective. In addition, qualitative and quantitative intramolecular charge transfer based on the frontier molecular orbitals provided the possibility of the ESIPT reaction. The potential energy curves of the ground and first excited states have been constructed to illustrate the ESIPT mechanism. Based on our calculations, we explain the equilibrium ESIPT processes observed in previous experiments.

Photophysical Properties of 4′-(p-aminophenyl)-2,2′:6′,2″-terpyridine

Spectral and photophysical investigations of 4′-(p-aminophenyl)-2,2′:6′,2″-terpyridine (APT) have been performed in various solvents with different polarity and hydrogen-bonding ability. The emission spectra of APT are found to exhibit dual fluorescence in polar solvents, which attributes to the local excited and intramolecular charge transfer states, respectively. The two-state model is proven out for APT in polar solvent by the time-correlated single photon counting emission decay measurement. Interestingly, the linear relationships of different emission maxima and solvent polarity parameter are found for APT in protic and aprotic solvents, because of the hydrogen bond formation between APT and alcohols at the amino nitrogen N25. Furthermore, the effects of the complexation of the metal ion with tpy group of APT and the hydrogen bond formation between APT with methanol at the terpyridine nitrogen N4-N8-N14 are also presented. The appearance of new long-wave absorption and fluorescence bands indicates that a new ground state of the complexes is formed.