Charge-Transfer-Induced Twisting of the Nitro Group

HSA over BSA: Selective detection of Human Serum Albumin via a naphtho [2,1-b] furan-based system

Human serum albumin (HSA) is an important biomarker that can be used for the early diagnosis of many diseases. In this work, a TICT probe bearing fused naphtho-furan scaffold (NPNF) was developed and employed in the selective turn-on sensing of HSA. The probe's selectivity towards HSA was observed using steady-state fluorescence experiments, with limit of quantitation in micromolar levels. NPNF's capability to exclusively detect HSA over BSA was further studied/rationalized using anisotropy and time-resolved studies. Molecular docking was used to shed light on the location of NPNF in the subdomain IB of HSA. The practical application of the probe was also demonstrated by the detection of HSA in urine and the HSA-assisted detection of cerium.

Solvent-polarity dependence of ultrafast excited-state dynamics of trans-4-nitrostilbene

Ultrafast excited-state dynamics of the simplest nitrostilbenes, namely trans-4-nitrostilbene (t-NSB), was studied in solvents of various polarities with ultrafast broadband time-resolved fluorescence and transient absorption spectroscopies, and by quantum-chemical computations. The results revealed that the initially excited S1(ππ*) state deactivation dynamics is strongly influenced by the solvent polarity. Specifically, the t-NSB S1-state lifetime decreases by three orders of magnitude from ∼60 ps in high-polarity solvents to ∼60 fs in nonpolar solvents. The strong solvent-polarity dependence arises from the differences in dipole moments among the S1 and relevant states, including the major intersystem crossing (ISC) receiver triplet states, and therefore, the solvent polarity can modulate their relative energies and ISC rates. In nonpolar solvents, the sub-100 fs lifetime is due to a combination of efficient ISC and internal conversion. In medium-polarity solvents, the S1-state population decays via a competing ISC relaxation mechanism in a biphasic manner, and the ISC rates are found to obey the inverse energy gap law of the strong coupling case. In high-polarity solvents, the S1 state is stabilized to a much lower energy such that ISC becomes energetically infeasible, and the S1 state decays via barrier crossing along the torsion angle of the central ethylenic bond to the nonfluorescent perpendicular configuration. Regardless of the initial S1-state deactivation pathways in various solvents, the excited-state population is ultimately trapped in the metastable T1-state perpendicular configuration, at which a slower ISC occurs to bring the system to the ground state and bifurcate into either trans or cis form of NSB.

Twisted intramolecular charge transfer of nitroaromatic push-pull chromophores

The structural changes during the intramolecular charge transfer (ICT) of nitroaromatic chromophores, 4-dimethylamino-4′-nitrobiphenyl (DNBP) and 4-dimethylamino-4′-nitrostilbene (DNS) were investigated by femtosecond stimulated Raman spectroscopy (FSRS) with both high spectral and temporal resolutions. The kinetically resolved Raman spectra of DNBP and DNS in the locally-excited and charge-transferred states of the S₁ state appear distinct, especially in the skeletal vibrational modes of biphenyl and stilbene including ν_8a and ν_C=C. The ν_8a of two phenyls and the ν_C=C of the central ethylene group (only for stilbene), which are strongly coupled in the planar geometries, are broken with the twist of nitrophenyl group with the ICT. Time-resolved vibrational spectroscopy measurements and the time-dependent density functional theory simulations support the ultrafast ICT dynamics of 220–480 fs with the twist of nitrophenyl group occurring in the S₁ state of the nitroaromatic chromophores. While the ICT of DNBP occurs via a barrier-less pathway, the ICT coordinates of DNS are strongly coupled to several low-frequency out-of-phase deformation modes relevant to the twist of the nitrophenyl group.

Functional Molecular System of Bis(pyrazolyl)pyridine Derivatives: Photophysics, Spectroscopy, Computation, and Ion Sensing

A new series of conjugated donor-π-acceptor type of 2,6-bis(pyrazolyl)pyridine derivatives (compounds IK-(3-9)) have been synthesized via Horner-Wadsworth-Emmons (HWE) reaction, starting from a common phosphonate precursor and diverse donor aromatic aldehydes and characterized by routine spectral analysis including elemental analysis. Compound IK-2, one of the starting precursors, and molecule IK-3, the first member of the donor-π-acceptor series, are additionally characterized by single-crystal X-ray structure determination. Compounds IK-2 and IK-3 are crystallized in P1̅ (triclinic) and P2₁/c (monoclinic) space groups, respectively. The absorption maxima in the electronic spectra of the title compounds shift mainly due to intramolecular charge transfer (ICT) between different donor (dibutyl and cyclic pyrrolidine) groups and the acceptor moiety [2,6-bis(pyrazolyl) pyridine]. Solution-state emission spectral studies of all these compounds show large solvent sensitive behavior with significant amounts of Stokes shifts. The large solvent dependence of the emission indicates that the excited state is stabilized in more polar solvents due to the ICT. All chromophores exhibit solid-state fluorescence behavior except compound IK-7. The role of the position and nature of the donor functionalities in the conjugated backbone of overall donor moiety of compounds IK-(3-9), on the electronic absorption properties of the title chromophores has been demonstrated, which has further been corroborated by density functional theory (DFT) and time-dependent DFT (TDDFT) computational studies. The emission spectral results of compounds IK-3, IK-5, and IK-7 have also been supported by the DFT and TDDFT calculations. A fluorescence lifetime study on this series also shows that the excited states are stabilized in more polar solvents. Finally, one of the chromophores (chromophore IK-4) in the title series has been shown to act as a selective molecular sensor (turn-off switch) for the Cu(II) ion.

Solvent-dependent dual fluorescence of the push-pull system 2-diethylamino-7-nitrofluorene

The solvent-dependent excited state behavior of the molecular push-pull system 2-diethylamino-7-nitrofluorene has been explored using femtosecond transient absorption spectroscopy in combination with density functional theory calculations. Several excited state minima have been identified computationally, all possessing significant intramolecular charge transfer character. The experimentally observed dual fluorescence is suggested to arise from a planar excited state minimum and another minimum reached by twisting of the aryl-nitrogen bond of the amino group. The majority of the excited state population, however, undergo non-radiative transitions and potential excited state deactivation pathways are assessed in the computational investigation. A third excited state conformer, characterized by twisting around the aryl-nitrogen bond of the nitro group, is reasoned to be responsible for the majority of the non-radiative decays and a crossing between the excited state and ground state is localized. Additionally, ultrafast intersystem crossing is observed in the apolar solvent cyclohexane and rationalized to occur via an El-Sayed assisted transition from one of the identified excited state minima. The solvent thus determines more than just the fluorescence lifetime and shapes the potential energy landscape, thereby dictating the available excited state pathways.

Long-living optical gain induced by solvent viscosity in a push–pull molecule

The combination of continuum and ultrafast pump–probe spectroscopy, in viscous and non-viscous environments, with DFT and TDDFT calculations, is effective in unraveling important features of the twisted intramolecular charge transfer mechanism in a new push–pull molecule that possesses aggregation induced emission properties.

Multisite constrained model of trans-4-(N,N-dimethylamino)-4'-nitrostilbene for structural elucidation of radiative and nonradiative excited states

A constrained model compound of trans-4-(N,N-dimethylamino)-4'-nitrostilbene (DNS), namely, compound DNS-B3 that is limited to torsions about the phenyl-nitro C-N bond and the central C═C bond, was prepared to investigate the structural nature of the radiative and nonradiative states of electronically excited DNS. The great similarities in solvent-dependent electronic spectra, fluorescence decay times, and quantum yields for fluorescence (Φf) and trans → cis photoisomerization (Φtc) between DNS and DNS-B3 indicate that the fluorescence is from a planar charge-transfer state and torsion of the nitro group is sufficient to account for the nonradiative decay of DNS. This conclusion is supported by TDDFT calculations on DNS-B3 in dichloromethane. The structure at the conical intersection for internal conversion is associated with not only a twisting but also a pyramidalization of the nitro group. The mechanism of the NO2 torsion is discussed in terms of the effects of solvent polarity, the substituents, and the volume demand. The differences and analogies of the NO2- vs amino-twisted intramolecular charge-transfer (TICT) state of trans-aminostilbenes are also discussed.

Decoupling fluorescence and photochromism in bifunctional azo derivatives for bulk emissive structures

Bifunctional molecules that combine independent push-pull fluorophores and azo photochromes have been synthesized to create fluorescent structures upon light-induced migration in neat thin films. Their photochromic and emissive properties have been systematically investigated and interpreted in light of those of the corresponding model compounds. Fluorescence lifetimes and photoisomerization and fluorescence quantum yields have been determined in toluene solution. Kinetic analyses of the femtosecond transient absorption spectra reveal that the fluorophores evolve in a few picoseconds into a distorted intramolecular charge-transfer excited state, strongly stabilized in energy. Radiative relaxation to the ground state occurred competitively with the energy-transfer process to the azo moiety. Introduction of a 10 Å-long rigid and nonconjugated bridge between the photoactive units efficiently inhibits the energy transfer while it imparts enhanced free volume, which favors photoactivated molecular migration in the solid state.

Femtosecond excited-state dynamics of 4-nitrophenyl pyrrolidinemethanol: evidence of twisted intramolecular charge transfer and intersystem crossing involving the nitro group

Ultrafast excited-state relaxation dynamics of a nonlinear optical (NLO) dye, (S)-(-)-1-(4-nitrophenyl)-2-pyrrolidinemethanol (NPP), was carried out under the regime of femtosecond fluorescence up-conversion measurements in augmentation with quantum chemical calculations. The primary concern was to trace the relaxation pathways which guide the depletion of the first singlet excited state upon photoexcitation, in such a way that it is virtually nonfluorescent. Ground- and excited-state (singlet and triplet) potential energy surfaces were calculated as a function of the -NO(2) torsional coordinate, which revealed the perpendicular orientation of -NO(2) in the excited state relative to the planar ground-state conformation. The fluorescence transients in the femtosecond regime show biexponential decay behavior. The first time component of a few hundred femtoseconds was ascribed to the ultrafast twisted intramolecular charge transfer (TICT). The occurrence of charge transfer (CT) is substantiated by the large dipole moment change during excitation. The construction of intensity- and area-normalized time-resolved emission spectra (TRES and TRANES) of NPP in acetonitrile exhibited a two-state emission on behalf of decay of the locally excited (LE) state and rise of the CT state with a Stokes shift of 2000 cm(-1) over a time scale of 1 ps. The second time component of a few picoseconds is attributed to the intersystem crossing (isc). In highly polar solvents both the processes occur on a much faster time scale compared to that in nonpolar solvents, credited to the differential stability of energy states in different polarity solvents. The shape of frontier molecular orbitals in the excited state dictates the shift of electron density from the phenyl ring to the -NO(2) group and is attributed to the charge-transfer process taking place in the molecule. The viscosity dependence of relaxation dynamics augments the proposition of considering the -NO(2) group torsional motion as the main excited-state relaxation coordinate.

Photophysical and structural properties of the fluorescent nucleobase analogues of the tricyclic cytosine (tC) family

Fundamental insight into the unique fluorescence and nucleobase-mimicking properties of the fluorescent nucleobase analogues of the tC family is not only vital in explaining the behaviour of these probes in nucleic acid environments, but will also be profitable in the development of new and improved fluorescent base analogues. Here, temperature-dependent fluorescence quantum yield measurements are used to successfully separate and quantify the temperature-dependent and temperature-independent non-radiative excited-state decay processes of the three nucleobase analogues tC, tC(O) and tC(nitro); all of which are derivatives of a phenothiazine or phenoxazine tricyclic framework. These results strongly suggest that the non-radiative decay process dominating the fast deactivation of tC(nitro) is an internal conversion of a different origin than the decay pathways of tC and tC(O). tC(nitro) is reported to be fluorescent only in less dipolar solvents at room temperature, which is explained by an increase in excited-state dipole moment along the main non-radiative decay pathway, a suggestion that applies in the photophysical discussion of large polycyclic nitroaromatics in general. New insight into the ground and excited-state potential energy surfaces of the isolated tC bases is obtained by means of high level DFT and TDDFT calculations. The S(0) potential energy surfaces of tC and tC(nitro) possess two global minima corresponding to geometries folded along the middle sulfur-nitrogen axis separated by an energy barrier of 0.05 eV as calculated at the B3LYP/6-311+G(2d,p) level. The ground-state potential energy surface of tC(O) is also predicted to be shallow along the bending coordinate but with an equilibrium geometry corresponding to the planar conformation of the tricyclic framework, which may explain some of the dissimilar properties of tC and tC(O) in various confined (biological) environments. The S(1) equilibrium geometries of all three base analogues are predicted to be planar. These results are discussed in the context of the tC bases positioned in double-stranded DNA scenarios.

Solvation oscillations and excited-state dynamics of 2-amino- and 2-hydroxy-7-nitrofluorene and its 2'-deoxyriboside

Push-pull substituted fluorenes are considered for use as dynamic solvation probes in polynucleotides. Their fluorescence band is predicted (by simulations) to show weak spectral oscillations on the subpicosecond time scale depending on the nucleotide sequence. The oscillations reflect the local far-infrared spectrum of the environment around the probe molecule. A connection is provided by the continuum theory of polar solvation which, however, neglects molecular aspects. We examine the latter using acetonitrile solution as a test case. A collective librational solvent mode at 100 cm(-1) is observed with 2-amino-7-nitrofluorene, 2-dimethylamino-7-nitrofluorene, 2-hydroxy-7-nitrofluorene, and its 2'-deoxyriboside. Different strengths of the oscillation indicate that rotational friction of nearby acetonitrile molecules depends on the solute structure or that H bonding is involved in launching the librational coherence. Polar solvation in methanol is used for comparison. With hydroxynitrofluorenes, the observation window is limited by intersystem crossing for which rates are reported. A prominent excited-state absorption band of nitrofluorenes at 430 nm can be used to monitor polar solvation. Structural and electronic relaxation pathways are discussed with the help of quantum chemical calculations.