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

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.

Amino-Terephthalonitrile-Based Single Benzene Fluorophores with Large Stokes Shifts and Solvatochromic Behavior

We have synthesized a small library of blue-to-green emissive single benzene-based fluorophores (SBFs) in a short synthetic sequence. The molecules exhibit good Stokes shift in the range of 60-110 nm and select examples also possess very high fluorescence quantum yields of up to 87%. Theoretical investigations into the ground state and excited state geometries of many of these compounds reveal that good degree of planarization between the electron donor secondary amines and electron accepting benzodinitrile units can be achieved under certain solvatochromic conditions, giving rise to the strongly fluorescent behavior. On the other hand, the excited state geometry which lacks co-planarity of the donor amine and the single benzene moiety can open up a non-fluorescent channel. Additionally, in molecules with a dinitrobenzene acceptor, the perpendicular nitro moieties render the molecules completely non-emissive.

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.

Probing twisted intramolecular charge transfer of pyrene derivatives as organic emitters in OLEDs

Intramolecular charge transfer (ICT) plays a critical role in determining the photophysical properties of organic molecules, including their luminescence efficiencies. Twisted intramolecular charge transfer (TICT) is a process in which...

Enhancing fluorescence of benzimidazole derivative via solvent-regulated ESIPT and TICT process: A TDDFT study

In this work, we use density functional theory and time dependent density functional theory to explore the ESIPT and TICT process of 6-(1H-Benzoimidazol-2-yl)-2,3-dimethoxy-phenol (BIDOP) in cyclohexane (CHX) and tetrahydrofuran (THF) solvent, respectively. It reveals that ESIPT process of BIDOP can occur in both CHX and THF solvent at the first excited state with similar reaction barrier. Remarkably, compared to barrierless from keto (K*) to TICT state of BIDOP in THF solvent, the reaction barrier between K* and TICT state is up to 20.28 kcal/mol for in CHX that TICT process is inhibited in CHX solvent. The absence of nonradiative decay TICT state of BIDOP in CHX solvent induces higher fluorescence in CHX compared to in THF solvent. These findings indicate that CHX solvent can effectively enhance fluorescence of BIDOP. Our study highlights a convenient approach for enhancing fluorescence and is significant for photophysics and photobiology field.

Visible-to-NIR-Light Activated Release: From Small Molecules to Nanomaterials

Photoactivatable (alternatively, photoremovable, photoreleasable, or photocleavable) protecting groups (PPGs), also known as caged or photocaged compounds, are used to enable non-invasive spatiotemporal photochemical control over the release of species of interest. Recent years have seen the development of PPGs activatable by biologically and chemically benign visible and near-infrared (NIR) light. These long-wavelength-absorbing moieties expand the applicability of this powerful method and its accessibility to non-specialist users. This review comprehensively covers organic and transition metal-containing photoactivatable compounds (complexes) that absorb in the visible- and NIR-range to release various leaving groups and gasotransmitters (carbon monoxide, nitric oxide, and hydrogen sulfide). The text also covers visible- and NIR-light-induced photosensitized release using molecular sensitizers, quantum dots, and upconversion and second-harmonic nanoparticles, as well as release via photodynamic (photooxygenation by singlet oxygen) and photothermal effects. Release from photoactivatable polymers, micelles, vesicles, and photoswitches, along with the related emerging field of photopharmacology, is discussed at the end of the review.

Fluorescent Chromophores Containing the Nitro Group: Relatively Unexplored Emissive Properties

Apart from numerous applications, for example in azo dye precursors, explosives, and industrial processes, the nitro group (-NO₂ ) appears on countless molecules in photochemical research owing to its unique characteristics such as a strong electron-withdrawing ability and facile conversion to the reduced substituent. Although it is well known as a fluorescence quencher, fluorescent chromophores that contain the nitro group have also emerged, with 3-nitrophenothiazine being recently reported to have 100 % emission quantum yield in nonpolar solvents. The diverse characters of nitro-containing chromophores motivated us to systematically review those chromophores with nitro substituents, their associated photophysical properties, and applications. In this Review, we succinctly elaborate the advance of the fluorescent nitro chromophores in fields of intramolecular charge transfer, fluorescent probes and nonlinear properties. Special attention is paid to the rationalization of the associated emission spectroscopy, so that the readers can gain insights into the structure-photophysics relationship and hence gain insights for the strategic design of nitro chromophores.

Optical properties of 3-substituted indoles

The optical properties of various donor or acceptor p-phenyl substituted ethenyl indoles were studied in solvents of varying polarity using absorption, fluorescence and TDDFT methods. Ethenyl indole exhibits non-linear optical properties (NLO) in a substituent dependent manner. Compound with a strong electron-attracting substituent, shows large NLO properties with charge transfer behavior, whereas ethenyls with moderate electron withdrawing or electron donating substituent exhibit lower NLO properties with non polar excited state. A highly dipolar excited state for p-nitro phenyl substituted ethenyl indoles (μ _e: 18.2-27.1 debye; Δμ: 9.4-17.8 debye) is observed as compared to other ethenyls (μ _e: 6.6-9.5 debye; Δμ: 4.2-6.2 debye). From TDDFT study, it is shown that the HOMO-LUMO energy of ethenyl is increased with increasing the electron donating ability of the p-phenyl substitution. The optical band gap of ethenyl 3 without substitution, is decreased upon p-phenyl substitution either with an electron withdrawing (Cl, NO₂) or an electron donating (OCH₃, OH, NH₂) substituent. The compound with a strong electron accepting, p-nitrophenyl ethenyl indole 1 shows 12 times better NLO response as compared to the reference ethenyl indole 3 (β: 1: 115 × 10^-30 esu^-1 cm⁵, 3: 9 × 10^-30 esu^-1 cm⁵). Ethenyls 2-6 bearing a weak or moderately electron withdrawing or electron accepting substituent, exhibit lower NLO response. The β of ethenyl is increased with increasing the order of electron withdrawing nature of phenyl ring. Overall, a correlation of β with the optical band gap, ground state dipole moment, % of charge transfer in the ground and excited state is found.

Push-Pull Stilbene: Visible Light Activated Photoremovable Protecting Group for Alcohols and Carboxylic Acids with Fluorescence Reporting Employed for Drug Delivery

For the first time we have utilized push-pull stilbene as a visible light activated photoremovable protecting group (PRPG) for the uncaging of alcohols and carboxylic acids. The PRPG efficiently release caged molecules with good photochemical quantum yield. It is capable of monitoring the release in real time owing to its fluorescence "turn on" phenomenon upon photorelease in polar medium. The efficient photorelease and real time monitoring abilities of push-pull stilbene were employed for in vitro drug delivery.

Tuning of optical properties of p-phenyl ethenyl-E-furans: A Solvatochromism and Density functional theory

p-Phenyl ethenyl-E-furans (1-6) with varied electron donor and acceptor substituent (NO₂, CN, Cl, H, OCH₃, NH₂) were synthesized and studied the substituent induced optical properties (dipole moment, transition dipole moment, oscillator strength, optical band gap, hyperpolarizability) using Solvatochromism and Density functional theory. It is shown that furan acts as a weak electron donor in presence of an electron withdrawing p‑phenyl substituent (NO₂, CN, Cl), whereas furan acts as a weak electron acceptor in presence of an electron donating p‑phenyl substituent (OCH₃, NH₂). In comparison to ethenylfuran 4, the HOMO-LUMO energy band gap is decreased upon increasing the electron donating or electron withdrawing nature of the phenyl ring. Calculation of excited state dipole moment and polarizability of 1-6 in solvent of varying polarity suggest that the nitro and amino compounds (1, 6) exhibit charge transfer excited state, whereas excited state of compounds 3-5 is non-polar in nature. As compared to the ethenylfuran (4), the first hyperpolarizability (β) is increased in presence of a strong electron withdrawing or strong electron donating p‑phenyl substituent. The higher β value is found for ethenylfuran with p‑nitrophenyl and p‑amino phenyl substitution. Overall, these studies provide useful information in tuning the optical properties of p‑phenyl substituted heterocyclic ethenyl systems.

Substituent Dependent Optical Properties of p-phenyl Substituted ethenyl-E-thiophenes

Various electron donor and acceptor substituted (NO₂, CN, Cl, H, OCH₃, NH₂) p-phenyl ethenyl-E- thiophenes (1-6) were synthesized and substituent dependent optical properties (dipole moment, transition dipole moment, oscillator strength, optical band gap, hyperpolarizability) were studied using Solvatochromism and Density functional theory. It is shown that thiophene acts as a weak electron donor in presence of an electron withdrawing p-phenyl substituent (NO₂, CN, Cl), whereas thiophene acts as a weak electron acceptor in presence of an electron donating p-phenyl substituent (OCH₃, NH₂). In comparison to ethenyl thiophene 4, the HOMO-LUMO energy band gap is decreased upon increasing the electron donating or electron withdrawing capacity of p-phenyl substituent. From the excited state dipole moment calculation, it is shown that the excited state is highly dipolar for nitro and amino compounds 1 and 6, whereas compounds 2-5 show a non-polar excited state. As compared to the ethenyl thiophene 4, the first hyperpolarizability (β) increases upon substitution either with a strong electron withdrawing or strong electron donating p-phenyl substituent. A large β value is found for p-nitro phenyl ethenyl-E-thiophene and p-amino phenyl ethenyl-E- thiophene. Overall, these studies provide useful information in understanding the optical properties of phenyl and heterocyclic based ethenyl systems.

Role of Solvent Polarity and Hydrogen-Bonding on Excited-State Fluorescence of 3-[(E)-{4-[Dimethylamino]benzylidene}amino]-2-naphthoic Acid (DMAMN): Isomerization vs Rotomerization

The present experimental and theoretical study on a new chromophore DMAMN of the type push-π-pull (push = dimethylaniline, π = imine, pull = 2-naphthoic acid), allows understanding of the mechanism by which the molecular conformational undergoes isomerization/rotomerization following electronic excitation. The steady-state fluorescence spectra of this compound, carried out in solvents of different polarities and proticities, showed significant changes in both the shape and peak positions. The wavelength and intensity change depend on the polarity and hydrogen-bonding environment. In highly polar solvents, the emission is weak and red-shifted compared to that for cyclohexane, but it is more red-shifted in moderate aprotic polar solvents. In hydroxyl solvents, a new weak low-energy emission band appears at ∼525 nm, attributed to the intermolecularly H-bonded open conformer. On the basis of the generated potential energy landscapes of the ground state and low-lying excited state in the gas phase and solution, we found that selective photon absorption, brings this molecule to a "bright" state, from which N═C isomerization Z → E, takes place. This isomerization in gas-phase and low-polarity solvents leads to two minima with a barrier, whereas in highly polar-protic media, it gives one minimum on the S₁ surface with low ΔE_S1/T1 (0.17 eV), facilitating deactivation via ISC.

Substituent Dependence Charge Transfer and Photochemical Properties of Donor-Acceptor Substituted Ethenyl Thiophenes

Donor-acceptor conjugated molecules with efficient light induced properties represent interesting material for electronic device application. In this context, we have calculated excited state dipole moment of three ethenyl thiophenes (1-3) bearing varied electron donor-acceptor substituent in p-phenyl unit using Lippert-Mataga, Bakhshiev and Kawski method. It is found that 1 with strong electron-withdrawing nitro substituent, is exhibiting charge transfer and highly dipolar excited state as compared to 2 and 3. Photochemical studies of 1-3, indicate towards the charge transfer dependence trans-cis photoisomerization under direct irradiation condition. Compound 1 exhibits charge transfer and less efficient towards photoisomerization, whereas 2 and 3 undergo efficient photoisomerization. Graphical Abstract Substituent dependence charge transfer and photochemical properties of donor-acceptor substituted ethenyl thiophenes.

Comparative photophysics and ultrafast dynamics of dimethylaminochalcone and a structurally rigid derivative: experimental identification of TICT coordinate

Restriction of torsional relaxation and solvent dependent competing photophysical dynamics of free and rigid dimethylaminochalcones were investigated by time resolved spectroscopy.

Two modes of photoinduced twisted intramolecular charge transfer in meso-arylaminated subporphyrins

Photoinduced twisted intramolecular charge transfer (TICT) of meso-(4-dimethylamino)phenylamino subporphyrin 2 and meso-(4-nitro)phenylamino subporphyrin 3 has been revealed by steady-state and time-resolved absorption/fluorescence experiments and quantum calculations.