State Photophysical Properties Research Articles

Pillar[5]arene-Based Ion-Pair Recognition for Encapsulation of a Stilbazolium-Type Dye with Enhanced Photophysical Properties and Nonlinear Optical Activity.

Constructing organic composite materials through molecular recognition has emerged as an important theme in materials science. Here we report an ion-pair recognition system involving the use of a propoxylated pillar[5]arene (PrP5) to modulate the solid-state photophysical properties of dye trans-4'-(dimethylamino)-N-methyl-4-stilbazolium hexafluorophosphate (DMASP). Single crystal X-ray diffraction analysis reveals that the dye guest DMASP is encapsulated by PrP5 to form a 2 : 1 host-guest complex 2PrP5⸧DMASP in the crystalline state. The macrocyclic skeleton of PrP5 imposes restrictions on the intramolecular motions of the dye guest, leading to a significant enhancement of its fluorescence emission. Additionally, within the 2PrP5⸧DMASP complex crystal structure, DMASP molecules are found to display two possible opposite orientations in the one-dimensional channels formed by PrP5 molecules. This arrangement is believed to alter the overall solid-state packing structure of DMASP, thereby activating its nonlinear optical activity. This work not only reports a novel ion-pair molecular recognition system based on pillararenes but also provides valuable insights into the modulation of the crystalline state photophysical properties of organic dyes via cocrystal engineering.

Effects of atomic electronegativity on the ESIPT process and photophysical properties of naphthalene derivatives

The effect of atomic electronegativity on the excited state intramolecular proton transfer (ESIPT) and photophysical properties of naphthalene derivatives containing DNHP, DPHP and DAHP have been researched through the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The potential energy curve (PEC) calculated shows that DNHP, DPHP and DAHP can generate ESIPT process and the ESIPT rate increases with the decrease of atomic electronegativity. Subsequently, we further explain the influence of atomic electronegativity on the ESIPT mechanism and photophysical properties of these molecules by electron-hole analysis. Eventually, we sincerely hope that these studies can be helpful in their application.

Theoretical study on the atomic substitution effect on the aromaticity and ESIPT behavior of a novel chalcone-based fluorophore

Recently, a new ESIPT-based fluorophore (E)-2-(4-(dimethylamino) benzylidene)-7-hydroxy-1-indanone (7HIN) was designed and obtained experimentally. We investigated the excited state intramolecular proton transfer (ESIPT) behavior and photophysical properties of 7HIN based on density functional theory (DFT) and time-dependent DFT. Two derivatives of 7HIN (4HID and 4HBF) were obtained by replacing the CH2 group in the five-membered ring with NH group and O atom to study the atomic substitution effect on the proton transfer reactions and fluorescent features of 7HIN. All data from structural parameters, infrared frequencies and electron density showed that the intramolecular hydrogen bonds (IHB) in 7HIN and its derivatives were strengthened in the excited state (S1), which will accelerate the ESIPT process. The potential energy curves (PECs) indicated that the proton transfer can easily occur in the S1 state because of the lower energy barrier. The electron-hole analysis revealed that the S0 → S1 transition is an intramolecular charge transfer transition. The NH- and O-substitution led to a weakening of IHB, increased the ESIPT barrier and red-shifted the absorption/emission wavelength of 7HIN. The PBE38 functional in this work is very reliable since the experimental absorption/fluorescence wavelength of 7HIN was reproduced well.

A Bichromophoric Approach to Induce Luminescence - A Blend of Experimental and Theoretical Studies.

Two homoleptic terpyridyl complexes of Ru(II), 1 and Fe(II), 2 were synthesized using a ligand L1 that contained a phenyl spacer between an anthracenyl (An) and a terpyridyl (tpy) moiety. An equilibrated-bichromophoric strategy was adopted to induce photoluminescence in 1 and 2. A glimpse into the excited state photophysical properties of 1 and 2 revealed that 1 exhibited NIR emission at ~700 nm with an excited state lifetime components of 1.33 and 6.52 ns. On the other hand, 2 was found to be non-luminescent. The origin of emission in case of 1 was attributed to the effect of phenyl spacer which rendered the 3An state to be nearly isoenergetic to the emissive 3MLCT state of 1 facilitating 3MLCT-3An equilibrium. This fact was supported by experimental (photocurrent generation) and theoretical (potential energy diagram) evidences.

Molecular Dyad vs Multi‐Component Approach‐Photocatalytic Hydrogen Evolution by Combining Oligothiophene Photosensitizers with [FeFe]‐Hydrogenase Mimics

AbstractA molecular dyad (PS‐CAT) consisting of a metal‐free photosensitiser (PS) and a [FeFe]‐hydrogenase‐inspired [Fe2S2] cluster (CAT) has been synthesized for hydrogen evolution under visible light irradiation. The electrochemical properties of PS‐CAT in the absence and presence of acid were investigated by CV and IR‐SEC. The ground state and excited state photophysical properties of PS and PS‐CAT, and the non‐chemically connected reference system [PS, CAT] were investigated to elucidate the effect of a covalent linkage between PS and CAT on the electronic communication and hence on the catalytic performance. The photo‐relaxation pathway of PS‐CAT was revealed computationally by scalar‐relativistic time‐dependent density functional theory simulations. These studies are complemented by comparative analyses of photocatalytic hydrogen evolution between PS‐CAT and [PS, CAT] under visible light irradiation.

Effect of different substituted groups on excited state intramolecular proton transfer of BOHMB

AbstractThe photophysical features of 3‐(benzo[d]oxazol‐2‐yl)‐2‐hydroxy‐5‐methoxy benzaldehyde (BOHMB) were investigated through experimental (J. Phys. Chem. A 2019, 123, 10,246–10,253) and theoretical (Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2022, 266, 120,406) methods. However, the effect of substituent groups on the excited state proton transfer process has not been studied. In this work, the excited state intramolecular proton transfer (ESIPT) dynamics and photophysical properties of BOHMB with different substituent groups were investigated by density‐functional theory (DFT) and time‐dependent DFT (TDDFT) methods at CAM‐B3LYP/6‐311G(d,p) level. The primary parameters related to hydrogen bonding and infrared vibration frequency were obtained to understand the ESIPT properties of BOHMB derivatives. The results indicate that the excited‐state intramolecular hydrogen bond (ESIHB) strengthening behaviors, and the intramolecular hydrogen bond O1–H2···O3 for 1a in the S1 state is the strongest among BOHMB derivatives. From the calculated potential energy curves, it can be inferred that the substitution and position of NH2 and NO2 groups will regulate the excited‐state energy barrier and thus affect the ESIPT process. The molecular absorption peak and fluorescence peak are affected by different substituting groups and different positions.

Fluorescent response mechanism based on ESIPT and TICT of novel probe H2QJI: A TD-DFT investigation

Recently, synthesized N-linked-disalicylaldehyde H2QJI probes have been used to detect heavy metal ions in the experiment conveniently. Nevertheless, there needs to be a more in-depth examination of the excited state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of the probe. This work remedied it based on quantum chemistry calculations. We contained due hydrogen bond (O1-H2 ⋯ N3 and O4-H5 ⋯ O6) and then analyzed bond parameters, IR vibration spectra, and non-covalent interaction. The bond strength is enhanced under photoexcitation, and the former is significantly stronger. The calculated electron spectra are in agreement with the experimental values. The results of the S0 and S1 potential energy curves and IRC calculations also confirm the unique ESIPT behavior, which isan excited stated stepwise double proton transfer. The fluorescence, internal conversion, and intersystem crossing rate of KD molecules (twisted-, double proton transfer) were calculated respectively to reveal the radiative and non-radiative pathways. It proved that the corresponding spectra are not obtained since the electrons are mainly deactivated by the ISC (S1->T1). Furthermore, the interfragment charge transfer (IFCT) approach indicates that the molecule possesses twisted intramolecular charge transfer (TICT) characteristics, which lead to the quenching of fluorescence introduction.

Impact of binary inorganic salt on micellization of pluronic block copolymers: A photophysical analysis

The impact of binary inorganic salt zinc chloride (ZnCl2) on the micellization behavior of triblock pluronic copolymer, F108 was investigated by various spectroscopic methods in the presence of Coumarin-6 (C6) as a fluorescent probe. Ground and excited state photophysical properties revealed that the microenvironment around the C6 molecule changes in the presence of ZnCl2 compared to that in a salt-free solution. The addition of salt to the medium increases the hydrophobicity and microviscosity of the micelle core. Due to the presence of ZnCl2, the central PPO core experiences dehydration, reduces the interaction between water molecules and PEO groups, and enhances the hydrophobic interaction between C6 and hydrophobic PPO parts in the micelle with more solubility of C6. This study helps to develop the property of copolymers as drug carriers in various pharmaceutical applications and drug delivery systems.

The effect of –CF3 substitution position on the aggregation-induced emission, solvatofluorochromism and mechanofluorochromism properties of the conjugated 9-butyl-9H-carbazole phenylacrylonitrile derivatives

A series of 9-butyl-9H-carbazole derivatives with intense solid-state emission were synthesised by the Knoevenagel reaction of 9-butyl-9H-carbazole-3-carbaldehyde with phenyl acetonitrile derivatives with –CF3 group at different substituent positions of the phenyl group. They exhibit remarkable aggregation-induced emission (AIE), solvatofluorochromism and mechanofluorochromic (MFC) properties depending on the –CF3 substitution site. The ortho-substituted derivative shows enhanced charge transfer properties with a distinct dual-state fluorescence emission. This peculiar property is further utilised for data encryption and anti-counterfeiting applications. The para-substituted derivative exhibits an increased solid-state fluorescence quantum yield of 88% with switchable MFC properties. The ground and excited state photophysical properties of the synthesised luminogens were explored using various spectroscopic techniques. The study provides an efficient strategy to understand the effect of the –CF3 substitution position in tuning and modifying the luminescence properties of the organic solids via subtle modifications in the molecular scaffolds.

Water soluble silicon quantum dot based fluorescence immunoassay probe for C-reactive protein (CRP) detection

Silicon quantum dot (SiQD) based water soluble fluorescence immunoassay probe with aggregation induced emission features (AIE) was developed for the sensitive detection of C-reactive protein (CRP) biomarker. The solid state and solution phase photophysical properties of SiQD attributed from AIE effect was studied successfully. Later the ability of CRP antibody functionalized SiQD against the detection CRP antigen was evaluated systematically. The method relies on the decrease in fluorescence intensity of CRP antibody functionalized SiQD (CRPAb) upon the gradual addition of CRP antigen (CRPAg) via antigen-antibody interaction. The intensity of the antibody functionalized fluorophore quenched linearly by the addition of CRPAg within the concentration range of 50–150 nM and the limit of detection (LOD) was as sensitive as about 0.89 nM. Detection mechanism was assigned further by evaluating the interaction that exists between SiQD-CRPAb and CRPAg, which suggests that the effective complexation through a lock and key mechanism could be the plausible reason behind the fluorescence quenching of SiQD-CRPAb in presence of CRPAg. The solid state fluorescence features of SiQD arises from AIE phenomena which suggest that as a promising alternative to conventional QDs and organic fluorophores, integration of SiQD with lateral flow strips could result in powerful eco-friendly point-of-care device for in vitro diagnosis and the detection of other biomarkers.

Regulating the photophysical properties of ESIPT-based fluorescent probes by functional group substitution: a DFT/TDDFT study.

In recent years, fluorescent probe technology has received more and more attention. However, the photophysical and photochemical properties of probe molecules still need to be further explored. This paper presents the excited state intramolecular proton transfer (ESIPT) processes and photophysical properties of the probe molecule 4-bromo-2-((E)-((Z)-((5-bromo-1H-indol-2-yl) methylene) hydrazono) methyl) phenol (BHPL) and its four derivatives (BHPL2, BHPL3, BHPL4, and BHPL5). Infrared spectra and geometric structure analyses revealed that introducing the -NH2 group on the benzene ring with the hydroxyl group will enhance the intramolecular hydrogen bond, which benefits the ESIPT process. Combining their absorption and fluorescence spectra, it can be concluded that BHPL2 and BHPL4 are both excellent probe candidates due to their large Stokes shift. The hole and electron and root mean square displacement analyses manifest that the fluorescence quenching of BHPL4 may be due to the intramolecular charge transfer process. Potential energy curves of BHPL and its four derivatives noted that ESIPT process of the BHPL2 is the most favorable to occur. The frontier molecular orbital and NBO analyses indicated that besides introducing electron-donating groups to reduce the energy gap and enhance fluorescence emission, introducing double electron-withdrawing groups can also achieve this effect, explaining why the energy barrier of ESIPT process for BHPL2 is lower than BHPL5. This work would provide the theoretical basis for designing novel fluorescence probes with more prominent properties. The ground (S0) and excited (S1) state structures of all compounds were optimized by density functional theory (DFT) and time-dependent (TDDFT) method, with B3LYP/6-311+G(d,p) level, respectively. The infrared spectra and potential energy curves were simulated at the same theoretical level. The reduced density gradient scatter plots and interaction region indicator isosurfaces were drawn using Multiwfn and VMD programs. The absorption and fluorescence spectra were simulated by the TDDFT/B3PW91/6-311+G(d,p) method. All the calculations in this work are carried out in Gaussian 16 program package.

Influence of Two- and Three-Dimensional Engineering on the Trap State Distribution and Photophysical Properties of Lead Halide Perovskite Polycrystals

Constructing a two- and three-dimensional (2D/3D) heterojunction structure on the surface of a 3D perovskite film, termed 2D/3D engineering, is effective in elevating the stability of perovskite polycrystal-based photovoltaic and photoelectronic devices; however, it remains controversial whether this protocol is favorable or detrimental to the device performance. Here, we prepare a series of 2D/3D perovskite films by post-treating the perovskite polycrystalline film with different concentrations of phenethylammonium iodide (PEAI). Systematic spectroscopy and electrochemical studies illustrate that PEAI can penetrate the 3D perovskite network and eliminate the intrinsic trap states of perovskite polycrystals, while the 2D perovskite nanosheets enriched on the top of the polycrystalline film may introduce additional trap states, which manipulate the photoluminescence performance and dynamics of the as-prepared perovskite films in an opposite manner. Based on this finding, the strategy of optimizing the photophysical properties of the host 3D perovskite through 2D/3D engineering is elaborated, paving the way for fabricating high-performance and high-stability perovskite polycrystalline films.

Correlating structure and photophysical properties in thiazolo[5,4-d]thiazole crystal derivatives for use in solid-state photonic and fluorescence-based optical devices.

There is a growing demand for new fluorescent small molecule dyes for solid state applications in the photonics and optoelectronics industry. Thiazolo[5,4-d]thiazole (TTz) is an organic heterocycle moiety which has previously shown remarkable properties as a conjugated polymer and in solution-based studies. For TTz-based small molecules to be incorporated in solid-state fluorescence-based optical devices, a thorough elucidation of their structure-photophysical properties needs to be established. Herein, we have studied four TTz-based materials functionalized with alkyl appendages of varying carbon chain lengths. We report the single crystal structures of the TTz derivatives, three of which were previously unknown. The packing modes of the crystals reveal that molecular arrangements are largely governed by a chorus of synergistic intermolecular non-covalent interactions. Three crystals packed in herringbone mode and one crystal packed in slipped stacks proving that alkyl appendages modulate structural organization in TTz-based materials. Steady state and time-resolved photophysical properties of these crystals were studied via diffuse-reflectance, micro-Raman, and photoluminescence spectroscopy. The crystals fluoresce from orange-red to blue spanning through the whole gamut of the visible spectrum. We have established that photophysical properties are a function of crystal packing in symmetrically substituted TTz-based materials. This correlation was then utilized to fabricate crystalline blends. We demonstrate, for the first time, that symmetrically substituted donor-acceptor-donor TTz-based materials can be used for phosphor-converted color-tuning and white-light emission. Given the cost effectiveness, ease of synthesis and now a structure-photophysics correlation, we present a compelling case for the adoption of TTz-based materials in solid-state photonic and fluorescence-based optical devices.

TD-DFT study of the excited state intramolecular proton transfer (ESIPT) mechanism and photophysical properties in coumarin–benzothiazole derivatives: substitution and solvent effects

The photo-switching mechanism in the coumarin–benzothiazole dyes (BT–Cou–R1–8; R1–8= H, NH2, OH, OCH3, CH3, CF3, CN and NO2) with potential application in drug delivery was investigated at the TD-PBE0/6-311++G(d,p) level of theory in the gas phase and three solvent media.

9,9-Bis[4-(N-aryl)phenyl]methylidene-xanthylidene Derivatives Displaying Mechano-, Crystallo-, and Thermochromism.

Tetraphenylethylene (TPE) derivatives bearing a xanthene moiety are of interest because they have novel optical properties. 9,9-Bis[4-(N,N-diphenylamino)phenyl] and 9,9-bis[4-(9-carbazolyl)-phenyl]methylidene-xanthylidenes 3 and 4 were synthesized using Suzuki-Miyaura coupling of 9,9-dibromomethylidene-xanthylidene with the corresponding boronic acids. Diphenylamino derivative 3 exhibits mechanochromism and mechanofluorochromism (MC and MFC) reflected in absorption and fluorescence color changes. In contrast, carbazolyl derivative 4 displays thermo- and crystallo-chromism in addition to MC and MFC in the solid state. Powder X-ray diffraction and single crystal X-ray crystallographic analysis reveal that the solid state photophysical properties of these substances are governed by conformational changes rather by the creation of planar π-conjugation extended geometries.

Different positions of cyano substitution controlled directionality of ESIPT processes with two asymmetric proton acceptors system: A TD-DFT study

In order to systematically investigate the effect of substitution position on excited state intramolecular proton transfer (ESIPT) process and photophysical properties, three 3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methoxy-benzaldehyde (BTHMB) derivatives (BTHMB-1, BTHMB-2 and BTHMB-3) were designed by introducing the strong electron-withdrawing group (–CN) at different positions. Based on the analyses of configurations, electron densities and infrared vibrational frequencies, the intramolecular hydrogen bond (IHB) of BTHMB derivatives are all enhanced in the excited state, which would promote the ESIPT process. All the BTHMB-1, BTHMB-2 and BTHMB-3 compounds can undergo ESIPT process via two possible pathways. The potential energy curves (PECs) showed that the ESIPT barriers decrease along the order of BTHMB-2-O > BTHMB-3-O > BTHMB-1-O, and BTHMB-3-N > BTHMB-2-N > BTHMB-1-N. Due to the lower energy barrier, the ESIPT process in BTHMB-2 tends to happen to the direction of proton acceptor O atom, while the ESIPT processes in BTHMB-1 and BTHMB-3 tend to happen to the direction of proton acceptor N atom. No matter where the substituted position was, the introduction of –CN group resulted in the obvious red-shift of absorption and fluorescence peaks. For BTHMB-1-O/BTHMB-2-O/BTHMB-3-O and BTHMB-1-N/ BTHMB-2-N/BTHMB-3-N, the red-shift values of absorption and fluorescence peaks in BTHMB-1-O and BTHMB-3-N are the largest, respectively. The more red-shift the absorption and emission peaks show, the smaller the S0 and S1 energy gap between HOMO and LUMO are.

2‐Aryl‐1H‐benz[de]isoquinolinium Ions: Cationic Dyes Displaying Mechanochromism and Crystallochromism

Abstract2‐Aryl‐5,8‐di‐tert‐butyl‐1H‐benz[de]isoquinolinium tetrafluoroborates were synthesized and crystalized to form yellow to orange crystals. Absorption and fluorescence maxima (λABS and λFL‐SL) of the compounds in CH2Cl2 depend on the electronic and steric properties of the aryl ring substituents and closely match those arising from theoretical calculations. λABS and λFL‐SL values of the compound possessing bulky 2,6‐diisopropylphenyl substituents are the shortest among the substances because of sterically promoted retardation of conjugation between the isoquinolinium and aryl ring planes. In contrast, solid‐state fluorescence maxima (λFL‐SD) of the compounds are not closely correlated with the electronic properties of the aryl substituents. Furthermore, one crystalline form of both the 4‐trifluoromethylphenyl and 4‐nitrophenyl derivatives exhibit mechanochromic and crystallochromic behavior reflected in absorption and fluorescence color changes. X‐ray diffraction and crystallographic analysis reveal that the solid state photophysical properties of these substances are related to different molecular stacking interactions occurring in different crystalline forms.

Unraveling excited state dynamics and photophysical properties for a series of phenol-quinoline derivatives by controlling hydrogen bond geometry

As a “site-specific” interaction, the hydrogen bonds play an important role in optical properties. Therefore, we used the time dependent density functional theory to study the excited state dynamics and photophysical properties of a series of phenol-quinoline derivatives (1-3 compounds) by controlling the hydrogen bond geometry. The calculated geometric parameters and reduced density gradient results show that the intramolecular hydrogen bond distance is inversely proportional to the hydrogen bond strength. Meanwhile, potential energy curves shows that the stronger the hydrogen bond strength, the smaller the energy barriers in the ground (S0) and excited (S1) states. In particular, we use not only the linear response solvation model but also the state-specific (SS) solvation model for the PES in the S1 state. The results show that the SS solvation model better establishes the relationship between the hydrogen bond strength and the energy barrier in the S1 state. In addition, increasing the dihedral angle of intramolecular hydrogen bond leads to more obvious intramolecular charge transfer during excitation transition. Moreover, we also found that the dihedral angle affects the excited state dynamics and the fluorescence emission properties. Our theoretical results indicate that the fluorescence quenching pathway of the 3 compound is caused by the photoinduced electron transfer process, which is different from the experimentally proposed non-radiative deactivation by conical intersections.

A theoretical perspective of the relationship between the structures and luminescence properties of red thermally activated delayed fluorescence molecules.

Orange and red thermally activated delayed fluorescence (TADF) emitters have shown promising applications in organic light emitting diodes (OLEDs) and the bio-medical field. However, both the species and amounts of orange and red molecules are far from meeting the requirement for practical applications; this is due to the lack of systematic studies on the relationship between molecular structures and luminescence properties. Herein, the excited state dynamic processes and photophysical properties of six donor-acceptor (D-A) type orange-red TADF molecules, which possess the same acceptor, are theoretically studied in toluene by using the polarizable continuum model (PCM). Based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations coupled with the thermal vibration correlation function (TVCF) method, the adiabatic singlet-triplet energy gaps, natural transition orbital properties, reorganization energies, hole and electron distributions, and the radiative and non-radiative as well as the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes are theoretically analyzed. The results indicate that remarkable geometric changes between the lowest singlet excited state (S1) and the ground state (S0) are mainly caused by the rotation of the donor unit for NAI-R2, NAI-R3 and NAI-DPAC, and the reorganization energy is mainly contributed by the dihedral angle. However, for NAI-DMAC, BTDMAc-NAI and BFDMAc-NAI, remarkable geometric changes are found in the acceptor unit with large contribution of reorganization energy by bond length. These variations bring different non-radiative energy consumption processes. Moreover, small energy gaps between S1 and the lowest triplet excited state (T1) are determined for all studied molecules and an efficient RISC process is detected. Furthermore, enhanced conjugacy in the donor unit and remarkable intramolecular interactions are determined for BTDMAc-NAI and BFDMAc-NAI, which is helpful to promote the up-conversion process. Our investigations give reasonable explanations for previous experimental measurements and the relationship between basic structures and luminescence properties is revealed, which could facilitate the development of new efficient TADF emitters.

Spectroscopic and computational studies of the solid state photophysical properties of a biscoumarin dye

The solid state photophysical properties of the 3,3′-paraphenyl bis[6,8-dimethoxy-2H-chromen-2-one] symmetrical biscoumarin material were investigated by optical spectroscopy techniques and by theoretical calculations. Vibrational analysis using IR absorption and Raman scattering techniques carried out together with DFT theoretical calculations have confirmed the structure of this biscoumarin. The geometry optimization using different functionals reveal a nonplanar equilibrium structure with a dihedral between the phenyl and the pyran rings of about 142°. The UV–Visible absorption measurements and the TDDFT simulation show that this biscoumarin is characterized by a bicomponent feature resulting from ππ* electronic transitions and Intramolecular Charge Transfer (ICT). Solid state photoluminescence showed a bright blue-green emission at 506 nm with a large stokes shift estimated at 146 nm, and the temperature dependence study of this emission reveals two thermal evolution regimes. Finally, these good optical properties, as well as the stability of the emission, make this biscoumarin dye of potential interest for optoelectronic applications.