Monomeric Fluorescence of H-Aggregates in a Series of 2-(Hydroxyphenyl)benzoxazoles Derivatives.

Uncovering photo-induced hydrogen bonding interaction and proton transfer mechanism for the novel salicylaldehyde azine derivative with para-position electrophilic cyano group

Part I: Detailed insights into the excited state intramolecular proton transfer (ESIPT) reaction in 2-(2’-hydroxy-4’-dietheylaminophenyl) benzothiazole (HABT) have been investigated via steady state and femtosecond fluorescence up-conversion approaches. In cyclohexane, in contrast to the ultrafast rate of ESIPT for the parent 2-(2’-hydroxyphenyl) benzothiazole (> 35 fs-1), HABT undergoes a resolvable, relatively slow rate (~1.8 ps-1) of ESIPT. In polar, aprotic solvents competitive rate of proton transfer and rate of solvent relaxation was resolved in the early dynamics. After reaching the equilibrium polarization in the normal state (N*), ESIPT takes place, associated with a solvent induced barrier due to different polarization equilibrium between normal (N*) and tautomer (T*) states. Supplementary support was also rendered via the study of 2-(2’-methoxy-4’-dietheylaminophenyl) benzothiazole (MABT), in which ESIPT is prohibited due to the lack of hydroxyl proton. The results are rationalized by a similar dipolar character between N and T* species, whereas due to the charge transfer effect N* possesses an appreciable dipolar change with respect to both N and T*. ESIPT is thus energetically favorable at the Franck-Condon excited N*, and its rate is competitive with respect to the solvation relaxation process. In CH3CN, due to the strong solvent stabilization there exists an equilibrium between N* and T* states in e.g. CH2Cl2, and both forward and reversed ESIPT dynamics are associated with a solvent induced barrier due to different polarization equilibrium between N* and T*. The N* ↔ T* equilibrium constant was sdeduced to be 24.5, 4.71 and 0.57 in cyclohexane, CH2Cl2 and CH3CN, respectively. Temperature dependent relaxation dynamics further resolved a solvent induced barrier of 1.88 kcal/mol with a rate of 6.8 ps-1 at 298 K for the forward reaction in CH2Cl2. Part II: A Azulenylocyanine dye (AC) has been synthesized to investigate its associated photophysical properties. AC is essentially nonluminescent (Φf < 10-6) in any solvents despite its very high absorption extinction coefficient (760 nm, e ~ 8.2×104 M-1cm-1 in methanol). Femtosecond fluorescence upconversion, anisotropy kinetics and transient absorption experiments, in combination with the theoretical TDDFT approach, lead us to conclude that the lowest S0 → S1 transition is partial optically forbidden in character, while the 760 nm absorption is ascribed to the fully allowed S0 → Sn (n ≥ 2) transition. The observed <130 fs decay component is attributed to the Sn → S1 internal conversion, while the S1 → S0, with a much slower radiative decay time (> 233 ns) undergoes a dominant radiationless deactivation 7 process (710 ± 70 fs) possibly governed by strong interaction between S1 and S0 potential energy surfaces. Part III: CdSe/ZnTe and CdTe/CdSe type-II quantum dots (QDs) are characterized in near-IR interband emission. Spectroscopic and femtosecond dynamic measurements reveal that the rate of photoinduced electron/hole spatial separation decreases with increases in the size of the core, and is independent of the thickness of the shell in the CdSe/ZnTe QDs. The results are consistent with the binding strength of the electron and hole confined at the center of CdSe. So far as CdTe/CdSe is concerned, the femtosecond fluorescence upconversion measurements on the relaxation dynamics of the CdTe core emission and CdTe/CdSe interband emission reveal that as the size of the core increases from 5.3, 6.1 to 6.9 nm, the rate of photoinduced electron separation decreases from 510, 690 to 930 fs. The finite rates of the initial charge separation are tentatively rationalized by the low electron-phonon coupling, causing small coupling between the initial and charge-separated states. The correlation between the core/shell size and the electron/hole spatial separation rate resolved in this study may provide valuable information for applications where rapid photoinduced carrier separation followed by charge transfer into a matrix or electrode is crucial, such as in photovoltaic devices. Tuning CdSe quantum dots (QDs) sizes and consequently their corresponding two-photon absorption (TPA) cross section have been systematically investigated. As increasing the size (diameter) of the quantum dots, the TPA cross section was found to be dependent on a 3.5 ± 0.5 and 5.6 ± 0.7 and 5.4 power of CdSe and CdTe QDs diameters, respectively. TPA cross section was measured to be as high as 1.0 × 10-46 cm4•s photon-1(104 GM) for CdSe QDs with a diameter of 4.8 nm. The results are rationalized on theoretical levels incorporating both one-photon and two-photon excitation properties on an exciton system.

Achieving multicolor emissions from a single molecule has been an active field of research, particularly in developing organic light‐emitting diodes. Reported herein is acenaphthylenedione (AcD), which displays multiple colors as a function of excitation wavelengths. Experimental and theoretical data reveal that the latter emits strongly from S3 and weakly from S2, while S1 remains as a dark state, thus violating Kasha's rule. The calculated large energy difference between S2 and S3 (i.e., 1.00 eV) promotes radiative decay from S3 rather than internal conversion (IC) to S1. Hydrazones derived from the same also possess excitation wavelength‐dependent emission. Time‐dependent density functional theory (TDDFT) calculations reveal that the longer wavelength emission can be assigned to enol‐form, produced through excited‐state intramolecular proton transfer (ESIPT) and locally excited (LE) keto‐form, while that of the shorter wavelength to LE S2, thus disobeying Kasha's rule. The calculated energy difference (ΔES1‐S2) is found to be 0.64 eV, which reduces the rate of IC (i.e., S2 → S1), resulting in the emission from the higher excited state. N‐methylated hydrazone, which blocks the ESIPT channel, also supports the hypothesis. Furthermore, all the compounds exhibit aggregation‐induced emission behavior, and nitro‐ and cyano‐substituted hydrazones are found as good candidates for antibacterial, antioxidant, and anticancer activities.

Background: Diarylethenes are well-known photochromic compounds, which undergo cyclization and cycloreversion reactions between open- and closed-ring isomers. Recently, diarylethene derivatives with photoswitchable fluorescent properties were prepared. They are applicable for fluorescence imaging including bio-imaging. On the other hand, a new system called “excited state intramolecular proton transfer (ESIPT)” is reported. In the system, absorption and emission bands are largely separated due to the proton transfer, hence it showed strong fluorescence even in the crystalline state. We aimed to construct the photochromic system incorporating the ESIPT mechanism.Results: A diarylethene incorporating a fluorescent moiety that exhibit ESIPT behavior was prepared. The ESIPT is one of the examples which express the mechanisms of aggregation-induced emission (AIE). This compound emits orange fluorescence with a large Stokes shift derived from ESIPT in aprotic solvents such as THF or hexane, while it exhibits only a photochromic reaction in protic solvents such as methanol. In addition, it shows turn-off type fluorescence switching in an aprotic solvent and in crystals. The fluorescence is quenched as the content of closed-ring isomers increases upon UV light irradiation.Conclusions: A diarylethene containing an ESIPT functional group was prepared. It showed fluorescent turn-off behavior during photochromism in aprotic solvents as well as in crystalline state upon UV light irradiation. Furthermore, it showed AIE in THF/water mixtures with blue-shift of the emission.

Density functional theory (DFT) and time‐dependent density functional theory (TDDFT) methods are adopted to explore the ground‐state and excited‐state intramolecular double hydrogen bonding interactions as well as the excited state intramolecular proton transfer (ESIPT) mechanism for 5,5′‐(9,9‐dihexyl‐9H‐fluorene‐2,7‐diyl)bis(2‐benzo[d]thiazol‐2‐yl)phenol) (abbreviated as Ia) system. The simulated electronic spectra of the Ia system (ie, absorption and fluorescence spectra) are reappeared by experimental results, which reveals the reasonability and correctness of the calculated theory adopted in this work. We firstly verify that the dual intramolecular hydrogen bonds of Ia should be enhanced in the first excited state via geometrical parameters (ie, bond lengths and bond angles) and infrared (IR) vibrational spectra. Then, insights into the photo‐excitation aspects, we confirm that the charge redistribution facilitates the ESIPT tendency. Particularly, the increased electronic densities around proton acceptors could play important roles in attracting proton H2 and H5 atoms for the Ia system. At last, via constructing potential energy surfaces (PESs), we clearly clarify the excited state intramolecular single proton transfer mechanism for the Ia system. This work not only clarifies the detailed ESIPT mechanism for the novel Ia system and makes up for the deficiencies in previous experiment but also promotes a deeper understanding about the excited state behaviors involved in multiple hydrogen bonding interactions.

In this work, a new excited state intramolecular proton transfer (ESIPT) mechanism of bis (salicylidene)‐1,5‐diaminonaphthalene (BSD) is proposed based on density functional theory (DFT) and time‐dependent DFT methods. Given the intramolecular dual hydrogen bonds of BSD, we explore the excited state hydrogen bonding interactions for BSD and verify that the dual intramolecular hydrogen bonds of BSD molecule should be strengthened in the first excited state. Further, in the analyses about charge distribution upon excitation, we confirm that the charge redistribution around hydrogen bonding moieties plays important roles in enhancing dual hydrogen bonds and in facilitating ESIPT process. To reveal the detailed ESIPT mechanism, we construct the potential energy surfaces (PESs) in both S0 and S1 states along with hydrogen bonds. Via comparing potential energy barriers among the stable structures on the S1‐state PES, we present the excited state intramolecular single proton transfer mechanism for BSD system. Moreover, the phenomenon and properties of fluorescence for BSD in experiment could be explained reasonably based on the ESIPT mechanism presented in this work.

Studies of excited state intramolecular proton transfer (ESIPT) coupled with charge transfer (ESICT) are presented here. As for a strategic design, we synthesized a series of derivatives based on the famous ESIPT molecules 2-(2’-hydroxy) benzoxazole (HBO) or 2-(2’-hydroxy) benzothiazole (HBT). In HBO and HBT, the lone pair electrons at the nitrogen atom have been incorporated into the resonance, i.e. the construction of aromaticity, such that its electron donating property, comparing with those of alkyl amines, is negligibly small. Upon Franck-Condon excitation, one can thus perceive its lack of ESICT. Instead, ESIPT takes place, forming a proton transfer tautomer, in which the central nitrogen atom acts as the secondary alkyl amine. If the para-position of the nitrogen is anchored by a strong electron withdrawing group, ESICT should proceed simultaneously at every moment ESIPT takes place. In other words, one can envisage such types of reaction as the photon induced ESICT associated with a nuclear (proton) motion, rendering an ideal system to study the role of solvent polarity for the ESICT/ESIPT coupled reaction, and hence a paradigm to test the “Marcus theory (modified by Hynes et al.) based on the proton transfer reaction. Although di-CN group is an electron acceptor anchored on the para position of the center nitrogen atom in diCN-HBO and diCN-HBT, the distance between di-CN group and hydroxy group is long enough to prevent affection in the ground state. Started by Franck-Condon excitation, the ESIPT takes place first to create a chance for electron delocalization. As electron motion is faster than nuclear motion, ESICT is initiated and finished during ESIPT. For ESIPT and ESICT is correlated, the combination rate is solvent polarity dependent. The enol form decay rate of diCN-HBO decreases with increasing solvent polarity. The phenomena provides us an ideal model different from our preceding proton transfer compounds such as 4’-(Dialkylamino)-3-hydroxyflavone, p-N,N-Ditolylaminosalicylaldehydes,and HABT, etc.

Unraveling photo-induced proton transfer mechanism and proposing solvent regulation manner for the two intramolecular proton-transfer-site BH-BA fluorophore

Excited state (formal) intramolecular proton transfer (ESIPT) in p-hydroxyphenyl ketones mediated by water

ABSTRACTGiven the importance of excited state relaxation in photochemical and photophysical behaviours, in this work, we mainly focus on the excited state dynamical behaviours for the novel 6-cyano-2-(2′-hydroxyphenyl)imidazo[1,2a]pyridine (6-CN-HPIP) system in different aprotic solvents. We find the ultrafast excited state intramolecular proton transfer (ESIPT) behaviour without potential energy barrier for 6-CN-HPIP in nonpolar solvents. Furthermore, we also explore the excited state intramolecular hydrogen bonding interactions and confirm the hydrogen bond O-HN of 6-CN-HPIP should be strengthened based photo-excitation process. It provides the possibility of ESIPT process. And the charge redistribution resulting from photo-excitation around hydrogen bonding moieties further reveals the ESIPT tendency for 6-CN-HPIP molecule. It is worth mentioning that the increased electronic densities around proton-acceptor moiety play important roles in attracting hydrogen proton, which directly promotes ESIPT reaction. Via constructing potential energy curves, we clarify the ESIPT dynamical process for 6-CN-HPIP system and present a novel mechanism that nonpolar aprotic solvents facilitate ESIPT behaviour for 6-CN-HPIP. We hope that novel applications and developments could be promoted for 6-CH-HPIP and its derivatives through regulating and controlling ESIPT behaviours in future.

Functional Scaffolds from AIE Building Blocks

Nanomaterials have been steadily gaining importance over the past few decades. Metal nanoclusters comprised of gold or silver have been thoroughly studied and well documented. Only recently, the focus has begun to shift to copper nanoclusters (CuNCs). However, there are very few studies that focus on the modulation of the photophysics of metal nanoclusters, especially CuNCs. Dimethyl sulfoxide (DMSO) is an industrially and biologically important solvent. Aqueous DMSO binary mixtures have been frequently used for modulation of excited-state dynamics of many conventional fluorophores. Here, by performing the time-resolved fluorescence spectroscopy of nucleotide-functionalized CuNCs in aqueous DMSO binary mixtures, we have demonstrated that DMSO negatively affects the excited-state decay dynamics of CuNCs by destabilizing the excited state, leading to faster decay dynamics. We have proposed that DMSO disrupts the water-mediated internanocluster (interNC) cuprophilic interactions, which are mostly responsible for stabilization of the CuNC excited state. These results will provide significant physical insight on techniques to modulate the radiative decay dynamics of CuNCs and other nanomaterials.

Excited-state intramolecular proton transfer (ESIPT) reactions are crucial in photoresponsive materials and fluorescent markers. The fluorescent compound 4-aminophthalimide (4-AP) has been reported to exhibit solvent-assisted ESIPT in protic solvents, such as methanol, wherein the solvent interacts with 4-AP to form a six-membered hydrogen-bonded ring that is strengthened upon excitation. Although the controversial observation of ESIPT in 4-AP has been extensively studied, the molecular mechanism has yet to be fully explored. In this study, femtosecond infrared spectroscopy was used to investigate the dynamics of 4-AP in methanol and acetonitrile after excitation at 350 and 300 nm, which promoted 4-AP to the S1 and S2 states, respectively. The excited 4-AP in the S1 state relaxed to the ground state, while 4-AP in the S2 state relaxed via the S1 state without the occurrence of ESIPT. The enol form of 4-AP (Enol 4-AP) in the S1 state was calculated to be ~10 kcal/mol higher in energy than the keto form in the S1 state, indicating that keto-to-enol tautomerization was endergonic, ultimately resulting in no observable ESIPT for 4-AP in the S1 state. Upon the excitation of 4-AP to the S2 state, the transition to Enol-4-AP in the S1 state was found to be exergonic; however, ESIPT must compete with an internal conversion from the S2 to the S1 state. The internal S2 → S1 conversion was significantly faster than the solvent-assisted ESIPT, resulting in a negligible ESIPT for the 4-AP excited to the S2 state. The detailed excitation dynamics of 4-AP clearly reveal the molecular mechanism underlying its negligible ESIPT, despite the fact that it forms a favorable structure for solvent-assisted ESIPT.

Multichannel luminescence has gained significant traction across diverse fields such as optics, electronics, biology, etc. However, achieving the selective manifestation of multichannel emissions, particularly those involving delayed emission like thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP), from a solitary molecule remains a formidable challenge. Herein, an innovative strategy based on excited‐state intramolecular proton transfer (ESIPT) facilitates the transition between TADF and RTP. The intermolecular hydrogen bonds between 2‐(2′‐hydroxyphenyl)benzothiazole (HBT) derivatives and polyvinyl alcohol (PVA) are utilized to modulate the ESIPT potential barrier. Only under excitation &lt;370 nm, higher excited states are allowed to proceed to luminescence through ESIPT prior to transitioning to the lowest singlet states, defying Kasha's rule. Thus, the adjustment of the excitation wavelength allows for precise control over both the RTP of the enol configuration and the TADF of the keto configuration. Further addition of a third component, sulforhodamine B, achieves a red‐shifted afterglow via energy transfer.

We report a comprehensive study on the spectrodynamic behavior of 4-dimethylamino-2'-hydroxychalcone (DMHC) in crystalline state and in aggregated state in water (pH 7.0) by the aid of steady-state and time-resolved spectroscopy. The studies reveal that the excited-state intramolecular proton transfer (ESIPT) process is slow in the crystalline state (τESIPT = 3.0 ns) with the emission dominated by ESIPT from 600 nm onwards, with little contribution (550-570 nm) from the local emission with lower lifetime of emission (τlocal-emission = 140 ps). Spectrodynamic studies in the aqueous solution of DMHC shows the emission lifetime values are both lowered in the aqueous solution with emission from aggregates and the solvated DMHC molecules in aqueous solution (τlocal-emission = 90 ps and τESIPT = 35 ps and 1.4 ns). It is concluded that DMHC shows ESIPT emission from the J-aggregated forms, whereas in aqueous solutions, the ESIPT emission in water occurs from the H-aggregates. The current report holds importance owing to the detailed spectroscopic dissection of the aggregation phenomenon of a model aggregation induced enhancement of emission (AIE)-coupled ESIPT active molecule and delineates how the spectroscopic behavior of each aggregated form differs in terms of the nature of the aggregates formed (J- and H-aggregates), which shall aid in the understanding and construction of tailor-made AIE active molecules.