Unraveling the effect of solvents on the excited state dynamics of C540A by experimental and theoretical study

In this work, the excited-state dynamics including intramolecular charge transfer (ICT) and the redshift of C540A have been investigated in a series of solvents on the basis of the Kamlet-Taft solvatochromic parameters (π*, α, β) using femtosecond transient absorption spectra and systematic theoretical calculation. We demonstrate that the redshift of the emission peak has a linear relationship with the α and π* scales and the effect of the π* scale is slightly stronger than that of the α scale. Meanwhile, the ICT rates can be suggested as relevant to not only the α scale but also the π* scale. Additionally, C540A-AN has proved that the excited state molecules have a unique inactivation mechanism because of the dark feature of the S1 (CT) state. The valuable mechanistic information gleaned from the excited-state dynamics by the experimental and theoretical study would facilitate the design of organic materials for prospective applications in photochemistry and photobiology.

Theoretical Investigation of the Excited-State Dynamics Mechanism of the Asymmetric Two-Way Proton Transfer Molecule BTHMB

An asymmetric two-way proton transfer molecule 3-(benzo[d]-thiazol-2-yl)-2-hydroxy-5-methoxybenzaldehyde (BTHMB) with the function of white-light emission was synthesized in a recent experiment (Bhattacharyya, A.; Mandal, S. K.; Guchhait, N. J. Phys. Chem. A 2019, 123, 10246). The particularity of this molecule is that there are two possible forms, one of which contained a six-membered H-bonded network toward a N atom (BTHMB-NH) present in the molecule as a proton acceptor and the other was toward an O atom (BTHMB-OH). Unfortunately, the experimental work lacked the theoretical explanation about the determination of the BTHMB-NH form and its excited-state intramolecular proton transfer (ESIPT) process under different solvents. Therefore, this study has explored these two points by means of the time-dependent density functional theory (TDDFT) method. The calculated relative energy and potential energy profile (PEP) of the transformation between BTHMB-NH and BTHMB-OH forms illustrated that BTHMB-NH was more stable, and the transfer from BTHMB-NH to BTHMB-OH was almost impossible at both S₀ and S₁ states under all solvents due to high potential energy barriers (PEBs) (11.67-21.59 kcal/mol). These calculated results provided the theoretical explanation and verification for the conclusion that the BTHMB molecule exists in the BTHMB-NH form in the experiment. Subsequently, the constructed PEPs of the ESIPT process for BTHMB-NH have proved that it was prone to the ESIPT process due to low PEBs (0.11-0.28 kcal/mol) at the S₁ state. In particular, as the solvent polarity increased, the intensity of the intramolecular hydrogen bond (IHB) (O₃-H₄···N₅) increased and the ESIPT process was more likely to occur. In addition, the twisted intramolecular charge-transfer (TICT) process was studied to explore the possible fluorescence quenching pathway of BTHMB-NH. Based on the PEPs of BTHMB-NH-T as a function of the N₅-C₆-C₇-C₈ dihedral angle at the S₀ and S₁ states, it is seen that the S₀ state TICT process was inhibited due to the large PEBs (16.45-23.93 kcal/mol). Although the S₁ state PEBs have been greatly reduced, they were still maintained at about 3.60 kcal/mol (3.60-3.84 kcal/mol), and hence, this process was still relatively difficult to occur. Due to the fact that BTHMB can be regarded as a standard in future designs involving red light and solvent-specific white-light emitters, a certain amount of investigative work on the ESIPT process was done in detail, and it paved the way for future research on the directionality of ESIPT in double ESIPT probes.

Effect of number and different types of proton donors on excited-state intramolecular single and double proton transfer in bipyridine derivatives: theoretical insights

Intra-HBs are strengthened upon photoexcitation, confirmed by red-shift in vibrational mode and topology analysis. Number and type of donors result in difference in photophysical properties. Occurrence of ESIPT depends on barrier and reaction energy.

Rational design of nanosystems for simultaneous drug delivery and photodynamic therapy by quantum mechanical modeling

Drug delivery systems are based on reversible interactions between carriers and drugs. Spacers are often introduced to tailor the type of interaction and to keep drugs intact. Here, we model a drug delivery system based on a functionalized curved TiO₂ nanoparticle of realistic size (700 atoms - 2.2 nm) by the neurotransmitter dopamine to carry the anticancer chemotherapeutic agent doxorubicin (DOX). The multiscale quantum chemical study aims at unraveling the nature and mechanism of the interactions between the components and the electronic properties of the composite system. We simulate the temperature effect through molecular dynamics runs of thermal annealing. Dopamine binds preferentially to low coordinated Ti sites on the nanoparticle through dissociated bidentate and chelate modes involving the diol groups. DOX is tethered by H-bonds, π-π stacking, dipole-dipole interactions and dispersion forces. Comparing different coverage densities of the spacer on the nanoparticle surface, we assess the best conditions for an effective drug transport and release: only at full coverage, DOX does not slip among the dopamine molecules to reach the nanoparticle surface, which is crucial to avoid the formation of stable coordinative bonds with under-coordinated Ti atoms. Finally, given the strong absorption properties and fluorescence of DOX and of the TiO₂ photocatalyst, we model the effect of light irradiation through excited state calculations to localize excitons and to follow the charge carrier's life path. This fundamental study on the nature and mechanism of drug/carrier interaction provides a solid ground for the rational design of new experimental protocols for a more efficient drug transport and release and its combination with photodynamic therapy.

Photophysics of proton transfer in hydrazides: a combined theoretical and experimental analysis towards OLED device application

Effect of conjugation to support ESIPT with impossible double proton transfer in structurally favored species.

Effects of π-expansion, an additional hydroxyl group, and substitution on the excited state single and double proton transfer of 2-hydroxybenzaldehyde and its relative compounds: TD-DFT static and dynamic study

The effects of π-expansion, an extra hydroxyl group, and substituents on the photophysical properties, the excited state single proton transfer and the double proton transfer of 2-hydroxybenzaldehyde and its relatives have been theoretically investigated using TD-DFT.