Theoretical study on the excited-state photoinduced electron transfer facilitated by hydrogen bonding strengthening in the C337–AN/MAN complexes

Individual control of singlet lifetime and triplet yield in halogen-substituted coumarin derivatives

The photophysical properties of three 3-diethylphosphonocoumarin derivatives are studied by transient absorption spectroscopy and DFT calculations. The measured lifetime of the first excited singlet state changes upon halogen substitution at the 6-position from 40 ps for the unsubstituted compound to 100 ps for Cl and 24 ps for Br. This observation is in clear contradiction with the estimated singlet–triplet quantum yield, which increases with atomic weight of the substituted atom and is usually referred as a heavy-atom effect. The DFT calculations give evidence that the main reason for this behavior is the different composition of the HOMO, while the LUMO is similar for all three compounds. The optical excitation leads to intramolecular charge transfer from the halogen lone pairs to the π* molecular orbital and thus to a significant change in the molecular dipole moment. Hence, the latter phenomenon in combination with the heavy-atom effect enables an independent control of singlet lifetime and singlet–triplet quantum yield in the studied 3-diethylphosphonocoumarin derivatives.

The photophysical properties of three 3-diethylphosphonocoumarin derivatives are studied by transient absorption spectroscopy and DFT calculations.

Fluorescence quenching by photoinduced electron transfer between 7-methoxycoumarin and guanine base facilitated by hydrogen bonds: an in silico study

In this study, the effects of hydrogen bond (H-bond) formation on fluorescence quenching of 7-methoxycoumarin (7MC) via photo-induced electron transfer from a guanine base (Gua) are investigated using a combined quantum mechanics/molecular mechanics simulation. The electronic structure is calculated by the floating occupation molecular orbital complete active space configuration interaction modification on a semiempirical method. Then the full multiple spawning method is employed for the dynamics simulations on multiple electronic states. The methods employed here are validated by simulating direct dynamics of 7MC (without Gua) and compared with available experimental results. Our computational results are in good agreement with the previously reported experimental results in terms of spectroscopic properties of 7MC. In the case of a H-bonded 7MC-Gua complex, the results from constrained dynamics simulations and single-point calculations suggest that the electron transfer occurs on the second excited state and it depends not only on the H-bond length but also on the intermolecular planarity between 7MC and Gua. Moreover, a proton coupled electron transfer can occur at ≈1 Å of H-bond length, where a proton from Gua is also transferred together with the electron to 7MC. The obtained simulations are expected to be greatly beneficial for designing effective fluorescently labeled nucleotide probes as well as providing information for precise fluorescence signal interpretation.

Anomalous modulation of photoinduced electron transfer of coumarin 102 in aniline-dimethylaniline mixture: dominant role of hydrogen bonding

In a previous study, we reported a striking observation that photoinduced electron transfer (PET) from aniline (AN) to photoexcited coumarin 102 (C102) can be accelerated by adding an inert component (cyclohexane or toluene) to the neat electron donor solvent AN (Phys. Chem. Chem. Phys., 2014, 16, 6159-6166). The H-bond linking the electron donor (D, AN) and the acceptor (A, C102) was proposed to dictate the PET process. To account for the unusual variation of quenching pattern with AN mole fraction, two possible reasons were cited - (1) the D-A (AN-C102) H-bonding may be modulated due to change in polarity of the medium or (2) the additional D-D (AN-AN) H-bonding may restrain the D-A H-bonding to adjust optimally for the PET. Here, we investigate the PET of C102 in an AN-dimethylaniline (DMA) mixture to negate the polarity variation. Since, both AN and DMA have similar polarities, the polarity of the mixture should remain invariant at all compositions. Nevertheless, we found that the fluorescence quantum yield and lifetime of C102 in the mixtures follows a similar unusual trend as observed earlier in the AN-toluene or AN-cyclohexane mixtures; it first decreases up to a particular mole fraction (XD) of the H-bond donor AN and, thereafter, increases on further enrichment of the donor. The observed PET modulation may be rationalized by considering efficient PET in the 1 : 1 H-bonded C102-AN complex but less efficient PET in higher order C102-(AN)n≥2 complexes, where additional D-D (AN-AN) H-bonding may influence the key C102-AN H-bonding and thus inhibit the PET process.

Faster photoinduced electron transfer in a diluted mixture than in a neat donor solvent: effect of excited-state H-bonding

In a neat electron-donating solvent (in this case aniline), photoinduced electron transfer (PET) from the solvent to an excited acceptor (e.g. a coumarin fluorophore) may be anticipated to be the most efficient because of the close contact of the acceptor with many donors. Addition of an inert component would most likely retard the PET process by replacing some donors from the neighbourhood of the acceptors. Surprisingly, we found dramatic acceleration of PET (6-10 fold enhancement compared to neat aniline), for coumarin 102 (C102) dissolved in a binary mixture of aniline and an inert solvent (cyclohexane or toluene). The PET induced fluorescence follows an anomalous trend against the mole fraction of aniline (XAN); first quenches up to certain XAN (0.075 for cyclohexane; 0.13 for toluene), thereafter, enhances with increase in XAN. Although the non-interacting component cannot directly participate in the PET process, it may modulate C102-aniline H-bonding association by changing the polarity of the medium or by disrupting the aniline-aniline H-bond. The study clearly illustrates the dominant role of hydrogen bonding in activating the electron transfer rate where standard thermodynamics predicts very weak donor-acceptor interaction.

Modulation of ultrafast photoinduced electron transfer in H-bonding environment: PET from aniline to coumarin 153 in the presence of an inert co-solvent cyclohexane

A passive component is found to boost H-bond assisted PET in a mixture using femtosecond fluorescence measurements and MD simulation.

Ultrafast hydrogen bond dynamics and partial electron transfer after photoexcitation of diethyl ester of 7-(diethylamino)-coumarin-3-phosphonic acid and its benzoxaphosphorin analog

The solvation dynamics after optical excitation of two phosphono-substituted coumarin derivatives dissolved in various solutions are studied by fluorescence up-conversion spectroscopy and quantum chemical simulations.

Time-dependent density functional theory study on the excited-state intramolecular proton transfer in salicylaldehyde

Time-dependent density functional theory method was performed to investigate the excited state intramolecular hydrogen bond dynamics of salicylaldehyde (SA). The geometric structures and IR spectra in the ground state S0 state and the excited state S1 state of SA are calculated using the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) methods, respectively. In addition, the absorption and fluorescence peaks are also calculated using TDDFT methods. It is noted that the calculated large Stokes shift is in good agreement with the experimental results. Furthermore, our results have demonstrated that the excited state intramolecular proton transfer (ESIPT) process happens upon photoexcitation, which are distinct monitored by the formation and disappearance of the characteristic peaks of IR spectra involved in the formation of hydrogen bonds in different states and in the potential energy curves. We find that the hydrogen bonded quasi-aromatic chelating ring in the excited state becomes smaller which can facilitate the ESIPT process. The results presented here suggest that the ESIPT process of the SA molecule in the excited state can be attributed to the electronegativity change of O1 induced by excitation.

Intermolecular hydrogen bond and twisted intramolecular charge transfer in excited state of fast violet B (FVB) in methanol solution

The excited state hydrogen bonding dynamics and corresponding photophysical processes of fast violet B (FVB) in hydrogen-donating methanol (MeOH) solution are investigated by using time-dependent density functional theory (TDDFT) method. In the FVB molecule, there are -C=O, -N-H groups which could act as hydrogen acceptor and donor. It is demonstrated that both the intramolecular hydrogen bond O⋯H-N in FVB and intermolecular hydrogen bond C=O⋯H-O between FVB and MeOH are formed in the ground state S0 and strengthened in the excited state S1. The absorption spectra are obviously red shifted for the hydrogen-bonded complex in comparison with FVB monomer in the low energy range. The theoretical investigation demonstrates that the twisted intramolecular charge transfer takes place in the excited states for both isolated FVB and hydrogen-bonded complex, and the dominant twisting is along N2-C3 bond. The potential energy curve is investigated to understand the photophysics process of FVB and hydrogen-bonded complex.

Multiple hydrogen bonding in excited states of aminopyrazine in methanol solution: time-dependent density functional theory study

Aminopyrazine (AP) and AP-methanol complexes have been theoretically studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The excited-state hydrogen bonds are discussed in detail. In the ground state the intermolecular multiple hydrogen bonds can be formed between AP molecule and protic solvents. The AP monomer and hydrogen-bonded complex of AP with one methanol are photoexcited initially to the S2 state, and then transferred to the S1 state via internal conversion. However the complex of AP with two methanol molecules is directly excited to the S1 state. From the calculated electronic excited energies and simulated absorption spectra, we find that the intermolecular hydrogen bonds are strengthened in the electronic excited states. The strengthening is confirmed by the optimized excited-state geometries. The photochemical processes in the electronic excited states are significantly influenced by the excited-state hydrogen bond strengthening.

Fluorescence of tryptophan in aqueous solution

In this work, the absorption and emission spectra of Tryptophan (Trp) in aqueous solution were studied. Moreover, a hydrogen-bonded zwitterionic Trp(H2O)9 model was proposed and its ground-state and excited-state properties were investigated using the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT) methods, respectively. All spectroscopic data in our experiments can be well explained by the hydrogen bond strengthening in the excited state of the model complex. The delocalization of electron density between indole moiety and neighboring H2O molecules in fluorescent state was proposed to be facilitated by the strengthened hydrogen-bond chain, and thus resulting in the large red-shift fluorescence of Trp in aqueous solution.