Solvent-Controlled Excited State Behavior: 2-(2‘-Pyridyl)indoles in Alcohols

Dipolar Microenvironment Engineering Enabled by Electron Beam Irradiation for Boosting Catalytic Performance

Creating a diverse dipolar microenvironment around the active site is of great significance for the targeted induction of intermediate behaviors to achieve complicated chemical transformations. Herein, an efficient and general strategy is reported to construct hypercross-linked polymers (HCPs) equipped with tunable dipolar microenvironments by knitting arene monomers together with dipolar functional groups into porous network skeletons. Benefiting from the electron beam irradiation modification technique, the catalytic sites are anchored in an efficient way in the vicinity of the microenvironment, which effectively facilitates the processing of the reactants delivered to the catalytic sites. By varying the composition of the microenvironment scaffold structure, the contact and interaction behavior with the reaction participants can be tuned, thereby affecting the catalytic activity and selectivity. As a result, the framework catalysts produced in this way exhibit excellent catalytic performance in the synthesis of glycinate esters and indole derivatives. This manipulation is reminiscent of enzymatic catalysis, which adjusts the internal polarity environment and controls the output of products by altering the scaffold structure.

Coupling between tautomerism and radiationless deactivation in porphycenes

Quantum yields of fluorescence of porphycenes – porphyrin isomers – can vary by orders of magnitude, even for very similar derivatives, such as meso-dimethyl- vs. meso-tetramethylporphycene. In weakly emitting porphycenes the fluorescence intensity strongly depends on viscosity and can be recovered by placing a molecule in a rigid environment. We postulate that the efficient nonradiative deactivation is due to the quantum effect, delocalization of the inner protons. The delocalization, which increases with the strength of intramolecular hydrogen bonds may induce structural changes that lead to distortion from planarity and, as a result, efficient S0 ← S1 internal conversion. The effect seems to be general, as indicated by good correlation between the quantum yield of fluorescence and the distance between H-bonded nitrogen atoms, the latter being a reliable measure of hydrogen bonding strength. Based on the available photophysical and X-ray data, such correlation was found so far for over 20 differently substituted porphycenes.

Solvent effects on the photooxidation of indolepyrazines

Photodestruction of 2-(pyrazin-2'-yl)-1H-indole and 2,5-di(1H-indol-2'-yl)pyrazine involves singlet oxygen generation and its rapid insertion into the indole ring with the formation of benzoxazinone derivatives: 2-(pyrazin-2-yl)-4H-3,1-benzoxazin-4-one and 2-[5-(1H-indol-2-yl)pyrazin-2-yl]-4H-3,1-benzoxazin-4-one. The quantum yield of this reaction strongly depends on the environment. It is definitely smaller in protic methanol than in aprotic acetonitrile or n-hexane. The observed effect of photostabilization is explained by formation of hydrogen bonded complexes between the chromophore and alcohol, which results in lower triplet formation efficiency and, in consequence, decrease of singlet oxygen formation quantum yield.

Triple Emission of 5'-(para-R-Phenylene)vinylene-2-(2'-hydroxyphenyl)benzoxazole (PVHBO). Part I: Dual Emission from the Neutral Species

The effects of 5'-(para-R-phenylene)vinylene (PV) substituents on the emission properties of 2-(2'-hydroxyphenyl)benzoxazole (HBO) are analyzed using steady-state and time-resolved absorption and emission spectroscopies in addition to quantum chemical calculations. All members in the series of PVHBOs are capable of excited-state intramolecular proton transfer (ESIPT) with a solvent sensitivity that is typical of a HBO derivative to produce a normal (aka enol) emission and an excited-state tautomer (aka keto) emission. These two emission bands of the neutral dyes are discussed in the current paper. The intermolecular proton transfer, i.e., the deprotonation, of a PVHBO results in the third band of the triple emission, which is described in the succeeding paper. The placement of an electron-withdrawing substituent R on the PVHBO scaffold increases the intensity of the keto emission relative to the enol emission in hydrogen-bonding solvents. The R substituents do not significantly alter the wavelengths of the enol and keto emission bands, which are located in the blue and green regions, respectively, of the visible spectrum. The ultrafast time-resolved spectroscopies and quantum chemical calculations offer explanations on how the R group and the solvent affect the enol and keto emission properties (i.e., wavelength, lifetime, fluorescence quantum yield, and relative ratio of their emissions). The key findings include the following: (1) the emission energies of both enol and keto forms are not sensitively dependent on the R substituent and (2) the solvent-engaged enol excited state is quenched more efficiently as the R substituent becomes more electron-withdrawing. A PVHBO acts as a fusion of HBO and stilbenoid that intersect at the hydroxyphenyl moiety. Depending on the solvent and other environmental conditions, PVHBOs may exhibit the ESIPT property of HBO or the substituent-dependent emission of stilbenoid. This paper and the succeeding article provide a photophysical model of PVHBOs to explain the wavelengths and relative abundances of the three emission bands (enol, keto, and anion) that these compounds are able to produce. Judicial selection of the environmental factors may drive the emission of a PVHBO into the spectral regions of blue, green, and, in a couple of cases, orange or red.

Combined effect of hydrogen bonding interactions and freezing of rotameric equilibrium on the enhancement of photostability

Rigid structure and hydrogen-bonding interactions provide a higher photostability of organic chromophores.

Non-typical fluorescence studies of excited and ground state proton and hydrogen transfer

Fluorescence studies of tautomerization have been carried out for various systems that exhibit single and double proton or hydrogen translocation in various environments, such as liquid and solid condensed phases, ultracold supersonic jets, and finally, polymer matrices with single emitters. We focus on less explored areas of application of fluorescence for tautomerization studies, using porphycene, a porphyrin isomer, as an example. Fluorescence anisotropy techniques allow investigations of self-exchange reactions, where the reactant and product are formally identical. Excitation with polarized light makes it possible to monitor tautomerization in single molecules and to detect their three-dimensional orientation. Analysis of fluorescence from single vibronic levels of jet-isolated porphycene not only demonstrates coherent tunneling of two internal protons, but also indicates that the process is vibrational mode-specific. Next, we present bifunctional proton donor-acceptor systems, molecules that are able, depending on the environment, to undergo excited state single intramolecular or double intermolecular proton transfer. For molecules that have donor and acceptor groups located in separate moieties linked by a single bond, excited state tautomerization can be coupled to mutual twisting of the two subunits.

Benzimidazolylquinoxalines: novel fluorophores with tuneable sensitivity to solvent effects

We report on the photophysical properties, conjugation, conformational behavior, intra- and intermolecular hydrogen bonds (HBs) of a series of novel fluorophores, consisting of 3-arylquinoxaline and benzimidazole moieties linked by a single CC bond. Computations employing density functional theory (DFT) reveal that conjugation between these moieties stabilizes syn-conformers with two HB centers located on the same side of the molecule. Anti-conformers form stronger intermolecular HBs with DMSO and DMF than syn-conformers, and this influences the energy gap between syn- and anti-forms, especially upon excitation of the molecules to the S₁ state. Substituents introduced in various positions of the molecules modify their conformational behavior, and mutual disposition of excited singlet states relative to the ground states. Various substitution patterns produce very different effects on relative quantum yield of luminescence: from a moderate increase in polar DMSO and DMF relative to 1,2-dichloroethane solutions to complete quenching of emission which is observable in polar media. The observed behavior is understood with the aid of DFT and time-dependent DFT calculations. The tuneability of the spectroscopic range of the luminescence and especially of its sensitivity to environmental effects allows rational design of the novel fluorophores of this family for various applications.

The role of hydrogen bonding in the fluorescence quenching of 2,6-bis((E)-2-(benzoxazol-2-yl)vinyl)naphthalene (BBVN) in methanol

The excited state hydrogen bonding dynamics of BBVN in hydrogen donating methanol solvent was explored at the TD-BMK/cc-pVDZ level of theory with accounting for the bulk environment effects at the polarizable continuum model (PCM). The heteroatoms of the BBVN laser dye form hydrogen bonds with four methanol molecules. In the formed BBVN-(MeOH)₄ complex, the A-type hydrogen bond (N…HO), of an average strength of 25kJmol^-1, is twofold stronger than the B-type (O…HO) one. Upon photon absorption, the total HB binding energy increases from 78.5kJmol^-1 in the ground state to 82.6kJmol^-1 in the first singlet (S₁) excited state. In consequence of the hydrogen bonding interaction, the absorption band maximum of the BBVN-(MeOH)₄ complex, which was anticipated at 398nm (exp. 397), is redshifted by 5nm relative to that of the free dye in methanol. The spectral shift of the stretching vibrational mode for the hydrogen bonded hydroxyl groups (with a maximum shift of 285cm^-1) from that of the free methanol indicated the elevated strengthening of hydrogen bonds in the excited state. The vibrational modes associated with hydrogen bonding provide effective accepting modes for the dissipation of the excitation energy, thus, decreasing the fluorescence quantum yield of BBVN in alcohols as compared to that in the polar aprotic solvents. Since there is no sign of photochemistry or phosphorescence, it seems reasonable in view of the outcomes of this study to assign the major decay process of the excited singlet (S₁) of BBVN in alcohols to vibronically induced internal conversion (IC) facilitated by hydrogen bonding.

New insights into the solvent-assisted excited-state double proton transfer of 2-(1H-pyrazol-5-yl)pyridine with alcoholic partners: a TDDFT investigation

Excited-state double proton transfer (ESDPT) in the hydrogen-bonded 2-(1H-pyrazol-5-yl)pyridine with propyl alcoholic partner (PPP) was theoretically investigated by time-dependent density functional theory (TDDFT) method. Great changes have taken place for the calculated geometric structures, the electron density features and vibrational spectrum of PPP system in S0 and S1 state. Our results have demonstrated that ESDPT reaction happens within the system upon photoexcitation. We also found that the ESDPT process is facilitated by the electronically excited state intermolecular hydrogen bond strengthening. Particularly, after the photoexcitation from HOMO(π) to the LUMO(π(∗)), the rearrangement of electronic density distribution of frontier molecular orbitals (MOs) on pyridine and the pyrazol moieties exhibits a very important positive factor for the ESDPT. Furthermore, by the investigation of the stretching vibrations of NH and OH groups, the infrared (IR) spectroscopic results provide us not only a theoretical evidence of ESDPT, but also a considerable clue to characterize the nature of intermolecular reaction. In addition, efforts have also been devoted towards calculating the absorption peak, which shows good consistency with the experimental result of the studied system.

TDDFT study on the excited-state proton transfer of 8-hydroxyquinoline: key role of the excited-state hydrogen-bond strengthening

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations have been employed to study the excited-state intramolecular proton transfer (ESIPT) reaction of 8-hydroxyquinoline (8HQ). Infrared spectra of 8HQ in both the ground and the lowest singlet excited states have been calculated, revealing a red-shift of the hydroxyl group (-OH) stretching band in the excited state. Hence, the intramolecular hydrogen bond (O-H···N) in 8HQ would be significantly strengthened upon photo-excitation to the S1 state. As the intramolecular proton-transfer reaction occurs through hydrogen bonding, the ESIPT reaction of 8HQ is effectively facilitated by strengthening of the electronic excited-state hydrogen bond (O-H···N). As a result, the intramolecular proton-transfer reaction would occur on an ultrafast timescale with a negligible barrier in the calculated potential energy curve for the ESIPT reaction. Therefore, although the intramolecular proton-transfer reaction is not favorable in the ground state, the ESIPT process is feasible in the excited state. Finally, we have identified that radiationless deactivation via internal conversion (IC) becomes the main dissipative channel for 8HQ by analyzing the energy gaps between the S1 and S0 states for the enol and keto forms.

Polytriazole bridged with 2,5-diphenyl-1,3,4-oxadiazole moieties: a highly sensitive and selective fluorescence chemosensor for Ag+

Fluorescent conjugated polytriazoles (FCP 1–4) containing both 2,5-diphenyl-1,3,5-oxadiazole (OXD) and 1,2,3-triazole moieties in the main chain were synthesized from aromatic diazide (1) and dialkynes (2–5) via click polymerization, respectively.

Photophysics, excited-state double-proton transfer and hydrogen-bonding properties of 5-deazaalloxazines

The photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine were studied in different solvents. These compounds have higher values of fluorescence quantum yields and longer fluorescence lifetimes, compared to those obtained for their alloxazine analogs. Electronic structure and S0 -Si transitions were investigated using the ab initio methods [MP2, CIS(D), EOM-CCSD] with the correlation-consistent basis sets. Also the time-dependent density functional theory (TD-DFT) has been employed. The lowest singlet excited states of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine are predicted to have the π, π* character, whereas similar alloxazines have two close-lying π, π* and n, π* transitions. Experimental steady-state and time-resolved spectral studies indicate formation of an isoalloxazinic excited state via excited-state double-proton transfer (ESDPT) catalyzed by an acetic acid molecule that forms a hydrogen bond complex with the 5-deazaalloxazine molecule. Solvatochromism of both 5-deazaalloxazine and its 1,3-dimethyl substituted derivative was analyzed using the Kamlet-Taft scale and four-parameter Catalán solvent scale. The most significant result of our studies is that the both scales show a strong influence of solvent acidity (hydrogen bond donating ability) on the emission properties of these compounds, indicating the importance of intermolecular solute-solvent hydrogen-bonding interactions in their excited state.

Investigation of excited-state intramolecular proton transfer coupled charge transfer reaction of paeonol

An excited-state intramolecular proton transfer (ESIPT) coupled charge transfer reaction of paeonol was investigated both experimentally and theoretically. The ESIPT reaction of paeonol was predicted based on the large Stokes shift, which is observed in steady-state absorption and fluorescence spectra in an ethanol solution. The steady-state spectra in some solutions, such as methanol, ethanol, propanol, dichloromethane, and n-hexane, illustrate that the ESIPT reaction of paeonol has no dependence on the solvent properties. Therefore, the excited-state intermolecular proton transfer cannot be generated in protic solvents. Using the density functional theory and time-dependent density functional theory methods, we make a subsequent theoretical calculation that indicates that the ESIPT reaction of paeonol occurs through the intramolecular hydrogen bond O−H···O=C. The excited-state potential energy curve of paeonol indicates that the ESIPT reaction is a barrierless process, and the fluorescence emission of paeonol at 493 nm in the ethanol solution was assigned to the keto isomer fluorescence. Additionally, we also found an intramolecular charge transfer in the excited state by analysing the frontier molecular orbitals of paeonol.

Excited state intramolecular proton transfer in π-expanded phenazine-derived phenols

Two previously inaccessible analogs of 10-hydroxybenzo[h]quinoline were prepared via a straightforward strategy comprising the formation of π-expanded phenazines skeleton followed by C-H acetoxylation at position 10. Two bis-phenols possessing C2 and D2 symmetry were obtained in yields of 52% and 15%, respectively. The occurrence of excited state intramolecular proton transfer (ESIPT) was detected in all cases because steady state emission was observed only from the excited keto-tautomer. Additionally, a short-lived, ∼0.1 ps, emission decay was resolved by the femtosecond up-conversion technique at the blue side of the keto-tautomer emission band, 610 nm, and was attributed to the ESIPT, i.e., conversion from enol to keto tautomer. In comparison with the corresponding 10-hydroxybenzo[h]quinoline emissions, the emission spectrum of the π-expanded phenazine analogues were weaker but displayed a characteristic bathochromically shift into NIR region. These phenazine analogues constitute one of largest heterocycles for which ESIPT was unambiguously detected.

Excited state intramolecular proton transfer in electron-rich and electron-poor derivatives of 10-hydroxybenzo[h]quinoline

Eight previously inaccessible derivatives of 10-hydroxybenzo[h]quinoline were prepared via a straightforward strategy comprising formation of the benzo[h]quinoline skeleton followed by C-H acetoxylation at position 10. The occurrence of excited state intramolecular proton transfer (ESIPT) was detected in all cases since emission was observed only from the excited keto-tautomer. Studies on derivatives bearing both electron-donating and electron-withdrawing groups adjacent to the pyridine ring allowed us to identify some design patterns giving rise to NIR emission and large Stokes shifts. For a derivative of 10-hydroxybenzo[c]acridine, emission at 745 nm was observed, one of the lowest energy fluorescence ever reported for ESIPT system. On the basis of time-resolved measurements, proton transfer was found to be extremely fast with time constants in the range (0.08-0.45 ps).

Three modes of proton transfer in one chromophore: photoinduced tautomerization in 2-(1H-pyrazol-5-yl)pyridines, their dimers and alcohol complexes

Studies of 2-(1H-pyrazol-5-yl)pyridine (PPP) and its derivatives 2-(4-methyl-1H-pyrazol-5-yl)pyridine (MPP) and 2-(3-bromo-1H-pyrazol-5-yl)pyridine (BPP) by stationary and time-resolved UV/Vis spectroscopic methods, and quantum chemical computations show that this class of compounds provides a rare example of molecules that exhibit three types of photoreactions: 1) excited-state intramolecular proton transfer (ESIPT) in the syn form of MPP, 2) excited-state intermolecular double-proton transfer (ESDPT) in the dimers of PPP in nonpolar media, as well as 3) solvent-assisted double-proton transfer in hydrogen-bonded 1:1 complexes of PPP and MPP with alcoholic partners. The excited-state processes are manifested by the appearance of a dual luminescence and a bimodal irreversible kinetic coupling of the two fluorescence bands. Ground-state syn-anti equilibria are detected and discussed. The fraction of the higher-energy anti form varies for different derivatives and is strongly dependent on the solvent polarity and hydrogen-bond donor or acceptor abilities.

TIME-DEPENDENT DENSITY FUNCTIONAL THEORY STUDY ON DYNAMICS OF HYDROGEN BONDING IN EXCITED STATES OF TRANS-ACETANILIDE IN METHANOL SOLVENT

The hydrogen-bonding dynamics in both singlet and triplet excited states of the trans-acetanilide ( AA ) in methanol ( MeOH ) solvent was investigated using the time-dependent density functional theory (TDDFT) method. Geometric optimizations of the hydrogen-bonded AA–MeOH complexes considered here as well as the isolated AA and MeOH molecules were performed using density functional theory (DFT) method. At the same time, the TDDFT method was performed to calculate the electronic transition energies and corresponding oscillation strengths of all the compounds in the low-lying electronically excited states. In this study, only the intermolecular hydrogen bonds C=O⋯H–O and N–H⋯O–H can be formed. A theoretical forecast that changes of hydrogen bonds in the low-lying electronic excited states was proposed. We discussed not only ground-state geometric structures and electronic excitation energies but also frontier molecular orbitals and electron density transition. The intermolecular hydrogen bonds between AA and MeOH molecules play an important role in the geometric structures and electronic excitation energies. Zhao et al. have put forward the relationship between the electronic spectra and hydrogen bonding dynamics for the first time. According to Zhao's rule, a redshift of the relevant electronic spectra will appear if hydrogen bond is strengthened, while the hydrogen bond weakening can make an electronic spectra shift to blue.

Partitioning of 2,6-Bis(1H-Benzimidazol-2-yl)pyridine fluorophore into a phospholipid bilayer: complementary use of fluorescence quenching studies and molecular dynamics simulations

Successful use of fluorescence sensing in elucidating the biophysical properties of lipid membranes requires knowledge of the distribution and location of an emitting molecule in the bilayer. We report here that 2,6-bis(1H-benzimidazol-2-yl)pyridine (BBP), which is almost non-fluorescent in aqueous solutions, reveals a strong emission enhancement in a hydrophobic environment of a phospholipid bilayer, making it interesting for fluorescence probing of water content in a lipid membrane. Comparing the fluorescence behavior of BBP in a wide variety of solvents with those in phospholipid vesicles, we suggest that the hydrogen bonding interactions between a BBP fluorophore and water molecules play a crucial role in the observed "light switch effect". Therefore, the loss of water-induced fluorescence quenching inside a membrane are thought to be due to deep penetration of BBP into the hydrophobic, water-free region of a bilayer. Characterized by strong quenching by transition metal ions in solution, BBP also demonstrated significant shielding from the action of the quencher in the presence of phospholipid vesicles. We used the increase in fluorescence intensity, measured upon titration of probe molecules with lipid vesicles, to estimate the partition constant and the Gibbs free energy (ΔG) of transfer of BBP from aqueous buffer into a membrane. Partitioning BBP revealed strongly favorable ΔG, which depends only slightly on the lipid composition of a bilayer, varying in a range from -6.5 to -7.0kcal/mol. To elucidate the binding interactions of the probe with a membrane on the molecular level, a distribution and favorable location of BBP in a POPC bilayer were modeled via atomistic molecular dynamics (MD) simulations using two different approaches: (i) free, diffusion-driven partitioning of the probe molecules into a bilayer and (ii) constrained umbrella sampling of a penetration profile of the dye molecule across a bilayer. Both of these MD approaches agreed with regard to the preferred location of a BBP fluorophore within the interfacial region of a bilayer, located between the hydrocarbon acyl tails and the initial portion of the lipid headgroups. MD simulations also revealed restricted permeability of water molecules into this region of a POPC bilayer, determining the strong fluorescence enhancement observed experimentally for the membrane-partitioned form of BBP.

Partitioning and localization of environment-sensitive 2-(2'-pyridyl)- and 2-(2'-pyrimidyl)-indoles in lipid membranes: a joint refinement using fluorescence measurements and molecular dynamics simulations

Fluorescence of environment-sensitive dyes is widely applied to monitor local structure and solvation dynamics of biomolecules. It has been shown that, in comparison with a parent indole fluorophore, fluorescence of 2-(2'-pyridyl)-5-methylindole (5M-PyIn-0) and 2-[2'-(4',6'-dimethylpyrimidyl)]-indole (DMPmIn-0) is remarkably sensitive to hydrogen bonding with protic partners. Strong fluorescence, observed for these compounds in nonpolar and polar aprotic solvents, is efficiently quenched in aqueous solution. This study demonstrates that 5M-PyIn-0 and DMPmIn-0, which are almost nonemitting in aqueous solution, become highly fluorescent upon titrating with phospholipid vesicles. The fluorescence enhancement is accompanied by a significant blue shift of emission maximum. The Gibbs free energy of membrane partitioning, measured by the increase in the steady-state fluorescence intensities during transfer from an aqueous environment to a lipid bilayer, is very favorable for both compounds, being in a range from -7.1 to -8.0 kcal/mol and depending only slightly on lipid composition of the membrane. The fluorescence enhancement upon membrane partitioning is indicative of the loss of the specific hydrogen-bonding interactions between the excited fluorophore and water molecules, causing efficient fluorescence quenching in bulk water. This conclusion is supported by atomistic molecular dynamics (MD) simulations, demonstrating that both 5M-PyIn-0 and DMPmIn-0 bind rapidly and partition deeply into a lipid bilayer. MD simulations also show a rapid, nanosecond-scale decrease in the probability of solute-solvent hydrogen bonding during passive diffusion of the probe molecules from bulk water into a lipid bilayer. At equilibrium conditions, both 5M-PyIn-0 and DMPmIn-0 prefer deep localization within the hydrophobic, water-free region of the bilayer. A free energy profile of penetration across a bilayer estimated using MD umbrella sampling shows that both indole derivatives favor residence in a rather wide potential energy well located 10-15 Å from the bilayer center.

On the origin of fluorescence quenching of pyridylindoles by hydroxylic solvents

Three isomeric 4'-pyridyl-substituted indoles, with the substituent in positions 2, 3 and 7, reveal strong fluorescence in aprotic solvents, both polar and nonpolar, whereas the emission is strongly quenched in water and alcohol solutions. Both viscosity and alcohol acidity play a role in efficient excited state deactivation. The process becomes faster for more acidic alcohols. It can be slowed down by increasing viscosity, which indicates that the proton movement is accompanied by large amplitude motions in the hydrogen-bonded complex. Quenching is not observed upon formation of solvates in which pyridylindoles act as hydrogen bond donors. The experimental results, combined with calculations can be explained by a model which assumes excited state protonation of the pyridine nitrogen atom, followed by twisting of the pyridyl group, leading to a low-energy structure. An alternative mechanism is also considered, in which the excited state proton transfer is accompanied by electron transfer from water or alcohol into a half-filled orbital of the chromophore, which leads to a conical intersection of the S(1) and S(0) energy surfaces.

Photochemical deuterium exchange in phenyl-substituted pyrroles and indoles in CD3CN-D2O

A new mechanism of photochemical deuteration of some phenylpyrroles and indoles is reported. Irradiation of 2-phenylpyrrole (8), 2-phenylindole (9), and 7-phenylindole (12) in CH(3)CN-D(2)O gives rise to deuterium exchange at the C-atoms of the heterocycle and the adjacent phenyl ring. Photolysis of indole (7), 8, 9, 12 and N-methyl-2-phenylindole (10) in CD(3)CN-D(2)O also leads to deuteration at C-atoms with significantly higher yield. The mechanism most probably involves ejection of an electron on excitation and formation of radical cations that abstract D-atoms from CD(3)CN or undergo bimolecular D-abstraction (photolysis in CH(3)CN-D(2)O). The other possible mechanism of deuterium exchange for 7, 8, 9 and 12 may take place via a homolytic N-D cleavage and recombination of the radical pair. Radical cations of 8, 9 and 10 were detected by laser flash photolysis. Steady state and time-resolved fluorescence of 8, 9, 10 and 12 showed that photoinduced intramolecular electron transfer probably does not occur. The fluorescence was quenched by acid and base and the rate constants determined by Stern-Volmer analyses. The estimated pK(a) values for the protonation and deprotonation of the singlet excited states indicated that in neutral aqueous solutions ESPT probably does not take place, additionally corroborating the radical or radical cationic mechanism of deuteration.