Stereoselectivity in the triplet decay of chiral benzophenone–naphthalene bichromophoric systems

Chiral recognition in the intramolecular induced quenching of the methoxynaphthalene triplet by benzophenone has been observed by using diastereomeric bichromophores

Chiral recognition is a crucial phenomenon in biochemical systems as well as in technological applications.

It enables the rational design of pharmaceuticals, chiral sensors and molecular devices.1

In asymmetric organic photochemistry, chiral recognition in the excited state is important to achieve enantioselectivity during photosensitization and quenching processes.2

Our group has recently succeeded in proving the existence of enantioselective discrimination in the intramolecular quenching of benzoylthiophene-derived triplets by phenols and indoles.3

In view of the current interest on stereoselectivity in photophysical/photochemical processes we decided to look for a possible chiral discrimination in systems where triplet decay is controlled by induced quenching (IQ).

This has been defined as a type of quenching different from energy or electron transfer and does not lead to any photochemical product;4 it is inherent to photoreactions via triplet exciplexes4–9 They can compete with other events such as electron transfer (ET); actually, using benzophenone–anisole systems it has been demonstrated that the relationship between IQ rate constants and oxidation potentials follows Rehm–Weller behavior.

This implies involvement of the same intermediate for IQ and ET processes.4

Bichromophoric compounds containing benzophenone and naphthalene units constitute classical textbook systems, that have been used to study singlet–singlet and triplet–triplet energy transfer processes.6

In this respect, Shizuka and co-workers have studied benzophenone–methoxynaphthalene bichromophores BP–(CH2)3–NPOMe.

They suggested that deactivation of the methoxynaphthalene triplet, generated by intramolecular benzophenone photosensitization, occurs by interaction with the ground-state BP-moiety through triplet exciplexes with loose sandwich-like structures and weak charge transfer character.7–10

This appeared to be an adequate model system in order to investigate the possible stereoselectivity in the process by introducing chirality in the bichromophores.

To this end, enantiomerically pure compounds were obtained by condensation of (R)- and (S)-naproxol [NPX, 2-(6-methoxy-2-naphthyl)propan-1-ol] with (S)-ketoprofen [KP, (2S)-2-(3-benzoylphenyl)propanoic acid] in the presence of dicyclohexylcarbodiimide and 4-(dimethylamino)pyridine, using dry methylene chloride as solvent (Scheme 1).

The UV spectra of the bichromophoric compounds were merely the sum of the spectra of the individual chromophores, indicating little electronic interaction between the chromophores in the ground state (Fig. 1).

All the processes involved in the photoexcitation–deactivation of the obtained KP-NPX bichromophores are summarized in Scheme 1.

Fluorescence spectra were obtained at λexc = 318 nm for (S,R)- and (S,S)-KP-NPX (2.5 × 10–4 M) in argon-saturated acetonitrile solutions and compared to those of naproxen [2-(6-methoxy-2-naphthyl)propanoic acid] under the same conditions (Fig. 2).

As expected, the shapes of the three emision spectra were identical due to the nonemissive nature of the benzophenone singlet state.

However, the emission intensity was considerably lower for the bichromophoric compounds, although the concentrations were adjusted to match the same absorption by the methoxynaphthalene chromophore.

Thus, in the bichromophoric compounds the naphthalene singlet state is quenched by the benzophenone group, due to singlet–singlet energy transfer.11

Laser excitation at 355 nm of (S,R)- and (S,S)-KP-NPX (3.0 × 10–3 M) in N2-saturated acetonitrile solutions led to the excited KP singlet state which undergoes fast intersystem crossing (isc) to the KP triplet.

Intramolecular4,7,10,11 triplet–triplet energy transfer from 3KP (ET = 69 kcal mol–1)12 to the NPX moiety (ET = 62 kcal mol–1)9 results in the NPX triplet (λabs = 430 nm).

This must occur within the laser pulse duration, as evidenced by the transient absorption spectrum that corresponds to the triplet naphthalene-like chromophore (Fig. 3).

No absorption for triplet benzophenone was observed at 525 nm, even at short delays.

The resulting triplets, KP-3NPX*, decay to the ground state; the triplet decay traces measured at 430 nm showed that the (R) and (S) NPX triplets were quenched differently by the (S)-KP moiety (Fig. 4).

Thus, the 430 nm band decayed with first-order rate constants kd of 8.5 × 105 and 5.3 × 105 s–1 for (S,R)- and (S,S)-KP-NPX, respectively.13

The stereoselectivity, as shown by the ratio between the decay rate constants, was 1.6.

The sensitivity to pair configuration is a strong evidence for the involvement of intermediate (KP-NPX)* exciplexes in the NPX deactivation (steps (4) and (5), Scheme 1).

Furthermore, it is indicative of specific structural requirements in the formation of the sandwich-like intermediates involved in the induced quenching processes.

Diastereomeric discrimination, though associated with charge transfer quenching (rather than exciplex formation) has been previously reported for 1,2,3,4-tetraphenyl-1,4-diketones.14

In summary, we have synthesized two new chiral bichromophoric diastereomers, which are good models to demonstrate the involvement of through-space methoxynaphthalene–benzophenone interactions in the decay of the NPX triplet.

This process has been found to be stereoselective.