Arun Kumar Sundaresan

Templation of the Excited-State Chemistry of α-(n-Alkyl) Dibenzyl Ketones: How Guest Packing within a Nanoscale Supramolecular Capsule Influences Photochemistry

Excited-state behavior of eight alpha-alkyl dibenzyl ketones (alkyl = CH3 through n-C8H17) that are capable of undergoing type II and/or type I photoreactions has been explored in isotropic solution and within a water-soluble capsule. The study consisted of two parts: photochemistry that explored the excited-state chemistry and an NMR analysis that revealed the packing of each guest within the capsule. The NMR data (COSY, NOESY, and TOCSY experiments) revealed that ternary complexes between alpha-alkyl dibenzyl ketones and the capsule formed by two cavitands are kinetically stable, and the guests fall into three packing motifs modulated by the length of the alpha-alkyl chain. In essence, the host is acting as an external template to promote the formation of distinct guest conformers. The major products from all eight guests upon irradiation either in hexane or in buffer solution resulted from the well-known Norrish type I reaction. However, within the capsule the excited-state chemistry of the eight ketones was dependent on the alkyl chain length. The first group consisted of alpha-hexyl, alpha-heptyl, and alpha-octyl dibenzyl ketones that yielded large amounts of Norrish type II products within the host, while in solution the major products were from Norrish type I reaction. The second group consists of alpha-butyl and alpha-pentyl dibenzyl ketones that yield equimolar amounts of two rearranged starting ketones within the capsule (combined yield of ca 60%), while in solution no such products were formed. The third group consisted of alpha-methyl, alpha-ethyl, and alpha-propyl dibenzyl ketones that within the capsule yielded only one (not two) rearranged starting ketone in larger amounts (21-35%) while in solution no rearrangement product was obtained. Variation in the photochemistry of the guest within the capsule, with respect to the alpha-alkyl chain length of the guest, highlights the importance of how a small variation in supramolecular structure can influence the selectivity within a confined nanoscale reactor.

Guest rotations within a capsuleplex probed by NMR and EPR techniques.

With the help of (1)H NMR and EPR techniques, we have probed the dynamics of guest molecules included within a water-soluble deep cavity cavitand known by the trivial name octa acid. All guest molecules investigated here form 2:1 (host/guest) complexes in water, and two host molecules encapsulate the guest molecule by forming a closed capsule. We have probed the dynamics of the guest molecule within this closed container through (1)H NMR and EPR techniques. The timescales offered by these two techniques are quite different, millisecond and nanosecond, respectively. For EPR studies, paramagnetic nitroxide guest molecules and for (1)H NMR studies, a wide variety of structurally diverse neutral organic guest molecules were employed. The guest molecules freely rotate along their x axis (long molecular axis and magnetic axis) on the NMR timescale; however, their rotation is slowed with respect to that in water on the EPR timescale. Rotation along the x axis is dependent on the length of the alkyl chain attached to the nitroxide probe. Overall rotation along the y or z axis was very much dependent on the structure of the guest molecule. The guests investigated could be classified into three groups: (a) those that do not rotate along the y or z axis both at room and elevated (55 degrees C) temperatures, (b) those that rotate freely at room temperature, and (c) those that do not rotate at room temperature but do so at higher temperatures. One should note that rotation here refers to the NMR timescale and it is quite possible that all molecules may rotate at much longer timescales than the one probed here. A slight variation in structure alters the rotational mobility of the guest molecules.

Consequences of controlling free space within a reaction cavity with a remote alkyl group: photochemistry of para-alkyl dibenzyl ketones within an organic capsule in water

With the aim of controlling free space available for the reactants, photochemical reactivity of several 4-alkyl dibenzyl ketones included within a water-soluble self-assembled organic capsule (octa acid) has been investigated. One and two-dimensional NMR spectroscopic techniques were employed to characterize the structure of the guest@host complexes. Free space controlled by the remote alkyl group influences the distribution of photoproducts. Correlation between the structures of the guest@host complexes and products selectivity suggested that the free space that could be controlled by a remote tether has a determining role during a photoreaction within the restricted space of the capsule. The fact that the same alkyl tether has no effect on the photobehavior of 4-alkyl dibenzyl ketones in hexane solution suggests that the role of substituents in free (solution) and restricted space (supramolecular assemblies) is different. The current observation suggests that rules of physical organic chemistry and photochemistry developed based on solution chemistry cannot be simply extended to supramolecular assemblies.

Chiral photochemistry within a confined space: diastereoselective photorearrangements of a tropolone and a cyclohexadienone included in a synthetic cavitand

The value of a supramolecular assembly to enforce a closer interaction between a chiral auxiliary and a reaction center has been established using photoreactions of tropolone and cyclohexadienone derivatives. Two probe molecules utilized to establish the concept undergo 4 e- electrocyclization and oxa-di-pi-methane rearrangement from excited singlet and triplet state, respectively. The chiral auxiliaries investigated here has no/little effect in acetonitrile solution during phototransformations of the probe molecules to yield products with new chiral centers. On the other hand the same ones are able to enforce diastereoselectivities to the extent of approximately 30% when the reactions occur within the restricted space of a capsule made up of a synthetic cavitand commonly known as octa acid. Extensive NMR studies have been utilized to characterize the guest-host supramolecular structures. The results presented here should be of value in the overall understanding of chiral induction in photochemical reactions.

Polymer-bound photobase generators and photoacid generators for pitch division lithography

The semiconductor industry is pursuing several process options that provide pathways to printing images smaller than the theoretical resolution limit of 193 nm projection scanners. These processes include double patterning, side wall deposition and pitch division. Pitch doubling lithography (PDL), the achievement of pitch division by addition of a photobase generator (PBG) to typical 193 nm resist formulations was recently presented.1 Controlling the net acid concentration as a function of dose by incorporating both a photoacid generator (PAG) and a PBG in the resist formulation imparts a resist dissolution rate response modulation at twice the frequency of the aerial image. Simulation and patterning of 45 nm half pitch L/S patterns produced using a 90 nm half pitch mask were reported.2 Pitch division was achieved, but the line edge roughness of the resulting images did not meet the current standard. To reduce line edge roughness, polymer bound PBGs and polymer bound PAGs were investigated in the PDL resist formulations. The synthesis, purification, analysis, and functional performance of various polymers containing PBG or PAG monomers are described herein. Both polymer bound PBG with monomeric PAG and polymer bound PAG with monomeric PBG showed a PDL response. The performance of the polymer bound formulations is compared to the same formulations with small molecule analogs of PAG and PBG.

Photobase generator enabled pitch division: a progress report

Pitch division lithography (PDL) with a photobase generator (PBG) allows printing of grating images with twice the pitch of a mask. The proof-of-concept has been published in the previous paper and demonstrated by others. Forty five nm half-pitch (HP) patterns were produced using a 90nm HP mask, but the image had line edge roughness (LER) that does not meet requirements. Efforts have been made to understand and improve the LER in this process. Challenges were summarized toward low LER and good performing pitch division. Simulations and analysis showed the necessity for an optical image that is uniform in the z direction in order for pitch division to be successful. Two-stage PBGs were designed for enhancement of resist chemical contrast. New pitch division resists with polymer-bound PAGs and PBGs, and various PBGs were tested. This paper focuses on analysis of the LER problems and efforts to improve patterning performance in pitch division lithography.

Adiabatic ring opening in tethered naphthalene and anthracene cycloadducts

The cycloadducts of tethered naphthalene and anthracene derivatives undergo photochemical ring opening to an electronically excited product with adiabatic yields up to 90%.

Photochemistry of 2-diphenylmethoxyacetophenone. Direct detection of a long-lived enol from a Norrish Type II photoreaction.

Photolysis of 2-diphenylmethoxyacetophenone (1) in deaerated acetonitrile at room temperature is expected to proceed via Norrish Type II reaction to generate benzophenone (2) and acetophenone (4) as the main isolated photoproducts. Although the acetophenone enol intermediate (3) is generally considered a putative intermediate that is a precursor of acetophenone, there are few reports of the direct spectroscopic detection of 3 when a Type II reaction is conducted in solution at room temperature. Presumably, rapid enol-ketone isomerisation under the reaction conditions causes 3 to have a very short lifetime that lowers its concentration below limits detectable by standard organic spectroscopic methods such as (1)H NMR. We report that 3 is readily observed at room temperature by (1)H NMR spectroscopy and showed a remarkably long lifetime of almost 1 h under our reaction conditions. It was found that the acetophenone enols from the classic Norrish Type II reactions of valerophenone and butyrophenone could also be readily detected by (1)H NMR in acetonitrile at room temperature, but that their lifetimes were similar (tens of minutes) from both precursors, but were considerably shorter than that of 1. The reason for the differences in the lifetime of the acetophenone enol is probably due to small amounts of adventitious catalysts that arise during the photolysis.