Side‐chain orientation of poly[N⁵‐(R and S)‐1‐(1‐naphthyl)ethyl‐L‐glutamines] in mixed solvents of 1,4‐dioxane and trifluoroacetic acid

Abstract

IntroductionThe photoprocesses of aromatic chromophores introduced in polymers differ, in many aspects, from those of low-molecular-weight analogues '). In solution, they may be regarded as covalently-bonded chromophore aggregates and possess unique photophysical as well as photochemical properties, e. g., intramolecular excitation energy migration and photosensitizati~n~*~). . Chromophores introduced in the side chains of helical polypeptides, in particular, are arranged at regular intervals and can be further oriented rigidly by side-chainhide-chain and/or side-chain/main-chain interactions. In fact, induced circular dichroism (CD) is often observed when aromatic chromophores are bound to the side chains of helical polypeptides4-'). Sisido et al.,for example, prepared poly(L-1-and L-2-naphthylalanines) and observed strong CD signals indicative of exciton coupling in the 'B, band of the naphthalene chromophore in trimethyl phosphate solution and attributed them to rigid orientation of the naphthalene chromophores along the helical polypeptide chain4x5). Ueno et al. ' ), on the other hand, examined induced CD in the 'B, band of poly(y-1-naphthylmethyl Lglutamate) and its dependence on the solvent composition in the mixed solvents of dichloromethane and hexafluoroisopropyl alcohol, but found no concrete evidence for exciton coupling. The flexible ester linkage, as well as the long spacer chain between the chromophore and the main chain, apparently causes looser chromophore orientation and thus weaker interaction among the chromophores. c=o NH

Side-Chain Chromophore Orientation and Excitation Energy Transport in Films Prepared from Poly[N5-(R)- and N5-(S)-1-(1-naphthyl)ethyl-l-glutamines]

“…Chromophores introduced in the side chains of polypeptides are arranged at regular intervals along the helical main chain and can be further oriented with respect to one another and with respect to the main chain by the side-chain/side-chain and side-chain/main-chain interactions. − In our previous studies, we prepared poly( l -glutamines), i.e., diastereomeric poly[ N 5 -( R )- and N 5 -( S )-1-(1-naphthyl)ethyl- l -glutamines] ( 1 and 2 ), where chiral ( R )- or ( S )-1-(1-naphthyl)ethyl groups are introduced in the side chains via amide linkage, and examined their fluorescence and CD spectra in dioxane as well as in mixed solvents of dioxane and trifluoroacetic acid (TFA). , The side-chain naphthalene chromophores in 1 are found to be rather rigidly oriented along the helical main chain in dioxane, suppressing excimer formation and giving rather strong CD signals indicative of exciton coupling in the 1 B b band of the naphthalene chromophore. The rigid nature of the side-chain amide linkage as well as the steric effects among the bulky side chains evidently plays an important role in orienting the naphthalene chromophores.…”

Intramolecular Hydrogen Bonding in Isotactic Poly(methacrylamide)s and Its Implications for Control of Side-Chain Orientation

“…5 Previously, we have prepared poly(L-glutamines) where naphthalene chromophores are introduced in the side chain via amide linkage and examined how the sidechain structure affects the hydrogen bonding interactions among the side-chain amide groups and subsequent ordering of the side-chain chromophores. 6 In the present study we prepared isotactic and atactic polymethacrylamides, i.e., poly(l-naphthylmethyl methacrylamide) (1) and poly[(S)-1-(1-naphthyl)ethyl methacrylamide] (2), where naphthalene chromophores are likewise introduced in the side chains via amide linkage and examined how the stereoregularity and the side-chain structure affect the hydrogen bonding interactions among the side-chain amide groups and the subsequent orientation of the side-chain chromophores. 1 To whom correspondence should be addressed.…”

Temperature Dependence of Circular Dichroism and Fluorescence Decay of Pyrene Appended to the Side Chains of Poly-l-glutamine

“…1 and 2 were prepared from poly( l -glutamic acid) (DP 385, Aldrich Co.) and ( R )- and ( S )-1-(1-pyrenyl)ethylamines with an amine to carboxylic acid molar ratio of 2 using N , N -dicyclohexylcarbodiimide and 1-hydroxybenzotriazole as condensing agents as previously described. , Full derivatization of the side-chain carboxyl groups was confirmed by 500 MHz 1 H NMR in DMSO- d 6 at 120 °C as shown in Figure . The DP 70 samples were similarly prepared for IR measurements.…”

“…The crystal structure of the photosystem in photosynthetic bacteria has been revealed by X-ray crystal analysis, showing that the arrangement of the light-harvesting antenna pigments is highly controlled by polypeptide frameworks so as to bring rapid and efficient excitation energy transport to the photochemical reaction center. − Polypeptides may thus be considered as promising molecular frameworks to arrange photofunctional chromophores in artificial photosynthesis and other photoelectronic processes. − If the orientation of the chromophores in the side chains of a helical polypeptide is highly controlled, we would have “molecular wires” capable of efficiently transporting excitation energy along the polymer backbone. − …”

“…In the previous studies, we prepared poly( l -glutamines) having naphthalene and pyrene chromophores in the side chains, i.e., poly[ N 5 -( R and S )-1-(1-naphthyl)ethyl- l -glutamines], , poly[ N 5 -1-naphthylmethyl- l -glutamine], and poly( N 5 -1-pyrenylmethyl- l -glutamine), and examined their side-chain chromophore orientation along the helical main chain by CD and fluorescence spectroscopy. Examining the CD signals in relation to the side-chain chromophore structure, we concluded that the chirality in the side chain, i.e., introduction of methyl groups near the chromophores, and the bulkiness of the chromophore itself contribute greatly to the side-chain chromophore orientation and suppression of excimer formation in poly( l -glutamines).…”