Peptide Magic: Interdistance-Sensitive Sign Inversion of Excimer Circularly Polarized Luminescence in Bipyrenyl Oligopeptides

Abstract

We prepared 16 novel chiral peptide-pyrene organic luminophores with different distances between the fluorescent pyrene groups and investigated their properties in CHCl 3 at room temperature. The peptide-pyrene organic luminophores bearing two pyrene groups emit strong excimer circularly polarized luminescence (CPL) in the solution state. Two pendant pyrenyl groups in a series of chiral oligopeptides clearly revealed excimer CPL in chloroform at 480-490 nm (j g em j % (2-8) 3 10 -3 ). When the distance between the two pyrenes increased, the sign of the CPL signals was inverted twice, while the sign of the corresponding circular dichroism (CD) signals was retained, (j g CD j % (3-8) 3 10 -5 ).Recently, chiral organic and organometallic luminophores that efficiently emit circularly polarized luminescence (CPL) in the near-ultraviolet, visible, and near-infrared regions have received much attention because of their potential optoelectronic and photonic applications. [1][2][3][4] However, several studies of chiral luminophores having conformational freedom have suggested that controlling photoexcited chirality is one of the most challenging, difficult issues in chiral photochemistry because the sign and absolute magnitude of the CPL signals in the photoexcited state often differ from those of the circular dichroism (CD) signals in the ground state. [3] Possibly, the limited knowledge and understanding of photoexcited-state chirality may lead to unpredictable CPL and CD characteristics, because most textbooks of stereochemistry mainly discuss the chirality of ground-and thermally excited-states. Steady-state CPL spectroscopy detects photoexcited, short-lived chiral species, whereas steady-state CD spectroscopy focuses on long-lived chiral species at ambient temperature.According to a modified Jablonski diagram of chiral luminophores, short-lived chiral species (S 1 , S 2 , …) upon photoexcitation are first generated according to the Franck-Condon scheme at~10 -15 sec, and undergo non-radiative relaxation associated with ro-vibrational modes to the lowest vibronic state (S 1 state with n = 0), at~10 -11 -10 -12 sec. These species relax to the chiral ground state with CPL radiation at~10 -9 -10 -6 sec. [3] To ensure identical chirality between the photoexcited and ground states, enantiopairs of rigid scaffolds bearing achiral luminophores can be designed when a pair of (+)-and (-)-sign CPL luminophores is needed.Several novel strategies that enable the inversion of the CPL signs of luminophores without significant changes in the chiral scaffolds have been reported. [4] These works showed that the CPL signs of chiral luminescent systems are not determined by the stereogenic centers and/or stereogenic bonds of the substances. The inversion characteristics of the CPL sign can be modified by the choice of solvent, [4a, k] host matrix, [4e] stir direction at the sol-gel transition temperature, [4b] geometrical modifications of luminophores, [4c, d, g] heating-and-cooling treatments, [4f, k] and aggregation-and-disag...

Solvent‐Sensitive Sign Inversion of Excimer Origin Circularly Polarized Luminescence in Bipyrenyl Peptides

“…The Φ f value of DD‐2 was 0.28 in CH 2 Cl 2 , 0.19 in MeOH, 0.18 in CH 3 CN, 0.23 in DMF, 0.13 in NMP, and 0.17 in DMAc. These PL characteristics were comparable to those in CHCl 3 (monomer λ em =398 nm, excimer λ em =484 nm, and Φ f =0.19) . The values of λ em and Φ f showed no significant solvent dependence.…”

“…The | g abs | values of DD‐2 at the first Cotton CD band, which is related to the excimer‐origin CPL bands, are on the order of 10 –4 : ∼0.7 × 10 −4 (345 nm) in CH 2 Cl 2 , ∼0.5 × 10 −4 (346 nm) in MeOH, ∼0.4 × 10 −4 (343 nm) in CH 3 CN, ∼0.3 × 10 −4 (345 nm) in DMF, ∼0.4 × 10 −4 (346 nm) in NMP, and ∼0.2 × 10 −4 (344 nm) in DMAc. The observed CD spectral profiles associated with these weak | g abs | values are similar to those in CHCl 3 (0.6 × 10 −4 (346 nm)) …”

“…Like DD‐1 , DD‐2 in CH 2 Cl 2 , MeOH, CH 3 CN, DMF, NMP, and DMAc (black solid lines) exhibits similar PL spectra to that in CHCl 3 (black dotted line) . Intense monomer‐origin and excimer‐origin PL bands are observed for DD‐2 in all solvents (Figure ). Like the excimer PL in CHCl 3 , the excimer PL bands of DD‐2 increased in intensity relative to those of DD‐1 .…”

“…DD‐2 and LL‐2 spectra are shown by black and grey solid lines, respectively. Previously reported d ‐configuration spectra in CHCl 3 are shown by black dotted lines . Path length=2 mm.…”

“…Recently, chiral oligopeptides with multiple pyrenes were found to emit intense excimer‐origin CPL signals . Additionally, the CPL signs of these pyrenyl peptides were controlled by the intramolecular distance between two pyrene units . However, the possibility of solvent‐controlled CPL sign inversion in chiral peptides has not yet been described.…”