Femtosecond Transient Absorption Spectroscopy of Nanocrystalline Polydiacetylene Colloids

Abstract

Femtosecond transient absorption spectral measurements have been performed for sizeantrolled polydiacetylene microcrystals (30, 100, and 150 nm mean diameter) dispersed in water. Ultrafast relaxation processes within 5 ps after excitation were attributed to formation and decay of self-trapped exciton, which was independent on the size. A time-dependent spectral shift of transient bleaching, corresponding to the stationary excitonic absorption band, was observed in the time region from 10 ps to a few ns. It is explained as a thermalization of absorbed light energy within the microcrystal and the succeeding cooling by comparing transient absorption spectra w i t h temperature difference spectra of the ground state absorption. Size-dependent cooling dynamics of a 'hot' microcrystal was successfully demonstrated; the time constant increases w i t h the crystal size.

Size-Dependent Optical Properties of Polydiacetylene Nanocrystal

“…This is consistent with the previously reported data. [20][21][22] It should be, however, noted that the optical extinction is a sum of optical absorption Q abs and scattering Q sct coefficients. In dipole moment approximation, the optical extinction of a small spherical particle with a diameter d can be expressed as 27 where N is the number of particles per unit of volume, n is the reflective index of the surrounding media, λ is the wavelength of light, x is the size factor defined by πnd/λ , and 1 and m are the permittivities of the particle and the surrounding medium, respectively.…”

“…This is consistent with the previously reported data. [20][21][22] It should be, however, noted that the optical extinction is a sum of optical absorption Q abs and scattering Q sct coefficients. In dipole moment approximation, the optical extinction of a small spherical particle with a diameter d can be expressed as 27…”

“…Both the steady-state and the time-resolved optical studies in aqueous suspensions of poly-DCHD nanocrystals revealed a number of interesting dependencies of extinction resonance energy and recovery times on the size of the prepared nanocrystals. − Reported phenomena raised questions concerning the nature of the excited state and its relaxation dynamics in this class of materials. In this work, we will answer the question by investigating optical properties of nanocrystals in ensemble and microscopically at a single-crystal level.…”

Cooperative Photochemical Reaction in Molecular Crystal Induced by Intense Femtosecond Laser Excitation: Photochromism of Spironaphthooxazine

“…High temperatures are attained immediately after organic solids are excited with an intense short-pulse laser. It was proved for some organic crystals by picosecond and femtosecond transient absorption spectroscopy. − Ultrashort laser excitation of organic solids leads to significant heat generation because of various nonradiative decay channels of the electronically excited state. In the case of the SNO crystal, the photoinduced bond breaking and the subsequent ring-closure reaction convert electric excitation energy into heat within a nanosecond time scale, resulting in a rapid temperature elevation of the crystals.…”

Photochromic reactions of crystalline spiropyrans and spirooxazines induced by intense femtosecond laser excitationDedicated to Professor Frank Wilkinson on the occasion of his retirement.

“…High temperature attained by an intense short-pulse laser excitation was proved for some organic crystals by femtosecond and picosecond transient absorption spectroscopy. [59][60][61][62][63] Ultrashort laser excitation of organic solids leads to significant heat generation due to various nonradiative decay channels of the electronically excited state. The photoinduced bond breaking in a ps time region and the ring-closure reaction in a ns time scale, also result in rapid temperature elevation of the crystals.…”