Pressure effects on the spectra of dye molecules in incommensurate and commensurate phases of biphenyl

Molecular probing of low-temperature incommensurate phases

“…3), where one observes convergence of the low-pressure triplet to a high-pressure singlet at pressures above 180 MPa, i.e. in the crystalline phase CI (for more detailed discussion of pressure effects, see [12]). This proves that we have only one substitutional site for dopant molecules in the phase CI.…”

“…By cooling a biphenyl sample it passes from the ordinary high-temperature crystalline phase CI to an incommensurate phase ICII at 40 K followed by transition to another incommensurate phase ICIII at 17 K, the spatially modulated property being the twist angle between the planes of two phenyl rings. Under pressure at liquid helium temperature, the phase transitions ICIII ® ICII and ICII ® CI occur at~20 MPa and~180 MPa, respectively [12]. As a possible reason for absence of the lock-in transition to a crystalline ground state at low temperatures, the metastability of the phase ICIII was proposed («incommensurate glass» [13]).…”

Optical study of terrylene molecules in crystalline biphenyl: effects of pressure and temperature on the luminescence spectra

“…One of such systems is crystalline biphenyl (C 12 H 10 ) doped with terrylene (C 30 H 16 ) molecules, a system that we have studied in recent years by spectroscopic methods including single-molecule spectroscopy [1][2][3]. Biphenyl is an interesting substance which has three different phases at ambient pressure (P) -a normal, commensurate phase C I with monoclinic structure, which is stable from the melting temperature (T ) of ∼70 • C down to 40 K, and two incommensurate phases: IC II between 40 and 17 K and IC III below 17 K. With increasing P, the phase transition temperatures T I −I I = 40 K and T I I −I I I = 17 K decrease rapidly and become zero at ∼1.8 kbar and ∼0.2 kbar, respectively (the detailed phase diagram of biphenyl can be found in our previous papers [4,5] The purpose of the present study was to establish how these phase transitions in biphenyl affect the photoluminescence (PL) of impurity terrylene molecules, which are known as popular probes in single-molecule spectroscopy. Another goal was to elucidate the mechanism of T -induced shifts of spectral bands (lines) in the system under study.…”

Selective spectroscopy of terrylene in incommensurate matrix of biphenyl

“…for chlorin the width is about 50 cm À1 [2]. An extremely broad (over 300 cm À1 ) inhomogeneous distribution can be explained by strong static interaction of incommensurate host matrix with terrylene impurity molecules, which are extremely sensitive to their local environment [4].…”

“…[1] These variations can be studied spectroscopically by doping biphenyl with impurity molecules, those spectral characteristics depend on their local environment [2].…”