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Time resolved optical spectroscopy was used to observe molecular rotation over more than 14 decades in time for six probes in o-terphenyl (OTP). In contrast to previous studies, probe rotation times are found to depend significantly upon probe size in the deeply supercooled regime. Systematic deviations from the temperature dependence of the Debye–Stokes–Einstein equation are observed, however, these deviations are relatively small. These observations are inconsistent with some models of cooperative molecular motion near Tg which invoke rigid aggregates or locally liquidlike regions. The width of the relaxation spectrum (characterized by the KWW β parameter) systematically decreases with increasing probe size. Near Tg, the largest probe (rubrene) rotates with nearly a single exponential correlation function. Based on the observed trend in β, it is estimated that OTP is homogeneous on length scales greater than 2.5 nm at Tg.

Measurements of the rotational and translational motion of several probe molecules in supercooled 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene (RR -TNB) are reported. Rotational correlation times from 10 -10 to 10 3 s were measured in the temperature range from 341 to 515 K (T g ) 343.5 K). Translational diffusion coefficients ranged from 10 -6 to 10 -15 cm 2 /s over a similar temperature range. The rotational correlation times of all probes have essentially the same temperature dependence as the viscosity of TNB. In contrast, the translational diffusion of the smallest probe, tetracene, is observed to have a significantly weaker temperature dependence. At T g , the translational diffusion coefficient of tetracene is 300 times larger than expected based on its rotational correlation time. Similar enhancements of translational diffusion near T g have recently been observed in other glass-forming liquids. These observations suggest that dynamics in TNB and other supercooled liquids are spatially heterogeneous. The probes used in this study were tetracene, rubrene, 9,10-diphenylanthracene, and 9,10-bis(phenylethynyl)anthracene. IntroductionWhile systematic studies of the glass transition and the properties of supercooled liquids began at least seven decades ago, 1 the last decade has seen a resurgence of interest in these problems. 2 Relaxation processes in fragile supercooled liquids are characterized by non-Arrhenius temperature dependences and nonexponential relaxation functions. 2 Despite vigorous effort, the fundamental origin of these and other features of fragile liquids is still in dispute. For example, nonexponential relaxation functions can be interpreted in two different ways. One can imagine that a heterogeneous set of environments exists in a supercooled liquid; relaxation in a given environment is nearly exponential but the relaxation time varies significantly among environments. Alternatively, one can imagine that supercooled liquids are homogeneous and that each molecule relaxes nearly identically in an intrinsically nonexponential manner. Of course, these two viewpoints are extreme positions, and it is possible that elements of both pictures are applicable.We report here measurements of the rotational and translational motion of several probe molecules in supercooled 1,3-bis(1-naphthyl)-5-(2-naphthyl)benzene (RR -TNB, T g ) 343.5 K). As explained below, these measurements suggest that nonexponential relaxation functions in fragile supercooled liquids are at least partially the result of a heterogeneous set of environments. Rotational correlation times from 10 -10 to 10 3 s are reported here for probe molecules in liquid and supercooled TNB; these data were acquired between 341 and 515 K. We also report translational diffusion coefficients from 10 -6 to 10 -15 cm 2 /s for some of the same probe molecules over a similar temperature range. The rotational correlation times of all probes have essentially the same temperature dependence as the viscosity of TNB. In contrast, the translational diffusion of th...

The photophysical and photochromic properties of 2,2-diphenyl-2H-naphtho [1,2-b]pyran and substituted derivatives were investigated by steady state and time-resolved optical absorption and emission spectroscopy in solution at room temperature and in a frozen matrix at 77 K. Fluorescence quantum yields, fluorescence lifetimes, and singlet energies depend strongly on the substitution patterns. Photoexcitation of the naphthopyrans (A) leads to efficient ring opening to produce the merocyanines (B) and (C). The optical absorption of the merocyanines can be tuned by the substituents and can cover most of the visible spectrum (400-700 nm). The decoloration kinetics of the open forms (merocyanines) B and C to produce naphthopyrans (A) depends strongly on the substituents. The ring closure rate constants (k BfA ) range from 0.0009 to 0.04 s -1 . IntroductionIn the past few years, photochromic dyes, which show reversible photoisomerization, have attracted considerable attention because of their implications in a great number of practical applications including data storage, optical filters, displays, sensor protection, waveguides, and ophthalmic plastic lenses. 1-3 Chromenes, a class of photochromic compounds, have been studied in their photochromic properties in the pioneering work by Becker et al. more than 30 years ago. [4][5][6][7][8][9][10][11] Naphthopyran is a chromene derivative (benzochromene) and can be divided into three isomers, 3H-naphtho[2,1-b]pyrans, 2H-naphtho[1,2-b]pyrans, and 2H-naphtho[2,3-b]pyrans (Scheme 1). The photochemistry and photophysics of 3H-naphtho[2,1-b]pyrans were extensively studied. 9,10,[12][13][14][15][16][17][18] Although, 2H-naphtho[1,2-b]pyrans are commercially used in plastic ophthalmic lenses, 19 systematic studies of the photophysics and photochemistry are surprisingly rare. 8,20,21 Most studies concentrated on synthetic aspects. 15,17,22,23 Therefore, we investigated in detail the photophysics and photochemistry of 2,2-diphenyl-2H-naphtho[1,2-b]pyran, 1(A), and four of its derivatives, 2(A)-5(A), (Scheme 1). The derivatives 2(A)-5(A) were chosen to study influences of substituents on the properties.Ottavi et al. showed that irradiation with UV light of 3,3-diphenyl-3H-naphtho[2,1-b]pyran produces two colored forms of ring opening products (cis-trans and trans-trans). 12 Applying this reaction mechanism to 1(A) leads to Scheme 2 in which A is the (uncolored) closed form (pyran form) and B (cis-trans) and C (trans-trans) are open (colored) forms (merocyanine forms). For this investigation, the photophysical properties of the closed form (A) and the open forms (B and C) were determined separately, and afterward, the ring closure kinetics (B and C to A) were investigated. Experimental SectionThe naphthopyrans 1(A)-5(A) (PPG Industries, Inc.) were recrystallized from ethanol. Acetonitrile, ethanol, and methylcyclohexane (Aldrich, spectroscopic grade) were used as received.The UV-vis spectra were recorded on a HP 8452A diode array spectrophotometer using quartz cells with path lengths...