252 J. Soc. Photogr. Sci. Technol. Japan. (2010) Vol. 73 No. 5: 252 256 Original Paper Tuning the Thermochemical Properties of Oxonol Dyes for Digital Versatile Disc Recordable: Reduction of Thermal Interference in High-Speed Recording ディジタル ヴァーサタイル ディスク レコーダブル用オキソノール色素の熱化学的性質の制御 : 高速記録時の熱的干渉の抑制 Shin-ichi MORISHIMA *, Koji WARIISHI *, Hisashi MIKOSHIBA *, Yoshio INAGAKI *,****, Michihiro SHIBATA **, Hirokazu HASHIMOTO ** and Hiroshi KUBO **,*** 森嶌慎一 * 割石幸司 * 御子柴尚 * 稲垣由夫 *,**** 柴田路宏 ** 橋本浩一 ** **,*** 久保裕史 Abstract To reduce thermal interference between adjacent recording marks on a recordable digital versatile disc, we examined the thermochemical behavior of oxonol dyes for digital versatile disc recordable (DVD-R). We found that oxonol dyes with Meldrum s acid skeleton exhibited an abrupt reduction in weight with increasing temperature without generating excessive heat that is the fundamental cause of thermal interference. DVD-R with the oxonol dyes suppressed fluctuation in the shapes of recorded marks, thereby attaining compatibility with high-speed recording. 要旨ディジタル ヴァーサタイル ディスク レコーダブルにおける隣接する記録マーク間の熱的干渉を抑制するため,DVD- R 用オキソノール色素の熱化学的挙動を調べた. メルドラム酸骨格を有する色素が, 熱的干渉の根本原因である過度の熱発生を伴わずに, 急激に熱分解することを見出した. このオキソノール色素の使用により,DVD-R の記録マークの形状が不規則になるのを防ぎ, 高速記録適性を付与した. Key words: optical disc, oxonol dye, heat of thermolysis, entropy キーワード : 光ディスク, オキソノール色素, 分解熱, エントロピー 1. Introduction The growing demand for optical information recording media with increasingly high recording densities and speeds has promoted attempts to improve dye-based optical recording discs, including compact disc recordable (CD-R), digital versatile disc recordable (DVD-R), and blu-ray disc recordable (BD-R). Preventing excessive heat accumulation at recording marks and thermal interference between adjacent recording marks has been a major challenge. Digital data are recorded on these recordable discs by heatmode optical recording in which an amorphous solid dye film (i.e., the recording layer) is irradiated by an intensity-modulated laser beam, which generates a rapid, local temperature rise due to photothermal conversion. The resulting thermal deformation of the recording layer and neighboring layers is detected as a change in the reflectance of a low-intensity reading beam and converted into a digital signal. In a recording at high density and high speed, the position of the recording mark edge is not determined by just the position of the laser pulse: heat diffusion and accumulation in the disc also affect it. To record at higher speeds, the time between two consecutive write pulses is reduced, which results in less efficient cooling. The shorter distance between adjacent recording marks and higher recording power causes heating of neighboring marks in the track, altering their shape and position. Heat accumulation is especially pronounced in long marks, leading to undesired expansion of the mark. Thermally balanced write Received 19th, August 2010, Accepted 24th, September 2010 平成 22 年 8 月 19 日受付平成 22 年 9 月 24 日受理 * Synthetic Organic Chemistry Laboratories, Research & Development Headquarters, Fujifilm Corporation, 577 Ushijima Kaiseimachi, Ashigarakamigun, Kanagawa 258-8577, Japan 富士フイルム株式会社,R&D 統括本部, 有機合成化学研究所 258-8577 神奈川県足柄上郡開成町牛島 577 ** Recording Media Research Laboratories, Research & Development Headquarters, Fujifilm Corporation, 2-12-1 Ohgi-cho Odawara, Kanagawa 250-0001, Japan 富士フイルム株式会社,R&D 統括本部, 記録メディア研究所 250-0001 神奈川県小田原市扇町 2-12-1 *** Present address: Department of Project Management, Faculty of Social Systems Science, Chiba Institute of Technology, 2-17-1, Tsudanuma, Narashino, Chiba 275-0016, Japan 千葉工業大学社会システム科学部プロジェクトマネジメント学科 275-0016 千葉県習志野市津田沼 2-17-1 **** To whom correspondence should be addressed: yoshio_inagaki@fujifilm.co.jp
S. MORISHIMA et al. Tuning the Thermochemical Properties of Oxonol Dyes for Digital Versatile Disc Recordable 253 strategies, which control power dissipation by dividing the writing pulse into a series of shorter pulses, have been proposed to reduce thermal interference 1 3) ; however, the writing behavior is still strongly influenced by the thermal properties of the disc. It is difficult to substantially alter the thermal conductivity of the dye layer because organic compounds tend to have similar thermal conductivities. Thus, the thermochemical properties of dyes need to be tuned to suppress heat generation. Through attempts to design new dyes for DVD-R, metalcomplexed azo dyes were found to be suitable for high-speed DVD-R recording 4). Their rapid and marked weight reduction on thermal decomposition and their abrupt light absorption threshold around the recording beam wavelength are indispensable for achieving high density and high speed DVD-R recording. Suzuki et al. reported that Ni-azo dyes exhibited low light absorption, moderate heat generation, and good recording characteristics, making them suitable for multiple speed recording 5). Because the fundamental cause of thermal interference is excessive heat generation, we focused on designing dye molecules that minimize the heat generated during photothermal decomposition. The free energy change (ΔG) must be negative for a reaction to occur spontaneously. Since we require a small negative enthalpy of decomposition (ΔH), the entropy change (ΔS) must be large, in accordance with the thermodynamic equation ΔG=ΔH TΔS. In addition, the entropy term (TΔS) becomes more significant at high temperatures generated by laser irradiation. A large positive entropy change is often observed for reactions involving extrusion of gaseous fragments. According to Pine s extrusivity scale 6), extrusion of nitrogen (N 2 ) is highly exothermic ( 42 kcal mol 1 ), extrusion of carbon dioxide (CO 2 ) is slightly exothermic ( 2.6 kcal mol 1 ), extrusion of sulfur dioxide (SO 2 ) is endothermic (14.2 kcal mol 1 ), and extrusion of carbon monoxide (CO) is endothermic (15.4 kcal mol 1 ). Extrusion of CO 2 seems most suitable; it has a large entropy change and a small negative ΔH, enabling sufficient thermal energy to be supplied to produce deformation at a spot irradiated by the recording beam. We therefore employed oxonol dyes with heterocyclic moieties that thermolytically release volatile fragments, including CO 2, CO, SO 2, acetone (CH 3 ) 2 CO, and cyclohexanone. In the present paper, we assess the thermochemical behavior of these oxonol dyes in relation to the recording characteristics of high-speed DVD-R recording. 2. Experimental Materials. Oxonol dyes (1 5) containing heterocycles that thermolytically release gaseous fragments were prepared as described previously 7 12). Each dye consists of two oxonol anions and one bipyridinium cation (see Fig. 1). For comparison, an azo dye (6) (see Fig. 2) used in a commercial DVD-R was prepared according to its patent 13). Fabrication of Recording Discs. DVD-R sample discs schematically illustrated in Fig. 3 were prepared by conventional manufacturing processes. A solution of a dye in 2,2,3,3-tetrafluoro-1- propanol was spin coated onto a polycarbonate (PC) substrate (0.6-mm-thick, 120 nm in diameter) with guide grooves to form the recording layer. Pure silver was sputtered on the dye layer to form a reflective layer and a polycarbonate substrate without guide grooves was bonded on the reflective layer. Fig. 1 Structures of oxonol dyes 1 5 used in this study. Dashed lines enclose volatile fragments.
254 J. Soc. Photogr. Sci. Technol. Japan Vol. 73 No. 5 (2010) 3. Results and Discussion Fig. 3 Fig. 2 A practical Ni-azo dye for DVD-R Schematic diagram of a DVD-R sample disc: (A) pristine, (B) recorded. Measurements. The recording characteristics were measured on a test drive equipped with an air spindle motor and a laser pick-up head with an objective lens (numerical aperture=0.6) 14). The recording wavelength was 657 nm and the maximum recording power was 40 mw at the disc surface. Playback testing was performed on a test device equipped with a laser pick-up head (numerical aperture=0.6). The reading wavelength was 651 nm and the reading power was 0.4 mw. Jitter, defined as the standard deviation of the time variation of the digitized data passed through the equalizer, was measured relative to the clock of the phase-lock loop and normalized by the channel bit clock period 15). Tolerance to fluctuations in the recording power is expressed by the power margin, which is defined as the ratio of the maximum to minimum powers in which the jitter is less than a certain percentage (e.g., 13%) of the optimum recording power (i.e., the power that minimizes the jitter at a particular recording speed). The shapes of recorded marks, which were deformations on the surface of the polycarbonate substrate, were observed by scanning electron microscopy after peeling the protective and reflective layers away and washing the dye layer off the polycarbonate substrate with ethanol. Thermogravimetry (TG) and differential scanning calorimetry (DSC) were performed using Seiko Instruments TG/DTA6200 and DSC6000 analyzers, respectively. We considered that the weight losses observed in the temperature range between the beginning and the end of the first exothermic event in differential thermal analysis were due to extrusion of volatile fragments. We therefore calculated the weight losses due to extrusion of volatile fragments by comparing the weights at the first and the second inflexions of the TG curve because these points correspond to the beginning and the end of the first exothermic event, respectively. 3.1 Thermogravimetric analysis To confirm extrusion of gaseous fragments from the oxonol dyes, we compared the thermogravimetric and calculated weight losses. We employed 1,1'-bis(2-methylpropyl)-4,4'-bipyridinium as the counter cation in 1, 2, 4, and 5, because the cation in its bromide did not decompose below a temperature of 300 C. The weight reductions observed for dyes 1 5 below 300 C is thus due to fragmentation of the oxonol anion moieties. The weight losses due to extrusion of volatile fragments from these oxonol dyes can be estimated by analogy with the thermolytic decomposition pathway for Meldrum s acid (Fig. 4) 16,17). For example, the primary weight reduction observed for dye 1 was 46%, which is nearly equal to 42%, the value calculated for extrusion of one CO 2 molecule and one acetone molecule per heterocyclic moiety (enclosed in the dashed line in Fig. 1). The calculated and observed values are in good agreement for the dyes listed in Table 1, confirming thermolytic evolution of the presumed gaseous fragments from the dyes. Fig. 5 shows the thermogravimetric profiles of oxonol dyes 1 5, together with a practical DVD-R azo dye 6. Azo dye 6 and oxonol dyes 1 3 exhibited very abrupt weight reductions as the temperature increased, implying a high reaction rate, whereas dyes 4 and 5 showed a rather gradual weight reduction, suggesting a slow reaction. Dyes 1 3 and 6 are thus suitable for highdensity and high-speed optical recording, whereas dyes 4 and 5 are unsuitable for high-speed recording according to the criteria proposed by Suzuki et al. 4) 3.2 DSC measurements Taking Pain s relative extrusivity scale into consideration 6), we estimated that the thermolytic heat generated by oxonol dyes is much smaller than that generated by azo dyes. To compare the heat evolution on thermolysis of a practical azo dye and Fig. 4 Thermolysis of alkylidene derivatives of Meldrum s acid Fig. 5 Thermogravimetric profiles of dyes 1 6
S. MORISHIMA et al. Tuning the Thermochemical Properties of Oxonol Dyes for Digital Versatile Disc Recordable 255 Table 1 Thermolytic weight losses through extrusion of volatile fragments Oxonol Dye Formula Weight (W F ) 1 968 2 1128 3 1352 4 1140 5 1160 Volatile Fragments (F V ) (CO 2 + acetone) 4 (44 + 58) 4=408 (CO 2 + cyclohexanone) 4 (44 + 98) 4=568 (CO 2 + cyclohexanone) 4 (44 + 98) 4=568 SO 2 4 64 4=256 CO 4 28 4=112 Calculated (F V /W F ) 100 42 50 42 22 10 Weight Loss (%) Observed (Temperature/ C) 46 (166 213) 55 (212 243) 40 (226 257) 25 (236 274) 7 (170 184) 0 Fig. 6 (A) Oxonol dye 3 (B) Ni-azo dye 6 Deformation of lands and grooves of recorded discs: (top) bird s eye views; (bottom) cross-sectional views of the groove floors. oxonol dyes, we performed DSC measurements on dyes 3 and 6 in tightly sealed stainless-steel cases. Azo dye 6 was highly exothermic ( 849 J g 1, 348 367 C), whereas oxonol dye 3 was far less exothermic ( 111 J g 1, 226 257 C). The reduced exothermicity of the oxonol dye will suppress thermal interference between recorded marks. 3.3 Improved shape of recorded marks Differences in the heat of thermal decomposition affected the shape of recorded marks. Fig. 6(A) and (B) are scanning electron microscopic images of recorded marks on DVD-R with oxonol dye 3 and azo dye 6 recorded at 4 speed, respectively. DVD-R with dye 3 (Fig. 6(A)) showed very sharp mark edges and no deformation outside the groove, whereas DVD-R with dye 6 (Fig. 6(B)) showed indistinct mark edges and irregular deformation of the floor and the banks of the groove. This contrast confirms that using moderately exothermic dyes effectively suppresses irregularities in the shapes of recorded marks. 3.4 Quality of recorded signals The modest heat evolution on thermolysis of oxonol dye 3 suppresses fluctuations in the positions of recorded mark edges, resulting in low jitter. As shown in Fig. 7, a DVD-R with oxonol dye 3 exhibited a low bottom jitter of less than 8% over a wide range of recording powers and speeds 15). Sufficiently wide power margins of 20 30% were observed for recording speeds in the range 1 to 16, suggesting compatibility with multi-speed recording. 4. Conclusion We have designed oxonol dyes based on Meldrum s acid for a DVD-R compatible with high-speed recording. This improvement is due to reduced thermal interference between adjacent recorded marks, thereby reducing fluctuations in the shape of recorded marks. Excessive heat generation (ΔH) due to thermolysis of the recording dye at the spot where the writing laser
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