The present invention relates to novel coloring phthalide compounds and recording materials using any of the phthalide compounds as coloring component capable of yielding images which absorb the light in the near infrared region by thermosensitive recording, pressure-sensitive recording or by use of laser beams.
A conventional pressure-sensitive recording material utilizes a chemical reaction between a colorless or light-colored leuco dye and a color developer capable of inducing color formation in the leuco dye to form colored images. More specifically, the pressure-sensitive recording material comprises (i) a coloring leuco dye sheet coated with a leuco dye which is dissolved in an organic solution and microcapsuled and (ii) a color developer sheet coated with a color developer for the leuco dye and a binder agent. The coloring leuco dye sheet is superimposed on the color developer sheet and pressure is applied thereto in such a manner that the microcapsules containing the leuco dye are ruptured so as to react with the color developer.
A conventional thermosensitive recording material generally comprises a support such as paper, synthetic paper or plastic film, and a thermosensitive coloring layer comprising as the main components a leuco dye and a color developer cable of inducing color formation in the leuco dye upon application of heat thereto to form colored images. The application of heat for such image formation is carried out, for instance, by a thermal head, a thermal pen, laser beams or a stroboscope.
These pressure-sensitive and thermosensitive recording materials are widely used because recording can be carried out more speedily by using a relatively simple device, without the complicated steps such as development and fixing of images, as compared with the recording materials for electrophotography and electrostatic recording.
Conventionally employed representative leuco dyes for use in such recording materials are Crystal Violet Lactone and Leuco Crystal Violet which are for blue-coloring, and fluorane compounds having an anilino substituent at its 7 position which are for black coloring. Further, in the conventional thermosensitive recording material, colorless or light-colored leuco dyes having, for example, lactone, lactam, or spiropyran rings, are employed, and organic acids and phenolic acid materials are employed as the color developers for the leuco dyes.
Recently optical character-reading apparatus have been developed, for example, for reading bar codes in bar code labels, and are used in a variety of fields. In these apparatus, light sources emitting light having a light wavelength of 700 nm or more, such as a light emit diode and a semi-conductor laser, are in general use. However, the above-mentioned leuco dyes, when colored, scarcely absorb the light in the near infrared region having a wavelength of 700 nm or more, so that a light source emitting light in the near infrared region cannot be effectively used when the above-mentioned leuco dyes are employed in the recording materials.
In order to improve this shortcoming of the conventional leuco dyes, leuco dyes capable of absorbing not only light in the visible region, but also the light in the near infrared region, have been proposed, for instance, in Japanese Laid-Open Patent Applications 51-121035, 51-121037 and 57-167979. The leuco dyes proposed in these Japanese Laid-Open Patent Applications, however, are insufficient in the power of absorbing the light in the rear infrared region for use in practice.