Benzpyrylium squarylium and croconylium dyes, and processes for their preparation and use

A process for generating heat in a medium comprising a dye, said dye comprising an inner salt of a compound of the formula:

• Q.sup.1 =Z-Q.sup.2
• wherein:
• Q.sup.1 is a 4-(benz[b]-4H-pyrylium)methylidene, 4-(benz[b]-4H-thiopyrylium)methylidene or 4-(benz[b]-4H-selenopyrylium)methylidene grouping;
• Z is a 1,3-(2-hydroxy-4-oxo-2-cyclobutylidene) hydroxide or 1,3-(2-hydroxy-4,5-dioxo-2-cyclopentylidene) hydroxide ring;
• and
• Q.sup.2 is a 4-(benz[b]-4H-pyran-4-ylidene)methyl, 4-(benz[b]-4H-thio-pyran-4-ylidene)methyl or 4-(benz[b]-4H-selenopyran-4-ylidene)methyl grouping;
• wherein at least one of the groupings Q.sup.1 and Q.sup.2 carries at its 2-position a substituent in which a non-aromatic carbon atom is bonded directly to the benzpyrylium, benzthiopyrylium or benzselenopyrylium nucleus, subject to the proviso that if said 2-substituent contains an aromatic nucleus, this aromatic nucleus is not conjugated with the benzpyrylium, benzthiopyrylium or benzselenopyrylium nucleus to which it is attached,
• which process comprises exposing at least part of the medium to infra-red actinic radiation of a frequency absorbed by the dye, whereby the radiation is absorbed by the dye and heat is generated within the parts of the medium exposed to the radiation.