US 10,356,899 B2  16  15  suitable solvent medium compnsmg one or more non› hydroxylic solvents (described below) at a general concen› tration of at least 0.1 mo!/! and up to and including 30 mo!/!;  and at room temperature, adding either one or more primary  alkylamines gradually to obtain a clear solution in the  resulting reaction solution. Specific details for these syn› thetic methods are provided below in the Examples.  Once prepared, the non-hydroxylic-solvent soluble silver  complexes can be stored in the form of solid (after the  solvent medium is removed by evaporative methods), or left  in the reaction solution under conditions that are optimum  for long-term stability (that is, negligible premature, non› photochemical reduction of silver ion to silver metal).  Some particularly useful non-hydroxylic-solvent soluble  silver complexes prepared according to the present invention  are represented by formula  (I):  (Ag•Ja(L)b(P)c  (I)  wherein:  a, b, and c are as defined above;  L has a molecular weight of 250 or less, and is represented  by either of the following formula (II) or (III):  wherein R$_{1 }$ is hydrogen or an alkyl group having 1 or 2  carbon atoms, R$_{2 }$ and R$_{3 }$ are independently branched or  linear alkyl groups having 1 to 8 carbon atoms, wherein any  of the hydrogen atoms in the alkyl groups can optionally be  replaced with a fluorine atom, and R$_{4 }$ is an alkyl group  having 1 to 8 carbon atoms;  Pis a 5- or 6-membered N-heteroaromatic compound that  is selected from the group consisting of pyridine, 2-meth› ylpyridine, 4-methylpyridine, 2,6-dimethylpyridine, 2,3-di› methylpyridine,  3,4-dimethylpyridine,  4-pyridylacetone,  3-chloropyridine, 3-fluoropyridine, oxazole, 4-methylox› azole, isoxazole, 3-methylisoxazole, pyrimidine, pyrazine,  pyridazine, and thiazole.  Photosensitive Reducible Silver Ion-Containing Composi› tions  An important characteristic of the present invention is to  ensure its general absorption and use in the UV to visible  part of the electromagnetic spectrum. Ideally, irradiating  energy should have wavelengths in the UV to visible region  (for example 350 nm to 450 nm) to allow selective excitation  of the photosensitizer in the presence of a large variety of  chromophores.  The photosensitive reducible silver ion-containing com› positions according to the present invention contain only  two essential components for purposes of providing silver  metal during photo-irradiation in a rapid manner: (a) one or  more non-hydroxylic-solvent soluble silver complexes  according to the present invention; and (b) a solvent medium  comprising one or more non-hydroxylic solvents as  described below. However, the presence of a third compo- nent, ( c) one or more photosensitizers as described below, is  very desirable in many embodiments, also can be considered  essential to assure rapid photoreduction of silver ion at room  or other low temperatures. No other components are pur› posely added to the photosensitive reducible silver ion› containing compositions according to the present invention  so that the present invention provides simplified composi› tions that exhibit rapid silver metal formation for various  methods and results. The incorporation of materials other  than the three components (a) through (c) is likely to impede  the generation of metallic silver.  The (a), (b), and (c) components can be put together in  general by mixing them under "safe light" (yellow light)  conditions if desired. This mixing can occur in suitable  solvent medium (as described below) comprising one or  more of the (b) solvents. The resulting photosensitive reduc- ible silver ion-containing composition can be provided in  liquid form having a viscosity of at least 1 centipoise and up  to and including 1,000 centipoises at 25°  C.  The one or more non-hydroxylic-solvent soluble silver  complexes can be present in an amount of at least 5 weight  % and up to and including 90 weight %, based on the total  weight of the photosensitive reducible silver ion-containing  composition.  Thus, in some embodiments, the photosensitive reducible  silver ion-containing composition can comprise two or more  different non-hydroxylic-solvent soluble silver complexes.  Such embodiments can also include two or more different  photosensitizers as described below.  The photosensitive reducible silver ion-containing com› position of claim  1  has exceptive long term stability as  demonstrated by a silver stability test such that when the  photosensitive reducible silver ion-containing composition  is held at ambient temperature (20°  C. to 25°  C.) and under  yellow safelight for 24 hours, less than 0.1 mo! % of its  original silver ion content is reduced to silver metal.  Solvent Medium:  The essential non-hydroxylic-solvent soluble silver com› plex (and photosensitizer if present) are generally solubi› lized in a suitable solvent medium that is predominantly  non-hydroxylic organic solvents that include but are not  limited to, acetonitrile, benzonitrile, acetone, methyl ethyl  ketone, butyronitrile, propylene carbonate, propionitrile,  isovaleronitrile, valeronitrile, and a mixture of two or more  thereof. It is also desirable that the non-hydroxylic solvents  do not participate in any redox reaction. That is, such  non-hydroxylic solvents should not be capable under general  preparation, storage, and use conditions to reduce silver ion  by transferring electrons or to oxidize an a-oxy carboxylate  by accepting electrons. Such reactions by non-hydroxylic  solvents would negatively impact the thermal stability of  photosensitive compositions according to the present inven› tion.  Water is not purposely added to the solvent medium, and  if water is present, it should be present at no more than 5  weight  % based on the total weight of the solvent medium  (not including the non-hydroxylic-solvent soluble silver  complex and photosensitizer).  Photo sensitizers:  The photosensitizers useful in the present invention ini› tiate the electrochemical transformation of the non-hydrox› ylic-solvent soluble silver complexes. The photosensitizer  must be capable of either reducing the silver ions or oxi› dizing the a-oxy carboxylates after the photosensitizer has  absorbed light (that is, photoinduced electron transfer) at an  electromagnetic radiation having a wavelength of at least  150 nm and up to and including 700 nm. Thus, the photo› sensitive compositions according to this invention are gen› erally sensitive to UV or visible electromagnetic radiation,  (II) 25  (III) 30 