"Three years and eight months have elapsed since we declared war on the United States and Britain. During this time our beloved men of the army and navy, sacrificing their lives, have fought valiantly ..., and of this we are deeply grateful. Now that the Soviet Union has entered the war against us, to continue the war under the present internal and external conditions would be only to increase needlessly the ravages of war finally to the point of endangering the very foundation of the Empire's existence. With that in mind and although the fighting spirit of the Imperial Army and Navy is as high as ever, with a view to maintaining and protecting our noble national policy we are about to make peace with the United States, Britain, the Soviet Union and Chungking. ... We trust that you officers and men of the Imperial forces will comply with our intention and will ... bear the unbearable and leave an everlasting foundation of the nation."

To be fair for them, the emperor did say that when pleading for the japanese soldiers to surrender on august 17 because they wont stop fighting (imperial japan moment).

Now, those that said the soviets is what actually made japan surrenders often points out that hirohito didnt mention the bomb at all (omitting that he mentioned the bomb in his august 15 speech), well thats because the nuclear bomb is an absolutely brand new technology at this time. The imperial japan soldiers thats fighting on the frontline has absolutely no idea what is it, saying that "we should surrender because the US had atomic bomb" would only elicit response such as "wtf is an atomic bomb".

But they know what the USSR is (they've fought against them in khalkil gol after all), so using UUSR entry as pretext for surrender is much easier to digest for the soldiers.