The pilot reported that during an agricultural application flight he had erroneously attempted a downwind takeoff after reloading product with the outside temperature estimated at 106 degrees. The airplane departed from the dirt strip westbound, and the pilot was looking directly into the setting sun. The airplane did not climb and impacted a ditch on the west end of the runway and nosed over. The airplane sustained substantial damage to the wings and fuselage. The pilot reported no abnormalities with the aircraft prior to the accident. He stated the wind had shifted while he was reloading and did not notice it until after the accident.