If there's a "charitable giving chemical," it's oxytocin. When produced in the brain, it enhances empathy. One of the most important drivers of giving.
A recent post at the HBR Blog reports some recent research on the type of storytelling that's best at producing oxytocin in the audience: Why Your Brain Loves Good Storytelling.
The "trick" is tension:
... in order to motivate a desire to help others, a story must first sustain attention -- a scarce resource in the brain -- by developing tension during the narrative. If the story is able to create that tension then it is likely that attentive viewers/listeners will come to share the emotions of the characters in it, and after it ends, likely to continue mimicking the feelings and behaviors of those characters.
When you want to motivate, persuade, or be remembered, start with a story of human struggle and eventual triumph. It will capture people's hearts -- by first attracting their brains.
Storytelling is almost like magic. But you have to tell the story right to get the magic!